Short-chain ceramide regulates hepatic methionine adenosyltransferase expression

Background: The metabolism of methionine plays an important role in regulating hepatic cellular function. Methionine adenosyltransferase (MAT) is the enzyme that catalyses the biosynthesis of S-adenosylmethionine (AdoMet) from ATP and methionine. Liver-specific MAT I/III levels are down-regulated in the regenerating rat liver after partial hepatectomy. Tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) are two cytokines fundamental for liver regeneration. TNF-α stimulates sphingomyelin metabolism and ceramide generation in a variety of cell systems. Aims: The role of exogenous cell-permeable ceramide in modifying MAT I/III mRNA levels and its association with TNF-α and IL-6 actions were investigated in rat hepatocytes and H35 hepatoma cells. Results: C2-ceramide (N-acetylsphingosine) at 1–10 μM decreased MAT I/III expression. The effect was maximum after 2 h of treatment and it was maintained up to 24 h. MAT I/III protein levels also decreased. IL-6 (1–10 ng/ml) potentiated C2-ceramide effects in cultured hepatocytes while decreasing by itself MAT I/III levels with a similar time-response curve in both cell types. C2-ceramide actions were not associated with an increase in cell death. TNF-α was also a potent antagonist for MAT I/III expression, at 1–20 ng/ml decreased MAT I/III levels and induced endogenous ceramide generation. The decrease of MAT I/III mRNA levels (in all the cases) was not due to a decrease in mRNA half-life which suggests a regulation at the transcriptional level. Finally, the decrease in MAT I/III mRNA levels correlated to a decrease in MAT activity. Conclusion: This work demonstrates that short-chain ceramide can be used as a novel exogenous agonist that can modulate hepatic methionine metabolism in association with cytokines.