Polyoma middle T-induced vascular tumor formation: the role of the plasminogen activator/plasmin system.

The middle T antigen of murine Polyomavirus (PymT) rapidly transforms endothelial cells, leading to the formation of vascular tumors in newborn mice. Transformed endothelial (End.) cell lines established from such tumors exhibit altered proteolytic activity as a result of increased expression of urokinase-type plasminogen activator (uPA) and are capable of inducing vascular tumors efficiently when injected into adult mice. In this study we have used mice lacking components of the PA/plasmin system to analyze the role of this system in the transformation process and in tumor growth. We found that the proteolytic status of the host is not a critical determinant for PymT-induced vascular tumor formation. In addition, the lack of either uPA or tissue-type PA (tPA) activity is not limiting for the establishment and proliferation of End. cells in vitro, although the combined loss of both PA activities leads to a marked reduction in proliferation rates. Furthermore, the in vitro morphogenetic properties of mutant End. cells in fibrin gels could only be correlated with an altered proteolytic status in cells lacking both uPA and tPA. However, in contrast with tumors induced by PymT itself, the tumorigenic potential of mutant and wild-type End. cell lines was found to be highly dependent on the proteolytic status of both the tumor cells and the host. Thus, genetic alterations in the PA/plasmin system affect vascular tumor development, indicating that this system is a causal component in PymTmediated oncogenesis.


Abstract.
The middle T antigen of murine Polyomavirus (PymT) rapidly transforms endothelial cells, leading to the formation of vascular tumors in newborn mice. Transformed endothelial (End.) cell lines established from such tumors exhibit altered proteolytic activity as a result of increased expression of urokinase-type plasminogen activator (uPA) and are capable of inducing vascular tumors efficiently when injected into adult mice. In this study we have used mice lacking components of the PA/plasmin system to analyze the role of this system in the transformation process and in tumor growth. We found that the proteolytic status of the host is not a critical determinant for PymT-induced vascular tumor formation. In addition, the lack of either uPA or tissue-type PA (tPA) activity is not limiting for the es-tablishment and proliferation of End. cells in vitro, although the combined loss of both PA activities leads to a marked reduction in proliferation rates. Furthermore, the in vitro morphogenetic properties of mutant End. cells in fibrin gels could only be correlated with an altered proteolytic status in cells lacking both uPA and tPA. However, in contrast with tumors induced by PymT itself, the tumorigenic potential of mutant and wild-type End. cell lines was found to be highly dependent on the proteolytic status of both the tumor cells and the host. Thus, genetic alterations in the PA/plasmin system affect vascular tumor development, indicating that this system is a causal component in PymTmediated oncogenesis.
1. Abbreviations used in this paper : End., endothelioma; MMP, matrix metalloproteinase; PA, plasminogen activator; uPA, urokinase-type PA; uPAR, uPA receptor; tPA, tissue-type PA; PAI, PA inhibitor; Plg, plasminogen; PTAH, phosphotungstic acid-hemotoxylin; PymT, Polyoma middle T antigen. O ncogenesis is a multistep process that requires a long latency period between the initiating event and the appearance of the tumor. However, a few viral oncogenes are known to rapidly transform target tissues without the need for additional genetic events; one such viral oncogene is the Polyoma middle T antigen (PymT). 1 The PymT antigen specifically transforms proliferating endothelial cells apparently in a single-step manner, resulting in the formation of hemorrhagic cystic tumors in embryonic and neonatal mice, but not in adult mice where endothelial cell proliferation has ceased (for review see Pepper et al., 1997). The potency with which PymT transforms endothelial cells is thought to be a consequence of its interaction with proteins of the signal transduction machinery (for review see Brizuela et al., 1994;Kiefer et al., 1994 a ). These tumors are functionally reminiscent of human vascular tumors in that they are endothelial specific and organ nonspecific (for review see Wagner and Risau, 1994;Pepper et al., 1997), and that tumor-bearing mice develop features of the Kasabach-Merritt syndrome, i.e., thrombocytopenia, anemia, and associated splenomegaly (Dubois-Stringfellow et al., 1994 a ).
Tumor formation and subsequent invasion involve the disruption of anatomical barriers including basement membranes and penetration of tumor cells into normal adjacent tissues. Several lines of evidence indicate that enhanced invasive and metastatic potential is correlated with increased protease activity. It has been shown that both tumor and stromal cells express enzymes that degrade components of the extracellular matrix. Many of the relevant enzymes belong to one of two families: the serine proteases, in particular the plasminogen activator (PA)/plasmin system, and the matrix metalloproteinases (MMPs). Urokinase and tissue-type PAs (uPA and tPA) are the principal activators of plasminogen (Plg), the zymogen from which plasmin is derived. uPA is secreted in the form of an inactive precursor that binds with high affinity to a specific glycosylphosphatidylinositol-anchored cell surface receptor. Plasmin is a protease of tryptic specificity that either directly, or indirectly through the activation of latent MMPs, hydrolyzes extracellular proteins. The existence of multiple specific physiological inhibitors of both plasmin, i.e., ␣ 2antiplasmin, and PAs, i.e., PA inhibitors 1 and 2 (PAI-1 and PAI-2), provides additional points of regulation along this protease cascade (for review see Danø et al., 1985;Vassalli et al., 1991;Pepper et al., 1996).
The recent advent of mice lacking uPA, uPA receptor (uPAR), tPA, PAI-1, and Plg has questioned much early dogma concerning the role of the PA/plasmin system in processes requiring extracellular matrix breakdown. Thus, uPA Ϫ / Ϫ , tPA Ϫ / Ϫ , and uPA Ϫ / Ϫ and tPA Ϫ / Ϫ (utPA Ϫ / Ϫ ) mice, as well as Plg Ϫ / Ϫ mice, develop normally in utero and are fertile. However, utPA Ϫ / Ϫ mice develop multiple organ failure starting at 2-3 mo of age as a result of extensive extravascular fibrin accumulation as well as generalized microvascular thrombosis (Carmeliet et al., 1994). utPA Ϫ / Ϫ mice thus have retarded postnatal growth, exhibit reduced fertility, and have a shortened life span. A similar phenotype is observed in Plg-deficient mice (Bugge et al., 1995 a ;Ploplis et al., 1995). The phenotype of both utPA Ϫ / Ϫ and Plg-deficient mice therefore appears to be due to a systemic defect in fibrinolysis. Although no overt phenotypic alterations have been observed in uPAR Ϫ / Ϫ mice with respect to development, fertility, and homeostasis (Bugge et al., 1995 b ), it has recently been reported that adult mice with a combined deficiency in tPA and uPAR display hepatic sinusoidal fibrin deposits, demonstrating that uPAR plays a role in physiological fibrinolysis (Bugge et al., 1996). Finally, development and fertility are unaffected in PAI-1-deficient mice, although the absence of PAI-1 induces a mild hyperfibrinolytic state (Carmeliet et al., 1993 a,b ). Therefore, neither PAs nor Plg are essential for mouse development, postnatal growth, or fertility, suggesting that the primary role for the PA/plasmin system in the mouse is fibrinolysis.
In previous studies with PymT-transformed endothelial (End.) cell lines in a fibrin gel system, we observed that End. cells formed cystlike structures rather than capillarylike tubes, which correlated with their increased proteolytic activity . Reduction of protease activity by the addition of broad spectrum serine protease inhibitors led to the formation of capillary-like tubes, resulting in the hypothesis that tightly controlled extracellular proteolysis is essential for normal capillary morphogenesis . Further support for the role of the PA/plasmin system in aberrant morphogenesis in vitro came from experiments in which depletion of Plg from serum or addition of neutralizing antibodies to murine uPA inhibited cyst formation (Dubois-Stringfellow et al., 1994 b ). These findings implicated increased proteolysis, and in particular increased uPA activity, in aberrant vascular morphogenesis in vitro and further suggested a causal role for PymT in the elevated proteolytic activity of End. cells. In this study, mice lacking uPA, tPA, utPA, PAI-1, or Plg were used to investigate whether the PA/plasmin system might be important for PymT-or End. cell-induced tumor formation.

Virus Induced Tumor Induction in Neonatal Mice
Helper-free stocks of the N-TKmT virus were obtained from the viral producer cell line GP+E subclone 108.4.2 as described previously (Williams et al., 1988). Neonatal mice of various genotypes were injected intraperitoneally with 1-2.5 ϫ 10 5 neo R -transferring particles. The mice were observed carefully and analyzed as soon as the first mouse of an injected litter showed signs of anemia.

Derivation and Maintenance of End. Cells
Macroscopically visible tumors from mice injected with PymT viral particles were explanted, and End. cell lines were established as previously described (Kiefer et al., 1994 b ). End. cells could be established with relative ease from heterogeneously growing primary cultures, since they outgrow contaminating fibroblasts. Within 5-10 passages, a homogeneously growing, mostly clonal population of End. cells was obtained, which was used for the experiments described. Cell lines were maintained in DME supplemented with 5% FCS, glutamine, and nonessential amino acids and subcultured at confluence every 4-8 d. The origin and nomenclature of these cell lines are summarized in Table I.

Determination of Proliferation Rates of End. Cells
End. cells were plated at a density of 4 ϫ 10 5 -1 ϫ 10 6 per 25-cm 2 flask, counted, and replated at the same density every 6-8 d. Total cumulative cell numbers are represented over 23 d in culture.

Tumor Induction by Injection of End. Cells in Adult Mice
For the induction of End. cell-induced tumors, C57Bl6/129 wild-type and mutant mice, derived from brother-sister matings and interbred for more than six generations (10-14 wk old), and CD1 nude mice (8-10 wk old) were injected subcutaneously into the flanks of the right leg with 1.5-2 ϫ 10 6 cells, and then scored for tumor formation. The times taken for the onset of tumors and the latency period to form tumors of 125 mm 2 (maximum size) were recorded. All mice were killed when they had 125-mm 2 tumors. The observation period was until 60 d after injection.

Northern Blot Hybridization
RNA analyses were carried out using standard Northern techniques (Kiefer et al., 1994 b ). The probes were kindly provided by D. Belin (University of Geneva, Switzerland) (uPA), S. Strickland (State University of New York, Stony Brook) (tPA), and R. Bravo (Bristol-Myers Squibb Pharmaceutical Research Institute, Princeton, NJ) (PAI-1).

Immunohistochemical Analysis and Immunocytochemistry
Tissue specimens were explanted, dissected, fixed overnight in 4% paraformaldehyde in PBS, and embedded in paraffin; 5m-thick sections were stained with hematoxylin and eosin. Adjacent sections were used for immunocytochemical reactions with antibody against factor VIII-related antigen/von Willebrand factor (vWF) (Dakopatts, Copenhagen, Den-mark). Some sections were processed using special staining techniques for fibrin. These included the Lendrum (Lendrum et al., 1962) and Mallory's phosphotungstic acid-hematoxylin (PTAH) techniques.

Zymography and Reverse Zymography
Confluent monolayers of End. cells in 35-mm tissue-culture dishes were washed twice with serum-free DME, and 1.5 ml serum-free DME containing 200 KIU/ml Trasylol (Bayer AG, Zurich, Switzerland) was added. 15 h later, cell extracts and culture supernatants were prepared and analyzed by zymography and reverse zymography as previously described (Vassalli et al., 1984;Montesano et al., 1990). Cell number was determined in a second set of dishes processed in parallel, and cell extract and culture supernatant samples were analyzed on the basis of cell equivalents.

In Vitro Fibrin Gel Assay
End. cells were seeded in suspension into 500-l fibrin gels at 1 ϫ 10 4 cells per gel. 500 l DME containing 10% FCS was added to each well above the fibrin gels. Fibrin gels were prepared as previously described . All experiments were performed in the absence or presence of 200 KIU/ml Trasylol, which was added both to the gel and to the medium at the time of embedding. Medium ( Ϯ Trasylol) was renewed every 2-3 d. Between 4 and 16 d after seeding, cultures were fixed in situ overnight in 2.5% glutaraldehyde in 0.1 M cacodylate buffer, pH 7.4, and photographed using a Phase Contrast-2 microscope (Nikon Inc., Garden City, NY).

PymT Efficiently Induces Vascular Tumors in the Absence of Individual Components of the PA/Plasmin System
To determine if PA/plasmin-dependent proteolytic activity is an essential requirement for the formation of vascular tumors by PymT, we inoculated newborn mice lacking uPA, tPA, utPA, PAI-1, or Plg with the PymT transducing retrovirus N-TkmT (Williams et al., 1988). The frequency of tumor formation was almost 100% in all mutant mice tested, and the time required for the development of lethal tumors (latency) varied only slightly (Table I). The high penetrance in Plg Ϫ / Ϫ mice was unexpected and no delay or inhibition of tumor formation was observed. Most mice were terminally ill within 8-10 d, and only the doublemutant pups lacking uPA and tPA succumbed after a slightly longer period of 12-14 d (Table I). These utPA Ϫ / Ϫ tumors retained the characteristic morphologic properties of PymT-induced tumors (Fig. 1, A and B ). All tumors appeared as blood-filled/hemorrhagic cysts lined by endothelial cells and were not different from similar tumors induced by PymT in control mice (Kiefer et al., 1994 b ). Representative sections through a typical lesion that formed in a double mutant (utPA Ϫ / Ϫ ) mouse as well as in a Plg Ϫ / Ϫ mouse are shown in Fig. 1. Staining with anti-vWF antibody shows endothelial cells lining these cysts (Fig. 1 B ). These results indicate that the PA/plasmin-dependent proteolytic status of the host is apparently not a critical determinant in the transformation of endothelial cells by PymT and in the formation of vascular tumors in newborn mice.

Derivation of End. Cell Lines Lacking uPA, tPA, utPA, and PAI-1
We have previously established transformed End. cell lines from tumor-bearing wild-type and various Src-kinase mutant mice, as well as by infection of primary endothelial cells by PymT in vitro (for review see Wagner and Risau, 1994;Pepper et al., 1997). To test whether the absence of components of the PA/plasmin system affects the capacity to establish permanent End. cell lines in vitro, we derived a number of independent cell lines from tumors induced in tPA-, uPA-, utPA-, and PAI-1-deficient mice ( Table I).
Each of these cell lines displayed the characteristic spindle-shaped, highly refractile morphology that is typical of endothelial cells (data not shown). The time required to establish the different mutant cell lines as homogeneously growing, characteristic End. cells was not significantly different using single knockout mice. However, the double mutant utPAϪ/Ϫ End. cells took three times as long to establish when compared with controls (data not shown). With the exception of ut1 and ut2, all End. cell lines exhibited roughly equivalent growth parameters in vitro as measured by cell doubling times after ‫01ف‬ passages (Fig. 2). The proliferation rate of ut1 and ut2 cell lines was Ͻ50% of controls. All End. cell lines obtained expressed the endothelial-specific tyrosine kinase receptor Flk-1 (Fig. 3 A) and CD31/PECAM-1 (data not shown), confirming their endothelial origin. Most of these cell lines were clonal, since only one integration site for the provirus was detected by Southern blotting (data not shown).

Morphogenetic Behavior of Mutant End. Cells in Fibrin Gels
We have previously reported that End. cells form large cystlike structures lined by a monolayer of endothelial cells when embedded in three-dimensional fibrin gels in vitro . In the present study we have observed that although all six wild-type cell lines have the capacity to form cysts in fibrin gels, there is heterogeneity with respect to the size of the cysts and the frequency with which they form. In addition to cysts, the presence of narrow or ectatic tubes was observed in four out of six lines ( Fig. 4 and Table II). When grown in fibrin gels in the presence of Trasylol, a broad-spectrum serine protease inhibitor, cyst formation was inhibited and tubelike structures were observed in most wild-type End. cell lines ( Fig. 4 and data not shown).
Both uPAϪ/Ϫ cell lines (u23 and u24) retained their capacity to form medium to small cysts ( Fig. 4 and Table II). Cyst formation was completely inhibited in the presence of Trasylol, demonstrating the requirement for serine protease activity (data not shown). However, in contrast with wild-type cells, tube formation was never observed in the presence of Trasylol in either uPAϪ/Ϫ cell line; instead these cells either formed multicellular aggregates or remained as single isolated cells (data not shown). The two tPAϪ/Ϫ cell lines (t37 and t38) formed small cysts and occasional tube-like structures ( Fig. 4 and Table II). Cyst and tube formation could be prevented by addition of Trasylol to t37 but not to t38 cells (data not shown). In striking contrast with most of the cell lines described thus far, both utPAϪ/Ϫ cell lines (ut1 and ut2) completely lost their capacity to form cysts ( Fig. 4 and Table II). In addition, two clearly distinguishable cell populations could be identified: cells either organized into a network of cell cords or remained isolated as single rounded cells ( Fig. 4 and Table  II). Finally, PAI-1-deficient cells formed cysts, although in later passage p1 cells, rapid and massive fibrinolysis precluded the formation of either cysts or tubes ( Fig. 4 and Table II). p3 cells appeared to be heterogeneous: in early passage cells, two cell populations that either formed cysts or cell cords could be identified. Cyst formation (in p1 and p3 cells) and fibrinolysis (in late passage p1 cells) could be inhibited by Trasylol. Cell cord formation by ut1, ut2, and p3 cells was unaffected by addition of Trasylol (data not shown). These findings show that, contrary to what might have been expected, uPAϪ/Ϫ and PAI-1Ϫ/Ϫ cells do not display a decreased or increased capacity to form cysts. The observation that the capacity to form cysts was re-duced to a greater extent in tPAϪ/Ϫ cells than in uPAϪ/Ϫ cells suggests that, although both enzymes are capable of fibrinolysis in this system, tPA is more efficient.

Tumorigenic Potential of uPA-, tPA-, and utPA-deficient End. Cells in Adult Wild-Type Mice
To define a causal role for the PAs in End. cell-induced vascular tumors, we have used cells lacking uPA, tPA, and both uPA and tPA (utPA) for tumor formation studies in adult wild-type mice. We have used immunologically competent adult mice in these studies to avoid variability that may arise from the use of newborn mice with a developing immune system. Tumors were visible as small outgrowths in all mice injected with wild-type End. cells between day 4 and 7 and grew in size to 125 mm 2 by 11-18 d (Fig. 5 A). Histologically, these tumors consisted of a central hemorrhagic/necrotic core surrounded by an intense inflammatory cell infiltrate with neovascularization and very little fibrin deposition (Fig. 6, A and B). Similar results were obtained in mice injected with tPA-deficient End. cells, and there was no significant delay in either the onset or growth rates of these tumors. In contrast, tumor induction with uPAϪ/Ϫ End. cells showed incomplete penetrance and a delay in formation (Fig. 5 A). Only 50% of the mice developed tumors by days 11-14, and it took between 18 and 25 d for these tumors to reach maximum size. The remaining mice did not develop tumors for the entire 60-d observation period. Similarly, about two-thirds of the mice injected with End. cells deficient in both uPA and tPA developed tumors at an extremely slow rate (Fig. 5 A). Tumors were visible between 19 and 24 d after injection, and it took between 40 and 60 d to form tumors of maximum size. The remaining mice did not develop tumors. Histologically, these tumors were similar to those induced by wild-type cells, with the notable exception that extensive fibrin deposition was observed with the Lendrum and PTAH staining techniques (Fig. 6, E and F; data not shown). These results indicate that End. cell lines lacking only tPA activity are not inhibited in their capacity to form tumors. However, cell lines lacking uPA activity (both uPAϪ/Ϫ and utPAϪ/Ϫ) display a reduced efficiency in tumor formation in wild-type mice; when tumors do arise, their growth rate is significantly retarded, indicating that uPA activity of tumor cells is critical for efficient tumor growth.
To rule out the possibility that the observed differences in tumor formation rates were due to various degrees of histoincompatibility, we performed the tumor formation studies in immunoincompetent nude mice. After injection of wild-type and tPAϪ/ϪEnd. cells, tumors were visible around day 5 and grew in size to 125 mm 2 by 10-11 d (Fig.  5 A). In contrast, tumors were only visible between 7 and 14 d after injection with uPAϪ/Ϫ End. cells, and these tumors grew in size to 125 mm 2 by 18-22 d (Fig. 5 A). Similar results were obtained when utPAϪ/Ϫ End. cells were used, with tumors being visible between 10 and 14 d and reaching maximum size by 22-26 d (Fig. 5 A). These results in immunoincompetent mice confirm that the lack of tPA activity in End. cell lines does not affect their tumorigenic potential, whereas lack of uPA activity retards the tumorigenic potential of both uPAϪ/Ϫ and utPAϪ/Ϫ End. cell lines.

Tumor Induction by Wild-Type and Mutant End. Cells in Adult uPA-, tPA-, and utPA-deficient Mice
We next investigated if adult mice lacking uPA, tPA, or both utPA are susceptible to tumor induction by wild-type End. cells. Cells were inoculated into the various mutant mice and tumor growth rates were scored (Fig. 5 B). No significant delay was observed in the onset of tumor development in tPA-deficient mice when compared with wildtype controls, although the time taken to reach maximumsized tumors was slightly delayed (between 14 and 20 d in tPA-deficient mice as compared with 11-18 d in wild-type mice). However, tumor growth was delayed and variation Figure 5. Incidence and latency of tumor formation. End. cellinduced tumor formation was investigated using mutant End. cells in wild-type and nude mice (A), wild-type End. cells in mutant mice (B), and utPAϪ/Ϫ End. cells in mutant mice (C). All mice were inbred for more than six generations and injected with the following cell lines; the time taken for the onset of tumors (days after injection) and the latency period for the formation of 125-mm 2 tumors was recorded: wild-type, s1 (p35 and 40) and s2 (p23, 34 and 40); uPAϪ/Ϫ, u23 (p33) and u24 (p23); tPAϪ/Ϫ, t37 (p25 and 35) and t38 (p6 and 25); and utPAϪ/Ϫ, ut1(p20) and ut2 (p28 and 40). Observation period was for up to 60 d after injection. n, number of mice from at least two independent experiments. Figure 6. Morphology of End. cell-induced tumors in wild-type and mutant adult mice. (A) s1-induced subcutaneous tumor in a wildtype mouse after 3 d; B is the consecutive section stained with the Lendrum technique: note the absence of fibrin deposition, (C) b3induced subcutaneous tumor in a tPAϪ/Ϫ mouse, and (D) s2-induced tumor in a uPAϪ/Ϫ mouse after 3 d. Note the presence of a cen-in tumor formation rates was observed in uPA-deficient mice as follows: (a) only 3 out of 11 uPA-deficient mice developed tumors 12 to 14 d after injection, which grew to maximum size by day 27; (b) tumors were also visible in two other uPA-deficient mice between 12 and 14 d, but these tumors subsequently regressed; (c) the remaining six mice did not develop tumors throughout the entire 60-d observation period. Of the utPA-deficient mice injected with wild-type cells, only two out of four had visible tumors 18-20 d after injection, which then progressed to 125mm 2 tumors by days 40-50. The remaining two mice did not develop tumors for the entire observation period. Only four utPAϪ/Ϫ mice could be used in this study since most of these mutant mice, even uninjected, became ill as they grew older (Carmeliet et al., 1994). Histologically, tumors induced by wild-type cells in mutant mice (Fig. 6, C and D) were similar to those induced in wild-type mice (Fig. 6, A and B), both at early (3 d; Fig. 6, C and D) and late (27 d; Fig. 6 G) time points. However, tumors induced by wild-type cells in utPAϪ/Ϫ mice were consistently associated with extensive fibrin deposition, as seen using the Lendrum and PTAH staining techniques ( Fig. 6 H and data not shown). These results indicate that the host's PA status, and in particular the presence of uPA activity, plays a critical role in determining if proteolytically competent tumor cells can proliferate in vivo.
Since lack of uPA activity in both the host and the injected tumor cells affected the efficient formation of tumors, we next investigated the potential of utPAϪ/Ϫ End. cells to form tumors in mice lacking either uPA or tPA. Tumor formation by utPAϪ/Ϫ End. cells was inhibited in tPAϪ/Ϫ mice, since only three of the seven tPAϪ/Ϫ mice initially developed tumors between days 16 and 21; however, these tumors regressed by day 40 in all cases (Fig.  5 C). The remaining mice remained tumor free. utPAϪ/Ϫ End. cells did not form tumors in any of the uPAϪ/Ϫ mice during the entire observation period (Fig. 5 C). These results suggest that, although the lack of tPA alone has no significant effect on tumor formation, the uPA status of both the host and invading tumor cell has an important role in determining the progression of End. cell tumors.

Discussion
The specific formation of vascular tumors by PymT antigen provides an excellent system to study the molecular mechanisms underlying endothelial cell transformation. One of the components of this process is extracellular matrix degradation and invasion of surrounding tissues, which in the case of PymT results in a profound subversion of normal vasculature. In an attempt to analyze the role of the PA/plasmin system in PymT-mediated oncogenesis, we have investigated its ability to transform endothelial cells in mice lacking uPA, tPA, both uPA and tPA (utPA), or Plg. We found that the proteolytic status (PA/plasmin system) is not a critical determinant, and hence is not limiting for PymT-induced tumor formation. Moreover, the lack of either uPA or tPA activity does not affect the establishment of End. cells in vitro, although the combined loss of both PA activities leads to a reduced proliferation rate. Cyst formation by End. cells in fibrin gels in vitro could be correlated with altered proteolytic activity only in cells lacking uPA and tPA. However, End. cell-induced tumor formation in vivo, as opposed to tumors induced by PymT itself, is dependent on the proteolytic status (PA/plasmin system) of both the tumor cells and the host.
The finding that PymT was able to induce vascular tumors efficiently in the absence of uPA, tPA, utPA, and Plg indicates that the primary transformation process is independent of the PA/plasmin proteolytic status of the endothelial cells. This suggests that PymT-induced tumors can only be used as an in vivo assay for determining the role of this oncogene and associated signaling components in the initial transformation process. To date, the only signaling molecule thus identified is the Src-like kinase, Yes, since PymT could not efficiently transform endothelial cells in Yes-deficient mice in which the latency for tumor formation was also increased (Kiefer et al, 1994b).
Mutant End. cells lacking either uPA or tPA show no proliferation defect in culture. However, End. cell lines lacking both uPA and tPA require a longer time for establishment (three times longer compared with wild-type End. cells) and, when established, exhibit a marked reduction in cell proliferation in vitro. This proliferation defect is also exemplified in End. cell-induced tumor formation in vivo where cells lacking both PAs do not form tumors efficiently, and, when they do, the time taken to reach maximum size is about three times that seen with wild-type End. cells.
Assessment of the morphogenetic behavior of mutant End. cells in fibrin gels in vitro revealed that, although tPA appeared to be more efficient, the absence of either uPA or tPA alone did not render End. cells incapable of forming cysts, indicating that one PA is sufficient. However, combined loss of both uPA and tPA prevented cyst formation, clearly indicating that PA activity is required. The previous prediction that the "proteolytic balance," as represented by the ratio of PA:PAI-1, would predict the behavior of End. cells in fibrin gels ) therefore appears to hold true only when both PAs are absent. These findings indicate that the in vitro system only partially mimics the in vivo situation, and that the behavior of End. cells in fibrin gels is a poor predictor of End. cell tumorigenesis.
The incidence and rate of tumor formation in wild-type mice using End. cells lacking tPA activity were not signifi- cantly affected. However, there was a marked decrease in the number of mice that developed tumors when injected with uPAϪ/Ϫ and utPAϪ/Ϫ End. cells. When tumors did form in a subset of these mice, the onset was delayed and the time taken to develop to maximum size was increased. In some cases, mice developed tumors initially, but these subsequently regressed completely. The data from tumor formation studies with nude mice ruled out the possibility that various degrees of histoincompatibility may have arisen during the establishment or the maintenance of mouse strains and tumor cell lines, thus affecting the experimental outcome. While wild-type and tPAϪ/Ϫ cell lines still formed tumors with 100% penetrance in nude mice, both uPAϪ/Ϫ and utPAϪ/Ϫ End. cell lines showed a delay in the onset of tumor formation as well as an increase in the time taken to form 125-mm 2 tumors. Furthermore, we have previously generated numerous End. cell lines lacking functional c-src, fyn, and yes genes in the same genetic background, C57Bl/6 ϫ 129. These cell lines have been used in similar tumorigenicity studies, and no variations in the tumorigenic profile were found (Kiefer et al., 1994). Thus, the observed delay in End. cell-induced tumor formation and the consequent low incidence of tumor-bearing mice are not likely to result from genetic vari-ations but could be from inefficient matrix degradation and a reduction in migratory/invasive properties of the tumor cells, which in turn lead to inefficient tumor expansion. This observation that End. cell-induced tumors cannot be efficiently formed when endogenous uPA is lacking but are independent of tPA may reflect the longstanding notion that uPA is required in situations of cell migration, morphogenesis, and tissue remodeling, while tPA is primarily implicated in intravascular thrombolysis.
The finding that mice lacking uPA activity (both uPA and utPA null mice) do not display efficient formation of wild-type End. cell-induced tumors was surprising. There was a marked decrease in the formation of tumors, and, when formed, there was a delay in the onset and latency. This indicates that the host's proteolytic status is also critical for the efficient formation of these tumors. Although the exact role of the host's uPA activity in tumor formation is presently unclear, it could be envisaged that the uPA proteolytic mechanism has to involve concerted interactions between both the invading tumor cells and the host's matrix for efficient fibrinolysis and subsequent matrix degradation. Thus, the lack of uPA activity in either would result in inefficient colonization by the tumor cells, as was shown here.
In conclusion, our studies have revealed that the pathogenesis of PymT-induced and End. cell-induced tumors is inherently different. In the former, the PymT virus infects endothelial cells in an existing vascular tree without the requirement for the PA/plasmin system in the initial transformation process (Fig. 7 A). Therefore, PymT-mediated vascular tumor formation appears to be PA/plasmin independent. On the other hand, in End. cell-induced tumors, the injected cells first have to invade host tissues and the basement membrane of existing blood vessels to gain access to the circulation to integrate within host vessels. This invasive process, which is reminiscent of tumor cell intravasation into the vascular tree, is dependent on both the host's and invading tumor cell's PA/plasmin status (Fig.  7 B). In addition, proliferation is impaired in cells entirely lacking PA activity. Defects in both invasion and proliferation, both of which appear to be PA dependent, are therefore likely responsible for the reduction in End. cell-induced tumor growth. Whether other proteases such as the MMPs, which have been shown to be involved in vascular tumor formation in vivo (Taraboletti et al., 1995), may play a role in this process has to be investigated in future experiments.