Abstract
Growth suppressive effect of diallyl trisulfide (DATS), a promising cancer chemopreventive constituent of garlic, against cultured human cancer cells correlates with checkpoint kinase 1 (Chk1)-mediated mitotic arrest, but the fate of the cells arrested in mitosis remains elusive. Using LNCaP and HCT-116 human cancer cells as a model, we now demonstrate that the Chk1-mediated mitotic arrest resulting from DATS exposure leads to apoptosis. The DATS exposure resulted in G(2) phase and mitotic arrest in both LNCaP and HCT-116 cell lines. The G(2) arrest was accompanied by downregulation of cyclin-dependent kinase 1 (Cdk1), cell division cycle (Cdc) 25B, and Cdc25C leading to Tyr15 phosphorylation of Cdk1 (inactivation). The DATS-mediated mitotic arrest correlated with inactivation of anaphase-promoting complex/cyclosome as evidenced by accumulation of its substrates cyclinB1 and securin. The DATS treatment increased activating phosphorylation of Chk1 (Ser317) and transient transfection with Chk1-targeted siRNA conferred significant protection against DATS-induced mitotic arrest in both cell lines. The Chk1 protein knockdown also afforded partial yet statistically significant protection against apoptotic DNA fragmentation and caspase-3 activation resulting from DATS exposure in both LNCaP and HCT-116 cells. Even though DATS treatment resulted in stabilization and Ser15 phosphorylation of p53, the knockdown of p53 protein failed to rescue DATS-induced mitotic arrest. In conclusion, the results of the present study indicate that Chk1 dependence of DATS-induced mitotic arrest in human cancer cells is not influenced by the p53 status and cells arrested in mitosis upon DATS exposure are driven to apoptotic DNA fragmentation.