Molecular mechanisms of cell cycle block by methionine restriction in human prostate cancer cells

Previous studies have shown that dietary or pharmacological methionine rest riction inhibits growth of human prostate cancer cells in vitro or as xenog rafts in mice. We undertook the present studies to clarify the molecular me chanisms by which methionine restriction inhibits prostate cancer cell grow th. We found that PC-3 and DU 145 cells stopped proliferating within six da ys in growth medium containing homocysteine in place of methionine. In cont rast, proliferation of LNCaP cells was only partially inhibited by the abse nce of methionine. Using flow cytometry, we found that methionine restricti on caused PC-3 cells to arrest in all phases of the cell cycle, but predomi nantly in the G(2)/M phase, whereas LNCaP cells accumulated exclusively in the G, phase. Methionine restriction led to accumulation of the cyclin-depe ndent kinase inhibitors p21 and p27, as determined by Western blot analysis , and inhibited the enzymatic activities of the cyclin-dependent kinases CD K2 and cdc2, as determined by an in vitro kinase assay. However, methionine restriction had little or no effect on CDK2 or cdc2 protein levels. Methio nine restriction also induced PC-3 cells to undergo apoptosis, as indicated by the appearance of a typical nucleosomal ladder on gel electrophoresis o f genomic DNA. We conclude that methionine restriction has potential as a n ovel treatment strategy for prostate cancer.