Differential effect of schisandrin b stereoisomers on ATR-mediated DNA damage checkpoint signaling

We have previously reported that schisandrin b (schb) is a specific inhibitor of atr (ataxia telangiectasia and rad-3-related) protein kinase. since schb consists of a mixture of its diastereomers gomisin n (gn) and γ-schisandrin (γ-sch),the inhibitory action of schb might result from a stereospecific interaction between one of the stereoisomers of schb and atr. therefore,we investigated the effect of gn and γ-sch on uv (uvc at 254 nm)-induced activation of dna damage checkpoint signaling in a549 cells. uv-induced cell death (25 - 75 j/m2) was amplified by the presence of the diastereomers,especially gn. at the same time,gn,but not γ-sch,inhibited the phosphorylation of checkpoint proteins such as p53,structural maintenance of chromosomes 1,and checkpoint kinase 1 in uv-irradiated cells. moreover,gn inhibited the g2/m checkpoint during uv-induced dna damage. the in vitro kinase activity of immunoaffinity- purified atr was dose-dependently inhibited by gn (ic50: 7.28 μm) but not by γ-sch. these results indicate that gn is the active component of schb and suggest that gn inhibits the dna damage checkpoint signaling by stereospecifically interacting with atr. © 2013 the japanese pharmacological society.