Date Published: March 22, 2017
Publisher: Public Library of Science
Author(s): Shilpa R. Nagarajan, Amanda E. Brandon, Jessie A. McKenna, Harrison C. Shtein, Thinh Q. Nguyen, Eurwin Suryana, Philip Poronnik, Gregory J. Cooney, Darren N. Saunders, Andrew J. Hoy, Manlio Vinciguerra.
Ubiquitin is a crucial post-translational modification regulating numerous cellular processes, but its role in metabolic disease is not well characterized. In this study, we identified the in vivo ubiquitin-modified proteome in rat liver and determined changes in this ubiquitome under acute insulin stimulation and high-fat and sucrose diet-induced insulin resistance. We identified 1267 ubiquitinated proteins in rat liver across diet and insulin-stimulated conditions, with 882 proteins common to all conditions. KEGG pathway analysis of these proteins identified enrichment of metabolic pathways, TCA cycle, glycolysis/gluconeogenesis, fatty acid metabolism, and carbon metabolism, with similar pathways altered by diet and insulin resistance. Thus, the rat liver ubiquitome is sensitive to diet and insulin stimulation and this is perturbed in insulin resistance.
The liver is exquisitely insulin-sensitive, and plays a critical role in glucose and lipid homeostasis as well as detoxification. Dysregulation of glucose and lipid metabolism in liver is a major factor in the pathogenesis of metabolic diseases including type 2 diabetes and non-alcoholic fatty liver disease (NAFLD). A primary characteristic of these metabolic diseases is the accumulation of excess lipid in the liver, known as fatty liver or hepatic steatosis . This scenario is linked with impaired whole-body and hepatic insulin-stimulated glucose metabolism . High-fat feeding studies with rodents demonstrates that impairment of insulin action in the liver precedes the development of insulin resistance in other glucoregulatory tissues, including skeletal muscle and adipose tissue [3, 4]. Fatty liver reduces tolerance to ischemic injury, impaired regeneration capacity  and increases the risk of developing hepatocellular carcinoma . As such, fatty liver is an initiating factor in the development of a range of pathologies.
Herein, we have described the systematic characterization of the rat liver ubiquitome and report the effects of acute in vivo insulin stimulation and high-fat, high-sucrose diet. Specifically, we observed ubiquitination of proteins involved in key metabolic pathways, in particular gluconeogenesis/glycolysis, oxidative phosphorylation and fatty acid metabolism. Hence, ubiquitination is likely a novel mechanism acting at multiple levels to regulate whole-body euglycemia and lipidemia. Further, widespread changes in the ubiquitin modified proteome may mediate the pathogenesis of fatty acid-associated diseases.