Enhanced beta cell function and anti-inflammatory effect after chronic treatment with the dipeptidyl peptidase-4 inhibitor vildagliptin in an advanced-aged diet-induced obesity mouse model.

AIMS/HYPOTHESIS: Studies have shown that dipeptidyl peptidase-4 (DPP4) inhibitors stimulate insulin secretion and increase beta cell mass in rodents. However, in these models hyperglycaemia has been induced early on in life and the treatment periods have been short. To explore the long-term effects of DPP4 inhibition on insulin secretion and beta cell mass, we have generated a high-fat diet (HFD)-induced-obesity model in mice of advanced age (10 months old). METHODS: After 1 month of HFD alone, the mice were given the DPP4 inhibitor vildagliptin for a further 11 months. At multiple time points throughout the study, OGTTs were performed and beta cell area and long-term survival were evaluated. RESULTS: Beta cell function and glucose tolerance were significantly improved by vildagliptin with both diets. In contrast, in spite of the long treatment period, beta cell area was not significantly different between vildagliptin-treated mice and controls. Mice of advanced age chronically fed an HFD displayed clear and extensive pancreatic inflammation and peri-insulitis, mainly formed by CD3-positive T cells, which were completely prevented by vildagliptin treatment. Chronic vildagliptin treatment also improved survival rates for HFD-fed mice. CONCLUSIONS/INTERPRETATION: In a unique advanced-aged HFD-induced-obesity mouse model, insulin secretion was improved and the extensive peri-insulitis prevented by chronic DPP4 inhibition. The improved survival rates for obese mice chronically treated with vildagliptin suggest that chronic DPP4 inhibition potentially results in additional quality-adjusted life-years for individuals with type 2 diabetes, which is the primary goal of any diabetes therapy.

Synergism by individual macronutrients explains the marked early GLP-1 and islet hormone responses to mixed meal challenge in mice.

Apart from glucose, proteins and lipids also stimulate incretin and islet hormone secretion. However, the glucoregulatory effect of macronutrients in combination is poorly understood. We therefore developed an oral mixed meal model in mice to 1) explore the glucagon-like peptide-1 (GLP-1) and islet hormone responses to mixed meal versus isocaloric glucose, and 2) characterize the relative contribution of individual macronutrients to these responses. Anesthetized C57BL/6J female mice were orally gavaged with 1) a mixed meal (0.285 kcal; glucose, whey protein and peanut oil; 60/20/20% kcal) versus an isocaloric glucose load (0.285 kcal), and 2) a mixed meal (0.285 kcal) versus glucose, whey protein or peanut oil administered individually in their mixed meal caloric quantity, i.e., 0.171, 0.055 and 0.055 kcal, respectively. Plasma was analyzed for glucose, insulin and intact GLP-1 before and during oral challenges. Plasma glucose was lower after mixed meal versus after isocaloric glucose ingestion. In spite of this, the peak insulin response (P=0.02), the peak intact GLP-1 levels (P=0.006) and the estimated ß-cell function (P=0.005) were higher. Furthermore, the peak insulin (P=0.004) and intact GLP-1 (P=0.006) levels were higher after mixed meal ingestion than the sum of responses to individual macronutrients. Compared to glucose alone, we conclude that there is a marked early insulin response to mixed meal ingestion, which emanates from a synergistic, rather than an additive, effect of the individual macronutrients in the mixed meal and is in part likely caused by increased levels of GLP-1.

Inhibitory effect of kisspeptins on insulin secretion from isolated mouse islets.

Islet hormone secretion is regulated by a variety of factors, and many of these signal through G protein-coupled receptors (GPCRs). A novel islet GPCR is GPR54, which couples to the Gq isoform of G proteins, which in turn signal through the phospholipase C pathway. Ligands for GPR54 are kisspeptins, which are peptides encoded in the KISS1 gene and also expressed in islet beta-cells. The KISS1 gene encodes a hydrophobic 145-amino acid protein that is cleaved into a 54-amino acid protein, kisspeptin-54 or KP54. Shorter kisspeptins also exist, such as kisspeptin-10 (KP10) and kisspeptin-13 (KP13). The involvement of GPR54 and kisspeptins in the regulation of islet function is not known. To address this problem, we incubated isolated mouse islets in the presence of KP13 and KP54 for 60 min and measured insulin secretion. We found that both KP13 and KP54 at 10 nM, 100 nM and 1microM inhibited insulin secretion in the presence of 2.8 mM glucose. However, by increasing the glucose concentration, this inhibitory action of the kisspeptins vanished. Thus, at 11.1 mM glucose, KP13 and KP54 inhibited insulin secretion only at high doses, and at 16.7 mM they no longer inhibited insulin secretion in any of the doses. We conclude that kisspeptins inhibit insulin secretion at glucose concentrations below 11.1 mM. This suggests that kisspeptins are regulating insulin secretion at physiological concentrations of glucose. The mechanisms by which kisspeptins regulate islet function and insulin secretion are unknown and will be further investigated.

Requirement for N-ethylmaleimide-sensitive factor for exocytosis of insulin-containing secretory granules in pancreatic beta-cells.

N-ethylmaleimide-sensitive factor (NSF) has an important role in fusion processes within intracellular compartments and at the plasma membrane, but the exact role of this protein in the exocytotic machinery has not yet been determined. NSF was found to be present in the cytosol of rat pancreatic beta-cells and rat insulinoma INS-1 cells. Capacitance measurements revealed that exocytosis of primed granules was not affected by the presence of a monoclonal antibody against NSF, mAb 2E5, suggesting that NSF is not involved in the fusion process. The antibody markedly decreased rapid refilling of new granules from a reserve pool during a first stimulation. However, slow refilling of primed granules occurred within a 2 min period between the first and second stimulations. We conclude that NSF is required in the exocytotic process in order to obtain a complete exocytotic response. Possible mechanisms by which NSF takes part in this process in insulin-secreting rat beta-cells are discussed.