Cathepsin G Promotes Hepatic Lipid Deposition by Regulating Key Genes Related to Lipid Metabolism.

Cathepsins play critical roles in various physiopathological processes, with several reported to be associated with nonalcoholic fatty liver disease (NAFLD). Herein, we investigated the expression patterns of the cathepsin family in human and mouse livers, cultured hepatocytes, and their roles in NAFLD. Public datasets of NAFLD patients and controls were analyzed to examine hepatic cathepsin expression in human livers. RT-qPCR assessed these genes in mouse livers, HepG2, Hepa1-6, and mouse primary cells. Despite different expression patterns, CTSA, CTSB, CTSD, CTSH, and CTSL were consistently highly expressed across all samples. Notably, steatosis patients and diabetic mice exhibited significantly increased hepatic expression of four cathepsins. Among these, we first observed elevated CTSG and CTSW, with CTSG showing the most pronounced increase. Moreover, hepatic CTSG was increased and positively correlated with disease severity in NASH patients. CTSG was also upregulated in HepG2 cells treated with high glucose or free fatty acids. In vitro, CTSG overexpression promoted, while its knockdown reduced lipid accumulation. In vivo, hepatic CTSG overexpression significantly induced lipid deposition, impaired glucose tolerance, and elevated HOMA-IR. Mechanistically, CTSG upregulated key lipid synthesis genes (ACC, SCD1) and downregulated those involved in lipid oxidation (PPARα, Lcad) and secretion (MTTP) by suppressing Akt. Furthermore, Akt activation alleviated lipid deposition induced by CTSG overexpression, while Akt inhibition abolished the beneficial effect of CTSG knockdown. This study is the first to reveal the expression patterns of the cathepsin family in human and mouse livers, and identifies that hepatic CTSG is elevated in NAFLD and can promote lipid deposition, supporting CTSG as a novel potential therapeutic target for NAFLD.