Abstract

Ubiquitin-proteasome dysfunction contributes to obesity-related metabolic disorders such as diabetes and fatty liver disease. However, the regulation of ubiquitin-proteasome activity by insulin remains to be elucidated. Here, we show that prolonged insulin stimulation reduces the ubiquitination of H4IIEC3 hepatocytes. Looking for a pathway by which insulin inhibits ubiquitination, we found that hepatic expression of ubiquitin-specific protease 14 (USP14) was upregulated in the liver of patients with insulin resistance. Indeed, the USP14-specific inhibitor IU1 canceled the insulin-mediated reduction of ubiquitinated proteins. Furthermore, insulin-induced endoplasmic reticulum (ER) stress, which was canceled by IU1, suggests that USP14 activity is involved in insulin-induced ER stress. Mixed insulin and IU1 stimulation for 2 h upregulated the nuclear translocation of the lipogenic transcription factor, sterol regulatory element binding protein-1c (SREBP-1c), increased the expression of the lipogenic gene, fatty acid synthase (Fasn), and repressed the gluconeogenic genes. Chronic administration of insulin to hepatocytes decreased ubiquitin protein through USP14 activation but increased proteasome activity independently of USP14 activation. Presumably, ubiquitination and proteasome activity can occur independently of each other. In summary, insulin inhibits ubiquitination by activating her USP14 in hepatocytes. Furthermore, activation of USP14 by long-term insulin stimulation inhibits mature SREBP-1c while upregulating the expression of genes involved in endoplasmic reticulum stress and gluconeogenesis. Understanding the mechanisms underlying the role of USP14 activation may lead to the development of therapeutics for obesity-related metabolic disorders such as diabetes and fatty liver disease.

Significance Statement This study shows that long-term insulin stimulation inhibits hepatocyte ubiquitination through activation of USP14, independent of proteasome activity. USP14 also increased the nuclear translocation of the lipogenic transcription factor SREBP-1c and the expression of genes involved in gluconeogenesis. Since USP14 is also elevated in the liver of insulin resistant patients, understanding the role of USP14 activation will be useful in developing treatments for metabolic disorders such as diabetes and fatty liver.