Ubiquitination of cytoplasmic HMGB1 by RNF186 regulates hepatic lipophagy in non-alcoholic fatty liver disease

Background: Lipophagy is a vital biological process that maintains the balance of intracellular lipid metabolism in nonalcoholic fatty liver disease (NAFLD). However, the precise regulatory mechanism of RNF186 in hepatic lipophagy is still unclear. This study investigates the roles and mechanisms of RNF186 in the regulation of lipophagy during the development of NAFLD. Methods: In this study, we employed RNF186 knockout mice as well as human liver cells and mouse primary hepatocytes (MPHs) to investigate the role and mechanisms of RNF186 in lipophagy during the progression of NAFLD. Additionally, liver specimens from individuals with NAFLD were examined to assess the expression of RNF186 and its associated factors. Results: Here, we provide evidence that depletion of RNF186 enhances lipophagy in hepatocytes of a NAFLD model. Mechanistically, RNF186 acts as an E3 ubiquitin ligase that targets cytoplasmic HMGB1 for lysine 48 (K48)- and K63-linked ubiquitination, leading to its subsequent proteasomal degradation. Importantly, the translocation of HMGB1 from the nucleus to the cytoplasm is responsible for inducing lipophagy in NAFLD samples. Knockdown of HMGB1 significantly reduces the activation of lipophagy and mediates the decrease in lipid accumulation caused by RNF186 depletion in hepatocytes. Furthermore, we find that maintaining the nuclear HMGB1 level and inhibiting its nuclear-cytoplasmic shuttling are critical for the proper function of RNF186 in NAFLD. Additionally, the expression of RNF186 and HMGB1 in human NAFLD samples, along with factors related to lipophagy, suggest that RNF186 may play a similar role in the pathogenesis of human fatty liver. Conclusion: RNF186 deficiency accelerates hepatic lipophagy in NAFLD through the inhibition of ubiquitination and degradation of cytoplasmic HMGB1. Consequently, targeting the RNF186-HMGB1 axis may offer a promising strategy for the prevention and treatment of NAFLD.