Anagliptin increases insulin-induced skeletal muscle glucose uptake via an NO-dependent mechanism in mice.

AIMS/HYPOTHESIS: Recently, incretin-related agents have been reported to attenuate insulin resistance in animal models, although the underlying mechanisms remain unclear. In this study, we investigated whether anagliptin, the dipeptidyl peptidase 4 (DPP-4) inhibitor, attenuates skeletal muscle insulin resistance through endothelial nitric oxide synthase (eNOS) activation in the endothelial cells. We used endothelium-specific Irs2-knockout (ETIrs2KO) mice, which show skeletal muscle insulin resistance resulting from a reduction of insulin-induced skeletal muscle capillary recruitment as a consequence of impaired eNOS activation. METHODS: In vivo, 8-week-old male ETIrs2KO mice were fed regular chow with or without 0.3% (wt/wt) DPP-4 inhibitor for 8 weeks to assess capillary recruitment and glucose uptake by the skeletal muscle. In vitro, human coronary arterial endothelial cells (HCAECs) were used to explore the effect of glucagon-like peptide 1 (GLP-1) on eNOS activity. RESULTS: Treatment with anagliptin ameliorated the impaired insulin-induced increase in capillary blood volume, interstitial insulin concentration and skeletal muscle glucose uptake in ETIrs2KO mice. This improvement in insulin-induced glucose uptake was almost completely abrogated by the GLP-1 receptor (GLP-1R) antagonist exendin-(9-39). Moreover, the increase in capillary blood volume with anagliptin treatment was also completely inhibited by the NOS inhibitor. GLP-1 augmented eNOS phosphorylation in HCAECs, with the effect completely disappearing after exposure to the protein kinase A (PKA) inhibitor H89. These data suggest that anagliptin treatment enhances insulin-induced capillary recruitment and interstitial insulin concentrations, resulting in improved skeletal muscle glucose uptake by directly acting on the endothelial cells via NO- and GLP-1-dependent mechanisms in vivo. CONCLUSIONS/INTERPRETATION: Anagliptin may be a promising agent to ameliorate skeletal muscle insulin resistance in obese patients with type 2 diabetes.

Insulin receptor substrate-2 (Irs2) in endothelial cells plays a crucial role in insulin secretion.

Endothelial cells are considered to be essential for normal pancreatic ß-cell function. The current study attempted to demonstrate the role of insulin receptor substrate-2 (Irs2) in endothelial cells with regard to insulin secretion. Endothelial cell-specific Irs2 knockout (ETIrs2KO) mice exhibited impaired glucose-induced, arginine-induced, and glucagon-induced insulin secretion and showed glucose intolerance. In batch incubation and perifusion experiments using isolated islets, glucose-induced insulin secretion was not significantly different between the control and the ETIrs2KO mice. In contrast, in perfusion experiments, glucose-induced insulin secretion was significantly impaired in the ETIrs2KO mice. The islet blood flow was significantly impaired in the ETIrs2KO mice. After the treatment of these knockout mice with enalapril maleate, which improved the islet blood flow, glucose-stimulated insulin secretion was almost completely restored to levels equal to those in the control mice. These data suggest that Irs2 deletion in endothelial cells leads to a decreased islet blood flow, which may cause impaired glucose-induced insulin secretion. Thus, Irs2 in endothelial cells may serve as a novel therapeutic target for preventing and ameliorating type 2 diabetes and metabolic syndrome.

Deregulation of pancreas-specific oxidoreductin ERO1ß in the pathogenesis of diabetes mellitus.

A growing body of evidence has underlined the significance of endoplasmic reticulum (ER) stress in the pathogenesis of diabetes mellitus. ER oxidoreductin 1ß (ERO1ß) is a pancreas-specific disulfide oxidase that is known to be upregulated in response to ER stress and to promote protein folding in pancreatic ß cells. It has recently been demonstrated that ERO1ß promotes insulin biogenesis in ß cells and thus contributes to physiological glucose homeostasis, though it is unknown if ERO1ß is involved in the pathogenesis of diabetes mellitus. Here we show that in diabetic model mice, ERO1ß expression is paradoxically decreased in ß cells despite the indications of increased ER stress. However, overexpression of ERO1ß in ß cells led to the upregulation of unfolded protein response genes and markedly enlarged ER lumens, indicating that ERO1ß overexpression caused ER stress in the ß cells. Insulin contents were decreased in the ß cells that overexpressed ERO1ß, leading to impaired insulin secretion in response to glucose stimulation. These data indicate the importance of the fine-tuning of the ER redox state, the disturbance of which would compromise the function of ß cells in insulin synthesis and thus contribute to the pathogenesis of diabetes mellitus.

TCF7L2 in mouse pancreatic beta cells plays a crucial role in glucose homeostasis by regulating beta cell mass.

AIMS/HYPOTHESIS: Common genetic variations of the transcription factor 7-like 2 gene (encoded by TCF7L2), one of the T cell factor/lymphoid enhancer-binding factor transcription factors for the converging wingless-type MMTV integration site family (Wnt)/ß-catenin signalling pathway, are known to be associated with type 2 diabetes. Individuals with at-risk alleles of TCF7L2 exhibit impaired insulin secretion. Although previous studies using animal models have revealed the existence of a relationship between the Wnt/ß-catenin signalling pathway and glucose homeostasis, it remains unclear whether TCF7L2 in the pancreatic beta cells might be causally involved in insulin secretion in vivo. In this study, we investigated the role of TCF7L2 expressed in the pancreatic beta cells in glucose homeostasis. METHODS: Three independent groups of genetically engineered mice (DN mice) were generated, in which expression of the dominant-negative form of Tcf7l2 was driven under a rat insulin promoter. Phenotypes of both adult and newborn mice were evaluated. The levels of genes and proteins expressed in isolated islets were determined by reverse transcription-quantitative PCR and western blot analysis, respectively. RESULTS: Adult DN mice showed impaired glucose tolerance and decreased insulin secretion in both oral and intraperitoneal glucose tolerance tests. Marked reduction of the beta cell area and whole-pancreas insulin content was observed in both the adult and newborn DN mice. Islets from the DN mice showed decreased gene expressions of Ccnd1, Ccnd2, Irs1, Irs2, Ins1, Ins2 and Mafa, consistent with the deleterious effects of the dominant-negative form of Tcf7l2 on beta cell proliferation and insulin production. CONCLUSIONS/INTERPRETATION: TCF7L2 expressed in the pancreatic beta cells plays a crucial role in glucose metabolism through regulation of the beta cell mass.

Dipeptidyl peptidase-4 inhibitor anagliptin ameliorates diabetes in mice with haploinsufficiency of glucokinase on a high-fat diet.

OBJECTIVE: Type 2 diabetes is a chronic metabolic disorder characterized by hyperglycemia with insulin resistance and impaired insulin secretion. DPP-4 inhibitors have attracted attention as a new class of anti-diabetic agents for the treatment of type 2 diabetes. We investigated the effects of anagliptin, a highly selective DPP-4 inhibitor, on insulin secretion and insulin resistance in high-fat diet-fed mice with haploinsufficiency of glucokinase (GckKO) as animal models of type 2 diabetes. MATERIALS/METHODS: Wild-type and GckKO mice were administered two doses of anagliptin by dietary admixture (0.05% and 0.3%) for 10weeks. RESULTS: Both doses of anagliptin significantly inhibited the plasma DPP-4 activity and increased the plasma active GLP-1 levels in both the wild-type and GckKO mice to a similar degree. After 10weeks of treatment with 0.3% anagliptin, body weight gain and food intake were significantly suppressed in both wild-type and GckKO mice. In addition, 0.3% anagliptin ameliorated insulin resistance and glucose intolerance in both genotypes of mice. On the other hand, treatment with 0.05% anagliptin was not associated with any significant change of the body weight, food intake or insulin sensitivity in either genotype of mice, but it did improve the glucose tolerance by enhancing insulin secretion and increasing the ß-cell mass in both genotypes of mice. CONCLUSIONS: High-dose anagliptin treatment improved glucose tolerance by suppression of body weight gain and amelioration of insulin resistance, whereas low-dose anagliptin treatment improved glucose tolerance by enhancing insulin secretion.

Characterization of the sol operon in butanol-hyperproducing Clostridium saccharoperbutylacetonicum strain N1-4 and its degeneration mechanism.

The solventogenic sol operon consisting of bld, ctfA, ctfB, and adc was cloned from Clostridium saccharoperbutylacetonicum strain N1-4. These genes share as high as 95-98% similarity with the corresponding sol genes of Clostridium beijerinckii NCIMB 8052. The N1-4 sol gene cluster was transcribed in a polycistronic manner under the control of two promoters, and its transcription was highly induced during solventogenesis. Strain DGN3-4, the degenerated strain derived from N1-4, maintained the sol genes, but transcription of the DGN3-4 sol operon was hardly induced during solventogenesis. A substance extracted from the culture supernatants of wild-type N1-4 allowed us to induce transcription of the sol operon in DGN3-4. These results suggest that the degeneration is caused by the incompetence of the induction mechanism of the sol operon, and that transcription might be under the control of a quorum-sensing mechanism.