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

Alpha4 contributes to the dysfunction of the pancreatic beta cell under metabolic stress

The current study examined the roles of Alpha4, a non-canonical subunit of protein phosphatase 2A, in the regulation of acute (insulin secretion) and chronic (cell dysfunction) effects of glucose in pancreatic beta cells. Alpha4 is expressed in human islets, rat islets and INS-1832/13 cells. Incubation of INS-1832/13 cells and rat islets with high glucose (HG) significantly increased the expression of Alpha4. C2-Ceramide, a biologically active sphingolipid, also increased the expression of Alpha4 in INS-1832/13 cells and rat islets. Subcellular distribution studies of Alpha4 in low glucose (LG) and HG exposed INS-1832/13 cells revealed that it is predominantly cytosolic, and its expression is significantly increased in the non-nuclear/cytosolic fractions in cells exposed to HG. siRNA-mediated knockdown of Alpha4 exerted minimal effects on glucose- or KCl-induced insulin secretion. siRNA-mediated deletion of Alpha4 significantly increased p38MAPK and JNK1/2 phosphorylation under LG conditions, comparable to the degree seen under HG conditions. Paradoxically, a significant potentiation of HG-induced p38MAPK and JNK2 phosphorylation was noted following Alpha4 deletion. HG-induced CHOP expression (ER stress marker) and caspase-3 activation were markedly attenuated in cells following Alpha4 knockdown. Deletion of Alpha4 in INS-1832/13 cells prevented HG-induced loss in the expression of Connexin36, a gap junction channel protein, which has been implicated in normal beta cell function. Lastly, depletion of endogenous Alpha4 significantly reduced HG-induced cell death in INS-1832/13 cells. Based on these findings we conclude that Alpha4 contributes to HG-induced metabolic dysfunction of the islet beta cell.

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

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

Emerging Roles of Small GTPases in Islet β-Cell Function

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

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

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

Specific deletion of CDC42 in pancreatic β cells attenuates glucose-induced insulin expression and secretion in mice

Insulin is a key hormone for maintaining glucose homeostasis in organisms. In general, deficiency of insulin synthesis and secretion results in type I diabetes, whereas insulin resistance leads to type 2 diabetes. Cell division cycle 42 (CDC42), a member of Rho GTPases family, has been shown as an essential regulator in the second phase of glucose-induced insulin secretion in pancreatic islets β cells in vitro. However, the effect of CDC42 on insulin expression has not been explored. Here we reported that the glucose-induced insulin expression and secretion were significantly inhibited in mice lacking CDC42 gene in pancreatic β cells (Rip-CDC42cKO) in vivo and in vitro. Deletion of CDC42 gene in pancreatic β cells did not affect survival or reproduction in mice. However, the Rip-CDC42cKO mice showed the systemic glucose intolerance and the decrease of glucose-induced insulin secretion without apparent alterations of peripheral tissues insulin sensitivity and the morphology of islets. Furthermore, we demonstrated that deletion of CDC42 gene in pancreatic β cells significantly attenuated the insulin expression through inhibiting the ERK1/2-NeuroD1 signaling pathway. Taken together, our study presents novel evidence that CDC42 is an important modulator in glucose-induced insulin expression as well as insulin secretion in pancreatic β cells.

The adaptor protein APPL2 controls glucose-stimulated insulin secretion via F-actin remodeling in pancreatic β-cells

Filamentous actin (F-actin) cytoskeletal remodeling is critical for glucose-stimulated insulin secretion (GSIS) in pancreatic β-cells, and its dysregulation causes type 2 diabetes. The adaptor protein APPL1 promotes first-phase GSIS by up-regulating soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein expression. However, whether APPL2 (a close homology of APPL1 with the same domain organization) plays a role in β-cell functions is unknown. Here, we show that APPL2 enhances GSIS by promoting F-actin remodeling via the small GTPase Rac1 in pancreatic β-cells. β-cell specific abrogation of APPL2 impaired GSIS, leading to glucose intolerance in mice. APPL2 deficiency largely abolished glucose-induced first- and second-phase insulin secretion in pancreatic islets. Real-time live-cell imaging and phalloidin staining revealed that APPL2 deficiency abolished glucose-induced F-actin depolymerization in pancreatic islets. Likewise, knockdown of APPL2 expression impaired glucose-stimulated F-actin depolymerization and subsequent insulin secretion in INS-1E cells, which were attributable to the impairment of Ras-related C3 botulinum toxin substrate 1 (Rac1) activation. Treatment with the F-actin depolymerization chemical compounds or overexpression of gelsolin (a F-actin remodeling protein) rescued APPL2 deficiency-induced defective GSIS. In addition, APPL2 interacted with Rac GTPase activating protein 1 (RacGAP1) in a glucose-dependent manner via the bin/amphiphysin/rvs-pleckstrin homology (BAR-PH) domain of APPL2 in INS-1E cells and HEK293 cells. Concomitant knockdown of RacGAP1 expression reverted APPL2 deficiency-induced defective GSIS, F-actin remodeling, and Rac1 activation in INS-1E cells. Our data indicate that APPL2 interacts with RacGAP1 and suppresses its negative action on Rac1 activity and F-actin depolymerization thereby enhancing GSIS in pancreatic β-cells.

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

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

Epigenetic alteration of Rho guanine nucleotide exchange Factor 11 (ARHGEF11) in cord blood samples in macrosomia exposed to intrauterine hyperglycemia

Background: Macrosomia at birth is associated with maternal hyperglycemia and leads to subsequent susceptibility to obesity, abnormal glucose metabolism, hypertension, and dyslipidemia in offspring. Epigenetic reprogramming has been reported to be involved in the development of human diseases caused by suboptimal environmental or nutritional factors. The study was aiming to explore epigenetic mechanism influences on macrosomic infants exposed to intrauterine hyperglycemia. We performed a genome-wide analysis of DNA methylation in cord blood from macrosomic infants born to women with gestational diabetes in order to identify genes related to fetal growth or early adipose tissue development.Methods: To analyze the epigenetic patterns in umbilical cord blood in gestational diabetes mellitus (GDM), we collected umbilical cord blood from women with GDM (mean pregestational BMI of 24.4 kg/m² and mean neonatal birth weight of 4366 g) and normal glucose-tolerant women (mean pregestational BMI of 19.8 kg/m² and mean neonatal birth weight of 3166 g). Differentially methylated genes in the GDM group were identified using the Infinium HumanMethylation450 BeadChip array.Results: A total of 1251 genes were differentially methylated compared to the controls (p < .01). The methylation microarray data showed that two specific CpG sites (cg12604331 and cg08480098) in the gene body of ARHGEF11 were significantly hypomethylated in the cord blood in macrosomic infants. Altered DNA methylation levels of ARHGEF11 were negatively correlated with glucose levels and neonatal birth weight.Conclusions: Exposure to adverse intrauterine environments can alter fetal development, such as by affecting the nutritional status of the fetus. Such exposure can also result in significant epigenetic modifications, including DNA methylation, which could serve as a potential marker for nutrition and metabolic conditions at the neonatal stage or even in the adult.

SAD-A Promotes Glucose-Stimulated Insulin Secretion Through Phosphorylation and Inhibition of GDIα in Male Islet β Cells

Rho GDP-dissociation inhibitor (GDIα) inhibits glucose-stimulated insulin secretion (GSIS) in part by locking Rho GTPases in an inactive GDP-bound form. The onset of GSIS causes phosphorylation of GDIα at Ser174, a critical inhibitory site for GDIα, leading to the release of Rho GTPases and their subsequent activation. However, the kinase regulator(s) that catalyzes the phosphorylation of GDIα in islet β cells remains elusive. We propose that SAD-A, a member of AMP-activated protein kinase-related kinases that promotes GSIS as an effector kinase for incretin signaling, interacts with and inhibits GDIα through phosphorylation of Ser174 during the onset GSIS from islet β cells. Coimmunoprecipitation and phosphorylation analyses were carried out to identify the physical interaction and phosphorylation site of GDIα by SAD-A in the context of GSIS from INS-1 β cells and primary islets. We identified GDIα directly binds to SAD-A kinase domain and phosphorylated by SAD-A on Ser174, leading to dissociation of Rho GTPases from GDIα complexes. Accordingly, overexpression of SAD-A significantly stimulated GDIα phosphorylation at Ser174 in response to GSIS, which is dramatically potentiated by glucagonlike peptide-1, an incretin hormone. Conversely, SAD-A deficiency, which is mediated by short hairpin RNA transfection in INS-1 cells, significantly attenuated endogenous GDIα phosphorylation at Ser174. Consequently, coexpression of SAD-A completely prevented the inhibitory effect of GDIα on insulin secretion in islets. In summary, glucose and incretin stimulate insulin secretion through the phosphorylation of GDIα at Ser174 by SAD-A, which leads to the activation of Rho GTPases, culminating in insulin exocytosis.

Calcineurin is an important factor involved in glucose uptake in human adipocytes

Calcineurin inhibitors are used in immunosuppressive therapy applied after transplantation, but they are associated with major metabolic side effects including the development of new onset diabetes. Previously, we have shown that the calcineurin inhibiting drugs tacrolimus and cyclosporin A reduce adipocyte and myocyte glucose uptakes by reducing the amount of glucose transporter type 4 (GLUT4) at the cell surface, due to an increased internalization rate. However, this happens without alteration in total protein and phosphorylation levels of key proteins involved in insulin signalling or in the total amount of GLUT4. The present study evaluates possible pathways involved in the altered internalization of GLUT4 and consequent reduction of glucose uptake provoked by calcineurin inhibitors in human subcutaneous adipose tissue. Short- and long-term treatments with tacrolimus, cyclosporin A or another CNI deltamethrin (herbicide) decreased basal and insulin-dependent glucose uptake in adipocytes, without any additive effects observed when added together. However, no tacrolimus effects were observed on glucose uptake when gene transcription and protein translation were inhibited. Investigation of genes potentially involved in GLUT4 trafficking showed only a small effect on ARHGEF11 gene expression (p < 0.05). In conlusion, the specific inhibition of calcineurin, but not that of protein phosphatases, decreases glucose uptake in human subcutaneous adipocytes, suggesting that calcineurin is an important regulator of glucose transport. This inhibitory effect is mediated via gene transcription or protein translation; however, expression of genes potentially involved in GLUT4 trafficking and endocytosis appears not to be involved in these effects.

Liver Proteome in Diabetes Type 1 Rat Model: Insulin-Dependent and -Independent Changes

Diabetes mellitus type 1 (DM1) is a major public health problem that continues to burden the healthcare systems worldwide, costing exponentially more as the epidemic grows. Innovative strategies and omics system diagnostics for earlier diagnosis or prognostication of DM1 are essential to prevent secondary complications and alleviate the associated economic burden. In a preclinical study design that involved streptozotocin (STZ)-induced DM1, insulin-treated STZ-induced DM1, and control rats, we characterized the insulin-dependent and -independent changes in protein profiles in liver samples. Digested proteins were subjected to LC-MS^E for proteomic data. Progenesis QI data processing and analysis of variance were utilized for statistical analyses. We found 305 proteins with significantly altered abundance among the control, DM1, and insulin-treated DM1 groups (p < 0.05). These differentially regulated proteins were related to enzymes that function in key metabolic pathways and stress responses. For example, gluconeogenesis appeared to return to control levels in the DM1 group after insulin treatment, with the restoration of gluconeogenesis regulatory enzyme, FBP1. Insulin administration to DM1 rats also restored the blood glucose levels and enzymes of general stress and antioxidant response systems. These observations are crucial for insights on DM1 pathophysiology and new molecular targets for future clinical biomarkers, drug discovery, and development. Additionally, we underscore that proteomics offers much potential in preclinical biomarker discovery for diabetes as well as common complex diseases such as cancer, dementia, and infectious disorders.

Fine tuning of insulin secretion by release of nerve growth factor from mouse and human islet β-cells

Nerve growth factor (NGF) is a protein required for neuronal development that also has regulatory functions in non-neuronal cells. Both NGF and its membrane receptors trkA and p75(NTR) are expressed by islet β-cells. In this study we dynamically profiled NGF secretion from islets and used selective trkA and p75(NTR) inhibitors to identify the role of endogenous NGF in β-cell stimulus-secretion coupling. NGF secretion from mouse islets was transient and did not accompany the sustained second phase of glucose-induced insulin secretion. Despite being present in human islets, NGF was not released at sufficient levels to be quantified. Inhibition of NGF signaling through trkA and p75(NTR) increased basal insulin secretion from both human and mouse islets, but impaired glucose-stimulated insulin secretion. These data support a role for islet NGF in fine-tuning insulin secretion, to both maintain a low basal level of insulin output and contribute to the biphasic secretory response to glucose.

Acute metabolic amplification of insulin secretion in mouse islets: Role of cytosolic acetyl-CoA

OBJECTIVE

Stimulation of the ß-cell metabolism by glucose and other fuels triggers insulin release by enhancing the mitochondrial ATP production and acutely amplifies the secretory response by increase in mitochondrial export of metabolites. We aimed to narrow down the uniform final reaction steps mediating fuel-induced acute amplification of insulin secretion.

MATERIAL/METHODS

Insulin secretion and metabolic parameters were measured in isolated mouse islets exposed to the sulfonylurea glipizide in high concentration (closing all ATP-sensitive K(+) channels) during the entire experiment. Fuel-induced effects were examined after treating the islets for one hour with medium devoid of fuels. This experimental design prevented acute amplification, but only when glucose was the sole fuel.

RESULTS

Strong amplification of insulin secretion by α-ketoisocaproate or glucose combined with α-ketoisovalerate (supplying mitochondrial oxaloacetate) was abolished within 14min after transition to medium devoid of fuels. After transition from medium containing glucose plus α-ketoisovalerate to medium containing solely glucose or α-ketoisovalerate, amplification (strong or weak, respectively) occurred until the end of the experiment. Glucose (alone or combined with α-ketoisovalerate) increased the total acetyl-CoA content as intensely as α-ketoisocaproate. Low concentrations of α-ketoisovalerate or α-ketoisocaproate were sufficient for saturation of acetyl-CoA increase, but caused no or only weak amplification, respectively. No acetyl-CoA increases occurred in the absence of glipizide.

CONCLUSIONS

Glucose and other fuels regulate acute amplification of insulin secretion by controlling the supply of acetyl-CoA to the ß-cell cytosol. Cytosolic acetyl-CoA does not amplify by serving as substrate for syntheses of metabolic intermediates, but amplifies by acting as substrate for cytosolic protein acetylation.

VAV2, a guanine nucleotide exchange factor for Rac1, regulates glucose-stimulated insulin secretion in pancreatic beta cells

AIMS/HYPOTHESIS

Rho GTPases (Ras-related C3 botulinum toxin substrate 1 [Rac1] and cell division cycle 42 [Cdc42]) have been shown to regulate glucose-stimulated insulin secretion (GSIS) via cytoskeletal remodelling, trafficking and fusion of insulin-secretory granules with the plasma membrane. GTP loading of these G proteins, which is facilitated by GDP/GTP exchange factors, is a requisite step in the regulation of downstream effector proteins. Guanine nucleotide exchange factor VAV2 (VAV2), a member of the Dbl family of proteins, has been identified as one of the GDP/GTP exchange factors for Rac1. Despite recent evidence on the regulatory roles of VAV2 in different cell types, roles of this guanine nucleotide exchange factor in the signalling events leading to GSIS remain undefined. Using immunological, short interfering RNA (siRNA), pharmacological and microscopic approaches we investigated the role of VAV2 in GSIS from islet beta cells.

METHODS

Co-localisation of Rac1 and VAV2 was determined by Triton X-114 phase partition and confocal microscopy. Glucose-induced actin remodelling was quantified by live cell imaging using the LifeAct-GFP fluorescent biosensor. Rac1 activation was determined by G protein linked immunosorbent assay (G-LISA).

RESULTS

Western blotting indicated that VAV2 is expressed in INS-1 832/13 beta cells, normal rat islets and human islets. Vav2 siRNA markedly attenuated GSIS in INS-1 832/13 cells. Ehop-016, a newly discovered small molecule inhibitor of the VAV2-Rac1 interaction, or siRNA-mediated knockdown of VAV2 markedly attenuated glucose-induced Rac1 activation and GSIS in INS-1 832/13 cells. Pharmacological findings were recapitulated in primary rat islets. A high glucose concentration promoted co-localisation of Rac1 and VAV2. Real-time imaging in live cells indicated a significant inhibition of glucose-induced cortical actin remodelling by Ehop-016.

CONCLUSIONS/INTERPRETATION

Our data provide the first evidence to implicate VAV2 in glucose-induced Rac1 activation, actin remodelling and GSIS in pancreatic beta cells.

Tiam1-Rac1 Axis Promotes Activation of p38 MAP Kinase in the Development of Diabetic Retinopathy: Evidence for a Requisite Role for Protein Palmitoylation

Background/Aims

Evidence in multiple tissues, including retina, suggests generation of reactive oxygen species (ROS) and the ensuing oxidative stress as triggers for mitochondrial defects and cell apoptosis. We recently reported novel roles for Tiam1-Rac1-Nox2 axis in retinal mitochondrial dysfunction and cell death leading to the development of diabetic retinopathy. Herein, we tested the hypothesis that activation of p38 MAP kinase, a stress kinase, represents the downstream signaling event to Rac1-Nox2 activation in diabetes-induced metabolic stress leading to capillary cell apoptosis.

Methods

Activation of p38 MAP kinase was quantified by Western blotting in retinal endothelial cells incubated with high glucose (20 mM) for up to 96 hours, a duration where mitochondrial dysfunction and capillary cell apoptosis can be observed. NSC23766 and 2-bromopalmitate (2-BP) were used to assess the roles of Tiam1-Rac1 and palmitoylation pathways, respectively.

Results

Activation of p38 MAP kinase was observed as early as 3 hours after high glucose exposure, and continued until 96 hours. Consistent with this, p38 MAP kinase activation was significantly higher in the retina from diabetic mice compared to age-matched normal mice. NSC23766 markedly attenuated hyperglycemia-induced activation of p38 MAP kinase. Lastly, 2-BP inhibited glucose-induced Rac1, Nox2 and p38 MAP kinase activation in endothelial cells.

Conclusions

Tiam1-Rac1-mediated activation of Nox2 and p38 MAP kinase constitutes early signaling events leading to mitochondrial dysfunction and the development of diabetic retinopathy. Our findings also provide the first evidence to implicate novel roles for protein palmitoylation in this signaling cascade.

EHT 1864, a small molecule inhibitor of Ras-related C3 botulinum toxin substrate 1 (Rac1), attenuates glucose-stimulated insulin secretion in pancreatic β-cells

Glucose-stimulated insulin secretion (GSIS) in the pancreatic β-cells entails a variety of signaling mechanisms including activation of small GTP-binding proteins (G-proteins). Previous studies from our laboratory in human islets, rodent islets and clonal β-cells have demonstrated that G-proteins (e.g., Arf6, Cdc42 and Rac1) play novel roles in cytoskeletal remodeling, which is a critical step in the trafficking of insulin-laden secretory granules for fusion with plasma membrane and release of insulin. To further understand regulatory roles of Rac1 in GSIS, we utilized, herein, EHT 1864, a small molecule inhibitor, which attenuates Rac1 activation by retaining the G-protein in an inert/inactive state, thereby preventing activation of its downstream effector proteins. We demonstrate that EHT 1864 markedly attenuated GSIS in INS-1 832/13 cells. In addition, EHT 1864 significantly reduced glucose-induced activation and membrane targeting of Rac1 in INS-1 832/13 cells. This Rac1 inhibitor also suppressed glucose-induced activation of ERK1/2 and p53, but not Akt. Lastly, unlike the inhibitors of protein prenylation (simvastatin), EHT 1864 did not exert any significant effects on cell morphology (cell rounding) under the conditions it attenuated Rac1-sensitive signaling steps leading to GSIS. Based on these findings, we conclude that EHT 1864 specifically inhibits glucose-induced Rac1 activation and membrane association and associated downstream signaling events culminating in inhibition of GSIS.

Human Rho guanine nucleotide exchange factor 11 gene is associated with schizophrenia in a Japanese population

OBJECTIVE

The human Rho guanine nucleotide exchange factor 11 (ARHGEF11) gene is one of the candidate genes for type 2 diabetes mellitus (T2DM). ARHGEF11 is mapped to chromosome 1q21, which has susceptible risk loci for T2DM and schizophrenia. We hypothesized that ARHGEF11 contributes to the pathogenesis of schizophrenia.

METHOD

We selected eight single nucleotide polymorphisms of ARHGEF11 that had significant associations with T2DM for a case-control association study of 490 patients with schizophrenia and 500 age-matched and sex-matched controls.

RESULTS

We did not find any differences in allelic, genotypic associations, or minor allele frequencies with schizophrenia. Analysis of the rs6427340-rs6427339 haplotype and the rs822585-rs6427340-rs6427339 haplotype combination provided significant evidence of an association with schizophrenia (global permutations p = 0.00047 and 0.0032, respectively). C-C of the rs6427340-rs6427339 haplotype and A-C-C of the rs822585-rs6427340-rs6427339 haplotype carried higher risk factors for schizophrenia (permutation p = 0.0010 and 0.0018, respectively). A-C-T of the rs822585-rs6427340-rs6427339 haplotype had a possible protective effect (permutation p = 0.031).

CONCLUSION

These results provide new evidence that ARHGEF11 may constitute a risk factor for schizophrenia.

TIAM1-RAC1 signalling axis-mediated activation of NADPH oxidase-2 initiates mitochondrial damage in the development of diabetic retinopathy

AIMS/HYPOTHESIS

In diabetes, increased retinal oxidative stress is seen before the mitochondria are damaged. Phagocyte-like NADPH oxidase-2 (NOX2) is the predominant cytosolic source of reactive oxygen species (ROS). Activation of Ras-related C3 botulinum toxin substrate 1 (RAC1), a NOX2 holoenzyme member, is necessary for NOX2 activation and ROS generation. In this study we assessed the role of T cell lymphoma invasion and metastasis (TIAM1), a guanine nucleotide exchange factor for RAC1, in RAC1 and NOX2 activation and the onset of mitochondrial dysfunction in in vitro and in vivo models of glucotoxicity and diabetes.

METHODS

RAC1 and NOX2 activation, ROS generation, mitochondrial damage and cell apoptosis were quantified in bovine retinal endothelial cells exposed to high glucose concentrations, in the retina from normal and streptozotocin-induced diabetic rats and mice, and the retina from human donors with diabetic retinopathy.

RESULTS

High glucose activated RAC1 and NOX2 (expression and activity) and increased ROS in endothelial cells before increasing mitochondrial ROS and mitochondrial DNA (mtDNA) damage. N6-[2-[[4-(diethylamino)-1-methylbutyl]amino]-6-methyl-4-pyrimidinyl]-2-methyl-4,6-quinolinediamine, trihydrochloride (NSC23766), a known inhibitor of TIAM1-RAC1, markedly attenuated RAC1 activation, total and mitochondrial ROS, mtDNA damage and cell apoptosis. An increase in NOX2 expression and membrane association of RAC1 and p47(phox) were also seen in diabetic rat retina. Administration of NSC23766 to diabetic mice attenuated retinal RAC1 activation and ROS generation. RAC1 activation and p47(phox) expression were also increased in the retinal microvasculature from human donors with diabetic retinopathy.

CONCLUSIONS/INTERPRETATION

The TIAM1-RAC1-NOX2 signalling axis is activated in the initial stages of diabetes to increase intracellular ROS leading to mitochondrial damage and accelerated capillary cell apoptosis. Strategies targeting TIAM1-RAC1 signalling could have the potential to halt the progression of diabetic retinopathy in the early stages of the disease.

Local mineralocorticoid receptor activation and the role of Rac1 in obesity-related diabetic kidney disease

BACKGROUND/AIMS

Obesity and diabetes are intimately interrelated, and are independent risk factors for kidney disease. Overactivation of mineralocorticoid receptor (MR) is implicated in end organ damage of both pathologies. But the underlying mechanism of MR activation in kidney remains uncertain. We explored the involvement of Rac1, which we previously identified as a ligand-independent MR activator, in renal MR activation in vitro and in vivo.

METHODS

We evaluated the MR activity and Rac1 activity under high-glucose stimulation using luciferase reporter system and glutathione S-transferase pull-down assay in cultured mesangial cells. To elucidate the role of Rac1 in vivo, we employed KKA(y), a mouse model of obesity-related type 2 diabetes, which spontaneously developed massive albuminuria and distinct glomerular lesions accompanied by increased plasma aldosterone concentration.

RESULTS

High-glucose stimulation increased Rac1 activity and MR transcriptional activity in cultured mesangial cells. Overexpression of constitutively active Rac1 activated MR, and glucose-induced MR activation was suppressed by overexpression of dominant negative Rac1 or Rac inhibitor EHT1864. In KKA(y), renal Rac1 was activated, and nuclear MR was increased. EHT1864 treatment suppressed renal Rac1 and MR activity and mitigated renal pathology of KKA(y) without changing plasma aldosterone concentration.

CONCLUSION

Our results suggest that MR activation plays an important role in the nephropathy of KKA(y) mice, and that glucose-induced Rac1 activation, in addition to hyperaldosteronemia, contributes to their renal MR activation. Along with MR blockade, Rac inhibition may potentially be a preferred option in the treatment of nephropathy in obesity-related diabetic patients.

High glucose exposure promotes activation of protein phosphatase 2A in rodent islets and INS-1 832/13 β-cells by increasing the posttranslational carboxylmethylation of its catalytic subunit

Existing evidence implicates regulatory roles for protein phosphatase 2A (PP2A) in a variety of cellular functions, including cytoskeletal remodeling, hormone secretion, and apoptosis. We report here activation of PP2A in normal rat islets and insulin-secreting INS-1 832/13 cells under the duress of hyperglycemic (HG) conditions. Small interfering RNA-mediated knockdown of the catalytic subunit of PP2A (PP2Ac) markedly attenuated glucose-induced activation of PP2A. HG, but not nonmetabolizable 3-O-methyl glucose or mannitol (osmotic control), significantly stimulated the methylation of PP2Ac at its C-terminal Leu-309, suggesting a novel role for this posttranslational modification in glucose-induced activation of PP2A. Moreover, knockdown of the cytosolic leucine carboxymethyl transferase 1 (LCMT1), which carboxymethylates PP2Ac, significantly attenuated PP2A activation under HG conditions. In addition, HG conditions, but not 3-O-methyl glucose or mannitol, markedly increased the expression of LCMT1. Furthermore, HG conditions significantly increased the expression of B55α, a regulatory subunit of PP2A, which has been implicated in islet dysfunction under conditions of oxidative stress and diabetes. Thapsigargin, a known inducer of endoplasmic reticulum stress, failed to exert any discernible effects on the carboxymethylation of PP2Ac, expression of LCMT1 and B55α, or PP2A activity, suggesting no clear role for endoplasmic reticulum stress in HG-induced activation of PP2A. Based on these findings, we conclude that exposure of the islet β-cell to HG leads to accelerated PP2A signaling pathway, leading to loss in glucose-induced insulin secretion.

Nm23-H1 regulates glucose-stimulated insulin secretion in pancreatic β-cells via Arf6-Rac1 signaling axis

BACKGROUND

A growing body of evidence implicates novel roles for nm23-like proteins in the regulation of cellular functions. However, roles of these proteins in islet function and glucose-stimulated insulin secretion (GSIS) remain largely unknown.

METHODS

siRNA-nm23-H1 and nucleoside diphosphate kinase and histidine kinase-deficient mutants of nm23-H1 (K12Q and H118F) were used to assess roles of nm23-H1 in GSIS.

RESULTS

siRNA-mediated knockdown of the expression of nm23-H1 markedly inhibited GSIS in INS-1 832/13 cells. Nm23-H1 knockdown also resulted in significant inhibition of glucose-mediated activation of Arf6, a small G-protein, which has been implicated in GSIS. Expression of K12Q and H118F mutants of nm23-H1 in INS-1 832/13 cells led to inhibition of glucose-induced translocation and membrane association of Rac1, another small G-protein, which is downstream to Arf6 in the signaling events leading to GSIS. A significant inhibition of GSIS was also seen in these cells expressing K12Q and H118F.

CONCLUSIONS

We conclude that the nm23-H1 activation step is upstream of Arf6 activation in signaling events leading to GSIS. NDP kinase and histidine kinase functions of nm23-H1 are necessary for glucose-induced membrane association of Rac1 and ensuing insulin secretion. We present the first evidence for regulation of GSIS by nm23-H1 in pancreatic β-cells.

Signaling mechanisms of glucose-induced F-actin remodeling in pancreatic islet β cells

The maintenance of whole-body glucose homeostasis is critical for survival, and is controlled by the coordination of multiple organs and endocrine systems. Pancreatic islet β cells secrete insulin in response to nutrient stimuli, and insulin then travels through the circulation promoting glucose uptake into insulin-responsive tissues such as liver, skeletal muscle and adipose. Many of the genes identified in human genome-wide association studies of diabetic individuals are directly associated with β cell survival and function, giving credence to the idea that β-cell dysfunction is central to the development of type 2 diabetes. As such, investigations into the mechanisms by which β cells sense glucose and secrete insulin in a regulated manner are a major focus of current diabetes research. In particular, recent discoveries of the detailed role and requirements for reorganization/remodeling of filamentous actin (F-actin) in the regulation of insulin release from the β cell have appeared at the forefront of islet function research, having lapsed in prior years due to technical limitations. Recent advances in live-cell imaging and specialized reagents have revealed localized F-actin remodeling to be a requisite for the normal biphasic pattern of nutrient-stimulated insulin secretion. This review will provide an historical look at the emergent focus on the role of the actin cytoskeleton and its regulation of insulin secretion, leading up to the cutting-edge research in progress in the field today.

Actin dynamics regulated by the balance of neuronal Wiskott-Aldrich syndrome protein (N-WASP) and cofilin activities determines the biphasic response of glucose-induced insulin secretion

Actin dynamics in pancreatic β-cells is involved in insulin secretion. However, the molecular mechanisms of the regulation of actin dynamics by intracellular signals in pancreatic β-cells and its role in phasic insulin secretion are largely unknown. In this study, we elucidate the regulation of actin dynamics by neuronal Wiskott-Aldrich syndrome protein (N-WASP) and cofilin in pancreatic β-cells and demonstrate its role in glucose-induced insulin secretion (GIIS). N-WASP, which promotes actin polymerization through activation of the actin nucleation factor Arp2/3 complex, was found to be activated by glucose stimulation in insulin-secreting clonal pancreatic β-cells (MIN6-K8 β-cells). Introduction of a dominant-negative mutant of N-WASP, which lacks G-actin and Arp2/3 complex-binding region VCA, into MIN6-K8 β-cells or knockdown of N-WASP suppressed GIIS, especially the second phase. We also found that cofilin, which severs F-actin in its dephosphorylated (active) form, is converted to the phosphorylated (inactive) form by glucose stimulation in MIN6-K8 β-cells, thereby promoting F-actin remodeling. In addition, the dominant-negative mutant of cofilin, which inhibits activation of endogenous cofilin, or knockdown of cofilin reduced the second phase of GIIS. However, the first phase of GIIS occurs in the G-actin predominant state, in which cofilin activity predominates over N-WASP activity. Thus, actin dynamics regulated by the balance of N-WASP and cofilin activities determines the biphasic response of GIIS.

Phagocytic NADPH oxidase links ARNO-Arf6 signaling pathway in glucose-stimulated insulin secretion from the pancreatic β-cell

BACKGROUND

Recent findings from our laboratory have demonstrated that glucose-stimulated insulin secretion (GSIS) involves interplay between a variety of small G proteins belonging to the Rho (e.g., Cdc42 and Rac1) and ADP-ribosylation factor (e.g., Arf6) subfamilies. Using immunological, pharmacological and molecular biological approaches, we have also identified guanine nucleotide exchange factors (GEFs) for Rac1 (e.g., Tiam1) and Arf6 (e.g., ARNO) in clonal INS-1 832/13 cells, normal rat islets and human islets. As a logical extension to these studies, we investigated, herein, potential downstream signaling steps involved in Arf6/ARNO-mediated GSIS.

METHODS

Using a selective pharmacological inhibitor of ARNO/Arf6 signaling axis (e.g., secinH3) we assessed regulatory roles for Arf6/ARNO in promoting phospholipase D (PLD), phagocytic NADPH oxidase (Nox2), reactive oxygen species (ROS), extracellular-regulated kinases (ERK 1/2) and cofilin (actin-severing protein] signaling steps in clonal INS-1 832/13 cells.

RESULTS

Our data suggested a marked inhibition by secinH3 of glucose-induced PLD activation, ERK1/2 phosphorylation and dephosphorylation of cofilin, suggesting that Arf6/ ARNO signaling mediates PLD, ERK1/2 and cofilin activation in beta-cells. In addition, secinH3 blocked glucose-induced Nox2 activation and associated ROS generation, thus placing Nox downstream to Arf6/ARNO signaling step. Lastly, we also demonstrate a significantly higher cofilin phosphorylation (inactive) in islets derived from type 2 diabetic human donors as well as the Zucker Diabetic Fatty (ZDF) rat, a model for type 2 diabetes.

CONCLUSION

Together, our current findings identify signaling steps downstream to ARNO/Arf6 axis leading to insulin secretion.

Cellular function of RhoGDI-α mediates the cycling of Rac1 and the regulation of pancreatic beta cell death

Mycophenolic acid (MPA) is an immunosuppressive agent that is widely used in clinical therapy, including pancreas and islet transplantation. Previously, we showed that MPA induces significant apoptosis of insulin-secreting cells by downregulating RhoGDI-α and increasing JNK expression. In this study, we investigated Rac1 directly associated with RhoGDI-α during MPA-induced apoptosis in INS-1E cells (an insulin-secreting cell line). Cells were treated with MPA for 24 and 36 hours. Immunoprecipitation was used to examine physical interactions between RhoGDI-α and Rac1. Activation and immunoprecipitation assays showed expressions of Rac1 and RhoGDI-α to be directly correlated. Rac1 binding to RhoGDI-α decreased after MPA treatment, and Rac1 was induced and subsequently activated by MPA. We concluded that this novel RhoGDI-α/Rac1/JNK pathway induced apoptosis of transplanted islet cells after MPA treatment.

Synapses of amphids defective (SAD-A) kinase promotes glucose-stimulated insulin secretion through activation of p21-activated kinase (PAK1) in pancreatic β-Cells

The p21-activated kinase-1 (PAK1) is implicated in regulation of insulin exocytosis as an effector of Rho GTPases. PAK1 is activated by the onset of glucose-stimulated insulin secretion (GSIS) through phosphorylation of Thr-423, a major activation site by Cdc42 and Rac1. However, the kinase(s) that phosphorylates PAK1 at Thr-423 in islet β-cells remains elusive. The present studies identified SAD-A (synapses of amphids defective), a member of AMP-activated protein kinase-related kinases exclusively expressed in brain and pancreas, as a key regulator of GSIS through activation of PAK1. We show that SAD-A directly binds to PAK1 through its kinase domain. The interaction is mediated by the p21-binding domain (PBD) of PAK1 and requires both kinases in an active conformation. The binding leads to direct phosphorylation of PAK1 at Thr-423 by SAD-A, triggering the onset of GSIS from islet β-cells. Consequently, ablation of PAK1 kinase activity or depletion of PAK1 expression completely abolishes the potentiating effect of SAD-A on GSIS. Consistent with its role in regulating GSIS, overexpression of SAD-A in MIN6 islet β-cells significantly stimulated cytoskeletal remodeling, which is required for insulin exocytosis. Together, the present studies identified a critical role of SAD-A in the activation of PAK1 during the onset of insulin exocytosis.

4-O-carboxymethyl ascochlorin causes ER stress and induced autophagy in human hepatocellular carcinoma cells

The synthetic derivative of ascochlorin, 4-O-carboxymethyl ascochlorin (AS-6) is an agonist of the nuclear hormone receptor PPARγ and has been shown to induce differentiation in mouse pre-adipocytes and to ameliorate type II diabetes in a murine model. AS-6 was cytotoxic when added at micromolar concentrations to cultures of three different human cancer cell lines. We used gel electrophoresis and mass spectrometry to identify proteins with altered expression in human hepatocarcinoma cells (HepG2) cells after 12 h in the presence of AS-6 and found 58 proteins that were differentially expressed. Many of the proteins showing increased expression in cells treated with AS-6 are involved in protein quality control, including glucose-regulated protein 78 (GRP78/BiP), a regulator of ER stress responses, and the transcriptional regulator CHOP, which mediates ER stress-induced apoptosis. Cells treated with AS-6 undergo an autophagic response accompanied by increased expression of beclin1, ATG5, and LC3-II and autophagosome formation marked by the appearance of large vesicles containing LC3-II. Grp78 induction was inhibited when the PPARγ antagonist, GW9662, was added together with AS-6, and autophagy and cell death were partially blocked. 3-methyl-adenine (3-MA), an inhibitor of phosphatidyl inositol 3-kinase (PI3-kinase) prevented induction of ATG5 and activation of LC3-II and blocked autophagosome formation. 3-MA also blocked induction of GRP78 and CHOP, suggesting that PI3-kinase, which is known to mediate ER stress-induced autophagy, also plays a role in initiating apoptosis in response to ER stress. Together these data establish that the cytotoxicity of AS-6 operates by a mechanism dependent on ER stress-induced autophagy and apoptosis.

Increased phagocyte-like NADPH oxidase and ROS generation in type 2 diabetic ZDF rat and human islets: role of Rac1-JNK1/2 signaling pathway in mitochondrial dysregulation in the diabetic islet

OBJECTIVE

To determine the subunit expression and functional activation of phagocyte-like NADPH oxidase (Nox), reactive oxygen species (ROS) generation and caspase-3 activation in the Zucker diabetic fatty (ZDF) rat and diabetic human islets.

RESEARCH DESIGN AND METHODS

Expression of core components of Nox was quantitated by Western blotting and densitometry. ROS levels were quantitated by the 2',7'-dichlorofluorescein diacetate method. Rac1 activation was quantitated using the gold-labeled immunosorbent assay kit.

RESULTS

Levels of phosphorylated p47(phox), active Rac1, Nox activity, ROS generation, Jun NH(2)-terminal kinase (JNK) 1/2 phosphorylation, and caspase-3 activity were significantly higher in the ZDF islets than the lean control rat islets. Chronic exposure of INS 832/13 cells to glucolipotoxic conditions resulted in increased JNK1/2 phosphorylation and caspase-3 activity; such effects were largely reversed by SP600125, a selective inhibitor of JNK. Incubation of normal human islets with high glucose also increased the activation of Rac1 and Nox. Lastly, in a manner akin to the ZDF diabetic rat islets, Rac1 expression, JNK1/2, and caspase-3 activation were also significantly increased in diabetic human islets.

CONCLUSIONS

We provide the first in vitro and in vivo evidence in support of an accelerated Rac1-Nox-ROS-JNK1/2 signaling pathway in the islet β-cell leading to the onset of mitochondrial dysregulation in diabetes.

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

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

Arf nucleotide binding site opener [ARNO] promotes sequential activation of Arf6, Cdc42 and Rac1 and insulin secretion in INS 832/13 β-cells and rat islets

Glucose-stimulated insulin secretion [GSIS] involves interplay between small G-proteins and their regulatory factors. Herein, we tested the hypothesis that Arf nucleotide binding site opener [ARNO], a guanine nucleotide-exchange factor [GEF] for the small G-protein Arf6, mediates the functional activation of Arf6, and that ARNO/Arf6 signaling axis, in turn, controls the activation of Cdc42 and Rac1, which have been implicated in GSIS. Molecular biological [i.e., expression of inactive mutants or siRNA] and pharmacological approaches were employed to assess the roles for ARNO/Arf6 signaling pathway in insulin secretion in normal rat islets and INS 832/13 cells. Degrees of activation of Arf6 and Cdc42/Rac1 were quantitated by GST-GGA3 and PAK-1 kinase pull-down assays, respectively. ARNO is expressed in INS 832/13 cells, rat islets and human islets. Expression of inactive mutants of Arf6 [Arf6-T27N] or ARNO [ARNO-E156K] or siRNA-ARNO markedly reduced GSIS in isolated β-cells. SecinH3, a selective inhibitor of ARNO/Arf6 signaling axis, also inhibited GSIS in INS 832/13 cells and rat islets. Stimulatory concentrations of glucose promoted Arf6 activation, which was inhibited by secinH3 or siRNA-ARNO, suggesting that ARNO/Arf6 signaling cascade is necessary for GSIS. SecinH3 or siRNA-ARNO also inhibited glucose-induced activation of Cdc42 and Rac1 suggesting that ARNO/Arf6 might be upstream to Cdc42 and Rac1 activation steps, which are necessary for GSIS. Lastly, co-immunoprecipitation and confocal microscopic studies suggested increased association between Arf6 and ARNO in glucose-stimulated β-cells. These findings provide the first evidence to implicate ARNO in the sequential activation of Arf6, Cdc42 and Rac1 culminating in GSIS.

R1467H Variants of Rho Guanine Nucleotide Exchange Factor 11 (ARHGEF11) are Associated with Type 2 Diabetes Mellitus in Koreans

Background

The human Rho guanine nucleotide exchange factor 11 (ARHGEF11) functions as an activator of Rho GTPases and is thought to influence insulin signaling. The R1467H variant of ARHGEF11 has been reported to be associated with susceptibility to type 2 diabetes mellitus (T2DM) in Western populations.

Methods

We investigated the effects of the R1467H variant on susceptibility to T2DM as well as related traits in a Korean population. We genotyped the R1467H (rs945508) of ARHGEF11 in 689 unrelated T2DM patients and 249 non-diabetic individuals and compared the clinical and biochemical characteristics according to different alleles.

Results

The H allele was significantly more frequent in T2DM cases than in controls (P = 0.037, 17.1% and 13.1%; respectively). H homozygocity was associated with a higher risk of T2DM compared to those with R/R or R/H genotype (odds ratio, 5.24; 95% confidence interval, 1.06 to 25.83; P = 0.042). The fasting plasma glucose, HbA1c, fasting insulin, HOMA2-IR and HOMA2-%β levels did not differ significantly between different genotypes.

Conclusion

Our study replicated associations of the ARHGEF11 polymorphism with increased risk of T2DM in a Korean population and thus supports previous data implicating a potential role of ARHGEF11 in the etiology of T2DM. Further studies revealing the underlying mechanism for this association are needed.

Regulation of glucose- and mitochondrial fuel-induced insulin secretion by a cytosolic protein histidine phosphatase in pancreatic beta-cells

We report localization of a cytosolic protein histidine phosphatase (PHP; approximately 16 kDa) in INS 832/13 cells, normal rat islets, and human islets. siRNA-mediated knockdown of PHP markedly reduced glucose- or mitochondrial fuel-induced but not KCl-induced insulin secretion. siRNA-mediated knockdown of PHP also attenuated mastoparan-induced insulin secretion, suggesting its participation in G protein-sensitive signaling steps, leading to insulin secretion. Functional assays revealed that the beta-cell PHP catalyzes the dephosphorylation of ATP-citrate lyase (ACL). Silencing of PHP expression markedly reduced ACL activity, suggesting functional regulation of ACL by PHP in beta-cells. Coimmunoprecipitation studies revealed modest effects of glucose on the interaction between PHP and ACL. Confocal microscopic evidence indicated that glucose promotes association between ACL and nm23-H1, a known kinase histidine kinase, but not between PHP and ACL. Furthermore, metabolic viability of INS 832/13 cells was resistant to siRNA-PHP, suggesting no regulatory roles of PHP in cell viability. Finally, long-term exposure (24 h) of INS 832/13 cells or rat islets to high glucose (30 mM) increased the expression of PHP. Such increases in PHP expression were also seen in islets derived from the Zucker diabetic fatty rat compared with islets from the lean control animals. Together, these data implicate regulatory roles for PHP in a G protein-sensitive step involved in nutrient-induced insulin secretion. In light of the current debate on putative regulatory roles of ACL in insulin secretion, additional studies are needed to precisely identify the phosphoprotein substrate(s) for PHP in the cascade of events leading to nutrient-induced insulin secretion.

Analysis of candidate genes on chromosome 20q12-13.1 reveals evidence for BMI mediated association of PREX1 with type 2 diabetes in European Americans

Chromosome 20q12-q13.1 has been linked to type 2 diabetes (T2D) in multiple populations. We examined the influence of genes in this region on T2D and BMI in two European American case-control populations. SNPs were genotyped in 300 diabetic patients and 310 controls. A subset of 72 SNPs were further genotyped in 470 cases and 442 controls. All genes examined showed evidence of association with T2D in the initial sample (additive P-value [P(a)]=0.00090-0.045). SNPs near PREX1 were also associated in the second case-control population (P(a)=0.017-0.042). The combined analysis resulted in the same SNPs, among others, associated with T2D (P(a)=0.0013-0.041). Stratification analysis by T2D status showed that association with BMI was observed solely in cases (P(a)=0.0018-0.041). Mediation testing revealed that 30-40% of the effects of these SNPs on T2D were significantly mediated by BMI. SNPs near PREX1 may contribute to T2D susceptibility mediated through effects of adiposity in European Americans.

Protein farnesylation-dependent Raf/extracellular signal-related kinase signaling links to cytoskeletal remodeling to facilitate glucose-induced insulin secretion in pancreatic beta-cells

OBJECTIVE

Posttranslational prenylation (e.g., farnesylation) of small G-proteins is felt to be requisite for cytoskeletal remodeling and fusion of secretory vesicles with the plasma membrane. Here, we investigated roles of protein farnesylation in the signaling steps involved in Raf-1/extracellular signal-related kinase (ERK1/2) signaling pathway in glucose-induced Rac1 activation and insulin secretion in the pancreatic beta-cell.

RESEARCH DESIGN AND METHODS

These studies were carried out in INS 832/13 cells and normal rat islets. Molecular biological (e.g., overexpression or small interfering RNA [siRNA]-mediated knockdown) and pharmacologic approaches were used to determine roles for farnesylation in glucose-mediated activation of ERK1/2, Rac1, and insulin secretion. Activation of ERK1/2 was determined by Western blotting. Rac1 activation (i.e., Rac1.GTP) was quantitated by p21-activated kinase pull-down assay. Insulin release was quantitated by enzyme-linked immunosorbent assay.

RESULTS

Coprovision of structure-specific inhibitors of farnesyl transferase (FTase; e.g., FTI-277 or FTI-2628) or siRNA-mediated knockdown of FTase beta-subunit resulted in a significant inhibition of glucose-stimulated ERK1/2 and Rac1 activation and insulin secretion. Pharmacologic inhibition of Raf-1 kinase using GW-5074 markedly reduced the stimulatory effects of glucose on ERK1/2 phosphorylation, Rac1 activation, and insulin secretion, suggesting that Raf-1 kinase activation may be upstream to ERK1/2 and Rac1 activation leading to glucose-induced insulin release. Lastly, siRNA-mediated silencing of endogenous expression of ERK1/2 markedly attenuated glucose-induced Rac1 activation and insulin secretion.

CONCLUSIONS

Together, our findings provide the first evidence of a role for protein farnesylation in glucose-mediated regulation of the Raf/ERK signaling pathway culminating in the activation of Rac1, which has been shown to be necessary for cytoskeletal reorganization and exocytotic secretion of insulin.

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.

Differential phosphorylation of RhoGDI mediates the distinct cycling of Cdc42 and Rac1 to regulate second-phase insulin secretion

Cdc42 cycling through GTP/GDP states is critical for its function in the second/granule mobilization phase of insulin granule exocytosis in pancreatic islet beta cells, although the identities of the Cdc42 cycling proteins involved remain incomplete. Using a tandem affinity purification-based mass spectrometry screen for Cdc42 cycling factors in beta cells, RhoGDI was identified. RNA interference-mediated depletion of RhoGDI from isolated islets selectively amplified the second phase of insulin release, consistent with the role of RhoGDI as a Cdc42 cycling factor. Replenishment of RhoGDI to RNA interference-depleted cells normalized secretion, confirming the action of RhoGDI to be that of a negative regulator of Cdc42 activation. Given that RhoGDI also regulates Rac1 activation in beta cells, and that Rac1 activation occurs in a Cdc42-dependent manner, the question as to how the beta cell utilized RhoGDI for differential Cdc42 and Rac1 cycling was explored. Co-immunoprecipitation was used to determine that RhoGDI-Cdc42 complexes dissociated upon stimulation of beta cells with glucose for 3 min, correlating with the timing of glucose-induced Cdc42 activation and the onset of RhoGDI tyrosine phosphorylation. Glucose-induced disruption of RhoGDI-Rac1 complexes occurred subsequent to this, coincident with Rac1 activation, which followed the onset of RhoGDI serine phosphorylation. RhoGDI-Cdc42 complex dissociation was blocked by mutation of RhoGDI residue Tyr-156, whereas RhoGDI-Rac1 dissociation was blocked by RhoGDI mutations Y156F and S101A/S174A. Finally, expression of a triple Y156F/S101A/S174A-RhoGDI mutant specifically inhibited only the second/granule mobilization phase of glucose-stimulated insulin secretion, overall supporting the integration of RhoGDI into the activation cycling mechanism of glucose-responsive small GTPases.

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

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

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.

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.

Bridging the gap between protein carboxyl methylation and phospholipid methylation to understand glucose-stimulated insulin secretion from the pancreatic beta cell

Recent findings have implicated post-translational modifications at C-terminal cysteines [e.g., methylation] of specific proteins [e.g., G-proteins] in glucose-stimulated insulin secretion [GSIS]. Furthermore, methylation at the C-terminal leucine of the catalytic subunit of protein phosphatase 2A [PP2Ac] has also been shown to be relevant for GSIS. In addition to these two classes of protein methyl transferases, a novel class of glucose-activated phospholipid methyl transferases have also been identified in the beta cell. These enzymes catalyze three successive methylations of phosphatidylethanolamine to yield phosphatidylcholine. The "newly formed" phosphatidylcholine is felt to induce alterations in the membrane fluidity, which might favor vesicular fusion with the plasma membrane for the exocytosis of insulin. The objectives of this commentary are to: (i) review the existing evidence on the regulation, by glucose and other insulin secretagogues, of post-translational carboxylmethylation [CML] of specific proteins in the beta cell; (ii) discuss the experimental evidence, which implicates regulation, by glucose and other insulin secretagogues, of phosphatidylethanolamine methylation in the islet beta cell; (iii) propose a model for potential cross-talk between the protein and lipid methylation pathways in the regulation of GSIS and (iv) highlight potential avenues for future research, including the development of specific pharmacological inhibitors to further decipher regulatory roles for these methylation reactions in islet beta cell function.

EphA-Ephrin-A-mediated beta cell communication regulates insulin secretion from pancreatic islets

In vertebrates, beta cells are aggregated in the form of pancreatic islets. Within these islets, communication between beta cells inhibits basal insulin secretion and enhances glucose-stimulated insulin secretion, thus contributing to glucose homeostasis during fasting and feeding. In the search for the underlying molecular mechanism, we have discovered that beta cells communicate via ephrin-As and EphAs. We provide evidence that ephrin-A5 is required for glucose-stimulated insulin secretion. We further show that EphA-ephrin-A-mediated beta cell communication is bidirectional: EphA forward signaling inhibits insulin secretion, whereas ephrin-A reverse signaling stimulates insulin secretion. EphA forward signaling is downregulated in response to glucose, which indicates that, under basal conditions, beta cells use EphA forward signaling to suppress insulin secretion and that, under stimulatory conditions, they shift to ephrin-A reverse signaling to enhance insulin secretion. Thus, we explain how beta cell communication in pancreatic islets conversely affects basal and glucose-stimulated insulin secretion to improve glucose homeostasis.

Depletion of the catalytic subunit of protein phosphatase-2A (PP2Ac) markedly attenuates glucose-stimulated insulin secretion in pancreatic beta-cells

Among various phosphatases, the protein phosphatase 2A (PP2A) is relatively well studied in the islet. Previously, we have demonstrated that the catalytic subunit of PP2A (PP2Ac) undergoes okadaic acid (OKA)-sensitive, reversible carboxylmethylation (CML), which appears to be requisite for glucose-stimulated insulin secretion (GSIS). Using the siRNA approach, we examined, herein, the contributory roles of PP2Ac in GSIS from insulin-secreting pancreatic beta-(INS-1 832/13) cells. Immunologically, PP2Ac was detectable in all the subcellular fractions studied in rank order of: cytosol > microsomes > secretory granules = nucleus > mitochondria. Transfection of PP2Ac-specific, but not scrambled-siRNA, markedly attenuated PP2A activity and GSIS in these cells. Together, our findings provide a direct evidence for a positive modulatory role for PP2Ac in signaling steps leading to GSIS.

Evidence that Rho guanine nucleotide exchange factor 11 (ARHGEF11) on 1q21 is a type 2 diabetes susceptibility gene in the Old Order Amish

Rho guanine nucleotide exchange factor 11 (ARHGEF11), located on chromosome 1q21, is involved in G protein signaling and is a pathway known to play a role in both insulin secretion and action. We genotyped 52 single nucleotide polymorphims (SNPs) in ARHGEF11 and compared the genotype frequencies of subjects with type 2 diabetes (n = 145) or type 2 diabetes/impaired glucose tolerance (IGT) (n = 293) with those of control subjects with normal glucose tolerance (NGT) (n = 358). Thirty SNPs, spanning the entire gene, were significantly associated with type 2 diabetes or type 2 diabetes/IGT. The most significantly associated SNP was rs6427340 (intron 2), in which the less common allele was the risk allele (odds ratio [OR] 1.82 [95% CI 1.20-2.70], P = 0.005 for type 2 diabetes vs. NGT and 1.79 [1.27-2.50], P = 0.0008 for type 2 diabetes/IGT vs. NGT). In an expanded set of nondiabetic subjects (n = 754), most of the type 2 diabetes-and IGT-associated SNPs were significantly associated with glucose levels during an oral glucose tolerance test, with the same SNP (rs6427340) showing the most significant associations (P = 0.007). All type 2 diabetes-and IGT-associated SNPs were in high linkage disequilibrium and constitute a single 133-kb haplotype block. These results, coupled with similar findings in Pima Indians, suggest that sequence variation in ARHGEF11 may influence risk of type 2 diabetes.

Regulatory roles for nm23/nucleoside diphosphate kinase-like enzymes in insulin secretion from the pancreatic islet beta cell

Recent studies from multiple laboratories, including our own, provided fresh insights into the contributory roles for GTP-binding proteins (G-proteins) in glucose-stimulated insulin secretion (GSIS) from the islet beta cell. However, the precise mechanisms underlying the activation of this class of signaling proteins by insulin secretagogues remain only partially understood. We recently proposed that nm23/nucleoside diphosphate kinase (NDPK) catalyzes an alternate, non-receptor-dependent activation of islet endogenous G-proteins. In further support of this proposal, we report, herein, that overexpression of wild type (WT) nm23-H1 mutant in INS cells markedly potentiated GSIS. However, an inactive mutant of nm23-H1(H118F), which is deficient in histidine kinase and NDPK activities, was considerably less effective in potentiating GSIS from these cells, suggesting that both of these activities may be relevant for the potentiating effects of nm23-H1. Potential significance of these findings in relation to contributory roles for nm23/NDPK-like enzymes in the stimulus-secretion coupling of GSIS is discussed.

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.

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.