Argininosuccinate synthase conditions the response to acute and chronic ethanol-induced liver injury in mice

Argininosuccinate synthase (ASS) is the rate-limiting enzyme in both the urea and the L-citrulline/nitric oxide (NO.) cycles regulating protein catabolism, ammonia levels, and NO. generation. Because a proteomics analysis identified ASS and nitric oxide synthase-2 (NOS2) as coinduced in rat hepatocytes by chronic ethanol consumption, which also occurred in alcoholic liver disease (ALD) and in cirrhosis patients, we hypothesized that ASS could play a role in ethanol binge and chronic ethanol-induced liver damage. To investigate the contribution of ASS to the pathophysiology of ALD, wildtype (WT) and Ass(+/-) mice (Ass(+/-) are lethal due to hyperammonemia) were exposed to an ethanol binge or to chronic ethanol drinking. Compared with WT, Ass(+/-) mice given an ethanol binge exhibited decreased steatosis, lower NOS2 induction, and less 3-nitrotyrosine (3-NT) protein residues, indicating that reducing nitrosative stress by way of the L-citrulline/NO. pathway plays a significant role in preventing liver damage. However, chronic ethanol-treated Ass(+/-) mice displayed enhanced liver injury compared with WT mice. This was due to hyperammonemia, lower phosphorylated AMP-activated protein kinase alpha (pAMPK alpha) to total AMPK alpha ratio, decreased sirtuin-1 (Sirt-1) and peroxisomal proliferator-activated receptor coactivator-1 alpha (Pgc1 alpha) messenger RNAs (mRNAs), lower fatty acid beta-oxidation due to down-regulation of carnitine palmitoyl transferase-II (CPT-II), decreased antioxidant defense, and elevated lipid peroxidation end-products in spite of comparable nitrosative stress but likely reduced NOS3. Conclusion: Partial Ass ablation protects only in acute ethanol-induced liver injury by decreasing nitrosative stress but not in a more chronic scenario where oxidative stress and impaired fatty acid beta-oxidation are key events. (HEPATOLOGY 2012)