Modulation of insulin secretion from normal rat islets by inhibitors of the post-translational modifications of GTP-binding proteins

Sesamol combats diabetogenic effects of atorvastatin through GLUT-4 expression and improved pancreatic viability

Statin-associated diabetes (SAD) is an issue that has come to light after a series of recent clinical trials that has led to the issue of a black box warning for statins by the US FDA. However, the benefit of statin outweighs its risk. Nevertheless, experiments have been conducted to identify the mechanism by which statins aggravate the risk of diabetes only in a select population who bear the risk factors of obesity, sedentary lifestyle, hypertension, and other associated risk factors of lifestyle disorders. In this study, the possibility of utilization of a phyto-molecule, sesamol, for its ability to combat statin-associated diabetes using atorvastatin as the agent of choice has been explored. MMP assay and western blot was conducted to investigate the effects of atorvastatin on apoptotic cascade with sesamol as a protective agent was conducted in MIN-6 cells. Effect of the combination was tested in L6 cells with 2-NBDG uptake assay and as well as western blot for GLUT-4. A diet-induced hypercholesterolemia model was developed in an in vivo model animals and treated with atorvastatin and sesamol with histopathological analysis being carried out to evaluate the apoptotic markers and GLUT-4 presence. It was found that sesamol can combat pancreatic beta cell apoptosis via the internal apoptotic pathway activated by atorvastatin. With regards to muscle cells, sesamol could improve the GLUT-4 vesical production, but not improve glucose uptake which is inhibited by atorvastatin. These findings are further confirmed by animal studies. These findings indicate that sesamol can serve as a prototype molecule for further development and investigation of similar compounds to tackle SAD.

Statins and risk of type 2 diabetes: mechanism and clinical implications

Statins are widely used to prevent cardiovascular disease events. Cardiovascular diseases and type 2 diabetes are tightly connected since type 2 diabetes is a major risk factor for cardiovascular diseases. Additionally, cardiovascular diseases often precede the development of type 2 diabetes. These two diseases have common genetic and environmental antecedents. Statins are effective in the lowering of cardiovascular disease events. However, they have also important side effects, including an increased risk of type 2 diabetes. The first study reporting an association of statin treatment with the risk of type 2 diabetes was the WOSCOPS trial (West of Scotland Coronary Prevention Study) in 2001. Other primary and secondary cardiovascular disease prevention studies as well as population-based studies have confirmed original findings. The purpose of our review is to examine and summarize the most important findings of these studies as well as to describe the mechanisms how statins increase the risk of type 2 diabetes.

Regulation of insulin secretion in mouse islets: metabolic amplification by alpha-ketoisocaproate coincides with rapid and sustained increase in acetyl-CoA content

Glucose and alpha-ketoisocaproate, the keto acid analogue of leucine, stimulate insulin secretion in the absence of other exogenous fuels. Their mitochondrial metabolism in the beta-cell raises the cytosolic ATP/ADP ratio, thereby providing the triggering signal for the exocytosis of the insulin granules. However, additional amplifying signals are required for the full extent of insulin secretion stimulated by these fuels. While it is generally recognized that the amplifying signals are also derived from the mitochondrial metabolism, their exact nature is still unclear. The current study tests the hypothesis that the supply of cytosolic acetyl-CoA is a signal in the amplifying pathway. The contents of acetyl-CoA and acetyl-CoA plus CoA-SH were measured in isolated mouse islets. Insulin secretion was recorded in isolated perifused islets. In islets, the ATP-sensitive K+ channels of which were pharmacologically closed and which were preincubated without exogenous fuel, 10 mmol/L alpha-ketoisocaproate enhanced the acetyl-CoA content after 5 and 20 min incubations and decreased the acetyl-CoA plus CoA-SH within 5 min, but not after 20 min. In islets not exposed to drugs, the preincubation with 3 mmol/L glucose, a non-triggering concentration, elevated the acetyl-CoA content. This content was further increased after 5 min and 20 min incubations with 30 mmol/L glucose, concurrent with a strong increase in insulin secretion. Alpha-ketoisocaproate and glucose increase the supply of acetyl-CoA in the beta-cell cytosol during both phases of insulin secretion. Most likely, this increase provides a signal for the metabolic amplification.

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.

β-Cell-Specific Deletion of HMG-CoA (3-hydroxy-3-methylglutaryl-coenzyme A) Reductase Causes Overt Diabetes due to Reduction of β-Cell Mass and Impaired Insulin Secretion

Inhibitors of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR), statins, which are used to prevent cardiovascular diseases, are associated with a modest increase in the risk of new-onset diabetes. To investigate the role of HMGCR in the development of β-cells and glucose homeostasis, we deleted Hmgcr in a β-cell-specific manner by using the Cre-loxP technique. Mice lacking Hmgcr in β-cells (β-KO) exhibited hypoinsulinemic hyperglycemia as early as postnatal day 9 (P9) due to decreases in both β-cell mass and insulin secretion. Ki67-positive cells were reduced in β-KO mice at P9; thus, β-cell mass reduction was caused by proliferation disorder immediately after birth. The mRNA expression of neurogenin3 (Ngn3), which is transiently expressed in endocrine progenitors of the embryonic pancreas, was maintained despite a striking reduction in the expression of β-cell-associated genes, such as insulin, pancreatic and duodenal homeobox 1 (Pdx1), and MAF BZIP transcription factor A (Mafa) in the islets from β-KO mice. Histological analyses revealed dysmorphic islets with markedly reduced numbers of β-cells, some of which were also positive for glucagon. In conclusion, HMGCR plays critical roles not only in insulin secretion but also in the development of β-cells in mice.

Statin Treatment-Induced Development of Type 2 Diabetes: From Clinical Evidence to Mechanistic Insights

Statins are the gold-standard treatment for the prevention of primary and secondary cardiovascular disease, which is the leading cause of mortality worldwide. Despite the safety and relative tolerability of statins, observational studies, clinical trials and meta-analyses indicate an increased risk of developing new-onset type 2 diabetes mellitus (T2DM) after long-term statin treatment. It has been shown that statins can impair insulin sensitivity and secretion by pancreatic β-cells and increase insulin resistance in peripheral tissues. The mechanisms involved in these processes include, among others, impaired Ca²⁺ signaling in pancreatic β-cells, down-regulation of GLUT-4 in adipocytes and compromised insulin signaling. In addition, it has also been described that statins’ impact on epigenetics may also contribute to statin-induced T2DM via differential expression of microRNAs. This review focuses on the evidence and mechanisms by which statin therapy is associated with the development of T2DM. This review describes the multifactorial combination of effects that most likely contributes to the diabetogenic effects of statins. Clinically, these findings should encourage clinicians to consider diabetes monitoring in patients receiving statin therapy in order to ensure early diagnosis and appropriate management.

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.

Atorvastatin but Not Pravastatin Impairs Mitochondrial Function in Human Pancreatic Islets and Rat β-Cells. Direct Effect of Oxidative Stress

Statins are a class of drugs widely prescribed as frontline therapy for lowering plasma LDL-cholesterol in cardiovascular risk prevention. Several clinical reports have recently suggested an increased risk of type 2 diabetes associated with chronic use of these drugs. The pathophysiology of this effect remains to be fully elucidated but impaired β-cell function constitutes a potential mechanism. The aim of this study was to explore the effect of a chronic treatment with lipophilic and hydrophilic statins on β-cell function, using human pancreatic islets and rat insulin-secreting INS-1 cells; we particularly focused on the role of mitochondria and oxidative stress. The present study demonstrates, for the first time, that atorvastatin (lipophilic) but not pravastatin (hydrophilic) affected insulin release and mitochondrial metabolism due to the suppression of antioxidant defense system and induction of ROS production in pancreatic β-cell models. Mevalonate addition and treatment with a specific antioxidant (N-AcetylCysteine) effectively reversed the observed defects. These data demonstrate that mitochondrial oxidative stress is a key element in the pathogenesis of statin-related diabetes and may have clinical relevance to design strategies for prevention or reduction of statin induced β-cell dysfunction and diabetes in patients treated with lipophilic statins.

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

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

Metabolic Stress Induces Caspase-3 Mediated Degradation and Inactivation of Farnesyl and Geranylgeranyl Transferase Activities in Pancreatic β-Cells

BACKGROUND/AIMS

At least 300 prenylated proteins are identified in the human genome; the majority of which partake in a variety of cellular processes including growth, differentiation, cytoskeletal organization/dynamics and vesicle trafficking. Aberrant prenylation of proteins is implicated in human pathologies including cancer; neurodegenerative diseases, retinitis pigmentosa, and premature ageing syndromes. Original observations from our laboratory have demonstrated that prenylation of proteins [small G-proteins and γ-subunits of trimeric G-proteins] is requisite for physiological insulin secretion. Herein, we assessed the impact of metabolic stress [gluco-, lipotoxicity and ER-stress] on the functional status of protein prenylation pathway in pancreatic β-cells.

METHODS

Farnesyltransferase [FTase] and geranylgeranyltransferase [GGTase] activities were quantified by radioisotopic methods. Caspase-3 activation and FTase/GGTase-α subunit degradation were determined by Western blotting.

RESULTS

We observed that metabolic stress activates caspase-3 and induces degradation of the common α-subunit of FTase and GGTase-I in INS-1 832/13 cells, normal rodent islets and human islets leading to functional defects [inactivation] in FTase and GGTase activities. Caspase-3 activation and FTase/GGTase-α degradation were also seen in islets from the Zucker diabetic fatty [ZDF] rat, a model for Type 2 diabetes. Consequential to defects in FTase/GGTase-α signaling, we observed significant accumulation of unprenylated proteins [Rap1] in β-cells exposed to glucotoxic conditions. These findings were replicated in β-cells following pharmacological inhibition of generation of prenylpyrophosphate substrates [Simvastatin] or catalytic activity of prenylating enzymes [GGTI-2147].

CONCLUSIONS

Our findings provide the first evidence to suggest that metabolic stress induced dysfunction of the islet β-cell may, in part, be due to defective protein prenylation signaling pathway.

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.

Statin use and the risk of developing diabetes: a network meta-analysis

PURPOSE

Randomized controlled trials have shown mixed findings regarding the association of statins and diabetes. This systematic literature review and network meta-analysis (NMA) was performed to update evidence on this association to possibly assist clinicians in making more informed treatment choices.

METHODS

We identified studies relevant to our NMA by performing study searches in databases like Embase, Cochrane, and PubMed, published between August 2010 and June 2014. Pre-2010 studies were identified from bibliography of previously published meta-analyses. Unpublished study data were found from clinicaltrial.gov. Data synthesis was performed by pairwise meta-analysis and NMA within a Frequentist framework.

RESULTS

Twenty nine trials in which 1 63 039 participants had been randomized were included in this review; among these 1 41 863 were non-diabetic patients. The direct meta-analysis showed that statins, as a class, significantly increased the likelihood of developing diabetes by 12% (pooled OR 1.12; 95%CI 1.05-1.21; I² 36%; p = 0.002; 18 RCTs). In the NMA, atorvastatin 80 mg was associated with a highest risk of diabetes, with OR of 1.34 (95%CI 1.14-1.57) followed by rosuvastatin (OR: 1.17; 95%CI: 1.02-1.35). The ORs (95%CIs) for simvastatin 80 mg, simvastatin, atorvastatin, pravastatin, lovastatin and pitavastatin were 1.21 (0.99-1.49), 1.13 (0.99-1.29), 1.13 (0.94-1.34), 1.04 (0.93-1.16), 0.98 (0.69-1.38) and 0.74 (0.31-1.77), respectively. High-dose atorvastatin increased the odds of developing diabetes even when compared with pravastatin, simvastatin and low-dose atorvastatin in the NMA.

CONCLUSIONS

Based on the results, statins, as a class, increased the risk of diabetes significantly in the pairwise meta-analysis. Overall, there appears to be a small increased risk of incident diabetes, particularly with more intensive statin therapy, although more data would be valuable to increase the robustness of this interpretation, given that the lower confidence intervals of our study analyses are close to, or just crossing one. Copyright © 2016 John Wiley & Sons, Ltd.

Impact of Statin Use on Development of New-Onset Diabetes Mellitus in Asian Population

There have been several reports showing that the statin use is associated with new-onset diabetes mellitus (DM). The aim of the present study was to evaluate the impact of chronic statin use on development of new-onset DM in a series of Asian population. The patients were retrospectively enrolled using the electronic database of Korea University Guro Hospital from January 2004 to February 2010. A total of 10,994 patients without a history of diabetes were analyzed. Baseline lipid profiles, fasting glucose, Hemoglobin (Hb) A1c, and glucose tolerance tests were measured in all patients before statin treatment. Included patients had HbA1c ≤ 5.7% and fasting glucose level ≤ 100 (mg/dl). The patients were divided into 2 groups according to the use of statins (the statin group, n = 2,324 patients and the nonstatin group, n = 8,670 patients). To adjust baseline potential confounders, a propensity score-matched analysis was performed using logistic regression model. After propensity score matching, 2 propensity-matched groups (1,699 pairs, n = 3,398, C statistic = 0.859) were generated and analyzed. After propensity score matching, baseline characteristics of both groups were balanced except that the statin group was older and had higher rate of coronary artery disease compared with the nonstatin group. During a 3-year follow-up, the statin group had higher incidence of new-onset DM compared with the nonstatin group (hazard ratio 1.99, 95% CI 1.36 to 2.92, p <0.001), but the statin group showed lower incidence of major adverse cerebral-cardiovascular events compared with the nonstatin group (hazard ratio 0.40, 95% CI 0.19 to 0.85, p <0.001). In the present study, although the use of statins was associated with higher rate of new-onset DM, it markedly improved 3-year cardiovascular outcomes in Asian population.

The diabetogenic action of statins - mechanisms and clinical implications

Treatment with statins has transformed primary and secondary prevention of cardiovascular disease (CVD), including thrombotic stroke. Evidence-based data demonstrate the benefits and safety of statin therapy and help to guide clinicians in the management of populations at high risk of CVD. Nevertheless, clinical trials, meta-analyses and observational studies highlight a 10-12% increase in new-onset diabetes mellitus (NODM) among patients receiving statins. The risk further increases with intensive therapy and among individuals with known risk factors for NODM. Mechanisms underpinning this effect are not yet fully understood; however, Mendelian randomization studies suggest that they are related to lowered activity of HMG-CoA reductase, the target of statin therapy. In vitro research indicates that statins potentially impair β-cell function and decrease insulin sensitivity but how these findings relate to patients is unknown. In the clinic, statins should be prescribed on the basis of CVD risk and individual patient characteristics. In addition, diet and lifestyle interventions should be emphasized to help mitigate the risk of NODM. Individuals who develop NODM while taking statins do not exhibit increased microvascular disease, which is reassuring. In diabetes mellitus of long duration, the effect of statins on glycaemic control is small and unlikely to be clinically important.

GGPPS-mediated Rab27A geranylgeranylation regulates β cell dysfunction during type 2 diabetes development by affecting insulin granule docked pool formation

Loss of first-phase insulin secretion associated with β cell dysfunction is an independent predictor of type 2 diabetes mellitus (T2DM) onset. Here we found that a critical enzyme involved in protein prenylation, geranylgeranyl pyrophosphate synthase (GGPPS), is required to maintain first-phase insulin secretion. GGPPS shows a biphasic expression pattern in islets of db/db mice during the progression of T2DM: GGPPS is increased during the insulin compensatory period, followed by a decrease during β cell dysfunction. Ggpps deletion in β cells results in typical T2DM β cell dysfunction, with blunted glucose-stimulated insulin secretion and consequent insulin secretion insufficiency. However, the number and size of islets and insulin biosynthesis are unaltered. Transmission electron microscopy shows a reduced number of insulin granules adjacent to the cellular membrane, suggesting a defect in docked granule pool formation, while the reserve pool is unaffected. Ggpps ablation depletes GGPP and impairs Rab27A geranylgeranylation, which is responsible for the docked pool deficiency in Ggpps-null mice. Moreover, GGPPS re-expression or GGPP administration restore glucose-stimulated insulin secretion in Ggpps-null islets. These results suggest that GGPPS-controlled protein geranylgeranylation, which regulates formation of the insulin granule docked pool, is critical for β cell function and insulin release during the development of T2DM.

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

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

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.

Statin therapy and related risk of new-onset type 2 diabetes mellitus

The use of statins for cardiovascular disease (CVD) prevention is clearly supported by clinical evidence. Although statin therapy is rather well tolerated, recent data from prospective and retrospective clinical trials and related meta-analyses suggest an increased incidence of new-onset type 2 diabetes mellitus (T2DM) in association with such treatment. The incidence of this adverse effect is not negligible, especially for specific subsets of patients, such as women, elderly, presence of familial history of T2DM and Asian ethnicity. Statin-driven T2DM appears to be a medication class-effect, mostly not related to potency nor to individual statin, as well as to be independent of previous history of CVD. Therefore, implementation of strategies for identification of patients using statins and at specific risk of incident T2DM, as well as of different therapeutic options is important and is discussed in this article. As most authors emphasized that benefits of CVD reduction by statin therapy seem to far exceed the risk of T2DM development itself, these medications remain the cornerstone for primary and secondary CVD prevention, although a specific attention to glucose metabolism and metabolic syndrome features should be payed before and during statin treatment, especially in cohorts at greater risk.

YES, a Src family kinase, is a proximal glucose-specific activator of cell division cycle control protein 42 (Cdc42) in pancreatic islet β cells

Second-phase insulin secretion sustains insulin release in the face of hyperglycemia associated with insulin resistance, requiring the continued mobilization of insulin secretory granules to the plasma membrane. Cdc42, the small Rho family GTPase recognized as the proximal glucose-specific trigger to elicit second-phase insulin secretion, signals downstream to activate the p21-activated kinase (PAK1), which then signals to Raf-1/MEK/ERK to induce filamentous actin (F-actin) remodeling, to ultimately mobilize insulin granules to the plasma membrane. However, the steps required to initiate Cdc42 activation in a glucose-specific manner in β cells have remained elusive. Toward this, we identified the involvement of the Src family kinases (SFKs), based upon the ability of SFK inhibitors to block glucose-stimulated Cdc42 and PAK1 activation events as well as the amplifying pathway of glucose-stimulated insulin release, in MIN6 β cells. Indeed, subsequent studies performed in human islets revealed that SFK phosphorylation was induced only by glucose and within 1 min of stimulation before the activation of Cdc42 at 3 min. Furthermore, pervanadate treatment validated the phosphorylation event to be tyrosine-specific. Although RT-PCR showed β cells to express five different SFK proteins, only two of these, YES and Fyn kinases, were found localized to the plasma membrane, and of these two, only YES kinase underwent glucose-stimulated tyrosine phosphorylation. Immunodetection and RNAi analyses further established YES kinase as a proximal glucose-specific signal in the Cdc42-signaling cascade. Identification of YES kinase provides new insight into the mechanisms underlying the sustainment of insulin secretion via granule mobilization/replenishment and F-actin remodeling.

Diabetogenic effect of statins: a double-edged sword?

Statins are widely prescribed cholesterol-lowering agents, which have been demonstrated to significantly reduce cardiovascular morbidity and mortality. However, recent trials have reported that statins cause worsening of hyperglycemia and increase the risk of new-onset diabetes. The association between the diabetogenic effect of statins with intensive dose and accompanying major risk factors for diabetes has been demonstrated. However, statins do not appear to have a class effect on insulin sensitivity in non-diabetic patients. Numerous mechanisms have been suggested to explain how statins cause β-cell insulin secretory dysfunction and peripheral insulin resistance leading to incident diabetes. According to findings from an aggregate of large clinical trials, the benefits of statin treatment appear to outweigh the risk of new-onset diabetes. Therefore, it would be inappropriate to discontinue the use of statins for prevention of cardiovascular events because of its potential risk for development of incident diabetes. This review addresses the currently available evidence related to statin use and new-onset diabetes from a clinical perspective.

Nifedipine prevents etoposide-induced caspase-3 activation, prenyl transferase degradation and loss in cell viability in pancreatic β-cells

Emerging evidence implicates novel roles for post-translational prenylation (i.e., farnesylation and geranylgeranylation) of various signaling proteins in a variety of cellular functions including hormone secretion, survival and apoptosis. In the context of cellular apoptosis, it has been shown previously that caspase-3 activation, a hallmark of mitochondrial dysregulation, promotes hydrolysis of several key cellular proteins. We report herein that exposure of insulin-secreting INS 832/13 cells or normal rat islets to etoposide leads to significant activation of caspase-3 and subsequent degradation of the common α-subunit of farnesyl/geranylgeranyl transferases (FTase/GGTase). Furthermore, the above stated signaling steps were prevented by Z-DEVD-FMK, a known inhibitor of caspase-3. In addition, treatment of cell lysates with recombinant caspase-3 also caused FTase/GGTase α-subunit degradation. Moreover, nifedipine, a calcium channel blocker, markedly attenuated etoposide-induced caspase-3 activation, FTase/GGTase α-subunit degradation in INS 832/13 cells and normal rat islets. Further, nifedipine significantly restored etoposide-induced loss in metabolic cell viability in INS 832/13 cells. Based on these findings, we conclude that etoposide induces loss in cell viability by inducing mitochondrial dysfunction, caspase-3 activation and degradation of FTase/GGTase α-subunit. Potential significance of these findings in the context of protein prenylation and β-cell survival are discussed.

Do statins cause diabetes?

A wealth of evidence has established that cholesterol-lowering statin drugs, widely used for the prevention of cardiovascular disease, do increase the risk of new-onset diabetes, possibly by impairing pancreatic beta cell function and decreasing peripheral insulin sensitivity. Groups at particular risk include the elderly, women, and Asians. The diabetogenic effect of statins appear directly related to statin dose and the degree of attained cholesterol lowering. Statins can cause hyperinsulinemia even in the absence of hyperglycemia and the potential mitogenic effects and implications of prolonged hyperinsulinemia are discussed. Suggestions are made as to how physicians might avert the hyperinsulinemic and diabetogenic effects of statin therapy in clinical practice, and modulate the detrimental effects of these drugs on exercise performance. Finally, long-term studies are needed to determine if the deleterious hyperinsulinemic and diabetogenic effects of statin therapy undermine the beneficial cardiovascular disease risk outcomes in various segments of the population.

Rosuvastatin and atorvastatin: comparative effects on glucose metabolism in non-diabetic patients with dyslipidaemia

The ever increasing interventional CVD outcome studies have resulted in statins being an essential factor of cardiovascular prevention strategies. The JUPITER study in 2008, despite reducing CVD and overall mortality, highlighted an increase in new onset diabetes in the rosuvastatin treated arm. Since then there have been many meta-analyses of the RCTs and the largest carried out by Sattar et al showed a significant increase in the incidence of diabetes during the trials. The findings from the individual studies when comparing the different statins were less clear. A higher statin dosage and risk factors associated with diabetes appeared to predict this phenomenon. There have been many studies investigating the effects of statins on glycaemic control, but again no clear conclusion is apparent. Despite the increase in new onset diabetes observed, the risk is clearly out-weighed by the CVD benefits observed in nearly all the statin trials. Thus, no change is required to any of the prevention guidelines regarding statins. However, it may be prudent to monitor glycaemic control after commencing statin therapy. This review will focus on atorvastatin which is the most widely used statin worldwide and rosuvastatin which is the most efficacious. This will be against a background of the effects of other statins on glucose metabolism in non-diabetic patients.

The antioxidant HDL-associated paraoxonase-1 (PON1) attenuates diabetes development and stimulates β-cell insulin release

OBJECTIVE

To analyze the direct effects of paraoxonase-1 (PON1) on diabetes development and on β-cell insulin release.

METHODS AND RESULTS

Injection of rePON1 to mice, prior to STZ-induced diabetes, resulted in reduced incidence of diabetes, as well as, in higher serum insulin levels. Incubation of β-cells with PON1 also dose-dependently increased insulin secretion and its cellular content. PON1 increased cell survival under high glucose levels, but not under high STZ concentrations. The addition of the PON1 carrier in the circulation - HDL, to βTC3 cell line, had an additive effect on PON1-induced insulin secretion. PON1 administration to mice or incubation with β-cells was associated with a substantial decreased oxidative stress. Just like PON1, the dietary anti-oxidants, pomegranate juice, punicalagin (major polyphenol in pomegranate) or vitamin E, also increased insulin release from βTC3, but unlike PON1, failed to increase insulin cellular content, suggesting a possible role for PON1 in insulin biosynthesis, separately from PON1 antioxidative effect. Both, PON1 catalytic activity and PON1 association to HDL, were not required for PON1 stimulation of insulin release from β-cells. However, the PON1 free sulfhydryl group was shown to be essential for insulin release by PON1, as blocking the PON1 SH group, abolished PON1 stimulatory effect on insulin secretion.

CONCLUSION

PON1 is a potent anti-diabetic enzyme that exerts this protection against diabetes through its antioxidative, as well as via its insulin stimulation properties on β-cells.

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.

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.

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.

Distinct effects of pravastatin, atorvastatin, and simvastatin on insulin secretion from a beta-cell line, MIN6 cells

In addition to the prevention of cardiovascular diseases by lowering plasma LDL cholesterol, recent studies suggest that statins could have some impact on insulin action. To estimate the direct effects of statins on insulin secretion from pancreatic beta-cells, MIN6 cells were treated with pravastatin, simvastatin, or atorvastatin. Basal insulin secretion at low glucose concentration was unexpectedly increased at very high doses of simvastatin or atorvastatin after 24- and 48-hour incubation. Insulin secretion at high glucose was not significantly changed, and thus, net glucose-stimulated insulin secretion was apparently decreased by these lipophilic statins. The changes in insulin secretion were highly associated with increased endogenous SREBP activities in response to HMG-CoA inhibition as estimated by SRE-luciferase assays, and finally after 48-hour incubation, accompanied by impaired cell viability as estimated by MTT assays. In contrast, these changes were much less prominent by the addition of pravastatin. Meanwhile, glucose-stimulated insulin secretion of islets isolated from C57BL/6 mice was not significantly changed by any of the statins. Overall, taken up by beta-cells, statins can affect insulin secretion through either HMG-CoA inhibition or cytotoxicity, as observed by the addition of extraordinary high doses of lipophilic statins, but not hydrophilic statins, to the medium.

Preferable effect of pravastatin compared to atorvastatin on beta cell function in Japanese early-state type 2 diabetes with hypercholesterolemia

While a large numbers of clinical trials using various kinds of statins has been reported, a possible preventive effect on new onset of type 2 diabetes mellitus was shown only by the subanalysis of The West of Scotland Coronary Prevention Study (WOSCOPS) using pravastatin. The aim of this study was to investigate whether pravastatin has a preferable effect on glucose tolerance among statins. An open-label prospective cross-over trial was performed to compare the effect of pravastatin (10 mg/day) or atorvastatin (10 mg/day) in Japanese early-state type 2 diabetes mellitus with hypercholesterolemia. The analyzed study subjects were treated with pravastatin (10 mg/day, n = 12) or atorvastatin (10 mg/day, n = 12) for 12 weeks. After a 4-week-washout period, the drugs were switched and treatment was continued for another 12 weeks. Oral glucose tolerance test (OGTT) was performed to evaluate several parameters including the appropriateness of beta cell function for the individual insulin sensitivity (disposition index: product of a validated secretion parameter and sensitivity) at the end of each therapy. HbA(1c) and 2 h-glucose levels during OGTT in the pravastatin treatment were significantly lower than atorvastatin treatment. Disposition index after pravastatin treatment was significantly higher than after atorvastatin treatment. In conclusion, our study suggests that pravastatin has a favorable effect on pancreatic beta cell function compared with atorvastatin.

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.

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

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

Down-regulation of autoreactive T-cells by HMG CoA reductase inhibitors

The inhibitors of HMG CoA reductase (statins) are widely used as cholesterol-lowering drugs with excellent safety records in hypercholesterolemic patients. Statins exert pleiotropic effects on a variety of cells, and they were recently described as a new class of immune modulators. Depending on their structure, dose, and route of administration, statins regulate the function of both the antigen-presenting cells and T-cells by HMG CoA reductase-dependent and independent mechanisms. Herein, we describe these mechanisms leading to prevention, amelioration, and reversal of autoimmune diseases. We also present data from our laboratories showing for the first time that in a double transgenic mouse model for autoimmune diabetes, atorvastatin (lipitor) prevented the onset of disease when administered in the neonatal period, and stabilized the glucose levels when administered in mice developing a mild form of diabetes.

Effect of pravastatin and atorvastatin on glucose metabolism in nondiabetic patients with hypercholesterolemia

OBJECTIVE

The aim of this study was to assess the effects of hydrophilic pravastatin and lipophilic atorvastatin on glucose metabolism and lipid metabolism in non-diabetic patients with hypercholesterolemia.

METHODS

Fasting plasma glucose (FPG), hemoglobin A(1c) (HbA(1c)), total cholesterol (TC), low-density lipoprotein cholesterol (LDLC), high-density lipoprotein cholesterol (HDLC), and triglyceride (TG) levels were determined before and after statin treatment.

PATIENTS

A total of 44 nondiabetic patients (FPG < or =125 mg/mL; HbA(1c) <5.8%) undergoing treatment with either pravastatin (n=21) or atorvastatin (n=23) for hypercholesterolemia were investigated.

RESULTS

FPG level in the pravastatin but not atorvastatin group was significantly lowered after vs before treatment. Accordingly, the HbA(1c) level in the atorvastatin but not in the pravastatin group was significantly increased. As expected, both TC and LDL-C levels were significantly lowered in both groups. In particular, the TC level in the atorvastatin group was more remarkably and significantly improved than in the pravastatin group. On the other hand, the HDL-C level in the pravastatin group but not in the atorvastatin group was significantly increased after the administration period. The TG level was unaffected in both groups.

CONCLUSION

Pravastatin was suggested to act favorably, while atorvastatin adversely, regarding it's effects on glucose metabolism in nondiabetic hypercholesterolemic patients, although atorvastatin exerted more potent cholesterol-lowering effects compared with pravastatin.

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.

Critical and distinct roles of amino- and carboxyl-terminal sequences in regulation of the biological activity of the Chp atypical Rho GTPase

Chp (Cdc42 homologous protein) shares significant sequence and functional identity with the human Cdc42 small GTPase, and like Cdc42, promotes formation of filopodia and activates the p21-activated kinase serine/threonine kinase. However, unlike Cdc42, Chp contains unique amino- and carboxyl-terminal extensions. Here we determined whether Chp, like Cdc42, can promote growth transformation and evaluated the role of the amino- and carboxyl-terminal sequences in Chp function. Surprisingly, we found that a GTPase-deficient mutant of Chp exhibited low transforming activity but that deletion of the amino terminus of Chp greatly enhanced its transforming activity. Thus, the amino terminus may serve as a negative regulator of Chp function. The carboxyl terminus of Cdc42 contains a CAAX (where C is cysteine, A is aliphatic amino acid, X is terminal amino acid) tetrapeptide sequence that signals for the posttranslational modification critical for Cdc42 membrane association and biological function. Although Chp lacks aCAAXmotif, we found that Chp showed carboxyl terminus-dependent localization to the plasma membrane and to endosomes. Furthermore, an intact carboxyl terminus was required for Chp transforming activity. However, treatment with inhibitors of protein palmitoylation, but not prenylation, caused Chp to mislocalize to the cytoplasm. Thus, Chp depends on palmitoylation, rather than isoprenylation, for membrane association and function. In summary, Chp is implicated in cell transformation, and the unique amino and carboxyl termini of Chp represent atypical mechanisms of regulation of Rho GTPase function.

Perspective: emerging evidence for signaling roles of mitochondrial anaplerotic products in insulin secretion

The importance of mitochondrial biosynthesis in stimulus secretion coupling in the insulin-producing beta-cell probably equals that of ATP production. In glucose-induced insulin secretion, the rate of pyruvate carboxylation is very high and correlates more strongly with the glucose concentration the beta-cell is exposed to (and thus with insulin release) than does pyruvate decarboxylation, which produces acetyl-CoA for metabolism in the citric acid cycle to produce ATP. The carboxylation pathway can increase the levels of citric acid cycle intermediates, and this indicates that anaplerosis, the net synthesis of cycle intermediates, is important for insulin secretion. Increased cycle intermediates will alter mitochondrial processes, and, therefore, the synthesized intermediates must be exported from mitochondria to the cytosol (cataplerosis). This further suggests that these intermediates have roles in signaling insulin secretion. Although evidence is quite good that all physiological fuel secretagogues stimulate insulin secretion via anaplerosis, evidence is just emerging about the possible extramitochondrial roles of exported citric acid cycle intermediates. This article speculates on their potential roles as signaling molecules themselves and as exporters of equivalents of NADPH, acetyl-CoA and malonyl-CoA, as well as alpha-ketoglutarate as a substrate for hydroxylases. We also discuss the "succinate mechanism," which hypothesizes that insulin secretagogues produce both NADPH and mevalonate. Finally, we discuss the role of mitochondria in causing oscillations in beta-cell citrate levels. These parallel oscillations in ATP and NAD(P)H. Oscillations in beta-cell plasma membrane electrical potential, ATP/ADP and NAD(P)/NAD(P)H ratios, and glycolytic flux are known to correlate with pulsatile insulin release. Citrate oscillations might synchronize oscillations of individual mitochondria with one another and mitochondrial oscillations with oscillations in glycolysis and, therefore, with flux of pyruvate into mitochondria. Thus citrate oscillations may synchronize mitochondrial ATP production and anaplerosis with other cellular oscillations.

P2Y purinergic potentiation of glucose-induced insulin secretion and pancreatic beta-cell metabolism

Purine nucleotides and their analogs increase insulin secretion through activation of pancreatic beta-cell P2Y receptors. The present study aimed at determining the role of glucose metabolism in the response to P2Y agonists and whether ATP-activated K+ channels (KATP channels) are involved in this response. The experiments were performed in the rat isolated pancreas, perfused with a Krebs-bicarbonate buffer supplemented with 2 g/l bovine serum albumin under dynamic glucose conditions from 5 mmol/l baseline to 11 mmol/l. ADPbetaS (0.5 micromol/l) was selected as a stable and selective P2Y agonist. This compound, ineffective on the 5 mmol/l glucose background, induced a significant threefold increase in insulin release triggered by the glucose challenge. The effect of ADPbetaS was markedly reduced (P <0.001) in the presence of an inhibitor of glucose metabolism. In addition to glucose, the ADP analog also amplified the beta-cell insulin response to 15 mmol/l methyl pyruvate (P <0.05), but it was ineffective on the insulin response to 2.5 mmol/l methyl succinate. A nonmetabolic stimulus was applied using tolbutamide (185 micromol/l). Insulin secretion induced by the KATP channel blocker was strongly reinforced by ADPbetaS (P <0.001), which prompted us to check a possible interplay of KATP channels in the effect of ADPbetaS. In the presence of diazoxide 250 micromol/l and 21 mmol/l KCl, ADPbetaS still amplified the second phase of glucose-induced insulin secretion (P <0.001). We conclude that P2Y receptor activation is able to promote insulin secretion through a mechanism, involving beta-cell metabolism and a rise in intracellular calcium; this effect does not result from a direct inhibitory effect on KATP channels.

Inhibition mechanism of S-adenosylmethionine-induced movement deficits by prenylcysteine analogs

We previously showed that S-adenosylmethionine (SAM) induces movement impairments similar to those observed in Parkinson's disease (PD) apparently by prenylated protein methylation; 5 kDa molecules being methylated and the symptoms being inhibited by prenylcysteine (PC) analogs. In the present study, we explore the biochemical mechanism of action of the PC analogs. N-acetylgeranylcysteine (AGC), N-acetylfarnesylcysteine (AFC), N-acetylgeranylgeranylcysteine (AGGC), farnesylthioacetic acid (FTA), farnesyl-2-ethanesulfonic acid (FTE) and farnesylsuccinic acid (FMS), but not farnesylthiotriazole (FTT) and farnesylthiolactic acid (FTL), inhibited the SAM-induced motor impairments. Incubation of the respective analogs with rat brain membranes containing prenylated protein methyltransferase (PPMTase) resulted in the methylation of AGC, AFC and AGGC. FTA, FTE, FMS and FTT, but not FTL, inhibited the enzyme activity. A single injection of the active analogs remained effective for at least 3 days against repeated injections of 1 micromol SAM. Amphetamine-induced hyperactivity in rats was inhibited by SAM but potentiated by FTE. During 60 min, the movement time for amphetamine-treated rats was 1477 s compared with 633 and 1664 s for amphetamine+SAM- and amphetamine+FTE-treated rats, respectively. The total distance for amphetamine+FTE-treated rats was 82% higher than for amphetamine. The horizontal activity was 30,728 (amphetamine), 15,430 (FTE), 18,526 (amphetamine+SAM), 41,736 (amphetamine+FTE) and 7004 (SAM) as compared to the PBS control (4726). The intricate relationship between the actions of SAM, which speeds up prenylated protein methylation and impairs movement, amphetamine, which increases synaptic dopamine levels and movement, and the PC analogs, which prevent the SAM-induced movement impairments, suggests a SAM-induced defect on dopamine signaling as the likely cause of the symptoms. The data reveal that interaction of PC analogs with PPMTase may not be an indicator of anti-PD-like activity.