OBJECTIVE: Obesity-related chronic inflammation plays an important role in the development of Metabolic Associated Fatty Liver Disease (MAFLD). Although the contribution of the pro-inflammatory NF-κB signaling pathway to the progression from simple steatosis to non-alcoholic steatohepatitis (NASH) is well-established, its role as an initiator of hepatic steatosis and the underlying mechanism remains unclear. Here, we investigated the hypothesis that the hepatocytic NF-κB signaling pathway acts as a metabolic regulator, thereby promoting hepatic steatosis development.
METHODS: A mouse model expressing a constitutively active form of IKKβ in hepatocytes (Hep-IKKβca) was used to activate hepatocyte NF-κB. In addition, IKKβca was also expressed in hepatocyte A20-deficient mice (IKKβca;A20LKO). A20 is a NF-κB-target gene and inhibits the activation of the NF-κB signaling pathway upstream of IKKβ. These mouse models were fed a sucrose-rich diet for 8 weeks. Hepatic lipid levels were measured and using [1-13C]-acetate de novo lipogenesis and cholesterol synthesis rate were determined. Gene expression analyses and immunoblotting were used to study the lipogenesis and cholesterol synthesis pathways.
RESULTS: Hepatocytic NF-κB activation by expressing IKKβca in hepatocytes resulted in hepatic steatosis without inflammation. Ablation of hepatocyte A20 in Hep-IKKβca mice (IKKβca;A20LKOmice) exacerbated hepatic steatosis, characterized by macrovesicular accumulation of triglycerides and cholesterol, and increased plasma cholesterol levels. De novo lipogenesis (DNL) and cholesterol synthesis were both elevated in IKKβca;A20LKO mice. Phosphorylation of AMP-activated kinase (AMPK) - a suppressor in lipogenesis and cholesterol synthesis - was decreased in IKKβca;A20LKO mice. This was paralleled by elevated protein levels of hydroxymethylglutaryl-CoA synthase (HMGCS1) and reduced phosphorylation of HMG-CoA reductase (HMGCR) both key enzymes in the cholesterol synthesis pathway. Whereas inflammation was not observed in young IKKβca;A20LKO mice sustained hepatic NF-κB activation resulted in liver inflammation, together with elevated hepatic and plasma cholesterol levels in middle-aged mice.
CONCLUSIONS: The hepatocytic IKK:NF-κB axis is a metabolic regulator by controlling DNL and cholesterol synthesis, independent of its central role in inflammation. The IKK:NF-κB axis controls the phosphorylation levels of AMPK and HMGCR and the protein levels of HMGCS1. Chronic IKK-mediated NF-κB activation may contribute to the initiation of hepatic steatosis and cardiovascular disease risk in MAFLD patients.