Regulation of insulin Secretion by the GLP-1 Receptor

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 DESCRIPTION (provided by applicant): The incretin glucagon-like peptide 1 (GLP-1) has been a major focus of diabetes research and drug development for the past 20 years. The incretin system is essential for normal glucose homeostasis and recent evidence indicates that there is specific dysfunction of this system in type 2 diabetes (T2DM). While initially proposed to act as a hormone, secreted into the circulation from enteroendocrine cells, recent work has raised questions as to whether this is an important mechanism of endogenous GLP-1 action. In fact, GLP-1 circulates at low levels and is rapidly inactivated by specific enzymatic metabolism. Emerging information suggests that blood borne GLP-1 may not account for many of its actions. We have recently observed an important effect of GLP-1 receptor activity in the fasting state when plasma levels are very low and unchanging. These studies, in mice and humans, suggest a role for -cell GLP-1 to regulate insulin secretion as a paracrine factor. This is currently an underdeveloped area of research, but has the potential to dramatically change the understanding of the incretin effect and physiology of GLP-1. Given that there is already a solid base of therapeutics based on GLP-1, new mechanistic understanding could be rapidly translated into clinical application. This proposal will take up the issue of fasting/paracrine GLP-1 action by addressing the following specific aims: 1. Determine the roles of fasting GLP-1 concentrations and alpha-cell secretion on insulin release; 2. Determine the role of basal GLP-1 action on the -cell response to insulin resistance; 3. Determine the role of basal GLP-1 action on fasting glucose regulation in lean, obese, prediabetic, and diabetic subjects. Each of these aims will be conducted in selected groups of human subjects. The studies proposed herein will use methods of human investigation that we have developed or refined in recent years and take advantage of INDs issued to our group by the FDA to use synthetic GLP-1 and a GLP-1r agonist, exendin-(9-39), in human research. At the conclusion of this project we will have a much clearer picture of how GLP-1 regulates β-cell function, and the role of fasting GLP-1r signaling and islet paracrine action in this process. A critical aspect of this project is the examination of -cell secretion of GLP-1 as central in the regulation of fasting insulin secretion. In addition, the results of the experiments proposed herei will examine a novel role for GLP-1 in the compensation for abnormal glucose metabolism. We will connect the findings from studies of healthy subjects made insulin resistant with parallel measures in diabetic and prediabetic subjects. These related hypotheses are driven by the goal of optimizing the GLP-1 system as a means of improving metabolic control. Overall, the outcomes of this project will be beneficial to the broad swath of diabetic patients currently needing better treatment, both inside and outside the VA system.