Herpesvirus-associated Ubiquitin-specific Protease (HAUSP) Modulates Peroxisome Proliferator-activated Receptor γ (PPARγ) Stability through Its Deubiquitinating Activity

Background: The increase of PPAR stability could contribute to lower blood glucose levels. Results: PPAR stability is increased by the deubiquitinating activity of HAUSP. Conclusion: HAUSP overexpression could decrease blood glucose and triglyceride levels at least in part by deubiquitinating and stabilizing PPAR in the liver. Significance: Identification of a novel enzyme (HAUSP) that deubiquitinates and stabilizes PPAR and its potential role in liver glucose and lipid metabolism are significant. The peroxisome proliferator-activated receptor (PPAR) is a central regulator of adipogenesis and modulates glucose and lipid metabolism. In this study, herpesvirus-associated ubiquitin-specific protease (HAUSP) was isolated as a binding partner of PPAR. Both endogenous and exogenous PPAR associated with HAUSP in co-immunoprecipitation analysis. HAUSP, but not the catalytically inactive HAUSP C223S mutant, increased the stability of both endogenous and exogenous PPAR through its deubiquitinating activity. Site-directed mutagenesis experiments showed that the Lys(462) residue of PPAR is critical for ubiquitination. HBX 41,108, a specific inhibitor of HAUSP, abolished the increase in PPAR stability induced by HAUSP. In addition, knockdown of endogenous HAUSP using siRNA decreased PPAR protein levels. HAUSP enhanced the transcriptional activity of both exogenous and endogenous PPAR in luciferase activity assays. Quantitative RT-PCR analysis showed that HAUSP increased the transcript levels of PPAR target genes in HepG2 cells, resulting in the enhanced uptake of glucose and fatty acids, and vice versa, upon siRNA knockdown of HAUSP. In vivo analysis using adenoviruses confirmed that HAUSP, but not the HAUSP C223S mutant, decreased blood glucose and triglyceride levels, which are associated with the increased expression of endogenous PPAR and lipid accumulation in the liver. Our results demonstrate that the stability and activity of PPAR are modulated by the deubiquitinating activity of HAUSP, which may be a target for the development of anti-diabetic drugs.