Complexity in Hepatic Insulin Resistance – Unraveling the Role of Ubiquitin-Specific Protease 14 in Protein Homeostasis of Metabolic Transcription Factors

Abstract

Hepatic glucagon and insulin action are central to the systemic regulation of glucose and lipid metabolism. Fasting hepatocytes undergo glycogenolysis and gluconeogenesis in response to a high glucagon-to-insulin ratio. Postprandially, hepatocytes halt glucose production and store excess nutrients as glycogen and lipids in response to a low glucagon-to-insulin ratio (Santoleri and Titchenell, 2019). During metabolic disease progression, pancreatic beta cells enhance insulin secretion, leading to hyperinsulinemia (Hudish et al., 2019). This chronic insulin stimulus reduces expression of the insulin receptor at the hepatocyte cell surface (Liu et al., 2022), where blunted phosphorylation of transcription factor forkhead box protein O1 (FOXO1) leads to continued nuclear access, thereby enabling glucose production in the postprandial state (Brown and Goldstein, 2008). Still, hepatocytes respond to insulin signaling in this state; they continue to accelerate lipid synthesis rates via the continued phosphorylation and nuclear localization of lipogenic transcription factor sterol regulatory element-binding protein 1C (SREBP-1C), rendering selective insulin resistance (Brown and Goldstein, 2008). As such, hepatic lipid accumulation often accompanies poor glucose regulation in metabolic disease.In addition to promoting the nuclear translocation of SREBP-1C, insulin signaling prevents proteasomal degradation of this lipogenic transcription factor (Botolin et al., 2006). Ubiquitin-specific protease 14 (USP14) is key to several canonical signaling pathways, including insulin signaling (Wang et al., 2022). Upon insulin stimulation, protein kinase B (Akt) phosphorylates USP14 at Ser432, which activates its C-terminal deubiquitinating activity, preventing proteasomal degradation of protein substrates (Xu et al., 2015). The N-terminal ubiquitin-like domain of USP14, however, activates proteasomal activity when bound to ubiquitinated substrates (Wang et al., 2022). This dual functionality of USP14 both protects protein substrates from degradation and promotes protein degradation (Wang et al., 2022). Dysregulation of USP14 activation and expression is linked to cancer, neurodegenerative diseases, immune responses, and viral infection (Wang et al., 2022). Growing evidence for the role of USP14 in metabolic disease includes the identification of fatty acid synthase (FASN) as a bona fide substrate of USP14, where hepatic overexpression and short hairpin RNA (shRNA) silencing of USP14 accelerates hepatic triglyceride accumulation and improves hepatic steatosis, respectively (Liu et al., 2018). Therefore, studies unraveling USP14's crystal structure and the pursuit of catalytic inhibitors are of therapeutic interest, including 1-[1-(4-fluorophenyl)-2,5-dimethyl-1H-pyrrol-3-yl]-2-(pyrrolidin-1-yl)ethan-1-one (IU1), which allosterically inhibits USP14 from binding to its ubiquitin substrate (Wang et al., 2018). Interestingly, in mouse models of obesity and insulin resistance (ob/ob and db/db), hepatic proteasome activity is reduced by ...