Hrd1-mediated ACLY ubiquitination alleviate NAFLD in db/db mice

The functions of Acly in regulating nonalcoholic fatty liver disease (NAFLD) have been identified; however, the dynamic control of Acly expression under the pathological state of metabolic disorders has not been fully elucidated. Previous studies reported an ubiquitin-proteasome-mediated degradation of Acly, but the mechanism is still largely unknown.

Co-IP-based mass spectrum (MS/MS) assays were performed in HepG2 and Hepa1-6 hepatocytes and mouse liver tissue. The protein-protein interaction and ubiquitin modification of Hrd1 on Acly were confirmed by co-IP based immuno-blotting. Acetyl-CoA levels and lipogenesis rates were determined. The roles of Hrd1 on NAFLD and insulin resistance were tested by adenovirus-mediated overexpression in db/db mice or in separated primary hepatocytes.

Hrd1, a subunit of the endoplasmic reticulum-associated degradation (ERAD) complex, interacted with and ubiquitinated Acly, thereby reducing its protein level. Hrd1 suppressed the acetyl-CoA level and inhibited lipogenesis through an Acly-dependent pathway. The expression of hepatic Hrd1 was negatively associated with NAFLD, whereas overexpression of Hrd1 ameliorated hepatic steatosis and enhanced insulin sensitivity, both in db/db mice and in separated mouse primary hepatocytes.

Our results suggest that Acly, a master enzyme that regulates lipogenesis, is degraded by Hrd1 through ubiquitin modification. The activation of Hrd1 in hepatocytes might therefore represent a strategic approach for NAFLD therapy.