RING Finger Protein 4 (RNF4) Reduces Nonalcoholic Fatty Liver Disease Accumulation by Promoting the SUMOylation of HIF-2α and Regulating the PPARα Signaling Pathway

RING finger protein 4 (RNF4) acts as a SUMO-targeted ubiquitin ligase, principally regulating protein stability and playing a crucial role in liver injury, inflammatory, and cholestatic diseases. In spite of this, it is unclear how it contributes to nonalcoholic fatty liver disease (NAFLD). The rat model of NAFLD was constructed by feeding a high-fat diet (HFD), and HepG2 cells were treated with 1 mmol/L oleic acid (OA) for 24 h. Real-time quantitative polymerase chain reaction (RT-qPCR) and western blotting were used to measure the expression of associated genes and proteins. Oil red O staining, enzyme-linked immunosorbent assay (ELISA), flow cytometry, and hematoxylin–eosin (HE) staining were used to assess damage to HepG2 cells and rat liver tissues. RNF4 expression is reduced in NAFLD. Overexpression of RNF4 in HepG2 cells reduced triglyceride (TG) and total cholesterol (TC) levels and increased high density lipoprotein cholesterol (HDL-C) levels. In addition, overexpression of RNF4 suppressed lipogenic genes liver X receptor alpha (LXRα), fatty acid synthase (FAS), stearoyl-CoA desaturase-1 (SCD1), and cytochrome P4A11 (Cyp4a11), inflammatory cytokines tumor necrosis factor-alpha (TNF-α), interleukin-1beta (IL-1β), and interleukin-6 (IL-6), and cell apoptosis; it also inhibited lipid accumulation in vivo and improved liver tissue pathology, thereby mitigating NAFLD progression. Mechanistically, RNF4 promotes SUMOylation and ubiquitin-mediated degradation of hypoxia inducible factor-2 alpha (HIF-2α), thereby enhancing peroxisome proliferator-activated receptor alpha (PPARα) expression, reducing lipid accumulation, inflammation, and cell apoptosis, ultimately alleviating NAFLD development. Our research indicates that RNF4 may be a novel therapeutic target for NAFLD.