1. ¹Division of Gastroenterology and Liver Diseases, Department of Medicine, USC Research Center for Liver Diseases, USC-UCLA Research Center for Alcoholic Liver and Pancreatic Diseases, Keck School of Medicine USC, Los Angeles, CA, USA
2. ²Department of Biochemistry, Vanderbilt University, Nashville, TN, USA
3. ³CIC bioGUNE, CIBERehd, Technology Park of Bizkaia, Bizkaia, Spain

Correspondence: Dr S Lu, MD, Division of Gastroenterology and Liver Diseases, USC Research Center for Liver Diseases, USC-UCLA Research Center for Alcoholic Liver and Pancreatic Diseases, Keck School of Medicine, 2011 Zonal Avenue, HMR 415A, Los Angeles, CA 90033, USA. E-mail: shellylu@usc.edu

Received 30 November 2007; Revised 18 April 2008; Accepted 4 May 2008; Published online 11 August 2008.

Abstract

Endotoxemia participates in the pathogenesis of many liver injuries. Lipopolysaccharide (LPS) was shown to inactivate hepatic methionine adenosyltransferase (MAT), the enzyme responsible for S-adenosylmethionine (SAMe) biosynthesis. SAMe treatment was shown to prevent the LPS-induced increase in tumor necrosis factor-, which may be one of its beneficial effects. SAMe is also an important precursor of glutathione (GSH) and GSH was shown to ameliorate LPS-induced hepatotoxicity. The aims of this work were to examine changes in SAMe and GSH homeostasis during endotoxemia and the effect of SAMe. Mice received SAMe or vehicle pretreatment followed by LPS and were killed up to18 h afterward. Unexpectedly, we found hepatic SAMe level increased 67% following LPS treatment while S-adenosylhomocysteine level fell by 26%, suggesting an increase in SAMe biosynthesis and/or block in transmethylation. The mRNA and protein levels of MAT1A and MAT2A were increased following LPS. However, despite increased MAT1A expression, MAT activity remained inhibited 18 h after LPS. The major methyltransferase that catabolizes hepatic SAMe is glycine N-methyltransferase, whose expression fell by 65% following LPS. Hepatic GSH level fell more than 50% following LPS, coinciding with a comparable fall in the mRNA and protein levels of glutamate-cysteine ligase (GCL) catalytic (GCLC) and modifier subunits (GCLM). SAMe pretreatment prevented the fall in GCLC and attenuated the fall in GCLM expression and GSH level. SAMe pretreatment prevented the LPS-induced increase in plasma alanine transaminases levels but not the LPS-induced increase in hepatic mRNA levels of proinflammatory cytokines. It further enhanced LPS-induced increase in interleukin-10 mRNA level. Taken together, the hepatic response to LPS is to upregulate MAT expression and inhibit SAMe utilization. GSH is markedly depleted largely due to lower expression of GCL. Interestingly, SAMe treatment prevented the fall in GCL and helped to preserve the GSH store and prevent liver injury.