Cyclic AMP/cAMP-GEF pathway amplifies insulin exocytosis induced by Ca2+ and ATP in rat islet beta-cells

Pharmacogenomics of GLP-1 receptor agonists: Focus on pharmacological profile

Type 2 Diabetes mellitus (T2DM) is a multifactorial metabolic disorder also known as a silent killer disease. Macrovascular and microvascular complications associated with diabetes worsen the condition leading to higher comorbidity and mortality rate. Currently, available treatment strategies for diabetes include biguanides, sulfonylureas, alpha-glucosidase inhibitors, thiazolidinediones, insulin and its analogs, DPP-4 (dipeptidyl-peptidase-4) inhibitors, SGLT-2 inhibitors, and Glucagon Like Peptide-1 receptor agonists (GLP-1RAs). Synthetic agonists of GLP-1 hormone, GLP-1RAs are an emerging class of anti-diabetic drugs which target the pathophysiology of diabetes through various mechanisms and at multiple sites. They promote insulin secretion from beta cells, and the proliferation of beta cells inhibits glucagon secretion, delays gastric emptying and induces satiety. However, treatment is reported to be associated with inter-individual variations and adverse drug reactions, which are also influenced by genetic variations. There have been a few pharmacogenetic studies have been carried out on this drug class. This review discusses all the available GLP-1RAs, their pharmacokinetics, pharmacodynamics and genetic variation affecting the inter-individual variation.

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

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

Cyclic AMP signaling in pancreatic islets

Cyclic 3'5'AMP (cAMP) is an important physiological amplifier of glucose-induced insulin secretion by the pancreatic islet beta-cell, where it is formed by the activity of adenylyl cyclases, which are stimulated by glucose, through elevation in intracellular calcium concentrations, and by the incretin hormones (GLP-1 and GIP). cAMP is rapidly degraded in the pancreatic islet beta-cell by various cyclic nucleotide phosphodiesterase (PDE) enzymes. Many steps involved in glucose-induced insulin secretion are modulated by cAMP, which is also important in regulating pancreatic islet beta-cell differentiation, growth and survival. This chapter discusses the formation, destruction and actions of cAMP in the islets with particular emphasis on the beta-cell.

PKA-dependent potentiation of glucose-stimulated insulin secretion by Epac activator 8-pCPT-2'-O-Me-cAMP-AM in human islets of Langerhans

Potential insulin secretagogue properties of an acetoxymethyl ester of a cAMP analog (8-pCPT-2'-O-Me-cAMP-AM) that activates the guanine nucleotide exchange factors Epac1 and Epac2 were assessed using isolated human islets of Langerhans. RT-QPCR demonstrated that the predominant variant of Epac expressed in human islets was Epac2, although Epac1 was detectable. Under conditions of islet perifusion, 8-pCPT-2'-O-Me-cAMP-AM (10 microM) potentiated first- and second-phase 10 mM glucose-stimulated insulin secretion (GSIS) while failing to influence insulin secretion measured in the presence of 3 mM glucose. The insulin secretagogue action of 8-pCPT-2'-O-Me-cAMP-AM was associated with depolarization and an increase of [Ca(2+)](i) that reflected both Ca(2+) influx and intracellular Ca(2+) mobilization in islet beta-cells. As expected for an Epac-selective cAMP analog, 8-pCPT-2'-O-Me-cAMP-AM (10 microM) failed to stimulate phosphorylation of PKA substrates CREB and Kemptide in human islets. Furthermore, 8-pCPT-2'-O-Me-cAMP-AM (10 microM) had no significant ability to activate AKAR3, a PKA-regulated biosensor expressed in human islet cells by viral transduction. Unexpectedly, treatment of human islets with an inhibitor of PKA activity (H-89) or treatment with a cAMP antagonist that blocks PKA activation (Rp-8-CPT-cAMPS) nearly abolished the action of 8-pCPT-2'-O-Me-cAMP-AM to potentiate GSIS. It is concluded that there exists a permissive role for PKA activity in support of human islet insulin secretion that is both glucose dependent and Epac regulated. This permissive action of PKA may be operative at the insulin secretory granule recruitment, priming, and/or postpriming steps of Ca(2+)-dependent exocytosis.

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.

Epac: defining a new mechanism for cAMP action

cAMP is a second messenger that is essential for relaying hormonal responses in many biological processes. The discovery of the cAMP target Epac explained various effects of cAMP that could not be attributed to the established targets PKA and cyclic nucleotide-gated ion channels. Epac1 and Epac2 function as guanine nucleotide exchange factors for the small G protein Rap. cAMP analogs that selectively activate Epac have helped to reveal a role for Epac in processes ranging from insulin secretion to cardiac contraction and vascular permeability. Advances in the understanding of the activation mechanism of Epac and its regulation by diverse anchoring mechanisms have helped to elucidate the means by which cAMP fulfills these functions via Epac.

Glucose-dependent potentiation of mouse islet insulin secretion by Epac activator 8-pCPT-2'-O-Me-cAMP-AM

Epac2 is a cAMP-regulated guanine nucleotide exchange factor (cAMP-GEF) that is proposed to mediate stimulatory actions of the second messenger cAMP on mouse islet insulin secretion. Here we have used methods of islet perifusion to demonstrate that the acetoxymethyl ester (AM-ester) of an Epac-selective cAMP analog (ESCA) penetrates into mouse islets and is capable of potentiating both first and second phases of glucose-stimulated insulin secretion (GSIS). When used at low concentrations (1-10 μM), 8-pCPT-2'-O-Me-cAMP-AM activates Rap1 GTPase but exhibits little or no ability to activate protein kinase A (PKA), as validated in assays of in vitro PKA activity (phosphorylation of Kemptide), Ser (133) CREB phosphorylation status, RIP1-CRE-Luc reporter gene activity, and PKA-dependent AKAR3 biosensor activation. Since quantitative PCR demonstrates Epac2 mRNA to be expressed at levels ca. 5.3-fold greater than that of Epac1, available evidence indicates that Epac2 does in fact mediate stimulatory actions of cAMP on mouse islet GSIS.

Insulinostatic activity of cerebellin--evidence from in vivo and in vitro studies in rats

Cerebellin (CER) is a neuromodulatory hexadecapeptide that originates from the precursor protein precerebellin (Cbln1). Four highly homologous isoforms of Cbln are known (Cbln1-Cbln4), which are expressed in the central nervous system (CNS) and in peripheral tissues. CER modulates synaptic structure formation in the CNS, whereas in the peripheral tissues CER regulates catecholamine secretion. Cbln is also expressed in the pancreas; however, its function in the pancreas is unknown. Here, we demonstrate the role of CER in regulating insulin secretion in vivo and in vitro. We identified Cbln1 and Cbln3 transcripts in rat pancreatic islets and detected Cbln-immunoreactivity, predominantly located in the periphery of the rat endocrine pancreas. In vivo, CER reduced plasma insulin levels in rats after 1 and 2 h. CER decreased insulin secretion from isolated rat pancreatic islets at high (11 mM), but not at low (3.33 mM) glucose concentration. CER inhibited stimulated insulin secretion from clonal rat insulinoma (INS-1) cells, reduced forskolin-induced production of cAMP and intracellular calcium concentration. Our study demonstrates for the first time that Cbln1 and Cbln3 are expressed in the rat endocrine pancreas. Furthermore, we identify CER as an insulinostatic factor, which decreases intracellular cAMP production and calcium in INS-1 cells.

Enhanced Rap1 activation and insulin secretagogue properties of an acetoxymethyl ester of an Epac-selective cyclic AMP analog in rat INS-1 cells: studies with 8-pCPT-2'-O-Me-cAMP-AM

To ascertain the identities of cyclic nucleotide-binding proteins that mediate the insulin secretagogue action of cAMP, the possible contributions of the exchange protein directly activated by cAMP (Epac) and protein kinase A (PKA) were evaluated in a pancreatic beta cell line (rat INS-1 cells). Assays of Rap1 activation, CREB phosphorylation, and PKA-dependent gene expression were performed in combination with live cell imaging and high throughput screening of a fluorescence resonance energy transfer-based cAMP sensor (Epac1-camps) to validate the selectivity with which acetoxymethyl esters (AM-esters) of cAMP analogs preferentially activate Epac or PKA. Selective activation of Epac or PKA was achieved following exposure of INS-1 cells to 8-pCPT-2'-O-Me-cAMP-AM or Bt(2)cAMP-AM, respectively. Both cAMP analogs exerted dose-dependent and glucose metabolism-dependent actions to stimulate insulin secretion, and when each was co-administered with the other, a supra-additive effect was observed. Because 2.4-fold more insulin was secreted in response to a saturating concentration (10 microm) of Bt(2)cAMP-AM as compared with 8-pCPT-2'-O-Me-cAMP-AM, and because the action of Bt(2)cAMP-AM but not 8-pCPT-2'-O-Me-cAMP-AM was nearly abrogated by treatment with 3 microm of the PKA inhibitor H-89, it is concluded that for INS-1 cells, it is PKA that acts as the dominant cAMP-binding protein in support of insulin secretion. Unexpectedly, 10-100 microm of the non-AM-ester of 8-pCPT-2'-O-Me-cAMP failed to stimulate insulin secretion and was a weak activator of Rap1 in INS-1 cells. Moreover, 10 microm of the AM-ester of 8-pCPT-2'-O-Me-cAMP stimulated insulin secretion from mouse islets, whereas the non-AM-ester did not. Thus, the membrane permeability of 8-pCPT-2'-O-Me-cAMP in insulin-secreting cells is so low as to limit its biological activity. It is concluded that prior reports documenting the failure of 8-pCPT-2'-O-Me-cAMP to act in beta cells, or other cell types, need to be re-evaluated through the use of the AM-ester of this cAMP analog.

P2 purinergic signalling in the pancreatic beta-cell: control of insulin secretion and pharmacology

Extracellular adenosine triphosphate is able to modulate pancreatic beta-cell function, acting on P2 purinergic ionotropic (P2X) and metabotropic (P2Y) receptors. Physiologically, ATP entrains beta-cells into a common rhythm by coordinating Ca(2+) oscillations; it plays a central role in insulin secretion pulsatility. ATP also triggers a positive feedback signal amplifying glucose-induced insulin release, which argues for a potential pharmacological application. ATP has consistently been shown to increase cytoplasmic free calcium concentration, notably in human tissue. Acting on P2X receptors, of which different molecular subtypes are expressed in beta-cells, it leads to a transient insulin release that may involve a closure of K(ATP) channels or a rapidly decaying inward current. Activation of G-protein-coupled P2Y receptors triggers different signalling pathways and amplifies insulin release in a glucose-dependent way. It has recently been shown that pancreatic beta-cells express different molecular subtypes of receptors, which may explain the complex interaction of P2Y ligands on high- and low-affinity binding sites. Despite the complexity of this purinergic pharmacology, consistent pre-clinical data suggest the potential of P2Y receptor agonists as drug candidates for type 2 diabetes.

Epac-selective cAMP analogs: new tools with which to evaluate the signal transduction properties of cAMP-regulated guanine nucleotide exchange factors

The identification of 2'-O-methyl substituted adenosine-3',5'-cyclic monophosphate (cAMP) analogs that activate the Epac family of cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs, also known as Epac1 and Epac2), has ushered in a new era of cyclic nucleotide research in which previously unrecognized signalling properties of the second messenger cAMP have been revealed. These Epac-Selective Cyclic AMP Analogs (ESCAs) incorporate a 2'-O-methyl substitution on the ribose ring of cAMP, a modification that impairs their ability to activate protein kinase A (PKA), while leaving intact their ability to activate Epac (the Exchange Protein directly Activated by Cyclic AMP). One such ESCA in wide-spread use is 8-pCPT-2'-O-Me-cAMP. It is a cell-permeant derivative of 2'-O-Me-cAMP, and it is a super activator of Epac. A wealth of newly published studies demonstrate that 8-pCPT-2'-O-Me-cAMP is a unique tool with which to asses atypical actions of cAMP that are PKA-independent. Particularly intriguing are recent reports demonstrating that ESCAs reproduce the PKA-independent actions of ligands known to stimulate Class I (Family A) and Class II (Family B) GTP-binding protein-coupled receptors (GPCRs). This topical review summarizes the current state of knowledge regarding the molecular pharmacology and signal transduction properties of Epac-selective cAMP analogs. Special attention is focused on the rational drug design of ESCAs in order to improve their Epac selectivity, membrane permeability, and stability. Also emphasized is the usefulness of ESCAs as new tools with which to assess the role of Epac as a determinant of intracellular Ca2+ signalling, ion channel function, neurotransmitter release, and hormone secretion.

A1 receptor deficiency causes increased insulin and glucagon secretion in mice

Adenosine influences metabolism and the adenosine receptor antagonist caffeine decreases the risk of type 2 diabetes. In this study the metabolic role of one adenosine receptor subtype, the adenosine A(1)R, was evaluated in mice lacking this receptor [A(1)R (-/-)]. The HbA1c levels and body weight were not significantly different between wild type [A(1)R (+/+)] and A(1)R (-/-) mice (3-4 months) fed normal lab chow. At rest, plasma levels of glucose, insulin and glucagon were similar in both genotypes. Following glucose injection, glucose tolerance was not appreciably altered in A(1)R (-/-) mice. Glucose injection induced sustained increases in plasma insulin and glucagon levels in A(1)R (-/-) mice, whereas A(1)R (+/+) control mice reacted with the expected transient increase in insulin and decrease in glucagon levels. Pancreas perfusion experiments showed that A(1)R (-/-) mice had a slightly higher basal insulin secretion than A(1)R (+/+) mice. The first phase insulin secretion (initiated with 16.7 mM glucose) was of the same magnitude in both genotypes, but the second phase was significantly enhanced in the A(1)R (-/-) pancreata compared with A(1)R (+/+). Insulin- and contraction-mediated glucose uptake in skeletal muscle were not significantly different between in A(1)R (-/-) and A(1)R (+/+) mice. All adenosine receptors were expressed at mRNA level in skeletal muscle in A(1)R (+/+) mice and the mRNA A(2A)R, A(2B)R and A(3)R levels were similar in A(1)R (-/-) and A(1)R (+/+) mice. In conclusion, the A(1)R minimally affects muscle glucose uptake, but is important in regulating pancreatic islet function.

Activation of exchange protein directly activated by cyclic adenosine monophosphate and protein kinase A regulate common and distinct steps in promoting plasma membrane exocytic and granule-to-granule fusions in rat islet beta cells

OBJECTIVES

Using FM1-43 epifluorescence imaging and electron microscopy, we recently reported that glucagon-like peptide (GLP-1)-mediated cyclic adenosine monophosphate (cAMP) potentiation of insulin secretion markedly promotes the number of plasma membrane (PM) exocytic sites and insulin secretory granule (SG)-to-granule fusions underlying compound and sequential exocytosis.

METHODS

Here, we used FM1-43 imaging to dissect the distinct contributions of putative GLP-1/cAMP activated substrates--exchange protein directly activated by cAMP (EPAC) and protein kinase A (PKA)--in mediating these exocytic events.

RESULTS

Like GLP-1, cAMP activation by forskolin increased the number of PM exocytic sites (2.3-fold), which were mainly of the robust-sustained (55.8%) and stepwise-multiphasic (37.7%) patterns corresponding to compound and sequential SG-SG exocytosis, respectively, with few monophasic hotspots (6.5%) corresponding to single-granule exocytosis. Direct activation of EPAC by 8-(4-chloro-phenylthio)-2'-O-methyladenosine-3',5'-cAMP also increased the number of exocytic sites, but which were mainly multiphasic (60%) and monophasic (40%) hotspots. Protein kinase A inhibition by H89 blocked forskolin-evoked robust-sustained hotspots, while retaining multiphasic (47%) and monophasic (53%) hotspots. Consistently, PKA activation (N6-benzoyladenosine-3',5'-cAMP) evoked only multiphasic (60%) and monophasic (40%) hotspots. These results suggested that PKA activation is required but alone is insufficient to promote compound SG-SG fusions. 8-(4-Chloro-phenylthio)-2'-O-methyladenosine-3',5'-cAMP plus N6-benzoyladenosine-3',5'-cAMP stimulation completely reconstituted the effects of forskolin, including increasing the number of exocytic sites, with a similar pattern of robust-sustained (42.6%) and stepwise (39.6%) hotspots and few monophasic (17.8%) hotspots.

CONCLUSIONS

The EPAC and PKA modulate both distinct and common exocytic steps to potentiate insulin exocytosis where (a) EPAC activation mobilizes SGs to fuse at the PM, thereby increasing number of PM exocytic sites; and (b) PKA and EPAC activation synergistically modulate SG-SG fusions underlying compound and sequential exocytoses.

Central insulin action in energy and glucose homeostasis

Insulin has pleiotropic biological effects in virtually all tissues. However, the relevance of insulin signaling in peripheral tissues has been studied far more extensively than its role in the brain. An evolving body of evidence indicates that in the brain, insulin is involved in multiple regulatory mechanisms including neuronal survival, learning, and memory, as well as in regulation of energy homeostasis and reproductive endocrinology. Here we review insulin's role as a central homeostatic signal with regard to energy and glucose homeostasis and discuss the mechanisms by which insulin communicates information about the body's energy status to the brain. Particular emphasis is placed on the controversial current debate about the similarities and differences between hypothalamic insulin and leptin signaling at the molecular level.

Cell physiology of cAMP sensor Epac

Epac is an acronym for the exchange proteins activated directly by cyclic AMP, a family of cAMP-regulated guanine nucleotide exchange factors (cAMPGEFs) that mediate protein kinase A (PKA)-independent signal transduction properties of the second messenger cAMP. Two variants of Epac exist (Epac1 and Epac2), both of which couple cAMP production to the activation of Rap, a small molecular weight GTPase of the Ras family. By activating Rap in an Epac-mediated manner, cAMP influences diverse cellular processes that include integrin-mediated cell adhesion, vascular endothelial cell barrier formation, and cardiac myocyte gap junction formation. Recently, the identification of previously unrecognized physiological processes regulated by Epac has been made possible by the development of Epac-selective cyclic AMP analogues (ESCAs). These cell-permeant analogues of cAMP activate both Epac1 and Epac2, whereas they fail to activate PKA when used at low concentrations. ESCAs such as 8-pCPT-2'-O-Me-cAMP and 8-pMeOPT-2'-O-Me-cAMP are reported to alter Na(+), K(+), Ca(2+) and Cl(-) channel function, intracellular [Ca(2+)], and Na(+)-H(+) transporter activity in multiple cell types. Moreover, new studies examining the actions of ESCAs on neurons, pancreatic beta cells, pituitary cells and sperm demonstrate a major role for Epac in the stimulation of exocytosis by cAMP. This topical review provides an update concerning novel PKA-independent features of cAMP signal transduction that are likely to be Epac-mediated. Emphasized is the emerging role of Epac in the cAMP-dependent regulation of ion channel function, intracellular Ca(2+) signalling, ion transporter activity and exocytosis.

cAMP sensor Epac as a determinant of ATP-sensitive potassium channel activity in human pancreatic beta cells and rat INS-1 cells

The Epac family of cAMP-regulated guanine nucleotide exchange factors (cAMPGEFs, also known as Epac1 and Epac2) mediate stimulatory actions of the second messenger cAMP on insulin secretion from pancreatic beta cells. Because Epac2 is reported to interact in vitro with the isolated nucleotide-binding fold-1 (NBF-1) of the beta-cell sulphonylurea receptor-1 (SUR1), we hypothesized that cAMP might act via Epac1 and/or Epac2 to inhibit beta-cell ATP-sensitive K+ channels (K(ATP) channels; a hetero-octomer of SUR1 and Kir6.2). If so, Epac-mediated inhibition of K(ATP) channels might explain prior reports that cAMP-elevating agents promote beta-cell depolarization, Ca2+ influx and insulin secretion. Here we report that Epac-selective cAMP analogues (2'-O-Me-cAMP; 8-pCPT-2'-O-Me-cAMP; 8-pMeOPT-2'-O-Me-cAMP), but not a cGMP analogue (2'-O-Me-cGMP), inhibit the function of K(ATP) channels in human beta cells and rat INS-1 insulin-secreting cells. Inhibition of K(ATP) channels is also observed when cAMP, itself, is administered intracellularly, whereas no such effect is observed upon administration N6-Bnz-cAMP, a cAMP analogue that activates protein kinase A (PKA) but not Epac. The inhibitory actions of Epac-selective cAMP analogues at K(ATP) channels are mimicked by a cAMP agonist (8-Bromoadenosine-3', 5'-cyclic monophosphorothioate, Sp-isomer, Sp-8-Br-cAMPS), but not a cAMP antagonist (8-Bromoadenosine-3', 5'-cyclic monophosphorothioate, Rp-isomer, Rp-8-Br-cAMPS), and are abrogated following transfection of INS-1 cells with a dominant-negative Epac1 that fails to bind cAMP. Because both Epac1 and Epac2 coimmunoprecipitate with full-length SUR1 in HEK cell lysates, such findings delineate a novel mechanism of second messenger signal transduction in which cAMP acts via Epac to modulate ion channel function, an effect measurable as the inhibition of K(ATP) channel activity in pancreatic beta cells.