Potential Roles of Angiotensin II, Glucagon Like Peptide-1 and Vitamin D Systems in Pancreatic Islet Function

Pancreatic islets perform critical biological activities by means of synthesizing and releasing islet peptide hormones, notably insulin that controls our glucose homeostasis. The present study is divided into three main parts: (1) the potential synergism of glucagon-like peptide 1 (GLP-1) and renin-angiotensin system (RAS) in islet function; (2) the modulatory effects of vitamin D on islet RAS expression and function; (3) The altered islet RAS and islet function under a hypovitaminosis D condition.;In the first part of our study, we examined the combined effect of blocking islet AT1 receptor (AT1 receptor blocker: valsartan) and enhancing GLP-1 actions (DPP IV inhibitor: LAF237) on islet function and glycemic control in a mouse model with type 2 diabetes, db/db mice. We compared the islet function in db/db mice with either valsartan or LAF237 mono treatment or combined treatment. Consistently, all these treatments improved glucose homeostasis in db/db mice while combined treatment resulted in a significant increase in islet beta-cell area by decreasing cell apoptosis and increasing proliferation, together with marked decreases of islet oxidative stress and fibrosis. In addition, a short-term effect on stimulating insulin secretion was also observed in isolated islets with combined treatment. These results indicate that the combination treatments with AT1 receptor blocker and DPP IV inhibitor has beneficial additive effects on islet structure and function in type 2 diabetes, compared with their monotherapeutic treatments.;It is reported that vitamin D is a hormone with anti-diabetic and anti-hypertension effects in human while inappropriate RAS activity has been known to reduce islet function and glucose tolerance. The direct suppressive effect of vitamin D on renal renin activity indicates vitamin D may acts as a regulator in RAS activity thus modulate islet physiology. In the second part of our study, it was aimed to study whether vitamin D vitamin D down-regulation of abnormal islet RAS activity improves beta-cell function using an isolated pancreatic islet model. VDR was localized in islet beta-cell nuclei and cytoplasm, mediated responses to active form of vitamin D calcitriol in a dose-dependent manner. This islet local vitamin D system may have its own feedback system as a marked increase of CYP24 transcription was triggered by calcitriol stimulation. In isolated islets exposed to prolonged high glucose environment, abnormal expressed islet RAS components could be reversed or protected by calcitriol at a specific concentration. However, the inhibition effect of calcitriol on islet RAS were not observed at physiological glucose concentrations. In addition, calcitriol increased islet proinsulin synthesis and insulin secretion with hyperglycemia. These results indicated that calcitriol modulate or protect the abnormal isolated islet RAS component expression against hyperglycemia and improve islet function via increasing insulin synthesis and secretion, which might provide an alternative mechanism by which vitamin D availability enhances islet function in hyperglycemia or diabetes.;The circulating vitamin D level is inversely related to blood glucose level and risks of diabetes. Results in the second part of experiments suggested the potential RAS-modulatory effect of vitamin D in isolated islets Therefore, in the third part of our study, we hypothesize that the insufficient vitamin D levels may lead to the inappropriate regulation of islet RAS expression and thus result in islet dysfunction. To achieve this, we examined the potential islet RAS-mediated effect of vitamin D on islet function by accessing glucose homeostasis, islet histomorphology, and local RAS expression and function by means of using a vitamin D receptor knockout and diet-induced vitamin D deficiency mouse models. Results showed that the islet RAS components were abnormally expressed when lacking a sufficient vitamin D level and normal vitamin D action. These observed effects of insufficient vitamin D might occur prior to onset of hyperglycemia thus modulating islet RAS expression, which in turn lead to islet failure and dysfunctional glucose homeostasis, together with decreased insulin actions in islet beta-cells. These results provide supports for the view that vitamin D physiologically exerts modulatory effects on islet function by downregulating islet RAS expression and function. (Abstract shortened by UMI.).