Mcl-1 depletion sensitizes the apoptosis of gastrointestinal cancer cells mediated by anti-cancer drugs.

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Apoptosis contributes to the regulation of the cell growth and regeneration and to the development of neoplasm. Mcl-1 (Myeloid cell leukemia-1) is an anti-apoptotic protein that regulates apoptosis sensitivity particularly in hematological and hepatobiliary malignancies. We have reported that Mcl-1 is uniquely overexpressed by cholangiocarcinoma cell lines, and mediates Tumor Necrosis Factor-related Apoptosis Inducing Ligand (TRAIL) resistance in those cells. Here we extend this study to evaluate if Mcl-1 contributes to apoptosis resistance in gastrointestinal carcinoma cells, i.e., gastric cancer and colorectal cancer. Therefore, our AIMs were; i) to examine Mcl-1 expression profiling among gastrointestinal carcinoma cell lines, and ii) to evaluate if Mcl-1 depletion sensitizes apoptosis by anti-cancer drugs. METHOD: Sixteen digestive organ cancer derived cell lines, 6 gastric cancer, 4 pancreatic cancer and 6 colorectal cancer were used. Expression of Mcl-1 was assessed by SDS-polyacrylamide gel electrophoresis and Western blot analysis. Cells were exposed to anti-cancer drugs such as 5-fluorouracil and cisplatin. Apoptosis was quantitated by a morphological observation (DAPI) and caspase activity measurement. Adenovirus-mediated RNA interference (RNAi) technology was used to knock down expression of Mcl-1. Releasing cytochrome C was evaluated by subcellular fractionation and immunoblot analysis. RESULTS: Mcl-1 expression levels were valuable in the cell lines. In gastric cancer cell lines, there was correlation between Mcl-1 protein level and chemosensitivity. Depletion of Mcl-1 protein by RNAi technology effectively sensitizes the Mcl-1-expressing cells to anti-cancer drug-induced apoptosis. Mcl-1-knockdowned apoptosis was associated with mitochondrial depolarization, cytochrome c release from mitochondria and caspase activation. In contrast, Mcl-1 knockdown failed to sensitize apoptosis in colorectal cancer cell lines as well as pancreatic cancer cell lines. IN CONCLUSION: These studies demonstrate that Mcl-1 mediates the resistance of apoptosis in gastric cancer cells by blocking the mitochondrial pathway of cell death. The results also identify some strategies for circumventing resistance to the anti-cancer drugs.