Previously, we showed that
sulforaphane (SFN), a naturally occurring
cancer chemopreventive agent, effectively inhibits proliferation of PC-3 human
prostate cancer cells by causing caspase-9- and caspase-8-mediated apoptosis. Here, we demonstrate that SFN treatment causes an irreversible arrest in the G(2)/M phase of the cell cycle. Cell cycle arrest induced by SFN was associated with a significant decrease in
protein levels of
cyclin B1, cell division cycle (Cdc) 25B, and Cdc25C, leading to accumulation of Tyr-15-phosphorylated (inactive)
cyclin-dependent kinase 1. The SFN-induced decline in Cdc25C
protein level was blocked in the presence of
proteasome inhibitor lactacystin, but
lactacystin did not confer protection against cell cycle arrest. Interestingly, SFN treatment also resulted in a rapid and sustained phosphorylation of Cdc25C at Ser-216, leading to its translocation from the nucleus to the cytoplasm because of increased binding with 14-3-3beta. Increased Ser-216 phosphorylation of Cdc25C upon treatment with SFN was the result of activation of
checkpoint kinase 2 (Chk2), which was associated with Ser-1981 phosphorylation of
ataxia telangiectasia-mutated, generation of
reactive oxygen species, and Ser-139 phosphorylation of
histone H2A.X, a sensitive marker for the presence of
DNA double-strand breaks. Transient transfection of PC-3 cells with Chk2-specific
small interfering RNA duplexes significantly attenuated SFN-induced G(2)/M arrest. HCT116 human
colon cancer-derived Chk2(-/-) cells were significantly more resistant to G(2)/M arrest by SFN compared with the wild type HCT116 cells. These findings indicate that Chk2-mediated phosphorylation of Cdc25C plays a major role in irreversible G(2)/M arrest by SFN. Activation of Chk2 in response to DNA damage is well documented, but the present study is the first published report to link Chk2 activation to cell cycle arrest by an
isothiocyanate.