Excess S-adenosylmethionine reroutes phosphatidylethanolamine towards phosphatidylcholine and triglyceride synthesis

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Methionine adenosyltransferase 1A (MAT1A) and glycineN-methyltransferase (GNMT) are the primary genes involved in hepatic S-adenosylmethionine (SAMe) synthesis and degradation, respectively.Mat1aablation in mice induces a decrease in hepatic SAMe, activation of lipogenesis, inhibition of triglyceride (TG) release, and steatosis.Gnmt-deficient mice, despite showing a large increase in hepatic SAMe, also develop steatosis. We hypothesized that as an adaptive response to hepatic SAMe accumulation, phosphatidylcholine (PC) synthesis by way of the phosphatidylethanolamine (PE)N-methyltransferase (PEMT) pathway is stimulated inGnmt−/−mice. We also propose that the excess PC thus generated is catabolized, leading to TG synthesis and steatosis by way of diglyceride (DG) generation. We observed thatGnmt−/−mice present with normal hepatic lipogenesis and increased TG release. We also observed that the flux from PE to PC is stimulated in the liver ofGnmt−/−mice and that this results in a reduction in PE content and a marked increase in DG and TG. Conversely, reduction of hepatic SAMe following the administration of a methionine-deficient diet reverted the flux from PE to PC ofGnmt−/−mice to that of wildtype animals and normalized DG and TG content preventing the development of steatosis.Gnmt−/−mice with an additional deletion of perilipin2, the predominant lipid droplet protein, maintain high SAMe levels, with a concurrent increased flux from PE to PC, but do not develop liver steatosis.Conclusion: These findings indicate that excess SAMe reroutes PE towards PC and TG synthesis and lipid sequestration. (Hepatology 2013;58:1296–1305)

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