MicroRNA-137 exerts protective effects on hypoxia-induced cell injury by inhibiting autophagy/mitophagy and maintaining mitochondrial function in breast cancer stem-like cells

Breast cancer stem-like cells (BCSCs) have been identified and proven to play critical roles in tumorigenesis and progression. Hypoxia is a common pathologic feature of breast cancer and potentially, at least in part, regulates the initiation, progression, and recurrence of breast cancer. However, less is known about how hypoxia regulates BCSCs. As several well-known microRNAs respond to hypoxia, we aimed to determine how hypoxia regulates the physiological processes of BCSCs by regulating the corresponding microRNAs. As expected, microRNA-137 (miRNA-137 or miR-137) was downregulated upon hypoxic exposure, indicating that it may play critical roles in BCSCs. Introduction of miR-137 mimics promoted cell cycle entry and inhibited hypoxia-induced cell apoptosis as determined by cell cycle assay and apoptosis assay. By detecting mitochondrial reactive oxygen species (ROS), it was found that miR-137 inhibited ROS accumulation induced by hypoxic exposure and thus suppressed cell apoptosis. Introduction of miR-137 mimics under hypoxia inhibited mitophagy/autophagy by targeting FUN14 domain containing 1 (Fundc1) and thus promoted mitochondrial functions, including mitochondrial mass, ATP synthesis and mitochondrial transcriptional activity, which was similar to the effects of Fundc1 knockdown by specific siRNA. Based on these observations, we hypothesized that the survival of BCSCs under hypoxia was mediated by miR-137 by regulating mitochondrial dysfunction. We demonstrated here that the introduction of exogenous miR-137 promoted mitochondrial function, indicating that it may be a potential therapeutic target in BCSCs.