Effects of protein kinase C inhibitors on insulin secretory responses from rodent pancreatic islets

Diacylglycerol-evoked activation of PKC and PKD isoforms in regulation of glucose and lipid metabolism: a review

Protein kinase C (PKC) and Protein kinase D (PKD) isoforms can sense diacylglycerol (DAG) generated in the different cellular compartments in various physiological processes. DAG accumulates in multiple organs of the obese subjects, which leads to the disruption of metabolic homeostasis and the development of diabetes as well as associated diseases. Multiple studies proved that aberrant activation of PKCs and PKDs contributes to the development of metabolic diseases. DAG-sensing PKC and PKD isoforms play a crucial role in the regulation of metabolic homeostasis and therefore might serve as targets for the treatment of metabolic disorders such as obesity and diabetes.

Dual effect of reduced type I diacylglycerol kinase activity on insulin secretion from MIN6 β-cells

The role of type I diacylglycerol kinases (DGKs) in the regulation of insulin secretion was investigated in MIN6 β-cells. In intracellular Ca²⁺ concentration ([Ca²⁺]_i) measurement experiments, 1 μM R59949, a type I DGK inhibitor, and 10 μM DiC₈, a diacylglycerol (DAG) analog, amplified 22.2 mM glucose-induced [Ca²⁺]_i oscillations in a protein kinase C (PKC)-dependent manner, whereas 10 μM R59949 and 100 μM DiC₈ decreased [Ca²⁺]_i independent of PKC. High concentrations of R59949 and DiC₈ attenuated voltage-dependent Ca²⁺ channel currents. According to these results, 22.2 mM glucose-stimulated insulin secretion (GSIS) was potentiated by 1 μM R59949 but suppressed by 10 μM of the same. The DGKα inhibitor R59022 showed a similar dual effect. Conversely, DiC₈ at 10 and 100 μM potentiated GSIS, although 100 μM DiC₈ decreased [Ca²⁺]_i. These results suggest that DAG accumulated through declined type I DGK activity shows a dual effect on insulin secretion depending on the degree of accumulation; a mild DAG accumulation induces a PKC-dependent stimulatory effect on insulin secretion, whereas an excessive DAG accumulation suppresses it in a PKC-independent manner, possibly via attenuation of VDCC activity.

Nutritional recovery from a low-protein diet during pregnancy does not restore the kinetics of insulin secretion and Ca2+ or alterations in the cAMP/PKA and PLC/PKC pathways in islets from adult rats

We investigated the insulin release induced by glucose, the Ca²⁺ oscillatory pattern, and the cyclic AMP (cAMP)/protein kinase A (PKA) and phospholipase C (PLC)/protein kinase C (PKC) pathways in islets from adult rats that were reared under diets with 17% protein (C) or 6% protein (LP) during gestation, suckling, and after weaning and in rats receiving diets with 6% protein during gestation and 17% protein after birth (R). First-phase glucose-induced insulin secretion was reduced in LP and R islets, and the second phase was partially restored in the R group. Glucose stimulation did not modify intracellular Ca²⁺ concentration, but it reduced the Ca²⁺ oscillatory frequency in the R group compared with the C group. Intracellular cAMP concentration was higher and PKA-Cα expression was lower in the R and LP groups compared with the C group. The PKCα content in islets from R rats was lower than that in C and LP rats. Thus, nutritional recovery from a low-protein diet during fetal life did not repair the kinetics of insulin release, impaired Ca²⁺ handling, and altered the cAMP/PKA and PLC/PKC pathways.

Lupin seed hydrolysate promotes G-protein-coupled receptor, intracellular Ca2+ and enhanced glycolytic metabolism-mediated insulin secretion from BRIN-BD11 pancreatic beta cells

Lupin seed proteins have been reported to exhibit hypoglycaemic effects in animals and humans following oral administration, however little is known about its mechanism of action. This study investigated the signalling pathway(s) responsible for the insulinotropic effect of the hydrolysate obtained from lupin (Lupinus angustifolius L.) seed extracts utilizing BRIN-BD11 β-cells. The extract was treated with digestive enzymes to give a hydrolysate rich in biomolecules ≤7 kDa. Cells exhibited hydrolysate induced dose-dependent stimulation of insulin secretion and enhanced intracellular Ca²⁺ and glucose metabolism. The stimulatory effect of the hydrolysate was potentiated by depolarizing concentrations of KCl and was blocked by inhibitors of the ATP sensitive K⁺ channel, Gα_q protein, phospholipase C (PLC) and protein kinase C (PKC). These findings reveal a novel mechanism for lupin hydrolysate stimulated insulin secretion via Gα_q mediated signal transduction (Gα_q/PLC/PKC) in the β-cells. Thus, lupin hydrolysates may have potential for nutraceutical treatment in type 2 diabetes.

Mechanisms of octanoic acid potentiation of insulin secretion in isolated islets

A potentiating effect of medium-chain triglycerides on glucose-stimulated insulin secretion (GSIS) has been observed since the 1960s. Subsequent observations identified octanoic acid (OA), the main component of medium-chain triglyceride, as the potentiator of GSIS, but the mechanism was unclear. We used wild-type (WT), short-chain 3-hydroxyacyl-CoA dehydrogenase knockout (Hadh^-/-), and sulfonylurea receptor 1 knockout (Sur1^-/-) mouse islets to define the mechanism of OA potentiation of insulin secretion. Application of OA alone induced a 2- to 3- fold increase of insulin secretion with an apparent threshold of 3 mM in WT mouse islets, suggesting that OA itself is a weak insulin secretagogue. However, OA at 1 mM strongly potentiated fuel-stimulated insulin secretion, especially GSIS. The potentiating effect on fuel-stimulated insulin secretion by OA did not require fatty acid β-oxidation because OA also potentiated amino acid-stimulated insulin secretion in islets isolated from Hadh^-/- mice, which cannot fully oxidize OA. Measurements using Sur1^-/- islets indicated that the potentiating effect of OA on fuel-stimulated insulin secretion is Ca²⁺ dependent and is often accompanied by β-cell membrane potential depolarization, and may also involve the Ca²⁺/calmodulin complex. Experiments using DCPIB, an ethacrynic acid derivative, to inhibit volume-sensitive anion channels (VSACs) in Sur1^-/- islets demonstrated that the potentiation effects of OA on insulin secretion are in part medicated by activation of VSAC. In addition, inhibition of IP3 receptor also abolishes the OA-induced intracellular Ca²⁺ increase in Sur1^-/- islets.

Feedback regulation of insulin secretion by extended synaptotagmin-1

Endoplasmic reticulum (ER)-plasma membrane (PM) contacts are dynamic structures with important roles in the regulation of calcium (Ca²⁺) and lipid homeostasis. The extended synaptotagmins (E-Syts) are ER-localized lipid transport proteins that interact with PM phosphatidylinositol 4,5-bisphosphate in a Ca²⁺-dependent manner. E-Syts bidirectionally transfer glycerolipids, including diacylglycerol (DAG), between the 2 juxtaposed membranes, but the biologic significance of this transport is still unclear. Using insulin-secreting cells and live-cell imaging, we now show that Ca²⁺-triggered exocytosis of insulin granules is followed, in sequence, by PM DAG formation and E-Syt1 recruitment. E-Syt1 counteracted the depolarization-induced DAG formation through a mechanism that required both voltage-dependent Ca²⁺ influx and Ca²⁺ release from the ER. E-Syt1 knockdown resulted in prolonged accumulation of DAG in the PM, resulting in increased glucose-stimulated insulin secretion. We conclude that Ca²⁺-triggered exocytosis is temporally coupled to Ca²⁺-triggered E-Syt1 PM recruitment and removal of DAG to negatively regulate the same process.-Xie, B., Nguyen, P. M., Idevall-Hagren, O. Feedback regulation of insulin secretion by extended synaptotagmin-1.

Regulation of insulin exocytosis by calcium-dependent protein kinase C in beta cells

The control of insulin release from pancreatic beta cells helps ensure proper blood glucose level, which is critical for human health. Protein kinase C has been shown to be one key control mechanism for this process. After glucose stimulation, calcium influx into beta cells triggers exocytosis of insulin-containing dense-core granules and activates protein kinase C via calcium-dependent phospholipase C-mediated generation of diacylglycerol. Activated protein kinase C potentiates insulin release by enhancing the calcium sensitivity of exocytosis, likely by affecting two main pathways that could be linked: (1) the reorganization of the cortical actin network, and (2) the direct phosphorylation of critical exocytotic proteins such as munc18, SNAP25, and synaptotagmin. Here, we review what is currently known about the molecular mechanisms of protein kinase C action on each of these pathways and how these effects relate to the control of insulin release by exocytosis. We identify remaining challenges in the field and suggest how these challenges might be addressed to advance our understanding of the regulation of insulin release in health and disease.

Coordinated activation of mitochondrial respiration and exocytosis mediated by PKC signaling in pancreatic β cells

Mitochondria play a central role in pancreatic β-cell nutrient sensing by coupling their metabolism to plasma membrane excitability and insulin granule exocytosis. Whether non-nutrient secretagogues stimulate mitochondria as part of the molecular mechanism to promote insulin secretion is not known. Here, we show that PKC signaling, which is employed by many non-nutrient secretagogues, augments mitochondrial respiration in INS-1E (rat insulinoma cell line clone 1E) and human pancreatic β cells. The phorbol ester, phorbol 12-myristate 13-acetate, accelerates mitochondrial respiration at both resting and stimulatory glucose concentrations. A range of inhibitors of novel PKC isoforms prevent phorbol ester-induced respiration. Respiratory response was blocked by oligomycin that demonstrated PKC-dependent acceleration of mitochondrial ATP synthesis. Enhanced respiration was observed even when glycolysis was bypassed or fatty acid transport was blocked, which suggested that PKC regulates mitochondrial processes rather than upstream catabolic fluxes. A phosphoproteome study of phorbol ester-stimulated INS-1E cells maintained under resting (2.5 mM) glucose revealed a large number of phosphorylation sites that were altered during short-term activation of PKC signaling. The data set was enriched for proteins that are involved in gene expression, cytoskeleton remodeling, secretory vesicle transport, and exocytosis. Interactome analysis identified PKC, C-Raf, and ERK1/2 as the central phosphointeraction cluster. Prevention of ERK1/2 signaling by using a MEK1 inhibitor caused a marked decreased in phorbol 12-myristate 13-acetate-induced mitochondrial respiration. ERK1/2 signaling module therefore links PKC activation to downstream mitochondrial activation. We conclude that non-nutrient secretagogues act, in part, via PKC and downstream ERK1/2 signaling to stimulate mitochondrial energy production to compensate for energy expenditure that is linked to β-cell activation.-Santo-Domingo, J., Chareyron, I., Dayon, L., Galindo, A. N., Cominetti, O., Giménez, M. P. G., De Marchi, U., Canto, C., Kussmann, M., Wiederkehr, A. Coordinated activation of mitochondrial respiration and exocytosis mediated by PKC signaling in pancreatic β cells.

Islet Insulin Secretion Measurements in the Mouse

This article describes detailed protocols for in vitro measurements of insulin function and secretion in isolated mouse islets for the analysis of glucose homeostasis. We specify a method of enzyme digestion and hand picking to isolate and release the greatest number of high quality islets from the pancreas of the mouse. We describe an effective method for generating dynamic measurements of insulin secretion using a perifusion assay including a detailed protocol for constructing a peristaltic pump and tubing assembly. In addition we describe an alternative and simple technique for measuring insulin secretion using static incubation of isolated islets. © 2016 by John Wiley & Sons, Inc.

Autocrine Signaling Underlies Fast Repetitive Plasma Membrane Translocation of Conventional and Novel Protein Kinase C Isoforms in β Cells

PKC signaling has been implicated in the regulation of many cell functions, including metabolism, cell death, proliferation, and secretion. Activation of conventional and novel PKC isoforms is associated with their Ca²⁺- and/or diacylglycerol (DAG)-dependent translocation to the plasma membrane. In β cells, exocytosis of insulin granules evokes brief (<10 s) local DAG elevations (“spiking”) at the plasma membrane because of autocrine activation of P2Y₁ purinoceptors by ATP co-released with insulin. Using total internal reflection microscopy, fluorescent protein-tagged PKCs, and signaling biosensors, we investigated whether DAG spiking causes membrane recruitment of PKCs and whether different classes of PKCs show characteristic responses. Glucose stimulation of MIN6 cells triggered DAG spiking with concomitant repetitive translocation of the novel isoforms PKCδ, PKCϵ, and PKCη. The conventional PKCα, PKCβI, and PKCβII isoforms showed a more complex pattern with both rapid and slow translocation. K⁺ depolarization-induced PKCϵ translocation entirely mirrored DAG spiking, whereas PKCβI translocation showed a sustained component, reflecting the subplasma membrane Ca²⁺ concentration ([Ca²⁺]_pm), with additional effect during DAG spikes. Interference with DAG spiking by purinoceptor inhibition prevented intermittent translocation of PKCs and reduced insulin secretion but did not affect [Ca²⁺]_pm elevation or sustained PKCβI translocation. The muscarinic agonist carbachol induced pronounced transient PKCβI translocation and sustained recruitment of PKCϵ. When rise of [Ca²⁺]_pm was prevented, the carbachol-induced DAG and PKCϵ responses were somewhat reduced, but PKCβI translocation was completely abolished. We conclude that exocytosis-induced DAG spikes efficiently recruit both conventional and novel PKCs to the β cell plasma membrane. PKC signaling is thus implicated in autocrine regulation of β cell function.

In vitro effects of bis(1,2-dimethyl-3-hydroxy-4-pyridinonato)oxidovanadium(IV), or VO(dmpp)2, on insulin secretion in pancreatic islets of type 2 diabetic Goto-Kakizaki rats

Vanadium compounds have been explored as therapy of diabetes, and most studies have focussed on insulin mimetic effects, i.e. reducing hyperglycemia by improving glucose sensitivity and thus glucose uptake in sensitive tissues. We have recently shown that bis(1,2-dimethyl-3-hydroxy-4-pyridinonato)oxidovanadium(IV), VO(dmpp)2, has promising effects when compared to another vanadium compound, bis(maltolato)oxidovanadium(IV), BMOV, and insulin itself, in isolated adipocytes and in vivo in Goto-Kakizaki (GK) rats, an animal model of hereditary type 2 diabetes (T2D).We now have investigated in GK rats whether VO(dmpp)2 also modulates another important defect in T2D, impaired insulin secretion. VO(dmpp)2, but not BMOV, stimulated insulin secretion from isolated GK rat pancreatic islets at high, 16.7mM, but not at low–normal, 3.3 mM, glucose concentration. Mechanistic studies demonstrate that the insulin releasing effect of VO(dmpp)2 is due to its interaction with several steps in the stimulus-secretion coupling for glucose, including islet glucose metabolism and K-ATP channels, L-type Ca2+ channels, modulation by protein kinases A and C, as well as the exocytotic machinery. In conclusion, VO(dmpp)2 exhibits properties of interest for treatment of the insulin secretory defect in T2D, in addition to its well-described insulin mimetic activity.

Amplification of insulin secretion by acetylcholine or phorbol ester is independent of β-cell microfilaments and distinct from metabolic amplification

Insulin secretion (IS) triggered by β-cell [Ca(2+)](c) is amplified by metabolic and receptor-generated signals. Diacylglycerol largely mediates acetylcholine (ACh) effects through protein-kinase C and other effectors, which can be directly activated by phorbol-ester (PMA). Using mouse islets, we investigated the possible role of microfilaments in ACh/PMA-mediated amplification of IS. PMA had no steady-state impact on actin microfilaments. Although ACh slightly augmented and PMA diminished glucose- and tolbutamide-induced increases in β-cell [Ca(2+)](c), both amplified IS in control islets and after microfilament disruption (latrunculin) or stabilization (jasplakinolide). Both phases of IS were larger in response to glucose than tolbutamide, although [Ca(2+)](c) was lower. This difference in secretion, which reflects metabolic amplification, persisted in presence of ACh/PMA and was independent of microfilaments. Amplification of IS by ACh/PMA is thus distinct from metabolic amplification, but both pathways promote acquisition of release competence by insulin granules, which can access exocytotic sites without intervention of microfilaments.

Nuciferine stimulates insulin secretion from beta cells-an in vitro comparison with glibenclamide

ETHNOPHARMACOLOGICAL RELEVANCE

Several Asian plants are known for their anti-diabetic properties and produce alkaloids and flavonoids that may stimulate insulin secretion.

MATERIALS AND METHODS

Using Vietnamese plants (Nelumbo nucifera, Gynostemma pentaphyllum, Smilax glabra, and Stemona tuberosa), we extracted two alkaloids (neotuberostemonine, nuciferine) and four flavonoids (astilbin, engeletin, smitilbin, and 3,5,3'-trihydroxy-7,4'-dimethoxyflavone), and studied their insulin stimulatory effects.

RESULTS

Nuciferine, extracted from Nelumbo nucifera, stimulated both phases of insulin secretion in isolated islets, whereas the other compounds had no effect. The effect of nuciferine was totally abolished by diazoxide and nimodipine, and diminished by protein kinase A and protein kinase C inhibition. Nuciferine and potassium had additive effects on insulin secretion. Nuciferine also stimulated insulin secretion in INS-1E cells at both 3.3 and 16.7 mM glucose concentrations. Compared with glibenclamide, nuciferine had a stronger effect on insulin secretion and less beta-cell toxicity. However, nuciferine did not compete with glibenclamide for binding to the sulfonylurea receptor.

CONCLUSIONS

Among several compounds extracted from anti-diabetic plants, nuciferine was found to stimulate insulin secretion by closing potassium-adenosine triphosphate channels, explaining anti-diabetic effects of Nelumbo nucifera.

Detailed protocol for evaluation of dynamic perifusion of human islets to assess β-cell function

The definitive measure of β-cell quality in an islet is the measurement of β-cell function, i.e., the ability of the islets to release insulin in a controlled manner in response to minute changes in ambient glucose levels. Continuous flow or dynamic perifusion of the solution containing glucose and secretagogues through the islets is the most accurate assessment of regulated insulin release in vitro. Here, we describe in detail a low cost, mini-perifusion system that can be adapted to any laboratory to assess islet function by examining dynamic insulin release in response to elevated glucose concentrations and addition of secretagogues. Human islets with purity > 80% and viability > 90% were perifused with low glucose (1 mM) and subsequently challenged with high glucose (16.8 mM ± KCl, 25 mM). A prototypical biphasic response to elevated glucose concentrations was observed with an average 8-fold (above basal) increase in insulin concentration at peak values. Similarly, perifusion with carbachol or exendin-4 (Byetta) with glucose (6 mM) resulted in 1.32- and 1.35-fold increase in insulin secretion above basal. Islets could be maintained in the perifusion apparatus and continued to respond to glucose for up to 3 h. At minimal financial cost and technical expertise, this apparatus can be set-up in any biological laboratory to evaluate regulated hormone release from many cell types in less than 6 h. This will allow other laboratories to measure insulin responses to their drug or modifier of interest in vitro, in a manner that better approximates islet function in vivo.

Identification of a small molecule activator of novel PKCs for promoting glucose-dependent insulin secretion

Using an image-based screen for small molecules that can affect Golgi morphology, we identify a small molecule, Sioc145, which can enlarge the Golgi compartments and promote protein secretion. More importantly, Sioc145 potentiates insulin secretion in a glucose-dependent manner. We show that Sioc145 selectively activates novel protein kinase Cs (nPKCs; δ and ɛ) but not conventional PKCs (cPKCs; α, βI and βII) in INS-1E insulinoma cells. In contrast, PMA, a non-selective activator of cPKCs and nPKCs, promotes insulin secretion independent of glucose concentrations. Furthermore, we demonstrate that Sioc145 and PMA show differential abilities in depolarizing the cell membrane, and suggest that Sioc145 promotes insulin secretion in the amplifying pathway downstream of K(ATP) channels. In pancreatic islets, the treatment with Sioc145 enhances the second phase of insulin secretion. Increased insulin granules close to the plasma membrane are observed after Sioc145 treatment. Finally, the administration of Sioc145 to diabetic GK rats increases their serum insulin levels and improves glucose tolerance. Collectively, our studies identify Sioc145 as a novel glucose-dependent insulinotropic compound via selectively activating nPKCs.

Microfluidic device for multimodal characterization of pancreatic islets

A microfluidic device to perfuse pancreatic islets while simultaneously characterizing their functionality through fluorescence imaging of the mitochondrial membrane potential and intracellular calcium ([Ca(2+)](i)) in addition to enzyme linked immunosorbent assay (ELISA) quantification of secreted insulin was developed and characterized. This multimodal characterization of islet function will facilitate rapid assessment of tissue quality immediately following isolation from donor pancreas and allow more informed transplantation decisions to be made which may improve transplantation outcomes. The microfluidic perfusion chamber allows flow rates of up to 1 mL min(-1), without any noticeable perturbation or shear of islets. This multimodal quantification was done on both mouse and human islets. The ability of this simple microfluidic device to detect subtle variations in islet responses in different functional assays performed in short time-periods demonstrates that the microfluidic perfusion chamber device can be used as a new gold standard to perform comprehensive islet analysis and obtain a more meaningful predictive value for islet functionality prior to transplantation into recipients, which is currently difficult to predict using a single functional assay.

The physiology of rodent beta-cells in pancreas slices

Beta-cells in pancreatic islets form complex syncytia. Sufficient cell-to-cell electrical coupling seems to ensure coordinated depolarization pattern and insulin release that can be further modulated by rich innervation. The complex structure and coordinated action develop after birth during fast proliferation of the endocrine tissue. These emergent properties can be lost due to various reasons later in life and can lead to glucose intolerance and diabetes mellitus. Pancreas slice is a novel method of choice to study the physiology of beta-cells still embedded in their normal cellulo-social context. I present major advantages, list drawbacks and provide an overview on recent advances in our understanding of the physiology of beta-cells using the pancreas slice approach.

The diverse roles of protein kinase C in pancreatic beta-cell function

Members of the serine/threonine PKC (protein kinase C) family perform diverse functions in multiple cell types. All members of the family are activated in signalling cascades triggered by occupation of cell surface receptors, but the cPKC (conventional PKC) and nPKC (novel PKC) isoforms are also responsive to fatty acid metabolites. PKC isoforms are involved in various aspects of pancreatic beta-cell function, including cell proliferation, differentiation and death, as well as regulation of secretion in response to glucose and muscarinic receptor agonists. Recently, the nPKC isoform, PKCepsilon, has also been implicated in the loss of insulin secretory responsiveness that underpins the development of Type 2 diabetes.

Multiple kinases regulate mafA expression in the pancreatic beta cell line MIN6

MafA is a basic leucine zipper transcription factor expressed within the beta cells of the pancreas and is required to maintain normal glucose homeostasis as it is involved in various aspects of beta cell biology. MafA protein levels are known to increase in response to high glucose through mechanisms that have yet to be fully characterized. We investigated whether discrete intracellular signaling events control mafA expression. We found that the general kinase inhibitor staurosporine induces mafA expression without altering the stability of the protein. Inhibition of the MAP-kinase JNK mimics the effects of staurosporine on the expression of mafA. Calmodulin kinase and calcium signaling are also important in stimulating mafA expression by high glucose. However, staurosporine, JNK, and calmodulin kinase have different effects on the induction of insulin expression. These data reveal that MafA levels are tightly controlled by the coordinated action of multiple kinase pathways.

Enhanced activation of phospholipase C and insulin secretion from islets incubated in fatty acid-free bovine serum albumin

Incubation in 100 micromol/L fatty acid-free bovine serum albumin (FAF-BSA) significantly amplifies insulin secretion from isolated, perifused rat islets. When compared with the responses of control islets incubated in 100 micromol/L radioimmunoassay-grade BSA, insulin secretion rates were increased 2- to 3-fold when these islets were stimulated with 10 mmol/L glucose alone or with the combination of 10 mmol/L glucose, 15 mmol/L KCl, and 100 micromol/L diazoxide. These amplified secretory responses were paralleled by significant increases in the phospholipase C (PLC) activation monitored by fractional increases in (3)H-inositol efflux from these same islets. Amplified PLC responses were also observed with the cholinergic agonist carbachol (50 micromol/L). No differences in the secretory responses to the protein kinase C activator phorbol 12-myristate 13-acetate (200 nmol/L) could be detected between control and FAF-BSA-pretreated rat islets. Mouse islets were also immune to the amplifying impact of this treatment protocol. These findings demonstrate that short-term incubation in FAF-BSA significantly augments the activation of PLC in rat islets by a number of agonists. This proximal event provides the impetus for the distal activation of protein kinase C. If applicable to human islets, this manipulation may provide a mechanism to enhance the secretory responses from islets destined for transplantation, thus improving their in vivo secretory capacity.

Ca2+-secretion coupling is impaired in diabetic Goto Kakizaki rats

The Goto Kakizaki (GK) rat is a widely used animal model to study defective glucose-stimulated insulin release in type-2 diabetes (T2D). As in T2D patients, the expression of several proteins involved in Ca(2+)-dependent exocytosis of insulin-containing large dense-core vesicles is dysregulated in this model. So far, a defect in late steps of insulin secretion could not be demonstrated. To resolve this apparent contradiction, we studied Ca(2+)-secretion coupling of healthy and GK rat beta cells in acute pancreatic tissue slices by assessing exocytosis with high time-resolution membrane capacitance measurements. We found that beta cells of GK rats respond to glucose stimulation with a normal increase in the cytosolic Ca(2+) concentration. During trains of depolarizing pulses, the secretory activity from GK rat beta cells was defective in spite of upregulated cell size and doubled voltage-activated Ca(2+) currents. In GK rat beta cells, evoked Ca(2+) entry was significantly less efficient in triggering release than in nondiabetic controls. This impairment was neither due to a decrease of functional vesicle pool sizes nor due to different kinetics of pool refilling. Strong stimulation with two successive trains of depolarizing pulses led to a prominent activity-dependent facilitation of release in GK rat beta cells, whereas secretion in controls was unaffected. Broad-spectrum inhibition of PKC sensitized Ca(2+)-dependent exocytosis, whereas it prevented the activity-dependent facilitation in GK rat beta cells. We conclude that a decrease in the sensitivity of the GK rat beta-cell to depolarization-evoked Ca(2+) influx is involved in defective glucose-stimulated insulin secretion. Furthermore, we discuss a role for constitutively increased activity of one or more PKC isoenzymes in diabetic rat beta cells.

Protein Kinase Cδ Plays a Non-redundant Role in Insulin Secretion in Pancreatic β Cells

Protein kinase C (PKC) is considered to modulate glucose-stimulated insulin secretion. Pancreatic beta cells express multiple isoforms of PKCs; however, the role of each isoform in glucose-stimulated insulin secretion remains controversial. In this study we investigated the role of PKCdelta, a major isoform expressed in pancreatic beta cells on beta cell function. Here, we showed that PKCdelta null mice manifested glucose intolerance with impaired insulin secretion. Insulin tolerance test showed no decrease in insulin sensitivity in PKCdelta null mice. Studies using islets isolated from these mice demonstrated decreased glucose- and KCl-stimulated insulin secretion. Perifusion studies indicated that mainly the second phase of insulin secretion was decreased. On the other hand, glucose-induced influx of Ca2+ into beta cells was not altered. Immunohistochemistry using total internal reflection fluorescence microscopy and electron microscopic analysis showed an increased number of insulin granules close to the plasma membrane in beta cells of PKCdelta null mice. Although PKC is thought to phosphorylate Munc18-1 and facilitate soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors complex formation, the phosphorylation of Munc18-1 by glucose stimulation was decreased in islets of PKCdelta null mice. We conclude that PKCdelta plays a non-redundant role in glucose-stimulated insulin secretion. The impaired insulin secretion in PKCdelta null mice is associated with reduced phosphorylation of Munc18-1.

Resveratrol inhibits insulin secretion from rat pancreatic islets

Resveratrol is a naturally occurring phytoalexin exerting cardioprotective, anticancer and antioxidant action. The most recent investigations have demonstrated that this compound plays a beneficial role alleviating some diabetic complications. However, resveratrols' influence on the endocrine function of the pancreas is unknown. The objective of the present study was to determine whether resveratrol affects insulin secretion from freshly isolated rat pancreatic islets. Incubations of pancreatic islets with resveratrol (1-100 microM, 90 min) revealed that the release of insulin induced by 6.6 and 16.6 mM glucose was substantially restricted by this compound in a concentration-dependent manner. This effect was not permanent and disappeared after resveratrol withdrawal from the buffer. However, the proper hormone secretion was not restored when glucose was replaced by other secretagogues - leucine with glutamine - indicating that disturbances other than the inhibition of glucose transport and glycolysis were responsible for the resveratrol-evoked reduction in insulin secretion. Glucose-induced insulin release tested in the presence of the sulfonylurea glibenclamide was also found to be reduced by resveratrol. Moreover, the activation of adenylyl cyclase by forskolin did not restrict the inhibitory effect of resveratrol on glucose-induced insulin release. In contrast, phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C, suppressed completely the inhibitory effect of 1 and 10 microM resveratrol on insulin release. However, this compound at the highest concentration tested diminished its secretion even in the presence of PMA. The perifusion studies revealed that the depression of insulin release caused by resveratrol began a few minutes after its addition to the medium. Results obtained in the present investigations demonstrate that resveratrol is a compound exerting a clear-cut, but reversible inhibitory effect on insulin secretion from isolated pancreatic islets.

Ca2+ microdomains and the control of insulin secretion

Nutrient-induced increases in intracellular free Ca(2+) concentrations are the key trigger for insulin release from pancreatic islet beta-cells. These Ca(2+) changes are tightly regulated temporally, occurring as Ca(2+) influx-dependent baseline oscillations. We explore here the concept that locally high [Ca(2+)] concentrations (i.e. Ca(2+) microdomains) may control exocytosis via the recruitment of key effector proteins to sites of exocytosis. Importantly, recent advances in the development of organelle- and membrane-targeted green fluorescent protein (GFP-) or aequorin-based Ca(2+) indicators, as well as in rapid imaging techniques, are providing new insights into the potential role of these Ca(2+) microdomains in beta-cells. We summarise here some of the evidence indicating that Ca(2+) microdomains beneath the plasma membrane and at the surface of large dense core vesicles may be important in the normal regulation of insulin secretion, and may conceivably contribute to "ATP-sensitive K(+)-channel independent" effects of glucose. We also discuss evidence that, in contrast to certain non-excitable cells, direct transfer of Ca(2+) from the ER to mitochondria via localised physical contacts between these organelles is relatively less important for efficient mitochondrial Ca(2+) uptake in beta-cells. Finally, we discuss evidence from single cell imaging that increases in cytosolic Ca(2+) are not required for the upstroke of oscillations in mitochondrial redox state, but may underlie the reoxidation process.

Munc13-1 is required for the sustained release of insulin from pancreatic beta cells

Munc13-1 is a presynaptic protein that is essential for synaptic vesicle priming. Deletion of Munc13-1/unc13 causes total arrest of synaptic transmission due to a complete loss of fusion-competent synaptic vesicles. The requirement of Munc13-1 for large dense-core vesicles (LDCVs), however, has not been established. In the present study, we use Munc13-1 knockout (KO) and diacylglycerol (DAG) binding-deficient Munc13-1(H567K) mutant knockin (KI) mice to determine the role of Munc13-1 in the secretion of insulin-containing LDCVs from primary cultured pancreatic beta cells. We show that Munc13-1 is required for the sustained insulin release upon prolonged stimulation. The sustained release involves signaling of DAG second messenger, since it is also reduced in KI mice. Insulin secretion in response to glucose stimulation is characterized by a biphasic time course. Our data show that Munc13-1 plays an essential role in the development of the second phase of insulin secretion by priming insulin-containing LDCVs.

Low-protein diets reduce PKAalpha expression in islets from pregnant rats

We investigated the effect of protein restriction on insulin secretion and the expression of protein kinase (PK)Aalpha and PKCalpha in islets from control and pregnant rats. Adult control nonpregnant (CN) and control pregnant (CP) rats were fed a normal-protein diet (17%), whereas low-protein nonpregnant (LPN) and low-protein pregnant (LPP) rats were fed a low-protein diet (6%) for 15 d. In the presence of 2.8 and 8.3 mmol glucose/L, insulin secretion by islets of CP rats was higher than that by islets of CN rats. Compared with the CN groups, insulin secretion by islets of LPN rats was lower with 8.3 but not with 2.8 mmol glucose/L. The insulin secretion by islets of LPP rats was higher than by LPN rats at both glucose concentrations. IBMX (1 mmol/L), a phosphodiesterase inhibitor, increased insulin secretion by islets from pregnant rats, and this effect was greater in islets of CP rats than in LPP rats. Forskolin (0.01-100 micromol/L), a stimulator of adenylyl cyclase, increased insulin secretion only in islets of CN and CP rats, with a higher 50% effective concentration in islets of CP rats compared with CN rats. The insulin secretion induced by phorbol 12-myristate 13-acetate (a stimulator of PKC) was higher in islets of LPN and LPP rats than in the respective controls, especially at 8.3 mmol glucose/L. PKAalpha, but not PKCalpha, expression was lower in islets of rats fed low protein than in the controls, regardless of the physiological status of the rats. All endocrine cells of the islets, including beta-cells, expressed the PKAalpha isoform. The cytoplasmic distribution of this enzyme in beta-cells was not modified by pregnancy and/or protein restriction. In conclusion, our results indicate that the response of islets from rats fed low protein during pregnancy is similar to that of control rats, at least for physiologic glucose concentration. However, the decreased response to IBMX and forskolin indicates decreased production and/or sensitivity to cAMP; this was associated with a decrease in PKA expression, which may result in lower PKA activity.

Involvement of novel protein kinase C isoforms in carbachol-stimulated insulin secretion from rat pancreatic islets

The roles of protein kinase C (PKC) isoforms in cholinergic potentiation of glucose-induced insulin secretion were investigated in rat pancreatic islets. Western-blot analysis showed the presence of PKC-alpha, betaII, delta, epsilon, eta, and zeta, but not PKC-betaI, gamma, or iota, in the islets. Carbachol (CCh) caused translocations of PKC-alpha, betaII, delta, and epsilon from the cytosol to the plasma membrane. CCh facilitated 7-mM glucose-induced insulin secretion from isolated rat islets. The CCh-stimulated insulin secretion was significantly suppressed by the generic PKC inhibitor chelerythrine. In contrast, Go 6976, an inhibitor of conventional PKC isoforms, had no effect on the insulin secretion stimulated by CCh, although it significantly inhibited that induced by phorbol 12-myristate 13-acetate. These results suggest that the novel PKC isoforms activated by CCh, i.e., PKC-delta and/or epsilon, participate in the stimulatory effect of CCh on insulin secretion.

Massive augmentation of stimulated insulin secretion induced by fatty acid-free BSA in rat pancreatic islets

Incubation of rat pancreatic islets for 4-6 h with 100 micromol/l fatty acid-free BSA induced a 3- to 10-fold enhancement of insulin release to a subsequent challenge with 16.7 mmol/l glucose, without changing the typical biphasic pattern of the response. A similar enhancement was observed with other stimuli, such as leucine, depolarizing concentrations of KCl and tolbutamide, pointing to a general phenomenon and common mechanism for the augmentation. Norepinephrine completely blocked the stimulated response. The protein kinase C (PKC) inhibitor Ro 31-8220, which acts at the ATP-binding site and inhibits all PKC isoforms, strongly inhibited the enhancement of a subsequent glucose challenge when present during the BSA pretreatment period. In contrast, Go 6976, an inhibitor of conventional PKC isoforms, was without effect, even at the high concentration of 1 micromol/l. Preincubation with calphostin C, which competes for the diacylglycerol (DAG)-binding site, therefore inhibiting conventional, novel, and PKC isoforms of the PKD type, completely abolished the enhancing effect of the BSA but did not affect secretion in islets treated with 10 micromol/l fatty acid-free BSA. We conclude that the remarkable enhancement of insulin release is due to a change in glucose signaling and activation of a novel PKC isoform or a DAG-binding protein.

Bimodal role of conventional protein kinase C in insulin secretion from rat pancreatic beta cells

The present study was conducted to evaluate the role of conventional protein kinase C (PKC) in calcium-evoked insulin secretion. In rat beta cells transfected with green fluorescent protein-tagged PKC-alpha (PKC-alpha-EGFP), a depolarizing concentration of potassium induced transient elevation of cytoplasmic free calcium ([Ca(2)(+)](c)), which was accompanied by transient translocation of PKC-alpha-EGFP from the cytosol to the plasma membrane. Potassium also induced transient translocation of PKC-theta-EGFP, the C1 domain of PKC-gamma and PKC-epsilon-GFP. A high concentration of glucose induced repetitive elevation of [Ca(2)(+)](c) and repetitive translocation of PKC-alpha-EGFP. Diazoxide completely blocked both elevation of [Ca(2)(+)](c) and translocation of PKC-alpha-EGFP. We then studied the role of conventional PKC in calcium-evoked insulin secretion using rat islets. When islets were incubated for 10 min with high potassium, Go-6976, an inhibitor of conventional PKC, and PKC-alpha pseudosubstrate fused to antennapedia peptide (Antp-PKC(19-31)) increased potassium induced secretion. Similarly, insulin release induced by high glucose for 10 min was enhanced by Gö-6976 and Antp-PKC(19-31). However, when islets were stimulated for 60 min with high glucose, both Gö-6976 and Antp-PKC(19-31) reduced glucose-induced insulin secretion. Similar results were obtained by transfection of dominant-negative PKC-alpha using adenovirus vector. Taken together, PKC-alpha is activated when cells are depolarized by a high concentration of potassium or glucose. Conventional PKC is inhibitory on depolarization-induced insulin secretion per se, but it also augments glucose-induced secretion.

Double incretin receptor knockout (DIRKO) mice reveal an essential role for the enteroinsular axis in transducing the glucoregulatory actions of DPP-IV inhibitors

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) are gut-derived incretins that potentiate glucose clearance following nutrient ingestion. Elimination of incretin receptor action in GIPR(-/-) or GLP-1R(-/-) mice produces only modest impairment in glucose homeostasis, perhaps due to compensatory upregulation of the remaining incretin. We have now studied glucose homeostasis in double incretin receptor knockout (DIRKO) mice. DIRKO mice exhibit normal body weight and fail to exhibit an improved glycemic response after exogenous administration of GIP or the GLP-1R agonist exendin-4. Plasma glucagon and the hypoglycemic response to exogenous insulin were normal in DIRKO mice. Glycemic excursion was abnormally increased and levels of glucose-stimulated insulin secretion were decreased following oral but not intraperitoneal glucose challenge in DIRKO compared with GIPR(-/-) or GLP-1R(-/-) mice. Similarly, glucose-stimulated insulin secretion and the response to forskolin were well preserved in perifused DIRKO islets. Although the dipeptidyl peptidase-IV (DPP-IV) inhibitors valine pyrrolidide (Val-Pyr) and SYR106124 lowered glucose and increased plasma insulin in wild-type and single incretin receptor knockout mice, the glucose-lowering actions of DPP-IV inhibitors were eliminated in DIRKO mice. These findings demonstrate that glucose-stimulated insulin secretion is maintained despite complete absence of both incretin receptors, and they delineate a critical role for incretin receptors as essential downstream targets for the acute glucoregulatory actions of DPP-IV inhibitors.

PKC alpha is activated but not required during glucose-induced insulin secretion from rat pancreatic islets

The role of protein kinase C (PKC) in glucose-stimulated insulin secretion (GSIS) is controversial. Using recombinant adenoviruses for overexpression of PKC alpha and PKC delta, in both wild-type (WT) and kinase-dead (KD) forms, we here demonstrate that activation of these two PKCs is neither necessary nor sufficient for GSIS from batch-incubated, rat pancreatic islets. In contrast, responses to the pharmacologic activator 12-O-tetradecanoylphorbol-13-acetate (TPA) were reciprocally modulated by overexpression of the PKC alpha WT or PKC alpha KD but not the corresponding PKC delta adenoviruses. The kinetics of the secretory response to glucose (monitored by perifusion) were not altered in either cultured islets overexpressing PKC alpha KD or freshly isolated islets stimulated in the presence of the conventional PKC (cPKC) inhibitor Go6976. However, the latter did inhibit the secretory response to TPA. Using phosphorylation state-specific antisera for consensus PKC phosphorylation sites, we also showed that (compared with TPA) glucose causes only a modest and transient functional activation of PKC (maximal at 2-5 min). However, glucose did promote a prolonged (15 min) phosphorylation of PKC substrates in the presence of the phosphatase inhibitor okadaic acid. Overall, the results demonstrate that glucose does stimulate PKC alpha in pancreatic islets but that this makes little overall contribution to GSIS.

Acylation stimulating protein stimulates insulin secretion

Acylation stimulating protein (ASP) is a hormone produced by adipocytes and is of importance for the storage of energy as fat. We examined whether ASP might also have effects on islet function. In clonal INS-1 cells, ASP dose-dependently augmented glucose-stimulated insulin secretion. The lowest effective dose of ASP at 10 mmol/l glucose was 5 micro mol/l. The effect was glucose-dependent because ASP did not increase insulin secretion at 1 mmol/l glucose but had clear effect at 10 and 20 mmol/l glucose. Similarly, ASP augmented glyceraldehyde-induced insulin secretion but the hormone did not enhance insulin secretion in response to depolarization by 20 mmol/l of KCl. ASP-induced insulin secretion was completely abolished by competitive inhibition of glucose phosphorylation by glucokinase with 5-thio-glucose and was partially inhibited by the calcium channel blocker, nifedipine, and by the protein kinase C inhibitor, GF109203. Furthermore, thapsigargin, an inhibitor of Ca(2+)-ATPase in the endoplasmic reticulum, did not affect ASP-induced insulin secretion. ASP (>5 micro mol/l) also augmented glucose-stimulated insulin secretion from islets isolated from C57BL/6J mice, and intravenous administration of ASP (50 nmol/kg) augmented the acute (1 and 5 min) insulin response to intravenous glucose (1 g/kg) in C57BL/6J mice. This was accompanied by an increased rate of glucose disposal. Minimal model analyses of data derived from the intravenous glucose tolerance test revealed that whereas ASP augmented insulin secretion, the hormone did not affect insulin sensitivity (S(I)) or glucose effectiveness (S(G)). We conclude that ASP augments glucose-stimulated insulin secretion through a direct action on the islet beta cells. The effect is dependent on glucose phosphorylation, calcium uptake and protein kinase C. Stimulation of insulin secretion by ASP in vivo results in augmented glucose disposal.

Pharmacological agents that directly modulate insulin secretion

Blood glucose levels are sensed and controlled by the release of hormones from the islets of Langerhans in the pancreas. The beta-cell, the insulin-secreting cell in the islet, can detect subtle increases in circulating glucose levels and a cascade of molecular events spanning the initial depolarization of the beta-cell membrane culminates in exocytosis and optimal insulin secretion. Here we review these processes in the context of pharmacological agents that have been shown to directly interact with any stage of insulin secretion. Drugs that modulate insulin secretion do so by opening the K(ATP) channels, by interacting with cell-surface receptors, by altering second-messenger responses, by disrupting the beta-cell cytoskeletal framework, by influencing the molecular reactions at the stages of transcription and translation of insulin, and/or by perturbing exocytosis of the insulin secretory vesicles. Drugs acting primarily at the K(ATP) channels are the sulfonylureas, the benzoic acid derivatives, the imidazolines, and the quinolines, which are channel openers, and finally diazoxide, which closes these channels. Methylxanthines also work at the cell membrane level by antagonizing the purinergic receptors and thus increase insulin secretion. Other drugs have effects at multiple levels, such as the calcineurin inhibitors and somatostatin. Some drugs used extensively in research, e.g., colchicine, which is used to study vesicular transport, have no effect at the pharmacological doses used in clinical practice. We also briefly discuss those drugs that have been shown to disrupt beta-cell function in a clinical setting but for which there is scant information on their mechanism of action.

Dynamics of glucose-induced membrane recruitment of protein kinase C beta II in living pancreatic islet beta-cells

The mechanisms by which glucose may affect protein kinase C (PKC) activity in the pancreatic islet beta-cell are presently unclear. By developing adenovirally expressed chimeras encoding fusion proteins between green fluorescent protein and conventional (betaII), novel (delta), or atypical (zeta) PKCs, we show that glucose selectively alters the subcellular localization of these enzymes dynamically in primary islet and MIN6 beta-cells. Examined by laser scanning confocal or total internal reflection fluorescence microscopy, elevated glucose concentrations induced oscillatory translocations of PKCbetaII to spatially confined regions of the plasma membrane. Suggesting that increases in free cytosolic Ca(2+) concentrations ([Ca(2+)](c)) were primarily responsible, prevention of [Ca(2+)](c) increases with EGTA or diazoxide completely eliminated membrane recruitment, whereas elevation of cytosolic [Ca(2+)](c) with KCl or tolbutamide was highly effective in redistributing PKCbetaII both to the plasma membrane and to the surface of dense core secretory vesicles. By contrast, the distribution of PKCdelta.EGFP, which binds diacylglycerol but not Ca(2+), was unaffected by glucose. Measurement of [Ca(2+)](c) immediately beneath the plasma membrane with a ratiometric "pericam," fused to synaptic vesicle-associated protein-25, revealed that depolarization induced significantly larger increases in [Ca(2+)](c) in this domain. These data demonstrate that nutrient stimulation of beta-cells causes spatially and temporally complex changes in the subcellular localization of PKCbetaII, possibly resulting from the generation of Ca(2+) microdomains. Localized changes in PKCbetaII activity may thus have a role in the spatial control of insulin exocytosis.

Nutrient-secretion coupling in the pancreatic islet beta-cell: recent advances

Insulin secretion from pancreatic islet beta-cells is a tightly regulated process, under the close control of blood glucose concentrations, and several hormones and neurotransmitters. Defects in glucose-triggered insulin secretion are ultimately responsible for the development of type II diabetes, a condition in which the total beta-cell mass is essentially unaltered, but beta-cells become progressively "glucose blind" and unable to meet the enhanced demand for insulin resulting for peripheral insulin resistance. At present, the mechanisms by which glucose (and other nutrients including certain amino acids) trigger insulin secretion in healthy individuals are understood only in part. It is clear, however, that the metabolism of nutrients, and the generation of intracellular signalling molecules including the products of mitochondrial metabolism, probably play a central role. Closure of ATP-sensitive K+(K(ATP)) channels in the plasma membrane, cell depolarisation, and influx of intracellular Ca2+, then prompt the "first phase" on insulin release. However, recent data indicate that glucose also enhances insulin secretion through mechanisms which do not involve a change in K(ATP) channel activity, and seem likely to underlie the second, sustained phase of glucose-stimulated insulin secretion. In this review, I will discuss recent advances in our understanding of each of these signalling processes.