Date of Award

Spring 2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Cellular and Molecular Physiology

First Advisor

Shulman, Gerald

Abstract

Obesity and Type 2 Diabetes mellitus (T2DM) are among the most serious global health problems as they are increasing in prevalence and related to many chronic diseases, including cardiovascular and cerebrovascular diseases, cancer and other metabolic disorders. Obesity increases the risk for T2DM through the development of low-grade inflammation and insulin resistance. Specifically, studies have shown that enhanced inflammation in adipose tissue is an essential player in the progression of insulin resistance and T2DM in obese individuals. However, how immune cells sense nutritional status and contribute to whole-body metabolism are largely unknown. In addition, most of the currently available therapies do not address the root cause of T2DM: insulin resistance. Pharmacological agents that improve diabetes have limited success due to side effects and decline in efficacy as most patients develop resistance over time. As such, understanding the pathogenesis of T2DM and finding new interventions to ameliorate insulin resistance are of great interest. In this doctoral dissertation, I describe work that elucidates the nutritional regulation of macrophage function and its contribution to whole-body metabolism, as well as the mechanisms by which a new potential treatment, adiponectin, ameliorates insulin resistance. Protein O-GlcNAcylation is thought to be a metabolic sensor that modulates cell signaling. I showed that overnutrition stimulated nutrient-sensing O-linked β-N-acetylglucosamine (O-GlcNAc) signaling in macrophages and O-GlcNAc signaling was down-regulated during macrophage pro-inflammatory polarization. Further, mice with O-GlcNAc transferase (Ogt) deletion in macrophages and other myeloid cells displayed enhanced macrophage pro-inflammatory activation in adipose tissue and lipolysis, increased ectopic lipid accumulation in peripheral tissues, and exacerbated tissue-specific and whole-body insulin resistance in diet-induced obese mice. O-GlcNAc signaling inhibited macrophage pro-inflammatory polarization by catalyzing ribosomal protein S6 kinase beta-1 (S6K1) serine 489 O-GlcNAcylation and suppressing S6K1 phosphorylation. These studies uncovered O-GlcNAc signaling as a novel homeostatic regulator at the interface of inflammation and metabolism and suggested that O-GlcNAc signaling may serve as a therapeutic target for obesity, diabetes, and other immune-related diseases. Finally, I examined the mechanisms for the anti-diabetic effect of adiponectin. Adiponectin has emerged as a promising insulin-sensitizing adipokine and a potential therapy to treat T2DM; however, the mechanisms by which adiponectin administration improves insulin sensitivity were unclear. To address this question, I examined the effects of a 2-week continuous subcutaneous infusion of globular adiponectin (gAcrp30) or saline on glucose and lipid metabolism in a high-fat diet (HFD) fed mouse model. Whole-body and tissue-specific insulin action was assessed by a hyperinsulinemic-euglycemic clamp (HEC). gAcrp30-treated mice displayed reduced fasting plasma glucose and insulin concentrations and increased glucose infusion rate during the HEC, reflecting increased whole-body insulin sensitivity. Increased insulin sensitivity could be attributed to reduced endogenous glucose production and increased glucose uptake in muscle and adipose tissues. We found that these liver and muscle sensitivity improvements were associated with reductions in the plasma membrane-associated diacylglycerol (DAG) content, and contrary to prior studies, were independent of reductions in total ceramide content. These effects in turn led to decreased protein kinase Cε (PKCε) activation in liver, decreased PKCε/PKCθ activity in muscle, and improved insulin signaling in these tissues. I further demonstrated that globular adiponectin (gAcrp30) and full-length adiponectin (Acrp30) reverse insulin resistance in HFD-fed mice through reductions in ectopic lipid in liver and muscle likely by stimulation of lipoprotein lipase (LPL) activity in white adipose tissue and increased epithelial nitric oxide synthase (eNOS)/ 5' AMP-activated protein kinase (AMPK) activation and fat oxidation in muscle. Taken together, the work presented in the dissertation provides novel mechanistic insight into the regulation and function of O-GlcNAc signaling in the immunometabolism and the mechanisms by which adiponectin reverses HFD-induced liver and muscle insulin resistance in mice. As such, adiponectin and O-GlcNAc signaling activators, such as glutamine and glucosamine, could serve as viable treatment options for T2DM, insulin resistance and other obesity-associated morbidities.

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