Diabetes is characterized by loss of pancreatic beta cell mass and over the course of this award we have established thioredoxin-interacting protein (TXNIP) as a critical factor in this process. Recently, we found that TXNIP regulates beta cell microRNA expression and particularly induces beta cell expression of a specific, beta cell-enriched microRNA, miR-204 and thereby inhibits insulin production. Now, we discovered that miR-204 also directly targets the glucagon-like peptide-1 (GLP-1) receptor (GLP1R), revealing a novel link between microRNA and incretin signaling. The GLP1R is essential for incretin-mediated insulin secretion and control of glucose homeostasis as well as for the function of GLP-1-based diabetes therapies that have become widely used in patients with type 2 diabetes. Intriguingly, our preliminary data further demonstrate that miR-204 overexpression downregulates GLP1R in INS-1 beta cells and primary mouse as well as human islets. Moreover, miR-204 as well as TXNIP deletion in vivo in mice promoted islet GLP1R expression and GLP-1-induced insulin secretion and glucose tolerance. In addition, miR-204 knockout (204KO) mice were found to retain a higher beta cell mass in the context of streptozotocin (STZ)-induced diabetes. Together with our extensive previous work on TXNIP and miR-204 and the well documented importance of GLP1R, these findings provide a strong scientific premise for the proposed studies. Our overall hypothesis is that inhibition of TXNIP-induced miR-204 enhances GLP1R expression and function as well as compensatory beta cell mass adaptation. To test this hypothesis, we propose 3 Specific Aims: 1. Elucidate the effects of miR-204 and TXNIP on beta cell GLP1R, using our established miR-204 mimics and inhibitors and TXNIP overexpression system as well as INS-1 cells, human islets and type 2 diabetic human islets. 2. Determine how deletion of miR-204 and TXNIP affect GLP1R expression and function in vivo. By taking advantage of the availability of our novel miR-204 knockout (204KO) mice and our beta cell-specific TXNIP knockout (bTKO) mice we will assess GLP1R expression, responsiveness to GLP1R agonists, glucose-stimulated insulin secretion and glucose homeostasis, under normal conditions and in the context of high fat diet (HFD)-induced insulin resistance and diabetes. 3. Study the mechanisms by which TXNIP-induced miR-204 regulates beta cell mass adaptation. We will investigate any potential differences in beta cell mass, apoptosis, proliferation and/or differentiation resulting from miR-204 deletion in the context of STZ-induced beta cell loss as well as HFD-induced insulin resistance and diabetes. The results of these studies will provide new knowledge in terms of the role TXNIP-induced miR-204 signaling plays in the control of beta cell GLP1R, insulin secretion, glucose homeostasis and beta cell mass adaptation and will shed new light onto the regulation of GLP1R, which currently serves as the target for a large number of diabetes drugs.
The results of the proposed studies will provide a better understanding of how a newly discovered regulatory mechanism controls pancreatic beta cell mass, function and insulin secretion by targeting a receptor that plays an important role in normal physiology as well as diabetes drug therapy. By revealing potential novel approaches to amplify and optimize current diabetes therapies, they will also provide the basis for the development of better treatment regimens for patients with diabetes.
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