Activation of Pparγ and Inhibition of Xiap in Human Cancer Cells: Synergistic Effects On Cell Proliferation, Apoptosis, and Tumorigenesis

Abstract

Background and objectives: In colon cancer, the anti-tumor effect of Peroxisome proliferatoractivated receptor gamma (PPARγ) on tumorigenesis is controversial. Over-expression of Xlinked inhibitor of apoptosis (XIAP) in colon cancer has been linked to treatment resistance and poor prognosis. The aims of our present study are to explore the synergistic effects of PPARγ activation and XIAP inhibition on the cell proliferation, apoptosis, and tumorigenesis in colon cancer In Vitro and In Vivo. Materials and methods: Colon cancer cell line HCT116 XIAP+/+ and HCT116XIAP-/- were cultured, and troglitazone (TGZ) was used as PPARγ agonist. Cell proliferation was measured by WST-1 assay. Apoptosis was determined by ELISA kit. Protein expression was detected by Western blot. In In Vivo tumorigenesis studies, Balb/c nude mice were subcutaneously inoculated with 1×107 cells to generate colon cancer xenograft. TGZ (200 ppm) was given on the day of tumor cell inoculation. The volume of tumor was measured every other day. Expressions of Ki67 and VEGF in tumor tissues were detected by Western blot. Results: TGZ induced apoptosis and inhibited the proliferation of HCT116XIAP+/+ and HCT116XIAP-/- cells in a time- and dose-dependent manner, and these effects were more significant in HCT116XIAP-/- cells. The IC50 values of TGZ against HCT116XIAP+/+ and HCT116XIAP-/- cells after 48h treatment were 88.80μM and 68.04μM, respectively. Caspase 3, 7, 8, and 9, as well as poly(ADP-ribose) polymerase were activated more obviously in HCT116XIAP-/- cells compared to HCT116XIAP+/+ cells. In addition, combined with XIAP inhibitor 1396-12 increased apoptosis and cell growth inhibition in HCT116XIAP+/+ cells when compared to TGZ alone. The study In Vivo showed there was no difference in the tumor development in HCT116XIAP+/+ and HCT116XIAP-/- cells xenograft model. TGZ retarded the growth of xenograft colon cancer, and such a growth inhibition was more significant in HCT116XIAP-/- cells-derived tumors than in HCT116XIAP+/+ cellsderived tumors, with a tumor volume reduction rate of 83.3% and 32.5% on day 33, respectively. This increased therapeutic benefit correlated with significant decreased expression of Ki67 and VEGF in tumors. Conclusion: Simultaneous activation of PPARγ and inhibition of XIAP may have synergistic anti-tumor effect against colon cancer.