Hepatic Sympathetic Denervation Reduces Hepatic Steatosis and Blunts Liver Lipid Acquisition Pathways in Diet‐Induced Obese Mice

Abstract

Non‐alcoholic fatty liver disease (NAFLD), characterized by hepatic steatosis, is directly associated with obesity and a significant contributor to chronic metabolic and cardiovascular diseases. We recently demonstrated that diet‐induced NAFLD is accompanied by robust elevations in hepatic sympathetic nerve activity (SNA), and that pharmacological or surgical reductions in hepatic SNA reduces hepatic steatosis in obese mice. However, the underlying molecular mechanisms through which liver SNA contributes to NAFLD remain unknown. We hypothesized that liver SNA contributes to hepatic steatosis via influences on both liver acquisition and disposal pathways. Starting at 6 wks of age, male C57B1/6 mice were fed a high fat diet (HFD, 60% fat) or remained on normal chow (NC, 5% fat) for 10 wks. Liver selective sympathetic denervation was then performed by applying 10% phenol to the hepatic nerve bundle, whereas saline served as a sham surgery. Seven days after surgery, histological examination (Oil Red O) confirmed widespread and severe lipid accumulation in the liver of HFD mice relative to NC counterparts (0.6±0.4% vs 38±7% stained area, NC Sham vs HFD Sham, p<0.05), which was reduced following liver denervation (16±5% stained area, HFD Denervation, p<0.05 vs HFD sham). Western blot analysis revealed that hepatic carnitine palmitoyltransferase 1a (CPT1a), as an indicator of mitochondria b‐oxidation, and acyl‐coenzyme A oxidase 1 (ACOX1), as a marker of peroxisomal b‐oxidation, were elevated following HFD feeding (CPT1a: 1.0±0.0 vs 1.5±0.1; ACOX1: 1.0±0.1 vs 1.2±0.1 fold NC, NC Sham vs HFD Sham, p<0.05 for both). Liver denervation did not affect hepatic CPT1a and ACOX1 expression in either diet group (e.g. CPT1a fold NC: NC Denervation 0.9±0.1; HFD Denervation 1.4±0.2, p>0.05 vs respective Sham). In addition, liver denervation did not alter microsomal triglyceride transfer protein, an endoplasmic reticulum protein involved in the processing of VLDL in either diet group. Real time PCR analysis showed robust obesity‐associated elevations in liver fatty acid transport protein 5, suggestive of increased liver FFA uptake, which was reduced by 30% following removal of liver SNA (2.9±1.1 vs 9.7±1.0 fold NC, NC Sham vs HFD Sham, p<0.05; 6.9±0.8 fold NC, HFD Denervation, p=0.2). Liver immunohistochemistry also indicated that the FFA transport protein cluster of differentiation 36 (CD36) was upregulated following HFD feeding and reduced in obese mice following removal of hepatic SNA (Figure). Lastly, HFD feeding resulted in an upregulation of mRNA indicators of triglyceride synthesis, (e.g. acyl‐CoA diacylglycerol acyltransferase 2: 1.0±0.2 vs 2.6±0.2 fold NC, NC Sham vs HFD Sham, p<0.05), which were reduced by liver denervation in HFD animals (1.4±0.2 fold NC, HFD Denervation, p<0.05 vs HFD sham). Collectively, these findings indicate that removal of the liver sympathetic nerves appears to beneficially alter hepatic triglyceride storage during HFD feeding by influencing lipid acquisition pathways, with less of an effect on triglyceride disposal mechanisms. These data also reinforce the idea that targeting hepatic sympathetic overactivity may be a novel therapeutic strategy to treat NAFLD.Support or Funding InformationHL116776This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.