Inhibition of mTOR suppresses human gallbladder carcinoma cell proliferation and enhances the cytotoxicity of 5-fluorouracil by downregulating MDR1 expression

OBJECTIVE: Although 5-fluorouracil (5-FU) is widely used in the treatment of various cancers, drug resistance remains a limitation for its anti-cancer activity. Mammalian target of rapamycin (mTOR) is deregulated in diverse human cancers, including gallbladder carcinoma and mTOR inhibitors show promising anti-cancer activities with proliferation inhibitory effects. This study aims to clarify the benefit of the combination of 5-FU and the mTOR inhibitor, OSI-027, on gallbladder carcinoma cell proliferation. MATERIALS AND METHODS: Two gallbladder carcinoma cell lines and two agents (5-FU and OSI-027) were used in the present study. Cell counting kit-8 assays and EdU staining were performed to examine the proliferation of cancer cells. The expression of MDR1 protein was determined by western blot analysis. RESULTS: The combination of OSI-027 with 5-FU showed a synergistic anti-proliferative effect on the gallbladder cancer cells, RBE and GBC-SD cells. Upon 5-FU treatment, MDR1 expression was upregulated and OSI-027 could reverse 5-FU-induced MDR1 upregulation. Moreover, MDR1 depletion sensitized gallbladder carcinoma cells to 5-FU stimulation and attenuated the synergistic effect of OSI-027 and 5-FU. Finally, we determined that OSI-027 downregulated MDR1 expression by suppressing its synthesis rather than by promoting its degradation. CONCLUSIONS: Dual mTORC1/mTORC2 inhibitors such as OSI-027 are promising therapeutic agents in combination with 5-FU for the treatment of human gallbladder cancer.