extfieldsofstudy: Medicine Biology

text:
Opposite Transcriptional Effects of Cyclic AMP-Responsive Elements in Confluent or p27KIP-Overexpressing Cells versus Serum-Starved or Growing Cells

ABSTRACT The minute virus of mice, an autonomous parvovirus, requires entry of host cells into the S phase of the cell cycle for its DNA to be amplified and its genes expressed. This work focuses on the P4 promoter of this parvovirus, which directs expression of the transcription unit encoding the parvoviral nonstructural polypeptides. These notably include protein NS1, necessary for the S-phase-dependent burst of parvoviral DNA amplification and gene expression. The activity of the P4 promoter is shown to be regulated in a cell cycle-dependent manner. At the G1/S-phase transition, the promoter is activated via a cis-acting DNA element which interacts with phase-specific complexes containing the cellular transcription factor E2F. It is inhibited, on the other hand, in cells arrested in G1 due to contact inhibition. This inhibitory effect is not observed in serum-starved cells. It is mediated in cis by cyclic AMP response elements (CREs). Unlike serum-starved cells, confluent cells accumulate the cyclin-dependent kinase inhibitor p27, suggesting that the switch from CRE-mediated activation to CRE-mediated repression involves the p27 protein. Accordingly, plasmid-driven overexpression of p27 causes down-modulation of promoter P4 in growing cells, depending on the presence of at least two functional CREs. No such effect is observed with two other cyclin-dependent kinase inhibitors, p16 and p21. Given the importance of P4-driven synthesis of protein NS1 in parvoviral DNA amplification and gene expression, the stringent S-phase dependency of promoter P4 is likely a major determinant of the absolute requirement of the minute virus of mice for host cell proliferation.

Cell cycle progression is coupled to the phase-dependent transcription of certain genes required for phase-specific metabolic activities. The varying transcription rate of such genes can be ascribed to various transcription factors whose activity is differentially regulated throughout the cell cycle. The best studied of these is E2F. In G 1 and late S/G 2 , E2F is downregulated by the so-called pocket proteins (pRB, p107, and p130). At these points in the cycle, the pocket proteins are in a hypophosphorylated state and can interact directly with E2F (reviewed recently in reference 26). As a result, E2F fails to induce and can even repress transcription of its target genes (4). Yet at the G 1 /S-phase transition, E2F exerts a strong activating effect. At this time, phosphorylation of pocket proteins by cyclin-cyclin-dependent kinase (cdk) complexes dissociates the complexes (34). cdk inhibitors (CKIs) such as p27 KIP1 , p16, and p21 suppress E2F-mediated promoter activation (44,48,62). Other factors, notably members of the ATF/cyclic AMP response element (CRE)-binding protein (CREB) and Sp1 transcription-factor families, which interact, respectively, with CREs and GC boxes, may also contribute to cell-cycle-dependent gene transcription. Activation of the adenovirus E2 promoter by G 1 cyclins is mediated by both CREand GC-box-binding proteins (46). Furthermore, recent data suggest that a CRE motif is involved in S-phase activation (13) and G 1 -phase repression (60) of cyclin A gene expression. Results conflicting with this finding were obtained by other authors (44), however, and so the involvement of other factors besides E2F in cell cycle-dependent transcription remains to be unraveled.
In this study, we used the minute virus of mice (prototype strain, MVMp), an autonomous parvovirus, as a model for investigating cell cycle regulation of promoter function. This choice was based on the low genetic complexity of parvoviruses and on the fact that their life cycle is highly dependent on host cell factors, notably ones transiently expressed during the S phase of the cell cycle (42,49,54). Unlike other DNA viruses, parvoviruses fail to induce resting cells to enter the S phase (54). Consequently, their multiplication is delayed until the host cells enter on their own a round of genomic DNA replication. MVMp virions contain a linear, single-stranded DNA genome of about 5 kb, comprising two overlapping transcription units. The focus of this study is P4, the early promoter of MVMp. P4 directs expression of the parvoviral nonstructural (NS) proteins, the major one being NS1, a multifunctional DNA-binding protein required for replication of the parvoviral DNA. NS1 also transactivates the second parvoviral promoter (P38), thereby controlling the viral capsid genes (9,27). The activity of the P4 promoter is modulated in cis via motifs known to bind a variety of cellular transcription factors ( Fig.  1), including CREs, a GC box, and a putative E2F-binding site (16). Since the burst of parvoviral gene expression occurs as host cells enter the S phase (11), we have investigated whether the activity of promoter P4 is differentially regulated in the course of the cell cycle.
The results presented here show that the P4 promoter of MVMp is indeed activated at the G 1 /S-phase transition, principally via the proximal E2F-binding site. Our in vitro data suggest that at this stage, the E2F-binding motif binds the so-called free form of E2F. We have further uncovered a novel pathway of transcriptional up-and down-regulation, mediated by CREs previously shown to constitute binding sites for ATF/ CREB family transcription factors (36): when present in at least two copies, the P4 CREs mediate promoter activation in growing and serum-starved cells but promoter repression in contact-inhibited cells. The switch from activation to repression can be triggered by overexpression of CKI p27 KIP1 , a known mediator of contact inhibition-induced G 1 arrest, while CKIs p16 and p21 have no detectable effect. The new regulatory system thus clearly differs from the above-mentioned E2Fpocket protein system.

MATERIALS AND METHODS
Cell cultures and cell synchronization. The established lines of Fisher rat fibroblasts (FR3T3) and Swiss mouse embryo fibroblasts (NIH 3T3) were grown at 37°C in an atmosphere containing 5% CO 2 in Dulbecco's modified Eagle's minimum essential medium supplemented with 10% aseptic donor calf serum (DCS) and 1% sodium pyruvate. The established A9 cell line was grown in minimum essential medium supplemented with 5% aseptic fetal calf serum.
Cells were synchronized in phase G 0 /G 1 by culturing them for 72 h in Dulbecco's modified Eagle's medium supplemented with 0.5% DCS (serum starvation) or by keeping them for 72 h at high density (contact inhibition). Synchronization in phase G 2 was obtained by incubating the cells in 2 mM thymidine for 12 h, after which the thymidine was removed and the cells were treated with nocodazole (50 ng/ml) for 14 h.
Plasmids. Plasmid P4wtLuc expressing the firefly luciferase gene under the control of the MVMp P4 promoter was obtained as follows. First the HindIII-EcoRI fragment of plasmid pLucDSS (18), containing the luciferase gene and its genuine polyadenylation signal, was cloned into the pBSK Ϫ vector (Stratagene). This yielded plasmid pBSK-Luc. The 38-bp HindIII-XbaI fragment of pBSK-Luc was then replaced with an oligonucleotide of identical sequence except that the bases surrounding the ATG codon were modified to create an NcoI restriction site (CCATGG). Subsequently, the ScaI-NcoI fragment of plasmid pP4Cat (52), containing promoter P4 and downstream leader sequences up to the ATG codon, was inserted in front of the firefly luciferase gene, thus generating plasmid P4wtLuc.
Linker-scanning mutants of promoter P4 were obtained by site-directed mutagenesis. To this end, the palindromic P4 promoter was excised from plasmid P4wtLuc and cut in two, yielding one fragment containing the left arm of the palindrome from nucleotide (nt) ϩ1 (downstream from the P4wtLuc plasmid XbaI site) of the MVM genome to the AflIII site (nt 75) and a second fragment containing the right arm of the palindrome from the AflIII site to the XbaI site at the very beginning of the luciferase gene. This separation avoided competition between mutation-bearing oligonucleotides and the complementary sequence of the palindrome during the annealing step of mutagenesis. After 3Ј-recessed end filling by means of the Klenow fragment of Escherichia coli DNA polymerase I, P4 fragments were cloned into the blunted SalI site of the pAlter vector (Promega), allowing site-directed mutagenesis with the Altered Sites System (Promega) according to the manufacturer's recommendations. For this purpose, we used 46-mer oligonucleotides in which the BglII recognition motif (AGATCT) was substituted for a P4 promoter sequence of equivalent length. Subsequently, FIG. 1. Schematic representation of the MVMp P4 promoter, of the mutants derived from it, and of the oligonucleotides used in this study. (A) The upper panel depicts the P4 promoter from nt 1 to the translation initiation site at nt 261 (numbering according to Astell et al. [2]). The arrow indicates the transcription initiation site. Symbols represent transcription factors shown to interact with specific P4 promoter sequence elements: TBP, TATA-box-binding protein (1,16); Sp1, GC-boxbinding proteins (1,17); Ets, Ets family of transcription factors (17); E2F, E2F-binding-site-specific protein complexes (reference 16 and this study); mut17, as yet unidentified proteins binding to and activating promoter P4 via the DNA element mutated in P4mut17 (15); NF-Y, Y-box-binding protein (18); USF, E-box-binding protein (19); CREB/ATF, CRE-binding proteins (36). The 40 contiguous BglII substitutions introduced in mutants P4mut01 to P4mut40, respectively, are located along the promoter. (B and C) The sequence elements analyzed in this study are framed and aligned beneath the line diagram of the promoter. Underlined sequences indicate mutations introduced in the various elements. (B) P4 promoter constructs driving expression of the luciferase reporter gene. (C) Oligonucleotides used as probes or competitors in electrophoretic mobility assays.
the mutated left and right P4 fragments were substituted for the equivalent wild-type ScaI-AflIII and AflIII-NcoI fragments of plasmid P4wtLuc, respectively, thus generating a linker-scanning series of full-length mutated P4 promoters. The resulting plasmids were named P4mutxLuc (Fig. 1A), x varying from 1 to 40 in contiguous mutants spanning the whole promoter from left to right.
The luciferase gene was placed under the thymidine kinase (tk) promoter of herpes simplex virus type 1 (HSV-1). This was done by replacing in plasmid pBLCat4 (28) the AvaI-EcoRI fragment containing the chloramphenicol acetyltransferase (cat) gene with the NcoI-EcoRI fragment of pLucDSS (18), containing the luciferase gene. This yielded plasmid HSVtkLuc.
Deletion of the BamHI-BglII fragment from P4mut26Luc yielded the minimal promoter construct TATALuc, retaining only the TATA box and downstream region of promoter P4 (including the transcription start site and leader sequence). Synthetic oligonucleotides flanked by BamHI and BglII restriction sites and containing either a P4 promoter-derived CRE motif, a consensus GC box, or a mutated P4 promoter-derived CRE motif were phosphorylated and self-ligated. Monomers, dimers, and trimers of each oligonucleotide were isolated on a 5% polyacrylamide gel and substituted for the upstream P4 region between the BamHI and BglII sites of P4mut26Luc. The minimal reporter constructs obtained in this way were named CRExLuc, GCxLuc, CREmxLuc, where x varies from 1 to 3.
DNA transfection and reporter protein analysis. Growing cells were transfected by calcium phosphate coprecipitation (7). Serum-starved and contactinhibited cells were transfected with the help of the DOTAP reagent (Boehringer Mannheim) in the presence of 0.5% DCS, as recommended by the manufacturer.
In some experiments, the Polybrene-dimethyl sulfoxide (30%) shock method (6,22) was used to transfect serum-starved cells. Cultures (5 ϫ 10 5 cells per 60mm-diameter petri dish) were cotransfected with 1 g of reporter construct and 1 g of plasmid pBLCat4 used as a standard. In some assays, the DNA inoculum also included a p27 KIP1 -expressing effector plasmid (40) or the corresponding empty vector (pX) as a control. The total amount of inoculated DNA was adjusted to 6 g per transfection by addition of salmon sperm DNA. In transient expression assays, CAT or luciferase activity was measured 48 h posttransfection in samples containing equal amounts of total protein from whole-cell extracts. CAT activities were determined according to the phase extraction protocol (45). Luciferase activities were measured with a Berthold luminometer (3). Only luciferase activity readings at least threefold above background were considered significant.
To generate stable transfectants, cells were cotransfected with 0.1 g of plasmid pSV2Neo (51) and a 10-fold molar excess of the plasmid of interest. Pools of stable transfectants were selected for their resistance to G418 (500 g/ml) over a 14-day period.
Flow cytometry. Cells were harvested by trypsinization, washed with phosphate-buffered saline (PBS), and fixed for 5 min at room temperature in paraformaldehyde (2%) and lysolecithin (20 g/ml). The cells were then washed twice in PBS and fixed in 80% methanol; 10 5 cells were transferred to an Eppendorf tube, washed in PBS, and resuspended in 500 l of PBS containing RNase A (0.1 mg/ml). After incubation for 30 min at 37°C, propidium iodide was added (500 l of a 50-g/ml solution). Analysis by flow cytometry (fluorescenceactivated cell sorting [FACS]) was performed with a Becton Dickinson FACSort. Cell cycle profiles were determined by using the CellFIT cell cycle analysis software package (Becton Dickinson).
For determining the cell cycle distribution of transfected cells, vector pXNS1404-405 (57) expressing an inactive parvoviral NS1 protein was used along with the effector plasmid (1:1 molar ratio) to cotransfect the cells. These were harvested, fixed, and incubated with the NS1-specific rabbit antiserum Sp8 (16). NS1-positive cells were detected with the help of fluorescein isothiocyanateconjugated goat anti-rabbit immunoglobulin G. After propidium iodide staining, 10 6 events were acquired on the FACSort, and cell cycle distribution of fluorescein isothiocyanate-positive cells was analyzed as described above.
Cell protein extraction and gel retardation assays. Extractions were performed as described by Kumar and Chambon (23), with previously described modifications (5). Briefly, FR3T3 cells were harvested with a rubber policeman, collected in ice-cold PBS, and washed twice. The pellet was resuspended in 1.5 volumes of lysis buffer (20 mM HEPES [pH 7.9], 0.4 M NaCl, 25% glycerol, 1 mM EDTA, 2.5 mM dithiothreitol, 1 mM phenylmethylsulfonyl fluoride). After incubation on ice for 20 min, the lysate was frozen at Ϫ70°C, thawed on ice, and vigorously vortexed. After centrifugation (18,000 ϫ g, 10 min, 4°C), the supernatant was recovered, frozen in liquid nitrogen, and used as whole-cell protein extract in gel retardation assays.
The DNA oligonucleotides used as probes or unlabeled competitors are depicted in Fig. 1. Double-stranded oligonucleotide probes were end labeled to a high specific activity by using the Klenow fragment of E. coli DNA polymerase I. About 250 pmol of labeled probe was incubated in the absence or presence of a 100-fold molar excess of unlabeled competitor oligonucleotide for 10 min at room temperature with 0.5 g salmon sperm DNA, 2 mM MgCl 2 , 10% glycerol, and whole-cell extract (3 to 6 g of protein) in a final volume of 10 l. For supershift assays, the extracts were first incubated for 1 h on ice with antibodies (1 l) prior to addition of oligonucleotide. Antibodies were purchased from Dianova (cdk2) and from Santa Cruz (p130 and p107) or were a generous gift from M. Pagano (cyclin A). Following incubation, the samples were loaded onto a 4.5% nondenaturing polyacrylamide gel. Electrophoresis was performed in 0.3ϫ Tris-borate-EDTA for 90 min at 4°C.
Protein analysis by Western blotting. Total protein was extracted from 10 5 cells as described above and fractionated by electrophoresis on a 12% polyacrylamide gel containing 1% sodium dodecyl sulfate. Proteins were transferred onto a Hybond ECL membrane (Amersham) in a Trans-Blot semidry electrophoretic transfer cell (Bio-Rad) (200 mA for 1 h) according to the manufacturer's instructions. The membranes were incubated first for 1 h in blocking buffer (5% powdered milk, 1% sodium caseinate) and then for 1 h with antibodies diluted in blocking buffer. Monoclonal antibodies specifically recognizing CKI p27 (Transduction Laboratories) were used at 1:1,000 dilution, and polyclonal antisera raised against either cyclin A (generous gift from M. Pagano), rat CREB in its nonphosphorylated or phosphorylated (both from Upstate Biotechnology Inc.) state, human CKI p16 (Pharmingen), or human CKI p21 (Transduction Laboratories) were used at 1:2,500 dilution. Membranes were washed three times for 5 min each in PBS containing 0.1% Tween 20 and then further incubated for 1 h with horseradish peroxidase-conjugated goat anti-mouse or anti-rabbit immunoglobulin G (1:1,000 dilution in blocking buffer; Dianova). All incubations were performed at room temperature. Immunocomplexes were detected with ECL reagent (Amersham) according to the manufacturer's instructions.

MVMp P4 promoter activity is host cell cycle dependent.
The focus of this study was the regulation of the MVMp P4 promoter. Since a burst of viral gene expression is observed in the S phase (54), the question was: is the activity of the early MVM P4 promoter subject to cell cycle-dependent regulation? To monitor P4 activity through the different phases of the host cell cycle, we stably transfected FR3T3 rat fibroblasts with a P4-driven firefly luciferase gene (P4wtLuc [ Fig. 1]). To avoid positional effects, we used pools of stably transfected cells throughout the study.
FR3T3 cells were chosen for the ease with which they are synchronized following serum starvation or contact inhibition. Both modes of synchronization were used to obtain cell populations arrested in G 0 /G 1 , allowing analysis of events occurring at the G 1 /S-phase transition upon release from growth arrest.
After serum starvation for 72 h in medium containing 0.5% DCS, stable FR3T3 transfectants were released from growth arrest by increasing the DCS concentration to 20%. Every 4 h postrelease, we analyzed the cells' cell cycle distribution and measured their luciferase activity. Up to 90% of the cells in serum-starved populations were found to be arrested in G 0 . Some 20 h after release, 50% of the cells were found to have left G 1 and entered the S phase (Fig. 2C). Some P4 promoter activity was detected during the first 15 h after serum addition, while the cells were still mostly in G 1 ( Fig. 2A), but this activity increased about 10-fold as the cells entered the S phase ( Fig.  2A and B). The same experiment was performed on a control pool of FR3T3 cells stably transfected with a reporter gene driven by the cell cycle-independent HSV-1 minimal tk promoter. These cells showed little change in luciferase activity as the cells transited from the G 1 phase of the cell cycle to the S phase ( Fig. 2A and B).
Due to contact inhibition, FR3T3 cells stably transfected with the wild-type P4 promoter construct became arrested in G 1 when grown at high density (Fig. 3C). After subculturing, the cells were analyzed every 4 h to determine their cell cycle distribution and luciferase activity. Under these conditions, the cells synchronously entered a new cell cycle (Fig. 3C) and showed an approximately 16-fold increase in promoter P4 activity at the G 1 /S transition ( Fig. 3A and B). Stably transfected control cells containing the reporter gene under the control of the HSV-1 minimal tk promoter again displayed little induction of promoter activity.
Around 25 h after release from either type of growth arrest, a decrease in P4 promoter activity was detected, correlating with the appearance of cells in G 2 ( Fig. 2 and 3). Since the cells became desynchronized later after release, it was necessary to confirm that P4 is indeed down-regulated during G 2 . To this end, G 2 -arrested cells were obtained by adding nocodazole, a drug that disturbs microtubule polymerization without affecting transcriptional processes (8). G 2 -blocked cells exhibited the same luciferase activity as cells released from nocodazoleimposed arrest and allowed to enter G 1 (data not shown). We conclude that promoter P4 activity is differentially regulated in the course of the cell cycle, being higher in the S phase than in phases G 1 and G 2 . These same cell cycle-specific variations in P4 promoter activity were demonstrated in P4wtLuc-transfected NIH 3T3 and A9 mouse cells synchronized by serum starvation (data not shown).
A cis-acting E2F-like binding site mediates S-phase activation of promoter P4. The P4 promoter can interact with a variety of cellular transcription factors (1,16,17,19,36) (Fig.  1). To see whether S-phase P4 activation involves known transcription factor-binding sites or perhaps other, as yet unidentified sequences, we constructed a series of BglII linker-scanning P4 mutants, each driving the luciferase reporter gene. In each mutant construct, the BglII recognition motif (AGATCT) was substituted for a P4 promoter sequence of equal length. The whole P4 promoter and immediate downstream region were scanned with consecutive substitutions spanning nt 19 to 25 in mutant 1 (P4mut01Luc) up to nt 255 to 260 in mutant 40 (P4mut40Luc) (Fig. 1). We then generated pools of FR3T3 cells stably transfected with the different mutant P4 reporter constructs. In these cells, luciferase activity was assayed at 4-h intervals after release from serum starvation or contact inhibition.
As shown in Fig. 2B, all of the P4 mutants (illustrated for P4mut04Luc and P4mut23Luc) but one (P4mut22Luc) retained the ability of the wild-type promoter to be activated during the S phase following release from serum starvation. In P4mut22Luc, which exhibited similar luciferase activities during G 1 and S, the BglII recognition motif has replaced a sequence resembling an E2F-binding site (59). To further test the involvement of this putative E2F-binding element in increasing P4 promoter activity during the S phase, we used another P4 promoter construct (P4mE2FLuc [ Fig. 1]) carrying point mutations previously shown to inactivate genuine E2Fbinding sites (35). This mutant also exhibited comparable activities in G 1 and S (data not shown), indicating that the E2F-like element is indeed important in cell cycle-dependent regulation of the P4 promoter and is probably a functional E2F-binding site.
Further confirmation of the involvement of this site came from experiments conducted in cells released from contact inhibition, where the above-mentioned P4mut22 derivative again exhibited little activation in the S phase compared to the wild-type promoter (Fig. 3A). Interestingly, two other sites besides the E2F-like element were found to impair S-phase P4 induction after release from contact inhibition. These additional sites correspond to substitutions mut04 and mut10 (Fig.  3B), which affect known CRE motifs (36). Each of these mutations taken independently has the same effect on the cell cycle regulation of the P4 promoter as both mutations combined (Fig. 3B). The different behaviors in G 1 of contact inhibition-and serum starvation-synchronized cells may reflect the fact, discussed below, that the CREs of promoter P4 exert opposite effects in contact-inhibited and serum-starved cells.
Experiments with stably transfected FR3T3 cells, like those just described, are suitable for determining changes in the activity of a given P4 promoter construct during the cell cycle, but for lack of an internal standard, they are unsuitable for comparing different promoter derivatives to determine their strengths. It is not possible, in such experiments, to determine whether the S-phase increase in P4 promoter activity mediated by the E2F-like site is due to stimulation of transcription in the S phase, inhibition in G 1 , or both. To address this question, we used a standardized transient transfection protocol to compare wild-type and mutant P4 promoter constructs in G 1 -and Sphase cells. Serum-starved FR3T3 cells were cotransfected with a wild-type or mutant reporter construct (all constructs were used in equal amounts) plus pBL4Cat, used as an internal standard to determine transfection efficiency. The cells were then released into the mitotic cycle. Luciferase activities were determined 4 and 20 h postrelease and adjusted for differences in transfection efficiency. As shown in Fig. 4 (P4mut22Luc panel), mutant P4 constructs with an altered E2F-like site behaved like wild-type constructs in arrested cells and during the G 1 phase following release but displayed less reporter gene expression than the wild type during the S phase. This was true in both starvation-synchronized and contact inhibition-synchronized cells and indicates that the E2F-like motif acts specifically in the S phase to activate promoter P4. As a control, a P4 promoter construct mutated in the Ets-binding site (P4mut23Luc) was analyzed in the same way. As illustrated in Fig. 4 (P4mut23Luc panel), the mut23 substitution impaired P4 functioning to similar extents in G 1 -and S-phase cells, confirming that the Ets element is involved in overall P4 promoter activation (17), independently of the point in the mitotic cycle (see above).
The P4 E2F-like motif interacts with different E2F-containing protein complexes in the course of the cell cycle. The E2Flike element of P4 lies in the proximal region of the promoter, immediately upstream from the Ets-and Sp1-binding sites (1,17). Recently, in vivo and in vitro footprinting experiments have demonstrated its interaction with cell proteins (16). To identify these proteins, we performed gel retardation assays with crude extracts of asynchronous FR3T3 cells, using a radiolabeled oligonucleotide probe (E2FP4 [Fig. 1C]) matching a region of the P4 promoter encompassing the E2F-like and Ets motifs. As illustrated in Fig. 5A, two major and several minor protein-DNA complexes were detected (lane 7). These complexes all proved specific, since their formation was inhibited by the homologous competitor oligonucleotide but not by heterologous competitors (data not shown). The protein constituent of the lower major complex has been shown to belong to the Ets family of transcription factors (17). Formation of this complex was specifically suppressed by the mE2F competitor (Fig. 5A, lane 6) containing an Ets-binding motif but no functional E2F recognition site (17) (Fig. 1C). We therefore included the mE2F competitor in all further assays so as to detect only E2F-specific DNA-protein complexes.
E2F is the generic name of DNA-binding heterodimers composed of a member of the E2F family and a member of the related DP family (25). In the course of the cell cycle, the transcriptional activity of E2F is differentially regulated through interactions with other proteins, in particular the product of the retinoblastoma tumor suppressor gene (pRB) and related proteins (p107 and p130), referred to as pocket proteins, as well as cyclins and cdks (24). To check that the E2F motif of promoter P4 is also the target of mitotic cycle-dependent E2F complexes, we performed gel retardation assays with the E2FP4 probe and crude extracts of synchronized FR3T3 cells harvested at different times after release from serum starvation (Fig. 5A, lanes 1 to 5). The specificity of the complexes observed under these conditions was ascertained in competition experiments with homologous and heterologous oligonucleotides (data not shown). We also performed immunoshift assays to identify the protein constituents of the resolved complexes (Fig. 5B). Extracts of serum-starved cells arrested in G 0 and sustaining basal P4 activity gave rise to one major E2F-specific complex called E2 (Fig. 5A, lane 1). The E2 band was specifically supershifted upon addition of polyclonal antibodies reacting with pocket protein p130 (Fig. 5B, lane 3) and thus appears to correspond with a multiprotein complex containing at least this polypeptide in addition to E2F. P4 promoter activation at the G 1 /S transition was accompanied by the disappearance of the G 1 -specific E2 complex and the appearance of three more rapidly migrating complexes named E3, E4, and E5 (Fig. 5A, lane 2). These complexes were supershifted upon addition of bacterially expressed glutathione S-transferase (GST)-Rb fusion polypeptide to the extract (Fig.  5B, lane 8). GST alone had no effect (lane 9). GST-Rb carries the pocket region of the pRB protein (21) which specifically interacts with so-called free E2F (the E2F-DP heterodimer). Furthermore, E3-E5 complexes migrate, with respect to E1 and E2, as do the free E2F complexes of a well-characterized E2F-binding site with respect to the corresponding pocket protein-E2F complexes (44). We thus attributed formation of complexes E3, E4, and E5 to binding of free E2F. Later in the S phase, we detected the slowly migrating complex E1 in addition to the three just mentioned (Fig. 5A, lanes 3 and 4). E1 appears to be a multiprotein complex comprising E2F, cyclin A (Fig. 5B, lane 7), p107 (Fig. 5B, lane 5), and cdk2 (data not shown). All complexes faded when the cells moved into the G 2 phase, i.e., when promoter P4 reverted to basal activity (Fig.  5A, lane 5). Hence, S-phase activation of P4 correlates with changes in the complex binding to the E2F site, from a G 1specific E2F-p130 complex to S-specific free E2F forms accompanied later by an E2F-cyclin A-p107-cdk2 complex.
CREs mediate P4 promoter repression in contact-inhibited cells. As illustrated in Fig. 3B, all three mutants affected in one or both CRE motifs of promoter P4 showed hyperactivity in stable transfectants arrested in mid-G 1 by contact inhibition. After dilution, this effect disappeared as the cells reentered the G 1 phase of the mitotic cycle. Whether one CRE, the other, or both were mutated, the G 1 effect was the same. To determine whether the CRE elements mediate promoter P4 repression in confluent cells, activation in growing cells, or both, we again performed transient transfection assays as described above for analysis of the E2F-like binding site, this time comparing the wild-type construct with ones carrying a mutated CRE. Since all three CRE mutants displayed similar regulation through the cell cycle, we limited our study to the P4mut04Luc construct.
As shown in Fig. 4B (P4mut04Luc panel), the CRE acted in cis both as a silencing element in contact-inhibited cells and as an activating element in growing cells released from confluence. CRE-mediated activation was also observed in growing cells released from serum starvation. In the latter case, the CRE contributed similarly to P4 activity during G 1 and S (Fig.  4A, P4mut04Luc panel). This means that it has little to do with S-phase P4 induction (cf. Fig. 2B). Interestingly, the CRE appeared to play a different role in serum-starved cells than in contact-inhibited cells: in the former, CRE-mediated activation rather than repression was observed (Fig. 4A,  P4mut04Luc panel). P4 silencing through the CREs thus appears unrelated to cell growth arrest per se but rather specific to contact inhibition. This was further ascertained by transiently transfecting asynchronous FR3T3 cell cultures with constructs driven by wild-type and CRE-mutant P4 promoters and measuring promoter activity at different cell densities during subsequent growth. In agreement with the activating role of CREB/ATF transcription factors in proliferating cells (see above and reference 36), the CRE mutant was two to three times weaker than the wild-type promoter in subconfluent cultures. As the cells reached confluence, the activity of the wildtype promoter progressively decreased, the CRE mutant being less affected and becoming up to 15 times more potent than the wild-type P4 (data not shown). Release from contact inhibition caused the CRE to switch from an inhibitory to an activating element (Fig. 4B, P4mut04Luc panel). This may explain why S-associated induction of promoter P4 is stronger in cells synchronized by contact inhibition than in cells synchronized by serum starvation (cf. Fig. 2B and 3B).
Due to their location within the left-hand terminal palindromic sequence of the parvoviral genome, the two CREs are mirror images of each other (36). Both interact with the same proteins of the ATF/CREB transcription factor family, forming three specific protein-DNA complexes (labeled A, B, and C in Fig. 6A) which can be visualized in gel retardation assays (36). The protein constituents of these three complexes have been analyzed by immunoshift assays and in vitro reconstitu- FIG. 5. Association of FR3T3 cell proteins with the P4 promoter E2F-binding site. Whole FR3T3 cell extracts were incubated with 32 P-end-labeled oligonucleotide E2FP4 containing the E2F-like motif of promoter P4 (Fig. 1). Specific DNA-protein complexes (arrows) were separated by electrophoresis and revealed by autoradiography. The free probe ran out of the gel. (A) Extracts were prepared from FR3T3 cultures at various intervals after release from confluence (corresponding to the indicated phases of the cell cycle) and supplemented with a 100-fold molar excess of unlabeled mE2FP4 competitor oligonucleotide. Asynchronous cell extracts (as) supplemented (or not) with a 100-fold molar excess of unlabeled mE2FP4 or mut23P4 competitor oligonucleotides were included for comparison. The mE2FP4 competitor prevents formation of the Ets-specific complex, whereas mut23P4 competes for formation of E2F-specific complexes. (B) Supershift assays were conducted, in the presence of a 100-fold molar excess of unlabeled mE2FP4 oligonucleotide, using either antibodies (␣) directed against proteins known to interact with E2F (p130, p107, and cyclins [cyc.] A and E) or recombinant proteins purified from E. coli (GST and GST-Rb fusion polypeptide). p.i., preimmune serum. tion experiments using ATF/CREB proteins purified from recombinant baculovirus-infected Spodoptera frugiperda SF9 cells. Complexes A and B result from binding of CREB1 homodimers and CREB1/ATF1 heterodimers, respectively, while C is a still elusive complex comprising ATF1 (36a). To detect possible cell cycle-dependent changes in protein-CRE interactions, we performed gel retardation assays with an oligonucleotide probe (CREP4 [Fig. 1C]) bearing the CRE a element of the P4 promoter, using FR3T3 whole-cell extracts prepared at different times after release from contact inhibition. Complexes A, B, and C were detected irrespective of the phase in which the cells were harvested (Fig. 6A, lanes 2 to 6). However, it is worth noting that the relative abundance of complex A was lower when the CREP4 oligonucleotide was incubated with Sand G 2 -specific extracts compared with G 1 extracts. DNA binding of transcription factor CREB1 (responsible for the formation of complex A) requires phosphorylation of a serine at position 133 (12). Western blotting analysis of the abovementioned cell extracts was performed to detect possible cell cycle variations in CREB1 production and/or phosphorylation. The total amount of CREB1 was found to decrease when cells reentered the mitotic cycle after contact inhibition (Fig. 6B, upper panel, lanes 1 to 4). Phosphorylated CREB1 was likewise down-regulated (Fig. 6B, lower panel, lanes 1 to 4), so that the ratio of phosphorylated to total CREB1 remained unchanged. Furthermore, serum-starved cells were similar to contact-inhibited cells with regard to their content in both total and phosphorylated CREB1 (Fig. 6B, lanes 5). We conclude that the CRE-mediated promoter P4 inhibition specific to contact-inhibited cells cannot be explained by the observed variation of CREB1 steady-state levels and does not correlate with a significant change in CREB1 phosphorylation.
CKI p27 KIP1 accumulates in FR3T3 cells grown to a high density and converts CRE motifs from activating to inhibitory elements. It is worth noting at this point that serum-starved and contact-inhibited cells are not arrested in exactly the same state. Growth arrest induced by serum starvation (best described as a G 0 state) is accompanied by down-regulation of genes encoding proteins required for cell cycle progression (such as cyclins [61]) and by stabilization of the CKIs p16 and p21 (61). In contrast, growth suppression at high cell density is mainly due to failure of another CKI, p27, to be degraded (39). Similar stabilization of p27 has been found to correlate with cell growth arrest in mid-G 1 following lovastatin treatment (20). Interestingly, we observed CRE-dependent inhibition of P4 promoter activity by lovastatin in the above-mentioned stable transfectants harboring the luciferase-encoding plasmid (data not shown). Furthermore, p27 was found to accumulate in confluent but not in serum-starved FR3T3 cells and to be quickly degraded upon cell release from contact inhibition, before the induction of cyclin A production that marks the S phase (Fig. 7A). These observations suggest that CRE-mediated inhibition of the P4 promoter in contact-inhibited cells could involve CKI p27.
To test this possibility directly, we cotransfected asynchronous FR3T3 cells with a CKI (p16, p21, or p27)-encoding plasmid and a construct bearing a reporter gene driven by wild-type or mutant promoter P4. As shown in Fig. 7B, plasmid-driven overexpression of all tested CKIs could be detected by Western blotting and led to cell cycle arrest in G 1 . Furthermore, overexpression of CKI p27 resulted in P4 promoter inhibition (Fig. 7C, column 7). This inhibitory effect of p27 required the presence of two functional CRE motifs (Fig. 7C, columns 8 and 9) but did not depend on an intact proximal GC box, as shown by the behavior of the mut24 derivative, in which this box is replaced by a BglII site (Fig. 1A). This derivative proved as sensitive to p27 overexpression as the wild-type promoter (Fig. 7C, column 10). In contrast, overexpression of CKIs p21 and p16 had little or no effect on the activity of the P4 promoter, whether wild type or mutated (Fig. 7C, columns 1 to 6). The slight repression observed when p21 was overexpressed proved to be CRE independent (Fig. 7C, columns 5  and 6). The CRE motifs thus appear as specific targets for p27-induced inhibition of the P4 promoter. In cells grown to confluence, plasmid-driven p27 synthesis resulted only in limited (yet significant and CRE-mediated) suppression of promoter activity (Fig. 7C, columns 11 to 14). This result is in keeping with the high level of p27 naturally accumulated by contact-inhibited cells (see above).
To confirm the ability of the P4 CREs to mediate p27induced promoter silencing, one to three copies of the CRE aЈ motif were cloned in front of a minimal promoter consisting of the P4 TATA box and downstream sequences. Constructs containing at least two copies of the CRE proved sensitive to plasmid-driven p27 overexpression (Fig. 7D, columns 5 and 6), while a construct comprising only one CRE did not respond (Fig. 7D, column 4). Minimal promoters supplemented with one to three repeats of the same length but containing a GC box (Fig. 7D, columns 1 to 3) or a mutated (BglII substitution) CRE (Fig. 7D, columns 7 to 9) likewise failed to respond to p27. When present in at least two copies, the P4 CREs are thus both necessary and sufficient to mediate the repressive effect of p27, even outside the context of the full P4 promoter.

DISCUSSION
We have shown here that regulation of the P4 promoter of parvovirus MVMp is host cell cycle dependent. Analysis of a series of linker-scanning mutants of promoter P4 has enabled us to distinguish two components of this regulation: activation at the G 1 /S transition and repression specific to growth arrest at high density (contact inhibition). We have further identified cis-acting elements within the promoter that mediate these two processes. A binding site for the E2F transcription factor is the major determinant of S-associated P4 activation, while two CREs mediate promoter silencing in contact-inhibited cells.
The E2F-binding site is required for P4 promoter activation when host cells enter the S phase. Promoter P4 drives the transcription unit encoding the parvoviral NS proteins, partic-FIG. 6. FR3T3 cell proteins interacting with the P4 promoter CRE motif. (A) Extracts from asynchronous FR3T3 cultures (as) or from FR3T3 cells harvested at different time points after release from contact inhibition were incubated with 32 P-end-labeled oligonucleotide CREP4 (Fig. 1). Specific DNA-protein complexes (arrows; see reference 36) were fractionated by electrophoretic mobility shift assays and revealed by autoradiography. The free probe ran out of the gel. (B) Extracts from FR3T3 cells released from contact inhibition (lanes 1 to 4) or blocked by serum starvation (lanes 5) were analyzed by Western blotting for their content of CREB1. Upper panel, total CREB1; lower panel, CREB1 phosphorylated on serine residue 133. The antibody specifically recognizing phosphorylated CREB1 did not react with CREBs when the extracts were pretreated with phage lambda phosphatase (data not shown). M, apparent molecular mass of proteins standards.
FIG. 7. Influence of p27 overexpression on whole and minimal P4 promoter construct activities. (A) Whole protein extracts were prepared from FR3T3 cells at increasing times after release from confluence or serum starvation (see Fig. 2C and 3C for cell cycle progression). Proteins were fractionated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and transferred to a nitrocellulose membrane. The upper and lower parts of the blot were incubated with antibodies directed against cyclin A and CKI p27, respectively. Immunocomplexes were revealed with the ECL detection system. M, apparent molecular masses of protein standards. (B) FACS analysis of FR3T3 cultures cotransfected with either of the CKI-expressing plasmid (p16, p21, or p27) or with the corresponding empty vector (pX). CKI overexpression was verified by Western blot analysis shown below the FACS profiles. (C) Growing or confluent FR3T3 cell cultures were cotransfected with each of the indicated P4Luc reporter plasmids (Fig. 1B) and either p16, p21, or p27 CKI-expressing vector. Transient expression assays were carried out at 48 h posttransfection and are presented as ratios of luciferase activities achieved in the presence of empty versus CKI-expressing vector (i.e., as factors of CKI-induced promoter repression). (D) Growing FR3T3 cell cultures were cotransfected with each of the indicated minimal promoter constructs (see Materials and Methods) and p27-expressing vector. Transient expression assays were carried out at 48 h posttransfection and are presented as ratios of luciferase activities achieved in the presence of empty versus p27-expressing vector (i.e., as factors of p27-induced promoter repression).
ularly the major product NS1, essential to amplification and expression of viral DNA (10). Hence E2F-mediated S-phase induction of NS gene expression may contribute, along with the S-phase specificity of host factors directly involved in the viral life cycle, to the well-known dependence of parvovirus replication upon host cell proliferation (11). The present results indicate that destruction of the E2F motif leads to an 80% reduction of promoter P4 activity in the S phase, without affecting the basal expression measured in G 1 . Furthermore, data to be presented elsewhere substantiate the importance of the E2F-binding site for the parvoviral life cycle, showing that S-phase induction of promoter P4 is necessary for the burst of parvoviral DNA replication and gene expression, and that mutations in E2F that abolish S-phase activation of P4 inactivate the virus unless the NS proteins are provided in trans (11a). Thus, S-phase activation of promoter P4 provides a clue to the S phase dependence of parvovirus multiplication.
The generic name E2F designates heterodimeric transcription factors composed of two polypeptides, an E2F and a DP family member. Currently, five distinct E2F and three DP proteins have been characterized (25,26,33). The transcriptional activity of E2F-DP dimers, often referred to as free E2F, is regulated according to the point in the cell cycle by formation of higher-order complexes. In particular, interaction with members of the pocket protein family (pRB, p130, and p107) prevents E2F from activating responsive promoters. It can even lead to repression of some promoters (50). Binding of pocket proteins to E2F heterodimers (and hence the activity of the E2F transcription factor) is regulated by pocket protein phosphorylation, apparently catalyzed mainly by cell cycle-regulated cdks (14,53). E2F proteins interact with cyclins and cdks either directly or through pocket proteins. After phosphorylation by cyclin-cdk complexes at the G 1 /S transition, pocket proteins can no longer bind to E2F proteins. This leads to the formation of transcriptionally active free E2F, known to transactivate target promoters at the G 1 /S transition (34). Accordingly, variations in P4 promoter activity throughout the cell cycle correlate with binding of different complexes to the E2F-binding site. During the G 1 phase, the site is occupied by an E2F-p130 protein complex. In agreement with the ability of p130 to prevent E2F from activating transcription (56), only basal P4 activity is detected in G 1 . Yet contrary to some other promoters (55), P4 does not appear to be a target of p130mediated repression. P4 activation at the G 1 /S transition is accompanied by replacement of the E2F-p130 complex by free E2F at the E2F-binding site, in keeping with the view that free dimers are the transcription-enhancing forms of E2F (4). Transient P4 hyperactivity during the S phase coincides with maintenance of free E2F at this site and the appearance of an additional higher-order complex containing p107, cyclin A, and cdk2. This complex is thought to be involved in the later inhibition of E2F-mediated transactivation (4). All of these complexes fade in late S and G 2 , when the P4 promoter reverts to its basal activity. Proteins p130 and p107 interact specifically with certain members of the E2F family, in particular E2F4 and E2F5 (4). Since these pocket proteins are present in some of the complexes formed with the P4 E2F-binding site, E2F4 and E2F5 are candidate constituents of these complexes.
We have further shown that CREs present within the P4 sequence mediate promoter down-regulation in contact-inhibited FR3T3 cells. This silencing effect is restricted to G 1 arrest at high cell density. Interestingly, this repression was observed only in the presence of two functional CRE motifs. When contact-inhibited cells are diluted and allowed to reenter the mitotic cycle, the CREs switch to mediating promoter activation. In most situations, in fact, these CREs appear to mediate up-regulation of the P4 promoter, notably in cells arrested in G 0 /G 1 by serum starvation or released from this block into the mitotic cycle and allowed to progress through the various phases. Contact inhibition thus represents a special situation in which the CRE-mediated effect on P4 promoter activity is negative rather than positive. Two regulatory pathways with opposite transcriptional effects thus appear to converge on the CREs of the P4 promoter, whose response clearly distinguishes contact inhibition from serum starvation.
Cell cycle progression is regulated by the sequential activation of different cyclins and cdks (37,38). Inhibitors of specific cyclin-cdk complexes (the CKIs) play key roles in this regulatory network (15,31). Serum starvation causes cycle arrest in the G 0 /G 1 phase, mainly as a result of reduced expression of a variety of proteins including the D, E, and A cyclins (58). Release from serum starvation allows cyclin reexpression and subsequent cell cycle progression (41). In contrast, contact inhibition causes induction of various CKIs, notably p27 KIP1 , leading to cell cycle arrest in mid-G 1 through inhibition of G 1 cyclin-cdk complexes (31,39). Release from contact inhibition is accompanied by degradation of p27 KIP1 , allowing reentry into the cell cycle. Hence, serum starvation and confluence cause growth arrest via different mechanisms. This may explain the opposite regulatory effects of CREs on the P4 promoter in these two situations. In keeping with this view, we have confirmed, first, that contact-inhibited FR3T3 cells have a much higher steady-state level of p27 KIP1 than serum-starved or growing cultures. We have further shown more directly that plasmid-driven overexpression of p27 KIP1 has an inhibitory effect on P4 promoter activity. This effect was striking in growing cultures but limited in contact-inhibited cells, which argues in favor of the view that it occurs naturally in the latter.
On the basis of sequence similarities, CKIs fall into one of two families. Proteins p16, p15, p18, and p19 belong to the INK4 family and specifically inhibit cyclin-cdk4 and cyclin-cdk6 complexes. Proteins p21, p27, and p57 belong to the CIP/KIP family and interfere with the activity of most cyclin-cdk complexes (47). The CIP/KIP inhibitors p21 and p27 carry out similar tasks in cell cycle control, inactivating the same cyclincdk complexes. They share a common N-terminal residue sequence believed to interact with the target complexes (30). They differ from the INK4 family CKIs in that they do not appear to require the retinoblastoma gene product pRB to inhibit cell-cycle progression (30). Experiments with p21 and p27 knockout mice suggest functional compensation between these CKIs in controlling cell proliferation during development (32). On the other hand, the activities of p21 and p27 do not overlap entirely, because their C-terminal sequences differ. Protein p21, but not p27, interacts with the proliferating cell nuclear antigen (PCNA) and inhibits PCNA-dependent DNA replication (29). The present data point to an additional functional difference between p21 and p27, i.e., the ability of the latter but not the former to convert CRE-binding transcriptional activators into repressors. Like promoter inhibition in confluent cells, this p27-mediated conversion was observed only when at least two CRE motifs were present.
In conclusion, the present work has uncovered two distinct cell cycle-related regulatory pathways affecting parvoviral gene expression. The first involves the E2F factor; it was known from other systems but is of special interest in parvovirus research because it imposes an early limitation on transcription, rendering progression of the viral life cycle dependent on entry of host cells into the S phase. The second is mediated by cis-acting CREs and appears specific to growth arrest by contact inhibition. It involves CRE-mediated promoter repression brought about by the CKI p27 KIP1 . This new function of p27 distinguishes it from p21, another member of the CIP/KIP kinase inhibitor family with which it shares several activities. The molecular mechanism by which p27 KIP1 interferes with the activity of CREBs remains to be unraveled. The P4 CREs have recently been shown, in rat fibroblasts, to interact with transcription factors of the ATF/CREB family (36), factors known to be regulated by various kinases (36,43). No correlation was observed between CRE-mediated repression and the phosphorylation state of CREB1. Given the multitude of proteins interacting with CREs and the cooperative action of the two CREs in p27-mediated repression, it could be interesting to identify new polypeptides binding to such CRE motif repeats. A worthy focus of future research might thus be the influence of kinase inhibitor p27 on the function of CRE-protein complexes and the phosphorylation of the protein constituents.


extfieldsofstudy: Biology Medicine

text:
Regulation of a TATA-binding protein-associated factor during cellular differentiation.

RNA polymerase III transcription is down-regulated when F9 embryonal carcinoma cells differentiate into parietal endoderm. This reflects a decrease in the activity of TFIIIB, a multisubunit complex that is required for all class III gene expression. Two essential components of TFIIIB are the TATA-binding protein (TBP) and an associated polypeptide called BRF that is specific to this complex. The abundance of both TBP and BRF decreases during F9 cell differentiation. Whereas the amount of TBP assembled into TFIIIB is down-regulated, this is not the case for all TBP-containing complexes. BRF levels show a more dramatic decline that appears sufficient to account for the overall change in transcriptional activity. Developmental regulation of a specific class of genes may therefore be achieved through changes in the availability of a TBP-associated factor.

The rapidly changing patterns of gene expression required for early development demand tight transcriptional control. In situ hybridization has revealed that RNA polymerase (pol) 1 III transcription is regulated strongly during mouse embryogenesis (1). After fertilization, the steady-state levels of pol III transcripts increase severalfold, and labeling is seen in the pronuclei and polar bodies, consistent with de novo transcription (1). Indeed, microinjection experiments using single cell embryos have demonstrated that the pol III machinery is active even before cell cleavage (2). The blastomeres of two-and four-cell morulae and of eight-cell blastocysts are heavily labeled (1). Hybridization decreases in the trophectoderm cells of expanding blastocysts but remains strong in the inner cell mass (1). By the late primitive streak stage, high expression is restricted to the ectoderm and mesoderm but has decreased substantially in the embryonic and extra-embryonic endoderm (1).
The decrease in pol III activity during differentiation into endoderm can be reproduced accurately using embryonal carcinoma (EC) cells, which mimic events within the early embryo (3)(4)(5). For example, pol III transcription decreases substantially both in vitro and in intact nuclei when F9 EC cells differentiate into parietal endoderm (PE) (4,5). Crude fractions containing the general pol III factor TFIIIB are sufficient to restore expression in PE cell extracts to undifferentiated levels (4). This suggested that the down-regulation of pol III during PE formation is achieved by a specific decrease in the availability of active TFIIIB (4). However, the molecular details of this event remain to be determined, since TFIIIB was largely uncharacterized at the time of the previous study.
It has since become clear that TFIIIB is a multisubunit complex that contains the TATA-binding protein TBP and at least two TBP-associated factors (TAFs) (reviewed in Ref. 6 -8).
One of these TAFs is related in both structure and function to the general pol II factor TFIIB (9 -14). This TAF has been variously named TDS4, PCF4, BRF, and TFIIIB90, but we shall refer to it as BRF, an acronym for TFIIB-related factor. Although yeast TFIIIB has been reconstituted using recombinant components (15)(16)(17), the mammalian factor is much less well characterized (reviewed in Refs. 6 -8). This is partly because of the instability of TFIIIB, which tends to dissociate during purification (18 -21). Although several groups have identified polypeptides as candidate TAFs for human TFIIIB, there is little consensus as yet (13, 14, 18 -21). To date, BRF is the only component of mammalian TFIIIB that has had its gene cloned and its function established unequivocally (13,14).
Although TFIIIB in higher organisms remains incompletely characterized, the identification of TBP and BRF as essential components has provided the opportunity to investigate regulatory events using molecular probes and affinity-purified fractions. We have therefore returned to the F9 system to investigate in more detail the mechanism(s) responsible for regulating pol III transcription during differentiation. We have confirmed the previous observation that TFIIIB activity decreases specifically when F9 cells differentiate. We demonstrate that this is associated with the down-regulation of one or more TAFs, and that TAF availability is limiting for pol III transcription in PE extracts. We have raised antibodies that recognize murine BRF and used these to demonstrate that BRF levels decrease substantially during F9 cell differentiation. The decrease in abundance of this essential TAF seems sufficient to account for the down-regulation of class III gene expression that accompanies PE formation.

EXPERIMENTAL PROCEDURES
Cell Culture-F9 cells were cultured and differentiated as described previously (4).
Preparation of Extracts and Protein Fractions-Whole cell extracts were prepared according to the method of Manley et al. (22) and also by a more direct freeze-thaw procedure (23). Both approaches yielded similar results.
Phosphocellulose chromatography, heparin chromatography of TFIIIC, DNA affinity purification of TFIIIC, Mono Q purification of TFIIIB, preparation of recombinant TBP, and TBP affinity purification of TFIIIB TAFs were carried out as described previously (23).
TFIIIB was also partially purified from EC and PE extract phosphocellulose (PC)-B fractions using Macro-Prep Q (Bio-Rad) and a protocol modified from Meyers and Sharp (24). PC-B was loaded onto Q columns in buffer A (20 mM Hepes (pH 7.9), 20% glycerol, 1 mM EDTA) containing 50 mM KCl. After washing with buffer A containing 200 mM KCl, TFIIIB was eluted using buffer A containing 600 mM KCl.
Transcription Assays-Transcription reactions were carried out as previously (4), except that pBR 322 was not included, and the incubations were for 60 min at 30°C.

F9 Cell Differentiation Is Accompanied by a Specific
Decrease in TFIIIB Activity-Whole cell extracts were prepared from undifferentiated F9 EC cells and from cells that had been induced to differentiate into parietal endoderm by culture in the presence of retinoic acid and dibutyryl cAMP. Extracts of PE cells were found to transcribe the VA I gene much less efficiently than extracts prepared in parallel from EC cells (Fig.  1A). This was consistently the case using several sets of extracts, two different extraction procedures, and PE cells harvested after 5, 7, or 9 days of differentiation. 2 We conclude that these cell-free systems accurately reproduce the down-regulation of pol III transcription that occurs in vivo.
Fractions containing partially purified TFIIIB were prepared from the F9 EC and PE extracts by sequential chromatography on phosphocellulose and MacroPrep-Q. These were then tested for their ability to reconstitute VA I transcription in the presence of a complementing PC-C fraction containing TFIIIC and pol III. TFIIIB activity was consistently found to be significantly reduced in the fractions prepared from PE extracts (Fig. 1B). Quantitation of six independent comparisons showed that, on average, the PE fractions had 6-fold lower TFIIIB activity than the corresponding EC fractions.
Complementation assays were also used to compare the TFIIIC activities of EC and PE extracts after chromatography on phosphocellulose. PC-C fractions from PE extracts were found to be at least as active as the equivalent fractions from EC extracts in reconstituting VA I expression (Fig. 1C). The observed decrease in TFIIIB activity is therefore a specific event.
TFIIIB Activity Limits the Rate of pol III Transcription in F9 PE Extracts-Add-back experiments were carried out to determine which factor is limiting for pol III transcription in differentiated F9 cell extracts. VA I expression was stimulated by the addition of PC-B but not by PC-C or affinity-purified TFIIIC fractions ( Fig. 2A). Indeed, the affinity-purified TFIIIC depressed the level of transcription; such over-titration effects may reflect the ability of excess TFIIIC to sequester limiting TFIIIB away from promoter sites. Since TFIIIB is the only known pol III factor that is specific to PC-B, these observations suggest that TFIIIB activity is limiting in the PE extracts. However, we cannot exclude the possibility of unknown class 2 H. Alzuherri and R. J. White, unpublished data. III factors that might also elute specifically in the PC-B. To reduce the possibility of such uncharacterized contaminants, we purified TFIIIB further by gradient chromatography on Mono Q. The resultant TFIIIB fraction was found to be extremely efficient at activating VA I transcription when added to PE extract (Fig. 2B). Indeed, this fraction was able to restore levels of expression in the PE extract to those obtained using an equal amount of EC extract. It is therefore likely that the expression of class III genes in differentiated F9 cell extracts is limited by the availability of active TFIIIB.
F9 Cell Differentiation Is Accompanied by a Decrease in the Activity of One or More TFIIIB TAFs-Since TFIIIB is a complex containing both TBP and TAFs, we tested which of these components is limiting for pol III transcription in PE extracts. Fractions containing TFIIIB TAFs were prepared from PC-B by affinity chromatography on columns carrying immobilized TBP. Such fractions are capable of reconstituting transcription in the presence of PC-C and recombinant TBP (23). Expression of the VA I gene in a PE extract was increased by up to 5-fold after the addition of affinity-purified TFIIIB TAFs (Fig. 3). In contrast, recombinant TBP produced no stimulation. These results suggest that it is the TAF component of TFIIIB that limits the rate of pol III transcription in extracts prepared from differentiated F9 cells.
We have previously devised a complementation assay that allows the direct comparison of TFIIIB TAF activity between unfractionated extracts (23). This assay exploits the fact that heating an extract at 47°C for 15 min inactivates endogenous TBP and TFIIIC but leaves the pol III TAFs unaffected (23, 26 -30). The activity of the TAF components of TFIIIB can then be measured by adding the heat-treated extract to a complementation reaction containing TBP, TFIIIC, and pol III but no TFIIIB (23,31,32). Fig. 4 shows that Mono Q-purified TFIIIB will reconstitute transcription in such a system after it has been heated in this way (lanes 10 -12). Using this assay, it was found that F9 EC extracts are substantially more active than PE extracts (lanes 1-9). Twenty g of PE extract are required to produce a signal in this assay that is comparable with that produced by 2.5 g of EC extract (compare lane 2 with lane 9). This dramatic decrease in TFIIIB TAF activity appears sufficient to account for the down-regulation of pol III transcription that accompanies F9 cell differentiation.
The Abundance of BRF Decreases Substantially When F9 Cells Differentiate-We carried out immunoblot analyses to look for qualitative or quantitative changes in the TFIIIBspecific TAF BRF. Available antisera against BRF were found to react poorly with murine extracts, giving a weak signal with  6 and 7, respectively). B, Mono Q-purified TFIIIB is sufficient to raise VA I transcription in PE extract to levels occurring in EC extract. pVA I template (250 ng) was transcribed using 14 g of EC (lane 1) or PE extract (lanes 2-5) in the presence of 2, 3, or 4 l (lanes 3, 4, and 5, respectively) of Mono Q-purified TFIIIB. multiple bands. 2 As the mouse gene for BRF had not been reported, we examined the available BRF sequences for regions that have been especially conserved through evolution. We have identified recently the gene for BRF from Caenorhabditis elegans (33). Comparison of this sequence with that of human and three different yeasts allowed us to identify a region near the C terminus of BRF that is particularly well conserved. We therefore raised an antiserum against a 15-residue epitope from this region. Western blot analysis revealed that this antiserum, named 128, recognizes human BRF that has been translated in vitro using reticulocyte lysate. 3 In addition, it reacts with BRF in the Mono Q-purified TFIIIB and affinitypurified TAF fractions used in Figs. 3 and 4. 2 To find out whether it can also recognize the murine protein, we tested its effect on pol III transcription in an F9 cell extract (Fig. 5A). Both human and mouse extracts that had been immunodepleted using antiserum 128 were found to transcribe the VA I gene up to 6-fold less actively than extracts immunodepleted using the corresponding preimmune serum or control extracts treated in parallel with protein A alone. Indeed, immunodepletion of the mouse extract was no less efficient than that obtained with a human extract. This result demonstrates the applicability of antiserum 128 to a murine system.
When antiserum 128 was used in western immunoblot analyses, the level of BRF was found to be substantially lower in PE than in EC extracts (Fig. 5B, lanes 1 and 2). Indeed, BRF was barely detectable in the crude extracts from PE cells. The BRF band was more readily observed when we tested TFIIIB fractions from MacroPrep-Q columns, on account of its having been significantly enriched by the purification protocol. Nevertheless, BRF was still found to be less abundant in the PE-derived fraction, after normalization for protein content (Fig. 5B, lanes  3 and 4). The same results were obtained using a second anti-BRF antiserum, whereas preimmune sera did not react with BRF. 2 We also analyzed the various step fractions from phosphocellulose (PC) columns. The BRF band was present in the PC-B fraction that contains TFIIIB, whereas little or none was detected in the other PC fractions that do not contain TFIIIB activity (Fig. 5C, upper panel). PC-B from EC extracts was found to have approximately 5-fold more BRF than PC-B from PE extracts. These data indicate that BRF levels decrease when F9 cells differentiate.
Apart from BRF, TBP is the only subunit of mammalian TFIIIB for which molecular probes are available. When PC-B fractions are compared, TBP levels are seen to be reduced in PE relative to EC fractions (Fig. 5C, lower panel). This result was confirmed using three different anti-TBP antibodies. 2 Quantitation of several experiments reveals that TBP in the PC-B fractions is 3-7-fold less abundant after differentiation. In contrast, the abundance of TBP in the PC-C and PC-D fractions shows little or no change after differentiation. The TBP present in PC-C and PC-D is associated with the complexes SNAPc, TFIID, and SL1/TIF-IB (6). Thus, the reduced TBP levels found in PE extracts are specific to a particular TBP-containing complex.
Although F9 cell differentiation is accompanied by a decrease in the abundance of both BRF and TBP, this is not the case for all components of the transcription apparatus. We have already shown by complementation assays that TFIIIC activity is not reduced in PE cell extracts. Western blot analysis reinforces this result, since we observe no change in the abundance of the ␤ or ␣ subunits of TFIIIC after F9 cell differentiation (Figs. 6, A and B, respectively). These data have been confirmed using alternative antisera against TFIIIC. 2 It is therefore clear that the down-regulation of TFIIIB is a specific regulatory event. DISCUSSION The results presented in this manuscript indicate that F9 cell differentiation is accompanied by a substantial decrease in the activity of TFIIIB, a general factor that is required for all pol III transcription. This is a specific phenomenon, since TFIIIC activity is not reduced in PE extracts. Indeed, TFIIIB seems to be the only general pol III factor that is deficient in differentiated F9 cells, since it is possible to restore VA I transcription to EC levels simply by adding TFIIIB to a PE extract. Direct assays and add-back experiments indicate that the loss of TFIIIB activity during differentiation reflects the downregulation of one or more pol III TAFs, which become limiting for VA I expression. This conclusion is substantiated by Western blot analyses, which demonstrate a large decrease in the abundance of the essential TAF BRF.
Mei␤ner et al. have also investigated the regulation of pol III transcription during F9 cell differentiation (5). These workers chromatographed EC and PE extracts on phosphocellulose and Q-Sepharose and then carried out complementation assays to compare the activities of TFIIIB and TFIIIC (5). They found that under some assay conditions TFIIIB fractions from PE extracts were substantially less active than those from EC extracts (5), as we have observed. However, such differences were reported to be inconsistent and dependent on the reaction conditions (5). In contrast, a PC-C fraction from PE cells was described as being "virtually unable to reconstitute transcription" in the presence of PC-B (5). Although this experiment was not quantitated, no detectable signal was obtained with the PE PC-C, whereas the EC PC-C fraction was active in the same assay (5). The almost complete inactivity of the PE PC-C fraction was especially striking, since the unfractionated PE starting material used in that study was only 5-fold less active than the EC extract (5). Mei␤ner et al. concluded that F9 cell differentiation is accompanied by a substantial reduction in TFIIIC with little or no change in TFIIIB activity (5).
We cannot reconcile this report with our current and earlier 3 C. G. C. Larminie, C. A. Cairns and R. J. White, unpublished data.
FIG. 4.TFIIIB TAF activity decreases during F9 cell differentiation. pVA I template (500 ng) was transcribed using 2 l of HeLa PC-C and 50 ng of recombinant TBP in the presence of 2.5, 5, 10, or 20 g of EC whole cell extract (lanes 2, 3, 4, and 5, respectively); 2.5, 5, 10, or 20 g of PE whole cell extract (lanes 6, 7, 8, and 9, respectively); or 1.6 or 2.7 g of Mono Q-purified TFIIIB (lanes 11 and 12, respectively). Before use, the whole cell extracts and TFIIIB were heat-treated for 15 min at 47°C to inactivate endogenous TBP and TFIIIC. findings (4). Mei␤ner et al. (5) have relied solely on complementation assays using fractionated factors. As the authors pointed out, such an approach is vulnerable to the possibilities of differential recovery or inactivation during chromatography. Although we have employed the same approach, we have also provided a variety of additional independent lines of evidence in order to reduce the danger of artifacts. Our conclusions are further strengthened by the use of affinity-purified reagents and molecular probes, none of which were used in the previous study. Our evidence that TFIIIC is not substantially deficient in F9 PE extracts is as follows. 1) Band shift assays using as probe VA I and B2 gene fragments or a B-block oligonucleotide demonstrated previously that the DNA binding activity of TFIIIC does not decrease when F9 cells differentiate (4). 2) Titration of transcription activity using a B-block oligonucleotide demonstrated that TFIIIC levels are similar in EC and PE extracts (4). 3) Mono Q-purified TFIIIB is sufficient to restore FIG. 5. BRF levels decrease during F9 cell differentiation. A, antibody 128 recognizes both human and mouse BRF. pVA I template (500 ng) was transcribed using 4 l of HeLa (lanes 1-3) or F9 EC (lanes 4 -6) cell extract that had been immunodepleted using either protein A-Sepharose alone (lanes 1 and 4), protein A-Sepharose prebound to anti-BRF antibody 128 (lanes 2 and 5), or protein A-Sepharose prebound to preimmune serum 128 (lanes 3 and 6). PI, preimmune B, PE extracts contain less BRF than EC extracts.  1, 3, 5, and 7) or PE (lanes 2, 4, 6, and 8) extract were resolved on an SDS-7.8% polyacrylamide gel and then analyzed by Western immunoblotting with anti-BRF antibody 128 (top panel) and anti-TBP antibody SI-1 (Santa Cruz) (bottom panel). The extra band running above TBP in the PC-C fractions is not recognized by other anti-TBP antibodies and is therefore presumed to be unrelated to TBP. pol III transcription in PE extracts, which would not be possible if TFIIIC were deficient. 4) The addition of PC-C or affinitypurified TFIIIC fractions does not stimulate transcription in a PE extract. 5) Complementation assays with PC-C fractions reveal no decrease in TFIIIC activity after F9 cell differentiation. 6) Western analysis reveals no change in the abundance of the ␣ or ␤ subunits of TFIIIC. Our contention that TFIIIB is down-regulated during PE formation is based on the following lines of evidence. 1) PC-B and Mono Q-purified TFIIIB fractions are sufficient to raise pol III transcription in PE extracts to levels occurring in EC extracts. 2) VA I expression in PE extracts is activated by the addition of affinity-purified TFIIIB TAF fractions, which contain BRF 2 . 3) Complementation assays with unfractionated extracts demonstrate a dramatic change in TFIIIB activity. 4) Complementation assays after fractionation on phosphocellulose and MacroPrep-Q show that much less TFIIIB activity is recovered from PE extracts than from EC extracts. 5) Western blotting demonstrates directly that the essential pol III TAF BRF becomes much less abundant when F9 cells differentiate. 6) TBP levels in the TFIIIB fractions also decrease during differentiation.
The observed decline in BRF levels seems sufficient to account for the down-regulation in tRNA, B1, and B2 gene transcription that occurs in vivo (4). However, we have not been able to restore expression in PE extracts by adding recombinant BRF. This would suggest that other components of TFIIIB may also be down-regulated as F9 cells differentiate. Coregulation of TAFs may be important, since partial complexes that lack an essential component such as BRF are unlikely to be of use to the cell and could sequester other transcription factors in unproductive interactions. The fact that TBP levels decrease in the PC-B fraction after differentiation supports the idea of a coordinate reduction in TFIIIB subunits and provides circumstantial evidence against the presence of a surplus of partial complexes in PE cells. Definitive resolution of this question will require the identification of the remaining mammalian pol III TAF(s) and the generation of molecular probes.
Although the decrease in BRF levels seems sufficient to account for the down-regulation of pol III transcription during F9 cell differentiation, we cannot at this stage conclude that it is the primary cause for the decrease in TFIIIB activity. It is entirely possible that a decrease in the synthesis of another pol III TAF reduces the opportunity for BRF to be assembled into complete TFIIIB complexes. Any surplus unassembled BRF may then be targeted for degradation, thereby leading to the observed decrease in abundance. A similar consideration applies to TBP; there may be less TBP assembled into TFIIIB as a result of limiting BRF, but it is also conceivable that a reduction in TBP triggers the decrease in BRF levels. Cause and effect are clearly difficult to distinguish in this case. At present we can only conclude that a specific decrease in the abundance of TFIIIB accounts for the reduced rate of transcription.
The coregulation of batteries of genes involved in producing a shared phenotypic goal must be essential for normal development. One mechanism by which this could be achieved is through changes in the activity of specific TAFs. Although a cell type-specific TAF has been described (34), this study provides, to our knowledge, the first documented example in which the regulation of a TAF has been shown to accompany a specific event that occurs during early embryogenesis. The decrease in abundance of BRF during PE formation is likely to account, at least in part, for the general decline in transcription of all pol III templates tested. This may provide a paradigm for a mechanism that allows the complex and rapidly changing patterns of developmental gene expression to be achieved in a coordinated fashion.


text:
Maximal proliferation of cytotoxic T lymphocytes requires reverse signaling through Fas ligand.

Fas ligand (FasL/CD95L) is best known for its role in delivering apoptotic signals through its receptor, Fas (APO-1/CD95). In this study, we present evidence that FasL has a second role as a signaling receptor. Alloantigen-specific proliferation by multiple FasL− murine CTL lines is depressed compared to that of FasL+ CTL lines. FasL− CTLs kill efficiently on a per recovered cell basis and can achieve wild-type levels of proliferation upon stimulation by optimal doses of anti-CD3, suggesting the lack of a costimulatory signal during antigen stimulation. To test this hypothesis directly, soluble FasIgG, a fusion protein of murine Fas and human IgG1, was added to FasL+ CTLs to demonstrate that blocking cell surface Fas–FasL interactions mimics the depression observed for FasL− CTLs. In addition, plate-bound FasIgG in conjunction with suboptimal anti-CD3 stimulation augments proliferative signals in FasL+ but not FasL− CTLs. In contrast to these results with CD8+ T cells, alloantigen-stimulated FasL− CD4+ T cells proliferate vigorously compared to FasL+ cells. These data demonstrate that reverse signaling through FasL is required for CTLs to achieve maximal proliferation and may provide clues to differences in the homeostatic regulation of activated CD4+ and CD8+ T cells during an immune response.

F as ligand (L) belongs to the TNF superfamily, a group of type II transmembrane or secreted molecules whose interactions with their receptors, members of the TNF receptor superfamily, regulate important functions in immune cell proliferation, activation, differentiation, and survival (1)(2)(3). FasL exists mainly in membrane-bound form, is generally expressed on cells of the T cell lineage, and is induced during T cell activation (1). FasL and members of this ligand superfamily primarily interact with their receptors by direct cell-cell contact (3). This observation, coupled with the cross-species sequence conservation of the cytoplasmic domains of these ligands, has led to the suggestion that signaling occurs in both directions for this family of ligand-receptor pairs (3). In support of this argument, family members including CD27L, 4-1BBL, OX40L, CD30L, and CD40L have recently been shown to transduce internally costimulatory signals to activated peripheral lymphocytes (2,4,5). As of now, no data exist concerning the reverse signaling capacity of ligands with known negative regulatory functions, such as FasL. To explore this added twist to the picture of reverse signaling, we used CTL lines generated from B6 wildtype and B6. lpr mice, which both express FasL, and from B6. gld mice, which lack FasL function, to demonstrate that FasL shares this capacity for reverse signaling in CD8 ϩ but not in CD4 ϩ T cells. Our data reveal that FasL, a molecule long associated with the induction of apoptosis in Fas ϩ target cells (6) can also positively regulate the proliferative capacity of CD8 ϩ T cells that express it. These data also offer some insight into how CD4 ϩ and CD8 ϩ T cells may be differentially regulated during an immune response.
Generation and Maintenance of T cell lines. Alloreactive H-2 kspecific CTLs were originally generated from age-matched B6 wildtype, B6. lpr , and B6. gld mice by incubating donor spleen and LN cells with an equal number of 3,000 rad C3H splenocytes in Reverse Signaling Through Fas Ligand RPMI 1640 supplemented with 10% FCS (RP10). Lines were maintained by restimulation every 8-10 d for 1-12 rounds. RP10 was supplemented after the third stimulation with 50 mM ␣ -methyl mannoside and 5% supernatant from rat cells stimulated for 2 d with 3 g/ml Con A. CD8 ϩ T cells for generating anti-K bm1 CTLs were purified from spleen and LN cells from age-matched B6 wild-type and B6. gld mice by nonadherence to nylon wool, treatment with anti-CD4 plus complement, and adherence to plate-bound anti-CD8 antibodies. Purity as monitored by flow cytometry was Ͼ 99%. All CTL lines were routinely monitored by flow cytometry and CTL assay. CD4 ϩ T cells for generating primary anti-H-2 bm12 responders were similarly nylon wool-purified, anti-CD8 killed, anti-CD4 panned spleen and LN cells, with Ͼ 95% purity by flow cytometry.
Cytotoxicity and Proliferation Assays. Serial dilutions of effector cells and 10 4 tumor targets labeled with 51 Cr sodium chromate were plated in round-bottomed 96-well plates in a final volume of 200 l. After a 4 h incubation at 37 Њ C, 100 l of supernatant was collected and specific lysis was determined as follows: percentage of specific lysis ϭ 100 ϫ [(cpm released by CTL Ϫ spontaneous release)/(cpm released by detergent Ϫ spontaneous release)]. Spontaneous release in the absence of CTL was Ͻ 20% in all experiments. For antigen-specific proliferation, 2 ϫ 10 4 CTL from long-term lines or 2 ϫ 10 5 primary T cells were cultured with 5 ϫ 10 5 irradiated allogeneic stimulator cells in a total volume of 200 l of RP10. For long-term lines, media were supplemented with 5% rat Con A supernatant or 50 U/ml recombinant human IL-2. For optimal anti-CD3 stimulation, responders were added to wells coated at 4 Њ C overnight with 15 g/ml goat antihamster IgG (GahIg) in PBS and then for 4 h with 5 g/ml anti-CD3 antibodies. For proliferation with FasL costimulation, responders were added to wells coated overnight at 4 Њ C with both 15 g/ml GahIg and 15 g/ml goat anti-human IgG and then for 4 h with a mixture of 1.0 g/ml anti-CD3 (suboptimal dose) with either 4.6 g/ml FasIgG or isotype-matched human IgG 1 (huIgG 1 ). Proliferation levels were measured by thymidine incorporation 18 h after pulsing with 1 Ci [ 3 H]TdR/well on day 3. Background counts (responders with only first stage antibodies) were subtracted from the values. Antigen-specific proliferation was also assayed for viable cells by trypan blue exclusion over the course of the culture period. Where appropriate, APCs were removed before counting viable cells by anti-H-2 k plus complement. For the FasIgG blocking assay, FasIgG was added to cultures at 2.8-10 g/ml. HuIgG 1 and/or B6.SCID preimmune sera were used as negative controls.

Results and Discussion
Antigenic Stimulation of B6.gld CTL Lines Is Characterized by Depressed Proliferation and Cell Recoveries Relative to CTL Lines Expressing Functional FasL. Proliferation assays using CTL lines derived from wild-type B6, B6. lpr , and B6. gld mice revealed that FasL ϩ CTLs proliferate better in response to allogeneic stimulation than do FasL Ϫ CTLs (Fig.  1, a and c ). APC titration from 5 ϫ 10 4 to 1 ϫ 10 6 cells cocultured per well with CTLs did not alter the relative depression in proliferation of B6. gld CTL lines (data not shown). In addition, kinetic analysis of viable cell counts revealed that the lack of FasL function also influences the ability of CTLs to increase in number over the course of an MLC ( Fig. 1 b ), and that this expansion coincides with FasL expression by the responding T cells. As determined by flow cytometry, FasL expression on the CTL lines is upregulated by 3 h after antigenic stimulation, peaks on day 3, and begins to decrease by day 4 (data not shown). This pattern of depressed proliferative capacity and number of recovered cells was reproducible for three independent CTL lines and was not reversed by the addition of exogenous IL-2. The depressed proliferation and recovered cell counts were also seen upon coculture of B6. gld CTLs with APCs from C3H. gld mice, demonstrating that this phenomenon is not caused by Fas-mediated killing of gld CTLs by FasL expressed on the APCs (Fig. 1 d ). Autocrine Fas-mediated suicide (10)(11)(12) can also be ruled out as a mechanism for depressed proliferation because gld CTLs do not express functional cell surface FasL. Analysis by flow cytometry (data not shown) also ensured that there is not an accumulation of Thy1 ϩ CD8 Ϫ CD4 Ϫ cells analogous to those found in vivo in gld and lpr mice (1).
FasL Ϫ CD4 ϩ T Cells Vigorously Proliferate upon Antigenic Stimulation and Do Not Contribute to the Depressed Proliferation in FasL Ϫ CTL Lines. To test the potential involvement of CD4 ϩ T cells in initiating or maintaining the depressed proliferative capacity of FasL Ϫ compared to wildtype CTLs, purified populations of CD8 ϩ T cells from B6 and B6.gld mice were tested for their response against the MHC class I molecule K bm1 . The same pattern of depres-  (a and b) [ 3 H]TdR uptake and viable cell counts over days 1-5 from long-term CTL lines from B6, B6.lpr, and B6.gld mice cultured with allogeneic H-2 k splenocytes. APCs were removed before counting by antibody depletion. (c) CTL responders were purified CD8 ϩ T cells derived from B6 and B6.gld mice cultured with H-2 bm1 splenocytes. (d) [ 3 H]TdR uptake by CTL responders used in a, cultured with C3H.gld splenocytes. Experiments were repeated two to seven times. All data are averages of triplicate wells and error bars represent the SD of the means. Percentages appearing to the right of the figures indicate normalized comparison to B6 wild-type. Brief Definitive Report sion was observed for gld relative to wild-type CTLs in proliferation and expansion of cell numbers over the course of an MLC (Fig. 1 c). These data indicate that the submaximal proliferation by FasL Ϫ CTL lines is not initiated or maintained by the lack of CD4 ϩ T cell help and further emphasize that the observed depression in proliferation is not merely an idiosyncracy of a single B6.gld-derived CTL line. In addition, this decreased proliferative capacity is not restricted to long-term lines, as primary cultures of anti-K bm1 lines also showed the same pattern of proliferation (data not shown).
In contrast to their CD8 ϩ counterparts, CD4 ϩ T cells lacking functional FasL proliferate vigorously upon alloantigenic stimulation. Purified CD4 ϩ cells from B6.gld mice cultured with class II different B6.bm12 stimulators proliferated better (Fig. 2 a) and generated more cells over the course of the 5 d of MLC (Fig. 2 b) than did CD4 ϩ cells from wildtype B6 mice. This observation demonstrates the differential requirement for functional FasL expression in the promotion of maximal proliferation by CD8 ϩ but not CD4 ϩ T cells.

FasL Ϫ CTL Lines Proliferate to Wild-type Levels upon Stimulation with an Optimal Dose of Anti-CD3 and Can Efficiently
Lyse Target Cells. Despite their submaximal proliferation in response to antigenic stimulation, gld CTLs do not differ from wild-type CTLs in their ability to proliferate to an optimal dose of anti-CD3 antibody (Fig. 3 a) or to lyse target cells (Fig. 3 b). Therefore, the depression seen with antigenic stimulation of gld CTLs cannot simply be explained as a characteristic of a weak line or a general defect in TCR signaling. Instead, it implies that gld CTLs are missing a costimulatory signal required to achieve maximal proliferation during an antigenic response.

Proliferation by FasL ϩ CTL Lines in the Presence of Soluble FasIgG Mimics the Depression Observed for FasL Ϫ CTLs, whereas Costimulatory Signals Are Provided by Plate-bound
FasIgG. To pinpoint FasL as the source of the proliferative signal, soluble FasIgG fusion protein was added in an effort to block cell surface FasL-Fas interactions. A dose-dependent decrease in the proliferation of B6 wild-type CTLs was observed upon the addition to the culture of increasing amounts of soluble FasIgG but not isotype-matched huIgG 1 (Fig. 4 a). Kinetic analyses also revealed that recovered cell numbers of both FasL ϩ CTLs (B6 wild-type and B6.lpr) were reduced to those of FasL Ϫ CTLs over the course of the MLC upon the addition of soluble FasIgG but not huIgG 1 (Fig. 4 b). The fact that Fas Ϫ CTL lines are susceptible to blocking by soluble FasIgG demonstrates that Fas-mediated killing is not involved, which implicates FasL in positive signaling. When plate-bound FasIgG fusion protein was used Purified CD4 ϩ T cells from B6 and B6.gld mice were cultured (in the absence of rIL-2) with H-2 bm12 splenocytes, and [ 3 H]TdR uptake and viable cell counts were measured over days 1-5 of culture. All data are presented as in Fig. 1.   Figure 3. Lack of FasL influences neither the ability of gld CTLs to proliferate to wild-type levels after stimulation by optimal levels of platebound anti-CD3 antibodies nor their ability to perform cytolytic activity on a per recovered cell basis. (a) [ 3 H]TdR incorporation by the long-term CTL lines used in Fig. 1 a, cultured over plate-bound anti-CD3 antibodies. Data are presented as in Fig. 1. (b) The same CTL responders were assayed for their ability to lyse 51 Cr-labeled EL4 (H-2 b , open symbols) and R1.1 (H-2 k , filled symbols) tumor targets in a 4 h chromium release assay.
in conjunction with suboptimal amounts of anti-CD3, a costimulatory signal was delivered for proliferation by FasL ϩ but not by FasL Ϫ CTLs (Fig. 4 c). These data demonstrate that the source of this positive signal is the FasL expressed on the wild-type and lpr CTL lines. Although it is important to note that the molecules mediating these signals have yet to be identified, the striking sequence conservation across species of the cytoplasmic domain of FasL suggests the presence of important functional domains. A prolinerich region of the cytoplasmic tail, in which 25 out of the 78 residues are prolines, including a stretch of 7 sequential proline residues (13,14), is reminiscent of an SH3 binding site (15).
These results provide insights into the observed differences in the regulation of activated CD8 ϩ and CD4 ϩ T cells. Apoptosis of CD4 ϩ T cells is primarily mediated through Fas (16), possibly both by fratricide and autocrine suicide (10)(11)(12). On the other hand, CD8 ϩ T cells are less sensitive to Fas-mediated death (16,17) and primarily undergo a slower death induced by the interaction of TNF with its receptor (16). However, neither mechanism operates exclusively in one T cell subpopulation or the other; TNFR-mediated apoptosis is measurable in CD4 ϩ T cells in the absence of Fas expression (18) and CD8 ϩ T cells are not completely resistant to Fas-mediated death (16). FasL Ϫ CD4 ϩ T cells proliferate more extensively to superantigenic (19,20) and class II-specific (Fig. 2) stimulation than do wild-type cells, while FasL Ϫ CD8 ϩ T cells proliferate less vigorously than do wild-type cells (Fig. 1). The difference in proliferation observed for these T cell subpopulations may be the sum of two separate events, positive reverse signaling through FasL in CD8 ϩ T cells and the sensitivity of CD4 ϩ T cells to Fas-mediated death. In addition, FasL and Fas are both found on the surface of CTLs, leading to a scenario in which both positive and negative signals may be received by the same cell. The perceived "reduced sensitivity" of CD8 ϩ T cells to Fas-mediated death may then be explained by the maintenance of a balance in the CD8 ϩ T cell population in which cell proliferation induced by the FasL signal offsets the number of cells undergoing Fas-mediated death. Survival of each cell may depend upon which signal occurs first, or the FasL signal may be able to override the Fas negative signal, for example through intracellular competition for signaling molecules.
The role of TCR engagement in conjunction with the FasL signal remains unclear. Other molecules known for their positive signaling capabilities have recently been implicated in the death of cells in the absence of a concomitant antigen receptor signal. For example, signaling through CD40 without concurrent engagement of the B cell receptor leads to Fas-mediated cell death (21,22), and may serve an immunoregulatory role by removing nonspecific B cells. It will be interesting to determine if FasL can still signal without engagement of the CD3/TCR complex and to analyze the consequences of such uncoupled signaling. Should FasL fail to deliver a positive signal without the coengagement of the TCR, the death signal through Fas may serve to remove activated T cells after antigen has been eliminated. In light of the discovery that CD40 signals can direct germinal center B cells to become memory B cells (23), one could speculate on the role of FasL in the clonal expansion of antigen-specific CTLs and the generation of memory T cells. Adoptive transfer of OT-1 (reference 24; class I-restricted OVA-specific TCR transgenic B6) spleen and LN cells into B6 wild-type, B6.lpr, and B6.gld mice followed by intraperitoneal injection of OVA peptide daily Experiments were repeated three times. All data are averages of triplicate wells and error bars represent the SD of the means. Brief Definitive Report for 3 d revealed that antigen-specific donor cells injected into B6.lpr mice were defective in their expansion compared to that demonstrated in the other two recipients (Martin, S., and M. Bevan, unpublished data). These data are consistent with the inability of the Fas Ϫ B6.lpr host to deliver a positive signal through FasL for clonal expansion. It will be interesting to determine whether this difference in expansion is related to the development of memory cells and if the FasL signal during an immune response influences the entry of CTLs into the memory pathway.
Several models that allow arrangement of the positive/ negative cell surface receptors into three categories have re-cently emerged: (a) surface receptors capable of both positive (survival or proliferation) and negative (death) signals such as Fas (25,26) and TNFR1 (27); (b) positively signaling receptors that augment the death receptor signal (28,29); and (c) induced receptors paired with constitutive receptors with the opposite signaling capacity, such as CD28 and CTLA-4 (30). Our data place Fas-FasL signaling into this third category. FasL, induced upon activation of CD8 ϩ T cells, sends a negative signal outward through its constitutively expressed receptor while also receiving a positive signal.