Selective inhibitors of GTP synthesis impede exocytotic insulin release from intact rat islets

Underappreciated roles for Rho GDP dissociation inhibitors (RhoGDIs) in cell function: Lessons learned from the pancreatic islet β-cell

Rho subfamily of G proteins (e.g., Rac1) have been implicated in glucose-stimulated insulin secretion from the pancreatic β-cell. Interestingly, metabolic stress (e.g., chronic exposure to high glucose) results in sustained activation of Rac1 leading to increased oxidative stress, impaired insulin secretion and β-cell dysfunction. Activation-deactivation of Rho G proteins is mediated by three classes of regulatory proteins, namely the guanine nucleotide exchange factors (GEFs), which facilitate the conversion of inactive G proteins to their active conformations; the GTPase-activating proteins (GAPs), which convert the active G proteins to their inactive forms); and the GDP-dissociation inhibitors (GDIs), which prevent the dissociation of GDP from G proteins. Contrary to a large number of GEFs (82 members) and GAPs (69 members), only three members of RhoGDIs (RhoGDIα, RhoGDIβ and RhoGDIγ) are expressed in mammalian cells.Even though relatively smaller in number, the GDIs appear to play essential roles in G protein function (e.g., subcellular targeting) for effector activation and cell regulation. Emerging evidence also suggests that the GDIs are functionally regulated via post-translational modification (e.g., phosphorylation) and by lipid second messengers, lipid kinases and lipid phosphatases. We highlight the underappreciated regulatory roles of RhoGDI-Rho G protein signalome in islet β-cell function in health and metabolic stress. Potential knowledge gaps in the field, and directions for future research for the identification of novel therapeutic targets to loss of functional β-cell mass under the duress of metabolic stress are highlighted.

Emerging Roles of Small GTPases in Islet β-Cell Function

Several small guanosine triphosphatases (GTPases) from the Ras protein superfamily regulate glucose-stimulated insulin secretion in the pancreatic islet β-cell. The Rho family GTPases Cdc42 and Rac1 are primarily involved in relaying key signals in several cellular functions, including vesicle trafficking, plasma membrane homeostasis, and cytoskeletal dynamics. They orchestrate specific changes at each spatiotemporal region within the β-cell by coordinating with signal transducers, guanine nucleotide exchange factors (GEFs), GTPase-activating factors (GAPs), and their effectors. The Arf family of small GTPases is involved in vesicular trafficking (exocytosis and endocytosis) and actin cytoskeletal dynamics. Rab-GTPases regulate pre-exocytotic and late endocytic membrane trafficking events in β-cells. Several additional functions for small GTPases include regulating transcription factor activity and mitochondrial dynamics. Importantly, defects in several of these GTPases have been found associated with type 2 diabetes (T2D) etiology. The purpose of this review is to systematically denote the identities and molecular mechanistic steps in the glucose-stimulated insulin secretion pathway that leads to the normal release of insulin. We will also note newly identified defects in these GTPases and their corresponding regulatory factors (e.g., GDP dissociation inhibitors (GDIs), GEFs, and GAPs) in the pancreatic β-cells, which contribute to the dysregulation of metabolism and the development of T2D.

Paradoxical regulation of glucose-induced Rac1 activation and insulin secretion by RhoGDIβ in pancreatic β-cells

Small GTPases (e.g., Rac1) play key roles in glucose-stimulated insulin secretion (GSIS) in the β-cell. We investigated regulation by RhoGDIβ of glucose-induced activation of Rac1 and insulin secretion. RhoGDIβ is expressed in INS-1 832/13 cells, rodent and human islets. siRNA-mediated knockdown of RhoGDIβ in INS-1 832/13 cells significantly attenuated glucose-induced Rac1 activation without affecting its translocation and membrane association. Further, suppression of RhoGDIβ expression exerted minimal effects on GSIS at the height of inhibition of Rac1 activation, suggesting divergent effects of RhoGDIβ on Rac1 activation and insulin secretion in the glucose-stimulated β-cell. We provide the first evidence for the expression of RhoGDIβ in rodent and human β-cells, and its differential regulatory roles of this protein in G protein activation and GSIS. Abbreviations: Arf6: ADP ribosylation factor; Cdc42: Cell Division Cycle; GAP: GTPase-activating protein; GDI: GDP dissociation inhibitor; GDIα: GDP dissociation inhibitorα; GDIβ: GDP dissociation inhibitorβ; GEF: Guanine nucleotide exchange factor; GSIS: Glucose-stimulated insulin secretion; Rac1: Ras-Related C3 Botulinum Toxin Substrate 1.

Roles of GTP and Rho GTPases in pancreatic islet beta cell function and dysfunction

A growing body of evidence implicates requisite roles for GTP and its binding proteins (Rho GTPases) in the cascade of events leading to physiological insulin secretion from the islet beta cell. Interestingly, chronic exposure of these cells to hyperglycaemic conditions appears to result in sustained activation of specific Rho GTPases (e.g. Rac1) leading to significant alterations in cellular functions including defects in mitochondrial function and nuclear collapse culminating in beta cell demise. One of the objectives of this review is to highlight our current understanding of the regulatory roles of GTP and Rho GTPases in normal islet function (e.g. proliferation and insulin secretion) as well potential defects in these signalling molecules and metabolic pathways that could contribute islet beta cell dysfunction and loss of functional beta cell mass leading to the onset of diabetes. Potential knowledge gaps in this field and possible avenues for future research are also highlighted.

ABBREVIATIONS

ARNO: ADP-ribosylation factor nucleotide binding site opener; CML: carboxyl methylation; Epac: exchange protein directly activated by cAMP; ER stress: endoplasmic reticulum stress; FTase: farnesyltransferase; GAP: GTPase activating protein; GDI: GDP dissociation inhibitor; GEF: guanine nucleotide exchange factor; GGTase: geranylgeranyltransferase; GGpp: geranylgeranylpyrophosphate; GGPPS: geranylgeranyl pyrophosphate synthase; GSIS: glucose-stimulated insulin secretion; HGPRTase: hypoxanthine-guanine phosphoribosyltransferase; IMPDH: inosine monophosphate dehydrogenase; α-KIC: α-ketoisocaproic acid; MPA: mycophenolic acid; MVA: mevalonic acid; NDPK: nucleoside diphosphate kinase; NMPK: nucleoside monophosphate kinase; Nox2: phagocyte-like NADPH oxidase; PAK-I: p21-activated kinase-I; β-PIX: β-Pak-interacting exchange factor; PRMT: protein arginine methyltransferase; Rac1: ras-related C3 botulinum toxin substrate 1; Tiam1: T-cell lymphoma invasion and metastasis-inducing protein 1; Trx-1: thioredoxin-1; Vav2: vav guanine nucleotide exchange factor 2.

GPCRs, G Proteins, and Their Impact on β-cell Function

Glucose-induced (physiological) insulin secretion from the islet β-cell involves interplay between cationic (i.e., changes in intracellular calcium) and metabolic (i.e., generation of hydrophobic and hydrophilic second messengers) events. A large body of evidence affirms support for novel regulation, by G proteins, of specific intracellular signaling events, including actin cytoskeletal remodeling, transport of insulin-containing granules to the plasma membrane for fusion, and secretion of insulin into the circulation. This article highlights the following aspects of GPCR-G protein biology of the islet. First, it overviews our current understanding of the identity of a wide variety of G protein regulators and their modulatory roles in GPCR-G protein-effector coupling, which is requisite for optimal β-cell function under physiological conditions. Second, it describes evidence in support of novel, noncanonical, GPCR-independent mechanisms of activation of G proteins in the islet. Third, it highlights the evidence indicating that abnormalities in G protein function lead to islet β-cell dysregulation and demise under the duress of metabolic stress and diabetes. Fourth, it summarizes observations of potential beneficial effects of GPCR agonists in preventing/halting metabolic defects in the islet β-cell under various pathological conditions (e.g., metabolic stress and inflammation). Lastly, it identifies knowledge gaps and potential avenues for future research in this evolving field of translational islet biology. Published 2020. Compr Physiol 10:453-490, 2020.

Role of G-proteins in islet function in health and diabetes

Glucose-stimulated insulin secretion (GSIS) involves interplay between metabolic and cationic events. Seminal contributions from multiple laboratories affirm essential roles for small G-proteins (Rac1, Cdc42, Arf6, Rab27A) in GSIS. Activation of these signalling proteins promotes cytoskeletal remodeling, transport and docking of insulin granules on the plasma membrane for exocytotic secretion of insulin. Evidence in rodent and human islets suggests key roles for lipidation (farnesylation and geranylgeranylation) of these G-proteins for their targeting to appropriate cellular compartments for optimal regulation of effectors leading to GSIS. Interestingly, however, inhibition of prenylation appears to cause mislocalization of non-prenylated, but (paradoxically) activated G-proteins, in "inappropriate" compartments leading to activation of stress kinases and onset of mitochondrial defects, loss in GSIS and apoptosis of the islet β-cell. This review highlights our current understanding of roles of G-proteins and their post-translational lipidation (prenylation) signalling networks in islet function in normal health, metabolic stress (glucolipotoxicity and ER stress) and diabetes. Critical knowledge gaps that need to be addressed for the development of therapeutics to halt defects in these signalling steps in β-cells in models of impaired insulin secretion and diabetes are also highlighted and discussed.

Tiam1/Vav2-Rac1 axis: A tug-of-war between islet function and dysfunction

Glucose-stimulated insulin secretion [GSIS] from the islet β-cell involves a well-orchestrated interplay between metabolic and cationic events. It is well established that intracellular generation of adenine and guanine nucleotide triphosphates [e.g., ATP and GTP] represents one of the requisite signaling steps in GSIS. The small molecular mass GTP-binding proteins [G-proteins; e.g., Rac1 and Cdc42] have been shown to regulate islet β-cell function including actin cytoskeletal remodeling and fusion of insulin granules with the plasma membrane for GSIS to occur. In this context, several regulatory factors for these G-proteins have been identified in the pancreatic β-cell; these include guanine nucleotide exchange factors [GEFs] and guanine nucleotide dissociation inhibitors [GDI]. Recent pharmacological and molecular biological evidence identified Tiam1 and Vav2 as GEFs for Rac1 in promoting physiological insulin secretion. Paradoxically, emerging evidence in multiple cell types, including the islet β-cell, suggests key roles for Rac1 in the onset of cellular dysfunction under conditions of metabolic stress and diabetes. Furthermore, functional inactivation of either Tiam1 or Vav2 appears to attenuate sustained activation of Rac1 and its downstream signaling events [activation of stress kinases] under conditions of metabolic stress. Together, these findings suggest both "friendly" and "non-friendly" roles for Tiam1/Vav2-Rac1 signaling axis in islet β-cell in health and diabetes. Our current understanding of the field and the knowledge gaps that exist in this area of islet biology are heighted herein. Furthermore, potential caveats in the specificity and selectivity of pharmacological inhibitors that are available currently are discussed in this Commentary.

Cytoophidium assembly reflects upregulation of IMPDH activity

Cytidine triphosphate synthase (CTPS) and inosine monophosphate dehydrogenase (IMPDH) (both of which have two isoforms) can form fiber-like subcellular structures termed 'cytoophidia' under certain circumstances in mammalian cells. Although it has been shown that filamentation of CTPS downregulates its activity by disturbing conformational changes, the activity of IMPDH within cytoophidia is still unclear. Most previous IMPDH cytoophidium studies were performed under conditions involving inhibitors that impair GTP synthesis. Here, we show that IMPDH forms cytoophidia without inhibition of GTP synthesis. First, we find that an elevated intracellular CTP concentration or treatment with 3'-deazauridine, a CTPS inhibitor, promotes IMPDH cytoophidium formation and increases the intracellular GTP pool size. Moreover, restriction of cell growth triggers the disassembly of IMPDH cytoophidia, implying that their presence is correlated with active cell metabolism. Finally, we show that the presence of IMPDH cytoophidia in mouse pancreatic islet cells might correlate with nutrient uptake in the animal. Collectively, our findings reveal that formation of IMPDH cytoophidia reflects upregulation of purine nucleotide synthesis, suggesting that the IMPDH cytoophidium plays a role distinct from that of the CTPS cytoophidium in controlling intracellular nucleotide homeostasis.

Protein prenylation in islet β-cell function in health and diabetes: Putting the pieces of the puzzle together

Post-translational prenylation involves incorporation of 15-(farnesyl) or 20-(geranylgeranyl) carbon derivatives of mevalonic acid into highly conserved C-terminal cysteines of proteins. The farnesyl transferase (FTase) and the geranylgeranyl transferase (GGTase) mediate incorporation of farnesyl and geranylgeranyl groups, respectively. At least 300 proteins are prenylated in the human genome; the majority of these are implicated in cellular processes including growth, differentiation, cytoskeletal function and vesicle trafficking. From a functional standpoint, isoprenylation is requisite for targeting of modified proteins to relevant cellular compartments for regulation of effector proteins. Pharmacological and molecular biological studies have provided compelling evidence for key roles of this signaling pathway in physiological insulin secretion in normal rodent and human islets. Recent evidence indicates that inhibition of prenylation results in mislocalization of unprenylated proteins, and surprisingly, they remain in active (GTP-bound) conformation. Sustained activation of G proteins has been reported in mice lacking GGTase, suggesting alternate mechanisms for the activation of non-prenylated G proteins. These findings further raise an interesting question if mislocalized, non-prenylated and functionally active G proteins cause cellular pathology since aberrant protein prenylation has been implicated in the onset of cardiovascular disease and diabetes. Herein, we overview the existing evidence to implicate prenylation in islet function and potential defects in this signaling pathways in the diabetic β-cell. We will also identify critical knowledge gaps that need to be addressed for the development of therapeutics to halt defects in these signaling steps in β cells in models of impaired insulin secretion, metabolic stress and diabetes.

Mizoribine-induced severe hyperglycemia in a patient with microscopic polyangiitis

A 77-year-old woman had been receiving prednisolone (PSL) for 5 months as induction therapy for microscopic polyangiitis. Because of repeated elevation of the antineutrophil cytoplasmic autoantibody titer, we added mizoribine (MZR), and, 2 months later, the patient developed severe hyperglycemia with low serum and urinary C-peptide reactivity (CPR). The MZR was, therefore, withdrawn and insulin therapy was started. One month later, the serum and urinary CPR had increased and postprandial hyperglycemia had improved. Previous in vitro studies have shown that MZR can induce hyperglycemia through at least two mechanisms. One is the alteration of insulin secretion from islet cells, and the other is action via the glucocorticoid receptor (GR). MZR reduces insulin secretion through the depletion of intracellular guanosine triphosphate (GTP), which leads to the inhibition of mitogenesis and induction of beta cell apoptosis. MZR affects insulin resistance by activating the GR through interaction with the 14-3-3 protein, leading to postprandial hyperglycemia. Although postprandial hyperglycemia generally appears between 3 and 7 weeks after PSL administration, that in our patient did not become apparent during 5 months of PSL monotherapy, but was manifested 2 months after the introduction of MZR, and improved after MZR had been withdrawn. We conclude from these findings that MZR had been responsible for the hyperglycemia in our patient.

Chemical screen identifies FDA-approved drugs and target pathways that induce precocious pancreatic endocrine differentiation

Pancreatic β-cells are an essential source of insulin and their destruction because of autoimmunity causes type I diabetes. We conducted a chemical screen to identify compounds that would induce the differentiation of insulin-producing β-cells in vivo. To do this screen, we brought together the use of transgenic zebrafish as a model of β-cell differentiation, a unique multiwell plate that allows easy visualization of lateral views of swimming larval fish and a library of clinical drugs. We identified six hits that can induce precocious differentiation of secondary islets in larval zebrafish. Three of these six hits were known drugs with a considerable background of published data on mechanism of action. Using pharmacological approaches, we have identified and characterized two unique pathways in β-cell differentiation in the zebrafish, including down-regulation of GTP production and retinoic acid biosynthesis.

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.

A farnesylated G-protein suppresses Akt phosphorylation in INS 832/13 cells and normal rat islets: regulation by pertussis toxin and PGE₂

Protein isoprenylation constitutes incorporation of either 15-carbon farnesyl or 20-carbon geranylgeranyl derivative of mevalonic acid onto the C-terminal cysteine, culminating in increased hydrophobicity of the modified proteins for optimal membrane anchoring and interaction with their respective effectors. Emerging evidence confirms the participatory role of prenylated proteins in pancreatic β-cell function including insulin secretion. Herein, we investigated the putative regulatory roles of protein farnesylation in cell survival signaling pathways in insulin-secreting INS 832/13 cells and normal rodent islets, specifically at the level of protein kinase-B/Akt phosphorylation induced by insulin-like growth factor [IGF-1]. Selective inhibitors of farnesylation [e.g., FTI-277 or FTI-2628] or knockdown of the β-subunit of farnesyl transferase by siRNA significantly increased Akt activation under basal and IGF-1-stimulated conditions. Consequentially, the relative abundance of phosphorylated FoxO1 and Bad were increased implicating inactivation of critical components of the cell death machinery. In addition, FTI-induced Akt activation was attenuated by the PI3-kinase inhibitor, LY294002. Exposure of INS 832/13 cells to pertussis toxin [PTx] markedly potentiated Akt phosphorylation suggesting involvement of a PTx-sensitive G-protein in this signaling axis. Furthermore, prostaglandin E₂, a known agonist of inhibitory G-proteins, significantly attenuated FTI-induced Akt phosphorylation. Taken together, our findings suggest expression of a farnesylated G-protein in INS 832/13 cells and normal rat islets, which appear to suppress Akt activation and subsequent cell survival signaling steps. Potential regulatory roles of the islet endogenous protein kinase-B inhibitory protein [Probin] in islet function are discussed.

Friendly, and not so friendly, roles of Rac1 in islet β-cell function: lessons learnt from pharmacological and molecular biological approaches

Glucose-stimulated insulin secretion [GSIS] involves a sequence of metabolic events leading to small G-protein [e.g., Rac1]-mediated cytoskeletal remodeling to promote granule mobilization toward the plasma membrane for fusion and release of insulin. Existing evidence supports a positive modulatory role for Rac1 in GSIS. Specific regulatory factors of Rac1 function, including the guanine nucleotide exchange factors [e.g., Tiam1] have also been identified and studied in the islet. Inhibition of Tiam1/Rac1 signaling axis attenuates GSIS suggesting its pivotal role in insulin secretion. In addition to its positive [i.e., friendly] roles in GSIS, Rac1 also plays "non-friendly" role[s] in the islet function. For example, it up-regulates the intracellular reactive oxygen species [ROS] levels via activation of phagocyte-like NADPH oxidase [Nox]. Despite the emerging evidence that a tonic increase in intracellular ROS is necessary for GSIS, experimental evidence also suggests that chronic exposure of β-cells to high glucose, palmitate or cytokines results in the onset of oxidative stress leading to reduction in mitochondrial membrane potential, cytosolic accumulation of cytochrome C and activation of caspase-3 leading to β-cell apoptosis. Pharmacological and molecular biological inhibition of Rac1 activation affords partial protection against Nox-induced oxidative stress and mitochondrial dysfunction induced by elevated glucose, lipids or cytokines. Herein, we overview the existing evidence to suggest positive as well as negative modulatory roles of Rac1 in islet function. Potential avenues for future research including development of inhibitors to halt the Rac1-Nox activation and generation of oxidative stress leading to the metabolic dysfunction of the β-cell are discussed.

Phagocyte-like NADPH oxidase generates ROS in INS 832/13 cells and rat islets: role of protein prenylation

Recent evidence suggests that an acute increase in the generation of phagocyte-like NADPH-oxidase (Nox)-mediated reactive oxygen species (ROS) may be necessary for glucose-stimulated insulin secretion. Using rat islets and INS 832/13 cells, we tested the hypothesis that activation of specific G proteins is necessary for nutrient-mediated intracellular generation of ROS. Stimulation of β-cells with glucose or a mixture of mitochondrial fuels (mono-methylsuccinate plus α-ketoisocaproic acid) markedly elevated intracellular accumulation of ROS, which was attenuated by selective inhibitors of Nox (e.g., apocynin or diphenyleneiodonium chloride) or short interfering RNA-mediated knockdown of p47(phox), one of the subunits of Nox. Selective inhibitors of protein prenylation (FTI-277 or GGTI-2147) markedly inhibited nutrient-induced ROS generation, suggesting that activation of one (or more) prenylated small G proteins and/or γ-subunits of trimeric G proteins is involved in this signaling axis. Depletion of endogenous GTP levels with mycophenolic acid significantly reduced glucose-induced activation of Rac1 and ROS generation in these cells. Other immunosuppressants, like cyclosporine A or rapamycin, which do not deplete endogenous GTP levels, failed to affect glucose-induced ROS generation, suggesting that endogenous GTP is necessary for glucose-induced Nox activation and ROS generation. Treatment of INS 832/13 cells or rat islets with pertussis toxin (Ptx), which ADP ribosylates and inhibits inhibitory class of trimeric G proteins (i.e., G(i) or G(o)), significantly attenuated glucose-induced ROS generation in these cells, implicating activation of a Ptx-sensitive G protein in these signaling cascade. Together, our findings suggest a prenylated Ptx-sensitive signaling step couples Rac1 activation in the signaling steps necessary for glucose-mediated generation of ROS in the pancreatic β-cells.

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.

Mechanisms of biphasic insulin-granule exocytosis - roles of the cytoskeleton, small GTPases and SNARE proteins

The release of insulin from pancreatic islets requires negative regulation to ensure low levels of insulin release under resting conditions, as well as positive regulation to facilitate robust responsiveness to conditions of elevated fuel or glucose. The first phase of release involves the plasma-membrane fusion of a small pool of granules, termed the readily releasable pool; these granules are already at the membrane under basal conditions, and discharge their cargo in response to nutrient and also non-nutrient secretagogues. By contrast, second-phase secretion is evoked exclusively by nutrients, and involves the mobilization of intracellular granules to t-SNARE sites at the plasma membrane to enable the distal docking and fusion steps of insulin exocytosis. Nearly 40 years ago, the actin cytoskeleton was first recognized as a key mediator of biphasic insulin release, and was originally presumed to act as a barrier to block granule docking at the cell periphery. More recently, however, the discovery of cycling GTPases that are involved in F-actin reorganization in the islet beta-cell, combined with the availability of reagents that are more specific and tools with which to study the mechanisms that underlie granule movement, have contributed greatly to our understanding of the role of the cytoskeleton in regulating biphasic insulin secretion. Herein, we provide historical perspective and review recent progress that has been made towards integrating cytoskeletal reorganization and cycling of small Rho-, Rab- and Ras-family GTPases into our current models of stimulus-secretion coupling and second-phase insulin release.

Down-regulation of expression and function of nucleoside diphosphate kinase in insulin-secreting beta-cells under in vitro conditions of glucolipotoxicity

Previously, we reported a significant reduction in expression and the activity of nucleoside diphosphate kinase (NDP kinase) in islets derived from the Goto-Kakizaki rat (GK rat), an animal model for type 2 diabetes. Herein, we examined the effects of chronic exposure of insulin-secreting beta-(INS 832/13) cells to high glucose (a model for glucotoxicity), palmitate (a model for lipotoxicity), or glucose plus palmitate (a model for glucolipotoxicity) on the expression and activity of nm23-H1 (NDP kinase A) and nm23-H2 (NDP kinase B). Our findings indicate a marked reduction in the expression of both nm23-H1 and nm23-H2 and the associated NDP kinase activity under each of these conditions. A cell-permeable analog of ceramide (CER) also mimicked the effects of palmitate in significantly reducing the expression of nm23-H1 and nm23-H2 and NDP kinase activity in these cells. These findings suggest that de novo generation of intracellular CER from palmitate might represent at least one of the signaling steps involved in lipid-induced effects on NDP kinase expression and function in beta-cells. Based on these data, we conclude that glucolipotoxic conditions significantly impair expression and function of NDP kinase in pancreatic beta-cells. Potential significance of these findings, specifically at the level of abnormal G-protein activation and impaired insulin secretion under glucolipotoxic conditions is discussed.

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.

Protein prenylation in glucose-induced insulin secretion from the pancreatic islet beta cell: a perspective

Insulin secretion from the pancreatic β cell is regulated principally by the ambient concentration of glucose. However, the molecular and cellular mechanisms underlying the stimulus – secretion coupling of glucose-stimulated insulin secretion (GSIS) remain only partially understood. Emerging evidence from multiple laboratories suggests key regulatory roles for GTP-binding proteins in the cascade of events leading to GSIS. This class of signalling proteins undergoes a series of requisite post-translational modifications (e.g. prenylation) at their C-terminal cysteines, which appear to be necessary for their targeting to respective membranous sites for optimal interaction with their respective effector proteins. This communication represents a perspective on potential regulatory roles for protein prenylation steps (i.e. protein farnesylation and protein geranylgeranylation) in GSIS from the islet β cell.Possible consequences of protein prenylation and potential mechanisms underlying glucose-induced regulation of prenylation, specifically in the context of GSIS, are also discussed.

Cell swelling-induced signaling for insulin secretion bypasses steps involving G proteins and PLA2 and is N-ethylmaleimide insensitive

BACKGROUND

This study was undertaken to examine putative mechanisms of calcium independent signal transduction pathway of cell swelling-induced insulin secretion.

METHODS

The role of phospholipase A(2), G proteins, and soluble N-ethylmaleimide-sensitive-factor attachment protein receptor (SNARE) in insulin secretion induced by 30% hypotonic medium was studied using isolated rat pancreatic islets.

RESULTS

In contrast to glucose stimulation, osmotically induced insulin secretion from pancreatic islets was not inhibited by 10 micromol/l bromoenol lactone, an iPLA(2) (Ca(2+) independent phospholipase) inhibitor. Similarly, preincubation of islets for 20 hours with 25 microg/ml mycophenolic acid to inhibit GTP synthesis fully abolished glucose-induced insulin secretion but was without effect on hypotonicity stimulated insulin release. Glucose-induced insulin secretion was prevented by preincubation with 20 nmol/l tetanus toxin (TeTx), a metalloprotease inactivating soluble SNARE. Cell swelling-induced insulin secretion was inhibited by TeTx in the presence of calcium ions but not in calcium depleted medium. The presence of N-ethylmaleimide (NEM, 5 mmol/l, another inhibitor of SNARE proteins) in the medium resulted in high basal insulin secretion and lacking response to glucose stimulation. In contrast, high basal insulin secretion from NEM treated islets further increased after hypotonic stimulation.

CONCLUSION

G proteins and iPLA(2) - putative mediators of Ca(2+) independent signaling pathway participate in glucose but not in hypotonicity-induced insulin secretion. Hypotonicity-induced insulin secretion is sensitive to clostridial neurotoxin TeTx but is resistant to NEM.

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.

Assessing biomarkers of oxidative stress: analysis of guanosine and oxidized guanosine nucleotide triphosphates by high performance liquid chromatography with electrochemical detection

Oxidation of the guanosine moiety in DNA has become a hallmark biomarker in assessing oxidative stress. The oxidation of guanosine in the nucleotide triphosphate pool has been overlooked due to the lack of a reliable methodology. This method describes a sample processing and high performance liquid chromatography with electrochemical detection protocol for the analysis of the cellular pool of guanosine triphosphates and oxidized guanosine triphosphates. Validation of this method is demonstrated along with evaluation of these analytes in control and oxidizing conditions in vitro and in HEK 293T cells. Oxidation of this triphosphate pool occurred independently of oxidation to DNA.

ATP enhances exocytosis of insulin secretory granules in pancreatic islets under Ca2+-depleted condition

Glucose and other nutrients have been shown to stimulate insulin release from pancreatic islets under Ca2+-depleted condition when protein kinase A (PKA) and protein kinase C (PKC) are activated simultaneously. We investigated the role of metabolic nucleotide signals including ATP, ADP, and GTP in exocytosis of insulin secretory granules under Ca2+-depleted condition using electrically permeabilized rat islets. ATP under PKC activation augmented insulin release concentration-dependently by 100 nM 12-O-tetradecanoyl-phorbol-13-acetate (TPA) in Ca2+-depleted condition, while ADP could not suppress ATP-dependent insulin release in this condition. Neither GTP nor activated PKA in the absence of PKC activation increased insulin release under Ca2+-depleted condition in the presence of ATP, but both enhanced insulin secretion in the presence of ATP when PKC was activated. In conclusion, activated PKC and the presence of ATP both are required in the insulin secretory process under Ca2+-depleted condition. While PKA activation and GTP cannot substitute for PKC activation and ATP, respectively, under Ca2+-depleted condition, they enhance ATP-dependent insulin secretion when PKC is activated.

Regulatory roles for small G proteins in the pancreatic beta-cell: lessons from models of impaired insulin secretion

Emerging evidence suggests that GTP-binding proteins (G proteins) play important regulatory roles in physiological insulin secretion from the islet beta-cell. Such conclusions were drawn primarily from experimental data derived through the use of specific inhibitors of G protein function. Data from gene depletion experiments appear to further substantiate key roles for these signaling proteins in the islet metabolism. The first part of this review will focus on findings supporting the hypothesis that activation of specific G proteins is essential for insulin secretion, including regulation of their function by posttranslational modifications at their COOH-terminal cysteines (e.g., isoprenylation). The second part will overview novel, non-receptor-dependent mechanism(s) whereby glucose might activate specific G proteins via protein histidine phosphorylation. The third section will review findings that appear to link abnormalities in the expression and/or functional activation of these key signaling proteins to impaired insulin secretion. It is hoped that this review will establish a basis for future research in this area of islet signal transduction, which presents a significant potential, not only in identifying key signaling proteins that are involved in physiological insulin secretion, but also in examining potential abnormalities in this signaling cascade that lead to islet dysfunction and onset of diabetes.

Role of tissue transglutaminase in GTP depletion-induced apoptosis of insulin-secreting (HIT-T15) cells

The role of tissue transglutaminase (tTG), a calcium-dependent and GTP-modulated enzyme, in apoptotic death induced by GTP depletion in islet beta-cells was investigated. GTP depletion and apoptosis were induced by mycophenolic acid (MPA) in insulin-secreting HIT-T15 cells. MPA treatment increased in situ tTG activity (but not protein levels) in a dose- and time-dependent manner in parallel with the induction of apoptosis. MPA-induced increases of both tTG activity and apoptosis were entirely blocked by co-provision of guanosine but not adenosine. MPA-enhanced tTG activity could be substantially reduced by co-exposure to monodansylcadaverine or putrescine (tTG inhibitors), and largely blocked by lowering free Ca(2+) concentrations in the culture medium. However, MPA-induced cell death was either not changed or was only slightly reduced under these conditions. By contrast, a pan-caspase inhibitor (Z-VAD-FMK) entirely prevented apoptosis induced by MPA, but did not block the enhanced tTG activity, indicating that GTP depletion can induce apoptosis and activate tTG either independently or as part of a cascade of events involving caspases. Importantly, the morphological changes accompanying apoptosis could be markedly prevented by tTG inhibitors. These findings suggest that the effect of the marked increase in tTG activity in GTP depletion-induced apoptosis of insulin-secreting cells may be restricted to some terminal morphological changes.

Activation of caspase-2 mediates the apoptosis induced by GTP-depletion in insulin-secreting (HIT-T15) cells

This study investigated the possible involvement of a specific caspase(s) (a family of aspartate-specific cysteine proteases) in programmed cell death of islet beta-cells due to sustained GTP depletion. Treatment (up to 48 h) with 3 microg/ml mycophenolic acid (MPA), which specifically depletes intracellular guanine nucleotides, reduced cell-cycle progression from G1 phase into S and G2/M phases (as assessed by flow cytometry) and, subsequently, induced apoptosis of HIT-15 cells (transformed pancreatic beta-cells). The latter was accompanied by a marked increase of caspase-2 activity (+343%) and moderate activation of caspase-9 (+150%) and caspase-3 (+145%). Importantly, only caspase-2 activation preceded induction of apoptosis. There was no change in activity of caspase-1, -4, -5, -6, and -8. Release of the mitochondrial protein cytochrome c into cytosol was also observed at a late stage. Cotreatment of cells with a permeable pan-caspase inhibitor (Z-VAD-FMK) blocked GTP depletion-induced cell death in a dose-dependent manner. A specific caspase-2 inhibitor (Z-VDVAD-FMK), but not a caspase-3 inhibitor (DEVD-CHO), was also capable of restoring cell viability. Interestingly, activation of caspase-2 leads to caspase-3 activation because the caspase-2 inhibitor abrogated caspase-3 activity. Our results indicate that, while activation of multiple caspases are involved in the execution phase of GTP depletion-induced apoptosis, caspase-2 appears to play the major role in the initiation of this program. This study revealed a novel, caspase-2 mediated form of apoptosis that may be consequent to impaired mitogenesis.

Triggering and augmentation mechanisms, granule pools, and biphasic insulin secretion

The insulin secretory response by pancreatic beta-cells to an acute "square wave" stimulation by glucose is characterized by a first phase that occurs promptly after exposure to glucose, followed by a decrease to a nadir, and a prolonged second phase. The first phase of release is due to the ATP-sensitive K(+) (K(ATP)) channel-dependent (triggering) pathway that increases [Ca(2+)](i) and has been thought to discharge the granules from a "readily releasable pool." It follows that the second phase entails the preparation of granules for release, perhaps including translocation and priming for fusion competency before exocytosis. The pathways responsible for the second phase include the K(ATP) channel-dependent pathway because of the need for elevated [Ca(2+)](i) and additional signals from K(ATP) channel-independent pathways. The mechanisms underlying these additional signals are unknown. Current hypotheses include increased cytosolic long-chain acyl-CoA, the pyruvate-malate shuttle, glutamate export from mitochondria, and an increased ATP/ADP ratio. In mouse islets, the beta-cell contains some 13,000 granules, of which approximately 100 are in a "readily releasable" pool. Rates of granule release are slow, e.g., one every 3 s, even at the peak of the first phase of glucose-stimulated release. As both phases of glucose-stimulated insulin secretion can be enhanced by agents such as glucagon-like peptide 1, which increases cyclic AMP levels and protein kinase A activity, or acetylcholine, which increases diacylglycerol levels and protein kinase C activity, a single "readily releasable pool" hypothesis is an inadequate explanation for insulin secretion. Multiple pools available for rapid release or rapid conversion of granules to a rapidly releasable state are required.

Inosine-5'-monophosphate dehydrogenase is required for mitogenic competence of transformed pancreatic beta cells

The relation of inosine-5'-monophosphate dehydrogenase (IMPDH; the rate-limiting enzyme in GTP synthesis) to mitogenesis was studied by enzymatic assay, immunoblots, and RT-PCR in several dissimilar transformed pancreatic ss-cell lines, using intact cells. Both of the two isoforms of IMPDH (constitutive type 1 and inducible type 2) were identified using RT-PCR in transformed beta cells or in intact islets. IMPDH 2 messenger RNA (mRNA) and IMPDH protein were both regulated reciprocally by changes in levels of their end-products. Flux through IMPDH was greatest in rapidly growing cells, due mostly to increased uptake of precursor. Glucose (but not 3-0-methylglucose, L-glucose, or fructose) further augmented substrate uptake and also increased IMPDH enzymatic activity after either 4 or 21 h of stimulation. Serum or ketoisocaproate also increased IMPDH activity (but not uptake). Two selective IMPDH inhibitors (mycophenolic acid and mizoribine) reduced IMPDH activity in all cell lines, and, with virtually identical concentration-response curves, inhibited DNA synthesis (assessed as bromodeoxyuridine incorporation) in response to glucose, serum, or ketoisocaproate. Inhibition of DNA synthesis was reversible, completely prevented by repletion of cellular guanine (but not adenine) nucleotides, and could not be attributed to toxic effects. Despite the fact that modulation of IMPDH expression by guanine nucleotides was readily detectable, glucose and/or serum failed to alter IMPDH mRNA or protein, indicating that their effects on IMPDH activity were largely at the enzyme level. Precursors of guanine nucleotides failed, by themselves, to induce mitogenesis. Thus, adequate IMPDH activity (and thereby, availability of GTP) is a critical requirement for beta-cell proliferation. Although it is unlikely that further increases in GTP can, by themselves, initiate DNA synthesis, such increments may be needed to sustain mitogenesis.

Augmentation of basal insulin release from rat islets by preexposure to a high concentration of glucose

We have found that preexposure to an elevated concentration of glucose reversibly induces an enhancement of basal insulin release from rat pancreatic islets dependent on glucose metabolism. This basal insulin release augmented by priming was not suppressed by reduction of the intracellular ATP or Ca(2+) concentration, because even in the absence of ATP at low Ca(2+), the augmentation was not abolished from primed electrically permeabilized islets. Moreover, it was not inhibited by an alpha-adrenergic antagonist, clonidine. A threshold level of GTP is required to induce these effects, because together with adenine, mycophenolic acid, a cytosolic GTP synthesis inhibitor, completely abolished the enhancement of basal insulin release due to the glucose-induced priming without affecting the glucose-induced increment in ATP content and ATP-to-ADP ratio. In addition, a GDP analog significantly suppressed the enhanced insulin release due to priming from permeabilized islets in the absence of ATP at low Ca(2+), suggesting that the GTP-sensitive site may play a role in the augmentation of basal insulin release due to the glucose-induced priming effect.

Effects of inhibitors of guanine nucleotide synthesis on membrane potential and cytosolic free Ca2+ levels in insulin-secreting cells

Adenine nucleotides play an important role in the control of membrane potential by acting on ATP-sensitive K+ (K(ATP)) channels and, in turn, modulating the open probability of voltage-gated Ca2+ channels in pancreatic islet beta-cells. Here, we provide evidence that guanine nucleotides (GNs) also may be involved in the modulation of these events in vivo. GNs were depleted by treatment of HIT-T15 cells with mycophenolic acid (MPA). Resting membrane potential was more depolarized in cells treated for 3 and 6 hr with MPA than in control cells, and this effect was inhibited by diazoxide. After 6 hr of exposure to MPA, basal cytosolic free Ca2+ concentrations ([Ca2+]i) were elevated by 20%. Increments in [Ca2+]i induced by submaximal concentrations of K+ (10-15 mM) or bombesin were enhanced by > 50%. Opening K(ATP) channels with diazoxide lowered basal [Ca2+]i in MPA-treated cells to normal and abrogated the enhanced [Ca2+]i responses. However, an L-type Ca2+ channel blocker only abolished the enhanced [Ca2+]i response to stimuli and had no effect on the elevated basal [Ca2+]i, in contrast to EGTA, which obliterated both, implying that the latter was due to Ca2+ influx via non-L-type Ca2+ channels. These effects on ion fluxes were attributable specifically to GN depletion, since guanosine, which restores GTP content and the GTP/GDP ratio, but not adenosine, prevented all MPA-induced ion changes; furthermore, the latter were mimicked by mizoribine (a structurally dissimilar GTP synthesis inhibitor). It is concluded that, in addition to adenine nucleotides, GNs might contribute to the modulation of K(ATP) channels in intact beta-cells. In addition, GN depletion appeared to be able to reduce stimulated insulin secretion by a mechanism largely independent of the changes of ion fluxes observed above.

Human nucleoside diphosphate kinase B (Nm23-H2) from melanoma cells shows altered phosphoryl transfer activity due to the S122P mutation

The Ser122 --> Pro mutation in human nucleoside diphosphate kinase (NDK)-B/Nm23-H2 was recently found in melanoma cells. In comparison to the wild-type enzyme, steady state activity of NDKS122P with ATP and TDP as substrates was slowed down 5-fold. We have utilized transient kinetic techniques to analyze phosphoryl transfer between the mutant enzyme and various pairs of nucleoside triphosphates and nucleoside diphosphates. The two half-reactions of phosphorylation and dephosphorylation of the active site histidine residue (His118) were studied separately by making use of the intrinsic fluorescence changes which occur during these reactions. All apparent second order rate constants are drastically reduced, falling 5-fold for phosphorylation and 40-200-fold for dephosphorylation. Also, the reactivity of the mutant with pyrimidine nucleotides and deoxy nucleotides is more than 100-fold reduced compared with the wild-type. Thus, the rate-limiting step of the NDK-BS122P-catalyzed reaction is phosphoryl transfer from the phospho-enzyme intermediate to the nucleoside diphosphate and not phosphoryl transfer from the nucleoside triphosphate to the enzyme as was found for the wild-type protein. This results in a pronounced shift of the equilibrium between unphosphorylated and phosphorylated enzyme. Moreover, like the Killer-of-prune mutation in Drosophila NDK and the neuroblastoma Ser120 --> Gly mutation in human NDK-A/Nm23-H1, the Ser122 --> Pro substitution in NDK-B affects the stability of the protein toward heat and urea. These significantly altered properties may be relevant to the role of the mutant enzyme in various intracellular processes.

Inosine monophosphate dehydrogenase: A molecular switch integrating pleiotropic GTP-dependent beta-cell functions

Studies of pancreatic islet function in the pathogenesis of type 2 diabetes mellitus have tended to focus on the short-term control of insulin secretion. However, the long-term control of beta-cell mass is also relevant to diabetes, since this parameter is reduced substantially even in non-insulin-dependent diabetes in humans. In animal models of type 2 diabetes, the normal balance between beta-cell proliferation and programmed cell death is perturbed. We take the perspective in this overview that inosine monophosphate dehydrogenase (IMPDH; EC 1.1.1. 205) may represent a previously neglected molecular integrator or sensor that exerts both functional (secretory) and anatomical (proliferative) effects within beta-cells. These properties reflect the fact that IMPDH is a rate-limiting enzyme in the new synthesis of the purine guanosine triphosphate (GTP), which modulates both exocytotic insulin secretion and DNA synthesis, as well as a number of other critical cellular functions within the beta-cell. Alterations in the expression or activity of IMPDH may be central to beta-cell replication, cell cycle progression, differentiation, and maintenance of adequate islet mass, effects that are probably mediated both by GTP directly, and indirectly via low molecular mass GTPases. If GTP becomes depleted, a hierarchy of beta-cell functions becomes progressively paralyzed, until eventually the effete cell is removed via apoptosis.

Prolonged depletion of guanosine triphosphate induces death of insulin-secreting cells by apoptosis

Inhibitors of IMP dehydrogenase, such as mycophenolic acid (MPA) and mizoribine, which deplete cellular GTP, are used clinically as immunosuppressive drugs. The prolonged effect of such agents on insulin-secreting beta-cells (HIT-T15 and INS-1) was investigated. Both MPA and mizoribine inhibited mitogenesis, as reflected by [3H]thymidine incorporation. Cell number, DNA and protein contents, and cell (metabolic) viability were decreased by about 30%, 60%, and 80% after treatment of HIT cells with clinically relevant concentrations (e.g. 1 microg/ml) of MPA for 1, 2, and 4 days, respectively. Mizoribine (48 h) similarly induced the death of HIT cells. INS-1 cells also were damaged by prolonged MPA treatment. MPA-treated HIT cells displayed a strong and localized staining with a DNA-binding dye (propidium iodide), suggesting condensation and fragmentation of DNA, which were confirmed by detection of DNA laddering in multiples of about 180 bp. DNA fragmentation was observed after 24-h MPA treatment and was dose dependent (29%, 49%, and 70% of cells were affected after 48-h exposure to 1, 3, and 10 microg/ml MPA, respectively). Examination of MPA-treated cells by electron microscopy revealed typical signs of apoptosis: condensed and marginated chromatin, apoptotic bodies, cytosolic vacuolization, and loss of microvilli. MPA-induced cell death was almost totally prevented by supplementation with guanosine, but not with adenosine or deoxyguanosine, indicating a specific effect of GTP depletion. An inhibitor of protein isoprenylation (lovastatin, 10-100 microM for 2-3 days) induced cell death and DNA degradation similar to those induced by sustained GTP depletion, suggesting a mediatory role of posttranslationally modified GTP-binding proteins. Indeed, impeding the function of G proteins of the Rho family (via glucosylation using Clostridium difficile toxin B), although not itself inducing apoptosis, potentiated cell death induced by MPA or lovastatin. These findings indicate that prolonged depletion of GTP induces beta-cell death compatible with apoptosis; this probably involves a direct impairment of GTP-dependent RNA-primed DNA synthesis, but also appears to be modulated by small GTP-binding proteins. Treatment of intact adult rat islets (the beta-cells of which replicate slowly) induced a modest, but definite, death by apoptosis over 1- to 3-day periods. Thus, more prolonged use of the new generation of immunosuppressive agents exemplified by MPA might have deleterious effects on the survival of islet or pancreas grafts.

Nutrient augmentation of Ca2+-dependent and Ca2+-independent pathways in stimulus-coupling to insulin secretion can be distinguished by their guanosine triphosphate requirements: studies on rat pancreatic islets

To delineate the underlying mechanisms by which glucose augments both Ca2+-dependent and Ca2+-independent insulin release, the latter induced by the simultaneous activation of protein kinases A and C, we examined the effects of GTP depletion by mycophenolic acid (MPA), an inhibitor of GTP synthesis, on the augmentation of insulin release from rat pancreatic islets. MPA treatment reduced GTP content by 30-40% and completely abolished glucose-induced augmentation of Ca2+-independent insulin release. Thus, this pathway is extremely sensitive to a decrease in cellular GTP content. Complete inhibition was also observed in islets treated with MPA plus adenine, to maintain ATP levels, under which conditions GTP is selectively depleted. Provision of guanine, which increases the activity of a salvage pathway for GTP synthesis and normalizes GTP content, completely reversed the inhibitory effect of MPA. Neither glucose utilization nor glucose oxidation was affected by MPA. The augmentation of Ca2+-independent insulin release by several other metabolizable nutrients including alpha-ketoisocaproic acid (KIC) was also inhibited by MPA. In sharp contrast, augmentation of Ca2+-dependent insulin release by KIC was resistant to GTP depletion, indicating that nutrient-induced augmentation of the Ca2+-dependent- and Ca2+-independent secretory pathways can be differentiated by GTP dependency. We interpret these data in accord with current knowledge concerning the two known stimuli for exocytosis, Ca2+ and GTP (independently of Ca2+). We propose that both Ca2+-dependent and Ca2+-independent augmentation occurs via one metabolic pathway acting upon Ca2+- and upon GTP-stimulated exocytosis. Activation of PKA and PKC stimulates the GTP-sensitive exocytosis.

Necessity of endogenous GTP derived from glucose-6-phosphate for insulin secretion augmented by glucose under protein kinase A activation

To investigate the possible involvement of some intracellular metabolic signaling other than the ATP derived from glucose metabolism under protein kinase A (PKA) activation, we measured the insulin secretory capacity stimulated by glucose and other fuel secretagogues using diazoxide-treated pancreatic islets. Under these conditions, we found a signal from a site proximal to glyceraldehyde-3-phosphate (GA-3-P) in the glycolysis to be necessary for glucose-induced insulin secretion. By using several different glycolytic enzyme inhibitors, we found that this proximal signal is derived from glucose-6-phosphate (G-6-P), and that metabolic signaling distal to GA-3-P also is necessary. Mycophenolic acid completely inhibited the augmented glucose-induced insulin secretion, which guanosine could reverse, indicating that the proximal signaling is coupling with endogenous GTP production. In this novel system of metabolic signaling, endogenous GTP derived from G-6-P in the glycolysis elicits the augmentation of glucose-induced insulin secretion under PKA activation in diazoxide-treated pancreatic islets.

Evidence for differential roles of the Rho subfamily of GTP-binding proteins in glucose- and calcium-induced insulin secretion from pancreatic beta cells

We utilized clostridial toxins (with known specificities for inhibition of GTPases) to ascertain the contribution of candidate GTPases in physiologic insulin secretion from beta cells. Exposure of normal rat islets or isolated beta (HIT-T15) cells to Clostridium difficile toxins A and B catalyzed the glucosylation (and thereby the inactivation) of Rac, Cdc42, and Rho endogenous to beta cells; concomitantly, either toxin reduced glucose- or potassium-induced insulin secretion from rat islets and HIT cells. Treatment of beta cells with Clostridium sordellii lethal toxin (LT; which modified only Ras, Rap, and Rac) also reduced glucose- or potassium-induced secretion. However, clostridial toxin C3-exoenzyme (which ADP-ribosylates and inactivates only Rho) was without any effect on either glucose- or potassium-induced insulin secretion. These data suggest that Cdc42, Rac, Ras, and/or Rap (but not Rho) may be needed for glucose- or potassium-mediated secretion. The effects of these toxins appear to be specific on stimulus-secretion coupling, since no difference in metabolic viability (assessed colorimetrically by quantitating the conversion of the tetrazolium salt into a formazan in a reduction reaction driven by nutrient metabolism) was demonstrable between control and toxin (A or LT)-treated beta cells. Toxin (A or LT) treatment also did not alter glucose- or potassium-mediated rises in cytosolic free calcium concentrations ([Ca2+]i), suggesting that these GTPases are involved in steps distal to elevations in [Ca2+]i. Recent findings indicate that the carboxyl methylation of Cdc42 is stimulated by only glucose, whereas that of Rap (Kowluru et al., J Clin Invest 98: 540-555, 1996) and Rac (present study) are regulated by glucose or potassium. Together, these findings provide direct evidence, for the first time, that the Rho subfamily of GTPases plays a key regulatory role(s) in insulin secretion, and they suggest that Cdc42 may be required for early steps in glucose stimulation of insulin release, whereas Rap and/or Rac may be required for a later step(s) in the stimulus-secretion coupling cascade (i.e. Ca2+-induced exocytosis of insulin).

Interleukin-1 beta inhibits phospholipase C and insulin secretion at sites apart from KATP channel

Although interleukin-1 beta (IL-1 beta) reduces pancreatic islet content of ATP and GTP, the distal events that mediate its inhibitory effects on insulin secretion remain poorly understood. Herein, the activation of phospholipase C (PLC) was quantified during islet perifusions. An 18-h exposure to IL-1 beta (100 pM) totally vitiated activation of PLC induced by glucose, an effect that requires ATP and GTP and closure of the ATP-dependent K+ (KATP) channel. Surprisingly, however, when islets were depolarized directly using either of two agonists, glyburide (which does not act via generation of purine nucleotides) or 40 mM K+ (which acts distal to KATP channel), PLC and insulin secretion were again obliterated by IL-1 beta. IL-1 beta also reduced the labeling of phosphoinositide substrates; however, this effect was insufficient to explain the inhibition of PLC, since the effects on substrate labeling, but not on PLC, were prevented by coprovision of guanosine or adenosine. Furthermore, when IL-1 beta-treated islets were exposed to 100 microM carbachol (which activates PLC partially independent of extracellular Ca2+), the effects were still obliterated by IL-1 beta. These data (together with the finding that IL-1 beta inhibited Ca(2+)-induced insulin release) suggest that, in addition to its effects on ATP synthesis and thereby on the KATP channel, IL-1 beta has at least two undescribed, distal effects to block both PLC as well as Ca(2+)-induced exocytosis. The latter correlated best with IL-1 beta's effect to impede phosphoinositide synthesis, since it also was reversed by guanosine or adenosine.

Inhibition of calcium-induced insulin secretion from intact HIT-T15 or INS-1 beta cells by GTP depletion

Using intact rat islets, we previously observed that GTP depletion (achieved through the use of mycophenolic acid or other synthesis inhibitors) impedes nutrient- but not K+-induced insulin secretion. It was concluded that a proximal nutrient-dependent step in stimulus-secretion coupling (but not the process of Ca2+-induced exocytosis itself) is modulated by ambient GTP levels. To examine Ca2+-dependent steps further in intact beta cells, INS-1 cells (which synthesize GTP and ATP similarly to rat islets) and HIT-T15 cells (whose synthesis of purine nucleotides is different) were studied following cell culture for 1-18 hr in various concentrations of mycophenolic acid (MPA) or mizoribine (MZ). Both agents profoundly reduced GTP content (mean: -78%) and lowered the GTP/GDP ratio by an average of -73%; concomitantly, MPA or MZ reduced insulin secretion induced by 10 mM glucose, 30 or 40 mM KCl, or 100 microM tolbutamide, independent of any changes in cell viability, insulin content, ATP content, the ATP/ADP ratio, or cytosolic free Ca2+ concentrations. In INS-1 cells (which appear to have normal nucleobase transport and "salvage" pathway activities), guanine (but not adenine) restored GTP content, the GTP/GDP ratio, and Ca2+-induced secretion. In HIT cells, the phosphoribosylation of exogenous guanine or hypoxanthine is defective; however, provision of 500 microM guanosine (but not adenosine) reversed the effects of MPA. We conclude that, at least in certain situations, a requisite role for GTP in the distal step(s) of exocytosis can be demonstrated.

Concentration dependence and time course of the effects of glucose on adenine and guanine nucleotides in mouse pancreatic islets

Changes in the ATP:ADP ratio in pancreatic B cells may participate in the regulation of insulin secretion by glucose. Here, we have investigated the possible role of guanine nucleotides. Mouse islets were incubated in a control medium (when K+-ATP channels are the major site of regulation) or in a high K+ medium (when glucose modulates the effectiveness of cytosolic Ca2+ on exocytosis). Glucose induced a concentration-dependent (0-20 m) increase in GTP and a decrease in GDP in both types of medium, thus causing a progressive rise of the GTP:GDP ratio. ATP and ADP levels were 4-5-fold higher but varied in a similar way as those of guanine nucleotides. Insulin secretion was inversely correlated with ADP and GDP levels and positively correlated with the ATP:ADP and GTP:GDP ratios between 6 and 20 m glucose in control medium and between 0 and 20 m glucose in high K+ medium. The increases in the GTP:GDP and ATP:ADP ratios induced by a rise of glucose were faster than the decreases induced by a fall in glucose, but the changes of both ratios were again parallel. In conclusion, glucose causes large, concentration-dependent changes in guanine as well as in adenine nucleotides in islet cells. This raises the possibility that both participate in the regulation of nutrient-induced insulin secretion.

Role for GTP in glucose-induced phospholipase C activation in pancreatic islets

We have previously demonstrated a permissive role for GTP in insulin secretion; in the current studies, we examined the effect of GTP on phospholipase C (PLC) activation to explore one possible mechanism for that observation. In rat islets preexposed to the GTP synthesis inhibitors mycophenolic acid (MPA) or mizoribine (MZ), PLC activation induced by 16.7 mM glucose (or by 20 mM alpha-ketoisocaproic acid) was inhibited 63% without altering the labeling of phosphoinositide substrates. Provision of guanine, which normalizes islet GTP content and insulin release, prevented the inhibition of PLC by MPA. Glucose-induced phosphoinositide hydrolysis was blocked by removal of extracellular Ca2+ or by diazoxide. PLC induced directly by Ca2+ influx (i.e., 40 mM K+) was reduced 42% in MPA-pretreated islets but without inhibition of the concomitant insulin release. These data indicate that glucose-induced PLC activation largely reflects Ca2+ entry and demonstrate (for the first time in intact cells) that adequate GTP is necessary for glucose (and Ca(2+)-)-induced PLC activation but not for maximal Ca(2+)-induced exocytosis.

ARF and PITP restore GTP gamma S-stimulated protein secretion from cytosol-depleted HL60 cells by promoting PIP2 synthesis

BACKGROUND

In many cell types, including neutrophils and HL60 cells, there is an absolute requirement for a GTP-dependent step to elicit Ca(2+)-regulated secretion. Neutrophils and HL60 cells secrete lysosomal enzymes from azurophilic granules; this secretion is inhibited by 1% ethanol, indicating that phosphatidate (PA) produced by phospholipase D (PLD) activity may be involved. PLD can use primary alcohols in preference to water during the hydrolytic step, generating the corresponding phosphatidylalcohol instead of PA, its normal product. As ARF (ADP-ribosylation factor) proteins regulate PLD activity and are implicated in constitutive vesicular traffic, we have investigated whether ARF is also required for GTP-dependent secretion in HL60 cells.

RESULTS

We have used a cell-permeabilization protocol that allows HL60 cells to become refractory to stimulation with GTP gamma S plus 10 microM Ca2+ with regard to secretion and PLD activity. Permeabilization with streptolysin O for 10 minutes permitted the loss of freely diffusable cytosolic proteins, including ARF proteins. Fractions derived from brain cytosol, enriched in ARF proteins, restored secretory function and PLD activity. The major contaminating protein present in these ARF-enriched fractions was identified as phosphatidylinositol transfer protein (PITP). Unexpectedly, PITP was also found to restore GTP gamma S-dependent secretion. Restoration of secretory function was characterized using recombinant proteins, rARF1 and rPITP alpha and rPITP beta. The rARF1 protein restored both secretory function and PLD activity, whereas PITP only restored secretory function. However, both ARF and PITP were capable of stimulating phosphatidylinositol bis phosphate (PIP2) synthesis.

CONCLUSIONS

ARF and PITP restore secretory function in cytosol-depleted cells when stimulated with GTP gamma S plus Ca2+. We have previously shown that PITP participates in the synthesis of PIP2. In comparison, ARF1 activates PLD, producing PA, which is a known activator of phosphatidylinositol-4-phosphate 5 kinase, the enzyme responsible for PIP2 synthesis. We propose that ARF and PITP both restore exocytosis by a common mechanism-promoting PIP2 synthesis.

Cytosolic biosynthesis of GTP and ATP in normal rat pancreatic islets

GTP and ATP are necessary for glucose-induced insulin secretion; however, the biosynthetic pathways of purine nucleotides have not been studied in pancreatic islets. The present work examines the cytosolic pathways of purine nucleotide synthesis using intact rat islets cultured overnight in RPMI 1640 medium containing either [14C]glycine (to label the de novo pathway) or [3H]hypoxanthine (to mark the salvage pathway), with or without mycophenolic acid or L-alanosine (selective inhibitors of cytosolic GTP and ATP synthesis, respectively). Addition of mycophenolic acid decreased total GTP content (mass) by 73-81%; although the incorporation of labeled hypoxanthine into GTP also fell by 87%, the incorporation of glycine did not change. Similarly, L-alanosine decreased ATP mass by 26-33% in the presence of either label; whereas the incorporation of hypoxanthine into ATP fell 59%, the incorporation of glycine was again not significantly decreased. Thus, both the de novo and salvage purine nucleotide biosynthetic pathways are present in rat islets; however, the salvage pathway appears to be quantitatively the more important source of nucleotides. This conclusion was supported by additional studies of the effects on nucleotide content and insulin secretion of various site-specific inhibitors of purine synthesis. These findings have potential relevance to the processes of mitogenesis, cell proliferation and differentiation of islet cells, as well as for the control of insulin secretion.

Non-specific stimulatory effects of mastoparan on pancreatic islet nucleoside diphosphokinase activity: dissociation from insulin secretion

We examined whether mastoparan (MAS)-induced insulin secretion might involve the activation of nucleoside diphosphokinase (NDP kinase), which catalyzes the conversion of GDP to GTP, a known permissive factor for insulin secretion. MAS and MAS 7 (which activate GTP-binding proteins), but not MAS 17 (an inactive analog), stimulated insulin secretion from normal rat islets. In contrast to their specific effects on insulin secretion, MAS, MAS 7 and MAS 17 each stimulated formation of the phosphoenzyme-intermediate of NDP kinase, as well as its catalytic activity. These effects were mimicked by several cationic drugs. Thus, caution is indicated in using MAS to study cellular regulation, since some of its effects appear to be non-specific, and may be due, in part, to its amphiphilic, cationic nature.

Subcellular localization and kinetic characterization of guanine nucleotide binding proteins in normal rat and human pancreatic islets and transformed beta cells

The subcellular localization and the kinetics of the GTPase activities of monomeric and heterotrimeric GTP-binding proteins were investigated in normal rat and human pancreatic islets and were compared to those obtained using a transformed hamster beta cell line (HIT cells). The [alpha-32P]GTP overlay technique revealed the presence of at least four low-molecular-mass proteins (approx. 20-27 kDa) in normal rat islets, which were enriched in the secretory granule fraction compared to the membrane fraction (with little abundance of these proteins in the cytosolic fraction). In contrast, in HIT cells, these proteins (at least six) were predominantly cytosolic. Three of these proteins were immunologically identified as rab3A, rac2, and CDC42Hs in islets as well as in HIT cells. In addition, pertussis toxin augmented the ribosylation of at least one heterotrimeric G-protein of about 39 kDa (probably G(i) and/or G(o)) in the membrane and secretory granule fractions of normal rat and human islets, whereas at least three such Ptx substrates (36-39 kDa) were found in HIT cell membranes. Kinetic activities revealed the presence of at least three such activities (Km for GTP of 372 nM, 2.2 microM, and 724 microM) in islet homogenates which were differentially distributed in various subcellular fractions; similar activities were also demonstrable in HIT cell homogenates. Thus, these studies demonstrate the presence of both monomeric G-proteins intrinsic to the secretory granules of normal rat islets which can be ascribed to beta cells; since these G-proteins are regulated by insulinotropic lipids (as described in the accompanying article), such proteins may couple the activation of phospholipases (endogenous to islets) to the exocytotic secretion of insulin. These findings also suggest that caution is necessary in extrapolating data concerning G-proteins from cultured, transformed beta cell lines to the physiology of normal islets, in view of both qualitative and quantitative differences between the two preparations.