Abstract
Normal differentiated hepatocytes primarily metabolize methionine, via homocysteine synthesis, through the transsulfuration pathway. In addition to glutathione, this pathway produces α-ketobutyrate that is further metabolized in the mitochondria. It is only under low methionine conditions that differentiated hepatocytes predominantly regenerate methionine from homocysteine. In contrast, proliferating hepatocytes and liver cancer cells regenerate methionine from homocysteine regardless of the availability of methionine. Here we propose that this less efficient metabolism of methionine in proliferating hepatocytes and cancer cells is an adaptation to the "Warburg effect" that is, to the well known phenomenon that cancer cells rely on aerobic glycolysis instead of oxidative phosphorylation to generate energy. The observation that knockout mice with impaired S-adenosylmethionine (SAMe) synthesis (the first step in methionine metabolism) or catabolism spontaneously develop fatty liver and hepatocellular carcinoma, together with the observation that SAMe administration induces apoptosis in hepatoma cells and prevents liver cancer support this hypothesis.