Mechanisms of glucocorticoid-induced metabolic dysfunction in skeletal muscle - Abstract/Project Summary Glucocorticoids are commonly prescribed as an essential drug regimen for anti-inflammatory modalities; however, a major side effect of such therapies is the development of insulin resistance and risk for hyperglycemia. Key contributors to insulin resistance induced by glucocorticoids are alterations in glucose and lipid metabolism that negatively impact skeletal muscle. Glucocorticoids induce endogenous breakdown of lipid from adipose tissue, releasing fatty acids into the circulation. Excess circulating lipids contribute to greater influx into skeletal muscle, accrual of intramuscular lipids (e.g., ceramides, DAGs, TAGs, etc.), and induce a pathological lipid burden on mitochondria. Excess intramuscular lipids also act as a competitive substrate to carbohydrate metabolism and re-route glucose into hexosamine biosynthesis. The net result of these metabolic changes ultimately impairs insulin signaling. While evidence supporting `lipotoxic' signaling molecules and mitochondrial lipid overload leading to dysfunction are linked to insulin resistance, this concept has not been investigated as part of the metabolic side effects of chronic glucocorticoid use. Our published and preliminary data support the hypothesis that glucocorticoids promote insulin resistance by increasing the lipid burden within skeletal muscle. To address this hypothesis, we propose the following strategies: In Specific Aim 1, we propose to limit lipolysis-derived fatty acids from flooding into skeletal muscle using genetic models, which will provide insights into glucocorticoid-mediated lipolytic events that influence insulin resistance. In complementary studies proposed in Specific Aim 2, we will build on our novel preliminary data using skeletal muscle-targeted deletion of Cpt1b (limiting mitochondrial fatty acid oxidation) to understand how restricting mitochondrial fatty acid oxidation directly protects against glucocorticoid-induced metabolic dysfunction. Studies in Specific Aim 2 also propose that excess cellular lipid uptake and hexosamine production act in a cooperative manner to potentiate insulin resistance. We will use genetically modified mouse models to A) limit cellular lipid uptake in skeletal muscle via deletion of the fatty acid transporter, Cd36, and B) limit hexosamine transfer to proteins in muscle using deletion of O-GlcNAc transferase (OGT). Our novel data also supports the premise that elevated insulin maintains lean mass and metabolic health in skeletal muscle. This will be tested using mice with skeletal muscle- specific deletion of the insulin receptor. Finally, we propose the in vivo use of two novel, chemically synthesized derivatives of hydrocortisone which function as glucocorticoid receptor agonists that retain anti-inflammatory efficacy but are predicted to not induce adverse metabolic outcomes. These highly innovative studies, supported by our preliminary data using both pharmacologic and genetic approaches, will provide unique mechanistic evidence explaining one of the most deleterious side effects of chronic glucocorticoid usage and reveal insights into strategies that can ideally be exploited to improve clinical outcomes of glucocorticoid therapy.
