Inhibition of the G(2) DNA damage checkpoint and of protein kinases Chk1 and Chk2 by the marine sponge alkaloid debromohymenialdisine

Cells can respond to DNA damage by activating checkpoints that delay cell c ycle progression and allow time for DNA repair. Chemical inhibitors of the G(2) phase DNA damage checkpoint may be used as tools to understand better how the checkpoint is regulated and may be used to sensitize cancer cells t o DNA-damaging therapies. However, few inhibitors are known. We used a cell -based assay to screen natural extracts for G(2) checkpoint inhibitors and identified debromohymenialdisine (DBH) from a marine sponge. DBH is distinc t structurally from previously known G(2) checkpoint inhibitors. It inhibit ed the G(2) checkpoint with an IC50 of 8 muM and showed moderate cytotoxici ty (IC50 = 25 muM) toward MCF-7 cells. DBH inhibited the checkpoint kinases Chk1 (IC50 = 3 muM) and Chk2 (IC50 = 3.5 muM) but not ataxia-telangiectasi a mutated (ATM), ATM-Rads-related protein, or DNA-dependent protein kinase in vitro, indicating that it blocks two major branches of the checkpoint pa thway downstream of ATM. It did not cause the activation or inhibition of d ifferent signal transduction proteins, as determined by mobility shift anal ysis in Western blots, suggesting that it inhibits a narrow range of protei n kinases in vivo.