Ionizing radiation-inducible microRNA miR-193a-3p induces apoptosis by directly targeting Mcl-1

The functions of microRNAs (miRNAs) as either oncogenes or tumor suppressors in regulating cancer-related events have been established. We analyzed the alterations in the miRNA expression profile of the glioma cell line U-251 caused by ionizing radiation (IR) by using an miRNA array and identified several miRNAs whose expression was significantly affected by IR. Among the IR-responsive miRNAs, we further examined the function of miR-193a-3p, which exhibited the most significant growth-inhibiting effect. miR-193a-3p was observed to induce apoptosis in both U-251 and HeLa cells. We also demonstrated that miR-193a-3p induces the accumulation of intracellular reactive oxygen species (ROS) and DNA damage as determined by the level of γH2AX and by performing the comet assay. The induction of both apoptosis and DNA damage by miR-193a-3p was blocked by antioxidant treatment, indicating the crucial role of ROS in the action of miR-193a-3p. Among the putative target proteins, the expression of Mcl-1, an anti-apoptotic Bcl-2 family member, decreased because of miR-193a-3p transfection. A reporter assay using a luciferase construct containing the 3'-untranslated region of Mcl-1 confirmed that Mcl-1 is a direct target of miR-193a-3p. Down-regulation of Mcl-1 by siRNA transfection closely mimicked the outcome of miR-193a-3p transfection showing increased ROS, DNA damage, cytochrome c release, and apoptosis. Ectopic expression of Mcl-1 suppressed the pro-apoptotic action of miR-193a-3p, suggesting that Mcl-1 depletion is critical for miR-193a-3p induced apoptosis. Collectively, our results suggest a novel function for miR-193a-3p and its potential application in cancer therapy.