Proteasome inhibitor MG132 induces Death Receptor 5 through CHOP in prostate cancer cell

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TNF-related apoptosis-inducing ligand (TRAIL) is one of promising candidates for cancer therapeutics due to its ability to induce apoptosis in cancer cells with little or no toxicity in normal cells. However, some tumor types exhibit resistance to TRAIL. Thus it is important to overcome this resistance. Combined treatment with a proteasome inhibitor and TRAIL is a promising strategy to overcome TRAIL resistance in cancer cells. Proteasome inhibitors induce the expression of Death Receptor 5 (DR5), a receptor for TRAIL, and sensitize cancer cells to TRAIL-induced apoptosis; however, the molecular mechanism of DR5 up-regulation has not been elucidated. In this study, we report that CHOP is a regulator of DR5 induction by proteasome inhibitor MG132. First, we show that the combination of MG132 and TRAIL effectively induced apoptosis even in hormone refractory prostate cancer DU145 cells. DU145 cells have a mutation of tumor suppressor p53 gene. Therefore our result suggests that the combination of MG132 and TRAIL is efficacious in malignant tumors carrying p53 mutation. MG132 induced DR5 expression at a protein and mRNA level in DU145 cells, indicating that the induction was also independent of p53. Furthermore, MG132 increased DR5 promoter activity. Using a series of deletion-mutant plasmids containing DR5 promoters of various sizes, we found that MG132 stimulated the promoter activity through the region of - 289 to - 253. This region contained a CHOP-binding site. Site-directed mutation of the site abrogated the promoter activity enhanced by MG132. An electrophoretic mobility shift assay demonstrated that CHOP directly bound to the MG132-responsive site on the DR5 promoter. Expression of the CHOP protein was increased with MG132 along with DR5 up-regulation. In addition, MG132 also increased CHOP mRNA and promoter activity. Furthermore, CHOP siRNA attenuated the DR5 up-regulation due to MG132. These results indicate that the proteasome inhibitor MG132 induces DR5 expression through CHOP up-regulation.