Glucagon‐like peptide 1‐potentiated insulin secretion and proliferation of pancreatic β‐cells GLP‐1促进胰岛素分泌和胰岛β细胞增殖的机制

Glucagon‐like peptide‐1 (GLP‐1) is the primary incretin hormone secreted from the intestine upon uptake of food to stimulate insulin secretion from pancreatic β‐cells. GLP‐1 exerts its effects by binding to its G‐protein coupled receptors and subsequently activating adenylate cyclase, leading to generation of cyclic adenosine monophosphate (cAMP). cAMP stimulates insulin secretion via activation of its effectors PKA and Epac2 in pancreatic β‐cells. In addition to its insulinotropic effects, GLP‐1 also preserves pancreatic β‐cell mass by stimulating β‐cell proliferation. Unlike the action of sulphonylureas in lowering blood glucose levels, action of GLP‐1 is affected by and interplays with glucose levels. Due to such advantages, GLP‐1‐based therapeutics have been rapidly developed and used clinically for treatment of type 2 diabetes. However, molecular mechanisms underlying how GLP‐1 potentiates diminished glucose‐stimulated insulin secretion and β‐cell proliferation under diabetic conditions are not well understood. Here, we review the actions of GLP‐1 in regulation of insulin secretion and pancreatic β‐cell proliferation.     

The incretin hormone GLP‐1 and mechanisms underlying its secretion

Glucagon‐like peptide‐1 (GLP‐1) is a cell type‐specific post‐translational product of proglucagon. It is encoded by the proglucagon gene and released primarily from intestinal endocrine L‐cells in response to hormonal, neuronal, and nutritional stimuli. In addition to serving as an incretin in mediating the effect of meal consumption on insulin secretion, GLP‐1 exerts other functions in pancreatic islets, including regulation of β‐cell proliferation and protection of β‐cells against metabolic stresses. Furthermore, GLP‐1 exerts numerous other functions in extrapancreatic organs, whereas brain GLP‐1 signaling controls satiety. Herein we review the discovery of two incretins and the development of GLP‐1‐based drugs. We also describe the development of cellular models for studying mechanisms underlying GLP‐1 secretion over the past 30 years. However, the main content of this review is a summary of studies on the exploration of mechanisms underlying GLP‐1 secretion. We not only summarize studies conducted over the past three decades on elucidating the role of nutritional components and hormonal factors in regulating GLP‐1 secretion, but also present a few very recent studies showing the possible role of dietary polyphenols. Finally, the emerging role of gut microbiota in metabolic homeostasis with the potential implication on GLP‐1 secretion is discussed.     

Plasma gastric inhibitory polypeptide and glucagon‐like peptide‐1 levels after glucose loading are associated with different factors in Japanese subjects

Aims/Introduction: Gastric inhibitory polypeptide (GIP) and glucagon‐like peptide‐1 (GLP‐1) are major incretins that potentiate insulin secretion from pancreatic β‐cells. The factors responsible for incretin secretion have been reported in Caucasian subjects, but have not been thoroughly evaluated in Japanese subjects. We evaluated the factors associated with incretin secretion during oral glucose tolerance test (OGTT) in Japanese subjects with normal glucose tolerance (NGT). Materials and Methods: We measured plasma GIP and GLP‐1 levels during OGTT in 17 Japanese NGT subjects and evaluated the factors associated with GIP and GLP‐1 secretion using simple and multiple regression analyses. Results: GIP secretion (AUC‐GIP) was positively associated with body mass index (P < 0.05), and area under the curve (AUC) of C‐peptide (P < 0.05) and glucagon (P < 0.01), whereas GLP‐1 secretion (AUC‐GLP‐1) was negatively associated with AUC of plasma glucose (P < 0.05). The insulinogenic index was most strongly associated with GIP secretion (P < 0.05); homeostasis model assessment β‐cell was the most the strongly associated factor in GLP‐1 secretion (P < 0.05) among the four indices of insulin secretion and insulin sensitivity. Conclusions: Several distinct factors might be associated with GIP and GLP‐1 secretion during OGTT in Japanese subjects. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2010.00078.x, 2011)     

Effect of single‐dose DPP‐4 inhibitor sitagliptin on β‐cell function and incretin hormone secretion after meal ingestion in healthy volunteers and drug‐naïve,well‐controlled type 2 diabetes subjects

To explore the effects of a single dose of the DPP‐4 inhibitor sitagliptin on glucose‐standardized insulin secretion and β‐cell glucose sensitivity after meal ingestion, 12 healthy and 12 drug‐naïve, well‐controlled type 2 diabetes (T2D) subjects (mean HbA1c 43 mmol/mol, 6.2%) received sitagliptin (100 mg) or placebo before a meal (525 kcal). β‐cell function was measured as the insulin secretory rate at a standardized glucose concentration and the β‐cell glucose sensitivity (the slope between glucose and insulin secretory rate). Incretin levels were also monitored. Sitagliptin increased standardized insulin secretion, in both healthy and T2D subjects, compared to placebo, but without increasing β‐cell glucose sensitivity. Sitagliptin also increased active glucose‐dependent insulinotropic polypeptide (GIP) and glucagon‐like peptide‐1 (GLP‐1) and reduced total (reflecting the secretion) GIP, but not total GLP‐1 levels. We conclude that a single dose of DPP‐4 inhibition induces dissociated effects on different aspects of β‐cell function and incretin hormones after meal ingestion in both healthy and well‐controlled T2D subjects.     

Incretin therapies: highlighting common features and differences in the modes of action of glucagon‐like peptide‐1 receptor agonists and dipeptidyl peptidase‐4 inhibitors

Over the last few years, incretin‐based therapies have emerged as important agents in the treatment of type 2 diabetes (T2D). These agents exert their effect via the incretin system, specifically targeting the receptor for the incretin hormone glucagon‐like peptide 1 (GLP‐1), which is partly responsible for augmenting glucose‐dependent insulin secretion in response to nutrient intake (the ‘incretin effect’). In patients with T2D, pharmacological doses/concentrations of GLP‐1 can compensate for the inability of diabetic β cells to respond to the main incretin hormone glucose‐dependent insulinotropic polypeptide, and this is therefore a suitable parent compound for incretin‐based glucose‐lowering medications. Two classes of incretin‐based therapies are available: GLP‐1 receptor agonists (GLP‐1RAs) and dipeptidyl peptidase‐4 (DPP‐4) inhibitors. GLP‐1RAs promote GLP‐1 receptor (GLP‐1R) signalling by providing GLP‐1R stimulation through ‘incretin mimetics’ circulating at pharmacological concentrations, whereas DPP‐4 inhibitors prevent the degradation of endogenously released GLP‐1. Both agents produce reductions in plasma glucose and, as a result of their glucose‐dependent mode of action, this is associated with low rates of hypoglycaemia; however, there are distinct modes of action resulting in differing efficacy and tolerability profiles. Furthermore, as their actions are not restricted to stimulating insulin secretion, these agents have also been associated with additional non‐glycaemic benefits such as weight loss, improvements in β‐cell function and cardiovascular risk markers. These attributes have made incretin therapies attractive treatments for the management of T2D and have presented physicians with an opportunity to tailor treatment plans. This review endeavours to outline the commonalities and differences among incretin‐based therapies and to provide guidance regarding agents most suitable for treating T2D in individual patients.     

GLP‐1 receptor agonist attenuates endoplasmic reticulum stress‐mediated β‐cell damage in Akita mice

Aims/Introduction: Endoplasmic reticulum (ER) stress is one of the contributing factors in the development of type 2 diabetes. To investigate the cytoprotective effect of glucagon‐like peptide 1 receptor (GLP‐1R) signaling in vivo, we examined the action of exendin‐4 (Ex‐4), a potent GLP‐1R agonist, on β‐cell apoptosis in Akita mice, an animal model of ER stress‐mediated diabetes. Materials and Methods: Ex‐4, phosphate‐buffered saline (PBS) or phlorizin were injected intraperitoneally twice a day from 3 to 5 weeks‐of‐age. We evaluated the changes in blood glucose levels, bodyweights, and pancreatic insulin‐positive area and number of islets. The effect of Ex‐4 on the numbers of C/EBP‐homologous protein (CHOP)‐, TdT‐mediated dUTP‐biotin nick‐end labeling (TUNEL)‐ or proliferating cell nuclear antigen‐positive β‐cells were also evaluated. Results: Ex‐4 significantly reduced blood glucose levels and increased both the insulin‐positive area and the number of islets compared with PBS‐treated mice. In contrast, there was no significant difference in the insulin‐positive area between PBS‐treated mice and phlorizin‐treated mice, in which blood glucose levels were controlled similarly to those in Ex‐4‐treated mice. Furthermore, treatment of Akita mice with Ex‐4 resulted in a significant decrease in the number of CHOP‐positive β‐cells and TUNEL‐positive β‐cells, and in CHOP mRNA levels in β‐cells, but there was no significant difference between the PBS‐treated group and the phlorizin‐treated group. Proliferating cell nuclear antigen staining showed no significant difference among the three groups in proliferation of β‐cells. Conclusions: These data suggest that Ex‐4 treatment can attenuate ER stress‐mediated β‐cell damage, mainly through a reduction of apoptotic cell death that is independent of lowered blood glucose levels. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2010.00075.x , 2010)     

Glucose‐dependent insulinotropic polypeptide signaling in pancreatic β‐cells and adipocytes

Glucose‐dependent insulinotropic polypeptide (GIP) was the first incretin to be identified. In addition to stimulating insulin secretion, GIP plays regulatory roles in the maintenance, growth and survival of pancreatic islets, as well as impacting on adipocyte function. The current review focuses on the intracellular signaling pathways by which GIP contributes to the regulation of β‐cell secretion and survival, and adipocyte differentiation and lipogenesis. Studies on signaling underlying the insulinotropic actions of the incretin hormones have largely been carried out with glucagon‐like peptide‐1. They have provided evidence for contributions by both protein kinase A (PKA) and exchange protein directly activated by cyclic adenosine monophosphate (EPAC2), and their probable role in GIP signaling is discussed. Recent studies have shown that inhibition of the kinase apoptosis signal‐regulating kinase 1 (ASK1) by GIP plays a key role in reducing mitochondria‐induced apoptosis in β‐cells through protein kinase B (PKB)‐mediated pathways, and that GIP‐induced post‐translational modification of voltage‐ dependent K⁺ (Kv) channels also contributes to its prosurvival role. Through regulation of gene expression, GIP tips the balance between pro‐ and anti‐apoptotic members of the B‐cell lymphoma‐2 (Bcl‐2) protein family towards β‐cell survival. GIP also plays important roles in the differentiation of pre‐adipocytes to adipocytes, and in the regulation of lipoprotein lipase expression and lipogenesis. These events involve interactions between GIP, insulin and resistin signaling pathways. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2012.00196.x, 2012)     

Glucose‐dependent insulinotropic polypeptide and glucagon‐like peptide‐1: Incretin actions beyond the pancreas

Glucose‐dependent insulinotropic polypeptide (GIP) and glucagon‐like peptide‐1 (GLP‐1) are the two primary incretin hormones secreted from the intestine on ingestion of various nutrients to stimulate insulin secretion from pancreatic β‐cells glucose‐dependently. GIP and GLP‐1 undergo degradation by dipeptidyl peptidase‐4 (DPP‐4), and rapidly lose their biological activities. The actions of GIP and GLP‐1 are mediated by their specific receptors, the GIP receptor (GIPR) and the GLP‐1 receptor (GLP‐1R), which are expressed in pancreatic β‐cells, as well as in various tissues and organs. A series of investigations using mice lacking GIPR and/or GLP‐1R, as well as mice lacking DPP‐4, showed involvement of GIP and GLP‐1 in divergent biological activities, some of which could have implications for preventing diabetes‐related microvascular complications (e.g., retinopathy, nephropathy and neuropathy) and macrovascular complications (e.g., coronary artery disease, peripheral artery disease and cerebrovascular disease), as well as diabetes‐related comorbidity (e.g., obesity, non‐alcoholic fatty liver disease, bone fracture and cognitive dysfunction). Furthermore, recent studies using incretin‐based drugs, such as GLP‐1 receptor agonists, which stably activate GLP‐1R signaling, and DPP‐4 inhibitors, which enhance both GLP‐1R and GIPR signaling, showed that GLP‐1 and GIP exert effects possibly linked to prevention or treatment of diabetes‐related complications and comorbidities independently of hyperglycemia. We review recent findings on the extrapancreatic effects of GIP and GLP‐1 on the heart, brain, kidney, eye and nerves, as well as in the liver, fat and several organs from the perspective of diabetes‐related complications and comorbidities.     

Current status of regeneration of pancreatic β‐cells

Newly generated insulin‐secreting cells for use in cell therapy for insulin‐deficient diabetes mellitus require properties similar to those of native pancreatic β‐cells. Pancreatic β‐cells are highly specialized cells that produce a large amount of insulin, and secrete insulin in a regulated manner in response to glucose and other stimuli. It is not yet explained how the β‐cells acquire this complex function during normal differentiation. So far, in vitro generation of insulin‐secreting cells from embryonic stem cells, induced‐pluripotent stem cells and adult stem/progenitor‐like cells has been reported. However, most of these cells are functionally immature and show poor glucose‐responsive insulin secretion compared to that of native pancreatic β‐cells (or islets). Strategies to generate functional β‐cells or a whole organ in vivo have also recently been proposed. Establishing a protocol to generate fully functional insulin‐secreting cells that closely resemble native β‐cells is a critical matter in regenerative medicine for diabetes. Understanding the physiological processes of differentiation, proliferation and regeneration of pancreatic β‐cells might open the path to cell therapy to cure patients with absolute insulin deficiency.     

GLP‐1 receptor activated insulin secretion from pancreatic β‐cells: mechanism and glucose dependence

The major goal in the treatment of type 2 diabetes mellitus is to control the hyperglycaemia characteristic of the disease. However, treatment with common therapies such as insulin or insulinotrophic sulphonylureas (SU), while effective in reducing hyperglycaemia, may impose a greater risk of hypoglycaemia, as neither therapy is self‐regulated by ambient blood glucose concentrations. Hypoglycaemia has been associated with adverse physical and psychological outcomes and may contribute to negative cardiovascular events; hence minimization of hypoglycaemia risk is clinically advantageous. Stimulation of insulin secretion from pancreatic β‐cells by glucagon‐like peptide 1 receptor (GLP‐1R) agonists is known to be glucose‐dependent. GLP‐1R agonists potentiate glucose‐stimulated insulin secretion and have little or no activity on insulin secretion in the absence of elevated blood glucose concentrations. This ‘glucose‐regulated’ activity of GLP‐1R agonists makes them useful and potentially safer therapeutics for overall glucose control compared to non‐regulated therapies; hyperglycaemia can be reduced with minimal hypoglycaemia. While the inherent mechanism of action of GLP‐1R agonists mediates their glucose dependence, studies in rats suggest that SUs may uncouple this dependence. This hypothesis is supported by clinical studies showing that the majority of events of hypoglycaemia in patients treated with GLP‐1R agonists occur in patients treated with a concomitant SU. This review aims to discuss the current understanding of the mechanisms by which GLP‐1R signalling promotes insulin secretion from pancreatic β‐cells via a glucose‐dependent process.     

Incretin concept revised: The origin of the insulinotropic function of glucagon‐like peptide‐1 – the gut,the islets or both?

Incretins comprise a pair of gut hormones, glucose‐dependent insulinotropic polypeptide (GIP) and glucagon‐like peptide‐1 (GLP‐1), which are secreted in response to food ingestion and enhance glucose‐dependent insulin secretion from pancreatic β‐cells. Immediately after secretion, GLP‐1 is degraded by dipeptidyl peptidase‐4 more rapidly than GIP, and circulating levels of biologically intact GLP‐1 are substantially lower than those of biologically intact GIP. Therefore, there has been a debate on how the gut‐derived GLP‐1 exerts insulinotropic actions. Recent publications have revealed two novel mechanisms by which GLP‐1 exerts insulinotropic actions: (i) the gut‐derived GLP‐1 activates receptors expressed in nodose ganglions, thereby potentiating glucose‐dependent insulin secretion through the vagus nerves; and (ii) the pancreatic α‐cell‐derived GLP‐1 activates receptors expressed in β‐cells in a paracrine manner. While the relative contributions of the two mechanisms under normal and pathological conditions remain unknown and mechanisms regulating GLP‐1 secretion from α‐cells need to be investigated, the available data strongly indicate that the effects of GLP‐1 on insulin secretion are far more complex than previously believed, and the classical incretin concept regarding GLP‐1 should be revised.     

Diabetes and obesity treatment based on dual incretin receptor activation: ‘twincretins’

The gut incretin hormones glucose‐dependent insulinotropic polypeptide (GIP) and glucagon‐like peptide‐1 (GLP‐1) are secreted after meal ingestion and work in concert to promote postprandial insulin secretion and regulate glucagon secretion. GLP‐1 also slows gastric emptying and suppresses appetite, whereas GIP seems to affect lipid metabolism. The introduction of selective GLP‐1 receptor (GLP‐1R) agonists for the treatment of type 2 diabetes and obesity has increased the scientific and clinical interest in incretins. Combining the body weight‐lowering and glucose‐lowering effects of GLP‐1 with a more potent improvement of β cell function through additional GIP action could potentially offer a more effective treatment of diabetes and obesity, with fewer adverse effects than selective GLP‐1R agonists; therefore, new drugs designed to co‐activate both the GIP receptor (GIPR) and the GLP‐1R simultaneously are under development. In the present review, we address advances in the field of GIPR and GLP‐1R co‐agonism and review in vitro studies, animal studies and human trials involving co‐administration of the two incretins, as well as results from a recently developed GIPR/GLP‐1R co‐agonist, and highlight promising areas and challenges within the field of incretin dual agonists.     

Weight loss and incretin responsiveness improve glucose control independently after gastric bypass surgery

Background: The aim of the present study was to determine the mechanisms underlying Type 2 diabetes remission after gastric bypass (GBP) surgery by characterizing the short‐ and long‐term changes in hormonal determinants of blood glucose. Methods: Eleven morbidly obese women with diabetes were studied before and 1, 6, and 12 months after GBP; eight non‐diabetic morbidly obese women were used as controls. The incretin effect was measured as the difference in insulin levels in response to oral glucose and to an isoglycemic intravenous challenge. Outcome measures were glucose, insulin, C‐peptide, proinsulin, amylin, glucagon, glucose‐dependent insulinotropic polypeptide (GIP), glucagon‐like peptide‐1 (GLP‐1) levels and the incretin effect on insulin secretion. Results: The decrease in fasting glucose (r = 0.724) and insulin (r = 0.576) was associated with weight loss up to 12 months after GBP. In contrast, the blunted incretin effect (calculated at 22%) that improved at 1 month remained unchanged with further weight loss at 6 (52%) and 12 (52%) months. The blunted incretin (GLP‐1 and GIP) levels, early phase insulin secretion, and other parameters of β‐cell function (amylin, proinsulin/insulin) followed the same pattern, with rapid improvement at 1 month that remained unchanged at 1 year. Conclusions: The data suggest that weight loss and incretins may contribute independently to improved glucose levels in the first year after GBP surgery.     

Gastrointestinal actions of glucagon‐like peptide‐1‐based therapies: glycaemic control beyond the pancreas

The gastrointestinal hormone glucagon‐like peptide‐1 (GLP‐1) lowers postprandial glucose concentrations by regulating pancreatic islet‐cell function, with stimulation of glucose‐dependent insulin and suppression of glucagon secretion. In addition to endocrine pancreatic effects, mounting evidence suggests that several gastrointestinal actions of GLP‐1 are at least as important for glucose‐lowering. GLP‐1 reduces gastric emptying rate and small bowel motility, thereby delaying glucose absorption and decreasing postprandial glucose excursions. Furthermore, it has been suggested that GLP‐1 directly stimulates hepatic glucose uptake, and suppresses hepatic glucose production, thereby adding to reduction of fasting and postprandial glucose levels. GLP‐1 receptor agonists, which mimic the effects of GLP‐1, have been developed for the treatment of type 2 diabetes. Based on their pharmacokinetic profile, GLP‐1 receptor agonists can be broadly categorized as short‐ or long‐acting, with each having unique islet‐cell and gastrointestinal effects that lower glucose levels. Short‐acting agonists predominantly lower postprandial glucose excursions, by inhibiting gastric emptying and intestinal glucose uptake, with little effect on insulin secretion. By contrast, long‐acting agonists mainly reduce fasting glucose levels, predominantly by increased insulin and reduced glucagon secretion, with potential additional direct inhibitory effects on hepatic glucose production. Understanding these pharmacokinetic and pharmacodynamic differences may allow personalized antihyperglycaemic therapy in type 2 diabetes. In addition, it may provide the rationale to explore treatment in patients with no or little residual β‐cell function.     

CRFR1 is expressed on pancreatic β cells,promotes β cell proliferation,and potentiates insulin secretion in a glucose-dependent manner

Corticotropin-releasing factor (CRF), originally characterized as the principal neuroregulator of the hypothalamus-pituitary-adrenal axis, has broad central and peripheral distribution and actions. We demonstrate the presence of CRF receptor type 1 (CRFR1) on primary β cells and show that activation of pancreatic CRFR1 promotes insulin secretion, thus contributing to the restoration of normoglycemic equilibrium. Stimulation of pancreatic CRFR1 initiates a cAMP response that promotes insulin secretion in vitro and in vivo and leads to the phosphorylation of cAMP response element binding and the induction of the expression of several immediate-early genes. Thus, the insulinotropic actions of pancreatic CRFR1 oppose the activation of CRFR1 on anterior pituitary corticotropes, leading to the release of glucocorticoids that functionally antagonize the actions of insulin. Stimulation of the MIN6 insulinoma line and primary rat islets with CRF also activates the MAPK signaling cascade leading to rapid phosphorylation of Erk1/2 in response to CRFR1-selective ligands, which induce proliferation in primary rat neonatal β cells. Importantly, CRFR1 stimulates insulin secretion only during conditions of intermediate to high ambient glucose, and the CRFR1-dependent phosphorylation of Erk1/2 is greater with elevated glucose concentrations. This response is reminiscent of the actions of the incretins, which potentiate insulin secretion only during elevated glucose conditions. The presence of CRFR1 on β cells adds another layer of complexity to the intricate network of paracrine and autocrine factors and their cognate receptors whose coordinated efforts can dictate islet hormone output and regulate β cell proliferation.     

Sorting nexin 19 regulates the number of dense core vesicles in pancreatic β‐cells

Aims/Introduction: Insulinoma‐associated protein 2 (IA‐2) regulates insulin secretion and the number of dense core vesicles (DCV). However, the mechanism of regulation of DCV number by IA‐2 is unknown. We examined the effect of sorting nexin 19 (SNX19), an IA‐2 interacting protein, on insulin secretion and the number of dense core vesicles (DCV). Materials and Methods: Stable SNX19 knockdown (SNX19KD) MIN6, a mouse pancreatic β‐cell line, and stable SNX19‐reintroduced SNX19KD MIN6 were established. Quantification of DCV, and lysosomes was carried out using electron micrographs. The half‐life of DCV was detected by pulse‐chase experiment. Results: Insulin secretion and content were decreased in stable SNX19KD MIN6 cells compared with those in control MIN6 cells. Electron micrographs showed that DCV number in SNX19KD cells was decreased by approximately 75% and that DCV size was decreased by approximately 40% compared with those in control cells, respectively. Furthermore, when SNX19 was reintroduced in SNX19KD cells, insulin content, insulin secretion and DCV number were increased. The half‐life of DCV was decreased in SNX19KD cells, but was increased in SNX19KD cells in which SNX19 was reintroduced. The number of lysosomes and the activity of lysosome enzyme cathepsin D were increased by approximately threefold in SNX19KD cells compared with those in control cells. In contrast, they were decreased to approximately half to one‐third in SNX19‐reintroduced SNX19KD cells. Conclusions: SNX19 regulates the number of DCV and insulin content by stabilizing DCV in β‐cells. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2011.00138.x, 2012)     

Glucose-incretin interaction revisited

Pancreatic beta cell dysfunction is pivotal to the development of diabetes, and restoration of insulin action is of primary importance. Here, we present a review of the mechanism of insulin secretion by pancreatic beta cells and discuss the mutual interaction of signaling pathways in stimulus-secretion coupling to better understand the scientific basis of pharmacological treatment for insulin secretion deficiency. Glucose stimulates insulin secretion via membrane depolarization by closure of ATP-sensitive K(+) channels (K(ATP) channels) and opening of L-type voltage-dependent Ca(2+) channels. The resultant elevation of cytosolic free Ca(2+) triggers insulin exocytosis. This is termed the "K(ATP)-dependent pathway" and is shared by sulfonylurea, which closes K(ATP) channels. Glucose also stimulates insulin release independent of its action on K(ATP) channels. This is referred to as the "K(ATP)-independent pathway," the molecular basis of which remains elusive. In the pancreatic beta cell, incretin hormones increase cAMP level, which enhances glucose-stimulated insulin release by protein kinase A-dependent and -independent mechanisms. Importantly, cAMP does not directly augment Ca(2+)-stimulated insulin release per se. The stimulatory level of ambient glucose is an absolute requirement for incretin to enhance insulin release. Therefore, incretin/cAMP enhances K(ATP)-independent insulinotropic action of glucose. The robust glucose-lowering effect of DPP4 inhibitor add-on in diabetic patients with sulfonylurea secondary failure is intriguing. With the clinical availability of DPP4 inhibitor and GLP-1 mimetics, the importance of the interactions between cAMP signaling and K(ATP) channel-independent actions of glucose is reappraised.