Expression and function of cyclooxygenase and lipoxygenase enzymes in human islets of Langerhans

The Rac2 GTPase contributes to cathepsin H-mediated protection against cytokine-induced apoptosis in insulin-secreting cells

The type 1 diabetes (T1D) risk locus on chromosome 15q25.1 harbors the candidate gene CTSH (cathepsin H). We previously demonstrated that CTSH regulates β-cell function in vitro and in vivo. CTSH overexpression protected insulin-secreting INS-1 cells against cytokine-induced apoptosis. The purpose of the present study was to identify the genes through which CTSH mediates its protective effects. Microarray analysis identified 63 annotated genes differentially expressed between CTSH-overexpressing INS-1 cells and control cells treated with interleukin-1β and interferon-γ for up to 16h. Permutation test identified 10 significant genes across all time-points: Elmod1, Fam49a, Gas7, Gna15, Msrb3, Nox1, Ptgs1, Rac2, Scn7a and Ttn. Pathway analysis identified the "Inflammation mediated by chemokine and cytokine signaling pathway" with Gna15, Ptgs1 and Rac2 as significant. Knockdown of Rac2 abolished the protective effect of CTSH overexpression on cytokine-induced apoptosis, suggesting that the small GTPase and T1D candidate gene Rac2 contributes to the anti-apoptotic effect of CTSH.

Indomethacin decreases insulin secretion by reducing KCa3.1 as a biomarker of pancreatic tumor and causes apoptotic cell death

Insulinomas originate from pancreatic β cells and it is the most widely known tumor. Indomethacin is a nonsteroidal anti-inflammatory drug, which is used for blocking the production of some natural substances that cause inflammation and decrease pain. In this study, I aimed to investigate the effects of indomethacin on rat insulinoma INS-1 cells. The relationship between cell death and insulin metabolism was determined with the administration of indomethacin. The cell viability by WST-1; the apoptosis and necrosis levels by ELISA kits; malondialdehyde levels by spectrophotometer; and beclin, intracellular insulin, insulin secretion, KCa3.1, insulin receptor (IR), glucose transporter type 2 (GLUT2), activating transcription factor 2 (ATF2), Elk1, c-Jun, Akt and phosphorylated ATF2, Elk1, c-Jun, Akt, intracellular betacellulin and betacellulin secretion levels by Western blot analysis investigated. The Ins1, Ins2, IR, GLUT2, ATF2, Elk1, c-Jun, Akt, and Betacellulin gene expression levels were determined by the real-time quantitative reverse transcription-polymerase chain reaction method. Apoptotic cell death was observed with the administration of indomethacin. The insulin secretion and Ins1, Ins2 gene expression levels decreased. The insulin receptor and GLUT2 levels increased, while KCa3.1 (KCNN4) levels decreased with the administration of indomethacin to insulinoma INS-1 cells. A decrease was observed in the total c-Jun, phosphorylated ATF2, Elk1, c-Jun, and Akt levels. Betacellulin secretion levels increased. In insulinoma INS-1 cells, apoptotic cell death occurred in the following manner: (i) indomethacin might decrease insulin secretion by reducing KCa3.1, (ii) might inactivate the JNK/ERK pathway with the inactivity of transcription factors.

The Combination of Whole Cell Lipidomics Analysis and Single Cell Confocal Imaging of Fluidity and Micropolarity Provides Insight into Stress-Induced Lipid Turnover in Subcellular Organelles of Pancreatic Beta Cells

Modern omics techniques reveal molecular structures and cellular networks of tissues and cells in unprecedented detail. Recent advances in single cell analysis have further revolutionized all disciplines in cellular and molecular biology. These methods have also been employed in current investigations on the structure and function of insulin secreting beta cells under normal and pathological conditions that lead to an impaired glucose tolerance and type 2 diabetes. Proteomic and transcriptomic analyses have pointed to significant alterations in protein expression and function in beta cells exposed to diabetes like conditions (e.g., high glucose and/or saturated fatty acids levels). These nutritional overload stressful conditions are often defined as glucolipotoxic due to the progressive damage they cause to the cells. Our recent studies on the rat insulinoma-derived INS-1E beta cell line point to differential effects of such conditions in the phospholipid bilayers in beta cells. This review focuses on confocal microscopy-based detection of these profound alterations in the plasma membrane and membranes of insulin granules and lipid droplets in single beta cells under such nutritional load conditions.

The role of Cox-2 and prostaglandin E2 receptor EP3 in pancreatic β-cell death

Elevated levels of lipids, in particular saturated fatty acids, are known to be associated with type 2 diabetes (T2D) and to have a negative effect on β-cell function and survival. We bring new evidence indicating that palmitate up-regulates cyclooxygenase-2 (COX-2) expression levels in human islets and in MIN6 β cells, and that it is elevated in islets isolated from T2D donors. Both small interfering specific cyclooxygenase-2 small interfering RNA (siRNA) or the COX-2 inhibitor celecoxib significantly inhibited apoptosis induced by palmitate. Prostaglandin E₂ (PGE₂), the predominant product of COX-2 enzymatic activity, activates membrane receptors, which are members of the GPCR-family (EP1-EP4). In the present study, elevated expression of the PGE₂ receptor subtype 3 (EP3) receptor was observed in β cells exposed to palmitate and in islets from individuals with T2D. Down-regulation of the pathway using EP3 siRNA or the specific L-798,106 antagonist markedly decreased the levels of palmitate-induced apoptosis. Altogether, our data put forward the COX-2-PGE₂-EP3 pathway as one of the mediators of palmitate-induced apoptosis in β-cells.-Amior, L., Srivastava, R., Nano, R., Bertuzzi, F., Melloul, D. The role of Cox-2 and prostaglandin E₂ receptor EP3 in pancreatic β-cell death.

Nutrient overload, lipid peroxidation and pancreatic beta cell function

Since the landmark discovery of α,β-unsaturated 4-hydroxyalkenals by Esterbauer and colleagues most studies have addressed the consequences of the tendency of these lipid peroxidation products to form covalent adducts with macromolecules and modify cellular functions. Many studies describe detrimental and cytotoxic effects of 4-hydroxy-2E-nonenal (4-HNE) in myriad tissues and organs and many pathologies. Other studies similarly assigned unfavorable effects to 4-hydroxy-2E-hexenal (4-HHE) and 4-hydroxy-2E,6Z-dodecadienal (4-HDDE). Nutrient overload (e.g., hyperglycemia, hyperlipidemia) modifies lipid metabolism in cells and promotes lipid peroxidation and the generation of α,β-unsaturated 4-hydroxyalkenals. Advances glycation- and lipoxidation end products (AGEs and ALEs) have been associated with the development of insulin resistance and pancreatic beta cell dysfunction and the etiology of type 2 diabetes and its peripheral complications. Less acknowledged are genuine signaling properties of 4-hydroxyalkenals in hormetic processes that provide defense against the consequences of nutrient overload. This review addresses recent findings on such lipohormetic mechanisms that are associated with lipid peroxidation in pancreatic beta cells. This article is part of a Special Issue entitled SI: LIPID OXIDATION PRODUCTS, edited by Giuseppe Poli.

Regulation of pancreatic β-cell function and mass dynamics by prostaglandin signaling

Prostaglandins (PGs) are signaling lipids derived from arachidonic acid (AA), which is metabolized by cyclooxygenase (COX)-1 or 2 and class-specific synthases to generate PGD₂, PGE₂, PGF_2α, PGI₂ (prostacyclin), and thromboxane A₂. PGs signal through G-protein coupled receptors (GPCRs) and are important modulators of an array of physiological functions, including systemic inflammation and insulin secretion from pancreatic islets. The role of PGs in β-cell function has been an active area of interest, beginning in the 1970s. Early studies demonstrated that PGE₂ inhibits glucose-stimulated insulin secretion (GSIS), although more recent studies have questioned this inhibitory action of PGE₂. The PGE₂ receptor EP3 and one of the G-proteins that couples to EP3, Gα_Z, have been identified as negative regulators of β-cell proliferation and survival. Conversely, PGI₂ and its receptor, IP, play a positive role in the β-cell by enhancing GSIS and preserving β-cell mass in response to the β-cell toxin streptozotocin (STZ). In comparison to PGE₂ and PGI₂, little is known about the function of the remaining PGs within islets. In this review, we discuss the roles of PGs, particularly PGE₂ and PGI₂, PG receptors, and downstream signaling events that alter β-cell function and regulation of β-cell mass.

Hormetic and regulatory effects of lipid peroxidation mediators in pancreatic beta cells

Nutrient sensing mechanisms of carbohydrates, amino acids and lipids operate distinct pathways that are essential for the adaptation to varying metabolic conditions. The role of nutrient-induced biosynthesis of hormones is paramount for attaining metabolic homeostasis in the organism. Nutrient overload attenuate key metabolic cellular functions and interfere with hormonal-regulated inter- and intra-organ communication, which may ultimately lead to metabolic derangements. Hyperglycemia and high levels of saturated free fatty acids induce excessive production of oxygen free radicals in tissues and cells. This phenomenon, which is accentuated in both type-1 and type-2 diabetic patients, has been associated with the development of impaired glucose tolerance and the etiology of peripheral complications. However, low levels of the same free radicals also induce hormetic responses that protect cells against deleterious effects of the same radicals. Of interest is the role of hydroxyl radicals in initiating peroxidation of polyunsaturated fatty acids (PUFA) and generation of α,β-unsaturated reactive 4-hydroxyalkenals that avidly form covalent adducts with nucleophilic moieties in proteins, phospholipids and nucleic acids. Numerous studies have linked the lipid peroxidation product 4-hydroxy-2E-nonenal (4-HNE) to different pathological and cytotoxic processes. Similarly, two other members of the family, 4-hydroxyl-2E-hexenal (4-HHE) and 4-hydroxy-2E,6Z-dodecadienal (4-HDDE), have also been identified as potential cytotoxic agents. It has been suggested that 4-HNE-induced modifications in macromolecules in cells may alter their cellular functions and modify signaling properties. Yet, it has also been acknowledged that these bioactive aldehydes also function as signaling molecules that directly modify cell functions in a hormetic fashion to enable cells adapt to various stressful stimuli. Recent studies have shown that 4-HNE and 4-HDDE, which activate peroxisome proliferator-activated receptor δ (PPARδ) in vascular endothelial cells and insulin secreting beta cells, promote such adaptive responses to ameliorate detrimental effects of high glucose and diabetes-like conditions. In addition, due to the electrophilic nature of these reactive aldehydes they form covalent adducts with electronegative moieties in proteins, phosphatidylethanolamine and nucleotides. Normally these non-enzymatic modifications are maintained below the cytotoxic range due to efficient cellular neutralization processes of 4-hydroxyalkenals. The major neutralizing enzymes include fatty aldehyde dehydrogenase (FALDH), aldose reductase (AR) and alcohol dehydrogenase (ADH), which transform the aldehyde to the corresponding carboxylic acid or alcohols, respectively, or by biding to the thiol group in glutathione (GSH) by the action of glutathione-S-transferase (GST). This review describes the hormetic and cytotoxic roles of oxygen free radicals and 4-hydroxyalkenals in beta cells exposed to nutritional challenges and the cellular mechanisms they employ to maintain their level at functional range below the cytotoxic threshold.

Isolation and characterisation of insulin-releasing compounds from Crotalus adamanteus, Crotalus vegrandis and Bitis nasicornis venom

Crude venom from three venomous snakes, Crotalus adamanteus, Crotalus vegrandis and Bitis nasicornis was fractionated by gel filtration chromatography, and selected fractions screened for in-vitro insulinotropic activity using clonal pancreatic BRIN-BD11 cells. Nineteen fractions stimulated insulin secretion and the structural identity of bioactive compounds responsible was probed using MALDI-ToF MS and N-terminal Edman degradation sequencing. Partial N-terminal sequences were determined and their homology to existing sequences identified using BLAST searching. The main insulinotropic peptide families identified were made up of snake venom serine proteinases, phospholipases A2 (PLA2) and disintegrins. Snake venom constituents may have therapeutic potential for diabetes, with each of the three viper venoms tested providing insulinotropic compounds from a range of different toxin families.

Effects of cyclooxygenase inhibition on insulin release and pancreatic islet blood flow in rats

OBJECTIVES

To examine the effects of inhibition of cyclooxygenase (COX) on islet hormone secretion in vitro and on pancreatic islet blood flow in vivo.

METHODS

Insulin release was measured in a static incubation system of islets isolated from Wistar-F rats after inhibition of COX-1 and COX-2 with SC 560 (COX-1), FR 122047 (COX-1), rofecoxib (COX-2), or indomethacin (both COX-1 and COX-2). In other rats organ blood flow values were measured with a microsphere technique during both normo- and hyperglycemia after administration of these enzyme inhibitors.

RESULTS

Serum insulin values were lower after pretreatment with a COX-1 inhibitor or a non-selective COX inhibitor in both control and glucose-injected rats in vivo, whereas COX-2 inhibition had no such effects. However, inhibition of COX had only minor effects on insulin release in vitro. Inhibition of COX affected neither total pancreatic nor islet blood flow in normoglycemic rats. Hyperglycemia caused an increase in both these flow values and in the duodenum. The increase in total pancreatic and duodenal blood flow was prevented by inhibition of COX-2 or non-selective COX inhibition. However, no effects on islet blood flow were seen after COX inhibition.

CONCLUSION

Inhibition of COX affects insulin release and blood glucose concentrations in vivo. However, COX inhibition has only minor effects on pancreatic islet blood flow, but prevents the glucose-induced increase in total pancreatic blood flow.

PTGS-2-PTGER2/4 signaling pathway partially protects from diabetogenic toxicity of streptozotocin in mice

Prostanoids are suggested to participate in diabetes pathology, but their roles are controversially discussed. The purpose of the current study was to examine the role of cyclooxygenase (prostaglandin synthase [PTGS]) enzymes and prostaglandin (PG) E(2) signaling pathways in streptozotocin (STZ)-induced type 1 diabetes. Blood glucose, insulin, and survival rate were studied in mice with targeted disruption of the genes for PTGS and PGE receptors (PTGERs). PGE(2) was found as the main prostanoid formed by the pancreas. Contrarily to PTGS-1, deficiency of PTGS-2 activity significantly amplified STZ effect, causing dramatic loss of insulin production and rise in blood glucose and death rate. STZ metabolism was unaffected by PTGS deficiency. Diabetogenicity of STZ in PTGER1(-/-), PTGER2(-/-), PTGER3(-/-), and PTGER4(-/-) mice was comparable to control mice. In striking contrast, combined knockout of PTGER2 and PTGER4 by blocking PTGER4 in PTGER2(-/-) mice strongly enhanced STZ pathology. Treatment of PTGS-2(-/-) and wild-type mice with PTGER2/PTGER4 agonists partially protected against STZ-induced diabetes and restored β-cell function. Our data uncover a previously unrecognized protective role of PTGS-2-derived PGE(2) in STZ-induced diabetes mediated by the receptor types PTGER2 and PTGER4. These findings offer the possibility to intervene in early progression of type 1 diabetes by using PTGER-selective agonists.

Pharmacological characterization of the cytoprotective effects of polyunsaturated fatty acids in insulin-secreting BRIN-BD11 cells

BACKGROUND AND PURPOSE

Free fatty acids are important metabolic fuels for mammalian cells but, recently, it has become clear that they can also fulfil signalling functions, which are independent of their metabolic fate. We are investigating the ability of unsaturated free fatty acids to exert a cytoprotective response during exposure of insulin-secreting cells to toxic stimuli. The majority of earlier studies have focussed on monounsaturated fatty acids but this has now been extended to define the structural requirements of the cytoprotective effects of polyunsaturated species.

EXPERIMENTAL APPROACH

Clonal rat insulin-secreting cell lines, BRIN-BD11 or INS-1, were exposed to fatty acids or their derivatives complexed with BSA and the viability of the cells was analysed by flow cytometry after staining with propidium iodide.

KEY RESULTS

A variety of polyunsaturated fatty acids with chain lengths between C18-C22 attenuated the cytotoxic actions of the saturated fatty acid, palmitate (C16:0) in BRIN-BD11 and INS-1 cells. These effects were dose-dependent and displayed potencies that were much higher than those achieved with monounsaturated fatty acids. Methyl esters of the polyunsaturates were also effective. The cytoprotective responses were not altered by incubation of cells with inhibitors of cyclooxygenase or lipoxygenase enzymes although they were antagonized dose-dependently by arachidonyltrifluoromethylketone (AACOCF(3)).

CONCLUSIONS AND IMPLICATIONS

The results are consistent with the involvement of a specific fatty acid binding site having loose, but defined, structural criteria, in mediating the cytoprotective effects of unsaturated fatty acids. AACOCF(3) may be of value in defining this site in molecular terms.

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.

Saturated and unsaturated (including arachidonic acid) non-esterified fatty acid modulation of insulin secretion from pancreatic beta-cells

Both stimulatory and detrimental effects of NEFAs (non-esterified fatty acids) on pancreatic beta-cells have been recognized. Acute exposure of the pancreatic beta-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 beta-cells during chronic exposure and can even exert positive protective effects. In the present review, we focus on exogenous and endogenous effects of NEFAs, including the polyunsaturated fatty acid, arachidonic acid (or its metabolites generated from cyclo-oxygenase activity), on beta-cell metabolism, and have explored the outcomes with respect to beta-cell insulin secretion.