The E3 Ubiquitin Ligase Ring Finger Protein 5 Ameliorates NASH Through Ubiquitin‐Mediated Degradation of 3‐Hydroxy‐3‐Methylglutaryl CoA Reductase Degradation Protein 1

Background and Aims NAFLD is the most prevalent chronic liver disease worldwide, but no effective pharmacological therapeutics are available for clinical use. NASH is the more severe stage of NAFLD. During this progress, dysregulation of endoplasmic reticulum (ER)–related pathways and proteins is one of the predominant hallmarks. We aimed to reveal the role of ring finger protein 5 (RNF5), an ER‐localized E3 ubiquitin‐protein ligase, in NASH and to explore its underlying mechanism. Approach and Results We first inspected the expression level of RNF5 and found that it was markedly decreased in livers with NASH in multiple species including humans. We then introduced adenoviruses for Rnf5 overexpression or knockdown into primary mouse hepatocytes and found that palmitic acid/oleic acid (PAOA)–induced lipid accumulation and inflammation in hepatocytes were markedly attenuated by Rnf5 overexpression but exacerbated by Rnf5 gene silencing. Hepatocyte‐specific Rnf5 knockout significantly exacerbated hepatic steatosis, inflammatory response, and fibrosis in mice challenged with diet‐induced NASH. Mechanistically, we identified 3‐hydroxy‐3‐methylglutaryl CoA reductase degradation protein 1 (HRD1) as a binding partner of RNF5 by systematic interactomics analysis. RNF5 directly bound to HRD1 and promoted its lysine 48 (K48)–linked and K33‐linked ubiquitination and subsequent proteasomal degradation. Furthermore, Hrd1 overexpression significantly exacerbated PAOA‐induced lipid accumulation and inflammation, and short hairpin RNA–mediated Hrd1 knockdown exerted the opposite effects. Notably, Hrd1 knockdown significantly diminished PAOA‐induced lipid deposition, and up‐regulation of related genes resulted from Rnf5 ablation in hepatocytes. Conclusions These data indicate that RNF5 inhibits NASH progression by targeting HRD1 in the ubiquitin‐mediated proteasomal pathway. Targeting the RNF5–HRD1 axis may provide insights into the pathogenesis of NASH and pave the way for developing strategies for NASH prevention and treatment.