PDIA4: A Novel ER Stress Protein Regulating Hepatic Lipogenesis in Non-Alcoholic Fatty Liver Disease

Abstract Hepatic steatosis is a histological characteristic of non-alcoholic fatty liver disease (NAFLD). Hepatocytes are enriched with the endoplasmic reticulum (ER), essential for lipid metabolism. ER stress modulates lipogenesis and hepatic steatosis in hepatocytes, indicating an involvement of unfolded protein response (UPR) components in the NAFLD pathogenesis. Our previous study demonstrated that serum levels of protein disulfide isomerase A4 (PDIA4) correlate with the components of NAFLD. Therefore, this study aimed to evaluate PDIA4 role in NAFLD development. We treated HepG2 cells with palmitate to generate an obesity-associated NAFLD cell model and a high-fat diet-induced obesity-associated NAFLD mouse model. We demonstrated that human serum PDIA4 concentration is an independent risk factor for the fatty liver index and is upregulated in the liver tissues of DIO mice and human NAFLD tissue arrays. ER stress and PDIA4 expression increase in palmitate-exposed HepG2 cells. Moreover, palmitate triggers lipid accumulation and increases intracellular triglyceride (TG) levels in HepG2 cells in a dose-dependent manner. PDIA4 silencing suppresses palmitate-induced hepatic steatosis in HepG2 cells. PDIA4 mediates hepatic steatosis through direct interaction and proteolytic activation of sterol regulatory element-binding protein 1c (SREBP1c), further inducing the expression of stearoyl-CoA desaturase-1 (SCD1) and hepatic lipogenesis in palmitate-treated HepG2 cells. Additionally, ATF6 acts as an upstream modulator of palmitate-induced PDIA4 upregulation, and its depletion suppresses the stimulatory effect of palmitate on PDIA4 expression and steatosis in HepG2 cells. Our findings indicate a pathological role of PDIA4 in the development of obesity-associated NAFLD.