Differential responsiveness of human hepatoma cells versus normal hepatocytes to TRAIL in combination with either histone deacetylase inhibitors or conventional cytostatics

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising candidate for the treatment of cancer because it elicits cell death in many tumor cells while sparing most normal cells. Liver cancer, however, is largely resistant to TRAIL and, thus, requires sensitization for TRAIL-mediated cytotoxicity. Sensitization may be achieved by cotreatment with chemotherapeutic agents. In this study, we comparatively investigated the treatment efficacy of TRAIL in combination with histone deacetylase inhibitors (HDI) versus TRAIL in combination with conventional cytostatics in the hepatocellular carcinoma cell line HepG2 and in the childhood hepatoblastoma cell line Huh6. We found that TRAIL resistance could be overcome by cotreatment with the HDI vorinostat, sodium butyrate and MS-275, but not by cotreatment with the cytostatics carboplatin and etoposide. However, TRAIL combination treatment bears the risk of sensitizing otherwise TRAIL-resistant normal cells. We thus explored a potential cytotoxic effect of combined HDI/TRAIL treatment in normal hepatocytes: TRAIL in conjunction with HDI did not impose any cytotoxicity on the non-malignant cells. In searching for the determinants of HDI-mediated TRAIL sensitization in hepatoma cells, we observed that HDI treatment did not increase cell-surface expression of proapoptotic TRAIL receptors. Instead, HDI treatment enhanced TRAIL-induced cleavage of Bid. In conclusion, our data suggest that HDI are potent sensitizers to TRAIL in hepatoma cells and that the combination of HDI and TRAIL is selectively active in hepatoma cells without affecting normal hepatocytes, indicating that the combination of HDI and TRAIL may be an effective approach for the treatment of advanced liver cancer.