Palmitate increases L-type Ca2+ currents and the size of the readily releasable granule pool in mouse pancreatic beta-cells

Glucolipotoxicity in INS-1E cells is counteracted by carnitine palmitoyltransferase 1 over-expression

Effects of non-esterified fatty acids (FAs) are accentuated when applied together with elevated glucose through preferential use of glucose as fuel, which leads to decreased oxidation of FAs. We examined how over-expression of the mitochondrial FA transporter carnitine palmitoyltransferase 1 (CPT1) affects glucose-stimulated insulin secretion (GSIS), apoptosis and ER stress in INS-1E cells cultured in the presence of elevated levels of glucose and palmitate. INS-1E cells were infected with Tet-ON regulated adenovirus containing CPT1 and cultured for 48h in the presence of 0.5mM palmitate and 20mM glucose. Over-expressing CPT1 lowered basal insulin secretion in a dose-dependent manner thereby improving GSIS from INS-1E cells. Also, apoptosis was alleviated and ER-stress markers p-eIF2alpha and CHOP were decreased in cells over-expressing CPT1. We conclude that regulated over-expression of CPT1 is beneficial for glucolipotoxic beta-cells.

Carboxypeptidase E mediates palmitate-induced beta-cell ER stress and apoptosis

Obesity is a principal risk factor for type 2 diabetes, and elevated fatty acids reduce beta-cell function and survival. An unbiased proteomic screen was used to identify targets of palmitate in beta-cell death. The most significantly altered protein in both human islets and MIN6 beta-cells treated with palmitate was carboxypeptidase E (CPE). Palmitate reduced CPE protein levels within 2 h, preceding endoplasmic reticulum (ER) stress and cell death, by a mechanism involving CPE translocation to Golgi and lysosomal degradation. Palmitate metabolism and Ca(2+) flux were also required for CPE proteolysis and beta-cell death. Chronic palmitate exposure increased the ratio of proinsulin to insulin. CPE null islets had increased apoptosis in vivo and in vitro. Reducing CPE by approximately 30% using shRNA also increased ER stress and apoptosis. Conversely, overexpression of CPE partially rescued beta-cells from palmitate-induced ER stress and apoptosis. Thus, carboxypeptidase E degradation contributes to palmitate-induced beta-cell ER stress and apoptosis. CPE is a major link between hyperlipidemia and beta-cell death pathways in diabetes.

Differential modulation of L-type calcium channel subunits by oleate

Nonesterified fatty acids such as oleate and palmitate acutely potentiate insulin secretion from pancreatic islets in a glucose-dependent manner. In addition, recent studies show that fatty acids elevate intracellular free Ca(2+) and increase voltage-gated Ca(2+) current in mouse beta-cells, although the mechanisms involved are poorly understood. Here we utilized a heterologous system to express subunit-defined voltage-dependent L-type Ca(2+) channels (LTCC) and demonstrate that beta-cell calcium may increase in part from an interaction between fatty acid and specific calcium channel subunits. Distinct functional LTCC were assembled in both COS-7 and HEK-293 cells by expressing either one of the EYFP-tagged L-type alpha(1)-subunits (beta-cell Cav1.3 or lung Cav1.2) and ERFP-tagged islet beta-subunits (ibeta(2a) or ibeta(3)). In COS-7 cells, elevations in intracellular Ca(2+) mediated by LTCC were enhanced by an oleate-BSA complex. To extend these findings, Ca(2+) current was measured in LTCC-expressing HEK-293 cells that revealed an increase in peak Ca(2+) current within 2 min after addition of the oleate complex, with maximal potentiation occurring at voltages <0 mV. Both Cav1.3 and Cav1.2 were modulated by oleate, and the presence of different auxiliary beta-subunits resulted in differential augmentation. The potentiating effect of oleate on Cav1.2 was abolished by the pretreatment of cells with triacsin C, suggesting that long-chain CoA synthesis is necessary for Ca(2+) channel modulation. These results show for the first time that two L-type Ca(2+) channels expressed in beta-cells (Cav1.3 and Cav1.2) appear to be targeted by nonesterified fatty acids. This effect may account in part for the acute potentiation of glucose-dependent insulin secretion by fatty acids.

Palmitate-induced beta-cell dysfunction is associated with excessive NO production and is reversed by thiazolidinedione-mediated inhibition of GPR40 transduction mechanisms

Background

Type 2 diabetes often displays hyperlipidemia. We examined palmitate effects on pancreatic islet function in relation to FFA receptor GPR40, NO generation, insulin release, and the PPARγ agonistic thiazolidinedione, rosiglitazone.

Principal Findings

Rosiglitazone suppressed acute palmitate-stimulated GPR40-transduced PI hydrolysis in HEK293 cells and insulin release from MIN6c cells and mouse islets. Culturing islets 24 h with palmitate at 5 mmol/l glucose induced β-cell iNOS expression as revealed by confocal microscopy and increased the activities of ncNOS and iNOS associated with suppression of glucose-stimulated insulin response. Rosiglitazone reversed these effects. The expression of iNOS after high-glucose culturing was unaffected by rosiglitazone. Downregulation of GPR40 by antisense treatment abrogated GPR40 expression and suppressed palmitate-induced iNOS activity and insulin release.

Conclusion

We conclude that, in addition to mediating acute FFA-stimulated insulin release, GPR40 is an important regulator of iNOS expression and dysfunctional insulin release during long-term exposure to FFA. The adverse effects of palmitate were counteracted by rosiglitazone at GPR40, suggesting that thiazolidinediones are beneficial for β-cell function in hyperlipidemic type 2 diabetes.

Suitability of commonly used excipients for electrophysiological in-vitro safety pharmacology assessment of effects on hERG potassium current and on rabbit Purkinje fiber action potential

INTRODUCTION

Regulatory guidelines require investigation of the liability for delayed ventricular repolarization by new chemical entities within a broad concentration range in-vitro. However, investigation can be limited by poor drug aqueous solubility, and by solvent physicochemical attributes that disrupt cell membrane integrity. Although excipients or solubilizing agents may aid to achieve the necessary high concentrations, no comprehensive overview on the suitability of solvents for in-vitro electrophysiological safety studies exists.

METHODS

Excipients were tested for potential interference with the hERG (human ether-a-go-go-related gene) K(+) current (whole-cell voltage-clamp, 23+/-2 degrees C), and the shape of rabbit Purkinje fiber action potentials (conventional glass microelectrode technique, 37+/-1 degrees C).

RESULTS AND DISCUSSION

Water-soluble complexation builders/carriers had little effect on hERG K(+) current at up to 50 mg/ml (BSA, bovine serum albumin) and 11 mg/ml (HP-beta-CD, hydroxypropyl-beta-cyclodextrin; IC(20), concentration of 20% inhibition). Water-soluble organic (co)solvents inhibited hERG K(+) currents (IC(20), %/mM): 0.7/152, ethanol; 0.9/67, Transcutol; 1.2/154, DMSO (dimethylsulfoxide); 1.6/389, acetonitrile; 1.9/48, polyethylene glycol 400; 2.1/660, methanol. Part of their inhibitory effect is attributed to the osmolality of extracellular solutions, because hERG IC(20) and extrapolated osmolality at the hERG IC(20) strongly correlate. Water-soluble non-ionic solubilizers/surfactants are potent inhibitors of hERG K(+) current with IC(20) concentrations of 0.07% (Cremophor EL) or lower (Tween 20, Tween 80: approximately 0.001%). Part of this inhibitory effect is attributed to their interaction with lipid membranes, because hERG inhibition occurs close to critical micelle concentrations (Cremophor, approximately 0.009%; Tween 20, approximately 0.007%). Purkinje fiber action potentials are little affected by HP-beta-CD at up to 2 mg/ml, while DMSO tends to shorten the action potential duration at 1%.

CONCLUSION

When conducting electrophysiological in-vitro assessments of drug effects, solubilizers/surfactants (Cremophor EL, Tween 20, Tween 80) should be avoided. Instead, water-soluble organic (co)solvents (methanol, acetonitrile, DMSO) or complexation builders/carriers (HP-beta-CD, BSA) appear to be more favorable.

Long-term exposure to glucose and lipids inhibits glucose-induced insulin secretion downstream of granule fusion with plasma membrane

Mouse beta-cells cultured at 15 mmol/l glucose for 72 h had reduced ATP-sensitive K+ (K(ATP)) channel activity (-30%), increased voltage-gated Ca2+ currents, higher intracellular free Ca2+ concentration ([Ca2+]i; +160%), more exocytosis (monitored by capacitance measurements, +100%), and greater insulin content (+230%) than those cultured at 4.5 mmol/l glucose. However, they released 20% less insulin when challenged with 20 mmol/l glucose. Glucose-induced (20 mmol/l) insulin secretion was reduced by 60-90% in islets cocultured at 4.5 or 15 mmol/l glucose and either oleate or palmitate (0.5 mmol/l). Free fatty acid (FFA)-induced inhibition of secretion was not associated with any major changes in [Ca2+]i or islet ATP content. Palmitate stimulated exocytosis by twofold or more but reduced K+-induced secretion by up to 60%. Basal (1 mmol/l glucose) K(ATP) channel activity was 40% lower in islets cultured at 4.5 mmol/l glucose plus palmitate and 60% lower in islets cultured at 15 mmol/l glucose plus either of the FFAs. Insulin content decreased by 75% in islets exposed to FFAs in the presence of high (15 mmol/l), but not low (4.5 mmol/l), glucose concentrations, but the number of secretory granules was unchanged. FFA-induced inhibition of insulin secretion was not associated with increased transcript levels of the apoptosis markers Bax (BclII-associated X protein) and caspase-3. We conclude that glucose and FFAs reduce insulin secretion by interference with the exit of insulin via the fusion pore.

Synthesis and biological evaluation of 3-aryl-3-(4-phenoxy)-propionic acid as a novel series of G protein-coupled receptor 40 agonists

High-throughput screening of a subset of the J&J compound library containing the carboxylic acid functional group uncovered a bromophenyl derivative as a moderate potent GPR40 agonist. Chemical elaboration of this bromophenyl led to the discovery of a novel series of GPR40 agonists with submicromolar potency. Among them, 22 and 24 behaved as full agonists when compared to the endogenous GPR40 ligand linolenic acid in a functional Ca+2 flux assay in HEK cells expressing GPR40 receptor. Several GPR40 agonists have also demonstrated the ability to induce glucose-mediated insulin secretion in the mouse MIN6 pancreatic beta-cell line. Our data supports the hypothesis that GPR40 may play an important role in fatty acid-mediated glucose-dependent insulin secretion. Compound 22 exhibited good pharmacokinetic profile in rat and may serve as a good candidate for in vivo study and may help to determine if GPR40 agonists would be beneficial in the treatment of type II diabetes.

Functional genomics of the beta-cell: short-chain 3-hydroxyacyl-coenzyme A dehydrogenase regulates insulin secretion independent of K+ currents

Recent advances in functional genomics afford the opportunity to interrogate the expression profiles of thousands of genes simultaneously and examine the function of these genes in a high-throughput manner. In this study, we describe a rational and efficient approach to identifying novel regulators of insulin secretion by the pancreatic beta-cell. Computational analysis of expression profiles of several mouse and cellular models of impaired insulin secretion identified 373 candidate genes involved in regulation of insulin secretion. Using RNA interference, we assessed the requirements of 10 of these candidates and identified four genes (40%) as being essential for normal insulin secretion. Among the genes identified was Hadhsc, which encodes short-chain 3-hydroxyacyl-coenzyme A dehydrogenase (SCHAD), an enzyme of mitochondrial beta-oxidation of fatty acids whose mutation results in congenital hyperinsulinism. RNA interference-mediated gene suppression of Hadhsc in insulinoma cells and primary rodent islets revealed enhanced basal but normal glucose-stimulated insulin secretion. This increase in basal insulin secretion was not attenuated by the opening of the KATP channel with diazoxide, suggesting that SCHAD regulates insulin secretion through a KATP channel-independent mechanism. Our results suggest a molecular explanation for the hyperinsulinemia hypoglycemic seen in patients with SCHAD deficiency.

The G-protein-coupled receptor 40 family (GPR40–GPR43) and its role in nutrient sensing

Recent deorphanization efforts have paired the G-protein-coupled receptors GPR40, GPR41 and GPR43 with fatty acids as endogenous ligands. While carboxylic acids have been historically known to serve as fuel sources and biomarkers of disease, these studies demonstrate that fatty acids can act as signalling molecules at the cell-surface level. This receptor subfamily shares approx. 30% identity among members, with some limited cross-over between ligand activities. Generalized expression patterns within the pancreatic β-cell, adipose depots and the gastrointestinal tract infer involvement in energy source recognition, absorption, storage and/or metabolism. GPR40, activated by medium and long-chain fatty acids, has been shown to potentiate insulin secretion at the β-cell, and has been hypothesized to participate in the detrimental effects of chronic fatty acid exposure on β-cell function. GPR41 and GPR43 have been reported to stimulate leptin release and adipogenesis respectively via activation by short-chain fatty acids. These common themes implicate GPR40, GPR41 and GPR43 in playing significant roles in metabolic diseases, such as diabetes, obesity and the metabolic syndrome.

The role of voltage-gated calcium channels in pancreatic beta-cell physiology and pathophysiology

Voltage-gated calcium (CaV) channels are ubiquitously expressed in various cell types throughout the body. In principle, the molecular identity, biophysical profile, and pharmacological property of CaV channels are independent of the cell type where they reside, whereas these channels execute unique functions in different cell types, such as muscle contraction, neurotransmitter release, and hormone secretion. At least six CaValpha1 subunits, including CaV1.2, CaV1.3, CaV2.1, CaV2.2, CaV2.3, and CaV3.1, have been identified in pancreatic beta-cells. These pore-forming subunits complex with certain auxiliary subunits to conduct L-, P/Q-, N-, R-, and T-type CaV currents, respectively. beta-Cell CaV channels take center stage in insulin secretion and play an important role in beta-cell physiology and pathophysiology. CaV3 channels become expressed in diabetes-prone mouse beta-cells. Point mutation in the human CaV1.2 gene results in excessive insulin secretion. Trinucleotide expansion in the human CaV1.3 and CaV2.1 gene is revealed in a subgroup of patients with type 2 diabetes. beta-Cell CaV channels are regulated by a wide range of mechanisms, either shared by other cell types or specific to beta-cells, to always guarantee a satisfactory concentration of Ca2+. Inappropriate regulation of beta-cell CaV channels causes beta-cell dysfunction and even death manifested in both type 1 and type 2 diabetes. This review summarizes current knowledge of CaV channels in beta-cell physiology and pathophysiology.

Glucose-sensing mechanisms in pancreatic beta-cells

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

Pharmacological regulation of insulin secretion in MIN6 cells through the fatty acid receptor GPR40: identification of agonist and antagonist small molecules

1. Long chain fatty acids have recently been identified as agonists for the G protein-coupled receptors GPR40 and GPR120. Here, we present the first description of GW9508, a small-molecule agonist of the fatty acid receptors GPR40 and GPR120. In addition, we also describe the pharmacology of GW1100, a selective GPR40 antagonist. These molecules were used to further investigate the role of GPR40 in glucose-stimulated insulin secretion in the MIN6 mouse pancreatic beta-cell line. 2. GW9508 and linoleic acid both stimulated intracellular Ca2+ mobilization in human embryonic kidney (HEK)293 cells expressing GPR40 (pEC50 values of 7.32+/-0.03 and 5.65+/-0.06, respectively) or GPR120 (pEC50 values of 5.46+/-0.09 and 5.89+/-0.04, respectively), but not in the parent HEK-293 cell line. 3. GW1100 dose dependently inhibited GPR40-mediated Ca2+ elevations stimulated by GW9508 and linoleic acid (pIC50 values of 5.99+/-0.03 and 5.99+/-0.06, respectively). GW1100 had no effect on the GPR120-mediated stimulation of intracellular Ca2+ release produced by either GW9508 or linoleic acid. 4. GW9508 dose dependently potentiated glucose-stimulated insulin secretion in MIN6 cells, but not in primary rat or mouse islets. Furthermore, GW9508 was able to potentiate the KCl-mediated increase in insulin secretion in MIN6 cells. The effects of GW9508 on insulin secretion were reversed by GW1100, while linoleic acid-stimulated insulin secretion was partially attenuated by GW1100. 5. These results add further evidence to a link between GPR40 and the ability of fatty acids to acutely potentiate insulin secretion and demonstrate that small-molecule GPR40 agonists are glucose-sensitive insulin secretagogues.

Contribution of adipocyte-derived factors to beta-cell dysfunction in diabetes

In addition to serving as an energy reservoir, the adipocyte has been characterized as an endocrine cell, secreting many bioactive factors which influence energy homeostasis. Being overweight, with excessive adipose tissue, is considered to be part of the pathogenesis of type 2 diabetes. Insulin resistance and beta-cell dysfunction are two major pathophysiological changes seen in type 2 diabetes. In addition to inducing insulin resistance in insulin-responsive tissues, adipocyte-derived factors play an important role in the pathogenesis of beta-cell dysfunction. Leptin, free fatty acids, adiponectin, tumor necrosis factor-alpha and interleukin-6 are all produced and secreted by adipocytes, and may directly influence aspects of beta-cell function, including insulin synthesis and secretion, insulin cell survival and apoptosis. During the progression from normal weight to obesity and on to overt diabetes, the adipocyte-derived factors contribute to the occurrence and development of beta-cell dysfunction and type 2 diabetes.

New Insights into Fatty Acid Modulation of Pancreatic β‐Cell Function

Insulin resistance states as found in type 2 diabetes and obesity are frequently associated with hyperlipidemia. Both stimulatory and detrimental effects of free fatty acids (FFA) on pancreatic beta cells have long been recognized. Acute exposure of the pancreatic beta cell to both high glucose concentrations and saturated FFA results in a substantial increase of insulin release, whereas a chronic exposure results in desensitization and suppression of secretion. Reduction of plasma FFA levels in fasted rats or humans severely impairs glucose-induced insulin release but palmitate can augment insulin release in the presence of nonstimulatory concentrations of glucose. These results imply that changes in physiological plasma levels of FFA are important for regulation of beta-cell function. Although it is widely accepted that fatty acid (FA) metabolism (notably FA synthesis and/or formation of LC-acyl-CoA) is necessary for stimulation of insulin secretion, the key regulatory molecular mechanisms controlling the interplay between glucose and fatty acid metabolism and thus insulin secretion are not well understood but are now described in detail in this review. Indeed the correct control of switching between FA synthesis or oxidation may have critical implications for beta-cell function and integrity both in vivo and in vitro. LC-acyl-CoA (formed from either endogenously synthesized or exogenous FA) controls several aspects of beta-cell function including activation of certain types of PKC, modulation of ion channels, protein acylation, ceramide- and/or NO-mediated apoptosis, and binding to and activating nuclear transcriptional factors. The present review also describes the possible effects of FAs on insulin signaling. We have previously reported that acute exposure of islets to palmitate up-regulates some key components of the intracellular insulin signaling pathway in pancreatic islets. Another aspect considered in this review is the potential source of fatty acids for pancreatic islets in addition to supply in the blood. Lipids can be transferred from leukocytes (macrophages) to pancreatic islets in coculture. This latter process may provide an additional source of FAs that may play a significant role in the regulation of insulin secretion.

Free fatty acid receptor 1 (FFA(1)R/GPR40) and its involvement in fatty-acid-stimulated insulin secretion

Free fatty acids (FFA) have generally been proposed to regulate pancreatic insulin release by an intracellular mechanism involving inhibition of CPT-1. The recently de-orphanized G-protein coupled receptor, FFA(1)R/GPR40, has been shown to be essential for fatty-acid-stimulated insulin release in MIN6 mouse insulinoma cells. The CPT-1 inhibitor, 2-bromo palmitate (2BrP), was investigated for its ability to interact with mouse FFA(1)R/GPR40. It was found to inhibit phosphatidyl inositol hydrolysis induced by linoleic acid (LA) (100 muM in all experiments) in HEK293 cells transfected with FFA(1)R/GPR40 and in the MIN6 subclone, MIN6c4. 2BrP also inhibited LA-stimulated insulin release from mouse pancreatic islets. Mouse islets were subjected to antisense intervention by treatment with a FFA(1)R/GPR40-specific morpholino oligonucleotide for 48 h. Antisense treatment of islets suppressed LA-stimulated insulin release by 50% and by almost 100% when islets were pretreated with LA for 30 min before applying the antisense. Antisense treatment had no effect on tolbutamide-stimulated insulin release. Confocal microscopy using an FFA(1)R/GPR40-specific antibody revealed receptor expression largely localized to the plasma membrane of insulin-producing cells. Pretreating the islets with LA for 30 min followed by antisense oligonucleotide treatment for 48 h reduced the FFA(1)R/GPR40 immunoreactivity to background levels. The results demonstrate that FFA(1)R/GPR40 is inhibited by the CPT-1 inhibitor, 2BrP, and confirm that FFA(1)R/GPR40 is indeed necessary, at least in part, for fatty-acid-stimulated insulin release.

Role of GPR40 in fatty acid action on the beta cell line INS-1E

GPR40 is a G protein-coupled receptor expressed preferentially in beta cells, that has been implicated in mediating free fatty acid-stimulated insulin release. GPR40 RNAi impaired the ability of palmitic acid (PA) to increase both insulin secretion and intracellular calcium ([Ca2+]i). The PA-dependent [Ca2+]i increase was attenuated by inhibitors of Galphaq, PLC, and SERCA. Thus GPR40 activates the Galphaq pathway, leading to release of Ca2+ from the ER. Yet the GPR40-dependent [Ca2+]i rise was dependent on extracellular Ca2+ and elevated glucose, and was blocked by inhibition of L-type calcium channels (LTCC) or opening of the K(ATP) channel; this suggests that GPR40 promotes Ca2+ influx through up-regulation of LTCC pre-activated by glucose and membrane depolarization. Taken together, the data indicate that GPR40 mediates the increase in [Ca2+]i and insulin secretion through the Galphaq-PLC pathway, resulting in release of Ca2+ from the ER and leading to up-regulation of Ca2+ influx via LTCC.

Hyperinsulinism of infancy associated with a novel splice site mutation in the SCHAD gene

Fatty acids play an important role in regulating insulin secretion, but the mechanisms are unclear. We report a case of a novel splice site mutation in the short-chain 3-hydroxyacyl-CoA dehydrogenase (SCHAD) gene associated with hyperinsulinism. This mutation resulted in a nearly complete absence of immunoreactive protein and a decrease in fibroblast SCHAD activity.

Rab27a: a new face in beta cell metabolism-secretion coupling

In pancreatic beta cells, not only insulin exocytosis per se, but translocation of beta granules toward the plasma membrane--an event upstream of exocytosis--are under the control of glucose. However, the molecular basis of this translocation has been poorly understood. Rab27a-mediated translocation of glucose-induced beta granules is reported in this issue of the JCI. Rab27a or its effector molecule may constitute a novel pharmacological target because potentiation of the Rab27a pathway is expected to restore beta cell glucose competency in patients with diabetes mellitus.

Fatty acid translocase (FAT/CD36) is localized on insulin-containing granules in human pancreatic beta-cells and mediates fatty acid effects on insulin secretion

The membrane receptor FAT/CD36 facilitates the major fraction of long-chain fatty acid (FA) uptake by muscle and adipose tissues. In line with the well-known effects of FA metabolism on carbohydrate utilization and insulin responsiveness, altered expression of CD36 has been linked to phenotypic features of the metabolic syndrome including insulin resistance and dyslipidemia. FA metabolism is also known to significantly affect insulin secretion. However, the role of CD36 in this process remains unknown, since its expression levels and function in the pancreas have not been explored. In the present study, freshly isolated human islets and a mouse-derived beta-cell line (MIN6) were shown positive for CD36 expression by RT-PCR, Western blot, and immunofluorescence. The identity of the PCR product was confirmed by microsequencing. The identified transcript was translated and the protein was expressed and subjected to the known posttranslational glycosylation. Fluorescence resonance energy transfer analysis and subcellular protein fractionation indicated that insulin and CD36 are colocalized in the secretory granules of beta-cells. Islet CD36 functioned in FA uptake because this process was blocked by the irreversible CD36 inhibitor sulfosuccinimidyl-oleate. More importantly, sulfosuccinimidyl-oleate reversed enhancing and inhibiting effects, respectively, of acute and long-term palmitate incubations on glucose-dependent insulin secretion. In conclusion, our study demonstrates that human islets express CD36 in the plasma membrane as well as in the insulin secretory granules. CD36 activity appears important for uptake of FA into beta-cells as well as for mediating their modulatory effects on insulin secretion.

Palmitate stimulation of glucagon secretion in mouse pancreatic alpha-cells results from activation of L-type calcium channels and elevation of cytoplasmic calcium

We have investigated the short-term effects of the saturated free fatty acid (FFA) palmitate on pancreatic alpha-cells. Palmitate (0.5 or 1 mmol/l bound to fatty acid-free albumin) stimulated glucagon secretion from intact mouse islets 1.5- to 2-fold when added in the presence of 1-15 mmol/l glucose. Palmitate remained stimulatory in islets depolarized with 30 mmol/l extracellular K(+) or exposed to forskolin, but it did not remain stimulatory after treatment with isradipine or triacsin C. The stimulatory action of palmitate on secretion correlated with a 3.5-fold elevation of intracellular free Ca(2+) when applied in the presence of 15 mmol/l glucose, a 40% stimulation of exocytosis (measured as increases in cell capacitance), and a 25% increase in whole-cell Ca(2+) current. The latter effect was abolished by isradipine, suggesting that palmitate selectively modulates l-type Ca(2+) channels. The effect of palmitate on exocytosis was not mediated by palmitoyl-CoA, and intracellular application of this FFA metabolite decreased rather than enhanced Ca(2+)-induced exocytosis. The stimulatory effects of palmitate on glucagon secretion were paralleled by a approximately 50% inhibition of somatostatin release. We conclude that palmitate increases alpha-cell exocytosis principally by enhanced Ca(2+) entry via l-type Ca(2+) channels and, possibly, relief from paracrine inhibition by somatostatin released by neighboring delta-cells.