The role of hepatocellular eNOS in NAFLD development and hepatic mitochondrial adaptations to exercise

Abstract

Whole body loss of endothelial nitric oxide synthase (eNOS) worsens hepatic mitochondrial function and exacerbates nonalcoholic fatty liver disease/steatohepatitis (NAFLD/NASH) development and progression. However, the precise role of eNOS in hepatocytes in the contribution to NAFLD has not been established. Here, we use gain- and loss- of-function approaches including a hepatocyte-specific eNOS knockout mouse model (eNOShep-/-), and lifestyle interventions (diet and exercise), to investigate the role of hepatocellular eNOS in NAFLD/NASH development and hepatic mitochondrial adaptations to exercise. Ablation of hepatocellular eNOS via genetic and viral knockout exacerbated hepatic steatosis and inflammation, decreased hepatic mitochondrial fatty acid oxidation and respiration, and impaired mitophagy. Conversely, overexpressing hepatocellular eNOS via viral approaches increased hepatocyte respiration, markers of mitophagy, while attenuating NASH progression. Interestingly, these detriments were not rescued by BNIP3 overexpression or nitric oxide (NO) donors in eNOS deficient hepatocytes. In addition, elevated H2O2 emission and hepatic steatosis in eNOShep-/- mice was completely ablated with 10 weeks of voluntary wheel running exercise. Interestingly, eNOShep-/- male mice had a blunted exercise-induced increase in hepatic fatty acid oxidation. eNOShep-/- mice also had impaired markers of energy sensing ability of the cell and attenuated activation of the autophagy initiating factor ULK1. While mitochondrial respiration and markers of mitochondrial content were not increased with exercise, female mice showed markers of mitochondrial biogenesis. Collectively, these data uncover the important and novel role of hepatocellular eNOS in exercise-induced hepatic mitochondrial adaptations, and help to further the understanding behind the mechanistic role of eNOS in NAFLD development.