Cellular Inhibitor of Apoptosis Protein 1 (cIAP1) Stability Contributes to YM155 Resistance in Human Gastric Cancer Cells*

Background: YM155, a survivin suppressant, has been shown anticancer efficacy in various cancer cells. Results: Human gastric cancer cells differentially displayed the sensitivity to YM155 dependent to degradation of cIAP1. Conclusion: Cellular inhibitor of apoptosis protein 1 (cIAP1) interacts with survivin to control their mutual stability and determine sensitivity to YM155. Significance: The regulation of cIAP1 enhances the efficacy for YM155 treatment in gastric cancer. YM155, which blocks the expression of survivin, a member of the inhibitor of apoptosis (IAP) family, induces cell death in a variety of cancer types, including prostate, bladder, breast, leukemia, and non-small lung cancer. However, the mechanism underlying gastric cancer susceptibility and resistance to YM155 is yet to be specified. Here, we demonstrate that cIAP1 stability dictates resistance to YM155 in human gastric cancer cells. Treatment of human gastric cancer cells with YM155 differentially induced cell death dependent on the stability of cIAP1 as well as survivin. Transfection with cIAP1 expression plasmids decreased cell sensitivity to YM155, whereas knockdown of endogenous cIAP1 using RNA interference enhanced sensitivity to YM155. In addition, double knockdown of survivin and cIAP1 significantly induced cell death in the YM155-resistant cell line, MKN45. We also showed that YM155 induced autoubiquitination and proteasome-dependent degradation of cIAP1. Surprisingly, survivin affected the stability of cIAP1 through binding, contributing to cell sensitivity to YM155. Thus, our findings reveal that YM155 sensitizes human gastric cancer cells to apoptotic cell death by degrading cIAP1, and furthermore, cIAP1 in gastric cancer cells may act as a PD marker for YM155 treatment.

YM155, which blocks the expression of survivin, a member of the inhibitor of apoptosis (IAP) family, induces cell death in a variety of cancer types, including prostate, bladder, breast, leukemia, and non-small lung cancer. However, the mechanism underlying gastric cancer susceptibility and resistance to YM155 is yet to be specified. Here, we demonstrate that cIAP1 stability dictates resistance to YM155 in human gastric cancer cells. Treatment of human gastric cancer cells with YM155 differentially induced cell death dependent on the stability of cIAP1 as well as survivin. Transfection with cIAP1 expression plasmids decreased cell sensitivity to YM155, whereas knockdown of endogenous cIAP1 using RNA interference enhanced sensitivity to YM155. In addition, double knockdown of survivin and cIAP1 significantly induced cell death in the YM155-resistant cell line, MKN45. We also showed that YM155 induced autoubiquitination and proteasome-dependent degradation of cIAP1. Surprisingly, survivin affected the stability of cIAP1 through binding, contributing to cell sensitivity to YM155. Thus, our findings reveal that YM155 sensitizes human gastric cancer cells to apoptotic cell death by degrading cIAP1, and furthermore, cIAP1 in gastric cancer cells may act as a PD marker for YM155 treatment.
Gastric cancer is a major health problem worldwide. In 2011, ϳ989,600 new cases (ϳ7.8% of all cancers) and 738,000 deaths (ϳ9.7% of all cancers) occurred worldwide (1). Asian and South American countries have higher incidence rates of gastric cancer than Western Europe and the United States. Unfortunately, most gastric cancer patients still present at an advanced stage, and, although there have been advances in diagnostic and treatment strategies, the mortality rates remain poor. Conventional therapies, such as radiotherapy, surgery, and chemotherapy, for the treatment of gastric cancer patients can improve survival. However, numerous gastric cancer patients either do not respond to these therapies or relapse after surgery.
Targeted therapies have been utilized as a treatment strategy for many solid tumors, including colorectal, breast, lung, and renal cancers (2)(3)(4)(5). Target therapeutic advances in gastric cancer partially increased ORR and OS rates through the inhibition of cell proliferation and angiogenesis and changes to the tumor microenvironment (6). Recently, targeted therapies for gastric cancer can be divided into two categories (7): those that target the epidermal growth factor receptor family and those that target the vascular endothelial growth factor (VEGF). However, the survival rate of gastric cancer patients receiving these therapeutic strategies remains poor.
YM155, a small imidazolium-based compound, has recently been reported to suppress the expression of survivin through the repression of its promoter activity (8). YM155 may also suppress survivin expression through disrupting the binding of SP1, a zinc finger transcription factor, to the survivin promoter (9). An initial screen was performed to identify YM155 in the human cervical cancer cell line HeLa and in Chinese hamster ovary cells (8). Until now, many reports have shown the efficacy of YM155 alone and in combinational application against cultured cancer cells, including breast, NSCLC, pancreas, prostate, glioblastoma, and head and neck cancers (8, 10 -14). However, the mechanism relating to SP1 and YM155 in cancer cells has remained unclear. Moreover, the inhibitory mechanism of YM155 targeting survivin on the growth of various cancer cells is yet unclear.
In this investigation, the inhibitory mechanisms of YM155 treatment against human gastric cancer cells were studied. We first tested whether YM155-induced apoptosis was associated with inhibition of survivin expression. We next focused on the cIAP1 4 change in anti-apoptotic proteins that related to YM155-induced apoptosis. To our knowledge, this is the first report demonstrating a relationship between survivin and cIAP1 in gastric cancer cells after YM155 treatment.
Cell Death and Annexin V Staining Assay-Cells were treated with YM155 at the indicated doses for 48 h, and cell death was determined using the trypan blue exclusion method. Cells were seeded in 60-mm dishes at a density of 5 ϫ 10 5 cells for the annexin V staining assay. Then, cells were treated with YM155 at the indicated doses for 48 h. The cells were washed with PBS, trypsinized, stained with an annexin V staining solution (BD Biosciences, San Jose, CA), and analyzed with a fluorescenceactivated cell sorter (BD Biosciences).
Immunoprecipitation and GST Pulldown Assay-For the in vivo binding assay, cell lysates (0.5 mg) were incubated with anti-survivin (or GFP) or anti-cIAP1 (or HA) antibodies at 4°C for 12 h. The mixture was added to protein G Plus-Sepharose beads (Santa Cruz Biotechnology) and then incubated for an additional 2 h at 4°C. The immunoprecipitates were washed with Nonidet P-40 RIPA lysis buffer, boiled in 2ϫ SDS sample buffer, and then analyzed with anti-survivin (or GFP) or anti-cIAP1 (or HA) antibodies. Purified His-survivin, GST-cIAP1, and GST proteins were purchased from Abnova (Taipei, Taiwan) for in vitro GST pulldown assays. Briefly, 500 ng of GST or GST-cIAP1 proteins were incubated with 500 ng of His-survivin protein in reaction buffer (20 mM Tris-HCl, pH 7.5, and 120 mM NaCl) at 30°C for 1 h, added to GST pulldown buffer (20 mM Tris-HCl, pH 8.0, 500 mM NaCl, 1% Triton X-100, 0.02% bovine serum albumin, and 5 mM 2-mercaptoethanol) to terminate the reaction and then glutathione-Sepharose beads were added (Cell Signaling Technology) for another 1 h at 4°C. The mixtures were washed five times with GST pulldown buffer and heated with 2ϫ SDS sample buffer. The binding of survivin and cIAP1 was analyzed via Western blotting with anti-His and anti-GST antibodies, respectively.
In Vivo and in Vitro Ubiquitination Assay-For the in vivo ubiquitination assays, cell lysates were precipitated using anti-cIAP1 or anti-HA antibodies at 4°C for 12 h and then added to protein G-Sepharose beads for another 2 h. The precipitates were prepared for Western blot analysis using anti-ubiquitin antibody. For the in vitro cIAP1 ubiquitination assay, 500 ng of purified GST-cIAP1 proteins were incubated with 8 ng of human E1, 500 ng of human His-UbcH5a, 2 mM Mg-ATP, and 5 g of ubiquitin (Boston Biochem Inc., Cambridge, MA) in ubiquitination reaction buffer (50 mM Tris-HCl, pH 7.4, 50 mM NaCl) and treated with 10 or 20 nM YM155 for 1 h at 37°C. The mixtures were pulled down using a GST buffer and then analyzed by Western blot assay using an anti-ubiquitin antibody.
Surface Plasmon Resonance Analysis-Proteon TM XPR36 (Bio-Rad) was used to determine the binding of YM155 to cIAP1. GST and GST-cIAP1 proteins were captured on a Proteon GLH sensor chip (Bio-Rad). GST or GST-cIAP1 proteins were captured to 2600 or 7000 response units after immobilization, respectively. YM155 was injected at various concentrations at a flow rate of 100 l/min for 60 s and allowed to dissociate for an additional 300 s.
Native PAGE-Native gels were prepared using an 8 -15% acrylamide mixture without SDS. Cell lysates (30 g/well) were loaded onto native gels without heating, and run in Tris glycine electrophoresis buffer (47 mM Tris base, 364 mM glycine) without SDS for 12 h on ice at 30 V.
Non-radioactive Pulse-Chase Assay-Newly synthesized survivin or cIAP1 protein was labeled using the Click-it metabolic labeling reagents (Invitrogen). Briefly, KATOIII cells were transfected with survivin or cIAP1 expressing plasmids for 48 h and then cells were depleted with methionine-free RPMI 1640 medium for 1 h. The cells were incubated with methionine-free RPMI 1640 medium containing 50 M L-azidohomoalanine, a methionine analog (Invitrogen), for 4 h. The cells were washed with PBS followed by the addition of complete media. The cells were then chased for the indicated times. L-Azidohomoalanine incorporated protein was biotinylated using the Click-it protein reaction buffer kit (Invitrogen). The biotinylated proteins were precipitated using anti-biotin and then analyzed by Western blotting using anti-survivin or anti-cIAP1 antibodies.
Statistical Analyses-All data were statistically analyzed using a two-tailed Student's t test. The significance in the text was verified by p values, and p values Ͻ0.05 were deemed significant.

Human Gastric Cancer Cells Display Differential Sensitivity to YM155-induced Apoptotic Cell
Death-It has recently been reported that YM155 inhibits cell growth and proliferation in various tumor cells (15). Treatment of p53-mutant or -deficient prostate cancer cells with YM155 decreases survivin expression (16). In mesothelioma cells, YM155 inhibits MCL-1 expression as well as survivin (17). Thus, YM155 has an anti-tumor effect on various cancer cells. However, the mechanism that regulates survivin and sensitivity to YM155 in human gastric cancer cells remain largely unexplored.
To this aim, we analyzed the inhibitory effect of YM155 on multiple gastric cancer cells (Fig. 1A). The viability of most gastric cancer cells was not affected, with the exception of KATOIII and AGS cells followed by YM155 treatment. To analyze the effect of YM155, two human gastric cancer cell lines with high expression levels of survivin were selected. First, the inhibitory effect of YM155 on these cells was analyzed. Each cell line was treated with different doses of YM155 for different times (Fig. 1). Interestingly, differences in sensitivity to YM155 were observed in the cancer cells (Fig. 1B). Consistently, increased annexin V staining was observed in KATO III cells after exposure to YM155, but not in MKN45 cells (Fig. 1C). To investigate whether the difference in sensitivity to YM155 was a result of different levels of survivin, we analyzed survivin levels after exposure to YM155. The survivin levels in KATO III cells, which were sensitive to YM155, decreased in a dose-and timedependent manner in response to YM155. However, survivin levels in MKN45 cells, which were resistant to YM155, were not changed (Fig. 1D). In addition, YM155 induced the cleavage of pro-caspase-3 to yield active caspase-3 in KATO III cells, but not in MKN45 cells. These results indicate that human gastric cancer cells display differential sensitivities to YM155.
Next, we examined the effects of survivin silencing via small interfering RNA (siRNA) on two gastric cancer cell lines, KATO III and MKN45 (Fig. 1E). The knockdown effects of the survivin siRNA on two gastric cancer cells were confirmed.
Dead cell populations were dose-dependently increased in KATO III cells after YM155 treatment. However, although the siRNA decreased survivin levels, MKN45 cell death did not increase after exposure to YM155. Consistently, caspase-3 cleavage was significantly induced in KATO III cells, whereas that in MKN45 cells was weakly observed. Moreover, the levels of survivin were clearly decreased in only sensitive AGS cells, whereas survivin levels in the other cells did not decrease (Fig.  1F). These results suggest that YM155 may induce cell death through the regulation of an alternative molecule as well as survivin in human gastric cancer cells.
YM155 Targets cIAP1as Well as Survivin-As shown in Fig.  1E, YM155 may target another factor as well as survivin. It was recently reported that YM155 inhibits XIAP in pancreatic cancer cells and cIAP2 in cancer cells (18). To confirm this hypothesis, we initially analyzed the effects of YM155 on XIAP, cIAP1, and 2 other IAP family members. Surprisingly, the levels of cIAP1 protein were significantly decreased in KATO III cells after treatment with YM155, whereas that in MKN45 cells was not decreased ( Fig. 2A, i and ii). In addition, XIAP and cIAP2 levels did not change in either cancer cell line, implying that difference in sensitivity to YM155 may relate to cIAP1 as well as survivin. Also, YM155 induced the decrease of cIAP1 expression through protein levels but not mRNA levels ( Fig. 2A, iii). Accordingly, we further analyzed the effects of cIAP1 on sensitivity to YM155. Therefore, we first constructed a plasmid expressing HA-tagged cIAP1 cDNA. Transfection with a construct expressing cIAP1 cDNA decreased YM155-induced apoptotic cell death in KATOIII cells but not MKN45 cells (Fig.  2B). Moreover, cIAP1 inhibited the YM155-induced decrease in survivin. In contrast, cIAP1 knockdown via siRNA decreased survivin levels in KATO III cells after exposure to YM155, but not in MKN45 cells (Fig. 2C, i). The dead cell population was also increased in KATO III cells that expressed cIAP1 siRNA (Fig. 2C, ii), indicating that YM155 targets cIAP1 and affects survivin in YM155-sensitive gastric cancer cells.
Based on the above data, we analyzed the effects of survivin on sensitivity to YM155 and cIAP1. To this end, we constructed a plasmid expressing GFP-tagged survivin cDNA. Survivin expression inhibited the YM155-induced decrease in cIAP1 protein levels and cell death in KATO III cells but not MKN45 cells (Fig. 2D). Conversely, knockdown of survivin decreased cIAP1 levels, but not in MKN45 cells (Fig. 2E, i). Consistently, cell death was also increased in KATO III cells only (Fig. 2E, ii), suggesting that YM155 induces a decrease of cIAP1 as well as survivin. However, YM155 does not decrease cIAP1 levels in YM155-resistant cells.  both survivin and cIAP1 siRNA on the YM155-resistant cell line MKN45. Double knockdown of survivin and cIAP1 significantly induced cell death in MKN45 cells (Fig. 3). Our results strongly suggest that sensitivity to YM155 depends on decreased survivin and cIAP1 protein levels in human gastric cancer cells.
Survivin and cIAP1 Affect the Protein Stability of Each Other-As shown in Fig. 2, survivin affected the expression of cIAP1 protein. Conversely, cIAP1 affected the expression of survivin protein. Thus, the proteins affected the expression of each other. Expression of survivin inhibited the YM155-induced decrease of cIAP1 protein levels. This result implies that survivin may affect cIAP1 protein stability. To confirm this hypothesis, we have performed the in vivo ubiquitination assays using ubiquitin chain-specific antibodies to analyze the effects of YM155 on survivin ubiquitination. Cells were treated with MG132 followed by YM155 treatment. Cell lysates were prepared for Western blot analysis and immunoprecipitated using an anti-survivin antibody. The level of survivin protein clearly increased in response to MG132. Consistently, survivin ubiquitination was also induced (Fig. 4A, i). Also, we analyzed whether the degradation of cIAP1 protein after exposure to YM155 was proteasome dependent using the proteasome inhibitor, MG132. Treatment of the YM155-sensitive cell line, KATO III, with MG132 significantly increased cIAP1 protein levels (Fig. 4A, ii), whereas cIAP1 levels in MKN45 cells were not affected by MG132 treatment followed by YM155 (data not shown). Consistent with these data, endogenous cIAP1 was ubiquitinated in response to YM155 in a time-dependent manner such as TNF-␣ treatment (Fig. 4B, i and ii). We further analyzed the effect of YM155 on cIAP1 ubiquitination through a in vitro ubiquitination assay. Ubiquitination of purified GST-cIAP1 gradually increased after exposure to YM155 in a dosedependent manner (Fig. 4B, iii). We analyzed the interaction between YM155 and cIAP1 for the effect of YM155 on cIAP1 ubiquitination. We performed a surface plasmon resonance assay. A peak of YM155-bound cIAP1 protein was observed by surface plasmon resonance analyses of YM155 that was incubated with purified GST-cIAP1 protein (Fig. 4C). These results suggest that YM155 induces the abrogation of cIAP1 autoinhibition through a direct binding interaction. We next analyzed whether cIAP1 ubiquitination was affected by survivin expression; we transfected KATO III cells with survivin-cDNA. Ectopic expression of survivin did not induce cIAP1 ubiquitination (Fig. 4B, i). Conversely, the knockdown of survivin led to cIAP1 ubiquitination (Fig. 4D). These results suggest that survivin inhibits cIAP1 ubiquitination and its subsequent proteasome-mediated degradation.
On the basis of these results, we further explored the role of survivin in controlling cIAP1 turnover by treating with the protein synthesis inhibitor cycloheximide and measuring changes in exogenously expressed survivin protein levels over time. In a cycloheximide-chase experiment, ectopic expression of GFPtagged survivin did not induce changes in cIAP1 protein levels in KATO III cells, whereas transfection with an empty vector led to diminished cIAP1 protein levels (Fig. 4E). Conversely,  knockdown of survivin significantly decreased cIAP1 protein levels (Fig. 4F), indicating that survivin positively controls the protein stability of cIAP1. In view of these data, we also analyzed the effect of cIAP1 expression on survivin protein stability. Ectopic expression of cIAP1 did not decrease survivin pro-tein levels after treatment with cycloheximide, whereas survivin protein levels were significantly decreased in cells expressing the control vector (Fig. 4G). Conversely, survivin protein levels decreased more than control (Fig. 4H). We additionally performed a non-radioactive pulse-chase assay to re- confirm the effects of cIAP1 or survivin overexpression on protein stability. Cells were transfected with either a survivin or cIAP1 construct or empty vector. After the indicated times, cell lysates were prepared and immunoprecipitated using an antibiotin antibody. The level of cIAP1 or survivin protein was detected via Western blotting using an anti-cIAP1 or survivin antibody. The overexpression of survivin affected the protein stability of cIAP1, and cIAP1 overexpression also affected survivin protein stability (Fig. 4I). These results indicate that survivin and cIAP1 affect the protein stability of each other.
Survivin Directly Binds to cIAP1-Based on the above data, we examined the direct interaction between endogenous cIAP1 and survivin. Immunoprecipitation and Western blotting analyses revealed both cIAP1-bound survivin and survivin-bound cIAP1 (Fig. 5A, i). To further confirm this, we performed GST pulldown assays. Western blot analyses of purified GST-cIAP1 or GST proteins incubated with purified His-tagged survivin. GST pulldown assays followed by Western blotting detected survivin-bound cIAP1 proteins, but did not detect survivinbound GST proteins. These results suggest that survivin directly interacts with cIAP1 (Fig. 5A, ii). To further investigate the interaction between cIAP1 and survivin, we constructed HA-tagged cIAP1, GFP-tagged wild-type survivin, and its deleted mutant, ␦BIR. In 293 cells co-transfected with these plasmids, HA-tagged cIAP1 was detected in GFP-tagged wildtype survivin immunoprecipitates, but not in GFP-tagged BIRdeleted survivin mutant immunoprecipitates (Fig. 5B), indicating that cIAP1 directly interacts with the BIR domain of survivin. Thus, cIAP1 may affect the stability of survivin through interaction with the BIR domain. To confirm this, we first constructed a GFP-tagged survivin T34A mutant (threonine was mutated to alanine in the BIR domain) (19), or C84A mutant (cysteine was mutated to alanine in the BIR domain) (20). cIAP1 ubiquitination was induced independently of the expression of each mutant after exposure to YM155 (Fig. 5C). Consistently, cIAP1 directly interacted with wild-type survivin, but not with other mutants (Fig. 5D, i), implying that cIAP1 may bind to specific amino acid resides that are only present in the BIR domain of survivin. These results suggest that cIAP1 affects the stability of survivin through an interaction with the BIR domain. We additionally analyzed the effect both survivin mutants had on stability of the cIAP1 protein. Each survivin mutant also led to a decrease in ectopic cIAP1 protein levels compared with cells transfected with wild-type survivin after exposure to the protein synthesis inhibitor cycloheximide (Fig.  5D, ii). Based on the above results, whether survivin point mutants specifically affect cell death in KATOIII cells, which are sensitive to YM155, was examined. Cell death was significantly increased in KATOIII cells but not MKN45 cells expressing T34A, C84A, or both, after exposure to YM155, whereas ectopic expression of wild-type survivin partially inhibited YM155-induced cell death (Fig. 5E), indicating that T34A/ C84A, which has a point mutation in the BIR domain of survivin, affects YM155-induced cell death. These results suggest that cIAP1 is stabilized through binding to the BIR domain of survivin and thereby can induce resistance to YM155.
As shown in Fig. 5A, cIAP1 also directly interacted with survivin. In view of these data, the interaction of survivin with cIAP1 was further investigated. To this end, we first constructed a plasmid for HA-tagged cIAP1 cDNA and its point mutant, H588A, which is an alanine substitution mutation that is solely present in the RING domain of cIAP1 (21). In 293 cells co-transfected with these plasmids, HA-tagged cIAP1 wildtype was detected in GFP-tagged survivin immunoprecipitates, but not in HA-tagged cIAP1 H588A immunoprecipitates (Fig.  5F, i), indicating that survivin interacts with the RING domain of cIAP1. We further analyzed the effect of the cIAP1 mutant, H588A, on the stability of survivin protein. Consistent with the two survivin mutants, ectopic expression of the cIAP1 mutant, H588A, resulted in decreased levels of ectopic survivin protein compared with cells treated with wild-type cIAP1. Furthermore, dimerization of the cIAP1 mutant, H588A, was not observed (Fig. 5F, ii and iii). These results suggest that survivin may interact with the RING domain of cIAP1 and subsequently increase its stability. In addition, we analyzed the RING domain of cIAP1 responsible for the interaction with survivin. To this end, we constructed an HA-tagged mutant construct (delta RING cIAP1) lacking the RING domain of cIAP1. Cells were co-transfected with wild-type survivin and HA-tagged wildtype cIAP1 or ␦RING cIAP1. Wild-type survivin was readily detected via Western blotting following incubation with extracts from cells expressing wild-type cIAP1, but not ␦RING cIAP1 (Fig. 5G). Thus, the RING domain of cIAP1 is crucial for the interaction with survivin as well as dimerization. Transfection of HA-tagged cIAP1 H588A in KATOIII cells but not MKN45 cells led to cell death after exposure to YM155, unlike in cells expressing wild-type cIAP1 (Fig. 5H), indicating that the stabilization of cIAP1 by survivin affects YM155-induced cell

DISCUSSION
Several studies have elucidated the mechanisms by which YM155, a novel small-molecule survivin suppressant, exerts anticancer effects in a variety of human solid tumors (15). However, the mechanism underlying human cancer susceptibility to YM155 remains to be fully elucidated. Recently, it was reported that YM155 exerts an antiproliferative effect, inducing cell death through suppression of X-linked inhibitor of apoptosis (XIAP) in human pancreatic cancer cells (18). Thus, YM155 may be an effective therapeutic agent, showing that its action on other IAP superfamily members as well as survivin. Here, we present evidence that differences in sensitivity to YM155 depend on the protein stability of cIAP1 as well as survivin in human gastric cancer cells. We further showed that their protein stability is interactively controlled through a direct interaction of survivin with cIAP1.
Over the past decade, targeted therapy for cancer treatment has progressed. Unfortunately, despite a wide variety of approaches, human gastric cancer remains a major challenge with fewer effective targeted therapies than other cancers. Recently, it was reported that YM155 promotes cisplatin-induced apoptosis in gastric cancer (22). Our research focused on the inhibitory effect of YM155 alone on gastric cancer. We investigated whether YM155 could induce apoptotic cell death through the down-regulation of cIAP1 protein as well as survivin in human gastric cancer cells, but not mRNA levels (Fig.  2). In addition, decreased levels of XIAP and cIAP2 were not observed ( Figs. 1 and 2). On the basis of these results, we focused on a decrease of cIAP1 protein after exposure to YM155. For this, we first analyzed the stability of cIAP1 and survivin with regard to their expression. As shown in Fig. 4, the decrease in the levels of cIAP1 or survivin by YM155 was blocked by the ectopic expression of each protein. Conversely, survivin knockdown led to cIAP1 ubiquitination, suggesting that survivin and cIAP1 each affect the protein stability of the other and subsequently determine cell sensitivity to YM155.
We further confirmed the mechanism underlying the degradation of cIAP1 and survivin by YM155. As shown in Fig. 5, cIAP1 directly interacted with the BIR domain of survivin, and survivin bound to the RING domain of cIAP1. We also analyzed the RING or combined RING-CARD on survivin-bound cIAP1.
To this end, we first constructed HA-tagged constructs of the RING domain and the RING-CARD domain. We assessed the levels of survivin-bound cIAP1 after co-transfecting 293 cells with GFP-tagged wild-type survivin and HA-tagged wild-type cIAP1, HA-tagged RING, or RING-CARD. The levels of wildtype survivin-bound cIAP1 clearly decreased in the cells transfected with HA-tagged RING or RING-CARD (data not shown). These results suggest that the RING or combined RING-CARD acts as an antagonist to the survivin-cIAP1 interaction. Interactive control of cIAP1 and survivin affects the sensitivity to YM155. However, these interactions were also observed in MKN45 cells, which are resistant to YM155 (Fig.  6A). A survivin heteromer disappeared in KATOIII cells, which are sensitive to YM155, after exposure to YM155, but an increase of the heteromer was observed in MKN45 cells (Fig.  6B). Also, we analyzed the interaction between survivin and the cIAP1 dimer and monomer. We prepared lysates from YM155treated cells and immunoprecipitated them using an anti-survivin antibody. Immunoprecipitation and native gel analyses revealed that survivin interacts with the monomer of cIAP1. In addition, treatment of YM155 led to a decrease in the cIAP1 monomer in KATOIII cells, but not in MKN45 cells (Fig. 6C). These results suggest that the survivin-cIAP1 complex is regulated by other factors in MKN45. That is, sensitivity to YM155 determine to the presence of other factor regulators for the survivin-cIAP1 complex. Therefore, knockdown of survivin or cIAP1 induces cell death through each others regulation in KATOIII, which is not regulated by other factors for the survivin-cIAP1 complex, but only double knockdown of survivin and cIAP1 induces cell death in MKN45, which is regulated by other factors for the survivin-cIAP1 complex (Fig. 3). The results coincided with data shown in Fig. 2. Next, we analyzed whether cIAP1 or survivin point mutants specifically affect cell death in MKN45 cells. Cell death was not observed in MKN45 cells expressing survivin T34A, C84A, both, or cIAP1 H588A after exposure to YM155, unlike KATOIII cells (Figs. 4 and 5), implying that there may be another factor that interacts with cIAP1 or/and survivin. Future research will be focused on the detailed mechanisms or unknown molecules that can directly regulate the stability of cIAP1 or/and survivin in human gastric cancer cells that are resistant to YM155.
Although we clearly established the important role of cIAP1 as well as survivin for the anti-tumor effects of YM155 in human gastric cancer cells, the regulatory mechanisms of cIAP1 and survivin in YM155-resistant cells were not addressed. Our findings highlight the therapeutic potential of YM155 for gastric cancer treatment through the degradation of cIAP1 as well as survivin.