The beneficial effects of a muscarinic agonist on pancreatic ß-cells.

The brain and nervous system play an important role in pancreatic ß-cell function. This study investigated the role of muscarinic agonists or acetylcholine, which is the major neurotransmitter in the vagal nerve, in regulating pancreatic ß-cell mass and glucose homeostasis. Administration of the muscarinic agonist bethanechol increased insulin secretion and improved glucose tolerance in insulin-receptor substrate 2 (IRS2)-knockout (IRS-2-/-) mice and diet-induced obesity mice. Oral administration of bethanechol increased ß-cell mass and proliferation in wild-type mice, but not IRS-2-/- mice. The muscarinic agonist also increased the incorporation of 5-bromo-2'-deoxyuridine (BrdU) into islets isolated from wild-type mice and pancreatic ß-cell line MIN6. The phosphorylation of protein kinase B (Akt) induced by oral administration of bethanechol was observed in wild-type mice, but not IRS-2-/- mice. The secretion of glucagon-like peptide-1 (GLP-1) was also stimulated by bethanechol in wild-type mice, and a GLP-1 antagonist partially inhibited the bethanechol-induced increase in ß-cell mass. These results suggest that the muscarinic agonist exerted direct and indirect effects on ß-cell proliferation that were dependent on the IRS-2/Akt pathway. The bethanechol-stimulated release of GLP-1 may be indirectly associated with ß-cell proliferation.

DPP-4 inhibition improves early mortality, ß cell function, and adipose tissue inflammation in db/db mice fed a diet containing sucrose and linoleic acid.

BACKGROUND: Diabetes therapy that not only lowers glucose levels but also lengthens life spans is required. We previously demonstrated that DPP-4 inhibition ameliorated ß cell apoptosis and adipose tissue inflammation in ß cell-specific glucokinase haploinsufficient mice fed a diet containing a combination of sucrose and linoleic acid (SL). METHODS: In this study, we investigated the effects of DPP-4 inhibition in obese diabetic db/db mice fed an SL diet or a control diet containing sucrose and oleic acid (SO). We also examined the effects of DPP-4 inhibition in IRS-1-deficient mice fed an SL or SO diet as a model of insulin resistance. RESULTS: DPP-4 inhibition efficiently increases the active GLP-1 levels in db/db mice. Unexpectedly, the SL diet, but not the SO diet, markedly increases mortality in the db/db mice. DPP-4 inhibition reduces the early lethality in SL-fed db/db mice. DPP-4 inhibition improves glucose tolerance, ß cell function, and adipose tissue inflammation in db/db mice fed either diet. No significant changes in glycemic control or ß cell mass were observed in any of the IRS-1-deficient mouse groups. CONCLUSIONS: A diet containing a combination of sucrose and linoleic acid causes early lethality in obese diabetic db/db mice, but not in lean and insulin resistant IRS-1 knockout mice. DPP-4 inhibition has protective effects against the diet-induced lethality in db/db mice.

ß-Cell proliferation after a partial pancreatectomy is independent of IRS-2 in mice.

The glucokinase-induced up-regulation of insulin receptor substrate 2 (IRS-2) plays an important role in ß-cell adaptive proliferation in response to high-fat diet-induced insulin resistance. This study aimed to investigate the role of IRS-2 in the proliferation of ß-cells after a 60% partial pancreatectomy. IRS-2-deficient (IRS-2(-/-)) mice or wild-type mice were subjected to a pancreatectomy (60% partial pancreatectomy) or a sham operation (Sham). The ß-cell proliferation and gene expression profiles of the islets were then assessed. Gene expression in islets from pancreatectomized and Sham C57BL/6J male mice was analyzed using a cDNA microarray analysis. To compare with ß-cell proliferation induced by a high-fat diet, Gck(+/-) mice subjected to a pancreatectomy were also analyzed. The IRS-2(-/-) mice exhibited ß-cell expansion and a significant increase in ß-cell proliferation after the pancreatectomy, compared with the Sham group. Although glucose-stimulated insulin secretion from islets was not impaired, IRS-2(-/-) mice manifested severe hyperglycemia after the pancreatectomy. The expression levels of Aurora kinase B, Cyclin A, and Cyclin B1 in the pancreatectomized islets were also enhanced in the IRS-2(-/-) mice. A gene set enrichment analysis suggested an association between the genes that were up-regulated in the pancreatectomized islets and those involved in M phase progression in the cell cycle. ß-Cell proliferation after a pancreatectomy was observed even in the Gck(+/-) mice. In conclusion, IRS-2 was not required for ß-cell proliferation but might be needed for functional ß-cell mass, after a pancreatectomy. A partial pancreatectomy in mice may be an attractive model for the development of new strategy for exploring the unique nature of ß-cell proliferation.

Glucokinase activation ameliorates ER stress-induced apoptosis in pancreatic ß-cells.

The derangement of endoplasmic reticulum (ER) homeostasis triggers ß-cell apoptosis, leading to diabetes. Glucokinase upregulates insulin receptor substrate 2 (IRS-2) expression in ß-cells, but the role of glucokinase and IRS-2 in ER stress has been unclear. In this study, we investigated the impact of glucokinase activation by glucokinase activator (GKA) on ER stress in ß-cells. GKA administration improved ß-cell apoptosis in Akita mice, a model of ER stress-mediated diabetes. GKA increased the expression of IRS-2 in ß-cells, even under ER stress. Both glucokinase-deficient Akita mice and IRS-2-deficient Akita mice exhibited an increase in ß-cell apoptosis, compared with Akita mice. ß-cell-specific IRS-2-overexpressing (ßIRS-2-Tg) Akita mice showed less ß-cell apoptosis than Akita mice. IRS-2-deficient islets were vulnerable, but ßIRS-2-Tg islets were resistant to ER stress-induced apoptosis. Meanwhile, GKA regulated the expressions of C/EBP homologous protein (CHOP) and other ER stress-related genes in an IRS-2-independent fashion in islets. GKA suppressed the expressions of CHOP and Bcl2-associated X protein (Bax) and protected against ß-cell apoptosis under ER stress in an ERK1/2-dependent, IRS-2-independent manner. Taken together, GKA ameliorated ER stress-mediated apoptosis by harmonizing IRS-2 upregulation and the IRS-2-independent control of apoptosis in ß-cells.

AMPK is involved in the regulation of incretin receptors expression in pancreatic islets under a low glucose concentration.

The precise role of AMP-activated protein kinase (AMPK), a target of metformin, in pancreatic ß cells remains controversial, even though metformin was recently shown to enhance the expression of incretin receptors (GLP-1 and GIP receptors) in pancreatic ß cells. In this study, we investigated the effect of AMPK in the regulation of incretin receptors expression in pancreatic islets. The phosphorylation of AMPK in the mouse islets was decreased by increasing glucose concentrations. We showed the expression of incretin receptors in bell-shaped response to glucose. Expression of the incretin receptors in the isolated islets showed higher levels under a medium glucose concentration (11.1 mM) than that under a low glucose concentration (2.8 mM), but was suppressed under a high glucose concentration (22.2 mM). Both treatment with an AMPK inhibitor and DN-AMPK expression produced a significant increase of the incretin receptors expression under a low glucose concentration. By contrast, in hyperglycemic db/db islets, the enhancing effect of the AMPK inhibitor on the expression of incretin receptors was diminished under a low glucose concentration. Taken together, AMPK is involved in the regulation of incretin receptors expression in pancreatic islets under a low glucose concentration.

Diet-induced adipose tissue inflammation and liver steatosis are prevented by DPP-4 inhibition in diabetic mice.

OBJECTIVE: Diet composition alters the metabolic states of adipocytes and hepatocytes in diabetes. The effects of dipeptidyl peptidase-4 (DPP-4) inhibition on adipose tissue inflammation and fatty liver have been obscure. We investigated the extrapancreatic effects of DPP-4 inhibition on visceral fat and the liver. RESEARCH DESIGN AND METHODS: We investigated diet-induced metabolic changes in ß-cell-specific glucokinase haploinsufficient (Gck(+/-)) diabetic mice. We challenged animals with a diet containing a combination of sucrose and oleic acid (SO) or sucrose and linoleic acid (SL). Next, we assessed the effects of a DPP-4 inhibitor, des-fluoro-sitagliptin, on adipose tissue inflammation and hepatic steatosis. RESULTS: The epididymal fat weight and serum leptin level were significantly higher in Gck(+/-) mice fed SL than in mice fed SO, although no significant differences in body weight or adipocyte size were noted. Compared with SO, SL increased the numbers of CD11c(+) M1 macrophages and CD8(+) T-cells in visceral adipose tissue and the expression of E-selectin, P-selectin, and plasminogen activator inhibitor-1 (PAI-1). DPP-4 inhibition significantly prevented adipose tissue infiltration by CD8(+) T-cells and M1 macrophages and decreased the expression of PAI-1. The production of cytokines by activated T-cells was not affected by DPP-4 inhibition. Furthermore, DPP-4 inhibition prevented fatty liver in both wild-type and Gck(+/-) mice. DPP-4 inhibition also decreased the expressions of sterol regulatory element-binding protein-1c, stearoyl-CoA desaturase-1, and fatty acid synthase, and increased the expression of peroxisome proliferator-activated receptor-α in the liver. CONCLUSIONS: Our findings indicated that DPP-4 inhibition has extrapancreatic protective effects against diet-induced adipose tissue inflammation and hepatic steatosis.