The role of the A2B adenosine receptor in adipogenesis and in obesity-induced type 2 diabetes mellitus
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Obesity is a significant health care problem, affecting more than one third of the United States population and is an important risk factor for Type 2 Diabetes Mellitus (T2D). Adipose tissue expansion results in the recruitment and accumulation of macrophages, which secrete proinflammatory cytokines that impair insulin signaling. Adenosine regulates inflammation by signaling through G-protein coupled receptors (GPCRs), such as the A2b adenosine receptor (A2bAR). Recently a role for adenosine receptors has been described in the differentiation of osteoblasts and adipocytes. This thesis tests the hypothesis that the A2bAR regulates adipose tissue dynamics at the level of preadipocyte differentiation and macrophage inflammation. This thesis showed that activation of the A2bAR inhibited preadipocyte differentiation. A2bAR-induced adipocyte inhibition was dependent on the expression of Krüppel-like factor 4 (KLF4), which is important for stem cell maintenance and renewal. A2bAR knockdown enhanced adipogenesis in vitro and A2bAR knockout (KO) mice had more adipocytes as compared to wild type (WT) mice, suggesting enhanced adipogenesis in the absence of the A2bAR. The translational potential of this work is strengthened by the previous finding of elevated A2bAR expression in adipose tissue of obese individuals as well as our new finding of a close correlation between the expression of A2bAR and KLF4 in adipose tissue of obese individuals. A2bAR KO mice have impaired insulin resistance, in part due to reduced levels of insulin receptor substrate-2 (IRS-2). Proinflammatory cytokines have been shown to reduce IRS-2 levels. Given the role of the A2bAR in regulating inflammation, the contribution of A2bAR signaling in macrophages to insulin resistance was elucidated. Transgenic mice that express A2bAR only in macrophages were generated. Intriguingly, restoration of A2bAR signaling in macrophages ameliorated insulin resistance, glucose tolerance, and fat and liver tissue insulin signaling. As expected, tissue and plasma proinflammatory cytokine levels were reduced to that of WT mice. This suggested that the protective effect of A2bAR signaling on insulin resistance was due in large part to A2bAR control of macrophage cytokine expression. This thesis highlights the importance of A2bAR signaling in adipogenesis and in regulating inflammation in the setting of obesity and T2D.