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 VIII levels are down-regulated in the regenerating rat liver after partial hepatectomy. Tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6) are two cytokines fundamental for liver regeneration. TNF-alpha 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-alpha and IL-6 actions were investigated in rat hepatocytes and H35 hepatoma cells. Results: C-2-ceramide (N-acetylsphingosine) at 1-10 muM decreased MAT I/III expression. The effect was maximum after 2 h of treatment and it was maintained up to 24 h. MAT VIII protein levels also decreased, IL-6 (1-10 ng/ml) potentiated C-2-ceramide effects in cultured hepatocytes while decreasing by itself MAT VIII levels with a similar time-response curve in both cell types. C-2-ceramide actions were not associated with an increase in cell death. TNF-alpha was also a potent antagonist for MAT VIII expression, at 1-20 ng/ml decreased MAT I/III levels and induced endogenous ceramide generation. The decrease of MAT VIII 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. (C) 2001 European Association for the Study of the Liver. Published by Elsevier Science B.V. All rights reserved.