Role of S-adenosylmethionine in two experimental models of pancreatitis.

Severe necrotizing pancreatitis occurs in young female mice fed a choline-deficient and ethionine-supplemented (CDE) diet. Although the mechanism of the pancreatitis is unknown, one consequence of this diet is depletion of hepatic S-adenosylmethionine (SAM). SAM formation is catalyzed by methionine adenosyltransferases (MATs), which are encoded by liver-specific (MAT1A) and non-liver-specific (MAT2A) genes. In this work, we examined changes in pancreatic SAM homeostasis in mice receiving the CDE diet and the effect of SAM treatment. We found that both MAT forms are expressed in normal pancreas and pancreatic acini. After 48 h of the CDE diet, SAM levels decreased 50% and MAT1A-encoded protein disappeared via post-translational mechanisms, whereas MAT2A-encoded protein increased via pretranslational mechanisms. CDE-fed mice exhibited extensive necrosis, edema, and acute pancreatic inflammatory infiltration, which were prevented by SAM treatment. However, old female mice consuming the CDE diet that do not develop pancreatitis showed a similar fall in pancreatic SAM level. SAM was also protective in cerulein-induced pancreatitis in the rat, but the protection was limited. Although the pancreatic SAM level fell by more than 80% in the MAT1A knockout mice, no pancreatitis developed. This study thus provides several novel findings. First, the so-called liver-specific MAT1A is highly expressed in the normal pancreas and pancreatic acini. Second, the CDE diet causes dramatic changes in the expression of MAT isozymes by different mechanisms. Third, in contrast to the situation in the liver, where absence of MAT1A and decreased hepatic SAM level can lead to spontaneous tissue injury, in the pancreas the roles of SAM and MAT1A appear more complex and remain to be defined.

Spontaneous oxidative stress and liver tumors in mice lacking methionine adenosyltransferase 1A.

In mammals, methionine metabolism occurs mainly in the liver via methionine adenosyltransferase-catalyzed conversion to S-adenosylmethionine. Of the two genes that encode methionine adenosyltransferase(MAT1Aand MAT2A), MAT1A is mainly expressed in adult liver whereas MAT2A is expressed in all extrahepatic tissues. Mice lacking MAT1A have reduced hepatic S-adenosylmethionine content and hyperplasia and spontaneously develop nonalcoholic steatohepatitis. In this study, we examined whether chronic hepatic S-adenosylmethionine deficiency generates oxidative stress and predisposes to injury and malignant transformation. Differential gene expression in MAT1A knockout mice was analyzed following the criteria of the Gene Ontology Consortium. Susceptibility of MAT1A knockout mice to CCl4-induced hepatotoxicity and malignant transformation was determined in 3- and 18-month-old mice, respectively. Analysis of gene expression profiles revealed an abnormal expression of genes involved in the metabolism of lipids and carbohydrates in MAT1A knockout mice, a situation that is reminiscent of that found in diabetes, obesity, and other conditions associated with nonalcoholic steatohepatitis. This aberrant expression of metabolic genes in the knockout mice was associated with hyperglycemia, increased hepatic CYP2E1 and UCP2 expression and triglyceride levels, and reduced hepatic glutathione content. The knockout animals have increased lipid peroxidation and enhanced sensitivity to CCl4-induced liver damage, which was largely due to increased CYP2E1 expression because diallyl sulfide, an inhibitor of CYP2E1, prevented CCl4-induced liver injury. Hepatocellular carcinoma developed in more than half of the knockout mice by 18 months of age. Taken together, our findings define a critical role for S-adenosylmethionine in maintaining normal hepatic function and tumorigenesis of the liver.

S-adenosylmethionine and methylthioadenosine are antiapoptotic in cultured rat hepatocytes but proapoptotic in human hepatoma cells.

S-adenosylmethionine (AdoMet) is an essential compound in cellular transmethylation reactions and a precursor of polyamine and glutathione synthesis in the liver. In liver injury, the synthesis of AdoMet is impaired and its availability limited. AdoMet administration attenuates experimental liver damage, improves survival of alcoholic patients with cirrhosis, and prevents experimental hepatocarcinogenesis. Apoptosis contributes to different liver injuries, many of which are protected by AdoMet. The mechanism of AdoMet's hepatoprotective and chemopreventive effects are largely unknown. The effect of AdoMet on okadaic acid (OA)-induced apoptosis was evaluated using primary cultures of rat hepatocytes and human hepatoma cell lines. AdoMet protected rat hepatocytes from OA-induced apoptosis dose dependently. It attenuated mitochondrial cytochrome c release, caspase 3 activation, and poly(ADP-ribose) polymerase cleavage. These effects were independent from AdoMet-dependent glutathione synthesis, and mimicked by 5'-methylthioadenosine (MTA), which is derived from AdoMet. Interestingly, AdoMet and MTA did not protect HuH7 cells from OA-induced apoptosis; conversely both compounds behaved as proapoptotic agents. AdoMet's proapoptotic effect was dose dependent and observed also in HepG2 cells. In conclusion, AdoMet exerts opposing effects on apoptosis in normal versus transformed hepatocytes that could be mediated through its conversion to MTA. These effects may participate in the hepatoprotective and chemopreventive properties of this safe and well-tolerated drug.