Protein kinases, protein phosphorylation, and the regulation of insulin secretion from pancreatic beta-cells

The triggering pathway, the metabolic amplifying pathway, and cellular transduction in regulation of glucose-dependent biphasic insulin secretion

Introduction: Insulin secretion is a highly regulated process critical for maintaining glucose homeostasis. This abstract explores the intricate interplay between three essential pathways: The Triggering Pathway, The Metabolic Amplifying Pathway, and Cellular Transduction, in orchestrating glucose-dependent biphasic insulin secretion.Mechanism: During the triggering pathway, glucose metabolism in pancreatic beta-cells leads to ATP production, closing ATP-sensitive potassium channels and initiating insulin exocytosis. The metabolic amplifying pathway enhances insulin secretion via key metabolites like NADH and glutamate, enhancing calcium influx and insulin granule exocytosis. Additionally, the cellular transduction pathway involves G-protein coupled receptors and cyclic AMP, modulating insulin secretion.Result and Conclusion: These interconnected pathways ensure a dynamic insulin response to fluctuating glucose levels, with the initial rapid phase and the subsequent sustained phase. Understanding these pathways' complexities provides crucial insights into insulin dysregulation in diabetes and highlights potential therapeutic targets to restore glucose-dependent insulin secretion.

Potential use of the Asteraceae family as a cure for diabetes: A review of ethnopharmacology to modern day drug and nutraceuticals developments

The diabetes-associated mortality rate is increasing annually, along with the severity of its accompanying disorders that impair human health. Worldwide, several medicinal plants are frequently urged for the management of diabetes. Reports are available on the use of medicinal plants by traditional healers for their blood-sugar-lowering effects, along with scientific evidence to support such claims. The Asteraceae family is one of the most diverse flowering plants, with about 1,690 genera and 32,000 species. Since ancient times, people have consumed various herbs of the Asteraceae family as food and employed them as medicine. Despite the wide variety of members within the family, most of them are rich in naturally occurring polysaccharides that possess potent prebiotic effects, which trigger their use as potential nutraceuticals. This review provides detailed information on the reported Asteraceae plants traditionally used as antidiabetic agents, with a major focus on the plants of this family that are known to exert antioxidant, hepatoprotective, vasodilation, and wound healing effects, which further action for the prevention of major diseases like cardiovascular disease (CVD), liver cirrhosis, and diabetes mellitus (DM). Moreover, this review highlights the potential of Asteraceae plants to counteract diabetic conditions when used as food and nutraceuticals. The information documented in this review article can serve as a pioneer for developing research initiatives directed at the exploration of Asteraceae and, at the forefront, the development of a botanical drug for the treatment of DM.

Biosynthetic Activity Differs Between Islet Cell Types and in Beta Cells Is Modulated by Glucose and Not by Secretion

A correct biosynthetic activity is thought to be essential for the long-term function and survival of islet cells in culture and possibly also after islet transplantation. Compared to the secretory activity, biosynthetic activity has been poorly studied in pancreatic islet cells. Here we aimed to assess biosynthetic activity at the single cell level to investigate if protein synthesis is dependent on secretagogues and increased as a consequence of hormonal secretion. Biosynthetic activity in rat islet cells was studied at the single cell level using O-propargyl-puromycin (OPP) that incorporates into newly translated proteins and chemically ligates to a fluorescent dye by "click" reaction. Heterogeneous biosynthetic activity was observed between the four islet cell types, with delta cells showing the higher relative protein biosynthesis. Beta cells protein biosynthesis was increased in response to glucose while 3-isobutyl-1-methylxanthine and phorbol-12-myristate-13-acetate, 2 drugs known to stimulate insulin secretion, had no similar effect on protein biosynthesis. However, after several hours of secretion, protein biosynthesis remained high even when cells were challenged to basal conditions. These results suggest that mechanisms regulating secretion and biosynthesis in islet cells are different, with glucose directly triggering beta cells protein biosynthesis, independently of insulin secretion. Furthermore, this OPP labeling approach is a promising method to identify newly synthesized proteins under various physiological and pathological conditions.

Glucose-dependent phosphorylation signaling pathways and crosstalk to mitochondrial respiration in insulin secreting cells

Background

Glucose is the main secretagogue of pancreatic beta-cells. Uptake and metabolism of the nutrient stimulates the beta-cell to release the blood glucose lowering hormone insulin. This metabolic activation is associated with a pronounced increase in mitochondrial respiration. Glucose stimulation also initiates a number of signal transduction pathways for the coordinated regulation of multiple biological processes required for insulin secretion.

Methods

Shotgun proteomics including TiO₂ enrichment of phosphorylated peptides followed by liquid chromatography tandem mass spectrometry on lysates from glucose-stimulated INS-1E cells was used to identify glucose regulated phosphorylated proteins and signal transduction pathways. Kinase substrate enrichment analysis (KSEA) was applied to identify key regulated kinases and phosphatases. Glucose-induced oxygen consumption was measured using a XF96 Seahorse instrument to reveal cross talk between glucose-regulated kinases and mitochondrial activation.

Results

Our kinetic analysis of substrate phosphorylation reveal the molecular mechanism leading to rapid activation of insulin biogenesis, vesicle trafficking, insulin granule exocytosis and cytoskeleton remodeling. Kinase-substrate enrichment identified upstream kinases and phosphatases and time-dependent activity changes during glucose stimulation. Activity trajectories of well-known glucose-regulated kinases and phosphatases are described. In addition, we predict activity changes in a number of kinases including NUAK1, not or only poorly studied in the context of the pancreatic beta-cell. Furthermore, we pharmacologically tested whether signaling pathways predicted by kinase-substrate enrichment analysis affected glucose-dependent acceleration of mitochondrial respiration. We find that phosphoinositide 3-kinase, Ca2+/calmodulin dependent protein kinase and protein kinase C contribute to short-term regulation of energy metabolism.

Conclusions

Our results provide a global view into the regulation of kinases and phosphatases in insulin secreting cells and suggest cross talk between glucose-induced signal transduction and mitochondrial activation.

Electronic supplementary material

The online version of this article (10.1186/s12964-019-0326-6) contains supplementary material, which is available to authorized users.

JONES P. Determining the insulin secretion potential for certain specific G-protein coupled receptors in MIN6 pancreatic beta cells

Background/aim

The polypeptide hormone insulin is essential for the maintenance of whole-body fuel homeostasis, and defects in insulin secretion and/or action are associated with the development of type 1 and type 2 diabetes. The aim of this study was to assess the role of some G-protein coupled receptors (GPCRs), GPR54, GPR56, and GPR75, and cannabinoid receptors CB1R and CB2R, in the regulation of pancreatic β-cell function.

Materials and methods

Insulin secretion from mouse insulinoma β-cell line (MIN6) monolayers was assessed via insulin radioimmunoassay (RIA). Reverse transcription-polymerase chain reaction (RT-PCR) was used to assess the expression of some specific GPCRs and the other receptors by MIN6 pancreatic β-cells.

Results

The agonists were not found to be toxic for the MIN6 pancreatic β-cells within the range of the doses used in this study, whereas insulin secretion altered depending on the ligands and receptors. In addition, arachidonyl-2’-chloroethylamide (ACEA), carbachol, chemokine (C-C motif) ligand-5 (CCL5), and exendin as well as phorbol myristate acetate (PMA) ligands showed significant increases in the insulin secretion of MIN6 pancreatic β-cells.

Conclusion

Understanding the normal β-cell function and identifying the defects in β-cell function that lead to the development of diabetes will generate new therapeutic targets.

Bioluminescence Assays for Monitoring Chondrogenic Differentiation and Cartilage Regeneration

Since articular cartilage has a limited regeneration potential, for developing biological therapies for cartilage regeneration it is important to study the mechanisms underlying chondrogenesis of stem cells. Bioluminescence assays can visualize a wide range of biological phenomena such as gene expression, signaling, metabolism, development, cellular movements, and molecular interactions by using visible light and thus contribute substantially to elucidation of their biological functions. This article gives a concise review to introduce basic principles of bioluminescence assays and applications of the technology to visualize the processes of chondrogenesis and cartilage regeneration. Applications of bioluminescence assays have been highlighted in the methods of real-time monitoring of gene expression and intracellular levels of biomolecules and noninvasive cell tracking within animal models. This review suggests that bioluminescence assays can be applied towards a visual understanding of chondrogenesis and cartilage regeneration.

Systematic Synergy of Glucose and GLP-1 to Stimulate Insulin Secretion Revealed by Quantitative Phosphoproteomics

GLP-1 synergizes with glucose in regulating pancreatic β-cell function, including facilitating β-cell survival and insulin secretion. Though it has been widely accepted that phosphorylation is extremely important in regulating β-cell functions, our knowledge to the global mechanism is still limited. Here we performed a quantitative phosphoproteomics study to systematically present the synergistic regulation of INS-1E cell phosphoproteome mediated by glucose and GLP-1. We generated the largest pancreatic β-cell phosphoproteome by identifying 25,327 accurately localized phosphorylation sites on 5,389 proteins. Our results discovered several novel kinases regulated by glucose, GLP-1 or their synergism, and some of these kinases might act as downstream molecules of GLP-1 mediated PKA signaling cascade. A few phosphosites were regulated by both GLP-1 and glucose alone, and these target proteins were highly related to their biological function on pancreatic β-cells. Finally, we found glucose and GLP-1 executed their synergistic effect at multiple levels, especially at pathway level. Both GLP-1 and glucose participated in regulating every single step of the secretion pathway, and systematically synergized their effects in inducing insulin secretion.

Transgenic Expression of Glucagon-Like Peptide-1 (GLP-1) and Activated Muscarinic Receptor (M3R) Significantly Improves Pig Islet Secretory Function

Porcine islets show notoriously low insulin secretion levels in response to glucose stimulation. While this is somehow expected in the case of immature islets isolated from fetal and neonatal pigs, disappointingly low secretory responses are frequently reported in studies using in vitro-maturated fetal and neonatal islets and even fully differentiated adult islets. Herein we show that β-cell-specific expression of a modified glucagon-like peptide-1 (GLP-1) and of a constitutively activated type 3 muscarinic receptor (M3R) efficiently amplifies glucose-stimulated insulin secretion (GSIS). Both adult and neonatal isolated pig islets were treated with adenoviral expression vectors carrying sequences encoding for GLP-1 and/or M3R. GSIS from transduced and control islets was evaluated during static incubation and dynamic perifusion assays. While expression of GLP-1 did not affect basal or stimulated insulin secretion, activated M3R produced a twofold increase in both first and second phases of GSIS. Coexpression of GLP-1 and M3R caused an even greater increase in the secretory response, which was amplified fourfold compared to controls. In conclusion, our work highlights pig islet insulin secretion deficiencies and proposes concomitant activation of cAMP-dependent and cholinergic pathways as a solution to ameliorate GSIS from pig islets used for transplantation.

The Physiological Effects of Dandelion (Taraxacum Officinale) in Type 2 Diabetes

The tremendous rise in the economic burden of type 2 diabetes (T2D) has prompted a search for alternative and less expensive medicines. Dandelion offers a compelling profile of bioactive components with potential anti-diabetic properties. The Taraxacum genus from the Asteraceae family is found in the temperate zone of the Northern hemisphere. It is available in several areas around the world. In many countries, it is used as food and in some countries as therapeutics for the control and treatment of T2D. The anti-diabetic properties of dandelion are attributed to bioactive chemical components; these include chicoric acid, taraxasterol (TS), chlorogenic acid, and sesquiterpene lactones. Studies have outlined the useful pharmacological profile of dandelion for the treatment of an array of diseases, although little attention has been paid to the effects of its bioactive components on T2D to date. This review recapitulates previous work on dandelion and its potential for the treatment and prevention of T2D, highlighting its anti-diabetic properties, the structures of its chemical components, and their potential mechanisms of action in T2D. Although initial research appears promising, data on the cellular impact of dandelion are limited, necessitating further work on clonal β-cell lines (INS-1E), α-cell lines, and human skeletal cell lines for better identification of the active components that could be of use in the control and treatment of T2D. In fact, extensive in-vitro, in-vivo, and clinical research is required to investigate further the pharmacological, physiological, and biochemical mechanisms underlying the effects of dandelion-derived compounds on T2D.

PI3 kinases p110α and PI3K-C2β negatively regulate cAMP via PDE3/8 to control insulin secretion in mouse and human islets

OBJECTIVES

Phosphatidylinositol-3-OH kinase (PI3K) signalling in the endocrine pancreas contributes to glycaemic control. However, the mechanism by which PI3K modulates insulin secretion from the pancreatic beta cell is poorly understood. Thus, our objective was two-fold; to determine the signalling pathway by which acute PI3K inhibition enhances glucose-stimulated insulin secretion (GSIS) and to examine the role of this pathway in islets from type-2 diabetic (T2D) donors.

METHODS

Isolated islets from mice and non-diabetic or T2D human donors, or INS 832/13 cells, were treated with inhibitors of PI3K and/or phosphodiesterases (PDEs). The expression of PI3K-C2β was knocked down using siRNA. We measured insulin release, single-cell exocytosis, intracellular Ca(2+) responses ([Ca(2+)]i) and Ca(2+) channel currents, intracellular cAMP concentrations ([cAMP]i), and activation of cAMP-dependent protein kinase A (PKA) and protein kinase B (PKB/AKT).

RESULTS

The non-specific PI3K inhibitor wortmannin amplifies GSIS, raises [cAMP]i and activates PKA, but is without effect in T2D islets. Direct inhibition of specific PDE isoforms demonstrates a role for PDE3 (in humans and mice) and PDE8 (in mice) downstream of PI3K, and restores glucose-responsiveness of T2D islets. We implicate a role for the Class II PI3K catalytic isoform PI3K-C2β in this effect by limiting beta cell exocytosis.

CONCLUSIONS

PI3K limits GSIS via PDE3 in human islets. While inhibition of p110α or PIK-C2β signalling per se, may promote nutrient-stimulated insulin release, we now suggest that this signalling pathway is perturbed in islets from T2D donors.

Quantitative proteomics reveals novel protein interaction partners of PP2A catalytic subunit in pancreatic β-cells

Protein phosphatase 2A (PP2A) is one of the major serine/threonine phosphatases. We hypothesize that PP2A regulates signaling cascades in pancreatic β-cells in the context of glucose-stimulated insulin secretion (GSIS). Using co-immunoprecipitation (co-IP) and tandem mass spectrometry, we globally identified the protein interaction partners of the PP2A catalytic subunit (PP2Ac) in insulin-secreting pancreatic β-cells. Among the 514 identified PP2Ac interaction partners, 476 were novel. This represents the first global view of PP2Ac protein-protein interactions caused by hyperglycemic conditions. Additionally, numerous PP2Ac partners were found involved in a variety of signaling pathways in the β-cell function, such as insulin secretion. Our data suggest that PP2A interacts with various signaling proteins necessary for physiological insulin secretion as well as signaling proteins known to regulate cell dysfunction and apoptosis in the pancreatic β-cells.

Dual Monitoring of Secretion and ATP Levels during Chondrogenesis Using Perfusion Culture-Combined Bioluminescence Monitoring System

Skeletal pattern formation in limb development depends on prechondrogenic condensation which prefigures the cartilage template. However, although morphogens such as TGF-βs and BMPs have been known to play essential roles in skeletal patterning, how the morphogens induce prechondrogenic cells to aggregate and determine patterns of cartilage elements has remained unclear. Our previous study reported that ATP oscillations are induced during chondrogenesis. This result suggests the possibility that ATP oscillations lead to the oscillatory secretion of morphogens, due to the fact that secretion process requires ATP. To examine the correlation between ATP oscillations and secretion levels of morphogens, we have developed perfusion culture-combined bioluminescence monitoring system to simultaneously monitor intracellular ATP levels and secretion levels. Using this system, we found that secretory activity oscillates in phase with ATP oscillations and that secretion levels of TGF-β1 and BMP2 oscillate during chondrogenesis. The oscillatory secretion of the morphogens would contribute to amplifying the fluctuation of the morphogens, underlie the spatial patterning of morphogens, and consequently lead to skeletal pattern formation.

Activators of PKA and Epac distinctly influence insulin secretion and cytosolic Ca2+ in female mouse islets stimulated by glucose and tolbutamide

Amplification of insulin secretion by cAMP is mediated by protein kinase A (PKA) and exchange protein directly activated by cAMP (Epac). Using selective activators, we determined how each effector influences the cytosolic free Ca(2+) concentration ([Ca(2+)]c) and insulin secretion in mouse islets. Alone PKA activator amplified glucose- and tolbutamide-induced insulin secretion, with a greater impact on second than first phase. Epac activator strongly amplified both phases in response to either secretagogue. Amplification was even greater when activators were combined. Although both activators similarly amplified glucose-induced insulin secretion, Epac activator was particularly efficient on tolbutamide-induced insulin secretion. That greater efficacy is attributed to higher [Ca(2+)]c rather than interaction of tolbutamide with Epac, because it was also observed during KCl stimulation. Moreover, in contrast to Epac activator, tolbutamide was inactive when insulin secretion was increased by gliclazide, and its effect on glucose-induced insulin secretion was unaffected by an inhibitor of Epac2. PKA activator increased [Ca(2+)]c during acute or steady-state glucose stimulation, whereas Epac activator had no effect alone or in combination. Neither activator affected [Ca(2+)]c response to tolbutamide or KCl. Metabolic (glucose-mediated) amplification of insulin secretion was unaffected by PKA activator. It was attenuated when insulin secretion was augmented by Epac activator but insensitive to Epac2 inhibitor, which suggests distinct although somewhat overlapping mechanisms. In conclusion, activators of PKA and Epac amplify insulin secretion by augmenting the action of Ca(2+) on exocytosis and, for PKA only, slightly increasing glucose-induced [Ca(2+)]c rise. The influence of Epac seems more important than that of PKA during first phase.

Protein phosphatases in pancreatic islets

The prevalence of diabetes is increasing rapidly worldwide. A cardinal feature of most forms of diabetes is the lack of insulin-producing capability, due to the loss of insulin-producing β-cells, impaired glucose-sensitive insulin secretion from the β-cell, or a combination thereof, the reasons for which largely remain elusive. Reversible phosphorylation is an important and versatile mechanism for regulating the biological activity of many intracellular proteins, which, in turn, controls a variety of cellular functions. For instance, significant changes in protein kinase activities and in protein phosphorylation patterns occur subsequent to the stimulation of insulin release by glucose. Therefore, the molecular mechanisms regulating the phosphorylation of proteins involved in the insulin secretory process by the β-cell have been extensively investigated. However, far less is known about the role and regulation of protein dephosphorylation by various protein phosphatases. Herein, we review extant data implicating serine/threonine and tyrosine phosphatases in various aspects of healthy and diabetic islet biology, ranging from control of hormonal stimulus-secretion coupling to mitogenesis and apoptosis.

Establishment of new clonal pancreatic β-cell lines (MIN6-K) useful for study of incretin/cyclic adenosine monophosphate signaling

Incretin/cyclic adenosine monophosphate (cAMP) signaling is critical for potentiation of insulin secretion. Although several cell lines of pancreatic β‐cells are currently available, there are no cell lines suitable for investigation of incretin/cAMP signaling. In the present study, we have newly established pancreatic β‐cell lines (named MIN6‐K) from the IT6 mouse, which develops insulinoma. MIN6‐K8 cells respond to both glucose and incretins, such as glucagon‐like peptide‐1 (GLP‐1) and glucose‐dependent insulinotropic polypeptide (GIP), as is the case in pancreatic islets, whereas MIN6‐K20 cells respond to glucose, but not to incretins. Despite the difference in incretin‐potentiated insulin secretion between these two cell lines, the accumulation of cAMP after stimulation of GLP‐1 is comparable in these cells. Interestingly, we also found that incretin responsiveness is drastically induced by the formation of pseudoislets from MIN6‐K20 cells to a level comparable to that of pancreatic islets. Thus, these cell lines are useful for studying incretin/cAMP signaling in β‐cells. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2010.00026.x, 2010)

Protein Fractions from Korean Mistletoe (Viscum Album coloratum) Extract Induce Insulin Secretion from Pancreatic Beta Cells

Mistletoe (Viscum Album coloratum) has been known as a medicinal plant in European and Asian countries. Recent data show that biological activity of mistletoe alleviates hypertension, heart disease, renal failure, and cancer development. In this study, we report the antidiabetic effect of Korean mistletoe extract (KME). KME treatments enhanced the insulin secretion from the pancreatic β-cell without any effects of cytotoxicity. PDX-1 and beta2/neuroD known as transcription factors that regulate the expression of insulin gene were upregulated by treatment of the KME protein fractions isolated by ion-exchange chromatography after ammonium sulfate precipitation. Furthermore, these KME protein fractions significantly lowered the blood glucose level and the volume of drinking water in alloxan induced hyperglycemic mice. Taken together with the findings, it provides new insight that KME might be served as a useful source for the development of medicinal reagent to reduce blood glucose level of type I diabetic patients.

High glucose exposure promotes activation of protein phosphatase 2A in rodent islets and INS-1 832/13 β-cells by increasing the posttranslational carboxylmethylation of its catalytic subunit

Existing evidence implicates regulatory roles for protein phosphatase 2A (PP2A) in a variety of cellular functions, including cytoskeletal remodeling, hormone secretion, and apoptosis. We report here activation of PP2A in normal rat islets and insulin-secreting INS-1 832/13 cells under the duress of hyperglycemic (HG) conditions. Small interfering RNA-mediated knockdown of the catalytic subunit of PP2A (PP2Ac) markedly attenuated glucose-induced activation of PP2A. HG, but not nonmetabolizable 3-O-methyl glucose or mannitol (osmotic control), significantly stimulated the methylation of PP2Ac at its C-terminal Leu-309, suggesting a novel role for this posttranslational modification in glucose-induced activation of PP2A. Moreover, knockdown of the cytosolic leucine carboxymethyl transferase 1 (LCMT1), which carboxymethylates PP2Ac, significantly attenuated PP2A activation under HG conditions. In addition, HG conditions, but not 3-O-methyl glucose or mannitol, markedly increased the expression of LCMT1. Furthermore, HG conditions significantly increased the expression of B55α, a regulatory subunit of PP2A, which has been implicated in islet dysfunction under conditions of oxidative stress and diabetes. Thapsigargin, a known inducer of endoplasmic reticulum stress, failed to exert any discernible effects on the carboxymethylation of PP2Ac, expression of LCMT1 and B55α, or PP2A activity, suggesting no clear role for endoplasmic reticulum stress in HG-induced activation of PP2A. Based on these findings, we conclude that exposure of the islet β-cell to HG leads to accelerated PP2A signaling pathway, leading to loss in glucose-induced insulin secretion.

The novel chemokine receptor, G-protein-coupled receptor 75, is expressed by islets and is coupled to stimulation of insulin secretion and improved glucose homeostasis

AIMS/HYPOTHESIS

Chemokine (C-C motif) ligand 5 (CCL5) acts at C-C chemokine receptors (CCRs) to promote immune cell recruitment to sites of inflammation, but is also an agonist at G-protein-coupled receptor 75 (GPR75), which has very limited homology with CCRs. GPR75 is coupled to Gq to elevate intracellular calcium, so we investigated whether islets express this receptor and whether its activation by CCL5 increases beta cell calcium levels and insulin secretion.

METHODS

Islet CCL5 receptor mRNA expression was measured by quantitative RT-PCR and GPR75 was detected in islets by western blotting and immunohistochemistry. In some experiments GPR75 was downregulated by transient transfection with small interfering RNA. Real-time changes in intracellular calcium were determined by single-cell microfluorimetry. Dynamic insulin secretion from perifused islets was quantified by radioimmunoassay. Glucose homeostasis in lean and obese mice was determined by measuring glucose and insulin tolerance, and insulin secretion in vivo.

RESULTS

Mouse and human islets express GPR75 and its ligand CCL5. Exogenous CCL5 reversibly increased intracellular calcium in beta cells via GPR75, this phenomenon being dependent on phospholipase C activation and calcium influx. CCL5 also stimulated insulin secretion from mouse and human islets in vitro, and improved glucose tolerance in lean mice and in a mouse model of hyperglycaemia and insulin resistance (ob/ob). The improvement in glucose tolerance was associated with enhanced insulin secretion in vivo, without changes in insulin sensitivity.

CONCLUSIONS/INTERPRETATION

Although CCL5 is implicated in the pathogenesis of diabetes through activation of CCRs, it has beneficial effects on beta cells through GPR75 activation.

The permissive effects of glucose on receptor-operated potentiation of insulin secretion from mouse islets: a role for ERK1/2 activation and cytoskeletal remodelling

AIMS/HYPOTHESIS

Glucose plays two distinct roles in regulating insulin secretion from beta cells--an initiatory role, and a permissive role enabling receptor-operated secretagogues to potentiate glucose-induced insulin secretion. The molecular mechanisms underlying the permissive effects of glucose on receptor-operated insulin secretion remain uncertain. We have investigated the role of extracellular signal-regulated kinase 1/2 (ERK1/2) activation and consequent cytoskeletal remodelling in this process.

METHODS

Insulin release was measured from groups of isolated mouse islets using static incubation experiments and subsequent radioimmunoassay of samples. ERK1/2 activation was measured by western blotting of islet protein samples for both phosphorylated and total ERK1/2. Rhodamine-phalloidin staining was used to measure filamentous actin in dispersed primary beta cells.

RESULTS

Inhibition of ERK1/2 blocked potentiation of glucose-induced insulin release by the receptor-operated secretagogues kisspeptin, A568, exendin-4 and JWH015, although the agonists alone had minimal effects on ERK1/2 activation, suggesting a permissive rather than causal role for ERK1/2 activation in receptor-operated insulin release. Following pharmacological activation of ERK1/2 all agonists caused a significant increase in insulin release from islets incubated with sub-stimulatory levels of glucose. ERK1/2 inhibition significantly reduced the glucose-dependent decreases in filamentous actin observed in primary beta cells, while pharmacological dissociation of actin filaments enabled all receptor-operated secretagogues tested to significantly stimulate insulin release from islets at a sub-stimulatory glucose concentration.

CONCLUSIONS/INTERPRETATION

Glucose-induced ERK1/2 activation in beta cells mediates the permissive effects of stimulatory glucose concentrations on receptor-operated insulin secretagogues, at least in part through effects on actin depolymerisation and cytoskeletal remodelling.

Cyclic AMP dynamics in the pancreatic β-cell

Insulin secretion from pancreatic β-cells is tightly regulated by glucose and other nutrients, hormones, and neural factors. The exocytosis of insulin granules is triggered by an elevation of the cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) and is further amplified by cyclic AMP (cAMP). Cyclic AMP is formed primarily in response to glucoincretin hormones and other G(s)-coupled receptor agonists, but generation of the nucleotide is critical also for an optimal insulin secretory response to glucose. Nutrient and receptor stimuli trigger oscillations of the cAMP concentration in β-cells. The oscillations arise from variations in adenylyl cyclase-mediated cAMP production and phosphodiesterase-mediated degradation, processes controlled by factors like cell metabolism and [Ca(2+)](i). Protein kinase A and the guanine nucleotide exchange factor Epac2 mediate the actions of cAMP in β-cells and operate at multiple levels to promote exocytosis and pulsatile insulin secretion. The cAMP signaling system contains important targets for pharmacological improvement of insulin secretion in type 2 diabetes.

The CaMK4/CREB/IRS-2 cascade stimulates proliferation and inhibits apoptosis of β-cells

Progressive reduction in β-cell mass is responsible for the development of type 2 diabetes mellitus, and alteration in insulin receptor substrate 2 (IRS-2) abundance plays a critical role in this process. IRS-2 expression is stimulated by the transcription factor cAMP response element-binding protein (CREB) and we recently demonstrated that Ca²⁺/calmodulin dependent kinase 4 (CaMK4) is upstream of CREB activation in β-cells. This study investigated whether CaMK4 is also a potential target to increase β-cell mass through CREB-mediated IRS-2 expression, by quantifying mouse MIN6 β-cell proliferation and apoptosis following IRS-2 knockdown, CaMKs inhibition and alterations in CaMK4 and CREB expression. Expression of constitutively active CaMK4 (ΔCaMK4) and CREB (CREB_DIEDLM) significantly stimulated β-cell proliferation and survival. In contrast, expression of their corresponding dominant negative forms (Δ^K75ECaMK4 and CREB_M1) and silencing of IRS-2 increased apoptosis and reduced β-cell division. Moreover, CREB_DIEDLM and CREB_M1 expression completely abolished the effects of Δ^K75ECaMK4 and of ΔCaMK4, respectively. Our results indicate that CaMK4 regulates β-cell proliferation and apoptosis in a CREB-dependent manner and that CaMK4-induced IRS-2 expression is important in these processes.

Deregulation of CREB signaling pathway induced by chronic hyperglycemia downregulates NeuroD transcription

CREB mediates the transcriptional effects of glucose and incretin hormones in insulin-target cells and insulin-producing β-cells. Although the inhibition of CREB activity is known to decrease the β-cell mass, it is still unknown what factors inversely alter the CREB signaling pathway in β-cells. Here, we show that β-cell dysfunctions occurring in chronic hyperglycemia are not caused by simple inhibition of CREB activity but rather by the persistent activation of CREB due to decreases in protein phophatase PP2A. When freshly isolated rat pancreatic islets were chronically exposed to 25 mM (high) glucose, the PP2A activity was reduced with a concomitant increase in active pCREB. Brief challenges with 15 mM glucose or 30 µM forskolin after 2 hour fasting further increased the level of pCREB and consequently induced the persistent expression of ICER. The excessively produced ICER was sufficient to repress the transcription of NeuroD, insulin, and SUR1 genes. In contrast, when islets were grown in 5 mM (low) glucose, CREB was transiently activated in response to glucose or forskolin stimuli. Thus, ICER expression was transient and insufficient to repress those target genes. Importantly, overexpression of PP2A reversed the adverse effects of chronic hyperglycemia and successfully restored the transient activation of CREB and ICER. Conversely, depletion of PP2A with siRNA was sufficient to disrupt the negative feedback regulation of CREB and induce hyperglycemic phenotypes even under low glucose conditions. Our findings suggest that the failure of the negative feedback regulation of CREB is the primary cause for β-cell dysfunctions under conditions of pathogenic hyperglycemia, and PP2A can be a novel target for future therapies aiming to protect β-cells mass in the late transitional phase of non-insulin dependent type 2 diabetes (NIDDM).

Synchronized ATP oscillations have a critical role in prechondrogenic condensation during chondrogenesis

The skeletal elements of embryonic limb are prefigured by prechondrogenic condensation in which secreted molecules such as adhesion molecules and extracellular matrix have crucial roles. However, how the secreted molecules are controlled to organize the condensation remains unclear. In this study, we examined metabolic regulation of secretion in prechondrogenic condensation, using bioluminescent monitoring systems. We here report on ATP oscillations in the early step of chondrogenesis. The ATP oscillations depended on both glycolysis and mitochondrial respiration, and their synchronization among cells were achieved via gap junctions. In addition, the ATP oscillations were driven by Ca(2+) oscillations and led to oscillatory secretion in chondrogenesis. Blockade of the ATP oscillations prevented cellular condensation. Furthermore, the degree of cellular condensation increased with the frequency of ATP oscillations. We conclude that ATP oscillations have a critical role in prechondrogenic condensation by inducing oscillatory secretion.

Protein farnesylation is requisite for mitochondrial fuel-induced insulin release: further evidence to link reactive oxygen species generation to insulin secretion in pancreatic β-cells

Several lines of recent evidence implicate regulatory roles for reactive oxygen species (ROS) in islet function and insulin secretion. The phagocyte-like NADPH oxidase (Nox2) has recently been shown to be one of the sources of ROS in the signaling events leading to glucose stimulated insulin secretion (GSIS). We recently reported inhibition of glucose- or mitochondrial fuel-induced Nox2-derived ROS by a specific inhibitor of protein farnesyl transferse (FTase; FTI-277), suggesting that activation of FTase might represent one of the upstream signaling events to Nox2 activation. Furthermore, FTase inhibitors (FTI-277 and FTI-2628) have also been shown to attenuate GSIS in INS 832/13 cells and normal rodent islets. Herein, we provide further evidence to suggest that inhibition of FTase either by pharmacological (e.g., FTI-277) or gene silencing (siRNA-FTase) approaches markedly attenuates mitochondrial fuel-stimulated insulin secretion (MSIS) in INS 832/13 cells. Together, our findings further establish a link between nutrient-induced Nox2 activation, ROS generation and insulin secretion in the pancreatic β-cell.

Gastric inhibitory peptide controls adipose insulin sensitivity via activation of cAMP-response element-binding protein and p110β isoform of phosphatidylinositol 3-kinase

Gastric inhibitory peptide (GIP) is an incretin hormone secreted in response to food intake. The best known function of GIP is to enhance glucose-dependent insulin secretion from pancreatic β-cells. Extra-pancreatic effects of GIP primarily occur in adipose tissues. Here, we demonstrate that GIP increases insulin-dependent translocation of the Glut4 glucose transporter to the plasma membrane and exclusion of FoxO1 transcription factor from the nucleus in adipocytes, establishing that GIP has a general effect on insulin action in adipocytes. Stimulation of adipocytes with GIP alone has no effect on these processes. Using pharmacologic and molecular genetic approaches, we show that the effect of GIP on adipocyte insulin sensitivity requires activation of both the cAMP/protein kinase A/CREB signaling module and p110β phosphoinositol-3' kinase, establishing a novel signal transduction pathway modulating insulin action in adipocytes. This insulin-sensitizing effect is specific for GIP because isoproterenol, which elevates adipocyte cAMP and activates PKA/CREB signaling, does not affect adipocyte insulin sensitivity. The insulin-sensitizing activity points to a more central role for GIP in intestinal regulation of peripheral tissue metabolism, an emerging feature of inter-organ communication in the control of metabolism.

Dynamics of insulin secretion and the clinical implications for obesity and diabetes

Insulin secretion is a highly dynamic process regulated by various factors including nutrients, hormones, and neuronal inputs. The dynamics of insulin secretion can be studied at different levels: the single β cell, pancreatic islet, whole pancreas, and the intact organism. Studies have begun to analyze cellular and molecular mechanisms underlying dynamics of insulin secretion. This review focuses on our current understanding of the dynamics of insulin secretion in vitro and in vivo and discusses their clinical relevance.

Environmental pollutants and type 2 diabetes: a review of mechanisms that can disrupt beta cell function

The prevalence of diabetes mellitus is currently at epidemic proportions and it is estimated that it will increase even further over the next decades. Although genetic predisposition and lifestyle choices are commonly accepted reasons for the occurrence of type 2 diabetes, it has recently been suggested that environmental pollutants are additional risk factors for diabetes development and this review aims to give an overview of the current evidence for this. More specifically, because of the crucial role of pancreatic beta cells in the development and progression of type 2 diabetes, the present work summarises the known effects of several compounds on beta cell function with reference to mechanistic studies that have elucidated how these compounds interfere with the insulin secreting capacity of beta cells. Oestrogenic compounds, organophosphorus compounds, persistent organic pollutants and heavy metals are discussed, and a critical reflection on the relevance of the concentrations used in mechanistic studies relative to the levels found in the human population is given. It is clear that some environmental pollutants affect pancreatic beta cell function, as both epidemiological and experimental research is accumulating. This supports the need to develop a solid and structured platform to fully explore the diabetes-inducing potential of pollutants.

Protein histidine [de]phosphorylation in insulin secretion: abnormalities in models of impaired insulin secretion

In the majority of cell types, including the islet β-cell, transduction of extracellular signals involves ligand binding to a receptor, often followed by the activation G proteins and their effector modules. The islet β-cell is unusual in that glucose lacks an extracellular receptor. Instead, events consequent to glucose metabolism promote insulin secretion via the generation of diffusible second messengers and mobilization of calcium. A selective increase in intracellular calcium has been shown to regulate the phosphorylation status key islet proteins thereby facilitating insulin secretion. In addition to classical protein kinases [e.g., protein kinases A and C], recent studies from our laboratory have focused on the expression and function of various forms of NDPK/nm23-like histidine kinases in clonal β-cells, normal rodent, and human islets. Further, we recently reported localization of a cytosolic protein histidine phosphatase [PHP] in INS 832/13 cells, normal rat islets, and human islets. siRNA-mediated knock down of nm23-H1 and PHP in insulin-secreting INS 832/13 cells significantly attenuated glucose-induced insulin secretion. We also observed significant alterations in the expression and function of nm23-H1/PHP in β-cells chronically exposed to elevated levels of glucose and saturated fatty acids, such as palmitate (i.e., glucolipotoxicity). Similar changes were also noted in islets from the Goto-Kakizaki and Zucker Diabetic Fatty rats, two known models for type 2 diabetes. It is concluded that protein histidine phosphorylation-dephosphorylation cycles play novel regulatory roles in G protein-mediated physiological insulin secretion and that abnormalities in this signaling axis lead to impaired insulin secretion in glucolipotoxicity and type 2 diabetes.

Deletion of protein kinase Cδ in mice modulates stability of inflammatory genes and protects against cytokine-stimulated beta cell death in vitro and in vivo

AIMS/HYPOTHESIS

Proinflammatory cytokines contribute to beta cell destruction in type 1 diabetes, but the mechanisms are incompletely understood. The aim of the current study was to address the role of the protein kinase C (PKC) isoform PKCδ, a diverse regulator of cell death, in cytokine-stimulated apoptosis in primary beta cells.

METHODS

Islets isolated from wild-type or Prkcd(-/-) mice were treated with IL-1β, TNF-α and IFNγ and assayed for apoptosis, nitric oxide (NO) generation and insulin secretion. Activation of signalling pathways, apoptosis and endoplasmic reticulum (ER) stress were determined by immunoblotting. Stabilisation of mRNA transcripts was measured by RT-PCR following transcriptional arrest. Mice were injected with multiple low doses of streptozotocin (MLD-STZ) and fasting blood glucose monitored.

RESULTS

Deletion of Prkcd inhibited apoptosis and NO generation in islets stimulated ex vivo with cytokines. It also delayed the onset of hyperglycaemia in MLD-STZ-treated mice. Activation of ERK, p38, JNK, AKT1, the ER stress markers DDIT3 and phospho-EIF2α and the intrinsic apoptotic markers BCL2 and MCL1 was not different between genotypes. However, deletion of Prkcd destabilised mRNA transcripts for Nos2, and for multiple components of the toll-like receptor 2 (TLR2) signalling complex, which resulted in disrupted TLR2 signalling.

CONCLUSIONS/INTERPRETATION

Loss of PKCδ partially protects against hyperglycaemia in the MLD-STZ model in vivo, and against cytokine-mediated apoptosis in vitro. This is accompanied by reduced NO generation and destabilisation of Nos2 and components of the TLR2 signalling pathway. The results highlight a mechanism for regulating proinflammatory gene expression in beta cells independently of transcription.

Regulation of glucose- and mitochondrial fuel-induced insulin secretion by a cytosolic protein histidine phosphatase in pancreatic beta-cells

We report localization of a cytosolic protein histidine phosphatase (PHP; approximately 16 kDa) in INS 832/13 cells, normal rat islets, and human islets. siRNA-mediated knockdown of PHP markedly reduced glucose- or mitochondrial fuel-induced but not KCl-induced insulin secretion. siRNA-mediated knockdown of PHP also attenuated mastoparan-induced insulin secretion, suggesting its participation in G protein-sensitive signaling steps, leading to insulin secretion. Functional assays revealed that the beta-cell PHP catalyzes the dephosphorylation of ATP-citrate lyase (ACL). Silencing of PHP expression markedly reduced ACL activity, suggesting functional regulation of ACL by PHP in beta-cells. Coimmunoprecipitation studies revealed modest effects of glucose on the interaction between PHP and ACL. Confocal microscopic evidence indicated that glucose promotes association between ACL and nm23-H1, a known kinase histidine kinase, but not between PHP and ACL. Furthermore, metabolic viability of INS 832/13 cells was resistant to siRNA-PHP, suggesting no regulatory roles of PHP in cell viability. Finally, long-term exposure (24 h) of INS 832/13 cells or rat islets to high glucose (30 mM) increased the expression of PHP. Such increases in PHP expression were also seen in islets derived from the Zucker diabetic fatty rat compared with islets from the lean control animals. Together, these data implicate regulatory roles for PHP in a G protein-sensitive step involved in nutrient-induced insulin secretion. In light of the current debate on putative regulatory roles of ACL in insulin secretion, additional studies are needed to precisely identify the phosphoprotein substrate(s) for PHP in the cascade of events leading to nutrient-induced insulin secretion.

Noradrenaline inhibits exocytosis via the G protein βγ subunit and refilling of the readily releasable granule pool via the α(i1/2) subunit

The molecular mechanisms responsible for the 'distal' effect by which noradrenaline (NA) blocks exocytosis in the β-cell were examined by whole-cell and cell-attached patch clamp capacitance measurements in INS 832/13 β-cells. NA inhibited Ca(2+)-evoked exocytosis by reducing the number of exocytotic events, without modifying vesicle size. Fusion pore properties also were unaffected. NA-induced inhibition of exocytosis was abolished by a high level of Ca(2+) influx, by intracellular application of antibodies against the G protein subunit Gβ and was mimicked by the myristoylated βγ-binding/activating peptide mSIRK. NA-induced inhibition was also abolished by treatment with BoNT/A, which cleaves the C-terminal nine amino acids of SNAP-25, and also by a SNAP-25 C-terminal-blocking peptide containing the BoNT/A cleavage site. These data indicate that inhibition of exocytosis by NA is downstream of increased [Ca(2+)](i) and is mediated by an interaction between Gβγ and the C-terminus of SNAP-25, as is the case for inhibition of neurotransmitter release. Remarkably, in the course of this work, a novel effect of NA was discovered. NA induced a marked retardation of the rate of refilling of the readily releasable pool (RRP) of secretory granules. This retardation was specifically abolished by a Gα(i1/2) blocking peptide demonstrating that the effect is mediated via activation of Gα(i1) and/or Gα(i2).

cAMP mediators of pulsatile insulin secretion from glucose-stimulated single beta-cells

Pulsatile insulin release from glucose-stimulated beta-cells is driven by oscillations of the Ca(2+) and cAMP concentrations in the subplasma membrane space ([Ca(2+)](pm) and [cAMP](pm)). To clarify mechanisms by which cAMP regulates insulin secretion, we performed parallel evanescent wave fluorescence imaging of [cAMP](pm), [Ca(2+)](pm), and phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) in the plasma membrane. This lipid is formed by autocrine insulin receptor activation and was used to monitor insulin release kinetics from single MIN6 beta-cells. Elevation of the glucose concentration from 3 to 11 mm induced, after a 2.7-min delay, coordinated oscillations of [Ca(2+)](pm), [cAMP](pm), and PIP(3). Inhibitors of protein kinase A (PKA) markedly diminished the PIP(3) response when applied before glucose stimulation, but did not affect already manifested PIP(3) oscillations. The reduced PIP(3) response could be attributed to accelerated depolarization causing early rise of [Ca(2+)](pm) that preceded the elevation of [cAMP](pm). However, the amplitude of the PIP(3) response after PKA inhibition was restored by a specific agonist to the cAMP-dependent guanine nucleotide exchange factor Epac. Suppression of cAMP formation with adenylyl cyclase inhibitors reduced already established PIP(3) oscillations in glucose-stimulated cells, and this effect was almost completely counteracted by the Epac agonist. In cells treated with small interfering RNA targeting Epac2, the amplitudes of the glucose-induced PIP(3) oscillations were reduced, and the Epac agonist was without effect. The data indicate that temporal coordination of the triggering [Ca(2+)](pm) and amplifying [cAMP](pm) signals is important for glucose-induced pulsatile insulin release. Although both PKA and Epac2 partake in initiating insulin secretion, the cAMP dependence of established pulsatility is mediated by Epac2.

Protein farnesylation-dependent Raf/extracellular signal-related kinase signaling links to cytoskeletal remodeling to facilitate glucose-induced insulin secretion in pancreatic beta-cells

OBJECTIVE

Posttranslational prenylation (e.g., farnesylation) of small G-proteins is felt to be requisite for cytoskeletal remodeling and fusion of secretory vesicles with the plasma membrane. Here, we investigated roles of protein farnesylation in the signaling steps involved in Raf-1/extracellular signal-related kinase (ERK1/2) signaling pathway in glucose-induced Rac1 activation and insulin secretion in the pancreatic beta-cell.

RESEARCH DESIGN AND METHODS

These studies were carried out in INS 832/13 cells and normal rat islets. Molecular biological (e.g., overexpression or small interfering RNA [siRNA]-mediated knockdown) and pharmacologic approaches were used to determine roles for farnesylation in glucose-mediated activation of ERK1/2, Rac1, and insulin secretion. Activation of ERK1/2 was determined by Western blotting. Rac1 activation (i.e., Rac1.GTP) was quantitated by p21-activated kinase pull-down assay. Insulin release was quantitated by enzyme-linked immunosorbent assay.

RESULTS

Coprovision of structure-specific inhibitors of farnesyl transferase (FTase; e.g., FTI-277 or FTI-2628) or siRNA-mediated knockdown of FTase beta-subunit resulted in a significant inhibition of glucose-stimulated ERK1/2 and Rac1 activation and insulin secretion. Pharmacologic inhibition of Raf-1 kinase using GW-5074 markedly reduced the stimulatory effects of glucose on ERK1/2 phosphorylation, Rac1 activation, and insulin secretion, suggesting that Raf-1 kinase activation may be upstream to ERK1/2 and Rac1 activation leading to glucose-induced insulin release. Lastly, siRNA-mediated silencing of endogenous expression of ERK1/2 markedly attenuated glucose-induced Rac1 activation and insulin secretion.

CONCLUSIONS

Together, our findings provide the first evidence of a role for protein farnesylation in glucose-mediated regulation of the Raf/ERK signaling pathway culminating in the activation of Rac1, which has been shown to be necessary for cytoskeletal reorganization and exocytotic secretion of insulin.

Small G proteins in islet beta-cell function

Glucose-stimulated insulin secretion from the islet beta-cell involves a sequence of metabolic events and an interplay between a wide range of signaling pathways leading to the generation of second messengers (e.g., cyclic nucleotides, adenine and guanine nucleotides, soluble lipid messengers) and mobilization of calcium ions. Consequent to the generation of necessary signals, the insulin-laden secretory granules are transported from distal sites to the plasma membrane for fusion and release of their cargo into the circulation. The secretory granule transport underlies precise changes in cytoskeletal architecture involving a well-coordinated cross-talk between various signaling proteins, including small molecular mass GTP-binding proteins (G proteins) and their respective effector proteins. The purpose of this article is to provide an overview of current understanding of the identity of small G proteins (e.g., Cdc42, Rac1, and ARF-6) and their corresponding regulatory factors (e.g., GDP/GTP-exchange factors, GDP-dissociation inhibitors) in the pancreatic beta-cell. Plausible mechanisms underlying regulation of these signaling proteins by insulin secretagogues are also discussed. In addition to their positive modulatory roles, certain small G proteins also contribute to the metabolic dysfunction and demise of the islet beta-cell seen in in vitro and in vivo models of impaired insulin secretion and diabetes. Emerging evidence also suggests significant insulin secretory abnormalities in small G protein knockout animals, further emphasizing vital roles for these proteins in normal health and function of the islet beta-cell. Potential significance of these experimental observations from multiple laboratories and possible avenues for future research in this area of islet research are highlighted.

Mitochondrial glutamate carrier GC1 as a newly identified player in the control of glucose-stimulated insulin secretion

The SLC25 carrier family mediates solute transport across the inner mitochondrial membrane, a process that is still poorly characterized regarding both the mechanisms and proteins implicated. This study investigated mitochondrial glutamate carrier GC1 in insulin-secreting beta-cells. GC1 was cloned from insulin-secreting cells, and sequence analysis revealed hydropathy profile of a six-transmembrane protein, characteristic of mitochondrial solute carriers. GC1 was found to be expressed at the mRNA and protein levels in INS-1E beta-cells and pancreatic rat islets. Immunohistochemistry showed that GC1 was present in mitochondria, and ultrastructural analysis by electron microscopy revealed inner mitochondrial membrane localization of the transporter. Silencing of GC1 in INS-1E beta-cells, mediated by adenoviral delivery of short hairpin RNA, reduced mitochondrial glutamate transport by 48% (p < 0.001). Insulin secretion at basal 2.5 mM glucose and stimulated either by intermediate 7.5 mM glucose or non-nutrient 30 mM KCl was not modified by GC1 silencing. Conversely, insulin secretion stimulated with optimal 15 mM glucose was reduced by 23% (p < 0.005) in GC1 knocked down cells compared with controls. Adjunct of cell-permeant glutamate (5 mM dimethyl glutamate) fully restored the secretory response at 15 mM glucose (p < 0.005). Kinetics of insulin secretion were investigated in perifused isolated rat islets. GC1 silencing in islets inhibited the secretory response induced by 16.7 mM glucose, both during first (-25%, p < 0.05) and second (-33%, p < 0.05) phases. This study demonstrates that insulin-secreting cells depend on GC1 for maximal glucose response, thereby assigning a physiological function to this newly identified mitochondrial glutamate carrier.

ROS signaling, oxidative stress and Nrf2 in pancreatic beta-cell function

This review focuses on the emerging evidence that reactive oxygen species (ROS) derived from glucose metabolism, such as H(2)O(2), act as metabolic signaling molecules for glucose-stimulated insulin secretion (GSIS) in pancreatic beta-cells. Particular emphasis is placed on the potential inhibitory role of endogenous antioxidants, which rise in response to oxidative stress, in glucose-triggered ROS and GSIS. We propose that cellular adaptive response to oxidative stress challenge, such as nuclear factor E2-related factor 2 (Nrf2)-mediated antioxidant induction, plays paradoxical roles in pancreatic beta-cell function. On the one hand, induction of antioxidant enzymes protects beta-cells from oxidative damage and possible cell death, thus minimizing oxidative damage-related impairment of insulin secretion. On the other hand, the induction of antioxidant enzymes by Nrf2 activation blunts glucose-triggered ROS signaling, thus resulting in reduced GSIS. These two premises are potentially relevant to impairment of beta-cells occurring in the late and early stage of Type 2 diabetes, respectively. In addition, we summarized our recent findings that persistent oxidative stress due to absence of uncoupling protein 2 activates cellular adaptive response which is associated with impaired pancreatic beta-cell function.

Kisspeptin stimulation of insulin secretion: mechanisms of action in mouse islets and rats

AIMS/HYPOTHESIS

Kisspeptin is a novel peptide identified as an endogenous ligand of the G-protein-coupled receptor 54 (GPR-54), which plays a crucial role in puberty and reproductive function. High levels of GPR-54 and kisspeptin have been reported in the pancreas and we have previously shown that kisspeptin potentiates glucose-induced insulin release from isolated islets, although the mechanisms underlying this effect were unclear.

METHODS

Insulin secretion from isolated mouse islets was measured to characterise the effects of kisspeptin. The effects of kisspeptin on both p42/44 mitogen-activated protein kinase (MAPK) phosphorylation and intracellular Ca(2+)([Ca(2+)](i)) in mouse islets were also investigated. Furthermore, kisspeptin was administered to rats in vivo and effects on plasma insulin levels measured.

RESULTS

In the current study, kisspeptin induced a concentration-dependent potentiation of glucose-induced (20 mmol/l) insulin secretion from mouse islets, with maximal effects at 1 micromol/l, but had no effect on insulin secretion at a substimulatory concentration of glucose (2 mmol/l). Activation of GPR-54 by kisspeptin also caused reversible increases in [Ca(2+)](i) in Fura-2 loaded dispersed islet cells. The kisspeptin-induced potentiation of glucose-induced insulin secretion was completely abolished by inhibitors of phospholipase C and p42/44 MAPK, but not by inhibitors of protein kinase C or p38 MAPK. Intravenous administration of kisspeptin into conscious, unrestrained rats caused an increase in circulating insulin levels, whilst central administration of kisspeptin had no effect, indicating a peripheral site of action.

CONCLUSIONS/INTERPRETATION

These observations suggest that neither typical protein kinase C isoforms nor p38 MAPK are involved in the potentiation of glucose-induced insulin release by kisspeptin, but intracellular signalling pathways involving phospholipase C, p42/44 MAPK and increased [Ca(2+)](i) are required for the stimulatory effects on insulin secretion. The observation that kisspeptin is also capable of stimulating insulin release in vivo supports the conclusion that kisspeptin is a regulator of beta cell function.

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.

Emerging roles for protein histidine phosphorylation in cellular signal transduction: lessons from the islet beta-cell

Protein phosphorylation represents one of the key regulatory events in physiological insulin secretion from the islet β-cell. In this context, several classes of protein kinases (e.g. calcium-, cyclic nucleotide- and phospholipid-dependent protein kinases and tyrosine kinases) have been characterized in the β-cell. The majority of phosphorylated amino acids identified include phosphoserine, phosphothreonine and phosphotyrosine. Protein histidine phosphorylation has been implicated in the prokaryotic and eukaryotic cellular signal transduction. Most notably, phoshohistidine accounts for 6% of total protein phosphorylation in eukaryotes, which makes it nearly 100-fold more abundant than phosphotyrosine, but less abundant than phosphoserine and phosphothreonine. However, very little is known about the number of proteins with phosphohistidines, since they are highly labile and are rapidly lost during phosphoamino acid identification under standard experimental conditions. The overall objectives of this review are to: (i) summarize the existing evidence indicating the subcellular distribution and characterization of various histidine kinases in the islet β-cell, (ii) describe evidence for functional regulation of these kinases by agonists of insulin secretion, (iii) present a working model to implicate novel regulatory roles for histidine kinases in the receptor-independent activation, by glucose, of G-proteins endogenous to the β-cell, (iv) summarize evidence supporting the localization of protein histidine phosphatases in the islet β-cell and (v) highlight experimental evidence suggesting potential defects in the histidine kinase signalling cascade in islets derived from the Goto-Kakizaki (GK) rat, a model for type 2 diabetes. Potential avenues for future research to further decipher regulatory roles for protein histidine phosphorylation in physiological insulin secretion are also discussed.

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.

Bridging the gap between protein carboxyl methylation and phospholipid methylation to understand glucose-stimulated insulin secretion from the pancreatic beta cell

Recent findings have implicated post-translational modifications at C-terminal cysteines [e.g., methylation] of specific proteins [e.g., G-proteins] in glucose-stimulated insulin secretion [GSIS]. Furthermore, methylation at the C-terminal leucine of the catalytic subunit of protein phosphatase 2A [PP2Ac] has also been shown to be relevant for GSIS. In addition to these two classes of protein methyl transferases, a novel class of glucose-activated phospholipid methyl transferases have also been identified in the beta cell. These enzymes catalyze three successive methylations of phosphatidylethanolamine to yield phosphatidylcholine. The "newly formed" phosphatidylcholine is felt to induce alterations in the membrane fluidity, which might favor vesicular fusion with the plasma membrane for the exocytosis of insulin. The objectives of this commentary are to: (i) review the existing evidence on the regulation, by glucose and other insulin secretagogues, of post-translational carboxylmethylation [CML] of specific proteins in the beta cell; (ii) discuss the experimental evidence, which implicates regulation, by glucose and other insulin secretagogues, of phosphatidylethanolamine methylation in the islet beta cell; (iii) propose a model for potential cross-talk between the protein and lipid methylation pathways in the regulation of GSIS and (iv) highlight potential avenues for future research, including the development of specific pharmacological inhibitors to further decipher regulatory roles for these methylation reactions in islet beta cell function.

ERK1/2 control phosphorylation and protein level of cAMP-responsive element-binding protein: a key role in glucose-mediated pancreatic beta-cell survival

cAMP-responsive element-binding protein (CREB) is required for beta-cell survival by regulating expression of crucial genes such as bcl-2 and IRS-2. Using MIN6 cells and isolated rat pancreatic islets, we investigated the signaling pathway that controls phosphorylation and protein level of CREB. We observed that 10 mmol/l glucose-induced CREB phosphorylation was totally inhibited by the protein kinase A (PKA) inhibitor H89 (2 micromol/l) and reduced by 50% with the extracellular signal-regulated kinase (ERK)1/2 inhibitor PD98059 (20 micromol/l). This indicates that ERK1/2, reported to be located downstream of PKA, participates in the PKA-mediated CREB phosphorylation elicited by glucose. In ERK1/2-downregulated MIN6 cells by siRNA, glucose-stimulated CREB phosphorylation was highly reduced and CREB protein content was decreased by 60%. In MIN6 cells and islets cultured for 24-48 h in optimal glucose concentration (10 mmol/l), which promotes survival, blockade of ERK1/2 activity with PD98059 caused a significant decrease in CREB protein level, whereas CREB mRNA remained unaffected (measured by real-time quantitative PCR). This was associated with loss of bcl-2 mRNA and protein contents, caspase-3 activation, and emergence of ultrastructural apoptotic features detected by electron microscopy. Our results indicate that ERK1 and -2 control the phosphorylation and protein level of CREB and play a key role in glucose-mediated pancreatic beta-cell survival.

Dynamics of glucose-induced localization of PKC isoenzymes in pancreatic beta-cells: diabetes-related changes in the GK rat

Glucose metabolism affects most major signal pathways in pancreatic beta-cells. Multiple protein kinases, including protein kinase C (PKC) isoenzymes, are involved in these effects; however, their role is poorly defined. Moreover, the dynamics of kinase isoenzyme activation in reference to the biphasic insulin secretion is unknown. In perfused pancreas of Wistar rats, PKCalpha staining was strongly associated with insulin staining, jointly accumulating in the vicinity of the plasma membrane during early first-phase insulin response. The signal declined before the onset of second phase and reappeared during second-phase insulin release as foci, only weekly associated with insulin staining; this signal persisted for at least 15 min after glucose stimulation. In the GK rat, glucose had minimal effect on beta-cell PKCalpha. In control beta-cells, PKCdelta stained as granulated foci with partial association with insulin staining; however, no glucose-dependent translocation was observed. In the GK rat, only minimal staining for PKCdelta was observed, increasing exclusively during early first-phase secretion. In Wistar beta-cells, PKCepsilon concentrated near the nucleus, strongly associated with insulin staining, with dynamics resembling that of biphasic insulin response, but persisting for 15 min after cessation of stimulation. In GK rats, PKCepsilon staining lacked glucose-dependent changes or association with insulin. PKCzeta exhibited bimodal dynamics in control beta-cells: during early first phase, accumulation near the cell membrane was observed, dispersing thereafter. This was followed by a gradual accumulation near the nucleus; 15 min after glucose stimulus, clear PKCzeta staining was observed within the nucleus. In the GK rat, a similar response was only occasionally observed. In control beta-cells, glucose stimulation led to a transient recruitment of PKCtheta, associated with first-phase insulin release, not seen in GK beta-cell. Data from this and related studies support a role for PKCalpha in glucose-induced insulin granule recruitment for exocytosis; a role for PKCepsilon in activation of insulin granules for exocytosis and/or in the glucose-generated time-dependent potentiation signal for insulin release; and a dual function for PKCzeta in initiating insulin release and in a regulatory role in the transcriptional machinery. Furthermore, diminished levels and/or activation of PKCalpha, PKCepsilon, PKCtheta, and PKCzeta could be part of the defective signals downstream to glucose metabolism responsible for the deranged insulin secretion in the GK rat.

A highly Ca2+-sensitive pool of granules is regulated by glucose and protein kinases in insulin-secreting INS-1 cells

We have used membrane capacitance measurements and carbon-fiber amperometry to assay exocytosis triggered by photorelease of caged Ca²⁺ to directly measure the Ca²⁺ sensitivity of exocytosis from the INS-1 insulin-secreting cell line. We find heterogeneity of the Ca²⁺ sensitivity of release in that a small proportion of granules makes up a highly Ca²⁺-sensitive pool (HCSP), whereas the bulk of granules have a lower sensitivity to Ca²⁺. A substantial HCSP remains after brief membrane depolarization, suggesting that the majority of granules with high sensitivity to Ca²⁺ are not located close to Ca²⁺ channels. The HCSP is enhanced in size by glucose, cAMP, and a phorbol ester, whereas the Ca²⁺-sensitive rate constant of exocytosis from the HCSP is unaffected by cAMP and phorbol ester. The effects of cAMP and phorbol ester on the HCSP are mediated by PKA and PKC, respectively, because they can be blocked with specific protein kinase inhibitors. The size of the HCSP can be enhanced by glucose even in the presence of high concentrations of phorbol ester or cAMP, suggesting that glucose can increase granule pool sizes independently of activation of PKA or PKC. The effects of PKA and PKC on the size of the HCSP are not additive, suggesting they converge on a common mechanism. Carbon-fiber amperometry was used to assay quantal exocytosis of serotonin (5-HT) from insulin-containing granules following preincubation of INS-1 cells with 5-HT and a precursor. The amount or kinetics of release of 5-HT from each granule is not significantly different between granules with higher or lower sensitivity to Ca²⁺, suggesting that granules in these two pools do not differ in morphology or fusion kinetics. We conclude that glucose and second messengers can modulate insulin release triggered by a high-affinity Ca²⁺ sensor that is poised to respond to modest, global elevations of [Ca²⁺]_i.

The G115S mutation associated with maturity-onset diabetes of the young impairs hepatocyte nuclear factor 4alpha activities and introduces a PKA phosphorylation site in its DNA-binding domain

HNF4alpha (hepatocyte nuclear factor 4alpha) belongs to a complex transcription factor network that is crucial for the function of hepatocytes and pancreatic beta-cells. In these cells, it activates the expression of a very large number of genes, including genes involved in the transport and metabolism of glucose and lipids. Mutations in the HNF4alpha gene correlate with MODY1 (maturity-onset diabetes of the young 1), a form of type II diabetes characterized by an impaired glucose-induced insulin secretion. The MODY1 G115S (Gly115-->Ser) HNF4alpha mutation is located in the DNA-binding domain of this nuclear receptor. We show here that the G115S mutation failed to affect HNF4alpha-mediated transcription on apolipoprotein promoters in HepG2 cells. Conversely, in pancreatic beta-cell lines, this mutation resulted in strong impairments of HNF4alpha transcriptional activity on the promoters of LPK (liver pyruvate kinase) and HNF1alpha, with this transcription factor playing a key role in endocrine pancreas. We show as well that the G115S mutation creates a PKA (protein kinase A) phosphorylation site, and that PKA-mediated phosphorylation results in a decreased transcriptional activity of the mutant. Moreover, the G115E (Gly115-->Glu) mutation mimicking phosphorylation reduced HNF4alpha DNA-binding and transcriptional activities. Our results may account for the 100% penetrance of diabetes in human carriers of this mutation. In addition, they suggest that introduction of a phosphorylation site in the DNA-binding domain may represent a new mechanism by which a MODY1 mutation leads to loss of HNF4alpha function.

Differential regulation by fatty acids of protein histidine phosphorylation in rat pancreatic islets

Long-chain fatty acids (e.g. arachidonic acid) have been implicated in physiological control of insulin secretion. We previously reported histidine phosphorylation of at least two islet proteins (e.g., NDP kinase and the beta subunit of trimeric G-proteins), and suggested that such a signalling step may have regulatory roles in beta cell signal transduction, specifically at the level of G-protein activation. Since our earlier findings also indicated potential regulation by long-chain fatty acids of islet G-proteins, we undertook the current study to verify putative regulation, by fatty acids, of protein histidine phosphorylation of NDP kinase and Gbeta subunit in normal rat islets. The phosphoenzyme formation of NDP kinase was stimulated by various fatty acids in the following rank order: linoleic acid > arachidonic acid > oleic acid > palmitic acid = stearic acid = control. Furthermore, the catalytic activity of NDP kinase was stimulated by these fatty acids in the rank order of: oleic acid > arachidonic acid > linoleic acid > palmitic acid = stearic acid = control. Arachidonic acid methyl ester, an inactive analog of arachidonic acid, did not significantly affect either the phosphoenzyme formation or the catalytic activity of NDP kinase. Interestingly, arachidonic acid exerted dual effects on the histidine phosphorylation of beta subunit; it significantly stimulated the phosphorylation at 33 microM beyond which it was inhibitory. Together, these findings identify additional loci (e.g., NDP kinase and Gbeta subunit) at which unsaturated, but not saturated, fatty acids could exert their intracellular effects leading to exocytotic secretion of insulin.

Pathways in beta-cell stimulus-secretion coupling as targets for therapeutic insulin secretagogues

Physiologically, insulin secretion is subject to a dual, hierarchal control by triggering and amplifying pathways. By closing ATP-sensitive K+ channels (KATP channels) in the plasma membrane, glucose and other metabolized nutrients depolarize beta-cells, stimulate Ca2+ influx, and increase the cytosolic concentration of free Ca2+ ([Ca2+]i), which constitutes the indispensable triggering signal to induce exocytosis of insulin granules. The increase in beta-cell metabolism also generates amplifying signals that augment the efficacy of Ca2+ on the exocytotic machinery. Stimulatory hormones and neurotransmitters modestly increase the triggering signal and strongly activate amplifying pathways biochemically distinct from that set into operation by nutrients. Many drugs can increase insulin secretion in vitro, but only few have a therapeutic potential. This review identifies six major pathways or sites of stimulus-secretion coupling that could be aimed by potential insulin-secreting drugs and describes several strategies to reach these targets. It also discusses whether these perspectives are realistic or theoretical only. These six possible beta-cell targets are 1) stimulation of metabolism, 2) increase of [Ca2+]i by closure of K+ ATP channels, 3) increase of [Ca2+]i by other means, 4) stimulation of amplifying pathways, 5) action on membrane receptors, and 6) action on nuclear receptors. The theoretical risk of inappropriate insulin secretion and, hence, of hypoglycemia linked to these different approaches is also envisaged.

The role of cytosolic phospholipase A(2) in insulin secretion

Cytosolic phospholipase A(2) (cPLA(2)) comprises a widely expressed family of enzymes, some members of which have the properties required of signal transduction elements in electrically excitable cells. Thus, alpha- and beta-isoforms of cPLA(2) are activated by the increases in intracellular Ca(2+) concentration ([Ca(2+)](i)) achieved in depolarized cells. Activation is associated with a redistribution of the enzyme within the cell; activation of cPLA(2) generates arachidonic acid (AA), a biologically active unsaturated fatty acid that can be further metabolized to generate a plethora of biologically active molecules. Studies using relatively nonselective pharmacological inhibitors have implicated cPLA(2) in insulin secretory responses to stimuli that elevate beta-cell [Ca(2+)](i); therefore, we have investigated the role of cPLA(2) in beta-cell function by generating beta-cell lines that under- or overexpress the alpha-isoform of cPLA(2). The functional phenotype of the modified cells was assessed by observation of cellular ultrastructure, by measuring insulin gene expression and insulin protein content, and by measuring the effects of insulin secretagogues on cPLA(2) distribution, on changes in [Ca(2+)](i), and on the rate and pattern of insulin secretion. Our results suggest that cPLA(2) is not required for the initiation of insulin secretion from beta-cells, but that it plays an important role in the maintenance of beta-cell insulin stores. Our data also demonstrate that excessive production of, or exposure to, AA is deleterious to normal beta-cell secretory function through metabolic dysfunction.

Protein kinase A translocation and insulin secretion in pancreatic beta-cells: studies with adenylate cyclase toxin from Bordetella pertussis

Activation of protein kinase A (cAMP-dependent protein kinase; PKA) triggers insulin secretion in the beta-cell. Adenylate cyclase toxin (ACT), a bacterial exotoxin with adenylate cyclase activity, and forskolin, an activator of adenylate cyclase, both dose-dependently increased insulin secretion in the presence, but not the absence, of glucose in insulin-secreting betaTC3 cells. The stimulation of cAMP release by either agent was dose-dependent but glucose-independent. Omission of extracellular Ca(2+) totally abolished the effects of ACT on insulin secretion and cytosolic cAMP accumulation. ACT and forskolin caused rapid and dramatic increases in cytosolic Ca(2+), which were blocked by nifedipine and the omission of extracellular Ca(2+). Omission of glucose completely blocked the effects of forskolin and partially blocked the effects of ACT on cytosolic Ca(2+). PKA alpha, beta and gamma catalytic subunits (Calpha, Cbeta and Cgamma respectively) were identified in betaTC6 cells by confocal microscopy. Glucose and glucagon-like polypeptide-1 (GLP-1) caused translocation of Calpha to the nucleus and of Cbeta to the plasma membrane and the nucleus, but did not affect the distribution of Cgamma. In conclusion, glucose and GLP-1 amplify insulin secretion via cAMP production and PKAbeta activation.