The Deubiquitinase USP28 Maintains the Expression of PPARγ and Its Inactivation Protects Mice from Diet-Induced NASH and Hepatocarcinoma

Nonalcoholic steatohepatitis (NASH), a progressive form of nonalcoholic fatty liver disease, is one of the leading causes of liver disease worldwide and may further progress to cirrhosis and cancer. However, the pathogenesis of NASH remains unclear, and there is currently only one FDA-approved drug for its treatment; therefore, new therapeutic approaches are urgently needed. Under high-fat diet or obese conditions, PPARγ is activated in the liver to promote lipid storage as droplets, inducing the progression of NASH. We showed that the expression level of USP28 is elevated in the livers of NAFLD/NASH patients. Through dietary induction, including a methionine-choline deficient (MCD) diet and a Western diet (WD) combined with intraperitoneal injection of carbon tetrachloride (CCl4), we established two severe mouse models of NASH and revealed the role of the deubiquitinase USP28. Mechanistically, the hepatic deubiquitinase USP28 directly binds to PPARγ and prevents its own ubiquitination and degradation, thereby regulating its downstream signaling pathways. In the absence of Usp28 or if the DUB is inhibited, PPARγ is downregulated, and the PPAR signaling pathway is inhibited, allowing the cells to mount a defensive response to excessive fat. Both genetic and pharmacological inactivation of Usp28 significantly attenuated the NASH phenotype induced by the MCD diet or WD-CCl4 regimen, as well as WD-CCl4-induced hepatocellular carcinoma in mice.