Glucose regulates the cortical actin network through modulation of Cdc42 cycling to stimulate insulin secretion

Insulin vesicle release: walk, kiss, pause ... then run

The mechanisms by which insulin-containing dense core secretory vesicles approach and finally fuse with the plasma membrane are of considerable current interest: defects in these processes may be one of the contributing factors to Type 2 diabetes. In this review, we discuss the molecular mechanisms involved in vesicle trafficking within the pancreatic beta-cell and the mechanisms whereby these may be regulated. We then go on to describe recent evidence that suggests that vesicle fusion at the plasma membrane is a partly reversible process ("kiss and run" or "cavity recapture"). We propose that vesicles may participate in a exo-endocytotic cycle in which a proportion of those that have already undergone an interaction with the plasma membrane may exchange exocytotic machinery with maturing vesicles.

Dominant-negative alpha-subunit of farnesyl- and geranyltransferase inhibits glucose-stimulated, but not KCl-stimulated, insulin secretion in INS 832/13 cells

The majority of small G-proteins undergo posttranslational modifications (e.g., isoprenylation) at their C-terminal cysteine residues. Such modifications increase their hydrophobicity, culminating in translocation of the modified proteins to their relevant membranous sites for interaction with their respective effectors. Previously, we reported glucose-dependent activation and membrane association of Rac1 in INS 832/13 cells. We also demonstrated modulatory roles for Rac1/GDP dissociation inhibitor in glucose-stimulated insulin secretion (GSIS) in INS 832/13 cells, further affirming roles for Rac1 in GSIS. Herein, we demonstrate that geranylgeranyltransferase inhibitor-2147 (GGTI-2147), an inhibitor of protein prenylation, markedly increased cytosolic accumulation of Rac1 and elicited significant inhibition of GSIS from INS 832/13 cells. In the current study, we also examined the localization of protein prenyltransferases (PPTases) and regulation of GSIS by PPTases in INS 832/13 cells. Western blot analyses indicated that the regulatory alpha-subunit and the structural beta-subunit of PPTase holoenzyme are predominantly cytosolic in their distribution. Overexpression of an inactive mutant of the regulatory alpha-subunit of PPTase markedly attenuated glucose- but not KCl-induced insulin secretion from INS 832/13 cells. Together, our findings provide the first evidence for the regulation of GSIS by PPTase in INS 832/13 cells. Furthermore, they support our original hypothesis that prenylation of specific G-proteins may be necessary for GSIS.

Limited role for SREBP-1c in defective glucose-induced insulin secretion from Zucker diabetic fatty rat islets: a functional and gene profiling analysis

Accumulation of intracellular lipid may contribute to defective insulin secretion in type 2 diabetes. Although Zucker diabetic fatty (ZDF; fa/fa) rat islets are fat-laden and overexpress the lipogenic master gene, sterol regulatory element binding protein 1c (SREBP-1c), the contribution of SREBP-1c to the secretory defects observed in this model remains unclear. Here we compare the gene expression profile of lean control (fa/+) and ZDF rat islets in the absence or presence of dominant-negative SREBP-1c (SREBP-1c DN). ZDF islets displayed elevated basal insulin secretion at 3 mmol/l glucose but a severely depressed response to 17 mmol/l glucose. While SREBP-1c DN reduced basal insulin secretion from ZDF islets, glucose-stimulated insulin secretion was not improved. Of 57 genes differentially regulated in ZDF islets and implicated in glucose metabolism, vesicle trafficking, ion fluxes, and/or exocytosis, 21 were upregulated and 5 were suppressed by SREBP-1c DN. Genes underrepresented in ZDF islets were either unaffected (Glut-2, Kir6.2, Rab3), stimulated (voltage-dependent Ca(2+) channel subunit alpha1D, CPT2, SUR2, rab9, syt13), or inhibited (syntaxin 7, secretogranin-2) by SREBP-1c inhibition. Correspondingly, SREBP-1c DN largely corrected decreases in the expression of the transcription factors Pdx-1 and MafA but did not affect the abnormalities in Pax6, Arx, hepatic nuclear factor-1alpha (HNF1alpha), HNF3beta/Forkhead box-a2 (Foxa2), inducible cyclic AMP early repressor (ICER), or transcription factor 7-like 2 (TCF7L2) expression observed in ZDF islets. We conclude that upregulation of SREBP-1c and mild increases in triglyceride content do not explain defective glucose-stimulated insulin secretion from ZDF rats. However, overexpression of SREBP-1c may contribute to enhanced basal insulin secretion in this model.

Biologically active lipids promote trafficking and membrane association of Rac1 in insulin-secreting INS 832/13 cells

Despite emerging evidence to suggest that glucose-stimulated insulin secretion (GSIS) requires membrane targeting of specific small G proteins (e.g., Rac1), very little is known with regard to the precise mechanisms underlying subcellular trafficking of these proteins in the glucose-stimulated islet beta-cell. We previously reported activation of small G proteins by biologically active lipids via potentiation of relevant GDP/GTP exchange activities within the beta-cell. Herein, we studied putative regulatory roles for these lipids in the trafficking and membrane association of Rac1 in cell-free preparations derived from INS 832/13 beta-cells. Incubation of INS 832/13 cell lysates with polyphosphoinositides (e.g., PIP(2)), phosphatidic acid, phosphatidylcholine, and phosphatidylserine significantly promoted trafficking of cytosolic Rac1 to the membrane fraction. Lysophosphatidic acid, but not lysophosphatidylcholine or lysophosphatidylserine, also promoted translocation and membrane association of Rac1. Arachidonic acid, diacylglycerol, calcium, and cAMP failed to exert any clear effects on Rac1 translocation to the membrane. Together, our findings provide the first direct evidence in support of our recent hypothesis (Kowluru A, Veluthakal R. Diabetes 54: 3523-3529, 2005), which states that generation of biologically active lipids, known to occur in the glucose-stimulated beta-cell, may mediate targeting of Rac1 to the membrane for optimal interaction with its putative effector proteins leading to GSIS.

Inhibition of vesicular stomatitis virus infection in epithelial cells by alpha interferon-induced soluble secreted proteins

Interferons (IFNs) are potent antiviral cytokines that inhibit infection by a wide spectrum of viruses by activating the Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathway. Several IFN-induced antiviral proteins including 2',5'-oligoadenylate synthetase, dsRNA-activated protein kinase and Mx play a critical role in conferring the antiviral properties of IFN. However, studies have shown that additional antiviral factors are involved in addition to these proteins during IFN-mediated antiviral action. In an effort to characterize these novel antiviral factors, the antiviral mechanism of alpha IFN (IFN-alpha) against vesicular stomatitis virus (VSV) was investigated in human lung epithelial A549 cells. These studies demonstrated that soluble secreted antiviral proteins as the constituents of conditioned medium prepared from IFN-alpha-treated cells reduced VSV infectivity by more than 2 logs, compared with a 4 log inhibition observed following treatment of cells with IFN-alpha. The antiviral mechanism of these secreted proteins appeared to act at the level of cellular entry of VSV. Interestingly, the IFN-alpha-induced antiviral proteins were secreted independently of STAT1 (an essential component of the JAK/STAT pathway), demonstrating that the release of such extracellular soluble antiviral proteins from cells may represent an alternative mechanism of the antiviral defence strategy of IFN towards VSV infection.

Exocytosis in neuroendocrine cells: new tasks for actin

Most secretory cells undergoing calcium-regulated exocytosis in response to cell surface receptor stimulation display a dense subplasmalemmal actin network, which is remodeled during the exocytotic process. This review summarizes new insights into the role of the cortical actin cytoskeleton in exocytosis. Many earlier findings support the actin-physical-barrier model whereby transient depolymerization of cortical actin filaments permits vesicles to gain access to their appropriate docking and fusion sites at the plasma membrane. On the other hand, data from our laboratory and others now indicate that actin polymerization also plays a positive role in the exocytotic process. Here, we discuss the potential functions attributed to the actin cytoskeleton at each major step of the exocytotic process, including recruitment, docking and fusion of secretory granules with the plasma membrane. Moreover, we present actin-binding proteins, which are likely to link actin organization to calcium signals along the exocytotic pathway. The results cited in this review are derived primarily from investigations of the adrenal medullary chromaffin cell, a cell model that is since many years a source of information concerning the molecular machinery underlying exocytosis.

Intersectin-1L nucleotide exchange factor regulates secretory granule exocytosis by activating Cdc42

Rho GTPases are key regulators of the actin cytoskeleton in membrane trafficking events. We previously reported that Cdc42 facilitates exocytosis in neuroendocrine cells by stimulating actin assembly at docking sites for secretory granules. These findings raise the question of the mechanism activating Cdc42 in exocytosis. The neuronal guanine nucleotide exchange factor, intersectin-1L, which specifically activates Cdc42 and is at an interface between membrane trafficking and actin dynamics, appears as an ideal candidate to fulfill this function. Using PC12 and chromaffin cells, we now show the presence of intersectin-1 at exocytotic sites. Moreover, through an RNA interference strategy coupled with expression of various constructs encoding the guanine nucleotide exchange domain, we demonstrate that intersectin-1L is an essential component of the exocytotic machinery. Silencing of intersectin-1 prevents secretagogue-induced activation of Cdc42 revealing intersectin-1L as the factor integrating Cdc42 activation to the exocytotic pathway. Our results extend the current role of intersectin-1L in endocytosis to a function in exocytosis and support the idea that intersectin-1L is an adaptor that coordinates exo-endocytotic membrane trafficking in secretory cells.

Glucose-sensing mechanisms in pancreatic beta-cells

The appropriate secretion of insulin from pancreatic beta-cells is critically important to the maintenance of energy homeostasis. The beta-cells must sense and respond suitably to postprandial increases of blood glucose, and perturbation of glucose-sensing in these cells can lead to hypoglycaemia or hyperglycaemias and ultimately diabetes. Here, we review beta-cell glucose-sensing with a particular focus on the regulation of cellular excitability and exocytosis. We examine in turn: (i) the generation of metabolic signalling molecules; (ii) the regulation of beta-cell membrane potential; and (iii) insulin granule dynamics and exocytosis. We further discuss the role of well known and putative candidate metabolic signals as regulators of insulin secretion.

Caveolin-1 functions as a novel Cdc42 guanine nucleotide dissociation inhibitor in pancreatic beta-cells

The cycling of the small Rho family GTPase Cdc42 is required for insulin granule exocytosis, although the regulatory proteins involved in Cdc42 cycling in pancreatic beta-cells are unknown. Here we demonstrate that the caveolar protein caveolin-1 (Cav-1) is a Cdc42-binding protein in beta-cells. Cav-1 associated with Cdc42-VAMP2-bound granules present near the plasma membrane under basal conditions. However, stimulation with glucose induced the dissociation of Cav-1 from Cdc42-VAMP2 complexes, coordinate with the timing of Cdc42 activation. Analyses of the Cav-1 scaffolding domain revealed a motif conserved in guanine nucleotide dissociation inhibitors (GDIs), which suggested a novel role for Cav-1 as a Cdc42 GDI in beta-cells. The novel role was further supported by: 1) in vitro binding analyses that demonstrated a direct interaction between Cav-1 and Cdc42; 2) GST-Cdc42 interaction assays showing preferential Cav-1 binding to GDP-Cdc42 over that of GTP-Cdc42; 3) Cav-1 depletion studies resulting in an inappropriate 40% induction of activated Cdc42 in the absence of stimuli and also a 40% increase in basal insulin release from both MIN6 cells and islets. Expression of wild-type Cav-1 in Cav-1-depleted cells restored basal level secretion to normal, whereas expression of a scaffolding domain mutant of Cav-1 failed to normalize secretion. Taken together, these data suggest that Cav-1 functions as a Cdc42 GDI in beta-cells, maintaining Cdc42 in an inactive state and regulating basal secretion in the absence of stimuli. Through its interaction with the Cdc42-VAMP2-bound insulin granule complex, Cav-1 may contribute to the specific targeting of granules to "active sites" of exocytosis organized by caveolae.

The role of cytoskeleton in glucose regulation

Cytoskeleton plays an important role in glucose regulation, mainly in the following three aspects. First, cytoskeleton regulates insulin secretion by guiding intracellular transport of insulin-containing vesicles and regulating release of insulin. Second, cytoskeleton is involved in insulin action by regulating distribution of insulin receptor substrate, GLUT4 translocation, and internalization of insulin receptor. In addition, cytoskeleton directs the intracellular distribution of glucose metabolism related enzymes including glycogen synthase and many glycolysis enzymes.

Regulation of pancreatic beta-cell insulin secretion by actin cytoskeleton remodelling: role of gelsolin and cooperation with the MAPK signalling pathway

We have previously isolated two MIN6 beta-cell sublines, B1, highly responsive to glucose-stimulated insulin secretion, and C3, markedly refractory (Lilla, V., Webb, G., Rickenbach, K., Maturana, A., Steiner, D. F., Halban, P. A. and Irminger, J. C. (2003) Endocrinology 144, 1368-1379). We now demonstrate that C3 cells have substantially increased amounts of F-actin stress fibres whereas B1 cells have shorter cortical F-actin. Consistent with these data, B1 cells display glucose-dependent actin remodelling whereas, in C3 cells, F-actin is refractory to this secretagogue. Furthermore, F-actin depolymerisation with latrunculin B restores glucose-stimulated insulin secretion in C3 cells. In parallel, glucose-stimulated ERK1/2 activation is greater in B1 than in C3 cells, and is potentiated in both sublines following F-actin depolymerisation. Glucose-activated phosphoERK1/2 accumulates at actin filament tips adjacent to the plasma membrane, indicating that these are the main sites of action for this kinase during insulin secretion. In addition, B1 cell expression of the calcium-dependent F-actin severing protein gelsolin is >100-fold higher than that of C3 cells. Knock-down of gelsolin reduced glucose-stimulated insulin secretion, whereas gelsolin over-expression potentiated secretion from B1 cells. Gelsolin localised along depolymerised actin fibres after glucose stimulation. Taken together, these data demonstrate that F-actin reorganization prior to insulin secretion requires gelsolin and plays a role in the glucose-dependent MAPK signal transduction that regulates beta-cell insulin secretion.

Syntaxin 4 Facilitates Biphasic Glucose-Stimulated Insulin Secretion from Pancreatic β-Cells

Numerous overexpression studies have recently implicated Syntaxin 4 as an effector of insulin secretion, although its requirement in insulin granule exocytosis is unknown. To address this, islets from Syntaxin 4 heterozygous (−/+) knockout mice were isolated and compared with islets from wild-type mice. Under static incubation conditions, Syntaxin 4 (−/+) islets showed a 60% reduction in glucose-stimulated insulin secretion compared with wild-type islets. Perifusion analyses revealed that Syntaxin 4 (−/+) islets secreted 50% less insulin during the first phase of glucose-stimulated insulin secretion and that this defect could be fully restored by the specific replenishment of recombinant Syntaxin 4. This essential role for Syntaxin 4 in secretion from the islet was localized to the β-cells because small interfering RNA-mediated depletion of Syntaxin 4 in MIN6 β-cells abolished glucose-stimulated insulin secretion. Moreover, immunofluorescent confocal microscopy revealed that Syntaxin 4 was principally localized to the β-cells and not the α-cells of the mouse islet. Remarkably, islets isolated from transgenic mice that express 2.4-fold higher levels of Syntaxin 4 relative to wild-type mice secreted approximately 35% more insulin during both phases of insulin secretion, suggesting that increased Syntaxin 4 may be beneficial for enhancing biphasic insulin secretion in a regulated manner. Taken together, these data support the notion that Syntaxin 4-based SNARE complexes are essential for biphasic insulin granule fusion in pancreatic β-cells.

Rho guanosine diphosphate-dissociation inhibitor plays a negative modulatory role in glucose-stimulated insulin secretion

Extant studies have implicated the Rho subfamily of guanosine triphosphate-binding proteins (G-proteins; e.g., Rac1) in physiological insulin secretion from isolated beta-cells. However, very little is known with regard to potential regulation by G-protein regulatory factors (e.g., the guanosine diphosphate-dissociation inhibitor [GDI]) of insulin secretion from the islet beta-cell. To this end, using Triton X-114 phase partition, co-immunoprecipitation, and sucrose density gradient centrifugation approaches, we report coexistence of GDI with Rac1 in insulin-secreting beta-cells (INS cells). Overexpression of wild-type GDI significantly inhibited glucose-induced, but not KCl- or mastoparan-induced, insulin secretion from INS cells. Furthermore, glucose-stimulated insulin secretion (GSIS) was significantly increased in INS cells in which expression of GDI was inhibited via the small interfering RNA-mediated knockdown approach. Together, these data appear to suggest an inhibitory role for GDI in the glucose metabolic signaling cascade, which may be relevant for GSIS.

Activation of NF-kappaB by extracellular matrix is involved in spreading and glucose-stimulated insulin secretion of pancreatic beta cells

Laminin-5-rich extracellular matrix derived from 804G cells (804G-ECM) engages beta1 integrins to induce spreading, improve glucose-stimulated insulin secretion (GSIS), and increase survival of pancreatic beta cells. The present study examines whether 804G-ECM activates the transcriptional activity of NF-kappaB and the involvement of NF-kappaB in those effects of 804G-ECM on pancreatic beta cells. 804G-ECM induces nuclear translocation and the DNA binding activity of the p65 subunit of NF-kappaB. 804G-ECM-induced nuclear translocation of NF-kappaB was weak as compared with that induced by interleukin-1beta. Transient 804G-ECM-induced DNA binding activity of NF-kappaB (peak at 2 h) and overexpression of NF-kappaB target genes IkappaB alpha and NF-kappaB1(p105) (peak at 4 h) were observed. When NF-kappaB was inhibited by an inhibitor of IkappaB alpha phosphorylation (Bay 11-7082) or by a recombinant adenovirus expressing the nonphosphorylatable form of IkappaB alpha, 804G-ECM-induced cell spreading and actin cytoskeleton organization were reduced. GSIS from cells on 804G-ECM was inhibited 5-fold, whereas cell survival was not affected. In summary, the results indicate that 804G-ECM induces a transient and moderate NF-kappaB activity. This study shows for the first time that ECM-induced NF-kappaB activity is necessary in maintaining GSIS, although it does not affect survival of pancreatic beta cells. The effects of ECM-induced NF-kappaB activity contrast with the deleterious effects of cytokine-induced NF-kappaB activity. It is proposed that transient and moderate NF-kappaB activity is essential for proper function of the pancreatic beta cell.

Myristoylated Alanine-rich C Kinase Substrate-mediated Neurotensin Release via Protein Kinase C-δ Downstream of the Rho/ROK Pathway

Myristoylated alanine-rich protein kinase C substrate (MARCKS) is a cellular substrate for protein kinase C (PKC). Recently, we have shown that PKC isoforms-alpha and -delta, as well as the Rho/Rho kinase (ROK) pathway, play a role in phorbol 12-myristate 13-acetate (PMA)-mediated secretion of the gut peptide neurotensin (NT) in the BON human endocrine cell line. Here, we demonstrate that activation of MARCKS protein is important for PMA- and bombesin (BBS)-mediated NT secretion in BON cells. Small interfering RNA (siRNA) to MARCKS significantly inhibited, whereas overexpression of wild-type MARCKS significantly increased PMA-mediated NT secretion. Endogenous MARCKS and green fluorescent protein-tagged wild-type MARCKS were translocated from membrane to cytosol upon PMA treatment, further confirming MARCKS activation. MARCKS phosphorylation was inhibited by PKC-delta siRNA, ROKalpha siRNA, and C3 toxin (a Rho protein inhibitor), suggesting that the PKC-delta and the Rho/ROK pathways are necessary for MARCKS activation. The phosphorylation of PKC-delta was inhibited by C3 toxin, demonstrating that the role of MARCKS in NT secretion was regulated by PKC-delta downstream of the Rho/ROK pathway. BON cell clones stably transfected with the receptor for gastrin releasing peptide, a physiologic stimulant of NT, and treated with BBS, the amphibian equivalent of gastrin releasing peptide, demonstrated a similar MARCKS phosphorylation as noted with PMA. BBS-mediated NT secretion was attenuated by MARCKS siRNA. Collectively, these findings provide evidence for novel signaling pathways, including the sequential regulation of MARCKS activity by Rho/ROK and PKC-delta proteins, in stimulated gut peptide secretion.

A direct interaction between Cdc42 and vesicle-associated membrane protein 2 regulates SNARE-dependent insulin exocytosis

In pancreatic beta cells, insulin granule exocytosis is regulated by SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor protein (SNAP) receptor) proteins, and this is coupled to cortical F-actin reorganization via the Rho family GTPase Cdc42 by an unknown mechanism. We investigated interactions among the target SNARE protein Syntaxin 1A and the vesicle-associated membrane SNARE protein (VAMP2) with Cdc42 and compared these structural interactions with their functional importance to glucose-stimulated insulin secretion in MIN6 beta cells. Subcellular fractionation analyses revealed a parallel redistribution of Cdc42 and VAMP2 from the granule fraction to the plasma membrane in response to glucose that temporally corresponded with the glucose-induced activation of Cdc42. Moreover, within these fractions Cdc42 and VAMP2 were found to co-immunoprecipitate under basal and glucose-stimulated conditions, suggesting that they moved as a complex. Furthermore, VAMP2 bound both GST-Cdc42-GTPgammaS and GST-Cdc42-GDP, indicating that the Cdc42-VAMP2 complex could form under both cytosolic GDP-bound Cdc42 and plasma membrane GTP-bound Cdc42 conformational conditions. In vitro binding analyses showed that VAMP2 bound directly to Cdc42 and that a heterotrimeric complex with Syntaxin 1A could also be formed. Deletion analyses of VAMP2 revealed that only the N-terminal 28 residues were required for Cdc42 binding. Expression of this 28-residue VAMP2 peptide in MIN6 beta cells resulted in the specific impairment of glucose-stimulated insulin secretion, indicating a functional importance for the Cdc42-VAMP2 interaction. Taken together, these data suggest a mechanism whereby glucose activates Cdc42 to induce the targeting of intracellular Cdc42-VAMP2-insulin granule complexes to Syntaxin 1A at the plasma membrane.

The Role of Protein Kinase D in Neurotensin Secretion Mediated by Protein Kinase C-α/-δ and Rho/Rho Kinase

Neurotensin (NT) is a gut peptide that plays an important role in gastrointestinal (GI) secretion, motility, and growth as well as the proliferation of NT receptor positive cancers. Secretion of NT is regulated by phorbol ester-sensitive protein kinase C (PKC) isoforms-alpha and -delta and may involve protein kinase D (PKD). The purpose of our present study was: (i) to define the role of PKD in NT release from BON endocrine cells and (ii) to delineate the upstream signaling mechanisms mediating this effect. Here, we demonstrate that small interfering RNA (siRNA) targeted against PKD dramatically inhibited both basal and PMA-stimulated NT secretion; NT release is significantly increased by overexpression of PKD. PKC-alpha and -delta siRNA attenuated PKD activity, whereas overexpression of PKC-alpha and -delta enhanced PKD activity. Rho kinase (ROK) siRNA significantly inhibited NT secretion, whereas overexpression of ROKalpha effectively increased NT release. Rho protein inhibitor C3 dramatically inhibited both NT secretion and PKD activity. In conclusion, our results demonstrate that PKD activation plays a central role in NT peptide secretion; upstream regulators of PKD include PKC-alpha and -delta and Rho/ROK. Importantly, our results identify novel signaling pathways, which culminate in gut peptide release.

Kinesin I and cytoplasmic dynein orchestrate glucose-stimulated insulin-containing vesicle movements in clonal MIN6 beta-cells

Glucose-stimulated mobilization of large dense-core vesicles (LDCVs) to the plasma membrane is essential for sustained insulin secretion. At present, the cytoskeletal structures and molecular motors involved in vesicle trafficking in beta-cells are poorly defined. Here, we describe simultaneous imaging of enhanced green fluorescent protein (EGFP)-tagged LDCVs and microtubules in beta-cells. Microtubules exist as a tangled array, along which vesicles describe complex directional movements. Whilst LDCVs frequently changed direction, implying the involvement of both plus- and minus-end directed motors, inactivation of the minus-end motor, cytoplasmic dynein, inhibited only a small fraction of all vesicle movements which were involved in vesicle recovery after glucose-stimulated exocytosis. By contrast, selective silencing of the plus-end motor, kinesin I, with small interfering RNAs substantially inhibited all vesicle movements. We conclude that the majority of LDCV transport in beta-cells is mediated by kinesin I, whilst dynein probably contributes to the recovery of vesicles after rapid kiss-and-run exocytosis.