Changes of fatty acid composition in incubated rat pancreatic islets

Changes in pancreatic histology, insulin secretion and oxidative status in diabetic rats following treatment with Ficus deltoidea and vitexin

BACKGROUND

The potential application of Ficus deltoidea and vitexin for the management of symptomatologies associated with diabetes mellitus (DM) has gained much attention. However, less firm evidence comes from data to augment our understanding of the role of F. deltoidea and vitexin in protecting pancreatic β-cells. The aim of this study was to assess histological and oxidative stress changes in the pancreas of streptozotocin (STZ)-induced diabetic rats following F. deltoidea extract and vitexin treatment.

METHODS

F. deltoidea and vitexin was administrated orally to six-weeks STZ-induced diabetic rats over 8 weeks period. The glucose and insulin tolerances were assessed by intraperitoneal glucose (2 g/kg) tolerance test (IPGTT) and intraperitoneal insulin (0.65 U/kg) tolerance test (IPITT), respectively. Subsequently, insulin resistance was assessed by homeostasis assessment model of insulin resistance (HOMA-IR), quantitative insulin sensitivity check index (QUICKI) and the insulin/triglyceride-derived McAuley index. The histological changes in the pancreas were then observed by hematoxylin-eosin (H&E) staining. Further, the pattern of fatty acid composition and infrared (IR) spectra of the serum and pancreas were monitored by gas chromatography (GC) method and Fourier Transform Infrared (FT-IR) spectroscopy.

RESULTS

F. deltoidea and vitexin increased pancreatic antioxidant enzymes and promoted islet regeneration. However, a significant increase in insulin secretion was observed only in rats treated with F. deltoidea. More importantly, reduction of fasting blood glucose is consistent with reduced FT-IR peaks at 1200-1000 cm^-1.

CONCLUSIONS

These results accentuate that F. deltoidea and vitexin could be a potential agent to attenuate pancreatic oxidative damage and advocate their therapeutic potential for treating DM.

Metabolic fate of glucose and candidate signaling and excess-fuel detoxification pathways in pancreatic β-cells

Glucose metabolism promotes insulin secretion in β-cells via metabolic coupling factors that are incompletely defined. Moreover, chronically elevated glucose causes β-cell dysfunction, but little is known about how cells handle excess fuels to avoid toxicity. Here we sought to determine which among the candidate pathways and coupling factors best correlates with glucose-stimulated insulin secretion (GSIS), define the fate of glucose in the β-cell, and identify pathways possibly involved in excess-fuel detoxification. We exposed isolated rat islets for 1 h to increasing glucose concentrations and measured various pathways and metabolites. Glucose oxidation, oxygen consumption, and ATP production correlated well with GSIS and saturated at 16 mm glucose. However, glucose utilization, glycerol release, triglyceride and glycogen contents, free fatty acid (FFA) content and release, and cholesterol and cholesterol esters increased linearly up to 25 mm glucose. Besides being oxidized, glucose was mainly metabolized via glycerol production and release and lipid synthesis (particularly FFA, triglycerides, and cholesterol), whereas glycogen production was comparatively low. Using targeted metabolomics in INS-1(832/13) cells, we found that several metabolites correlated well with GSIS, in particular some Krebs cycle intermediates, malonyl-CoA, and lower ADP levels. Glucose dose-dependently increased the dihydroxyacetone phosphate/glycerol 3-phosphate ratio in INS-1(832/13) cells, indicating a more oxidized state of NAD in the cytosol upon glucose stimulation. Overall, the data support a role for accelerated oxidative mitochondrial metabolism, anaplerosis, and malonyl-CoA/lipid signaling in β-cell metabolic signaling and suggest that a decrease in ADP levels is important in GSIS. The results also suggest that excess-fuel detoxification pathways in β-cells possibly comprise glycerol and FFA formation and release extracellularly and the diversion of glucose carbons to triglycerides and cholesterol esters.

Characterization of Acyl-CoA synthetase isoforms in pancreatic beta cells: Gene silencing shows participation of ACSL3 and ACSL4 in insulin secretion

Long-chain acyl-CoA synthetases (ACSLs) convert fatty acids to fatty acyl-CoAs to regulate various physiologic processes. We characterized the ACSL isoforms in a cell line of homogeneous rat beta cells (INS-1 832/13 cells) and human pancreatic islets. ACSL4 and ACSL3 proteins were present in the beta cells and human and rat pancreatic islets and concentrated in insulin secretory granules and less in mitochondria and negligible in other intracellular organelles. ACSL1 and ACSL6 proteins were not seen in INS-1 832/13 cells or pancreatic islets. ACSL5 protein was seen only in INS-1 832/13 cells. With shRNA-mediated gene silencing we developed stable ACSL knockdown cell lines from INS-1 832/13 cells. Glucose-stimulated insulin release was inhibited ∼50% with ACSL4 and ACSL3 knockdown and unaffected in cell lines with knockdown of ACSL5, ACLS6 and ACSL1. Lentivirus shRNA-mediated gene silencing of ACSL4 and ACSL3 in human pancreatic islets inhibited glucose-stimulated insulin release. ACSL4 and ACSL3 knockdown cells showed inhibition of ACSL enzyme activity more with arachidonate than with palmitate as a substrate, consistent with their preference for unsaturated fatty acids as substrates. ACSL4 knockdown changed the patterns of fatty acids in phosphatidylserines and phosphatidylethanolamines. The results show the involvement of ACLS4 and ACLS3 in insulin secretion.

Diets Containing α-Linolenic (ω3) or Oleic (ω9) Fatty Acids Rescues Obese Mice From Insulin Resistance

Subclinical systemic inflammation is a hallmark of obesity and insulin resistance. The results obtained from a number of experimental studies suggest that targeting different components of the inflammatory machinery may result in the improvement of the metabolic phenotype. Unsaturated fatty acids exert antiinflammatory activity through several distinct mechanisms. Here, we tested the capacity of ω3 and ω9 fatty acids, directly from their food matrix, to exert antiinflammatory activity through the G protein-coupled receptor (GPR)120 and GPR40 pathways. GPR120 was activated in liver, skeletal muscle, and adipose tissues, reverting inflammation and insulin resistance in obese mice. Part of this action was also mediated by GPR40 on muscle, as a novel mechanism described. Pair-feeding and immunoneutralization experiments reinforced the pivotal role of GPR120 as a mediator in the response to the nutrients. The improvement in insulin sensitivity in the high-fat substituted diets was associated with a marked reduction in tissue inflammation, decreased macrophage infiltration, and increased IL-10 levels. Furthermore, improved glucose homeostasis was accompanied by the reduced expression of hepatic gluconeogenic enzymes and reduced body mass. Thus, our data indicate that GPR120 and GPR40 play a critical role as mediators of the beneficial effects of dietary unsaturated fatty acids in the context of obesity-induced insulin resistance.

Metabolic signaling in fuel-induced insulin secretion

The pancreatic islet β cell senses circulating levels of calorigenic nutrients to secrete insulin according to the needs of the organism. Altered insulin secretion is linked to various disorders such as diabetes, hypoglycemic states, and cardiometabolic diseases. Fuel stimuli, including glucose, free fatty acids, and amino acids, promote insulin granule exocytosis primarily via their metabolism in β cells and the production of key signaling metabolites. This paper reviews our current knowledge of the pathways involved in both positive and negative metabolic signaling for insulin secretion and assesses the role of established and candidate metabolic coupling factors, keeping recent developments in focus.

Evidence for the involvement of GPR40 and NADPH oxidase in palmitic acid-induced superoxide production and insulin secretion

G protein coupled receptor 40 (GPR40) and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex have been shown to be involved in the fatty acid amplification of glucose-stimulated insulin secretion (GSIS). The effect of palmitic acid on superoxide production and insulin secretion by INS-1E cells and the possible involvement of GPR40 and NADPH oxidase in these processes were examined in this study. Cells were incubated during 1 h with palmitic acid in low and high glucose concentrations, a GPR40 agonist (GW9508) and inhibitors of NADPH oxidase (diphenyleneiodonium, DPI) and PKC (calphostin C). GW9508 induced superoxide production at 2.8 and 5.6 mM glucose concentrations and stimulated insulin secretion at 16.7 mM glucose concentration involving both PKC and NADPH oxidase activation. Palmitic acid induced superoxide production through NADPH oxidase and GPR40-dependent pathways and the stimulation of insulin secretion in the presence of a high glucose concentration was reduced by knockdown of GPR40 using siRNA. Our results suggest that palmitic acid induces superoxide production and potentiates GSIS through NADPH oxidase and GPR40 pathways in pancreatic ? cells.

Knockdown of pyruvate carboxylase or fatty acid synthase lowers numerous lipids and glucose-stimulated insulin release in insulinoma cells

We previously showed that knockdown of the anaplerotic enzyme pyruvate carboxylase in the INS-1 832/13 insulinoma cell line inhibited glucose-stimulated insulin release and glucose carbon incorporation into lipids. We now show that knockdown of fatty acid synthase (FAS) mRNA and protein also inhibits glucose-stimulated insulin release in this cell line. Levels of numerous phospholipids, cholesterol esters, diacylglycerol, triglycerides and individual fatty acids with C14-C24 side chains were acutely lowered about 20% in glucose-stimulated pyruvate carboxylase knockdown cells over a time course that coincides with insulin secretion. In FAS knockdown cells glucose carbon incorporation into lipids and the levels of the subclasses of phospholipids and cholesterol ester species were lower by 20-30% without inhibition of glucose oxidation. These studies suggest that rapid lipid modification is essential for normal glucose-stimulated insulin secretion.

Inhibition of hypothalamic inflammation reverses diet-induced insulin resistance in the liver

Defective liver gluconeogenesis is the main mechanism leading to fasting hyperglycemia in type 2 diabetes, and, in concert with steatosis, it is the hallmark of hepatic insulin resistance. Experimental obesity results, at least in part, from hypothalamic inflammation, which leads to leptin resistance and defective regulation of energy homeostasis. Pharmacological or genetic disruption of hypothalamic inflammation restores leptin sensitivity and reduces adiposity. Here, we evaluate the effect of a hypothalamic anti-inflammatory approach to regulating hepatic responsiveness to insulin. Obese rodents were treated by intracerebroventricular injections, with immunoneutralizing antibodies against Toll-like receptor (TLR)4 or tumor necrosis factor (TNF)α, and insulin signal transduction, hepatic steatosis, and gluconeogenesis were evaluated. The inhibition of either TLR4 or TNFα reduced hypothalamic inflammation, which was accompanied by the reduction of hypothalamic resistance to leptin and improved insulin signal transduction in the liver. This was accompanied by reduced liver steatosis and reduced hepatic expression of markers of steatosis. Furthermore, the inhibition of hypothalamic inflammation restored defective liver glucose production. All these beneficial effects were abrogated by vagotomy. Thus, the inhibition of hypothalamic inflammation in obesity results in improved hepatic insulin signal transduction, leading to reduced steatosis and reduced gluconeogenesis. All these effects are mediated by parasympathetic signals delivered by the vagus nerve.

Glycerolipid/free fatty acid cycle and islet β-cell function in health, obesity and diabetes

Pancreatic β-cells secrete insulin in response to fluctuations in blood fuel concentrations, in particular glucose and fatty acids. However, chronic fuel surfeit can overwhelm the metabolic, signaling and secretory capacity of the β-cell leading to its dysfunction and death - often referred to as glucolipotoxicity. In β-cells and many other cells, glucose and lipid metabolic pathways converge into a glycerolipid/free fatty acid (GL/FFA) cycle, which is driven by the substrates, glycerol-3-phosphate and fatty acyl-CoA, derived from glucose and fatty acids, respectively. Although the overall operation of GL/FFA cycle, consisting of lipolysis and lipogenesis, is "futile" in terms of energy expenditure, this metabolic cycle likely plays an indispensable role for various β-cell functions, in particular insulin secretion and excess fuel detoxification. In this review, we discuss the significance of GL/FFA cycle in the β-cell, its regulation and role in generating essential metabolic signals that participate in the lipid amplification arm of glucose stimulated insulin secretion and in β-cell growth. We propose the novel concept that the lipolytic segment of GL/FFA cycle is instrumental in producing signals for insulin secretion, whereas, the lipogenic segment generates signals relevant for β-cell survival/death and growth/proliferation.

Analysis of fatty acid composition in insulin secreting cells by comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry

A comprehensive two-dimensional gas chromatography (GC×GC) time-of-flight mass spectrometry method was developed for determination of fatty acids (irrespective of origin, i.e., both free fatty acids and fatty acids bound in sources such as triglycerides) in cultured mammalian cells. The method was applied to INS-1 cells, an insulin-secreting cell line commonly used as a model in diabetes studies. In the method, lipids were extracted and transformed to fatty acid methyl esters for analysis. GC×GC analysis revealed the presence of 30 identifiable fatty acids in the extract. This result doubles the number of fatty acids previously identified in these cells. The method yielded linear calibrations and an average relative standard deviation of 8.4% for replicate injections of samples and 12.4% for replicate analysis of different samples. The method was used to demonstrate changes in fatty acid content as a function of glucose concentration on the cells. These results demonstrate the utility of this method for analysis of fatty acids in mammalian cell cultures.

Unsaturated fatty acids revert diet-induced hypothalamic inflammation in obesity

BACKGROUND

In experimental models, hypothalamic inflammation is an early and determining factor in the installation and progression of obesity. Pharmacological and gene-based approaches have proven efficient in restraining inflammation and correcting the obese phenotypes. However, the role of nutrients in the modulation of hypothalamic inflammation is unknown.

METHODOLOGY/PRINCIPAL FINDINGS

Here we show that, in a mouse model of diet-induced obesity, partial substitution of the fatty acid component of the diet by flax seed oil (rich in C18:3) or olive oil (rich in C18:1) corrects hypothalamic inflammation, hypothalamic and whole body insulin resistance, and body adiposity. In addition, upon icv injection in obese rats, both ω3 and ω9 pure fatty acids reduce spontaneous food intake and body mass gain. These effects are accompanied by the reversal of functional and molecular hypothalamic resistance to leptin/insulin and increased POMC and CART expressions. In addition, both, ω3 and ω9 fatty acids inhibit the AMPK/ACC pathway and increase CPT1 and SCD1 expression in the hypothalamus. Finally, acute hypothalamic injection of ω3 and ω9 fatty acids activate signal transduction through the recently identified GPR120 unsaturated fatty acid receptor.

CONCLUSIONS/SIGNIFICANCE

Unsaturated fatty acids can act either as nutrients or directly in the hypothalamus, reverting diet-induced inflammation and reducing body adiposity. These data show that, in addition to pharmacological and genetic approaches, nutrients can also be attractive candidates for controlling hypothalamic inflammation in obesity.

NAD(P)H oxidase participates in the palmitate-induced superoxide production and insulin secretion by rat pancreatic islets

Nicotinamide adenine dinucleotide phosphate [NAD(P)H] oxidase complex has been shown to be involved in the process of glucose-stimulated insulin secretion (GSIS). In this study, we examined the effect of palmitic acid on superoxide production and insulin secretion by rat pancreatic islets and the mechanism involved. Rat pancreatic islets were incubated during 1 h with 1 mM palmitate, 1% fatty acid free-albumin, 5.6 or 10 mM glucose and in the presence of inhibitors of NAD(P)H oxidase (DPI--diphenyleneiodonium), PKC (calphostin C) and carnitine palmitoyl transferase-I (CPT-I) (etomoxir). Superoxide content was determined by hydroethidine assays. Palmitate increased superoxide production in the presence of 5.6 and 10 mM glucose. This effect was dependent on activation of PKC and NAD(P)H oxidase. Palmitic acid oxidation was demonstrated to contribute for the fatty acid induction of superoxide production in the presence of 5.6 mM glucose. In fact, palmitate caused p47(PHOX) translocation to plasma membrane, as shown by immunohistochemistry. Exposure to palmitate for 1 h up-regulated the protein content of p47(PHOX) and the mRNA levels of p22(PHOX), gp91(PHOX), p47(PHOX), proinsulin and the G protein-coupled receptor 40 (GPR40). Fatty acid stimulation of insulin secretion in the presence of high glucose concentration was reduced by inhibition of NAD(P)H oxidase activity. In conclusion, NAD(P)H oxidase is an important source of superoxide in pancreatic islets and the activity of NAD(P)H oxidase is involved in the control of insulin secretion by palmitate.

Palmitate activates insulin signaling pathway in pancreatic rat islets

OBJECTIVE

To investigate the action of palmitate on insulin receptor (IR) signaling pathway in rat pancreatic islets. The following proteins were studied: IR substrate-1 and -2 (IRS1 and IRS2), phosphatidylinositol 3-kinase, extracellular signal-regulated protein kinase-1 and -2 (ERK1/2), and signal transducer and activator of transcription 3 (STAT3).

METHODS

Immunoblotting and immunoprecipitation assays were used to evaluate the phosphorylation states of IRS1 and IRS2 (tyrosine [Tyr]), ERK1/2 (threonine 202 [Thr202]/Tyr204), and STAT3 (serine [Ser727]).

RESULTS

The exposure of rat pancreatic islets to 0.1-mmol/L palmitate for up to 30 minutes produced a significant increase of Tyr phosphorylation in IRS2 but not in IRS1. The association of phosphatidylinositol 3-kinase with IRS2 was also upregulated by palmitate. Exposure to 5.6-mmol/L glucose caused a gradual decrease in ERK1/2 (Thr202/Tyr204) and STAT3 (serine [Ser727]) phosphorylations after 30-minute incubation. The addition of palmitate (0.1 mmol/L), associated with 5.6-mmol/L glucose, abolished these latter effects of glucose after 15-minute incubation.

CONCLUSIONS

Palmitate at physiological concentration associated with 5.6-mmol/L glucose activates IR signaling pathway in pancreatic beta cells.

Saturated fatty acids produce an inflammatory response predominantly through the activation of TLR4 signaling in hypothalamus: implications for the pathogenesis of obesity

In animal models of diet-induced obesity, the activation of an inflammatory response in the hypothalamus produces molecular and functional resistance to the anorexigenic hormones insulin and leptin. The primary events triggered by dietary fats that ultimately lead to hypothalamic cytokine expression and inflammatory signaling are unknown. Here, we test the hypothesis that dietary fats act through the activation of toll-like receptors 2/4 and endoplasmic reticulum stress to induce cytokine expression in the hypothalamus of rodents. According to our results, long-chain saturated fatty acids activate predominantly toll-like receptor 4 signaling, which determines not only the induction of local cytokine expression but also promotes endoplasmic reticulum stress. Rats fed on a monounsaturated fat-rich diet do not develop hypothalamic leptin resistance, whereas toll-like receptor 4 loss-of-function mutation and immunopharmacological inhibition of toll-like receptor 4 protects mice from diet-induced obesity. Thus, toll-like receptor 4 acts as a predominant molecular target for saturated fatty acids in the hypothalamus, triggering the intracellular signaling network that induces an inflammatory response, and determines the resistance to anorexigenic signals.

Glycerolipid metabolism and signaling in health and disease

Maintenance of body temperature is achieved partly by modulating lipolysis by a network of complex regulatory mechanisms. Lipolysis is an integral part of the glycerolipid/free fatty acid (GL/FFA) cycle, which is the focus of this review, and we discuss the significance of this pathway in the regulation of many physiological processes besides thermogenesis. GL/FFA cycle is referred to as a "futile" cycle because it involves continuous formation and hydrolysis of GL with the release of heat, at the expense of ATP. However, we present evidence underscoring the "vital" cellular signaling roles of the GL/FFA cycle for many biological processes. Probably because of its importance in many cellular functions, GL/FFA cycling is under stringent control and is organized as several composite short substrate/product cycles where forward and backward reactions are catalyzed by separate enzymes. We believe that the renaissance of the GL/FFA cycle is timely, considering the emerging view that many of the neutral lipids are in fact key signaling molecules whose production is closely linked to GL/FFA cycling processes. The evidence supporting the view that alterations in GL/FFA cycling are involved in the pathogenesis of "fatal" conditions such as obesity, type 2 diabetes, and cancer is discussed. We also review the different enzymatic and transport steps that encompass the GL/FFA cycle leading to the generation of several metabolic signals possibly implicated in the regulation of biological processes ranging from energy homeostasis, insulin secretion and appetite control to aging and longevity. Finally, we present a perspective of the possible therapeutic implications of targeting this cycling.

The islet beta-cell: fuel responsive and vulnerable

The pancreatic beta-cell senses blood nutrient levels and is modulated by neurohormonal signals so that it secretes insulin according to the need of the organism. Nutrient sensing involves marked metabolic activation, resulting in the production of coupling signals that promote insulin biosynthesis and secretion. The beta-cell's high capacity for nutrient sensing, however, necessitates reduced protection to nutrient toxicity. This potentially explains why in susceptible individuals, chronic fuel surfeit results in beta-cell failure and type 2 diabetes. Here we discuss recent insights into first, the biochemical basis of beta-cell signaling in response to glucose, amino acids and fatty acids, and second, beta-cell nutrient detoxification. We emphasize the emerging role of glycerolipid/fatty acid cycling in these processes.

The role of rapid lipogenesis in insulin secretion: Insulin secretagogues acutely alter lipid composition of INS-1 832/13 cells

Pancreatic beta cell mitochondria convert insulin secretagogues into products that support insulin exocytosis. We explored the idea that lipids are some of these products formed from acyl group transfer out of mitochondria to the cytosol, the site of lipid synthesis. There are two isoforms of acetyl-CoA carboxylase, the enzyme that forms malonyl-CoA from which C(2) units for lipid synthesis are formed. We found that ACC1, the isoform seen in lipogenic tissues, is the only isoform present in human and rat pancreatic islets and INS-1 832/13 cells. Inhibitors of ACC and fatty acid synthase inhibited insulin release in islets and INS-1 cells. Carbon from glucose and pyruvate were rapidly incorporated into many lipid classes in INS-1 cells. Glucose and other insulin secretagogues acutely increased many lipids with C14-C24 chains including individual cholesterol esters, phospholipids and fatty acids. Many phosphatidylcholines and phosphatidylserines were increased and many phosphatidylinositols and several phosphatidylethanolamines were decreased. The results suggest that lipid remodeling and rapid lipogenesis from secretagogue carbon support insulin secretion.

Propionate inhibits glucose-induced insulin secretion in isolated rat pancreatic islets

Dietary fibers, probably by generating short chain fatty acids (SCFA) through enterobacterial fermentation, have a beneficial effect on the control of glycemia in patients with peripheral insulin resistance. We studied the effect of propionate on glucose-induced insulin secretion in isolated rat pancreatic islets. Evidence is presented that propionate, one of the major SCFA produced in the gut, inhibits insulin secretion induced by high glucose concentrations (11.1 and 16.7 mM) in incubated and perfused pancreatic islets. This short chain fatty acid reduces [U-(14)C]-glucose decarboxylation and raises the conversion of glucose to lactate. Propionate causes a significant decrease of both [1-(14)C]- (84%) and [2-(14)C]-pyruvate (49%) decarboxylation. These findings indicate pyruvate dehydrogenase as the major site for the propionate effect. These observations led us to postulate that the reduction in glucose oxidation and the consequent decrease in the ATP/ADP ratio may be the major mechanism for the lower insulin secretion to glucose stimulus induced by propionate.

Fatty acid signaling in the beta-cell and insulin secretion

Fatty acids (FAs) and other lipid molecules are important for many cellular functions, including vesicle exocytosis. For the pancreatic beta-cell, while the presence of some FAs is essential for glucose-stimulated insulin secretion, FAs have enormous capacity to amplify glucose-stimulated insulin secretion, which is particularly operative in situations of beta-cell compensation for insulin resistance. In this review, we propose that FAs do this via three interdependent processes, which we have assigned to a "trident model" of beta-cell lipid signaling. The first two arms of the model implicate intracellular metabolism of FAs, whereas the third is related to membrane free fatty acid receptor (FFAR) activation. The first arm involves the AMP-activated protein kinase/malonyl-CoA/long-chain acyl-CoA (LC-CoA) signaling network in which glucose, together with other anaplerotic fuels, increases cytosolic malonyl-CoA, which inhibits FA partitioning into oxidation, thus increasing the availability of LC-CoA for signaling purposes. The second involves glucose-responsive triglyceride (TG)/free fatty acid (FFA) cycling. In this pathway, glucose promotes LC-CoA esterification to complex lipids such as TG and diacylglycerol, concomitant with glucose stimulation of lipolysis of the esterification products, with renewal of the intracellular FFA pool for reactivation to LC-CoA. The third arm involves FFA stimulation of the G-protein-coupled receptor GPR40/FFAR1, which results in enhancement of glucose-stimulated accumulation of cytosolic Ca2+ and consequently insulin secretion. It is possible that FFA released by the lipolysis arm of TG/FFA cycling is partly "secreted" and, via an autocrine/paracrine mechanism, is additive to exogenous FFAs in activating the FFAR1 pathway. Glucose-stimulated release of arachidonic acid from phospholipids by calcium-independent phospholipase A2 and/or from TG/FFA cycling may also be involved. Improved knowledge of lipid signaling in the beta-cell will allow a better understanding of the mechanisms of beta-cell compensation and failure in diabetes.

Life and death decisions of the pancreatic β-cell: the role of fatty acids

Both stimulatory and detrimental effects of NEFAs (non-esterified fatty acids) on pancreatic β-cells have been recognized. Acute exposure of the pancreatic β-cell to high glucose concentrations and/or saturated NEFAs results in a substantial increase in insulin release, whereas chronic exposure results in desensitization and suppression of secretion, followed by induction of apoptosis. Some unsaturated NEFAs also promote insulin release acutely, but they are less toxic to β-cells during chronic exposure and can even exert positive protective effects. Therefore changes in the levels of NEFAs are likely to be important for the regulation of β-cell function and viability under physiological conditions. In addition, the switching between endogenous fatty acid synthesis or oxidation in the β-cell, together with alterations in neutral lipid accumulation, may have critical implications for β-cell function and integrity. Long-chain acyl-CoA (formed from either endogenously synthesized or exogenous fatty acids) controls several aspects of β-cell function, including activation of specific isoenzymes of PKC (protein kinase C), modulation of ion channels, protein acylation, ceramide formation and/or NO-mediated apoptosis, and transcription factor activity. In this review, we describe the effects of exogenous and endogenous fatty acids on β-cell metabolism and gene and protein expression, and have explored the outcomes with respect to insulin secretion and β-cell integrity.

Fatty acid-induced toxicity and neutral lipid accumulation in insulin-producing RINm5F cells

Fatty acids have been shown to cause death of rat and human primary pancreatic beta cells and of insulin-producing cell lines. These studies focused mainly on saturated and monounsaturated FA such as palmitic, stearic and oleic acids. In this study, we have performed a comparison of the toxicity of a wider range of FA. The toxicity of different FA to insulin-producing RINm5F cells was assessed by flow cytometry measuring loss of plasma membrane integrity and increase in DNA fragmentation. Additionally, the FA induced neutral lipid accumulation and the FA composition were determined. Palmitic, linoleic, gamma-linolenic, oleic, stearic, and eicosapentaenoic acid caused DNA fragmentation of insulin-producing RINm5F cells. Loss of membrane integrity was mainly caused by linoleic and gamma-linolenic acid. There was no correlation between cytotoxicity and the abundance of the FA in the cells as determined by HPLC analysis. Taken as whole, the toxic effect of the FA on insulin-producing RINm5F cells varied irrespective of the chain length and the degree of unsaturation. In these cells PA and LA exhibited the highest toxicity, whereas AA was not toxic. In addition, the toxicity of most tested FA was inversely related to low NLA, except for AA and EPA. The results of this study contribute to the understanding of the role of FA in the impairment of pancreatic beta cell function that occurs in type 2 diabetes and obesity.

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.