S-adenosyl-L-methionine (SAM) is synthesized by methionine adenosyltransferases (MATs). Ablation of the liver-specific MAT1A gene results in liver neoplasia and sensitivity to oxidant injury. Here we show that acidic sphingomyelinase (ASMase) mediates the downregulation of MAT1A by TNF-α. The levels of MAT1A mRNA as well as MAT I/III protein decreased in cultured rat hepatocytes by in situ generation of ceramide from exogenous human placenta ASMase. Hepatocytes lacking the ASMase gene (ASMase–/–) were insensitive to TNF-α but were responsive to exogenous ASMase-induced downregulation of MAT1A. In an in vivo model of lethal hepatitis by TNF-α, depletion of SAM preceded activation of caspases 8 and 3, massive liver damage, and death of the mice. In contrast, minimal hepatic SAM depletion, caspase activation, and liver damage were seen in ASMase–/– mice. Moreover, therapeutic treatment with SAM abrogated caspase activation and liver injury, thus rescuing ASMase+/+ mice from TNF-α–induced lethality. Thus, we have demonstrated a new role for ASMase in TNF-α–induced liver failure through downregulation of MAT1A, and maintenance of SAM may be useful in the treatment of acute and chronic liver diseases.
Montserrat Marí, Anna Colell, Albert Morales, Covadonga Pañeda, Isabel Varela-Nieto, Carmen García-Ruiz, José C. Fernández-Checa