Type II diabetes mellitus (T2D) is a major risk factor for multiple diseases and a significant contributor to morbidity and mortality in the United States T2D is defined by an inability of the body to respond to changing blood glucose levels due to reduced responsiveness of tissues to the glucose regulatory hormone, insulin, and/or an insufficient secretion of insulin. Obesity, a significant risk factor for T2D, is associated with chronic inflammation and it has become clear that inflammation can contribute to insulin resistance. The macrophage is a key mediator of obesity-induced inflammation. However, the mechanism connecting obesity, inflammation and T2D is not clearly established. One interesting signaling molecule that may play a role in this triad is adenosine, a metabolite that is elevated following cellular stress and inflammation. Adenosine signaling through the A2b adenosine receptor (A2bAR) has been shown to dampen the release of inflammatory cytokines. Furthermore, administration of a high fat, high cholesterol diet (HFD) vastly upregulates the expression of the A2b adenosine receptor (A2bAR), and A2bAR KO mice subjected to HFD have elevated plasma cytokine levels compared to wild type (WT) mice. Intriguingly, A2bAR KO mice also have delayed glucose clearance and augmented insulin levels following HFD. Adipose tissue and liver insulin signaling is reduced in A2bAR KO mice relative to WT mice. One important mediator of insulin signaling, the insulin receptor substrate 2 (IRS-2), is reduced by the action of inflammatory cytokines. In search for a mechanism of impaired glucose handling in A2bAR KO mice, the level of IRS-2 was shown to be decreased in the liver and adipose tissue of A2bAR KO mice. Importantly, pharmacological activation of A2bAR in WT mice (with an A2bAR ligand) fed a HFD restores IRS-2 levels and ameliorates T2D. Importantly, A2bAR expression is elevated in adipose tissue from obese human subjects as compared to lean subjects and A2bAR expression correlates strongly with IRS-2 expression. Considering the control of inflammatory cytokines by A2bAR, and the regulation of IRS-2 and insulin signaling by these cytokines, we hypothesize a link between macrophage A2bAR, adipose tissue inflammation, and insulin signaling. With access to genetically modified mice and to human adipose tissue from diabetic and non-diabetic obese individuals, this proposal will examine the novel contentions that metabolically healthy adipose tissue from obese humans have a strong expression of A2bAR and IRS-2 as compared to metabolically unhealthy obese humans (Aim 1), and that fat and/or macrophage A2bAR signaling contributes to regulation of IRS-2 and to glucose homeostasis (Aim 2). Proposed studies might open new receptor-based therapeutic approaches, facilitated by the collaboration and support of my co- mentors on this project, Dr. Ravid, a Biochemist and expert in vascular and adenosine biology, and Dr. Gokce, a Clinician Scientist with access to human samples and expertise in studying human adipose tissue.
Type 2 diabetes mellitus (T2D) affects more than 20 million people in the United States and leads to significant long-term complications. Contributors to the development of T2D include chronic inflammation and obesity. The proposed research addresses the newly identified ability of the A2b adenosine receptor, a membrane protein activated by the metabolite adenosine, to modulate adipose tissue inflammation and T2D.
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