Regulatory Role of Human AP-endonuclease (APE1/Ref-1) in YB-1-Mediated Activation of Multi Drug Resistance (MDR1) Gene

Running Title: APE1 modulates YB-1-mediated activation of MDR1 ABSTRACT The human AP-endonuclease (APE1/Ref-1), a central enzyme involved in repair of oxidative base damage and DNA strand breaks, has a second activity as a transcriptional regulator that binds to several transacting factors. APE1 overexpression is often observed in tumor cells, and confers resistance to various anticancer drugs; its downregulation sensitizes tumor cells to such agents. Because APE1's involvement in repairing the DNA damage induced by many of these drugs is unlikely, the drug resistance may be linked to APE1's transcriptional regulatory function. Here we show that APE1, preferably in acetylated form, stably interacts with the Y-box-binding protein-1 (YB-1), and enhances its binding to the Y-box element, leading to activation of the multidrug resistance gene MDR1. Enhanced MDR1 level due to ectopic expression of WT APE1, but not of its nonacetylable mutant, underscores the importance of APE1's acetylation in its co-activator function. APE1 downregulation sensitizes MDR1-overexpressing tumor cells to cisplatin or doxorubicin, showing APE1's critical role in YB-1-mediated gene expression and thus drug resistance in tumor cells. A systematic increase in both APE1 and MDR1 expression was observed in non-small cell lung cancer tissue samples. Thus our study has established the novel role of acetylation-mediated transcriptional regulatory function of APE1, making it a potential target for drug sensitization of tumor cells.