Methionine Stress Induces Downregulation of Cdc6 and Pre-Replication Complexes and Activates Intrinsic Apoptosis in Breast Cancer Cells

Methionine and homocysteine are metabolites in the transmethylation pathway leading to synthesis of the methyl-donor S-adenosylmethionine (SAM). Most cancer cells stop proliferating during methionine stress conditions - when methionine is replaced in the growth media by its immediate metabolic precursor homocysteine (Met-Hcy+). Non-transformed cells proliferate in Met-Hcy+ media making the methionine metabolic requirement of cancer cells an attractive target for therapy. To study this phenomenon in breast cancer cells we selected methionine independent resistant cell lines derived from MDAMB468 breast cancer cells. Resistant cells grew normally in Met-Hcy+ media, whereas their parental MDAMB468 cells rapidly arrested in the G1 phase. Remarkably, supplementing Met-Hcy+ growth media with S adenosylmethionine suppressed the cell proliferation defects, indicating that methionine stress is a consequence of SAM limitation rather than low amino acid concentrations. Accordingly, mTOR activity, the primary effector responding to amino acid limitation, remained high. However, we found that Cdc6 protein levels decreased and pre-replication complexes were destabilized in methionine stressed MDAMB468 but not resistant cells. The methionine stress induced G1 cell cycle arrest was maintained for several days before induction of intrinsic apoptosis and downstream caspase activation led to a significant decrease in cell number. We also created a mathematical model of methionine metabolism in yeast cells as a first step toward studying the equivalent pathway in silico in cancer cells. Our study characterizes metabolite requirements, cell cycle changes, and apoptosis induction during methionine stress and helps explain the metabolic uniqueness of cancer cells.