Altered Methionine Metabolism in Cancer Cells

Many different types of cancer cells have been shown to be methionine (MET) dependent. Cancer cells, unlike normal cells, grow poorly or not at all when MET is restricted. Cancer cells have an elevated requirement for exogenous MET for growth, despite high levels of endogenous synthesis. This requirement reflects increased utilization of MET by cancer cells, analogous to increased utilization glucose by cancer cells (Warburg effect). To answer the critical question of whether MET-dependent cancer cells synthesize normal amounts of MET, we determined the levels of MET, S-adenosylmethionine (AdoMET), and S-adenosylhomocysteine (AdoHCY) that were synthesized by MET-dependent cancer cells under conditions of MET restriction. We demonstrated that MET-dependent cells synthesize a normal amount of endogenously synthesized MET but are still deficient in AdoMET. In contrast, exogenously supplied MET results in normal AdoMET levels. The ratio of AdoMET to AdoHCY is low in MET-dependent cells growing in MET-restricted medium but is normal when MET is supplied. Under conditions of MET restriction, the low AdoMET/AdoHCY ratio probably limits proliferation of MET-dependent cancer cells. The amount of free MET is also low in MET-dependent cancer cells under MET restriction. The elevated MET requirement for cancer cells may be due to enhanced overall rates of transmethylation compared to normal human cells. Thus, MET-dependent cancer cells have low levels of free MET, low levels of AdoMET, and elevated levels of AdoHCY under conditions of MET restriction probably due to overuse of MET for transmethylation reactions ("Hoffman effect"), thereby blocking cellular proliferation.