The overall goal of this grant is to understand the molecular mechanisms for insulin resistance in Type 2 diabetes and, in particular, how changes in adipose biology contribute to diabetes risk, with the long-term goal of finding new therapeutic targets for T2D. Since our discovery in 2005 that Retinol Binding Protein 4 (RBP4) is elevated in serum and white adipose tissue in many insulin-resistant states in humans, RBP4 elevation causes insulin resistance, and lowering serum RBP4 levels improves insulin sensitivity, many studies worldwide have extended these observations to show that elevated RBP4 is a biomarker and a potential cause of insulin resistance and metabolic syndrome. A breakthrough in the last grant cycle was our discovery that RBP4 causes insulin resistance by inducing a proinflammatory state in adipose tissue mediated by Toll Like Receptor 4. We also showed that overexpression of RBP4 selectively in adipose tissue has detrimental systemic metabolic effects and causes hepatic steatosis. In the next grant cycle, we will investigate novel mechanisms by which RBP4 induces adipose tissue inflammation and systemic insulin resistance including the role of lipolysis. Using ?omics? approaches in adipose tissue from the same adipose-Glut4 overexpressing and knockout mice in which we found RBP4, we discovered a novel class of endogenous lipids with anti-diabetic and anti-inflammatory effects - Branched Fatty Acid esters of Hydroxy Fatty Acids (FAHFAs). Since a subclass of FAHFAs, Palmitic Acid esters of Hydroxy Stearic Acids (PAHSAs), are low in insulin-resistant people and in RBP4-overexpressing mice and our new data show that PAHSAs can block the actions of RBP4 at the cellular level and improve insulin sensitivity in RBP4-overexpressing mice, we will investigate a possible mechanistic relationship between RBP4 and PAHSAs. We will also build on our studies to determine the molecular mechanisms for increased RBP4 retention in serum in insulin-resistant states by investigating the role of post- translational modifications of transthyretin, the serum binding partner for RBP4. Significance: These studies will provide novel insights into the mechanisms by which adipose tissue regulates systemic insulin sensitivity and glucose homeostasis. Discovering the interactions among these molecules will lead to a more comprehensive understanding of the cellular and tissue networks by which Glut4 downregulation in adipocytes increases Type 2 diabetes risk. Since RBP4 is elevated in many insulin-resistant people, understanding the mechanisms underlying RBP4-induced insulin resistance and retention of RBP4 in serum may lead to new strategies to lower RBP4 levels to prevent and treat Type 2 diabetes.
The proposed research is relevant to public health because the discovery of the cellular and physiological mechanisms by which Retinol Binding Protein 4 activates the immune system in adipose tissue and causes systemic insulin resistance, and the mechanisms by which it is increased in insulin-resistant states, could help identify new therapeutic approaches for type 2 diabetes and the metabolic syndrome. The proposed research is relevant to the NIH's mission pertaining to developing fundamental knowledge that will help reduce the health burdens of obesity, diabetes, and maladaptive behavior, which are prominent ongoing public health and safety concerns.
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