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 gamma-schisandrin (gamma-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 gamma-Sch on UV (UVC at 254 nm)-induced activation of DNA damage checkpoint signaling in A549 cells. UV-induced cell death (25 - 75 J/m(2)) was amplified by the presence of the diastereomers, especially GN. At the same time, GN, but not gamma-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 immuno-affinity-purified ATR was dose-dependently inhibited by GN (IC50: 7.28 mu M) but not by gamma-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.