Low extracellular pH enhances TRAIL-induced apoptosis by downregulating Mcl-1 expression

We previously reported that low extracellular pH promotes tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis through the mitochondria-mediated caspase signal transduction pathway. In this study, we further investigated the mechanism of low extracellular pH on TRAIL-induced apoptosis. When human colorectal carcinoma HCT116 cells were treated with TRAIL for 4 h, significant cytotoxicity was observed at pH 6.3, while cytotoxic effects were notably reduced at pH 7.2. These findings suggest that TRAIL's cytotoxic effects on human colorectal cancer cells are enhanced in low pH environments following TRAIL treatment. Similar results were observed in human pancreatic adenocarcinoma BxPC-3 cells. Interestingly, TRAIL was found to downregulate the levels of anti-apoptotic proteins, such as Mcl-1. This was confirmed by the knock-in (KI) of an Mcl-1 phosphorylation site mutant in HCT116 cells, which blocked TRAIL-induced Mcl-1 downregulation and the subsequent apoptotic response. These results indicate that Mcl-1 mediates TRAIL resistance in the Mcl-1 KI cells. Additionally, our results revealed that TRAIL significantly induced JNK phosphorylation in HCT116 cells, suggesting the involvement of JNK in TRAIL-induced cell death in colorectal cancer cells. Our findings demonstrate that low extracellular pH enhances TRAIL-induced cytotoxicity, particularly at pH 6.3 and 6.6. Moreover, the anti-apoptotic Bcl-2 family member Mcl-1 is an important target of TRAIL in colorectal carcinoma HCT116 cells under different low pH conditions. TRAIL triggered a rapid decline in Mcl-1, suggesting that Mcl-1 downregulation is crucial for TRAIL-induced apoptosis.