GTP-binding proteins in cell survival and demise: the emerging picture in the pancreatic beta-cell

Upregulation of phagocyte-like NADPH oxidase by cytokines in pancreatic beta-cells: attenuation of oxidative and nitrosative stress by 2-bromopalmitate

Phagocyte-like NADPH oxidase (Nox2) has been shown to play regulatory roles in the metabolic dysfunction of the islet β-cell under the duress of glucolipotoxic conditions and exposure to proinflammatory cytokines. However, the precise mechanisms underlying Nox2 activation by these stimuli remain less understood. To this end, we report a time-dependent phosphorylation of p47phox, a cytosolic subunit of Nox2, by cytomix (IL-1β+TNFα+IFNγ) in insulin-secreting INS-1 832/13 cells. Furthermore, cytomix induced the expression of gp91phox, a membrane component of Nox2. 2-Bromopalmitate (2-BP), a known inhibitor of protein palmitoylation, markedly attenuated cytokine-induced, Nox2-mediated reactive oxygen species (ROS) generation and inducible nitric oxide synthase (iNOS)-mediated nitric oxide (NO) generation. However, 2-BP failed to exert any significant effects on cytomix-induced CHOP expression, a marker for endoplasmic reticulum stress. Together, our findings identify palmitoyltransferase as a target for inhibition of cytomix-induced oxidative (ROS generation) and nitrosative (NO generation) stress in the pancreatic β-cell.

Essential role of the small GTPase Ran in postnatal pancreatic islet development

The small GTPase Ran orchestrates pleiotropic cellular responses of nucleo-cytoplasmic shuttling, mitosis and subcellular trafficking, but whether deregulation of these pathways contributes to disease pathogenesis has remained elusive. Here, we generated transgenic mice expressing wild type (WT) Ran, loss-of-function Ran T24N mutant or constitutively active Ran G19V mutant in pancreatic islet β cells under the control of the rat insulin promoter. Embryonic pancreas and islet development, including emergence of insulin⁺ β cells, was indistinguishable in control or transgenic mice. However, by one month after birth, transgenic mice expressing any of the three Ran variants exhibited overt diabetes, with hyperglycemia, reduced insulin production, and nearly complete loss of islet number and islet mass, in vivo. Deregulated Ran signaling in transgenic mice, adenoviral over-expression of WT or mutant Ran in isolated islets, or short hairpin RNA (shRNA) silencing of endogenous Ran in model insulinoma INS-1 cells, all resulted in decreased expression of the pancreatic and duodenal homeobox transcription factor, PDX-1, and reduced β cell proliferation, in vivo. These data demonstrate that a finely-tuned balance of Ran GTPase signaling is essential for postnatal pancreatic islet development and glucose homeostasis, in vivo.

Comparative analysis of the role of small G proteins in cell migration and cell death: cytoprotective and promigratory effects of RalA

Small G protein superfamily consists of more than 150 members, and is classified into six families: the Ras, Rho, Rab, Arf, Ran, and RGK families. They regulate a wide variety of cell functions such as cell proliferation/differentiation, cytoskeletal reorganization, vesicle trafficking, nucleocytoplasmic transport and microtubule organization. The small G proteins have also been shown to regulate cell death/survival and cell shape. In this study, we compared the role of representative members of the six families of small G proteins in cell migration and cell death/survival, two cellular phenotypes that are associated with inflammation, tumorigenesis, and metastasis. Our results show that small G proteins of the six families differentially regulate cell death and cell cycle distribution. In particular, our results indicate that Rho family of small G proteins is antiapoptotic. Ras, Rho, and Ran families promoted cell migration. There was no significant correlation between the cell death- and cell migration-regulating activities of the small G proteins. Nevertheless, RalA was not only cytoprotective against multiple chemotherapeutic drugs, but also promigratory inducing stress fiber formation, which was accompanied by the activation of Akt and Erk pathways. Our study provides a framework for further systematic investigation of small G proteins in the perspectives of cell death/survival and motility in inflammation and cancer.

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.

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.

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.

Mycophenolic Acid Induces Islet Apoptosis by Regulating Mitogen-Activated Protein Kinase Activation

Mycophenolic acid (MPA), an inosine monophosphate dehydrogenase inhibitor, is widely used as an immunosuppressive drug after transplantations including those of pancreas islet cells. However, recent reports have indicated that MPA has apoptotic effects on islet cells in vitro. To study the effect of MPA on islet cells and determine its mechanism, we used an insulin secreting cell-line, HIT-T15. We examined mitogen-activated protein kinase (MAPK) activation after MPA treatment, and determining cell death levels using methylthiazdetetrazolium assays. The activations of extracellular signal-regulated protein kinase (ERK), c-jun N-terminal kinase (JNK), and p38 MAPK and caspase-3 cleavage were measured by Western blotting. MPA (1, 10, 30 micromol/L) increased cell death and caspase-3 cleavage within 24 hours. Exogenous 500 micromol/L guanosine reversed the MPA-induced islet cell death, but exogenous adenosine did not. MPA 10 micromol/L induced cell apoptosis and increased the activations of JNK, ERK, and p38 MAPK. Furthermore, exogenous guanosine, but not exogenous adenosine, reversed these effects induced by MPA. This study demonstrated that MPA may induce islet apoptosis in HIT-T15 cells by increasing activations of JNK, ERK, and p38 MAPK in a guanosine-dependent manner.

Essential role for membrane lipid rafts in interleukin-1beta-induced nitric oxide release from insulin-secreting cells: potential regulation by caveolin-1+

We recently reported that the activation of H-Ras represents one of the signaling steps underlying the interleukin-1beta (IL-1beta)-mediated metabolic dysfunction of the islet beta-cell. In the present study, we examined potential contributory roles of membrane-associated, cholesterol-enriched lipid rafts/caveolae and their constituent proteins (e.g., caveolin-1 [Cav-1]) as potential sites for IL-1beta-induced nitric oxide (NO) release in the isolated beta-cell. Disruption of lipid rafts (e.g., with cyclodextrin) markedly reduced IL-1beta-induced gene expression of inducible NO synthase (iNOS) and NO release from beta-cells. Immunologic and confocal microscopic evidence also suggested a transient but significant stimulation of tyrosine phosphorylation of Cav-1 in beta-cells briefly (for 15 min) exposed to IL-1beta that was markedly attenuated by three structurally distinct inhibitors of protein tyrosine phosphorylation. Overexpression of an inactive mutant of Cav-1 lacking the tyrosine phosphorylation site (Y14F) or an siRNA-mediated Cav-1 knock down also resulted in marked attenuation of IL-1beta-induced iNOS gene expression and NO release from these cells, thus further implicating Cav-1 in this signaling cascade. IL-1beta treatment also increased (within 20 min) the translocation of H-Ras into lipid rafts. Here we provide the first evidence to suggest that tyrosine phosphorylation of Cav-1 and subsequent interaction among members of the Ras signaling pathway within the membrane lipid microdomains represent early signaling mechanisms of IL-1beta in beta-cells.

Antifungal protein PAF severely affects the integrity of the plasma membrane of Aspergillus nidulans and induces an apoptosis-like phenotype

The small, basic, and cysteine-rich antifungal protein PAF is abundantly secreted into the supernatant by the beta-lactam producer Penicillium chrysogenum. PAF inhibits the growth of various important plant and zoopathogenic filamentous fungi. Previous studies revealed the active internalization of the antifungal protein and the induction of multifactorial detrimental effects, which finally resulted in morphological changes and growth inhibition in target fungi. In the present study, we offer detailed insights into the mechanism of action of PAF and give evidence for the induction of a programmed cell death-like phenotype. We proved the hyperpolarization of the plasma membrane in PAF-treated Aspergillus nidulans hyphae by using the aminonaphtylethenylpyridinium dye di-8-ANEPPS. The exposure of phosphatidylserine on the surface of A. nidulans protoplasts by Annexin V staining and the detection of DNA strand breaks by TUNEL (terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling) gave evidence for a PAF-induced apoptotic-like mechanism in A. nidulans. The localization of reactive oxygen species (ROS) by dichlorodihydrofluorescein diacetate and the abnormal cellular ultrastructure analyzed by transmission electron microscopy suggested that ROS-elicited membrane damage and the disintegration of mitochondria played a major role in the cytotoxicity of PAF. Finally, the reduced PAF sensitivity of A. nidulans strain FGSC1053, which carries a dominant-interfering mutation in fadA, supported our assumption that G-protein signaling was involved in PAF-mediated toxicity.

Involvement of the cGMP signalling pathway in the regulation of viability in insulin-secreting BRIN-BD11 cells

We have evaluated the hypothesis that cGMP may serve as an intracellular messenger regulating the viability of pancreatic beta-cells. A direct activator of soluble guanylyl cyclase, YC-1, caused a time- and dose-dependent loss of viability in clonal BRIN-BD11 beta-cells. This was accompanied by a rise in cGMP and was antagonised by Rp-8-pCPT-cGMPS, a selective inhibitor of protein kinase G (PKG). Reverse transcription polymerase chain reaction analysis confirmed that BRIN-BD11 cells (and human islets) express all three known isoforms of PKG (PKG-Ialpha, -Ibeta and II). Cell death induced by YC-1 was not sensitive to cell-permeable caspase inhibitors and was not accompanied by oligonucleosomal DNA fragmentation. The response was, however, inhibited by actinomycin D, suggesting that a transcription-dependent pathway of programmed cell death is involved in the actions of cGMP.

Novel roles for palmitoylation of Ras in IL-1β-induced nitric oxide release and caspase 3 activation in insulin-secreting β cells

We recently demonstrated that functional inactivation of H-Ras results in significant reduction in interleukin 1 beta (IL-1 beta)-mediated effects on isolated beta cells. Since palmitoylation of Ras has been implicated in its membrane targeting, we examined the contributory roles of palmitoylation of Ras in IL-1 beta-induced nitric oxide (NO) release and subsequent activation of caspases. Preincubation of HIT-T15 or INS-1 cells with cerulenin (CER, 134 microM; 3 hr), an inhibitor of protein palmitoylation, significantly reduced (-95%) IL-1 beta-induced NO release from these cells. 2-Bromopalmitate, a structurally distinct inhibitor of protein palmitoylation, but not 2-hydroxymyristic acid, an inhibitor of protein myristoylation, also reduced (-67%) IL-1 beta-induced NO release from HIT cells. IL-induced inducible nitric oxide synthase gene expression was markedly attenuated by CER. Further, CER markedly reduced incorporation of [3H]palmitate into H-Ras and caused significant accumulation of Ras in the cytosolic fraction. CER-treatment also prevented IL-1 beta-induced activation of caspase 3 in these cells. Moreover, N-monomethyl-L-arginine, a known inhibitor of inducible nitric oxide synthase, markedly inhibited IL-induced activation of caspase 3, thus establishing a link between IL-induced NO release and caspase 3 activation. Depletion of membrane-bound cholesterol using methyl-beta-cyclodextrin, which also disrupts caveolar organization within the plasma membrane, abolished IL-1 beta-induced NO release suggesting that IL-1 beta-mediated Ras-dependent signaling in these cells involves the intermediacy of caveolae and their key constituents (e.g. caveolin-1) in isolated beta cells. Confocal light microscopic evidence indicated significant colocalization of Ras with caveolin-1. Taken together, our data provide the first evidence to indicate that palmitoylation of Ras is essential for IL-1 beta-induced cytotoxic effects on the islet beta cell.

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.

Mastoparan-induced insulin secretion from insulin-secreting betaTC3 and INS-1 cells: evidence for its regulation by Rho subfamily of G proteins

Mastoparan, a tetradecapeptide from wasp venom, stimulates insulin secretion from the islet beta-cells, presumably via activation of trimeric G proteins. Herein, we used Clostridial toxins, which selectively modify and inactivate the Rho subfamily of G proteins, to examine whether mastoparan-induced insulin secretion also involves activation of these signaling proteins. Mastoparan, but not mastoparan 17 (an inactive analog of mastoparan), significantly stimulated insulin secretion from betaTC3 and INS-1 cells. Preincubation of betaTC3 cells with either Clostridium difficille toxin B, which inactivates Rho, Cdc42, and Rac, or Clostridium sordellii toxin, which inactivates Ras, Rap, and Rac, markedly attenuated the mastoparan-induced insulin secretion, implicating Rac in this phenomenon. Mastoparan-stimulated insulin secretion was resistant to GGTI-2147, a specific inhibitor of geranylgeranylation of Rho G proteins (e.g. Rac), suggesting that mastoparan induces direct activation of Rac via GTP/GDP exchange. This was confirmed by a pull-down assay that quantifies the binding of activated (i.e. GTP-bound) Rac to p21-activated kinase. However, glucose-induced insulin secretion from these cells was abolished by toxin B or GGTI-2147, suggesting that the geranylgeranylation step is critical for glucose-stimulated secretion. Mastoparan significantly increased the translocation of cytosolic Rac and Cdc42 to the membrane fraction. Confocal light microscopy revealed a substantial degree of colocalization of Rac (and, to a lesser degree, Cdc42) with insulin in beta-cells exposed to mastoparan. Further, stable expression of a dominant negative (N17Rac) form of Rac into INS-1 cells resulted in a significant reduction in mastoparan-stimulated insulin secretion from these cells. Taken together, our findings implicate Rho G proteins, specifically Rac, in mastoparan-induced insulin release.

Novel roles for the rho subfamily of GTP-binding proteins in succinate-induced insulin secretion from betaTC3 cells: further evidence in support of the succinate mechanism of insulin release

We have previously demonstrated regulatory roles for Rho subfamily of G-proteins in glucose- and calcium-induced insulin secretion. Herein, we examined regulation by these proteins of insulin secretion from betaTC3 cells elicited by mitochondrial fuels, such as the succinic acid methyl ester (SAME). Preincubation of these cells with Clostridium difficile toxin-B (200 ng/mL), which monoglucosylates and inactivates Cdc42 and Rac1, markedly decreased (> 70%) SAME-induced insulin secretion. Furthermore, exposure of betaTC3 cells to GGTI-2147 (20 microM), a selective inhibitor of the requisite prenylation of Rac1 and Cdc42, significantly reduced (> 80%) SAME-induced insulin release, suggesting that post-translational prenylation of these proteins is necessary for SAME-induced insulin release. Western blot analysis indicated localization of Cdc42, Rac1, and Ras in the beta cell mitochondrial fraction. Confocal microscopy revealed a modest, but inconsistent, increase in the association of either Rac1 or Cdc42 with Mitotracker, a mitochondrial marker, following exposure to SAME. These data suggest that activation of preexisting intramitochondrial Rac1 and Cdc42 may be sufficient to regulate SAME-induced insulin secretion. Together, our findings support a role for G-proteins in insulin secretion at a step dependent on mitochondrial metabolism. They also identify mevalonate-derived, isoprenoid modified Rho G-proteins as specific signaling molecules in recently proposed succinate mechanism of insulin release.

Glucagon-like peptide-1 receptor signaling modulates beta cell apoptosis

Glucagon-like peptide-1 (GLP-1) stimulates insulin secretion and augments beta cell mass via activation of beta cell proliferation and islet neogenesis. We examined whether GLP-1 receptor signaling modifies the cellular susceptibility to apoptosis. Mice administered streptozotocin (STZ), an agent known to induce beta cell apoptosis, exhibit sustained improvement in glycemic control and increased levels of plasma insulin with concomitant administration of the GLP-1 agonist exendin-4 (Ex-4). Blood glucose remained significantly lower for weeks after cessation of exendin-4. STZ induced beta cell apoptosis, which was significantly reduced by co-administration of Ex-4. Conversely, mice with a targeted disruption of the GLP-1 receptor gene exhibited increased beta cell apoptosis after STZ administration. Exendin-4 directly reduced cytokine-induced apoptosis in purified rat beta cells exposed to interleukin 1beta, tumor necrosis fator alpha, and interferon gamma in vitro. Furthermore, Ex-4-treated BHK-GLP-1R cells exhibited significantly increased cell viability, reduced caspase activity, and decreased cleavage of beta-catenin after treatment with cycloheximide in vitro. These findings demonstrate that GLP-1 receptor signaling directly modifies the susceptibility to apoptotic injury, and provides a new potential mechanism linking GLP-1 receptor activation to preservation or enhancement of beta cell mass in vivo.

Inhibition of glucose- and calcium-induced insulin secretion from betaTC3 cells by novel inhibitors of protein isoprenylation

The majority of low molecular weight G proteins undergoes a series of post-translational modification steps, e.g., isoprenylation, at their C-terminal cysteine, which seem to be critical for the transport of the modified proteins to the membrane sites for interaction with their respective effector proteins. Using lovastatin, an inhibitor of mevalonic acid, and hence, isoprenoid biosynthesis, we demonstrated previously that protein isoprenylation is critical for physiological insulin secretion from normal rat islets. Herein, we used more selective synthetic inhibitors of protein prenylation to examine their effects on glucose- and calcium-mediated insulin secretion from betaTC3 cells. Both 3-allyl- and vinylfarnesols, which inhibit and/or modulate protein farnesyl transferases, significantly (80-95%) inhibited glucose- and KCl-stimulated insulin secretion from these cells. In a similar manner, the allyl and vinyl forms of geranylgeraniol, reagents targeted toward protein geranylation, attenuated insulin secretion elicited by glucose and KCl. Furthermore, manumycin A, a natural inhibitor of protein farnesylation, and geranylgeranyl transferase inhibitor-2147 (GGTI-2147), a peptidomimetic inhibitor of protein geranylgeranylation, also inhibited glucose- and KCl-induced insulin secretion to comparable degrees. Treatment of betaTC3 cells with either 3-vinylfarnesol or 3-vinyl geranylgeraniol resulted in accumulation of unprenylated proteins in the cytosolic fraction. These data further support our original formulation that inhibition of isoprenylation of small molecular weight G proteins might impede their interaction with their putative effectors, which may be required for physiological insulin secretion.