Restoration of first-phase insulin secretion by the imidazoline compound LY374284 in pancreatic islets of diabetic db/db mice

Insulin secretion from beta cells within intact islets: location matters

The control of hormone secretion is central to body homeostasis, and its dysfunction is important in many diseases. The key cellular steps that lead to hormone secretion have been identified, and the stimulus-secretion pathway is understood in outline for many endocrine cells. In the case of insulin secretion from pancreatic beta cells, this pathway involves the uptake of glucose, cell depolarization, calcium entry, and the triggering of the fusion of insulin-containing granules with the cell membrane. The wealth of information on the control of insulin secretion has largely been obtained from isolated single-cell studies. However, physiologically, beta cells exist within the islets of Langerhans, with structural and functional specializations that are not preserved in single-cell cultures. This review focuses on recent work that is revealing distinct aspects of insulin secretion from beta cells within the islet.

The secretory deficit in islets from db/db mice is mainly due to a loss of responding beta cells

AIMS/HYPOTHESIS

We used the db/db mouse to determine the nature of the secretory defect in intact islets.

METHODS

Glucose tolerance was compared in db/db and wild-type (WT) mice. Isolated islets were used: to measure insulin secretion and calcium in a two-photon assay of single-insulin-granule fusion; and for immunofluorescence of soluble N-ethylmaleimide-sensitive factor attachment proteins (SNAREs).

RESULTS

The 13-18-week-old db/db mice showed a diabetic phenotype. Isolated db/db islets showed a 77% reduction in insulin secretion induced by 15 mmol/l glucose and reductions in the amplitude and rise-time of the calcium response to glucose. Ionomycin-induced insulin secretion in WT but not db/db islets. Immunofluorescence showed an increase in the levels of the SNAREs synaptosomal-associated protein 25 (SNAP25) and vesicle-associated membrane protein 2 (VAMP2) in db/db islets, but reduced syntaxin-1A. Therefore, db/db islets have both a compromised calcium response to glucose and a compromised secretory response to calcium. Two-photon microscopy of isolated islets determined the number and distribution of insulin granule exocytic events. Compared with WT, db/db islets showed far fewer exocytic events (an 83% decline at 15 mmol/l glucose). This decline was due to a 73% loss of responding cells and, in the remaining responsive cells, a 50% loss of exocytic responses per cell. An assay measuring granule re-acidification showed evidence for more recaptured granules in db/db islets compared with WT.

CONCLUSIONS/INTERPRETATION

We showed that db/db islets had a reduced calcium response to glucose and a reduction in syntaxin-1A. Within the db/db islets, changes were manifest as both a reduction in responding cells and a reduction in fusing insulin granules per cell.

Basement membrane extract preserves islet viability and activity in vitro by up-regulating α3 integrin and its signal

OBJECTIVE

Survival of transplanted islets is limited partly because of the disruption of the islet basement membrane (BM) occurring during isolation. We hypothesized that the embedment of BM extract (BME) could induce a viable cell mass and prolong islet functionality before transplantation.

METHODS

A special reconstituted BME that solidifies into a gel at 37°C was used to embed isolated islets in this study. The strategy was used to re-establish the interaction between the islets and peri-islet BM.

RESULTS

Islets embedded in BME showed lower caspase-3 levels and higher Akt activity than those in suspension. Moreover, we found for the first time that the expression of α3 integrin and focal adhesion kinase (FAK) and FAK activity was up-regulated in islets after BME embedment. The reverse effect was observed on islet apoptosis when islets rescued from a 24-hour suspension culture were embedded in BME for the next 24 hours. In addition, expression of pancreatic duodenal homeobox factor-1 and phospho-extracellular signal-regulated kinase 1/2 was partially preserved, suggesting the positive effect of BME on islet development.

CONCLUSIONS

These results indicate that BME embedment of islets can up-regulate the expression of α3 integrin and its signal transduction, which may improve islet viability.

Intracellular transport of insulin granules is a subordinated random walk

We quantitatively analyzed particle tracking data on insulin granules expressing fluorescent fusion proteins in MIN6 cells to better understand the motions contributing to intracellular transport and, more generally, the means for characterizing systems far from equilibrium. Care was taken to ensure that the statistics reflected intrinsic features of the individual granules rather than details of the measurement and overall cell state. We find anomalous diffusion. Interpreting such data conventionally requires assuming that a process is either ergodic with particles working against fluctuating obstacles (fractional brownian motion) or nonergodic with a broad distribution of dwell times for traps (continuous-time random walk). However, we find that statistical tests based on these two models give conflicting results. We resolve this issue by introducing a subordinated scheme in which particles in cages with random dwell times undergo correlated motions owing to interactions with a fluctuating environment. We relate this picture to the underlying microtubule structure by imaging in the presence of vinblastine. Our results provide a simple physical picture for how diverse pools of insulin granules and, in turn, biphasic secretion could arise.

Apolipoprotein A-IV improves glucose homeostasis by enhancing insulin secretion

Apolipoprotein A-IV (apoA-IV) is secreted by the small intestine in response to fat absorption. Here we demonstrate a potential role for apoA-IV in regulating glucose homeostasis. ApoA-IV-treated isolated pancreatic islets had enhanced insulin secretion under conditions of high glucose but not of low glucose, suggesting a direct effect of apoA-IV to enhance glucose-stimulated insulin release. This enhancement involves cAMP at a level distal to Ca(2+) influx into the β cells. Knockout of apoA-IV results in compromised insulin secretion and impaired glucose tolerance compared with WT mice. Challenging apoA-IV(-/-) mice with a high-fat diet led to fasting hyperglycemia and more severe glucose intolerance associated with defective insulin secretion than occurred in WT mice. Administration of exogenous apoA-IV to apoA-IV(-/-) mice improved glucose tolerance by enhancing insulin secretion in mice fed either chow or a high-fat diet. Finally, we demonstrate that exogenous apoA-IV injection decreases blood glucose levels and stimulates a transient increase in insulin secretion in KKAy diabetic mice. These results suggest that apoA-IV may provide a therapeutic target for the regulation of glucose-stimulated insulin secretion and treatment of diabetes.

DPP-4 inhibitor des-F-sitagliptin treatment increased insulin exocytosis from db/db mice β cells

Incretin promotes insulin secretion acutely. Recently, orally-administered DPP-4 inhibitors represent a new class of anti-hyperglycemic agents. Indeed, inhibitors of dipeptidyl peptidase-IV (DPP-4), sitagliptin, has just begun to be widely used as therapeutics for type 2 diabetes. However, the effects of sitagliptin-treatment on insulin exocytosis from single β-cells are yet unknown. We therefore investigated how sitagliptin-treatment in db/db mice affects insulin exocytosis by treating db/db mice with des-F-sitagliptin for 2 weeks. Perfusion studies showed that 2 weeks-sitagliptin treatment potentiated insulin secretion. We then analyzed insulin granule motion and SNARE protein, syntaxin 1, by TIRF imaging system. TIRF imaging of insulin exocytosis showed the increased number of docked insulin granules and increased fusion events from them during first-phase release. In accord with insulin exocytosis data, des-F-sitagliptin-treatment increased the number of syntaxin 1 clusters on the plasma membrane. Thus, our data demonstrated that 2-weeks des-F-sitagliptin-treatment increased the fusion events of insulin granules, probably via increased number of docked insulin granules and that of syntaxin 1 clusters.

Open innovation for phenotypic drug discovery: The PD2 assay panel

Phenotypic lead generation strategies seek to identify compounds that modulate complex, physiologically relevant systems, an approach that is complementary to traditional, target-directed strategies. Unlike gene-specific assays, phenotypic assays interrogate multiple molecular targets and signaling pathways in a target "agnostic" fashion, which may reveal novel functions for well-studied proteins and discover new pathways of therapeutic value. Significantly, existing compound libraries may not have sufficient chemical diversity to fully leverage a phenotypic strategy. To address this issue, Eli Lilly and Company launched the Phenotypic Drug Discovery Initiative (PD(2)), a model of open innovation whereby external research groups can submit compounds for testing in a panel of Lilly phenotypic assays. This communication describes the statistical validation, operations, and initial screening results from the first PD(2) assay panel. Analysis of PD(2) submissions indicates that chemical diversity from open source collaborations complements internal sources. Screening results for the first 4691 compounds submitted to PD(2) have confirmed hit rates from 1.6% to 10%, with the majority of active compounds exhibiting acceptable potency and selectivity. Phenotypic lead generation strategies, in conjunction with novel chemical diversity obtained via open-source initiatives such as PD(2), may provide a means to identify compounds that modulate biology by novel mechanisms and expand the innovation potential of drug discovery.

Novel small molecule glucagon-like peptide-1 receptor agonist stimulates insulin secretion in rodents and from human islets

OBJECTIVE

The clinical effectiveness of parenterally-administered glucagon-like peptide-1 (GLP-1) mimetics to improve glucose control in patients suffering from type 2 diabetes strongly supports discovery pursuits aimed at identifying and developing orally active, small molecule GLP-1 receptor agonists. The purpose of these studies was to identify and characterize novel nonpeptide agonists of the GLP-1 receptor.

RESEARCH DESIGN AND METHODS

Screening using cells expressing the GLP-1 receptor and insulin secretion assays with rodent and human islets were used to identify novel molecules. The intravenous glucose tolerance test (IVGTT) and hyperglycemic clamp characterized the insulinotropic effects of compounds in vivo.

RESULTS

Novel low molecular weight pyrimidine-based compounds that activate the GLP-1 receptor and stimulate glucose-dependent insulin secretion are described. These molecules induce GLP-1 receptor-mediated cAMP signaling in HEK293 cells expressing the GLP-1 receptor and increase insulin secretion from rodent islets in a dose-dependent manner. The compounds activate GLP-1 receptor signaling, both alone or in an additive fashion when combined with the endogenous GLP-1 peptide; however, these agonists do not compete with radiolabeled GLP-1 in receptor-binding assays. In vivo studies using the IVGTT and the hyperglycemic clamp in Sprague Dawley rats demonstrate increased insulin secretion in compound-treated animals. Further, perifusion assays with human islets isolated from a donor with type 2 diabetes show near-normalization of insulin secretion upon compound treatment.

CONCLUSIONS

These studies characterize the insulinotropic effects of an early-stage, small molecule GLP-1 receptor agonist and provide compelling evidence to support pharmaceutical optimization.

Identifying the targets of the amplifying pathway for insulin secretion in pancreatic beta-cells by kinetic modeling of granule exocytosis

A kinetic model for insulin secretion in pancreatic beta-cells is adapted from a model for fast exocytosis in chromaffin cells. The fusion of primed granules with the plasma membrane is assumed to occur only in the "microdomain" near voltage-sensitive L-type Ca(2+)-channels, where [Ca(2+)] can reach micromolar levels. In contrast, resupply and priming of granules are assumed to depend on the cytosolic [Ca(2+)]. Adding a two-compartment model to handle the temporal distribution of Ca(2+) between the microdomain and the cytosol, we obtain a unified model that can generate both the fast granule fusion and the slow insulin secretion found experimentally in response to a step of membrane potential. The model can simulate the potentiation induced in islets by preincubation with glucose and the reduction in second-phase insulin secretion induced by blocking R-type Ca(2+)-channels (Ca(V)2.3). The model indicates that increased second-phase insulin secretion induced by the amplifying signal is controlled by the "resupply" step of the exocytosis cascade. In contrast, enhancement of priming is a good candidate for amplification of first-phase secretion by glucose, cyclic adenosine 3':5'-cyclic monophosphate, and protein kinase C. Finally, insulin secretion is enhanced when the amplifying signal oscillates in phase with the triggering Ca(2+)-signal.

Synthesis and antidiabetic activity of 2,5-disubstituted-3-imidazol-2-yl-pyrrolo[2,3-b]pyridines and thieno[2,3-b]pyridines

In the present investigation, two series of 2,5-disubstituted-3-imidazol-2-yl-pyrrolo[2,3-b]pyridines (2a-l) and thieno[2,3-b]pyridines (3a-l) were designed as analogs of BL 11282 (1). The in vitro glucose dependent insulinotropic activity of all the test compounds was evaluated using RIN5F cell based assay and all the test compounds showed glucose and concentration dependent insulin secretion. The in vivo antidiabetic activities of most potent compounds from each series (2c and 3c) were assessed in C57BL/6J mice. Compounds 2c and 3c showed dose dependent insulin secretion and significant glucose reduction in vivo. In general, compounds 2c and 3c were found to be equipotent at all the three different doses selected and with respect to BL 11282, both the test compounds were found to be more potent, at all the time points.

Design, synthesis, and biological evaluation of substituted-N-(thieno[2,3-b]pyridin-3-yl)-guanidines, N-(1H-pyrrolo[2,3-b]pyridin-3-yl)-guanidines, and N-(1H-indol-3-yl)-guanidines

Sulfonylureas stimulate insulin secretion independent of the blood glucose concentration and therefore cause hypoglycemia in type 2 diabetic patients. Over the last years, a number of aryl-imidazoline derivatives have been identified that stimulate insulin secretion in a glucose-dependent manner. In the present study, we have developed three series of substituted N-(thieno[2,3-b]pyridin-3-yl)-guanidine (2a-l), N-(1H-pyrrolo[2,3-b]pyridin-3-yl)-guanidine (3a-l), and N-(1H-indol-3-yl)-guanidine (4a-l) as new class of antidiabetic agents. In vitro glucose-dependent insulinotropic activity of test compounds 2a-l, 3a-l, and 4a-l was evaluated using RIN5F (Rat Insulinoma cell) based assay. All the test compounds showed concentration-dependent insulin secretion, only in presence of glucose load (16.7mmol). Some of the test compounds (2c, 3c, and 4c) from each series were found to be equipotent to BL 11282 (standard aryl-imidazoline), which indicated that the guanidine group acts as a bioisostere of imidazoline ring system.

Fibroblast growth factor-21 improves pancreatic beta-cell function and survival by activation of extracellular signal-regulated kinase 1/2 and Akt signaling pathways

Fibroblast growth factor-21 (FGF-21) is a recently discovered metabolic regulator. Here, we investigated the effects of FGF-21 in the pancreatic beta-cell. In rat islets and INS-1E cells, FGF-21 activated extracellular signal-regulated kinase 1/2 and Akt signaling pathways. In islets isolated from healthy rats, FGF-21 increased insulin mRNA and protein levels but did not potentiate glucose-induced insulin secretion. Islets and INS-1E cells treated with FGF-21 were partially protected from glucolipotoxicity and cytokine-induced apoptosis. In islets isolated from diabetic rodents, FGF-21 treatment increased islet insulin content and glucose-induced insulin secretion. Short-term treatment of normal or db/db mice with FGF-21 lowered plasma levels of insulin and improved glucose clearance compared with vehicle after oral glucose tolerance testing. Constant infusion of FGF-21 for 8 weeks in db/db mice nearly normalized fed blood glucose levels and increased plasma insulin levels. Immunohistochemistry of pancreata from db/db mice showed a substantial increase in the intensity of insulin staining in islets from FGF-21-treated animals as well as a higher number of islets per pancreas section and of insulin-positive cells per islet compared with control. No effect of FGF-21 was observed on islet cell proliferation. In conclusion, preservation of beta-cell function and survival by FGF-21 may contribute to the beneficial effects of this protein on glucose homeostasis observed in diabetic animals.

Sterol regulatory element-binding protein 1 mediates liver X receptor-beta-induced increases in insulin secretion and insulin messenger ribonucleic acid levels

Liver X receptors (LXRalpha and LXRbeta) regulate glucose and lipid metabolism. Pancreatic beta-cells and INS-1E insulinoma cells express only the LXRbeta isoform. Activation of LXRbeta with the synthetic agonist T0901317 increased glucose-induced insulin secretion and insulin content, whereas deletion of the receptor in LXRbeta knockout mice severely blunted insulin secretion. Analysis of gene expression in LXR agonist-treated INS-1E cells and islets from LXRbeta-deficient mice revealed that LXRbeta positively regulated expression of ATP-binding cassette transporter A1 (ABCA1), sterol regulatory element-binding protein 1 (SREBP-1), insulin, PDX-1, glucokinase, and glucose transporter 2 (Glut2). Down-regulation of SREBP-1 expression with the specific small interfering RNA blocked basal and LXRbeta-induced expression of pancreatic duodenal homeobox 1 (PDX-1), insulin, and Glut2 genes. SREBP-1 small interfering RNA also prevented an increase in insulin secretion and insulin content induced by T0901317. Moreover, 5-(tetradecyloxy)-2-furoic acid, an inhibitor of the SREBP-1 target gene acetyl-coenzyme A carboxylase, blocked T0901317-induced stimulation of insulin secretion. In conclusion, activation of LXRbeta in pancreatic beta-cells increases insulin secretion and insulin mRNA expression via SREBP-1-regulated pathway. These data support the role of LXRbeta, SREBP-1, and cataplerosis/anaplerosis pathways in the control of insulin secretion in pancreatic beta-cells.

Dipeptidyl peptidase inhibitors as new drugs for the treatment of type 2 diabetes

Inhibitors of the regulatory protease dipeptidyl peptidase-IV (DPP-IV) are currently under development in preclinical and clinical studies (several pharmaceutical companies, now in Phase III) as potential drugs for the treatment of type 2 diabetes. Their development is based on the observation that DPP-IV rapidly inactivates the incretin hormone glucagon-like peptide-1 (GLP-1), which is released postprandially from the gut and increases insulin secretion. DPP-IV inhibitors stabilise endogenous GLP-1 at physiological concentrations, and induce insulin secretion in a glucose-dependent manner; therefore, they do not demonstrate any hypoglycaemic effects. Furthermore, they are orally bioavailable. In addition to their ability to protect GLP-1 against degradation, DPP-IV inhibitors also stabilise other incretins, including gastric inhibitory peptide and pituitary adenylate cyclase-activating peptide. They also reduce the antagonistic and desensitising effects of the fragments formed by truncation of the incretins. In clinical studies, when used for the treatment of diabetes over a 1-year period, DPP-IV inhibitors show improved efficacy over time. This finding can be explained by a GLP-1-induced increase in the number of beta cells. Potential risks associated with DPP-IV inhibitors include the prolongation of the action of other peptide hormones, neuropeptides and chemokines cleaved by the protease, and their interaction with DPP-IV-related proteases. Based on their mode of action, DPP-IV inhibitors seem to be of particular value in early forms of type 2 diabetes, either alone or in combination with other types of oral agents.