Suppression of TopBP1 function increases the efficacy of chemotherapeutic treatments by enhancing the induction of apoptosis

Abstract

AbstractThe DNA damage response (DDR) is an important mechanism to maintain genome integrity by arresting cell cycle progression and inducing DNA repair and/or apoptosis. Therefore, the activity of DDR is closely related to the drug sensitivity of cancer cells. Inhibitors of ATR, a key member of protein kinases functioning in DDR, are attractive candidates as sensitizers in chemotherapy. In this study, we explore another candidate of chemosensitizers and report DNA topoisomerase II binding protein 1 (TopBP1), a regulator of ATR‐mediated signaling, as a potential target to increase the efficacy of chemotherapeutic treatments. Suppression of TopBP1 using siRNA increased cancer cell sensitivity to cisplatin and an alkylating agent N‐methyl‐N‐nitrosourea (MNU), concomitant with a percentage increase in the sub‐G1 population and caspase‐9 activation. The immunoblotting analysis revealed that the phosphorylation of CHK1 was significantly reduced in TopBP1‐knockdown cells. Consequently, treatment with an ATR inhibitor dramatically increased the production of the sub‐G1 population compared to an ATM inhibitor. Phosphorylation of RPA2 increased after drug treatment in TopBP1‐knockdown cells. These results suggest that TopBP1 is involved in DDR protecting stalled forks from collapse and preventing apoptosis through the activation of an ATR/CHK1 signaling pathway.