Evidence for involvement of the proteasome complex (26S) and NFkappaB in IL-1beta-induced nitric oxide and prostaglandin production by rat islets and RINm5F cells

Interleukin-1beta (IL-1beta) has been implicated as an effector molecule of beta-cell destruction in autoimmune diabetes. IL-1beta inhibits insulin secretion from pancreatic beta-cells by stimulating the expression of inducible nitric oxide synthase (iNOS) that generates the free radical nitric oxide. IL-1beta also induces the coexpression of the inducible isoform of cyclooxygenase (COX-2) that results in the overproduction of proinflammatory prostaglandins. The current studies were designed to characterize the involvement of protease(s) in the signaling pathway of IL-1beta-induced iNOS and COX-2 expression by rat islets and transformed rat pancreatic beta-cells. Because of the limitations of cell numbers of purified primary beta-cells obtained from rat islets, biochemical and molecular studies were performed using the rat insulinoma beta-cell line RINm5F. A serine protease inhibitor, Nalpha-P-tosyl-L-lysine chloromethyl ketone (TLCK), and a proteasome complex (26S) inhibitor, MG 132, inhibited IL-1beta-induced nitrite formation, an oxidation product of nitric oxide produced by iNOS, in a concentration-dependent manner, with complete inhibition observed at 100 micromol/l and 10 micromol/l, respectively. Both TLCK and MG 132 also inhibited iNOS gene expression at the level of mRNA and protein. In an analogous manner, TLCK (100 micromol/l) and MG 132 (10 micromol/l) inhibited IL-1beta-induced COX-2 enzyme activity (PGE2 formation) and COX-2 gene expression at the level of mRNA and protein. In human islets, the proteasome inhibitor MG 132 also inhibited the formation of the products of iNOS and COX-2 enzyme activity, nitrite, and PGE2, respectively. These findings suggest that the inhibitory action of TLCK and MG 132 on iNOS and COX-2 expression precedes transcription. The transcription factor NFkappaB is essential for activation of a number of cytokine-inducible enzymes and was evaluated as a possible site of protease action necessary for IL-1beta-induced coexpression of iNOS and COX-2. TLCK and MG 132 inhibited both IL-1beta-induced activation of NFkappaB and degradation of IkappaBalpha by islets and RINm5F cells. These results implicate protease activation as an early signaling event in IL-1beta-induced inhibition of beta-cell function. This study also suggests that IL-1beta-induced iNOS and COX-2 coexpression by pancreatic beta-cells share a common signaling pathway in utilizing the proteasome complex (26S) and the transcription factor NFkappaB, and it identifies sites of intervention to prevent the overproduction of their inflammatory products.

Insulin mediates glucose-stimulated phosphorylation of PHAS-I by pancreatic beta cells. An insulin-receptor mechanism for autoregulation of protein synthesis by translation

Although glucose regulates the biosynthesis of a variety of beta cell proteins at the level of translation, the mechanism responsible for this effect is unknown. We demonstrate that incubation of pancreatic islets with elevated glucose levels results in rapid and concentration-dependent phosphorylation of PHAS-I, an inhibitor of mRNA cap-binding protein, eukaryotic initiation factor (eIF)-4E. Our initial approach was to determine if this effect is mediated by the metabolism of glucose and activation of islet cell protein kinases, or whether insulin secreted from the beta cell stimulates phosphorylation of PHAS-I via an insulin-receptor mechanism as described for insulin-sensitive cells. In support of the latter mechanism, inhibitors of islet cell protein kinases A and C exert no effect on glucose-stimulated phosphorylation of PHAS-I, whereas the phosphatidylinositol 3-kinase inhibitor, wortmannin, the immunosuppressant, rapamycin, and theophylline, a phosphodiesterase inhibitor, promote marked dephosphorylation of PHAS-I. In addition, exogenous insulin and endogenous insulin secreted by the beta cell line, betaTC6-F7, increase phosphorylation of PHAS-I, suggesting that beta cells of the islet, in part, mediate this effect. Studies with beta cell lines and islets indicate that amino acids are required for glucose or exogenous insulin to stimulate the phosphorylation of PHAS-I, and amino acids alone dose-dependently stimulate the phosphorylation of PHAS-I, which is further enhanced by insulin. Furthermore, rapamycin inhibits by approximately 62% the increase in total protein synthesis stimulated by high glucose concentrations. These results indicate that glucose stimulates PHAS-I phosphorylation via insulin interacting with its own receptor on the beta cell which may serve as an important mechanism for autoregulation of protein synthesis by translation.

Hyperglycemic levels of glucose inhibit interleukin 1 release from RAW 264.7 murine macrophages by activation of protein kinase C

Diabetic patients with hyperglycemia (high blood glucose) have frequent and persistent bacterial infections linked to significantly diminished bactericidal activity and macrophage function. Interleukin-1 (IL-1), released primarily from activated macrophages, is a key mediator of effective host defense against microorganisms. We observe that hyperglycemic levels of D-glucose (8-20 mM) inhibit the release of IL-1 by lipopolysaccharide-stimulated RAW 264.7 murine macrophage cells. An inhibitor of glucose transport and metabolism, 2-deoxyglucose, prevents this inhibition of IL-1 release. High levels (8-20 mM) of fructose and mannose (but not galactose or L-glucose) also inhibit the release of IL-1 activity, suggesting that metabolism is required for IL-1 inhibition. Immunoprecipitation and activity measurements demonstrate that high glucose levels block the release of IL-1 but do not inhibit IL-1 production. High glucose levels (20 mM) increase protein kinase C (PKC) activity, and inhibitors of PKC block the inhibitory effects of glucose. Phorbol 12-myristate 13-acetate, an agonist of PKC, mimics glucose-induced inhibition of IL-1 release. These results demonstrate that high glucose levels inhibit IL-1 release (but not production) by RAW 264. 7 murine macrophages, and this inhibition is mediated by PKC activation. These studies suggest that persistent infections in hyperglycemic patients may be due to an inhibition of IL-1 release from macrophages.

Evidence for the presence of type I IL-1 receptors on beta-cells of islets of Langerhans

The cytokine interleukin-1beta (IL-1beta) has been shown to inhibit insulin secretion and destroy pancreatic islets by a mechanism that involves the expression of inducible nitric oxide synthase (iNOS), and the production of nitric oxide (NO). Insulin containing beta-cells, selectively destroyed during the development of autoimmune diabetes, appear to be the islet cellular source of iNOS following treatment with IL-1beta. In this study we have evaluated the presence of type I IL-1 signaling receptors on purified pancreatic beta-cells. We show that the interleukin-1 receptor antagonist protein (IRAP) prevents IL-1beta-induced nitrite formation and IL-1beta-induced inhibition of insulin secretion by isolated islets and primary beta-cells purified by fluorescence-activated cell sorting (FACS). The protective effects of IRAP correlate with an inhibition of IL-1beta-induced iNOS expression by islets and FACS purified beta-cells. To provide direct evidence to support beta-cell expression of IL-1 type I signaling receptors, we show that antiserum specific for the type I IL-1 receptor neutralizes IL-1beta-induced nitrite formation by RINm5F cells, and that RINm5F cells express the type I IL-1 receptor at the protein level. Using reverse transcriptase-polymerase chain reaction (RT-PCR), the expression of type I IL-1 signaling receptors by FACS purified beta-cells and not alpha-cells is demonstrated. These results provide direct support for the expression of type I IL-1 receptors by primary pancreatic beta-cells, the cell type selectively destroyed during the development of autoimmune diabetes.

Effects of aspirin on nitric oxide formation and de novo protein synthesis by RINm5F cells and rat islets

Aspirin and aspirin-like drugs are the most commonly indicated agents for the treatment of inflammation. Mechanisms of action for these drugs, however, are not clearly understood. In this study, we examined the effects of aspirin on production of nitric oxide (NO), a proinflammatory mediator, and show that aspirin inhibits NO production by transformed pancreatic beta cells (RINm5F) and rat islets in a concentration-dependent manner with an IC50 value of approximately 3 mM. Therapeutic concentrations of aspirin (1-5 mM) that block NO production affected neither nuclear factor-kappaB activation nor inducible NO synthase (iNOS) mRNA transcription but potently inhibited iNOS protein expression by both RINm5F cells and rat islets. The effects of aspirin on islet function were examined by measuring glucose-stimulated insulin secretion in the presence of various concentrations of aspirin. Aspirin (1-5 mM) did not affect insulin secretion at basal or glucose-stimulated conditions, whereas higher concentrations of aspirin (10-20 mM) significantly increased basal insulin secretion. Aspirin at high concentrations of 10 and 20 mM inhibited de novo protein synthesis as demonstrated by inhibition of [35S]methionine incorporation into total islet protein and by inhibition of rabbit reticulocyte expression by Brome mosaic virus mRNA, suggesting that inhibition of iNOS expression at these high concentrations of aspirin may be due to the impairment of the translational machinery. These findings indicate that inhibition of iNOS expression and NO production may explain, in part, the beneficial effects of aspirin as an anti-inflammatory agent at therapeutic concentrations, whereas inhibition of de novo protein synthesis may possibly explain clinical and side effects of aspirin in the inflamed tissues and organs such as stomach and kidney that may accumulate high concentrations of aspirin.

Nitric oxide regulates interleukin 1 bioactivity released from murine macrophages

The bioactivity of interleukin-1 (IL-1), a major proinflammatory cytokine, can be modulated by a variety of factors including inhibitors of IL-1 production and release and receptor blockade by IL-1 receptor antagonist and by binding to nonsignaling soluble receptors. This study demonstrates that the free radical nitric oxide (NO) is also a regulator of IL-1 bioactivity. Lipopolysaccharide-activated murine macrophage RAW264.7 cells, and lipopolysaccharide plus interferon-gamma-activated murine peritoneal macrophages release IL-1 bioactivity, which is increased 10-fold over control levels by 24 h. NG-Monomethyl -arginine (NMMA), a nitric oxide synthase (NOS) inhibitor, almost completely inhibits the release of IL-1 bioactivity from activated macrophages in a time- and concentration-dependent manner with an IC50 of 50 microM. IL-1 activity was determined by thymocyte proliferation bioassay and by a new spectrophotometric bioassay based on IL-1-specific induction of NOS and NO production by an insulinoma cell line, RINm5F. Neither NO nor NOS inhibitors present in the macrophage supernatant interfere with the bioassays. Aminoguanidine and iodonium diphenyl, mechanistically unrelated NOS inhibitors, also prevent the release of IL-1 activity from RAW 264.7 cells. The addition of the NO donor S-nitroso-acetylpenicillamine reconstituted the release of IL-1 bioactivity inhibited by NMMA in a concentration-dependent manner. NO appears to increase the amount of IL-1 protein released by activated macrophages as determined by enzyme-linked immunosorbent assay, but not by mechanisms involving cell death nor modification of IL-1 precursor processing. A cGMP donor, 8-bromo-cGMP, dose-dependently reverses NMMA inhibition of bioactive IL-1 release, suggesting that NO regulates IL-1 release by a cGMP-dependent mechanism. These observations suggest that NO stimulation of the activity of IL-1, a key mediator of the immune response, may be a potentially important mechanism for control of IL-1 activity in vivo.

The Use of Aminoguanidine, a Selective iNOS Inhibitor, to Evaluate the Role of Nitric Oxide in the Development of Autoimmune Diabetes

Insulin-dependent diabetes mellitus is an autoimmune disease characterized by the selective destruction of insulin-secreting beta cells found in islets of Langerhans. The biochemical mechanisms associated with beta-cell destruction have remained elusive. Cytokines, released from T lymphocytes, macrophages, and monocytes during islet insulitis, have been implicated as effector molecules that participate in beta-cell death. Recently, cytokine-induced expression of inducible nitric oxide synthase (iNOS) and production of nitric oxide by beta cells has been suggested as one potential mechanism associated with beta-cell destruction. Treatment of rat islets with interleukin 1 (IL-1) results in a potent inhibition of insulin secretion followed by islet destruction. The inhibitory and destructive effects of this cytokine on islet function are completely prevented by the inhibition of iNOS enzymatic activity. Islets contain a heterogeneous population of both endocrine and nonendocrine cells including a low level of resident tissue macrophages ( approximately0.5% of all islet cells). The intraislet macrophage appears to one cellular source of IL-1. Activation of resident islet macrophages results in both the expression of iNOS and the release of IL-1. Intraislet macrophage production of nitric oxide (in the absence of IL-1) does not modulate beta-cell function; however, macrophage release of IL-1 and IL-1-induced iNOS expression by beta cells results in a potent inhibition of beta-cell function. These findings support a role for nitric oxide as a potential mediator of cytokine-induced inhibition of beta-cell function and implicate the intraislet macrophage as one cellular source of IL-1. Direct support for a role of nitric oxide in the development of diabetes includes the ability of inhibitors of iNOS to prevent or delay the development of this disease condition in animal models. Important to these studies has been the identification of selective inhibitors of iNOS. Many inhibitors of nitric oxide synthase have been developed; however, few selective inhibitors for the individual isoforms of NOS (inducible, endothelial, neuronal) have been described. Aminoguanidine has been identified as one of the first iNOS selective inhibitors. Aminoguanidine is over 50-fold more effective at inhibiting the enzymatic activity of iNOS than endothelial or neuronal NOS. The effects of aminoguanidine on the development of diabetes in the nonobese diabetic mouse using an adoptive transfer protocol has been evaluated. Aminoguanidine delays the onset of diabetes in this animal model by 7-10 days. These studies, which provide in vivo evidence implicating a role for nitric oxide in the development of autoimmune diabetes, also support the use of selective inhibitors of iNOS for the attenuation of disease conditions associated with the expression of iNOS and an increased production of nitric oxide.

Tyrosine kinase inhibitors prevent cytokine-induced expression of iNOS and COX-2 by human islets

Insulin-dependent diabetes mellitus (IDDM) is an autoimmune disease that is characterized by selective destruction of insulin-secreting beta-cells. Cytokines have been implicated as effector molecules that participate in both islet inflammation and beta-cell destruction during the development of IDDM. In this study, the effects of cytokines on the expression of inducible nitric oxide synthase (iNOS) and inducible cyclooxygenase (COX-2) by human islets were examined. In combination, the cytokines, human recombinant interleukin-1 beta (IL-1 beta), human recombinant tumor necrosis factor-alpha (TNF-alpha), and human recombinant interferon-gamma (IFN-gamma), induce the time-dependent formation of nitrite and prostaglandin E2 (PGE2) by human islets. The nitric oxide synthase inhibitor NG-monomethyl-L-arginine (L-NMMA) completely inhibits cytokine-induced nitrite formation and attenuates PGE2 production by human islets. L-NMMA does not inhibit cytokine-induced expression of COX-2 by human islets, suggesting that nitric oxide may directly activate cyclooxygenase, an effect that has been previously demonstrated for isolated rat islets. This combination of cytokines (IL-1 beta, TNF-alpha, and IFN-gamma) also induces the expression of iNOS mRNA by human islets as demonstrated by both reverse transcriptase-polymerase chain reaction and Northern blot analysis. We further show that the tyrosine kinase inhibitors genistein and herbimycin A prevent IL-1 beta plus IFN-gamma-induced expression of COX-2 and iNOS and the production of PGE2 and nitric oxide by human islets. These results demonstrate that cytokines induce the expression of iNOS and COX-2 by human islets and that cytokine-induced expression of both COX-2 and iNOS by human islets appears to require the activation of a tyrosine kinase(s).

Nitric oxide production by the rat insulinoma cell line, RINm5F, Is specific for IL-1: a spectrophotometric IL-1 bioassay

Cytokines inhibit glucose-induced insulin secretion from pancreatic beta-cells by stimulating the expression of nitric oxide synthase and the increased production of nitric oxide (NO). We have found that the rat insulinoma cell line, RINm5F, responds specifically and linearly to murine and human interleukin-1beta (IL-1beta) and IL-1alpha in the range of 0.1 to 1 unit/ml to produce nitric oxide. Other cytokines, including IL-2, IL-4, IL-6, IL-9, IL-11, IL-15, tumor necrosis factor-alpha, interferon-gamma, and lipopolysaccharide fail to stimulate nitric oxide formation by RINm5F cells either alone or in combination. In addition, these cytokines do not significantly potentiate or attenuate the IL-1 response. This unprecedented specificity to IL-1 has been further developed as a sensitive and specific assay for IL-1 bioactivity. Quantitation by this new bioassay of human IL-1beta and IL-1 released from activated murine peritoneal macrophages showed a close correlation with the quantitation of IL-1 by enzyme immunoassay (ELISA). This new bioassay, which is specific, nonradioactive and inexpensive, represents a significant improvement over current bioassays for IL-1.

Characterization of the sphingomyelin content of isolated pancreatic islets. Evaluation of the role of sphingomyelin hydrolysis in the action of interleukin-1 to induce islet overproduction of nitric oxide

Inflammatory cytokines may participate in the destruction of pancreatic islets during the pathogenesis of insulin-dependent diabetes mellitus, and the cytokine interleukin-1 (IL-1) strongly inhibits insulin secretion from rat pancreatic islets by a process which involves induction of expression of the inducible isoform of nitric oxide synthase and the overproduction of nitric oxide. The signaling events between IL-1 receptor occupancy and induction of nitric oxide synthase in rat islets involve activation of the transcriptional activator NFkappa B. Because sphingomyelin hydrolysis has been implicated as a signaling process both in NFkappa B activation and in IL-1 action in some cells, we have examined the potential involvement of sphingomyelin hydrolysis in the induction of islet nitric oxide overproduction by IL-1. Rat islet sphingomyelin pools were radiolabeled with [3H]choline, and sphingomyelin was then isolated by normal phase HPLC. Electrospray ionization-mass spectrometric analysis revealed islet sphingomyelin consists of at least 4 distinct molecular species, and the most abundant of them contained sphingosine as the long chain base and a residue of palmitic acid as the fatty acid substituent. Molecular species containing residues of stearic acid and arachidic acid were also observed. Neither interleukin-1 nor tumor necrosis factor-alpha was found to induce hydrolysis of islet sphingomyelin species, and neither an exogenous, cell-permeant ceramide species (N-acetyl-D-sphingosine) nor exogenous sphingomyelinase mimicked or potentiated the effect of IL-1 to increase rat islet nitric oxide generation, as reflected by nitrite production. Similar findings were obtained with RINm5F insulinoma cells and with mouse pancreatic islets. These findings provide the first information on the molecular species of sphingomyelin in pancreatic islets and suggest that sphingomyelin hydrolysis is not involved in the signaling pathway whereby IL-1 induces the overproduction of nitric oxide by pancreatic islets.

Interleukin 1-induced Fos and Jun do not regulate inducible nitric oxide synthase in rat islets of Langerhans and RINm5F cells

Recent evidence indicates that nitric oxide (NO) produced after expression of inducible NO synthase (iNOS) mediates cytokine-induced inhibition of insulin secretion by pancreatic islets. The current studies were designed to characterize the involvement of immediate-early response genes, c-fos and c-jun, in interleukin 1 (IL-1)-induced expression of iNOS. iNOS messenger RNA (mRNA) expression by both rat islets and RINm5F cells was time dependent, with maximal expression observed after an approximately 3- to 6-h exposure to IL-1. IL-1 also stimulated rapid and transient expression of c-fos and c-jun by both rat islets and RINm5F cells, with maximal mRNA accumulation detected 30-60 min after IL-1 treatment. IL-1-induced protein synthesis of Fos and Jun was observed as early as 30 min, peaked between 3-5 h, and decreased by 8 h after IL-1 treatment. Temporal correlation of Fos and Jun expression and iNOS gene induction suggested that Fos and Jun might regulate iNOS gene transcription by rodent pancreatic beta-cells. The present study, however, indicates that IL-1 induced expression of Fos and Jun does not seem to participate in the regulation of iNOS and mRNA expression, because: 1) cycloheximide (1 microM) completely inhibited Fos expression but had no inhibitory effect on iNOS mRNA levels; and 2) tyrosine kinase inhibitors genistein and herbimycin A completely inhibited IL-1 induced iNOS expression but did not block c-fos and c-jun expression. These results indicate that two separate signaling pathways may exist for induction of c-fos and c- jun and iNOS genes and that de novo synthesis of Fos and Jun does not participate in the regulation of iNOS gene expression.

Experimental allergic encephalomyelitis in the rat is inhibited by aminoguanidine, an inhibitor of nitric oxide synthase

This study assessed the role of de novo nitric oxide (NO) production in the pathogenesis of experimental allergic encephalomyelitis (EAE) by using aminoguanidine (AG), an inhibitor of nitric oxide synthase (NOS), which preferentially inhibits the cytokine- and endotoxin-inducible isoform of NOS versus the constitutive isoforms consisting of endothelial and neuronal NOS. The maximum clinical severity of EAE and the duration of illness were significantly reduced or totally inhibited by twice daily subcutaneous injection of 100 mg/kg body weight AG. Histochemical staining for NADPH diaphorase, which detects enzymatic activity of NOS, revealed positive reactivity in untreated EAE rats both in parenchymal blood vessel walls and in anterior horn cell neurons, while normal rats and rats with EAE treated with AG showed predominantly the neuronal positivity. Moreover, this NADPH staining pattern was further supported by the immunohistochemical findings that endothelial NOS (eNOS) expression was increased in blood vessels in the inflamed lesions of untreated EAE rats and that inducible NOS (iNOS) was detected in some inflammatory cells, while treatment with AG could significantly reduce both iNOS and eNOS production. These results suggest that: (i) both iNOS and eNOS are upregulated in inflamed areas of the rat central nervous system in EAE; (ii) increased NO production plays a role in the development of clinical signs in EAE; and (iii) selective inhibitors of iNOS and/or eNOS may have therapeutic potential for the treatment of certain autoimmune diseases.

Interleukin-1 enhances pancreatic islet arachidonic acid 12-lipoxygenase product generation by increasing substrate availability through a nitric oxide-dependent mechanism

Interleukin-1 (IL-1) impairs insulin secretion from pancreatic islets and may contribute to the pathogenesis of insulin-dependent diabetes mellitus. IL-1 increases islet expression of nitric oxide (NO) synthase, and the resultant overproduction of NO participates in inhibition of insulin secretion because NO synthase inhibitors, e.g. NG-monomethyl-arginine (NMMA), prevent this inhibition. While exploring effects of IL-1 on islet arachidonic acid metabolism, we found that IL-1 increases islet production of the 12-lipoxygenase product 12-hydroxyeicosatetraenoic acid 12-(HETE). This effect requires NO production and is prevented by NMMA. Exploration of the mechanism of this effect indicates that it involves increased availability of the substrate arachidonic acid rather than enhanced expression of 12-lipoxygenase. Evidence supporting this conclusion includes the facts that IL-1 does not increase islet 12-lipoxygenase protein or mRNA levels and does not enhance islet conversion of exogenous arachidonate to 12-HETE. Mass spectrometric stereochemical analyses nonetheless indicate that 12-HETE produced by IL-1-treated islets consists only of the S-enantiomer and thus arises from enzyme action. IL-1 does enhance release of nonesterified arachidonate from islets, as measured by isotope dilution mass spectrometry, and this effect is suppressed by NMMA and mimicked by the NO-releasing compound 3-morpholinosydnonimine. Although IL-1 increases neither islet phospholipase A2 (PLA2) activities nor mRNA levels for cytosolic or secretory PLA2, a suicide substrate which inhibits an islet Ca(2+)-independent PLA2 prevents enhancement of islet arachidonate release by IL-1. IL-1 also impairs esterification of [3H8]arachidonate into islet phospholipids, and this effect is prevented by NMMA and mimicked by the mitochondrial ATP-synthase inhibitor oligomycin. Experiments with exogenous substrates indicate that NMMA does not inhibit and that the NO-releasing compound does not activate islet 12-lipoxygenase or PLA2 activities. These results indicate that a novel action of NO is to increase levels of nonesterified arachidonic acid in islets.

Cytokines and nitric oxide in islet inflammation and diabetes

Cytokines released by both T lymphocytes and activated macrophages, in particular interleukin-1 (IL-1), have been implicated as immunological effector molecules that both inhibit insulin secretion from the pancreatic beta cell and induce beta-cell destruction. Recent findings have demonstrated that production of the free radical nitric oxide (NO), resulting from the expression of the cytokine-inducible isoform of NO synthase (iNOS), mediates these deleterious effects. The cellular mechanism responsible for inhibition of beta-cell function and destruction by NO involves, in part, inactivation of enzymes specifically localized to the beta-cell mitochondria that contain iron- sulfur centers or clusters. Intraislet release of IL-1 also inhibits beta-cell function by this same cellular mechanism involving the overproduction of NO. In addition, the cytokine, IL-1, induces the co-expression of both iNOS and the cytokine-inducible isoform of cyclooxygenase, COX-2. The expression of COX-2 results in the overproduction of the proinflammatory prostaglandins and thromboxanes. Furthermore, NO produced by iNOS directly stimulates the activities of both constitutive and inducible isoforms of COX, further augmenting the overproduction of these proinflammatory mediators, NO and prostaglandins, which may be important in initiating or maintaining the inflammatory response and destruction of the beta cell associated with autoimmune diabetes.

Interleukin-1 beta-induced nitric oxide synthase expression by rat pancreatic beta-cells: evidence for the involvement of nuclear factor kappa B in the signaling mechanism

Recent evidence indicates that overproduction of nitric oxide mediates cytokine-induced inhibition of insulin secretion by pancreatic islets. The current studies were designed to characterize signaling events involving the transcriptional factor NFkappaB in interleukin-1 (IL-1)-induced expression of inducible nitric oxide synthase (iNOS) by primary and transformed rat pancreatic beta-cells. Due to limitations of cell numbers of purified primary beta-cells, biochemical and molecular studies were performed primarily using the insulinoma cell line, RINm5F. Inhibitors of NFkappaB, diethyldithiocarbamate, pyrrolidine dithiocarbamate, and N-acetyl cysteine prevent IL-1-induced iNOS expression at the level of messenger RNA, protein, and nitrite generation. IL-1 induces a time-dependent translocation of NFkappaB from cytosol to nucleus, with maximal translocation observed approximately 15-30 min after IL-1 treatment, as determined by electrophoretic mobility shift assays. The specificity of the band containing the NF kappa B DNA-protein complex was shown by competition with a 150-fold excess of nonradiolabeled NF kappa B oligonucleotide. Supershift assays using immunoglobulins G against NF kappa b subunits p50 an p65 indicate that the protein complex contains a heterodimer of p50 and p65. IL-1-induced translocation of NF kappa B was blocked by 100 microns 100 microM diethyldithiocarbamate or 100 microM pyrrolidine dithiocarbamate, further establishing a critical role for NF kappa B in the induction of iNOS by IL-1 in rat pancreatic beta-cells. Activation of tyrosine kinase appears to precede NF kappa B activation, as the tyrosine kinase inhibitor genistein (100 microM) blocks IL-1-induced translocation of NF kappa B. An understanding of the signal transduction pathway of cytokine-induced nitric oxide generation by beta-cells will provide strategies of intervention to further evaluate the role of nitric oxide in mediating beta-cell dysfunction.

Intraislet release of interleukin 1 inhibits beta cell function by inducing beta cell expression of inducible nitric oxide synthase

Cytokines, released in and around pancreatic islets during insulitis, have been proposed to participate in beta-cell destruction associated with autoimmune diabetes. In this study we have evaluated the hypothesis that local release of the cytokine interleukin 1 (IL-1) by nonendocrine cells of the islet induce the expression of inducible nitric oxide synthase (iNOS) by beta cells which results in the inhibition of beta cell function. Treatment of rat islets with a combination of tumor necrosis factor (TNF) and lipopolysaccharide (LPS), conditions known to activate macrophages, stimulate the expression of iNOS and the formation of nitrite. Although TNF+LPS induce iNOS expression and inhibit insulin secretion by intact islets, this combination does not induce the expression of iNOS by beta or alpha cells purified by fluorescence activated cell sorting (Facs). In contrast, IL-1 beta induces the expression of iNOS and also inhibits insulin secretion by both intact islets and Facs-purified beta cells, whereas TNF+LPS have no inhibitory effects on insulin secretion by purified beta cells. Evidence suggests that TNF+LPS inhibit insulin secretion from islets by stimulating the release of IL-1 which subsequently induces the expression of iNOS by beta cells. The IL-1 receptor antagonist protein completely prevents TNF+LPS-induced inhibition of insulin secretion and attenuates nitrite formation from islets, and neutralization of IL-1 with antisera specific for IL-1 alpha and IL-1 beta attenuates TNF+LPS-induced nitrite formation by islets. Immunohistochemical localization of iNOS and insulin confirm that TNF+LPS induce the expression of iNOS by islet beta cells, and that a small percentage of noninsulin-containing cells also express iNOS. Local release of IL-1 within islets appears to be required for TNF+LPS-induced inhibition of insulin secretion because TNF+LPS do not stimulate nitrite formation from islets physically separated into individual cells. These findings provide the first evidence that a limited number of nonendocrine cells can release sufficient quantities of IL-1 in islets to induce iNOS expression and inhibit the function of the beta cell, which is selectively destroyed during the development of autoimmune diabetes.

Activation of calcium and calmodulin dependent protein kinase II during stimulation of insulin secretion

Pancreatic islets contain an alloxan sensitive, calcium and calmodulin dependent protein kinase (CaM-PK) which may play an important part in the cellular control of insulin secretion. We have studied this activity in islets and the insulin secreting tumor cell line RINm5f with particular interest in the changes in kinase activity that accompany stimulation of secretion. Initial experiments showed that the CaM-PK activity enriched in microsomal preparations from RIN cells was similar to the islet cell kinase in terms of apparent endogenous substrates, Ca2+ and calmodulin dependence, and inactivation by alloxan. For studies of protein substrate specificity, tumor cell CaM-PK was isolated from other kinase activities and substantially purified by affinity chromatography with calmodulin-agarose. The major protein substrates of CaM-PK (54 kD and 57 kD) co-purified with the kinase activity, representing autophosphorylation of subunits of the enzyme. Exogenous substrates phosphorylated by these preparations included microtubule-associated protein 2, synapsin, and glycogen synthase; this pattern of substrate utilization identified the kinase as the Type II multifunctional kinase which has been extensively characterized in brain. A polyclonal antibody to rat brain CaM-PK II was employed to immunoprecipitate the kinase from RINm5f cells incubated with secretagogues to measure the effect of stimulation of secretion on autophosphorylation of CaM-PK (which reflects kinase activation). D-Glyceraldehyde (22 mM) and depolarizing concentrations of potassium increased autophosphorylation and insulin secretion in a parallel fashion. Potassium stimulated autophosphorylation was dose dependent and saturable, and was increased to near maximal levels at times as short as 1 min. These studies demonstrate that pancreatic islets and RINm5f cells contain a Type II CaM-PK which is activated during the secretion process.

Tyrosine kinase involvement in IL-1 beta-induced expression of iNOS by beta-cells purified from islets of Langerhans

Nitric oxide is believed to mediate the inhibitory effects of cytokines on glucose-stimulated insulin secretion by both rat and human islets. The aims of this study were 1) to determine the cellular source of the cytokine-inducible isoform of nitric oxide synthase (iNOS) expressed in islets following cytokine stimulation and 2) to determine whether tyrosine kinase activity participates in cytokine-induced iNOS expression. In this report we demonstrate that the cytokine interleukin-1 beta (IL-1 beta) stimulates the expression of iNOS and the formation of nitric oxide (as determined by nitrite formation, a stable oxidative product of nitric oxide) by isolated intact rat islets and by primary beta-cells purified by fluorescence-activated cell sorting (FACS). Both the expression of iNOS and nitrite formation induced by IL-1 beta were prevented by the mRNA transcriptional inhibitor actinomycin D. IL-1 beta did not induce the expression of iNOS by FACS-purified alpha-cells, the other major endocrine cell type of the islet. The tyrosine kinase inhibitors genistein and herbimycin A prevented IL-1 beta-induced expression of immunoprecipitable iNOS and nitrite release by islets, by insulinoma RINm5F cells, and by FACS-purified beta-cells. Herbimycin A and genistein also prevented IL-1 beta-induced iNOS mRNA accumulation as determined by Northern blot analysis of total RNA isolated from RINm5F cells. These findings indicate tyrosine kinase activation participates in IL-1 beta-induced expression of iNOS by the insulin-secreting beta-cell.(ABSTRACT TRUNCATED AT 250 WORDS)

Reversibility of interleukin-1 beta-induced islet destruction and dysfunction by the inhibition of nitric oxide synthase

We have examined the reversibility of NO-mediated islet dysfunction and destruction induced by interleukin-1 beta (IL-1 beta). Previous studies have shown that pretreatment of islets for 18 h with IL-1 beta results in an inhibition of glucose-stimulated insulin secretion that requires 4 days incubation in the absence of IL-1 beta to restore islet secretory function. In this study we use a sequential experimental design in which islets are first exposed to IL-1 beta for 18 h, and then treated with the NO synthase inhibitor NG-monomethyl-L-arginine (NMMA). Insulin secretion is inhibited by 98% after the 18 h incubation with IL-1 beta, and this inhibition is reversed in a time-dependent fashion by NMMA, with complete recovery of insulin secretion observed 8 h after the inhibition of NO synthase. Inhibition of NO synthase also restores IL-1 beta-induced inhibition of mitochondrial aconitase activity in a time-dependent fashion that mimics the recovery of glucose-stimulated insulin secretion by islets. Ferrous iron and the reducing agents cysteine and thiosulphate accelerate the rate of recovery of insulin secretion, and ferrous iron and thiosulphate stimulate the recovery of islet aconitase activity, suggesting that iron-sulphurcentre reconstitution may be involved in the recovery process. The recovery process also appears to require mRNA transcription, because the transcriptional inhibitor actinomycin D prevents the recovery of insulin secretion by islets after the inhibition of NO synthase. Although IL-1 beta induces the co-expression of NO synthase and cyclo-oxygenase by islets, cyclo-oxygenase is not involved in the recovery of glucose-stimulated insulin secretion. Inhibition of NO synthase also prevents IL-1 beta-induced islet destruction, which otherwise occurs during a 96 h continuous exposure to this cytokine. The destructive effects of IL-1 beta on islet viability are prevented if NMMA is added to islet cultures during the first 24 h of exposure to IL-1 beta, but islet destruction is not prevented if NMMA is added after the first 48 h exposure to IL-1 beta. These results show that IL-1 beta-induced islet dysfunction is reversed by the inhibition of NO synthase, that recovery of insulin secretion is stimulated by iron and reducing agents, and that the recovery process appears to require mRNA transcription. We also show that it is possible to rescue islets from the destructive effects of IL-1 beta if NO synthase is inhibited early after its induction.

Potential autoantigens in IDDM. Expression of carboxypeptidase-H and insulin but not glutamate decarboxylase on the beta-cell surface

Insulin, carboxypeptidase-H (CP-H), and glutamate decarboxylase (GAD) have been identified as potential autoantigens in insulin-dependent diabetes mellitus (IDDM). Previous studies have described immunoreactive insulin as a surface molecule on the plasma membrane of rat islet cells and suggested that cell-surface insulin was derived during exocytosis by the fusion of insulin secretory granules with the beta-cell plasma membrane. These findings predict that insulin and other secretory granule-derived proteins such as the putative autoantigen CP-H may be colocalized with insulin at specific sites of exocytosis on the beta-cell surface. In studies to test this hypothesis, cell-surface staining of dispersed rat islet cells occurred in a granule-like pattern with antibodies for CP-H and insulin. The specificity of the CP-H antiserum was confirmed by immunoblotting and indicated that the antiserum was essentially monospecific for CP-H. Confocal laser microscopy confirmed that immunoreactive staining for CP-H and insulin was confined to the beta-cell surface. Colocalization of CP-H and insulin on the cell surface of beta-cells was demonstrated by double staining with antibodies to CP-H and insulin, and the percentage of beta-cells positive for both of these autoantigens increased twofold with increases in insulin secretion. In contrast, islet cells failed to reveal cell-surface staining for GAD65, another putative autoantigen in IDDM, under either basal or insulin stimulatory conditions or following exposure of islet cells to the cytokines interleukin-1 beta, tumor necrosis factor-alpha, and recombinant human interferon-gamma.(ABSTRACT TRUNCATED AT 250 WORDS)

Biochemical evidence for nitric oxide formation from streptozotocin in isolated pancreatic islets

Streptozotocin (STZ) is selectively toxic to insulin-secreting beta-cells of pancreatic islets and induces impairment of islet glucose oxidation and of glucose-induced insulin secretion. Similar effects are induced by Interleukin-1 (IL-1), and the deleterious effects of IL-1 on islets appear to be mediated by nitric oxide (NO). STZ contains a nitroso moiety and may liberate NO by processes analogous to those for the NO-releasing drug nitroprusside. NO is rapidly transformed to nitrite in aqueous solution, and NO activates heme-containing enzymes such as guanylyl cyclase and inhibits iron-sulfur enzymes such as mitochondrial aconitase. Data presented here indicate that incubation of rat islets with STZ at concentrations that impair insulin secretion results in generation of nitrite, stimulation of islet guanylyl cyclase and accumulation of cGMP, and inhibition of islet mitochondrial aconitase activity to a degree similar to that achieved by IL-1. Effects of STZ on beta-cells may be mediated by local liberation of NO from STZ within islets.

Interferon-gamma and interleukin-1 beta induce nitric oxide formation from primary mouse astrocytes

An inducible form of nitric oxide synthase (iNOS) capable of producing large quantities of nitric oxide (NO) exists in some cell types. We demonstrate by immunoprecipitation and nitrite formation that interleukin-1 beta (IL1 beta) plus interferon-gamma (INF gamma) induce the expression of nitric oxide synthase in primary cultures of murine cortical astrocytes. This induction is time and dose dependent, and inhibited by the NOS inhibitor NG-nitro-L-arginine and the protein synthesis inhibitor cycloheximide.

IL-1 beta induces the coexpression of both nitric oxide synthase and cyclooxygenase by islets of Langerhans: activation of cyclooxygenase by nitric oxide

Autoimmune diabetes is characterized by an early infiltration of lymphocytes into and around islets, which is followed by selective destruction of the insulin-secreting beta-cell. Cytokines released during this inflammatory reaction have been implicated as effector molecules which mediate beta-cell destruction. In vitro treatment of rat islets with the cytokine IL-1 beta results in an inhibition of glucose-stimulated insulin secretion that is mediated by the overproduction of nitric oxide. IL-1 beta also stimulates the production of the cyclooxygenase (COX) product prostaglandin E2 (PGE2). In this study we have examined the effects of IL-1 beta on both inducible nitric oxide synthase (iNOS) and inducible cyclooxygenase (iCOX) expression, and the direct effects of nitric oxide on the activity of COX. Treatment of rat islets with 5 units/mL IL-1 beta induces a similar time-dependent production of both nitrite and PGE2. IL-1 beta-induced nitrite and PGE2 production is attenuated by the NOS inhibitor NG-monomethyl-L-arginine (NMMA), but NMMA has no inhibitory effect on the expression of either iCOX or iNOS as determined by immunoprecipitation. Actinomycin D prevents IL-1 beta-induced iCOX and iNOS expression and the production of both nitrite and PGE2 by islets, suggesting that mRNA transcription is required for IL-1 beta-induced expression of both iNOS and iCOX. The effects of exogenous arachidonic acid on both constitutive COX (cCOX) and iCOX activity were also investigated.(ABSTRACT TRUNCATED AT 250 WORDS)