ADP-ribosylation factor 6 regulates insulin secretion through plasma membrane phosphatidylinositol 4,5-bisphosphate

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.

Phosphatidic acid regulation of phosphatidylinositol 4-phosphate 5-kinases

Phosphatidic acid (PA) production by receptor-stimulated phospholipase D is believed to play an important role in the regulation of cell function. The second messenger function of PA remains to be elucidated. PA can bind and affect the activities of different enzymes and here we summarise the current status of activation of Type I phosphatidylinositol 4-phosphate 5-kinase by PA. Type 1 phosphatidylinositol 4-phosphate 5-kinase is also regulated by ARF proteins as is phospholipase D and we discuss the contributions of ARF and PA towards phosphatidylinositol(4,5)bisphosphate synthesis at the plasma membrane.

Regulatory roles for Tiam1, a guanine nucleotide exchange factor for Rac1, in glucose-stimulated insulin secretion in pancreatic beta-cells

Using various biochemical, pharmacological and molecular biological approaches, we have recently reported regulatory roles for Rac1, a small G-protein, in glucose-stimulated insulin secretion (GSIS). However, little is understood with respect to localization of, and regulation by, specific regulatory factors of Rac1 in GSIS. Herein, we investigated regulatory roles for Tiam1, a specific nucleotide exchange factor (GEF) for Rac1, in GSIS in pancreatic beta-cells. Western blot analysis indicated that Tiam1 is predominantly cytosolic in distribution. NSC23766, a specific inhibitor of Tiam1-mediated activation of Rac1, markedly attenuated glucose-induced, but not KCl-induced insulin secretion in INS 832/13 cells and normal rat islets. Further, NSC23766 significantly reduced glucose-induced activation (i.e. GTP-bound form) and membrane association of Rac1 in INS 832/13 cells and rat islets. Moreover, siRNA-mediated knock-down of Tiam1 markedly inhibited glucose-induced membrane trafficking and activation of Rac1 in INS 832/13 cells. Interestingly, however, in contrast to the inhibitory effects of NSC23766, Tiam1 gene depletion potentiated GSIS in these cells; such a potentiation of GSIS was sensitive to extracellular calcium. Together, our studies present the first evidence for a regulatory role for Tiam1/Rac1-sensitive signaling step in GSIS. They also provide evidence for the existence of a potential Rac1/Tiam1-independent, but calcium-sensitive component for GSIS in these cells.

Clathrin-independent endocytosis: a unique platform for cell signaling and PM remodeling

There is increasing interest in endocytosis that occurs independently of clathrin coats and the fates of membrane proteins internalized by this mechanism. The appearance of clathrin-independent endocytic and membrane recycling pathways seems to vary with different cell types and cargo molecules. In this review we focus on studies that have been performed using HeLa and COS cells as model systems for understanding this membrane trafficking system. These endosomal membranes contain signaling molecules including H-Ras, Rac1, Arf6 and Rab proteins, and a lipid environment rich in cholesterol and PIP(2) providing a unique platform for cell signaling. Furthermore, activation of some of these signaling molecules (H-Ras, Rac and Arf6) can switch the constitutive form of clathrin-independent endocytosis into a stimulated one, associated with PM ruffling and macropinocytosis.

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.

Somatostatin receptors signal through EFA6A-ARF6 to activate phospholipase D in clonal beta-cells

Somatostatin (SS) is a peptide hormone that inhibits insulin secretion in beta-cells by activating its G(i/o)-coupled receptors. Our previous work indicated that a betagamma-dimer of G(i/o) coupled to SS receptors can activate phospholipase D1 (PLD1) (Cheng, H., Grodnitzky, J. A., Yibchok-anun, S., Ding, J., and Hsu, W. H. (2005) Mol. Pharmacol. 67, 2162-2172). The aim of the present study was to elucidate the mechanisms underlying SS-induced PLD activation. We demonstrated the presence of ADP-ribosylation factor Arf1 and Arf6 in clonal beta-cells, HIT-T15. We also determined that the activation of PLD1 was mediated through Arf6. Overexpression of dominant-negative (dn) Arf6 mutant, Arf6(T27N), and suppression of mRNA levels using siRNA, both abolished SS-induced PLD activation, while overexpression of wild type Arf6 further enhanced this PLD activation. In contrast, overexpression of dn-Arf1 mutant Arf1(T31N) or dn-Arf5 mutant Arf5(T31N) failed to reduce SS-induced PLD activation. These findings suggested that Arf6, but not Arf1 or Arf5, mediates the effect of SS. We further determined the involvement of the Arf6 guanine nucleotide exchange factor (GEF) EFA6A, a GEF previously thought to be found predominantly in the brain, in the activation of PLD1 in HIT-T15 cells. Using Northern and Western blot analyses, both mRNA and protein of EFA6A were found in these cells. Overexpression of dn-EFA6A mutant, EFA6A(E242K), and suppression of mRNA levels using siRNA, both abolished SS-induced PLD activation, whereas overexpression of dn-EFA6B mutant, EFA6B(E651K), failed to reduce SS-induced PLD activation. In addition, overexpression of dn-ARNO mutant, ARNO(E156K), another GEF of Arf6, had no effect on SS-induced activation of PLD. Taken together, these results suggest that SS signals through EFA6A to activate Arf6-PLD cascade.

Rapid turnover of phosphatidylinositol-4,5-bisphosphate in insulin-secreting cells mediated by Ca2+ and the ATP-to-ADP ratio

Phosphatidylinositol-4,5-bisphosphate (PIP(2)) is important for a variety of cellular processes as a precursor for second messengers and by regulating ion channels, the cytoskeleton, and vesicle traffic in many types of cells, including insulin-secreting beta-cells. Here, we applied evanescent wave microscopy and the PIP(2)-binding pleckstrin homology domain from phospholipase C (PLC)-delta fused to the green fluorescent protein to characterize the regulation of plasma membrane PIP(2) in individual insulin-secreting MIN6 beta-cells. Elevation of the glucose concentration from 3 to 11 mmol/l evoked antisynchronous oscillations of [PIP(2)] and cytoplasmic Ca(2+)concentration, consistent with PLC being periodically activated by the voltage-dependent Ca(2+) influx. The effect of adenine nucleotides on [PIP(2)] was studied in cells permeabilized with alpha-toxin. ATP dose- dependently stimulated PIP(2) synthesis with half-maximal effect at 300 mumol/l. Omission of the nucleotide resulted in rapid loss of PIP(2) with t(1/2) < 40 s. ADP also stimulated PIP(2) formation, but this effect reflected local ATP formation and was prevented by the adenylate kinase inhibitor diadenosine-pentaphosphate. The ATP-induced PIP(2) synthesis was counteracted by the ADP analog adenosine-5'-O-2-thiodiphosphate. We conclude that plasma membrane PIP(2) is dynamically regulated by intracellular Ca(2+) and the ATP-to-ADP ratio in insulin-secreting cells. The rapid turnover allows maintenance of PIP(2) levels while generating second messengers of critical importance for insulin secretion.

Glucose-stimulated Cdc42 signaling is essential for the second phase of insulin secretion

The small Rho family GTPases Cdc42 and Rac1 have each been shown to function in insulin exocytosis and are presumed to function in actin remodeling and insulin granule mobilization. However, whether either GTPase is required for the mobilization phase of insulin release (second phase) and are linked in a common signaling pathway has remained unknown. Here we demonstrate that small interfering RNA-mediated depletion of Cdc42 from isolated islets results in the selective loss of second phase insulin release. Consistent with a role in this nutrient-dependent phase, Cdc42 activation was detected exclusively in response to D-glucose and was unresponsive to KCl or non-metabolizable glucose analogs in MIN6 beta-cells. Cdc42 activation occurred early in secretion (3 min), whereas Rac1 activation required approximately 15-20 min, suggesting Cdc42 as proximal and Rac1 as distal regulators of second-phase secretion. Importantly, Rac1 activation and function was linked in a common pathway downstream of Cdc42; Cdc42 depletion ablated glucose-induced Rac1 activation, and expression of constitutively active Rac1 in Cdc42-depleted cells functionally restored glucose-stimulated insulin secretion. Occurring at a time midway between Cdc42 and Rac1 activations was the phosphorylation of p21-activated-kinase 1 (Pak1), and this phosphorylation event required Cdc42. Moreover, small interfering RNA-mediated Pak1 depletion abolished Rac1 activation and glucose-stimulated insulin release, suggesting that Pak1 may mediate the link between Cdc42 and Rac1 in this pathway. Taken together, these data substantiate the existence of a novel signaling pathway in the islet beta-cell whereby Cdc42 functions as a key proximal transmitter of the glucose signal early in stimulus-secretion coupling to support the later stage of insulin release.

Phospholipase D signaling: orchestration by PIP2 and small GTPases

Hydrolysis of phosphatidylcholine by phospholipase D (PLD) leads to the generation of the versatile lipid second messenger, phosphatidic acid (PA), which is involved in fundamental cellular processes, including membrane trafficking, actin cytoskeleton remodeling, cell proliferation and cell survival. PLD activity can be dramatically stimulated by a large number of cell surface receptors and is elaborately regulated by intracellular factors, including protein kinase C isoforms, small GTPases of the ARF, Rho and Ras families and, particularly, by the phosphoinositide, phosphatidylinositol 4,5-bisphosphate (PIP(2)). PIP(2) is well known as substrate for the generation of second messengers by phospholipase C, but is now also understood to recruit and/or activate a variety of actin regulatory proteins, ion channels and other signaling proteins, including PLD, by direct interaction. The synthesis of PIP(2) by phosphoinositide 5-kinase (PIP5K) isoforms is tightly regulated by small GTPases and, interestingly, by PA as well, and the concerted formation of PIP(2) and PA has been shown to mediate receptor-regulated cellular events. This review highlights the regulation of PLD by membrane receptors, and describes how the close encounter of PLD and PIP5K isoforms with small GTPases permits the execution of specific cellular functions.

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.

Dynamic phospholipid signaling by G protein-coupled receptors

G protein-coupled receptors (GPCRs) control a variety of fundamental cellular processes by regulating phospholipid signaling pathways. Essential for signaling by a large number of receptors is the hydrolysis of the membrane phosphoinositide PIP(2) by phospholipase C (PLC) into the second messengers IP(3) and DAG. Many receptors also stimulate phospholipase D (PLD), leading to the generation of the versatile lipid, phosphatidic acid. Particular PLC and PLD isoforms take differential positions in receptor signaling and are additionally regulated by small GTPases of the Ras, Rho and ARF families. It is now recognized that the PLC substrate, PIP(2), has signaling capacity by itself and can, by direct interaction, affect the activity and subcellular localization of PLD and several other proteins. As expected, the synthesis of PIP(2) by phosphoinositide 5-kinases is tightly regulated as well. In this review, we present an overview of how these signaling pathways are governed by GPCRs, explain the molecular basis for the spatially and temporally organized, highly dynamic quality of phospholipid signaling, and point to the functional connection of the pathways.

Supervised membrane swimming: small G-protein lifeguards regulate PIPK signalling and monitor intracellular PtdIns(4,5)P2 pools

Regulation of PIPK (phosphatidylinositol phosphate kinase) and PtdIns(4,5)P2 signalling by small G-proteins and their effectors is key to many biological functions. Through selective recruitment and activation of different PIPK isoforms, small G-proteins such as Rho, Rac and Cdc42 modulate actin dynamics and cytoskeleton-dependent cellular events in response to extracellular signalling. These activities affect a number of processes, including endocytosis, bacterial penetration into host cells and cytolytic granule-mediated targeted cell killing. Small G-proteins and their modulators are also regulated by phosphoinositides through translocation and conformational changes. Arf family small G-proteins act at multiple sites as regulators of membrane trafficking and actin cytoskeletal remodelling, and regulate a feedback loop comprising phospholipase D, phosphatidic acid, PIPKs and PtdIns(4,5)P2, contributing to enhancement of PtdIns(4,5)P2-mediated cellular events and receptor signalling. Na+, Kir (inwardly rectifying K+), Ca2+ and TRP (transient receptor potential) ion channels are regulated by small G-proteins and membrane pools of PtdIns(4,5)P2. Yeast phosphatidylinositol 4-phosphate 5-kinases Mss4 and Its3 are involved in resistance against disturbance of sphingolipid biosynthesis and maintenance of cell integrity through the synthesis of PtdIns(4,5)P2 and downstream signalling through the Rom2/Rho2 and Rgf1/Rho pathways. Here, we review models for regulated intracellular targeting of PIPKs by small G-proteins and other modulators in response to extracellular signalling. We also describe the spatial and temporal cross-regulation of PIPKs and small G-proteins that is critical for a number of cellular functions.

GEP100/BRAG2: activator of ADP-ribosylation factor 6 for regulation of cell adhesion and actin cytoskeleton via E-cadherin and alpha-catenin

GEP(100) (p100) was identified as an ADP-ribosylation factor (ARF) guanine nucleotide-exchange protein (GEP) that partially colocalized with ARF6 in the cell periphery. p100 preferentially accelerated guanosine 5[gamma-thio]triphosphate (GTPgammaS) binding by ARF6, which participates in protein trafficking near the plasma membrane, including receptor recycling, cell adhesion, and cell migration. Here we report that yeast two-hybrid screening of a human fetal brain cDNA library using p100 as bait revealed specific interaction with alpha-catenin, which is known as a regulator of adherens junctions and actin cytoskeleton remodeling. Interaction of p100 with alpha-catenin was confirmed by coimmunoprecipitation of the endogenous proteins from human HepG2 or CaSki cells, although colocalization was difficult to demonstrate microscopically. alpha-Catenin enhanced GTPgammaS binding by ARF6 in vitro in the presence of p100. Depletion of p100 by small interfering RNA (siRNA) treatment in HepG2 cells resulted in E-cadherin content 3-fold that in control cells and blocked hepatocyte growth factor-induced redistribution of E-cadherin, consistent with a known role of ARF6 in this process. F-actin was markedly decreased in normal rat kidney (NRK) cells overexpressing wild-type p100, but not its GEP-inactive mutants, also consistent with the conclusion that p100 has an important role in the activation of ARF6 for its functions in both E-cadherin recycling and actin remodeling.

Elimination of plasma membrane phosphatidylinositol (4,5)-bisphosphate is required for exocytosis from mast cells

The inositol lipid phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P2] is involved in a myriad of cellular processes, including the regulation of exocytosis and endocytosis. In this paper, we address the role of PtdIns(4,5)P2 in compound exocytosis from rat peritoneal mast cells. This process involves granule-plasma membrane fusion as well as homotypic granule membrane fusion and occurs without any immediate compensatory endocytosis. Using a novel quantitative immunofluorescence technique, we report that plasma membrane PtdIns(4,5)P2 becomes transiently depleted upon activation of exocytosis, and is not detected on the membranes of fusing granules. Depletion is caused by phospholipase C activity, and is mandatory for exocytosis. Although phospholipase C is required for Ca2+ release from internal stores, the majority of the requirement for PtdIns(4,5)P2 hydrolysis occurs downstream of Ca2+ signalling - as shown in permeabilised cells, where the inositol (1,4,5)-trisphosphate-Ca2+ pathway is bypassed. Neither generation of the PtdIns(4,5)P2 metabolite, diacylglycerol (DAG) or simple removal and/or sequestration of PtdIns(4,5)P2 are sufficient for exocytosis to occur. However, treatment of permeabilised cells with DAG induces a small potentiation of exocytosis, indicating that it may be required. We propose that a cycle of PtdIns(4,5)P2 synthesis and breakdown is crucial for exocytosis to occur in mast cells, and may have a more general role in all professional secretory cells.

Endothelin-1 inhibits adiponectin secretion through a phosphatidylinositol 4,5-bisphosphate/actin-dependent mechanism

Adiponectin is an adipokine with profound insulin-sensitizing, anti-inflammatory, and anti-atherogenic properties. Plasma levels of adiponectin are reduced in insulin resistant states such as obesity, type 2 diabetes and cardiovascular disease. However, the mechanism(s) by which adiponectin concentrations are decreased during disease development is unclear. Studies have shown that endothelin-1 (ET-1), a vasoconstrictor peptide, affects adipocyte glucose metabolism and secretion of adipokines such as leptin, resistin, and adiponectin. The goal of our study was to determine the mechanism by which ET-1 decreases adiponectin secretion. 3T3-L1 adipocytes were treated for 24h with ET-1 (10nM) and then stimulated with vehicle or insulin (100 nM) for a period of 1-2h. Chronic ET-1 (24h) treatment significantly decreased basal and insulin-stimulated adiponectin secretion by 66% and 47%, respectively. Inhibition of phosphatidylinositol 4,5-bisphosphate (PIP(2)) hydrolysis by the PLCbeta inhibitor, U73122, or exogenous addition of PIP(2):histone carrier complex (1.25:0.625 microM) ameliorated the decrease in basal and insulin-stimulated adiponectin secretion observed with ET-1. However, treatment with exogenous PIP(2):histone carrier complex and the actin depolymerizing agent latrunculin B (20 microM) did not reverse the ET-1-mediated decrease in adiponectin secretion. In conclusion, we demonstrate that ET-1 inhibits basal and insulin-stimulated adiponectin secretion through PIP(2) modulation of the actin cytoskeleton.

ADP-ribosylation factor 6 regulation of phosphatidylinositol-4,5-bisphosphate synthesis, endocytosis, and exocytosis

Unlike other members of the ADP-ribosylation factor (ARF) family, Arf6 is localized to the plasma membrane and endosomes, and regulates membrane traffic from and into the plasma membrane. Arf6 regulates a clathrin-independent endocytic membrane recycling pathway in nonpolarized cells and clathrin-dependent endocytosis in polarized cells. It also regulates recycling endosome traffic back to the plasma membrane as well as dense-core vesicle exocytosis in neuroendocrine cells. A key effector for Arf6 is phosphatidylinositol 4-monophosphate 5-kinase, which catalyzes plasma membrane synthesis of phosphatidylinositol-4,5-bisphosphate (PIP2), a common required cofactor for several endocytic and exocytic membrane trafficking pathways. Long-term expression of a constitutively active Arf6 mutant in cells can lead to the depletion of PIP2 from the plasma membrane, its accumulation in intracellular vacuoles, and the inhibition of PIP2-dependent membrane trafficking at the plasma membrane.

Role of Phosphoinositide Signaling in the Control of Insulin Exocytosis

Phosphoinositides (PI) are important signaling molecules involved in the regulation of vesicular trafficking. We found that phosphatidylinositol 4-phosphate (PI4P) and phosphatidylinositol 4,5-biphosphate [PI(4,5)P(2)] increase the secretory response triggered by 10 mum Ca(2+) in streptolysin-O-permeabilized insulin-secreting INS-1E cells. In addition, nutrient-induced exocytosis was diminished in intact cells expressing constructs that sequester PI(4,5)P(2) and in cells transfected with constructs that reduce by RNA interference the level of two enzymes involved in PI(4,5)P(2) production, type III PI4-kinase beta and type I phosphatidylinositol 4-bisphosphate 5-kinase-gamma. To clarify the mechanism of action of PI, we investigated the involvement in the regulation of insulin exocytosis of three potential PI targets, phospholipase D1, the Ca(2+)-dependent activator protein for secretion 1, and Munc18-interacting protein 1. Transfection of insulin-secreting cells with plasmids that direct the synthesis of small interfering RNAs capable of reducing the endogenous levels of these proteins inhibited hormone release elicited by glucose- and cAMP-elevating agents without affecting basal release. Our data indicate that the production of PI(4,5)P(2) is necessary for proper control of beta-cell secretion and suggest that at least part of the effect of PI on insulin exocytosis could be exerted through the activation of phospholipase D1, Ca(2+)-dependent activator protein for secretion 1, and Munc18-interacting protein 1.

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.

An effector domain mutant of Arf6 implicates phospholipase D in endosomal membrane recycling

In this study, we investigated the role of phospholipase D (PLD) in mediating Arf6 function in cells. Expression of Arf6 mutants that are defective in activating PLD, Arf6N48R and Arf6N48I, inhibited membrane recycling to the plasma membrane (PM), resulting in an accumulation of tubular endosomal membranes. Additionally, unlike wild-type Arf6, neither Arf6 mutant could generate protrusions or recruit the Arf6 GTPase activating protein (GAP) ACAP1 onto the endosome in the presence of aluminum fluoride. Remarkably, all of these phenotypes, including accumulated tubular endosomes, blocked recycling, and failure to make protrusions and recruit ACAP effectively, could be recreated in either untransfected cells or cells expressing wild-type Arf6 by treatment with 1-butanol to inhibit the formation of phosphatidic acid (PA), the product of PLD. Moreover, most of the defects present in cells expressing Arf6N48R or N48I could be reversed by treatment with agents expected to elevate PA levels in cells. Together, these observations provide compelling evidence that Arf6 stimulation of PLD is required for endosomal membrane recycling and GAP recruitment.

Membrane phosphoinositides control insulin secretion through their effects on ATP-sensitive K+ channel activity

ATP-sensitive K(+) channels (K(ATP) channels) of pancreatic beta-cells play key roles in glucose-stimulated insulin secretion by linking metabolic signals to cell excitability. Membrane phosphoinositides, in particular phosphatidylinositol 4,5-bisphosphates (PIP(2)), stimulate K(ATP) channels and decrease channel sensitivity to ATP inhibition; as such, they have been postulated as critical regulators of K(ATP) channels and hence of insulin secretion in beta-cells. Here, we tested this hypothesis by manipulating the interactions between K(ATP) channels and membrane phospholipids in a beta-cell line, INS-1, and assessing how the manipulations affect membrane excitability and insulin secretion. We demonstrate that disruption of channel interactions with PIP(2) by overexpressing PIP(2)-insensitive channel subunits leads to membrane depolarization and elevated basal level insulin secretion at low glucose concentrations. By contrast, facilitation of channel interactions with PIP(2) by upregulating PIP(2) levels via overexpression of a lipid kinase, phosphatidylinositol 4-phosphate 5 kinase, decreases the ATP sensitivity of endogenous K(ATP) channels by approximately 26-fold and renders INS-1 cells hyperpolarized, unable to secrete insulin properly in the face of high glucose. Our results establish an important role of the interaction between membrane phosphoinositides and K(ATP) channels in regulating insulin secretion.

The p38 mitogen-activated protein kinase pathway negatively regulates Ca2+-activated K+ channel trafficking in developing parasympathetic neurons

The trafficking of large-conductance Ca2+-activated K+ channels (K(Ca)) in chick ciliary ganglion neurons is regulated by growth factors. Here we show that a canonical p38 cascade inhibits K(Ca) trafficking in ciliary ganglion neurons. Two different p38 inhibitors (SB202190 or SB203580) or over-expression of dominant-negative forms of several components of the p38 cascade increased K(Ca) in ciliary neurons. Inhibition of protein synthesis or Golgi processing had no effect on this phenomenon, suggesting that p38 is acting at a distal step of the trafficking pathway. Depolymerization of filamentous actin (F-actin) increased functional expression of K(Ca), whereas stabilization of F-actin inhibited the effect of SB202190 on K(Ca) trafficking. SB202190 also caused an immunochemically detectable increase in K(Ca) on the plasma membrane. Inhibition of p38 decreased the extent of cortical F-actin in ciliary neurons. Macroscopic K(Ca) is suppressed by transforming growth factor (TGF) beta3. Application of TGFbeta3 increased the phosphorylation of p38 in ciliary neurons and increased cortical F-actin. Thus, the p38 signaling cascade endogenously suppresses development of functional K(Ca), in part by stabilizing an F-actin barrier that prevents plasma membrane insertion of functional channel complexes. This cascade also appears to mediate inhibitory effects of TGFbeta3 on the expression of K(Ca).

Localization and function of Arf family GTPases

Arfs are a family of Ras-related GTP-binding proteins that function in the regulation of membrane trafficking and structure. The six mammalian Arf proteins are expressed ubiquitously and so it is anticipated that each will have a distinct localization and function within the cell. It has been assumed that much of this specificity will be defined by determining which regulators of Arfs, the GEFs (guanine nucleotide-exchange factors) and GAPs (GTPase-activating proteins) function with which Arf proteins. Although in vitro assays may indicate Arf preferences for the numerous Arf GEFs and GAPs that have been identified, in the cell the different Arfs, GEFs and GAPs are targeted to specific compartments where they carry out their functions. We have embarked on studies to define regions of the Arf1 and Arf6 proteins that determine their sites of action and specific activities at the Golgi and plasma membrane respectively. Chimaeras were made between Arf1 and Arf6 in order to identify regions of the protein that contributed to targeting and function. Whereas Arf6 is targeted to the plasma membrane through multiple regions along the protein, we have found a Golgi-targeting region in Arf1 that is sufficient to target Arf6 to the Golgi complex.

Phosphatidylinositol phosphate kinase type I gamma regulates dynamics of large dense-core vesicle fusion

Phosphatidylinositol-4,5-bisphosphate was proposed to be an important regulator of large dense-core vesicle exocytosis from neuroendocrine tissues. Here, we have examined the kinetics of secretion in chromaffin cells from mice lacking phosphatidylinositol phosphate kinase type I gamma, the major neuronal phosphatidylinositol-4-phosphate 5-kinase. Absence of this enzyme caused a reduction of the readily releasable vesicle pool and its refilling rate, with a small increase in morphologically docked vesicles, indicating a defect in vesicle priming. Furthermore, amperometry revealed a delay in fusion pore expansion. These results provide direct genetic evidence for a key role of phosphatidylinositol-4,5-bisphosphate synthesis in the regulation of large dense-core vesicle fusion dynamics.

Insulin-stimulated plasma membrane fusion of Glut4 glucose transporter-containing vesicles is regulated by phospholipase D1

Insulin stimulates glucose uptake in fat and muscle by mobilizing Glut4 glucose transporters from intracellular membrane storage sites to the plasma membrane. This process requires the trafficking of Glut4-containing vesicles toward the cell periphery, docking at exocytic sites, and plasma membrane fusion. We show here that phospholipase D (PLD) production of the lipid phosphatidic acid (PA) is a key event in the fusion process. PLD1 is found on Glut4-containing vesicles, is activated by insulin signaling, and traffics with Glut4 to exocytic sites. Increasing PLD1 activity facilitates glucose uptake, whereas decreasing PLD1 activity is inhibitory. Diminished PA production does not substantially hinder trafficking of the vesicles or their docking at the plasma membrane, but it does impede fusion-mediated extracellular exposure of the transporter. The fusion block caused by RNA interference-mediated PLD1 deficiency is rescued by exogenous provision of a lipid that promotes fusion pore formation and expansion, suggesting that the step regulated by PA is late in the process of vesicle fusion.

Coupling actin and membrane dynamics during calcium-regulated exocytosis: a role for Rho and ARF GTPases

Release of neurotransmitters and hormones occurs by calcium-regulated exocytosis, a process that shares many similarities in neurons and neuroendocrine cells. Exocytosis is confined to specific regions in the plasma membrane, where actin remodelling, lipid modifications and protein-protein interactions take place to mediate vesicle/granule docking, priming and fusion. The spatial and temporal coordination of the various players to form a "fast and furious" machinery for secretion remain poorly understood. ARF and Rho GTPases play a central role in coupling actin dynamics to membrane trafficking events in eukaryotic cells. Here, we review the role of Rho and ARF GTPases in supplying actin and lipid structures required for synaptic vesicle and secretory granule exocytosis. Their possible functional interplay may provide the molecular cues for efficient and localized exocytotic fusion.

Regulation and cellular roles of phosphoinositide 5-kinases

The membrane phospholipid, phosphatidylinositol 4,5-bisphosphate (PIP(2)), plays a critical role in various, apparently very different cellular processes. As precursor for second messengers generated by phospholipase C isoforms and class I phosphoinositide 3-kinases, PIP(2) is indispensable for cellular signaling by membrane receptors. In addition, PIP(2) directly affects the localization and activity of many cellular proteins via specific interaction with unique phosphoinositide-binding domains and thereby regulates actin cytoskeletal dynamics, vesicle trafficking, ion channel activity, gene expression and cell survival. The activity and subcellular localization of phosphatidylinositol 4-phosphate 5-kinase (PIP5K) isoforms, which catalyze the formation of PIP(2), are actively regulated by membrane receptors, by phosphorylation and by small GTPases of the Rho and ARF families. Spatially and temporally organized regulation of PIP(2) synthesis by PIP5K enables dynamic and versatile PIP(2) signaling and represents an important link in the execution of cellular tasks by Rho and ARF GTPases.

Neomycin Prevents the Wortmannin Inhibition of Insulin-stimulated Glut4 Translocation and Glucose Transport in 3T3-L1 Adipocytes

Insulin stimulates the movement of the facilitative glucose transporter glucose transporter-4 (Glut4) from an intracellular compartment to the plasma membrane in adipocytes and muscle cells, resulting in an increased rate of glucose uptake. Insulin-stimulated Glut4 translocation and glucose transport are abolished by wortmannin, a specific inhibitor of phosphatidylinositol 3'-kinase (PI3K). Here, we demonstrate that neomycin, a drug that masks the cellular substrate of PI3K, phosphatidylinositol 4,5-bisphosphate (PIP), prevents wortmannin inhibition of insulin-stimulated (2)Glut4 translocation and glucose transport without activating protein kinase B, a downstream effector of PI3K. These results suggest that PIP(2) may have an important regulatory function in insulin-stimulated Glut4 translocation and glucose transport.

Phosphatidylinositol 4,5-bisphosphate regulates adipocyte actin dynamics and GLUT4 vesicle recycling

To investigate the potential role of phosphatidylinositol 4, 5-bisphosphate (PI(4,5)P2) in the regulation of actin polymerization and GLUT4 translocation, the type I phosphatidylinositol 4-phosphate 5-kinases (PIP5Ks) were expressed in 3T3L1 adipocytes. In preadipocytes (fibroblasts) PIP5K expression promoted actin polymerization on membrane-bound vesicles to form motile actin comets. In contrast, expression of PIP5K in differentiated 3T3L1 adipocytes resulted in the formation of enlarged vacuole-like structures coated with F-actin, cortactin, dynamin, and N-WASP. Treatment with either latrunculin B (an inhibitor for actin polymerization) or Clostridium difficile toxin B (a general Rho family inhibitor) resulted in a relatively slower disappearance of coated F-actin from these vacuoles, but the vacuoles themselves remained unaffected. Functionally, the increased PI(4,5)P2 levels resulted in an inhibition of transferrin receptor and GLUT4 endocytosis and a slow accumulation of these proteins in the PI(4,5)P2-enriched vacuoles along with the non-clathrin-derived endosome marker (caveolin) and the AP-2 adaptor complex. However, these structures were devoid of early endosome markers (EEA1, clathrin) and the biosynthetic membrane secretory machinery markers p115 (Golgi) and syntaxin 6 (trans-Golgi Network). Taken together, these data demonstrate that PI(4,5)P2 has distinct morphologic and functional properties depending upon specific cell context. In adipocytes, altered PI(4,5)P2 metabolism has marked effects on GLUT4 endocytosis and intracellular vesicle trafficking due to the derangement of actin dynamics.