Epidemiological data continues to support the notion that increased consumption of Allium vegetables may reduce the risk for different types of malignancies including cancer of the prostate. Moreover, allium vegetable derived organosulfur compounds, such as diallyl trisulfide (DATS; CH2=CH-CH2-(S)3-CH2-CH=CH2), are highly effective in affording protection against cancer development in animal models. Evidence is mounting to indicate that garlic-derived organosulfur compounds can suppress proliferation of cultured cancer cells by inhibiting cell cycle progression and causing apoptosis induction. Previous studies including those from our laboratory have offered novel insights into the mechanism of apoptosis induction by DATS. For instance we showed previously that DATS-induced apoptosis in PC-3 and DU145 human prostate cancer cells was associated with c-jun N-terminal kinase mediated phosphorylation of Bcl-2, which reduced Bax-Bcl-2 interaction to trigger activation of caspase-9/3. In the present study, we extended these observations and determined the mechanism by which DATS inhibits cell cycle progression using PC-3 cells as a cellular model. We found that exposure of PC-3 cells to 20 or 40 μM DATS resulted in a time-dependent enrichment of G2/M phase cells that was accompanied by accumulation of cyclin B1, degradation of cell division cycle 25C (Cdc25C), and increased phosphorylation of Cdc25C at Ser-216 and cyclin-dependent kinase 1 (Cdk1) at Tyr-15. DATS treatment also caused activating phosphorylation of checkpoint kinase 1 (Chk1; Ser-317) and Chk2 (Thr-68), which are intermediaries of DNA damage checkpoint and implicated in Ser-216 phosphorylation of Cdc25C. The Ser-216 phosphorylation of Cdc25C phosphatase, which plays a pivotal role in activation of Cdk1/cyclin B1, negatively regulates its activity. In addition, DATS treated PC-3 cells exhibited features characteristic of mitotic arrest including disruption of the tubulin network, chromatin condensation and increased Ser-10 phosphorylation of histone H3. DATS-induced Cdc25C and Cdk1 phosphorylation and mitotic arrest, but not G2 phase arrest, was significantly attenuated by depletion of Chk1 protein using Chk1 targeted siRNA duplexes. On the other hand, Chk2 protein knockdown did not have any appreciable effect on G2 or M phase arrest caused by DATS. In conclusion, these results suggest existence of a Chk1-Cdc25C-Cdk1 independent mechanism responsible for DATS-induced G2 arrest. Furthermore, we provide evidence to indicate that Chk1 kinase is necessary to arrest cells that escape G2 block at the early stages of mitosis (supported by NIH grants CA101753 and CA076348).
[Proc Amer Assoc Cancer Res, Volume 46, 2005]