The incretin effect describes the augmentation of insulin secretion by gut factors when nutrients are ingested, and is primarily mediated by glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1). 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). In recent studies we have determined that the incretin effect is stable and reproducible in humans, that there are important extra-incretin effects of endogenous GLP-1, and that correction of hyperglycemia in diabetic subjects specifically restores the incretin effect. There are now available two classes of drugs to treat diabetes based on signaling through the GLP-1 receptor. However, there are still important unanswered questions about the function of the incretin system that are essential to know if these new therapeutic directions are to be optimized. This proposal will take up these issues by addressing the following specific aims: 1) Determine the interaction of GLP-1 and GIP to regulate insulin secretion in nondiabetic subjects. We hypothesize that sensitivity to the incretin will be reproducible within nondiabetic subjects and will vary inversely such that either GIP or GLP-1 will be the dominant incretin in any given individual. 2) Determine the relative sensitivitie of ?-cell and non?-cell actions of GLP-1 in nondiabetic subjects. We hypothesize that individuals who are relatively sensitive to the effects of GLP-1 on ?-cell function will also be sensitive to the effects of GLP-1 on other tissues as well. 3) Determine the relative effects of hyperglycemia on the actions of the incretins to regulate islet hormone secretion in persons with T2DM. We hypothesize that in diabetic subjects with poor glycemic control, suppression of glucagon release will be the predominant action of GLP-1, but with improved glucose control insulinotropic effects of GLP-1 will increase. 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 incretins and a GLP-1r agonist in human research. At the conclusion of this project we will have a much clearer picture of how the incretins regulate ?-cell function, and the nature of the interactions of GIP and GLP-1 to control this process. In addition, the results of the experiments proposed herein will provide a model for the regulation of the multiple GLP-1 responses. We will connect the findings from studies of healthy subjects with parallel measures in T2DM subjects before and after glycemic correction. A critical aspect of this project is a focus on inter-individual variation in the workings of the incretin system. This approach is a critical first step to establish whether there is a physiologic basis on which to predict the existence of responders and non-responders to incretin therapies;such knowledge is imperative to advance the concept of personalized medicine and therapeutics. Overall, the outcomes of this project will provide important new information that will advance the treatment of persons with diabetes.
The control of insulin secretion after eating is strongly influenced by the secretion of two hormones from the intestine called incretins, GIP and GLP-1;the actions of these hormones are blunted in diabetes and this likely contributes to abnormal control of blood glucose. In this project we will determine the interactions of GIP and GLP-1 to control insulin secretion, how sensitivity to these hormones varies among individuals and how diabetes affects these processes. The incretins have already become models for new diabetes treatments and the results of this project will allow this strategy to be optimized.