Arachidonic acid metabolite regulation of insulin secretion

The prostaglandin E2 EP3 receptor has disparate effects on islet insulin secretion and content in β-cells in a high-fat diet-induced mouse model of obesity

Signaling through prostaglandin E2 EP3 receptor (EP3) actively contributes to the β-cell dysfunction of type 2 diabetes (T2D). In T2D models, full-body EP3 knockout mice have a significantly worse metabolic phenotype than wild-type controls due to hyperphagia and severe insulin resistance resulting from loss of EP3 in extra-pancreatic tissues, masking any potential beneficial effects of EP3 loss in the β cell. We hypothesized β-cell-specific EP3 knockout (EP3 βKO) mice would be protected from high-fat diet (HFD)-induced glucose intolerance, phenocopying mice lacking the EP3 effector, Gαz, which is much more limited in its tissue distribution. When fed a HFD for 16 wk, though, EP3 βKO mice were partially, but not fully, protected from glucose intolerance. In addition, exendin-4, an analog of the incretin hormone, glucagon-like peptide 1, more strongly potentiated glucose-stimulated insulin secretion in islets from both control diet- and HFD-fed EP3 βKO mice as compared with wild-type controls, with no effect of β-cell-specific EP3 loss on islet insulin content or markers of replication and survival. However, after 26 wk of diet feeding, islets from both control diet- and HFD-fed EP3 βKO mice secreted significantly less insulin as a percent of content in response to stimulatory glucose, with or without exendin-4, with elevated total insulin content unrelated to markers of β-cell replication and survival, revealing severe β-cell dysfunction. Our results suggest that EP3 serves a critical role in temporally regulating β-cell function along the progression to T2D and that there exist Gαz-independent mechanisms behind its effects.NEW & NOTEWORTHY The EP3 receptor is a strong inhibitor of β-cell function and replication, suggesting it as a potential therapeutic target for the disease. Yet, EP3 has protective roles in extrapancreatic tissues. To address this, we designed β-cell-specific EP3 knockout mice and subjected them to high-fat diet feeding to induce glucose intolerance. The negative metabolic phenotype of full-body knockout mice was ablated, and EP3 loss improved glucose tolerance, with converse effects on islet insulin secretion and content.

Rat prostaglandin EP3 receptor is highly promiscuous and is the sole prostanoid receptor family member that regulates INS-1 (832/3) cell glucose-stimulated insulin secretion

Chronic elevations in fatty acid metabolites termed prostaglandins can be found in circulation and in pancreatic islets from mice or humans with diabetes and have been suggested as contributing to the β‐cell dysfunction of the disease. Two‐series prostaglandins bind to a family of G‐protein‐coupled receptors, each with different biochemical and pharmacological properties. Prostaglandin E receptor (EP) subfamily agonists and antagonists have been shown to influence β‐cell insulin secretion, replication, and/or survival. Here, we define EP3 as the sole prostanoid receptor family member expressed in a rat β‐cell‐derived line that regulates glucose‐stimulated insulin secretion. Several other agonists classically understood as selective for other prostanoid receptor family members also reduce glucose‐stimulated insulin secretion, but these effects are only observed at relatively high concentrations, and, using a well‐characterized EP3‐specific antagonist, are mediated solely by cross‐reactivity with rat EP3. Our findings confirm the critical role of EP3 in regulating β‐cell function, but are also of general interest, as many agonists supposedly selective for other prostanoid receptor family members are also full and efficacious agonists of EP3. Therefore, care must be taken when interpreting experimental results from cells or cell lines that also express EP3.

The Eicosanoids, Redox-Regulated Lipid Mediators in Immunometabolic Disorders

SIGNIFICANCE

The oxidation of arachidonic acid via cyclooxygenase (COX) and lipoxygenase (LOX) activity to produce eicosanoids during inflammation is a well-known biosynthetic pathway. These lipid mediators are involved in fever, pain, and thrombosis and are produced from multiple cells as well as cell/cell interactions, for example, immune cells and epithelial/endothelial cells. Metabolic disorders, including hyperlipidemia, hypertension, and diabetes, are linked with chronic low-grade inflammation, impacting the immune system and promoting a variety of chronic diseases. Recent Advances: Multiple studies have corroborated the important function of eicosanoids and their receptors in (non)-inflammatory cells in immunometabolic disorders (e.g., insulin resistance, obesity, and cardiovascular and nonalcoholic fatty liver diseases). In this context, LOX and COX products are involved in both pro- and anti-inflammatory responses. In addition, recent work has elucidated the potent function of specialized proresolving mediators (i.e., lipoxins and resolvins) in resolving inflammation, protecting organs, and stimulating tissue repair and remodeling.

CRITICAL ISSUES

Inhibiting/stimulating selected eicosanoid pathways may result in anti-inflammatory and proresolution responses leading to multiple beneficial effects, including the abrogation of reactive oxygen species production, increased speed of resolution, and overall improvement of diseases related to immunometabolic perturbations.

FUTURE DIRECTIONS

Despite many achievements, it is crucial to understand the molecular and cellular mechanisms underlying immunological/metabolic cross talk to offer substantial therapeutic promise. Antioxid. Redox Signal. 29, 275-296.

Comparison of the efficacy of sitagliptin and glimepiride dose-up in Japanese patients with type 2 diabetes poorly controlled by sitagliptin and glimepiride in combination

Aims/Introduction

The goal of the study was to examine the effects of sitagliptin dose‐up or glimepiride dose‐up in Japanese patients with type 2 diabetes who were controlled inadequately by sitagliptin and glimepiride in combination.

Materials and Methods

A multicenter, prospective, randomized, open‐label study was carried out in 50 patients with type 2 diabetes treated with sitagliptin and low‐dose glimepiride. The patients were randomly assigned to receive the addition of 50 mg/day sitagliptin or 0.5 mg/day glimepiride. The primary end‐point was the percentage change in glycated hemoglobin (HbA1c).

Results

During a follow‐up period, the difference in the percentage changes in HbA1c between the two groups was not significant (P = 0.13). However, HbA1c was significantly decreased by glimepiride dose‐up (P < 0.01 vs baseline), but not by sitagliptin dose‐up (P = 0.74). Univariate linear regression analyses showed that the percentage change in HbA1c was significantly associated with the serum level of arachidonic acid (AA) in both groups.

Conclusions

There was no significant difference in the HbA1c‐lowering effects between the two groups. However, a significant HbA1c‐lowering effect from baseline of glimepiride dose‐up was found, and the AA level showed a negative correlation with the decrease in HbA1c in the sitagliptin dose‐up group, but a positive correlation in the glimepiride dose‐up group. These findings suggest that the AA level is associated with HbA1c reduction in response to dose‐up with these drugs in patients with type 2 diabetes in a combination therapy with sitagliptin and glimepiride. This trial was registered with UMIN (no. 000009544).

Overexpression of glutathione peroxidase 4 prevents β-cell dysfunction induced by prolonged elevation of lipids in vivo

We have shown that oxidative stress is a mechanism of free fatty acid (FFA)-induced β-cell dysfunction. Unsaturated fatty acids in membranes, including plasma and mitochondrial membranes, are substrates for lipid peroxidation, and lipid peroxidation products are known to cause impaired insulin secretion. Therefore, we hypothesized that mice overexpressing glutathione peroxidase-4 (GPx4), an enzyme that specifically reduces lipid peroxides, are protected from fat-induced β-cell dysfunction. GPx4-overexpressing mice and their wild-type littermate controls were infused intravenously with saline or oleate for 48 h, after which reactive oxygen species (ROS) were imaged, using dihydrodichlorofluorescein diacetate in isolated islets, and β-cell function was assessed ex vivo in isolated islets and in vivo during hyperglycemic clamps. Forty-eight-hour FFA elevation in wild-type mice increased ROS and the lipid peroxidation product malondialdehyde and impaired β-cell function ex vivo in isolated islets and in vivo, as assessed by decreased disposition index. Also, islets of wild-type mice exposed to oleate for 48 h had increased ROS and lipid peroxides and decreased β-cell function. In contrast, GPx4-overexpressing mice showed no FFA-induced increase in ROS and lipid peroxidation and were protected from the FFA-induced impairment of β-cell function assessed in vitro, ex vivo and in vivo. These results implicate lipid peroxidation in FFA-induced β-cell dysfunction.

Cyclooxygenase-2, not microsomal prostaglandin E synthase-1, is the mechanism for interleukin-1β-induced prostaglandin E2 production and inhibition of insulin secretion in pancreatic islets

Arachidonic acid is converted to prostaglandin E(2) (PGE(2)) by a sequential enzymatic reaction performed by two isoenzyme groups, cyclooxygenases (COX-1 and COX-2) and terminal prostaglandin E synthases (cPGES, mPGES-1, and mPGES-2). mPGES-1 is widely considered to be the final enzyme regulating COX-2-dependent PGE(2) synthesis. These generalizations have been based in most part on experiments utilizing gene expression analyses of cell lines and tumor tissue. To assess the relevance of these generalizations to a native mammalian tissue, we used isolated human and rodent pancreatic islets to examine interleukin (IL)-1β-induced PGE(2) production, because PGE(2) has been shown to mediate IL-1β inhibition of islet function. Rat islets constitutively expressed mRNAs of COX-1, COX-2, cPGES, and mPGES-1. As expected, IL-1β increased mRNA levels for COX-2 and mPGES-1, but not for COX-1 or cPGES. Basal protein levels of COX-1, cPGES, and mPGES-2 were readily detected in whole cell extracts but were not regulated by IL-1β. IL-1β increased protein levels of COX-2, but unexpectedly mPGES-1 protein levels were low and unaffected. In microsomal extracts, mPGES-1 protein was barely detectable in rat islets but clearly present in human islets; however, in neither case did IL-1β increase mPGES-1 protein levels. To further assess the importance of mPGES-1 to IL-1β regulation of an islet physiologic response, glucose-stimulated insulin secretion was examined in isolated islets of WT and mPGES-1-deficient mice. IL-1β inhibited glucose-stimulated insulin secretion equally in both WT and mPGES-1(-/-) islets, indicating that COX-2, not mPGES-1, mediates IL-1β-induced PGE(2) production and subsequent inhibition of insulin secretion.

Mechanism of prostacyclin-induced potentiation of glucose-induced insulin secretion

Arachidonic acid metabolites are crucial mediators of inflammation in diabetes. Although eicosanoids are established modulators of pancreatic β-cell function, the role of prostacyclin (prostaglandin I2) is unknown. Therefore, this study aimed to analyze the role of prostacyclin in β-cell function. Prostacyclin synthase (PGIS) was weakly expressed in rat islet cells but nevertheless significantly increased by incubation with 30 mM glucose, especially in non-β-cells. PGIS was overexpressed in INS1E cells, and the regulation of insulin secretion was analyzed. PGIS overexpression strongly potentiated glucose-induced insulin secretion along with increased insulin content and ATP production. Importantly, overexpression of PGIS potentiated only nutrient-induced insulin secretion. The effect of PGIS overexpression was mediated by prostacyclin released from insulin-secreting cells and dependent on prostacyclin receptor (IP receptor) activation, with concomitant cAMP production. The cAMP-mediated potentiation of glucose-induced insulin secretion by prostacyclin was independent of the protein kinase A pathway but strongly attenuated by the knockdown of the exchange protein directly activated by cAMP 2 (Epac2), pointing to a crucial role for Epac2 in this process. Thus, prostacyclin is a powerful potentiator of glucose-induced insulin secretion. It improves the secretory capacity by inducing insulin biosynthesis and probably by stimulating exocytosis. Our findings open a new therapeutical perspective for an improved treatment of type 2 diabetes.

Concordance of changes in metabolic pathways based on plasma metabolomics and skeletal muscle transcriptomics in type 1 diabetes

Insulin regulates many cellular processes, but the full impact of insulin deficiency on cellular functions remains to be defined. Applying a mass spectrometry-based nontargeted metabolomics approach, we report here alterations of 330 plasma metabolites representing 33 metabolic pathways during an 8-h insulin deprivation in type 1 diabetic individuals. These pathways included those known to be affected by insulin such as glucose, amino acid and lipid metabolism, Krebs cycle, and immune responses and those hitherto unknown to be altered including prostaglandin, arachidonic acid, leukotrienes, neurotransmitters, nucleotides, and anti-inflammatory responses. A significant concordance of metabolome and skeletal muscle transcriptome-based pathways supports an assumption that plasma metabolites are chemical fingerprints of cellular events. Although insulin treatment normalized plasma glucose and many other metabolites, there were 71 metabolites and 24 pathways that differed between nondiabetes and insulin-treated type 1 diabetes. Confirmation of many known pathways altered by insulin using a single blood test offers confidence in the current approach. Future research needs to be focused on newly discovered pathways affected by insulin deficiency and systemic insulin treatment to determine whether they contribute to the high morbidity and mortality in T1D despite insulin treatment.

Eicosanoids, β-cell function, and diabetes

Arachidonic acid (AA) is metabolized by cyclooxygenase (COX), lipoxygenase (LOX), and cytochrome P450 (CYP) enzymes into eicosanoids, which are involved in diverse diseases, including type 1 and type 2 diabetes. During the last 30 years, evidence has been accumulated that suggests important functions for eicosanoids in the control of pancreatic β-cell function and destruction. AA metabolites of the COX pathway, especially prostaglandin E(2) (PGE(2)), appear to be significant factors to β-cell dysfunction and destruction, participating in the pathogenesis of diabetes and its complications. Several elegant studies have contributed to the sorting out of the importance of 12-LOX eicosanoids in cytokine-mediated inflammation in pancreatic β cells. The role of CYP eicosanoids in diabetes is yet to be explored. A recent publication has demonstrated that stabilizing the levels of epoxyeicosatrienoic acids (EETs), CYP eicosanoids, by inhibiting or deleting soluble epoxide hydrolase (sEH) improves β-cell function and reduces β-cell apoptosis in diabetes. In this review we summarize recent findings implicating these eicosanoid pathways in diabetes and its complications. We also discuss the development of animal models with targeted gene deletion and specific enzymatic inhibitors in each pathway to identify potential targets for the treatment of diabetes and its complications.

Arachidonic acid fights palmitate: new insights into fatty acid toxicity in β-cells

Saturated fatty acids are toxic to pancreatic β-cells. By inducing apoptosis, they contribute to a decrease in β-cell mass, a hallmark of Type 2 diabetes. In the present issue of Clinical Science, Keane and co-workers show that the polyunsaturated fatty acid arachidonic acid protects the β-cell against the toxic effects of palmitate. As Type 2 diabetes is characterized by subclinical inflammation, and arachidonic acid and metabolites thereof are produced during states of inflammation, it is possible that pancreatic β-cells use arachidonic acid as a compound for self-protection.

Glutathione peroxidase protein expression and activity in human islets isolated for transplantation

BACKGROUND

Overexpression of antioxidant enzymes has been reported to protect rodent beta cells from oxidative stress. However, very little is known about protein expression and activity of antioxidant enzymes in human islets.

METHOD/RESULTS

Human islet protein levels by Western analysis and enzymatic activity for the key antioxidant enzymes superoxide dismutases (SODs), catalase, and glutathione peroxidase-1 (GPx) were examined. Enzyme protein expression and activity were in the order SODs > catalase > GPx. Human islet GPx protein expression was significantly less than that found for catalase (p < 0.0001) and levels of GPx activity were virtually undetectable. As glucose and estrogens have been proposed to alter antioxidant enzyme levels, we examined islet data from male and female donors separately and under varying glucose concentrations. We found significantly less (p < 0.001) GPx protein expression in islets from females compared to males, but no significant regulation by glucose in either gender.

CONCLUSIONS

Human islets have very low protein and activity levels for GPx, the essential enzyme for protection against excessive levels of intracellular lipid peroxides. GPx mimetics may be especially valuable in providing human islets with the broadest spectrum of protection against oxidative stress during isolation and transplantation.

Role of the renin-angiotensin system in the endocrine pancreas: implications for the development of diabetes

Activation of the renin-angiotensin system has a pivotal role in the pathogenesis of diabetic complications. However, recent evidence suggests that it may also contribute to the development of diabetes itself. In the endocrine pancreas, all the components of an active renin-angiotensin system are present, which modulate a range of activities including local blood flow, hormone release and prostaglandin synthesis. In both types 1 and 2 diabetes, there is an up-regulation of its expression and activity in the endocrine pancreas. Whether these changes have a direct pathogenetic role or reflect a response to local stress or tissue injury remains to be established. Angiotensin-mediated increases in oxidative stress, inflammation and free fatty acids levels potentially contribute to beta-cell dysfunction in diabetes. In addition, activation of the renin-angiotensin system appears to potentiate the action of other pathogenic pathways including glucotoxicity, lipotoxicity and advanced glycation. In experimental models of type 2 diabetes, blockade of the renin-angiotensin system with angiotensin converting enzyme inhibitors or angiotensin receptor antagonists results in the improvement of islet structure and function. Moreover, the incidence of de novo diabetes appears to be significantly reduced by blockade of the renin-angiotensin system in clinical studies. At least two large controlled trials are currently underway to study the role of renin-angiotensin system in the development of diabetes. It is hoped that these studies will demonstrate the true potential of the blockade of the renin-angiotensin system for the prevention of diabetes.

Role of cyclooxygenase-2 in cytokine-induced beta-cell dysfunction and damage by isolated rat and human islets

Type I diabetes mellitus is an autoimmune disease characterized by the selective destruction of the insulin-secreting beta-cell found in pancreatic islets of Langerhans. Cytokines such as interleukin-1 (IL-1), interferon-gamma (IFN-gamma), and tumor necrosis factor-alpha (TNF-alpha) mediate beta-cell dysfunction and islet degeneration, in part, through the induction of the inducible isoform of nitric-oxide synthase and the production of nitric oxide by beta-cells. Cytokines also stimulate the expression of the inducible isoform of cyclooxygenase, COX-2, and the production of prostaglandin E(2) (PGE(2)) by rat and human islets; however, the role of increased COX-2 expression and PGE(2) production in mediating cytokine-induced inhibition of islet metabolic function and viability has been incompletely characterized. In this study, we have shown that treatment of rat islets with IL-1beta or human islets with a cytokine mixture containing IL-1beta + IFN-gamma +/- TNF-alpha stimulates COX-2 expression and PGE(2) formation in a time-dependent manner. Co-incubation of rat and human islets with selective COX-2 inhibitors SC-58236 and Celecoxib, respectively, attenuated cytokine-induced PGE(2) formation. However, these inhibitors failed to prevent cytokine-mediated inhibition of insulin secretion or islet degeneration. These findings indicate that selective inhibition of COX-2 activity does not protect rat and human islets from cytokine-induced beta-cell dysfunction and islet degeneration and, furthermore, that islet production of PGE(2) does not mediate these inhibitory and destructive effects.

Cyclooxygenase-2 is overexpressed in chronic pancreatitis

INTRODUCTION

Cyclooxygenase enzymes catalyze a critical step in the conversion of arachidonic acid to prostaglandins, which are important mediators of acute and chronic inflammation. The constitutively expressed cyclooxygenase-1 (COX-1) appears to regulate many normal physiologic functions in several cell types, whereas the inducible cyclooxygenase-2 (COX-2) enzyme mediates the inflammatory response.

AIMS AND METHODOLOGY

We investigated the expression of COX-2 in tissues of 35 patients with chronic pancreatitis, 6 patients with pancreatic cancer, and 5 control patients by immunohistochemical analysis and correlations to clinicopathologic features.

RESULTS

We found an overexpression of COX-2 in the atrophic acinar cells (80% of patients), hyperplastic ductal cells (86% of patients), and islets cells (97% of patients) but not in normal pancreatic tissues. The COX-2 overexpression in the tissue of patients with chronic pancreatitis was significantly correlated with the frequency of acute attacks of pancreatitis. Tissue from patients who had more than five acute attacks of pancreatitis (n = 10) exhibited COX-2 immunoreactivity of a significantly higher score in atrophic acinar cells (p = 0.004). No correlation could be found with other examined clinical features such as duration of the disease, diabetes, alcohol consumption, smoking, or pain.

CONCLUSION

Our results support the hypothesis that COX-2 may be involved in inflammatory responses in chronic pancreatitis and in the progression of this chronic inflammatory disease.

Inhibition of interleukin-1beta-induced COX-2 and EP3 gene expression by sodium salicylate enhances pancreatic islet beta-cell function

Previous work has suggested that functional interrelationships may exist between inhibition of insulin secretion by interleukin (IL)-1beta and the endogenous synthesis of prostaglandin E(2) (PGE(2)) in the pancreatic islet. These studies were performed to ascertain the relative abundance of E prostaglandin (EP) receptor mRNAs in tissues that are major targets, or major degradative sites, of insulin; to identify which EP receptor type mediates PGE(2) inhibition of insulin secretion in pancreatic islets; and to examine possible sites of action through which sodium salicylate might affect IL-1beta/PGE(2) interactions. Real-time fluorescence-based RT-PCR indicated that EP3 is the most abundant EP receptor type in islets, liver, kidney, and epididymal fat. EP3 mRNA is the least, whereas EP2 mRNA is the most, abundant type in skeletal muscle. Misoprostol, an EP3 agonist, inhibited glucose-induced insulin secretion from islets, an event that was prevented by preincubation with pertussis toxin, by decreasing cAMP. Electromobility shift assays demonstrated that sodium salicylate inhibits IL-1beta-induced nuclear factor-kappaB (NF-kappaB) activation. Sodium salicylate also prevented IL-1beta from inducing EP3 and cyclooxygenase (COX)-2 gene expression in islets and thereby prevented IL-1beta from inhibiting glucose-induced insulin secretion. These findings indicate that the sites of action through which sodium salicylate inhibits these negative effects of IL-1beta on beta-cell function include activation of NF-kappaB as well as generation of PGE(2) by COX-2.

Mechanisms and physiological significance of the cholinergic control of pancreatic beta-cell function

Acetylcholine (ACh), the major parasympathetic neurotransmitter, is released by intrapancreatic nerve endings during the preabsorptive and absorptive phases of feeding. In beta-cells, ACh binds to muscarinic M(3) receptors and exerts complex effects, which culminate in an increase of glucose (nutrient)-induced insulin secretion. Activation of PLC generates diacylglycerol. Activation of PLA(2) produces arachidonic acid and lysophosphatidylcholine. These phospholipid-derived messengers, particularly diacylglycerol, activate PKC, thereby increasing the efficiency of free cytosolic Ca(2+) concentration ([Ca(2+)](c)) on exocytosis of insulin granules. IP3, also produced by PLC, causes a rapid elevation of [Ca(2+)](c) by mobilizing Ca(2+) from the endoplasmic reticulum; the resulting fall in Ca(2+) in the organelle produces a small capacitative Ca(2+) entry. ACh also depolarizes the plasma membrane of beta-cells by a Na(+)- dependent mechanism. When the plasma membrane is already depolarized by secretagogues such as glucose, this additional depolarization induces a sustained increase in [Ca(2+)](c). Surprisingly, ACh can also inhibit voltage-dependent Ca(2+) channels and stimulate Ca(2+) efflux when [Ca(2+)](c) is elevated. However, under physiological conditions, the net effect of ACh on [Ca(2+)](c) is always positive. The insulinotropic effect of ACh results from two mechanisms: one involves a rise in [Ca(2+)](c) and the other involves a marked, PKC-mediated increase in the efficiency of Ca(2+) on exocytosis. The paper also discusses the mechanisms explaining the glucose dependence of the effects of ACh on insulin release.

Prevention of glucose toxicity in HIT-T15 cells and Zucker diabetic fatty rats by antioxidants

Chronic exposure of pancreatic islets to supraphysiologic concentrations of glucose causes adverse alterations in beta cell function, a phenomenon termed glucose toxicity and one that may play a secondary pathogenic role in type 2 diabetes. However, no mechanism of action has been definitively identified for glucose toxicity in beta cells. To ascertain whether chronic oxidative stress might play a role, we chronically cultured the beta cell line, HIT-T15, in medium containing 11.1 mM glucose with and without the antioxidants, N-acetyl-L-cysteine (NAC) or aminoguanidine (AG). Addition of NAC or AG to the culture medium at least partially prevented decreases in insulin mRNA, insulin gene promoter activity, DNA binding of two important insulin promoter transcription factors (PDX-1/STF-1 and RIPE-3b1 activator), insulin content, and glucose-induced insulin secretion. These findings suggested that one mechanism of glucose toxicity in the beta cell may be chronic exposure to reactive oxygen species, i.e., chronic oxidative stress. To ascertain the effects of these drugs on diabetes, NAC or AG was given to Zucker diabetic fatty rats, a laboratory model of type 2 diabetes, from 6 through 12 weeks of age. Both drugs prevented a rise in blood oxidative stress markers (8-hydroxy-2'-deoxyguanosine and malondialdehyde + 4-hydroxy-2-nonenal), and partially prevented hyperglycemia, glucose intolerance, defective insulin secretion as well as decrements in beta cell insulin content, insulin gene expression, and PDX-1 (STF-1) binding to the insulin gene promoter. We conclude that chronic oxidative stress may play a role in glucose toxicity, which in turn may worsen the severity of type 2 diabetes.

Multiple effector pathways regulate the insulin secretory response to the imidazoline RX871024 in isolated rat pancreatic islets

When isolated rat islets were cultured for 18 h prior to use, the putative imidazoline binding site ligand, RX871024 caused a dose-dependent increase in insulin secretion at both 6 mM and 20 mM glucose. By contrast, a second ligand, efaroxan, was ineffective at 20 mM glucose whereas it did stimulate insulin secretion in response to 6 mM glucose. Exposure of islets to RX871024 (50 microM) for 18 h, resulted in loss of responsiveness to this reagent upon subsequent re-exposure. However, islets that were unresponsive to RX871024 still responded normally to efaroxan. The imidazoline antagonist, KU14R, blocked the insulin secretory response to efaroxan, but failed to prevent the stimulatory response to RX871024. By contrast with its effects in cultured islets, RX871024 inhibited glucose-induced insulin release from freshly isolated islets. Efaroxan did not inhibit insulin secretion under any conditions studied. In freshly isolated islets, the effects of RX871024 on insulin secretion could be converted from inhibitory to stimulatory, by starvation of the animals. Inhibition of insulin secretion by RX871024 in freshly isolated islets was prevented by the cyclo-oxygenase inhibitors indomethacin or flurbiprofen. Consistent with this, RX871024 caused a marked increase in islet PGE2 formation. Efaroxan did not alter islet PGE2 levels. The results suggest that RX871024 exerts multiple effects in the pancreatic beta-cell and that its effects on insulin secretion cannot be ascribed only to interaction with a putative imidazoline binding site.

Basal expression of cyclooxygenase-2 and nuclear factor-interleukin 6 are dominant and coordinately regulated by interleukin 1 in the pancreatic islet

The enzyme cyclooxygenase (COX)-1 is constitutive whereas COX-2 is regulated in virtually all tissues. To assess whether this dogma holds true in the pancreatic islet, we examined basal and interleukin (IL)-1-regulated expression of COX-2 in HIT-T15 cells, Syrian hamster and human islets, and other Syrian hamster tissues. We found that COX-2, and not COX-1, gene expression is dominant in pancreatic islet tissue under both basal and IL-1-stimulated conditions. Control tissues (liver, spleen, and kidney) showed the expected predominance of COX-1 gene expression. Basal and IL-1-stimulated prostaglandin E2 synthesis were blocked by a specific COX-2 inhibitor. IL-1 stimulation had a biphasic effect on COX-2 mRNA levels with an initial mild increase at 2-4 hr followed by a more dramatic decrease below basal level by 24 hr. The IL-1-induced increase in COX-2 mRNA levels was accompanied by a parallel increase in NF-kappaB binding to COX-2 promoter elements. The subsequent decrease in COX-2 mRNA levels was accompanied by a parallel decrease in NF-IL-6 binding activity and COX-2 promoter activity. Specific mutation of the NF-IL-6 binding motif within the COX-2 promoter reduced basal promoter activity by 50% whereas mutation of the NF-kappaB motif had no effect. These studies provide documentation of NF-IL-6 in the pancreatic islet and that COX-2, rather than COX-1, is dominantly expressed. They suggest coordinate regulation by IL-1 of COX-2 mRNA, NF-kappaB, and NF-IL-6 and raise the issue of whether intrinsically high levels of COX-2 gene expression predisposes the normal islet for microenvironmentally induced overproduction of islet prostaglandin E2.

Unsaturated fatty acids alter the insulin secretion response of the islets of Langerhans in vitro

The aim of the present study is to examine the influence which different concentrations of free fatty acids (FFAs) have on the insulin secretion response of the islets of Langerhans when the culture is prolonged over a week. Animals were killed by decapitation and pancreatic islets of Langerhans isolated. The islets were incubated in enriched RPMI and oleic or linoleic acid was added at concentrations of 0.04, 0.1, 0.375, and 1 mmol/l. The medium without FFAs was used as a control. The culture time was 7 days. For insulin secretion studies, islets were selected after preincubation in batches of six islets in 1 ml of KRB (Krebs-Ringer buffer) containing one of the following: 5.5, 11.1, 16 or 26.7 mmol/l glucose, 10 mumol/l forskolin or 20 mmol/l arginine. The results showed a significant increase in insulin secretion observed after culture with 1 mmol/l oleic and linoleic acid compared to the other concentrations and the control culture for all the secretagogues used. However, at this same concentrations no increase was observed in insulin secretion as the glucose concentration rose, and this was noticeable with linoleic acid at concentrations of 0.375 mmol/l. In conclusion, culture of islets of Langerhans for a week with high concentrations of unsaturated fatty acids produces a hypersecretion of insulin which is not influenced by secretagogues such as glucose, arginine, or forskolin. The loss of gluco-sensitivity may become greater as the degree of unsaturation of the fatty acid used increases.

Does dietary fat influence insulin action?

What is clear from the research thus far is that dietary fat intake does influence insulin action. However, whether the effect is good, bad, or indifferent is strongly related to the fatty acid profile of that dietary fat. The evidence has taken many forms, including in vitro evidence of differences in insulin binding and glucose transport in cells grown with different types of fat in the incubation medium, in vivo results in animals fed different fats, relationships demonstrated between the membrane structural lipid fatty acid profile and insulin resistance in humans, and finally epidemiological evidence linking particularly high saturated fat intake with hyperinsulinemia and increased risk of diabetes. This contrasts with the lack of relationship, or even possible protective effect, of polyunsaturated fats. In particular, habitual increased n-3 polyunsaturated dietary fat intake (as fish fats) would appear to be protective against the development of glucose intolerance. It is reassuring that the patterns of dietary fatty acids that appear beneficial for insulin action and energy balance are also the patterns that would seem appropriate in the fight against thrombosis and cardiovascular disease. Mechanisms, though, still need to be defined. However, there are strong indicators that defining the ways in which changes in the fatty acid profile of membrane structural lipids are achieved, and in turn influence relevant transport events, plus understanding the processes that control accumulation and availability of storage lipid in muscle may be fruitful avenues for future research. One of the problems of moving the knowledge gained from research at the cellular level through to the individual and on to populations is the need for more accommodating research designs. In vitro studies may provide in-depth insights into intricate mechanisms, but they do not give the "big picture" for practical recommendations. On the other hand, correlational studies tend to be fairly blunt instruments, requiring large numbers that are very often not feasible if a greater depth of understanding of the biological processes is to be incorporated. There may be benefit in turning to the clinical case study as a framework for a more comprehensive analysis of the links between dietary fats and insulin action. The real challenge is to keep the depth of analysis rigorous enough to be able to explain and accommodate individual variation (i.e., the diversity of both environmental and genetic backgrounds) while at the same time satisfying the cultural need to provide appropriate overall dietary guidelines. Finally, David Kritchevsky brought to our attention a delightful quote from Mark Twain: "There is something fascinating about science. One gets such a wholesale return of conjecture for such a trifling investment of fact." In the field of dietary fats and the Metabolic Syndrome, this quotation is, unfortunately, apt. Much more research is necessary to define how dietary fats really work to affect insulin action. Well designed, long-term studies in "free range" humans must be undertaken if dietary guidelines for the Metabolic Syndrome are to be based on anything more than a "trifling" amount of "fact."

Involvement of phospholipase A2 and arachidonic acid in cholecystokinin-8-induced insulin secretion in rat islets

Cholecystokinin (CCK) has been shown to stimulate insulin secretion through an effect which involves mediation by phospholipase C (PLC) and protein kinase C (PKC). However, data exist suggesting involvement also of other transduction pathways. We investigated possible involvement of phospholipase A2 (PLA2) and arachidonic acid (AA) in mechanisms of insulin secretion, induced by the C-terminal octapeptide of CCK (CCK-8) in isolated rat islets. At 5.6 mM glucose, the specific PLA2 inhibitor p-amylcinnamoylantranilic acid (ACA; 50 microM) diminished CCK-8 (100 nM)-stimulated insulin secretion (by 57 +/- 16%; P = 0.001). Furthermore, at 5.6 mM glucose, CCK-8 significantly increased the efflux of [3H]arachidonic acid from prelabelled islets (by 130 +/- 25%; P < 0.001). These results imply that CCK-8 activates PLA2 to form AA in islets. To study whether the insulinotropic effect of CCK-8 is due to AA per se or to its metabolites, the oxidative pathways of the AA metabolism were inhibited. However, the cyclooxygenase inhibitors, indomethacin (30 microM) and salicylate (1.25 mM) as well as the lipoxygenase inhibitors baicalein (1-100 microM) and esculetin (0.5-50 microM), did not affect CCK-8-induced insulin secretion. We conclude that CCK-8-induced insulin secretion is partially mediated by a pathway involving PLA2, and that the formed AA, rather than its metabolites, is of importance.

Glucose-induced phosphorylation and activation of a high molecular weight cytosolic phospholipase A2 in neonatal rat pancreatic islets

Previous studies have shown that stimulus-secretion coupling for the release of insulin from the pancreatic islet is potentiated by phospholipase A2 activity. Several biochemically distinct phospholipase A2 activities have been described in the islet. A recently identified cytosolic high molecular weight phospholipase A2, which requires Ca2+ for association with cellular membranes but not for catalytic activity can be activated in a protein kinase C-dependent manner in other cell-types. We determined its phosphorylation and activation in response to phorbol ester and glucose in cultured islet cells from neonatal rats. Islet cell monolayers were labelled to equilibrium with [32P]orthophosphate. Following stimulation cytosolic phospholipase A2 was immunoprecipitated and, after electrophoretic separation and transfer to nitrocellulose membrane, 32P-labelled protein was detected by autoradiography. Phospholipase A2 activity of islet cell cytosol was determined by hydrolysis of exogenous I-stearyl- 2[14C]arachidonyl phosphatidylcholine substrate. It could be shown that phosphorylation of immunoprecipitated phospholipase A2 was augmented by prolonged glucose exposure (> 1 hr) in a protein kinase C-dependent manner. Phosphorylation occurred concomitant with a glucose-induced increase in total cellular phospholipase A2 activity (177 +/- 3 nmol substrate hydrolysed/mg protein at glucose 5.6 mM vs 267 +/- 32 (SEM, n = 4) at glucose 25 mM, P < 0.05). Both acute protein kinase C (459 +/- 71) and glucose-activated phospholipase A2 activities were reduced in the presence of a specific arachidonic acid analogue inhibitor of cytosolic phospholipase A2 (to 231 +/- 10 and 161 +/- 17, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation by prostaglandin E2 of a high-affinity GTPase in the secretory granules of normal rat and human pancreatic islets

Recent reports of a pertussis-toxin (Ptx)-sensitive inhibition of glucose-induced insulin release by prostaglandin E2 (PGE2) in transformed beta-cells prompted us to look for the presence of prostaglandin-regulatable GTP-binding proteins (G-proteins) on the secretory granules of normal pancreatic islets. PGE2 (but not PGF2 alpha, PGA2, PGB2 or PGD2) stimulated in a concentration-dependent manner a high-affinity GTPase activity in the secretory-granule-enriched fractions of both normal rat and human islets. Similar results were found after sucrose-density-gradient-centrifugation-based isolation of secretory granules to those after a differential-centrifugation procedure. Half-maximal stimulation occurred at 800 nM PGE2, a concentration known to inhibit both phases of glucose-induced insulin secretion from pure beta-cell lines. The GTPase stimulatory effect of PGE2 was blocked virtually totally by Ptx pretreatment; it was not due to an effect on substrate binding since no measurable effect of PGE2 on binding of guanosine 5'-[gamma-[35S]thio]triphosphate was observed in cognate fractions. Other Ptx-sensitive inhibitors of insulin secretion (such as adrenaline or clonidine) also stimulated GTPase activity, suggesting that one (or more) inhibitory exocytotic G-proteins (i.e. a putative GEi) is located on the secretory granules. These studies demonstrate, for the first time in an endocrine gland, the presence of a regulatable G-protein, strategically located on the secretory granules where it might regulate the exocytotic cascade distal to both plasma-membrane events and the generation of soluble mediators of insulin secretion.

Significance of the N-6/N-3 ratio for insulin action in diabetes

This report supplies supportive data for the hypothesis that a high n-6/n-3 dietary fat ratio is a potent, negative variable in NIDDM. Further work is necessary to critically evaluate this hypothesis.

Carbachol stimulation of phospholipase A2 and insulin secretion in pancreatic islets

Arachidonic acid has been implicated as a second messenger in insulin secretion by islets of Langerhans. D-Glucose, the major physiological stimulus, increases unesterified arachidonate accumulation in islets. We now show, for the first time, that the muscarinic agonist carbachol, at concentrations which stimulate insulin secretion, causes a rapid and nearly 3-fold increase in arachidonic acid accumulation in islets. The combination of glucose and carbachol has an additive effect on unesterified arachidonate release. There is a large component of secretagogue-induced arachidonate accumulation that is independent of extracellular Ca2+. Carbachol stimulation of arachidonic acid release is mediated by activation of phospholipase A2, as demonstrated by early increases in endogenous lysophosphatidylcholine. In addition to phospholipase A2 activation, carbachol-induced arachidonic acid accumulation also appears to involve diacylglycerol hydrolysis, since the diacylglycerol lipase inhibitor RG80267 partly inhibited arachidonic acid accumulation. In contrast, glucose-induced arachidonic acid accumulation appears to reflect diacylglycerol hydrolysis entirely. Our observations indicate that phospholipase A2 has an important role in muscarinic-induced insulin secretion.

Physiological importance of omega-3/omega-6 polyunsaturated fatty acids in man. An overview of still unresolved and controversial questions

The 'essentiality' of (omega-6) and (omega-3) fatty acids in mammals is well known. Nevertheless, some important points remain unclear concerning their implication in physiology. After a short discussion about the definition of essential fatty acids deficiency, this brief overview deals with some of these points, pointing out some of the unresolved questions. Different subjects are approached concerning the (omega-6) and (omega-3) fatty acids metabolism: desaturases, eicosanoids, production, as well as some of their metabolic effects on cell membranes, intestinal function, glucose and lipid metabolism, haemorheology.

Pancreatic function in the essential fatty acid deficient rat

The influence of essential fatty acid (EFA) deficiency on pancreatic endocrine and exocrine function was studied in 120-day-old rats. The plasma insulin response was determined after in vivo administration of glucose and arginine. The plasma glucagon response was assessed after infusion of arginine. Islet peptides were examined by immunocytochemistry. The exocrine function of pancreas was studied by amylase secretion in isolated pancreatic acinar cells after stimulation with the cholinergic agonist carbacholine chloride. The EFA-deficient (EFAD) rats showed higher basal plasma insulin concentrations and lower basal glucose levels than control rats (P less than .01 and P less than .01, respectively). The plasma insulin response to glucose was potentiated in the EFAD rats (P less than .001). Both insulin and glucagon responses to arginine were normal. The isolated pancreatic acinar cells showed a low basal amylase secretion, but a normal response to carbacholine chloride. There were no overt morphological changes seen in the pancreas and the immunocytochemical staining pattern of insulin, glucagon, somatostatin, and pancreatic polypeptide cells did not differ from controls. The results of the study show that the secretory function of the endocrine and exocrine pancreas is operational in EFA deficiency. The EFA deficiency was accompanied by a basal hyperinsulinemia and hypoglycemia and an exaggerated insulin response to glucose, the pathophysiology of which has to be further studied.

Lipoxygenase-generated icosanoids inhibit glucose-induced insulin release from rat islets

Lipoxygenase-pathway metabolites of arachidonic acid are produced in pancreatic islets. They are are implicated in insulin release, since nonselective inhibitors of lipoxygenases inhibit glucose-induced insulin release. We studied the interplay in insulin release between glucose and selected icosanoids formed in 5-, 12- and 15-lipoxygenase pathways. Effects on immunoreactive insulin release of 10(7) to 10(6)-12-(R)-HETE, 12-(S)-HETE, hepoxilin A3, lipoxin B4, LTB4 or LTC4 were tested individually in 30-min incubations of freshly isolated young adult Wistar rat pancreatic islets, in the presence of 5.6 mM or 23 mM glucose. Basal insulin release (at 5.6 mM glucose) was stimulated by LTC4 and hepoxilin A3 (304% and 234% of controls at 5.6 mM glucose alone, respectively), inhibited by 12-(S)-HPETE (56%), and was not affected by 12-(R)-HETE, 12-(S)-HETE, lipoxin B4 or LTB4 (111%, 105%, 106% and 136%, respectively). Insulin release evoked by 23 mM glucose (190-320%) was inhibited (50-145%) by all icosanoids tested, except LTC4 (162%). We conclude that, among the lipoxygenase products tested, only leukotrienes and hepoxilin are candidates for a tonic-stimulatory influence on basal insulin release. Since glucose promotes icosanoid formation in islets, the observed inhibition of glucose-induced insulin release by lipoxygenase products suggests the existence of a negative-feedback system.

Beta-endorphin and islet hormone release in humans: evidence for interference with cAMP

The present studies were undertaken to characterize further the influence of synthetic human beta-endorphin (0.5 mg/h) on insulin and glucagon responses to intravenous glucose in humans. Infusion of beta-endorphin in 10 normal volunteers caused a clear-cut inhibition of the overall insulin responses to a glucose pulse (0.33 g/kg iv) with values of glucose disappearance rates in the diabetic range [0.89 +/- 0.09 (P less than 0.01) vs. saline 1.82 +/- 0.15%/min]. Glucose-induced glucagon suppression was significantly lower during beta-endorphin, a fact that could have contributed to the reduced glucose utilization rates. The infusion of theophylline (150 mg + 350 mg/h) to increase the intracellular cAMP activity by inhibiting phosphodiesterase completely reversed the inhibitory effect of beta-endorphin on glucose-induced insulin secretion. As a consequence, glucose disappearance rates rose to 1.77 +/- 0.18%/min. Theophylline did not influence significantly the glucagon-releasing effect of beta-endorphin as well as the reduced glucagon suppression. An infusion of exogenous calcium (100 mg as iv bolus + 5 mg/min) to raise serum calcium in the hypercalcemic range (15 mg/dl) and lysine acetylsalicylate (72 mg/min) to block the synthesis of endogenous prostaglandin E did not interfere with the inhibiting effect of beta-endorphin on insulin secretion. These data confirm that beta-endorphin stimulates glucagon and inhibits basal and glucose-stimulated insulin secretion and suggest that the opioid influences the intraislet adenylate cyclase activity.

Membrane phospholipid turnover as an intermediary step in insulin secretion. Putative roles of phospholipases in cell signaling

One or more phospholipases of the C and A2 types exist in rodent islets and may play a pivotal role in the cell signaling cascade culminating in exocytotic insulin release. Phospholipase C generates myo-inositol-1,4,5-trisphosphate, which mobilizes a "pool" of calcium in the endoplasmic reticulum and which may also secondarily facilitate calcium (Ca++) influx from the extracellular space to replenish that pool. Diacylglycerol is also generated by phospholipase C action and activates protein kinase C; it may thereby potentiate the cellular response to elevations in cytosolic free Ca++ concentration. Arachidonic acid may be released during the degradation of diacylglycerol and may also contribute to islet activation. Phospholipase C is activated by glucose, cholinergic agonists, and probably by Ca++ fluxes. Phospholipase A2 action generates arachidonic acid and lysophospholipids. Certain lysophospholipids mobilize cellular Ca++, at least in part from superficial, plasmalemmal stores. Native (unoxygenated) arachidonic acid also has the capability of mobilizing cellular Ca++ from membrane-bound stores; it may, in addition, activate protein kinase C, as suggested by recent indirect studies. The further metabolism of arachidonic acid via lipoxygenase and cyclo-oxygenase appears to provide positive and negative modulation, respectively, of stimulated insulin secretion. Many pieces of the puzzle remain, however, to be supplied. For example, it has not yet been unequivocally demonstrated that phospholipase A2 is activated by physiologic stimuli in intact islets. Furthermore, the absence of truly specific pharmacologic stimulators or inhibitors of these processes currently precludes precise delineation of the respective physiologic roles of each potential mediator in stimulus-secretion coupling. When such roles are elucidated, it can be asked whether the defects in insulin secretion in diabetes mellitus may be due in part to abnormalities in the turnover of beta-cell membrane phospholipids and the generation of intracellular lipid-derived signals.

Opiate-prostaglandin interactions in the regulation of insulin secretion from rat islets of Langerhans in vitro

The inadequate insulin secretory response to glucose stimulation in non-insulin dependent diabetes has been attributed to many factors including high PGE2 levels blunting the secretory response, and to the existence of inhibitory opiate activity in vivo. The purpose of the present work was to see if there was a connection between these two independent theories. Radioimmunoassayable PGE2 in islets of Langerhans was found to be proportional to islet number and protein content and was typically 4 to 5pg/micrograms islet protein. Indomethacin (2.8 X 10(-5) M), sodium salicylate (1.25 X 10(-3) M) and chlorpropamide (7.2 X 10(-5) M) all lowered islet PGE2 levels and stimulated insulin release in vitro. Dynorphin (1-13), stimulated insulin release at a concentration of 6 X 10(-9) M, while lowering islet PGE2. Conversely, at a higher concentration, (6 X 10(-7) M), dynorphin had no stimulatory effect on insulin secretion and did not lower PGE2 levels in islets or in the incubation media. The stimulatory effects of dynorphin and sodium salicylate on insulin secretion were blocked by exogenous PGE2 (10(-5) M). PGE2 at a lower concentration (10(-9) M) did not exert any inhibitory effect on dynorphin- or sodium salicylate-induced insulin release. This concentration of exogenous PGE2 stimulated insulin release in the presence of 6mM glucose. Results from these experiments suggest that since an opioid peptide can lower endogenous PGE2 production in islets and since the stimulatory effects of the opioid peptide are reversed by exogenous PGE2 there may be interactions between these two modulators of insulin secretion.