Abstract 1020: PPAR Gamma-regulated Expression of eNOS in Adipocytes Suppresses Lipolysis and Prevents High Fat Diet-induced Fatty Liver Disease via S-nitrosylation
Background: Abdominal obesity-associated fatty liver disease (FLD) causes insulin resistance and risk factor clustering, increasing the risk of diabetes and atherosclerosis. FLD is induced by excess free fatty acid secretion from visceral fat tissue through lipolysis. Very recently, endothelial nitric oxide synthase (eNOS) has been reported to be expressed in non-endothelial cells including adipocytes, but the regulation mechanism and functions of eNOS in adipocytes remain unclear. The aim of this study was to investigate the suppressive role of adipocyte-expressed eNOS in lipolysis and high fat diet (HFD)-induced FLD formation.
Methods and Results: eNOS expression was lacking in 3T3-L1 preadipocytes, whereas eNOS mRNA and protein were markedly upregulated during differentiation into mature adipocytes. Ciglitazone (PPAR gamma agonist: 10μmol/L) almost completely abolished eNOS expression in mature 3T3-L1 adipocytes, suggesting a regulatory role of PPAR gamma in eNOS expression. In hypertrophied adipocytes, eNOS level was lower than that in normal adipocytes. Isoproterenol enhanced lipolysis assayed by glycerol measurement in culture medium, accompanied by Akt and eNOS phosphorylation and NO production. Blockade of eNOS by L-NIO (10μmol/L) and siRNA augmented lipolysis (8-fold vs basal level). N-ethylmaleimide (1μmol/L) and auranofin (2μmol/L) significantly reduced lipolysis, suggesting the involvement of S-nitrosylation as a mechanism. Next, C57BL/6 mice (WT) and eNOS KO mice (eNOS KO) were fed normal chow or HFD for 12 weeks (n=5). Body weight and abdominal fat mass were significantly larger in eNOS KO (1.5 fold) than in WT on HFD. HFD induced marked histological fatty liver change with a significant increase in hepatic triglyceride and serum AST, and they were markedly increased in eNOS KO (1.5 fold) compared with those in WT. In WT, HFD significantly downregulated eNOS expression in epididymal fat and there was a negative correlation between hepatic triglyceride and adipose eNOS expression levels.
Conclusion: In conclusion, eNOS in adipocytes has an anti-lipolytic action, which may play a preventive role in the pathogenesis of FLD. These results could provide new insights into the mechanisms of obesity-associated disorders.