Evaluating the role of Mcl-1 and its interacting partners in radioresistance of oral cancer cells

Earlier studies from the lab have demonstrated over expression of anti-apoptotic Mcl-1 protein in human oral cancers and its association with radiotherapy resistance and poor outcome, implying it to be a potential therapeutic target in oral cancers. In the present study, we investigated the role of Mcl-1 and its interacting partners, USP9X and TCTP in radio-resistance of oral cancer cells. Expression of these three proteins was induced post radiation treatment, followed by their translocation to the nucleus and co-localization with gamma H2AX, indicating their possible role in DNA damage response. We confirmed the interaction between Mcl-1 and USP9X proteins and demonstrated that USP9X down regulation leads to a significant reduction in Mcl-1 protein levels, primarily due to its increased turnover. Interestingly, the oral cancer patients showing elevated USP9X levels also exhibited high levels of Mcl-1 protein and a significant decrease in disease free survival. Inhibition of USP9X by deubiquitinase inhibitor WP1130, potently induced apoptosis in oral cancer cells associated with a rapid decline of Mcl-1 protein and accumulation of aggresomes. The single agent in vivo efficacy of WP1130 was also demonstrated against oral cancer xenografts. Our studies thus demonstrate an important role of USP9X in stabilizing Mcl-1, thereby contributing to oral cancer radio-resistance. Thus inhibition of Mcl-1 and/or their interacting partner holds great promise in the improved management of oral cancer patients. (author)