Development of autoimmune diabetes in NOD mice is associated with the formation of peroxynitrite in pancreatic islet beta-cells

Immunohistochemical Demonstration of Nitrotyrosine, a Biomarker of Oxidative Stress, in Islet Cells of the NOD Mouse

Reactive oxygen species and nitric oxide generated within the beta cell or by intraislet immune cells may be major mediators of beta cell death during insulin-dependent diabetes mellitus. Here, the intraislet expression of nitrotyrosine, a biomarker of oxidative damage, and its cellular sources were examined in the islets of NOD mice at various stages of spontaneous and cyclophosphamide-accelerated diabetes. At day 30, nitrotyrosine-positive cells were undetectable; they were rare at day 40, being expressed in only a few beta cells and in macrophages located in the periphery of some islets. At day 90 and at onset of diabetes, an increasing number of macrophages and beta cells were nitrotyrosine positive. In the cyclophosphamide group at day 4, the number of beta cells and macrophages with positive immunolabeling declined slightly from day 0 (day of cyclophosphamide injection = day 90). This pattern of immunolabeling increased gradually by day 7 but increased markedly by days 11 and 14 after cyclophosphamide administration. In the spontaneous and cyclophosphamide groups, macrophages were the predominant source of nitrotyrosine and were present within the intraislet and periislet regions. Nitrotyrosine labeling was observed in a significant number of beta cells but less than in immunolabeled macrophages. We conclude that protein nitration in beta cells and macrophages is an important feature of the diabetogenic process in the NOD mouse. Whether this deleterious process also occurs during the very early stages of the disease and preceding insulitis is unclear.

HIV-1 Tat-mediated protein transduction of Cu,Zn-superoxide dismutase into pancreatic beta cells in vitro and in vivo

Reactive oxygen species (ROS) are considered an important mediator in pancreatic beta cell destruction, thereby triggering the development of insulin-dependent diabetes mellitus. In the present study, we investigated the HIV-1 Tat protein transduction domain-mediated transduction of Cu,Zn-superoxide dismutase (SOD), which supplies SOD activity exogenously in pancreatic beta cells under oxidative stress. Tat-SOD fusion protein was successfully delivered into insulin-producing RINm5F cells and rat islet cells. The intracellular dismutation activities of SOD were found to increase in line with the amount of protein delivered into the cells. ROS, nitric oxide-induced cell death, lipid peroxidation, and the DNA fragmentation of insulin-producing cells were found to be significantly reduced when the cells were pretreated with Tat-SOD. Next, we examined the in vivo transduction of Tat-SOD into streptozotocin-induced diabetic mice. A single intraperitoneal injection of Tat-SOD resulted in the delivery of this biologically active enzyme to the pancreas. Moreover, increased radical scavenging activity in the pancreas was induced by multiple injections of Tat-SOD, and this enhanced the tolerance of pancreatic beta cells to oxidative stress. These results suggest that the transduction of Tat-SOD offers a new strategy for protecting pancreatic beta cells from destruction by relieving oxidative stress in ROS-implicated diabetes.

Antioxidant enzyme activity and mRNA expression in the islets of Langerhans from the BB/S rat model of type 1 diabetes and an insulin-producing cell line

It has been proposed that low activities of antioxidant enzymes in pancreatic beta cells may increase their susceptibility to autoimmune attack. We have therefore used the spontaneously diabetic BB/S rat model of type 1 diabetes to compare islet catalase and superoxide dismutase activities in diabetes-prone and diabetes-resistant animals. In parallel studies, we employed the RINm5F beta cell line as a model system (previously validated) to investigate whether regulation of antioxidant enzyme activity by inflammatory mediators (cytokines, nitric oxide) occurs at the gene or protein expression level. Diabetes-prone rat islets had high insulin content at the age used (58-65 days) but showed increased amounts of DNA damage when subjected to cytokine or hydrogen peroxide treatments. There was clear evidence of oxidative damage in freshly isolated rat islets from diabetes-prone animals and significantly lower catalase and superoxide dismutase activities than in islets from age-matched diabetes-resistant BB/S and control Wistar rats. The mRNA expression of antioxidant enzymes in islets from diabetes-prone and diabetes-resistant BB/S rats and in RINm5F cells, treated with a combination of cytokines or a nitric oxide donor, DETA-NO, was analysed semi-quantitatively by real time PCR. The mRNA expression of catalase was lower, whereas MnSOD expression was higher, in diabetes-prone compared to diabetes-resistant BB/S rat islets, suggesting regulation at the level of gene expression as well as of the activities of these enzymes in diabetes. The protein expression of catalase, CuZnSOD and MnSOD was assessed by Western blotting and found to be unchanged in DETA-NO treated cells. Protein expression of MnSOD was increased by cytokines in RINm5F cells whereas the expression of CuZnSOD was slightly decreased and the level of catalase protein was unchanged. We conclude that there are some changes, mostly upregulation, in protein expression but no decreases in the mRNA expression of catalase, CuZnSOD or MnSOD enzymes in beta cells treated with either cytokines or DETA-NO. The lower antioxidant enzyme activities observed in islets from diabetes-prone BB/S rats could be a factor in the development of disease and in susceptibility to DNA damage in vitro and could reflect islet alterations prior to immune attack or inherent differences in the islets of diabetes-prone animals, but are not likely to result from cytokine or nitric oxide exposure in vivo at that stage.

Beta cell death and protection

Type 1 diabetes is an immune-mediated disease critically dependent upon the interaction between antigen-presenting cells and T cells. Clearly, both CD4+ and CD8+ T cells are required, but activated CD4+ T cells are both necessary and sufficient in causing disease. The mechanism of the Th1/Th2 immunoregulatory imbalance is unclear and needs to be further investigated. CD8+ T cells are not commonly sufficient in causing disease, but CD8 T cells are necessary in initiation (<14 weeks in the NOD mouse), but not in the later (>14 weeks) effector phase of the disease. It is still unclear whether the CD8+ T cell exerts its function as a classical effector cell or mainly as an immunomodulatory cell acting in synergy with the CD4+ T cell. The relative role of T cell effector mechanisms such as Fas/FasL, perforin/granzyme, and the TRAIL systems is unclear. Proinflammatory cytokines, reactive oxygen species, and other immune mediators seem to be involved in beta cell destruction, but much is to be learned about signaling, molecular mechanisms, and in vivo importance.

Metallothionein Protects Islets from Hypoxia and Extends Islet Graft Survival by Scavenging Most Kinds of Reactive Oxygen Species

Islet transplantation is a promising therapy for Type 1 diabetes, but many attempts have failed due to early graft hypoxia or immune rejection, which generate reactive oxygen species (ROS). In the current study, we determined that transgenic overexpression of the antioxidant metallothionein (MT) in pancreatic beta cells provided broad resistance to oxidative stress by scavenging most kinds of ROS including H2O2, peroxynitrite radical released from streptozotocin, 3-morpholinosydnonimine (SIN-1), and superoxide radical produced by xanthine/xanthine oxidase. MT also reduced nitric oxide-induced beta cell death. A direct test of hypoxia/reperfusion sensitivity was made by exposing FVB and MT islets to hypoxia (1% O2). MT markedly reduced ROS production and improved islet cell survival. Because MT protected beta cells from a broad spectrum of ROS and from hypoxia, we considered it to be an ideal candidate for improving islet transplantation. We first tested syngeneic transplantation by implanting islets under the kidney capsule of the same strain, FVB mice, thereby eliminating the immune rejection component. Under these conditions, MT islets maintained much greater insulin content than control islets. Allotransplantation was then tested. MT transgenic and normal FVB islets were implanted under the kidney capsule of BALB/c mice that were previously treated with streptozotocin to induce diabetes. We found that MT islets extended the duration of euglycemia 2-fold longer than nontransgenic islets. The benefit of MT was due to protection from ROS since nitrotyrosine staining, an indicator of free radical damage, was much lower in MT grafts than in FVB grafts. The time course of protection suggested that the major mode of MT action may have been protection from hypoxia or hypoxia/reperfusion. These data demonstrate that treatment with a broad spectrum antioxidant protects islets from ROS damage such as that produced during the early phase of islet transplantation.

Autoimmune Diabetes in the NOD Mouse: An Essential Role of Fas-FasL Signaling in β Cell Apoptosis

Despite evidence that both Fas and FasL can be expressed in pancreatic islets, there has been considerable controversy regarding the potential role of Fas signaling in autoimmune beta cell death. Using the HIPFasL model, we have been able to demonstrate that, in the presence of an inflammatory infiltrate, FasL-expressing beta cells are exquisitely sensitive to Fas-mediated apoptosis and that this can be blocked by preventing FasL-Fas interaction. This points to a highly important role of Fas-FasL interaction in autoimmune beta cell death.

Fructus Benincasae RecensExtract Prevents Cytokine‐Induced Nitric Oxide Formation and Cytotoxicity of RINm5F Cells

Cytokines produced by immune cells infiltrating pancreatic islets are important mediators of beta-cell destruction in insulin-dependent diabetes mellitus. In this study, the effects of Fructus Benincasae Recens (FBR) extract on cytokine-induced beta-cell dysfunction were examined. Fructus Benincasae Recens extract completely protected interleukin-1beta (IL-1beta) and interferon-gamma (IFN-gamma)-mediated cytotoxicity in rat insulinoma cell line (RINm5F). Incubation with FBR extract resulted in a significant reduction of IL-1beta and IFN-gamma-induced nitric oxide (NO) production, a finding that correlated well with reduced levels of the inducible form of NO synthase (iNOS) mRNA and protein. The molecular mechanism by which FBR extract inhibited iNOS gene expression appeared to involve the inhibition of NF-kappaB activation. Our results revealed the possible therapeutic value of FBR extract for the prevention of diabetes mellitus progression.

A defect in tryptophan catabolism impairs tolerance in nonobese diabetic mice

The predisposition of nonobese diabetic (NOD) mice to develop autoimmunity reflects deficiencies in both peripheral and central tolerance. Several defects have been described in these mice, among which aberrant antigen-presenting cell function and peroxynitrite formation. Prediabetes and diabetes in NOD mice have been targeted with different outcomes by a variety of immunotherapies, including interferon (IFN)-gamma. This cytokine may be instrumental in specific forms of tolerance by virtue of its ability to activate immunosuppressive tryptophan catabolism. Here, we provide evidence that IFN-gamma fails to induce tolerizing properties in dendritic cells from highly susceptible female mice early in prediabetes. This effect is associated with impaired tryptophan catabolism, is related to transient blockade of the Stat1 pathway of intracellular signaling by IFN-gamma, and is caused by peroxynitrite production. However, the use of a peroxynitrite inhibitor can rescue tryptophan catabolism and tolerance in those mice. This is the first report of an experimental autoimmune disease in which defective tolerance is causally linked to impaired tryptophan catabolism.

Selection of insulin-producing rat insulinoma (RINm) cells with improved resistance to oxidative stress

The defense system against reactive oxygen species is believed to be crucial for the survival of insulin-producing cells after various injuries. The aim of our study was to select a subpopulation of insulin-producing RINm cells with higher resistance to oxidative stress. The cells resistant to hydrogen peroxide (RINmHP) were obtained by repeated exposure of parental RINm cells to 100 and 200 microM hydrogen peroxide (HP). The increased resistance of RINmHP cells to HP was confirmed by three different cytotoxicity assays. In addition, the selected cells also were resistant to the cytotoxic effect of activated rat splenocytes compared to parental cells. The half-life of HP in the RINmHP cell culture medium was about 2.5 times lower than that of the parental cells, corresponding to the increased level of catalase expression and activity in selected cells. The increased defense property of the selected cells was not associated with any significant changes in insulin content and insulin response to a mixture of glucose with isobutyl methyl xanthine or potassium chloride. In conclusion, repeated exposure to HP induces selection of RINm cells with improved resistance to oxidative stress. This improved defense characteristic probably is due to an increased level of catalase expression and activity in the selected cells.

High glucose-induced oxidative stress causes apoptosis in proximal tubular epithelial cells and is mediated by multiple caspases

Diabetic nephropathy is the leading cause of end-stage renal disease in the Western world. Poor glycemic control contributes to the development of diabetic nephropathy, but the mechanisms underlying high glucose-induced tissue injury are not fully understood. In the present study, the effect of high glucose on a proximal tubular epithelial cell (PTEC) line was investigated. Reactive oxygen species (ROS) were detected using the fluorescent probes dichlorofluorescein diacetate, dihydrorhodamine 123, and 2,3-diaminonapthalene. Peroxynitrite (ONOO-) generation and nitrite concentrations were increased after 24 h of high glucose treatment (P<0.05). LLC-PK1 cells exposed to high D-glucose (25 mM) for up to 48 h had increased DNA fragmentation (P<0.01), caspase-3 activity (P<0.001), and annexin-V staining (P<0.05) as well as decreased expression of XIAP when compared with controls (5 mM D-glucose). The ONOO- scavenger ebselen reduced DNA fragmentation and caspase-3 activity as well as the high glucose-induced nitrite production and DCF fluorescence. High glucose-induced DNA fragmentation was completely prevented by an inhibitor of caspase-3 (P<0.01) and a pan-caspase inhibitor (P<0.001). Caspase inhibition did not affect ROS generation. This study, in a PTEC line, demonstrates that high glucose causes the generation of ONOO-, leading to caspase-mediated apoptosis. Ebselen and a caspase-3 inhibitor provided significant protection against high glucose-mediated apoptosis, implicating ONOO- as a proapoptotic ROS in early diabetic nephropathy.

Diabetes-associated nitration of tyrosine and inactivation of succinyl-CoA:3-oxoacid CoA-transferase

High levels of reactive species of nitrogen and oxygen in diabetes may cause modifications of proteins. Recently, an increase in protein tyrosine nitration was found in several diabetic tissues. To understand whether protein tyrosine nitration is the cause or the result of the associated diabetic complications, it is essential to identify specific proteins vulnerable to nitration with in vivo models of diabetes. In the present study, we have demonstrated that succinyl-CoA:3-oxoacid CoA-transferase (SCOT; EC 2.8.3.5) is susceptible to tyrosine nitration in hearts from streptozotocin-treated rats. After 4 and 8 wk of streptozotocin administration and diabetes progression, SCOT from rat hearts had a 24% and 39% decrease in catalytic activity, respectively. The decrease in SCOT catalytic activity is accompanied by an accumulation of nitrotyrosine in SCOT protein. SCOT is a mitochondrial matrix protein responsible for ketone body utilization. Ketone bodies provide an alternative source of energy during periods of glucose deficiency. Because diabetes results in profound derangements in myocardial substrate utilization, we suggest that SCOT tyrosine nitration is a contributing factor to this impairment in the diabetic heart.

Peroxynitrite is a mediator of cytokine-induced destruction of human pancreatic islet beta cells

The proinflammatory cytokines, interleukin-1beta (IL-1beta), tumor necrosis factor alpha (TNFalpha), and interferon gamma (IFNgamma), are cytotoxic to pancreatic islet beta cells, possibly by inducing nitric oxide and/or oxygen radical production in the beta cells. Peroxynitrite, the reaction product of nitric oxide and the superoxide radical, is a strong oxidant and cytotoxic mediator; therefore, we hypothesized that peroxynitrite might be a mediator of cytokine-induced islet beta-cell destruction. To test this hypothesis we incubated islets isolated from human pancreata with the cytokine combination of IL-1beta, TNFalpha, and IFNgamma. We found that these cytokines induced significant increases in nitrotyrosine, a marker of peroxynitrite, in islet beta cells, and the increase in nitrotyrosine preceded islet-cell destruction. Peroxynitrite mimicked the effects of cytokines on nitrotyrosine formation and islet beta-cell destruction. L-N(G)-monomethyl arginine, an inhibitor of nitric oxide synthase, prevented cytokine-induced nitric oxide production but not hydrogen peroxide production, nitrotyrosine formation, or islet beta-cell destruction. In contrast, guanidinoethyldisulphide, an inhibitor of inducible nitric oxide synthase and scavenger of peroxynitrite, prevented cytokine-induced nitric oxide and hydrogen peroxide production, nitrotyrosine formation, and islet beta-cell destruction. These results suggest that cytokine-induced peroxynitrite formation is dependent upon increased generation of superoxide (measured as hydrogen peroxide) and that peroxynitrite is a mediator of cytokine-induced destruction of human pancreatic islet beta cells.

Inhibition of poly (ADP-ribose) synthetase by gene disruption or inhibition with 5-iodo-6-amino-1,2-benzopyrone protects mice from multiple-low-dose-streptozotocin-induced diabetes

Activation of poly(ADP-ribose) synthetase (PARS, also termed polyADP-ribose polymerase or PARP) has been proposed as a major mechanism contributing to beta-cell destruction in type I diabetes. In the present study, we have investigated the role of PARS in mediating the induction of diabetes and beta-cell death in the multiple-low-dose-streptozotocin (MLDS) model of type I diabetes. Mice genetically deficient in PARS were found to be less sensitive to MLDS than wild type mice, with a lower incidence of diabetes and reduced hyperglycemia. A potent inhibitor of PARS, 5-iodo-6-amino-1,2-benzopyrone (INH(2)BP), was also found to protect mice from MLDS and prevent beta-cell loss, in a dose-dependent manner. Paradoxically, in the PARS deficient mice, the compound increased the onset of diabetes. In vitro the cytokine combination; interleukin-1beta, tumor necrosis factor-alpha and interferon-gamma inhibited glucose-stimulated insulin secretion from isolated rat islets of Langerhans and decreased RIN-5F cell viability. The PARS inhibitor, INH(2)BP, protected both the rat islets and the beta-cell line, RIN-5F, from these cytokine-mediated effects. These protective effects were not mediated by inhibition of cytokine-induced nitric oxide formation. Inhibition of PARS by INH(2)BP was unable to protect rat islet cells from cytokine-mediated apoptosis. Cytokines, peroxynitrite and streptozotocin were all shown to induce PARS activation in RIN-5F cells, an effect suppressed by INH(2)BP. The present study provides evidence for in vivo PARS activation contributing to beta-cell damage and death in the MLDS model of diabetes, and indicates a role for PARS activation in cytokine-mediated depression of insulin secretion and cell viability in vitro.

Tyrosine nitration: localisation, quantification, consequences for protein function and signal transduction

The nitration of free tyrosine or protein tyrosine residues generates 3-nitrotyrosine the detection of which has been utilised as a footprint for the in vivo formation of peroxynitrite and other reactive nitrogen species. The detection of 3-nitrotyrosine by analytical and immunological techniques has established that tyrosine nitration occurs under physiological conditions and levels increase in most disease states. This review provides an updated, comprehensive and detailed summary of the tissue, cellular and specific protein localisation of 3-nitrotyrosine and its quantification. The potential consequences of nitration to protein function and the pathogenesis of disease are also examined together with the possible effects of protein nitration on signal transduction pathways and on the metabolism of proteins.

An inhibitor of inducible nitric oxide synthase and scavenger of peroxynitrite prevents diabetes development in NOD mice

Peroxynitrite (ONOO(-)) is a highly reactive oxidant produced by the interaction of the free radicals superoxide (O*-2) and nitric oxide (NO(*)). In a previous study, we found that peroxynitrite is formed in islet beta-cells of nonobese diabetic (NOD) mice. Here, we report that guanidinoethyldisulphide (GED), a selective inhibitor of inducible nitric oxide synthase (iNOS) and scavenger of peroxynitrite prevents diabetes in NOD mice. GED treatment of female NOD mice, starting at age 5 weeks, delayed diabetes onset (from age 12 to 22 weeks) and significantly decreased diabetes incidence at 30 weeks (from 80% to 17%). GED did not prevent pancreatic islet infiltration by leukocytes; however, beta-cells that stained positive for nitrotyrosine (a marker of peroxynitrite) were significantly decreased in islets of GED-treated mice (1+/-1%) compared with vehicle-treated mice (30+/-9%). In addition, GED significantly inhibited nitric oxide and nitrotyrosine formation and decreased destruction of beta-cells in NOD mouse islets incubated in vitro with the combination of proinflammatory cytokines interleukin 1-beta (IL-1beta), tumour necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma). These findings indicate that both superoxide and nitric oxide radicals contribute to islet beta-cell destruction in autoimmune diabetes via peroxynitrite formation in the beta-cells.

Species differences in susceptibility of transplanted and cultured pancreatic islets to the beta-cell toxin alloxan

The beta-cell toxin alloxan, which produces oxygen radicals, is a model substance in studies of type 1 diabetes. Recently, human beta-cells have been found to be relatively resistant to this toxin. To clarify species differences in alloxan diabetogenicity, and oxygen radical toxicity, mouse, rat, rabbit, dog, pig, human and guinea pig islets have been studied after alloxan exposure. Using a standardized in vivo model, where islets were transplanted to nude mice, the different islets were compared. The results demonstrated that mouse and rat islet grafts were morphologically disturbed by alloxan and ROS. Rabbit and dog islet graft morphology was reasonably intact; and human, porcine, and guinea pig islet grafts were all well preserved. Furthermore, ultrastructural signs of apoptosis and necrosis, disturbances in the insulin secretory pattern during and after an alloxan perifusion, and islet lysosomal enzyme activities were studied in vitro in islets from some species. Guinea pig beta-cells were affected by alloxan, but a regeneration process compensated for the observed apoptotic and necrotic cell death. Human islets did not show any signs of alloxan-induced damage in the different models studied. Finally, no correlation between high alloxan sensitivity and high lysosomal enzyme activity was found. Thus, the beta-cell lysosomes are hardly specific targets for alloxan.

Protection of human islets from the effects of interleukin-1beta by adenoviral gene transfer of an Ikappa B repressor

Interleukin-1beta (IL-1beta) is a pro-inflammatory cytokine that inhibits beta cell function and promotes Fas-triggered apoptosis. IL-1beta is thought to act early in the initiation of the autoimmune destruction of pancreatic beta cells in type I diabetes. IL-1beta promotes beta cell impairment, in part, by activating NF-kappaB transcription factor-dependent signaling pathways. We have examined whether beta cells could be protected from the effects of IL-1beta by overexpressing an inhibitor of NF-kappaB activity, IkappaB, by adenoviral gene transfer to intact human islets in culture. Infection of islets with an adenoviral vector encoding a non-phosphorylatable, non-degradable variant of IkappaBalpha resulted in normal insulin responses to glucose in the presence of IL-1beta. Furthermore, nitric oxide production was prevented and, more importantly, Fas-triggered apoptosis was inhibited following IkappaBalpha gene transfer. These results suggest that blocking the NF-kappaB pathway might prevent cytokine-induced beta cell impairment as a means of facilitating islet transplantation.

Peroxynitrite-induced apoptosis involves activation of multiple caspases in HL-60 cells

In this study, we show that caspases 2, 3, 6, and 7 were activated during peroxynitrite-induced apoptosis in human leukemia HL-60 cells and that processing of these caspases was accompanied by cleavage of poly(ADP-ribose) polymerase and lamin B. Treatment of cells with DEVD-fluoromethyl ketone (FMK), a selective inhibitor for caspase 3-like proteases, resulted in a marked diminution of apoptotic cells. VAVAD-FMK, an inhibitor of caspase 2, partially inhibited the apoptotic response to peroxynitrite. However, selective inactivation of caspase 6 by VEID-FMK did not affect apoptosis rates. These data suggest that caspase 3-like proteases and caspase 2, but not caspase 6, are required for peroxynitrite-induced apoptosis in this cell type. Moreover, we demonstrate that peroxynitrite treatment stimulated activation of caspases 8 and 9, two initial caspases in the apoptotic signaling pathway, and preincubation of cells with their inhibitor, IETD-FMK, inhibited activation of caspase 3-like proteases and caspase 2 at the concentration that prevents the apoptosis. These observations, together, suggest that caspase 8 and/or caspase 9 mediates activation of caspase 3-like proteases and caspase 2 during the apoptosis induced by peroxynitrite in HL-60 cells.

Beta cell destruction in the development of autoimmune diabetes in the non-obese diabetic (NOD) mouse

In the non-obese diabetic (NOD) mouse model of Type 1 (insulin-dependent) diabetes, evidence suggests that pancreatic beta cells are destroyed in part by apoptotic mechanisms. The precise mechanisms of beta cell destruction leading to diabetes remain unclear. The NOD mouse has been studied to gain insight into the cellular and molecular mediators of beta cell death, which are discussed in this review. Perforin, secreted by CD8(+) T cells, remains one of the only molecules confirmed to be implicated in beta cell death in the NOD mouse. There are many other molecules, including Fas ligand and cytokines such as interferon-gamma, interleukin-1 and tumor necrosis factor-alpha, which may lead to beta cell destruction either directly or indirectly via regulation of toxic molecules such as nitric oxide. As beta cell death can occur in the absence of perforin, these other factors, in addition to other as yet unidentified factors, may be important in the development of diabetes. Effective protection of NOD mice from beta cell destruction may therefore require inhibition of multiple effector mechanisms.

Biology of nitric oxide signaling

The free radical nitric oxide (NO) has emerged in recent years as a fundamental signaling molecule for the maintenance of homeostasis, as well as a potent cytotoxic effector involved in the pathogenesis of a wide range of human diseases. Although this paradoxical fate has generated confusion, separating the biological actions of NO on the basis of its physiologic chemistry provides a conceptual framework which helps to distinguish between the beneficial and toxic consequences of NO, and to envision potential therapeutic strategies for the future. Under normal conditions, NO produced in low concentration acts as a messenger and cytoprotective (antioxidant) factor, via direct interactions with transition metals and other free radicals. Alternatively, when the circumstances allow the formation of substantial amounts of NO and modify the cellular microenvironment (formation of the superoxide radical), the chemistry of NO will turn into indirect effects consecutive to the formation of dinitrogen trioxide and peroxynitrite. These "reactive nitrogen species" will, in turn, mediate both oxidative and nitrosative stresses, which form the basis of the cytotoxicity generally attributed to NO, relevant to the pathophysiology of inflammation, circulatory shock, and ischemia-reperfusion injury.

A mouse CD8 T cell-mediated acute autoimmune diabetes independent of the perforin and Fas cytotoxic pathways: possible role of membrane TNF

Double transgenic mice [rat insulin promoter (RIP)-tumor necrosis factor (TNF) and RIP-CD80] whose pancreatic beta cells release TNF and bear CD80 all develop an acute early (6 wk) and lethal diabetes mediated by CD8 T cells. The first ultrastructural changes observed in beta cells, so far unreported, are focal lesions of endoplasmic reticulum swelling at the points of contact with islet-infiltrating lymphoblasts, followed by cytoplasmic, but not nuclear, apoptosis. Such double transgenic mice were made defective in either the perforin, Fas, or TNF pathways. Remarkably, diabetes was found to be totally independent of perforin and Fas. Mice lacking TNF receptor (TNFR) II had no or late diabetes, but only a minority had severe insulitis. Mice lacking the TNF-lymphotoxin (LTalpha) locus (whose sole source of TNF are the beta cells) all had insulitis comparable to that of nondefective mice, but no diabetes or a retarded and milder form, with lesions suggesting different mechanisms of injury. Because both TNFR II and TNF-LTalpha mutations have complex effects on the immune system, these data do not formally incriminate membrane TNF as the major T cell mediator of this acute autoimmune diabetes; nevertheless, in the absence of involvement of the perforin or Fas cytotoxic pathways, membrane TNF appears to be the likeliest candidate.

Human islets in mixed islet grafts protect mouse pancreatic beta-cells from alloxan toxicity

We have previously shown that human beta-cells are resistant to the toxic effects of alloxan. In order to further clarify this characteristic of human islets, we investigated whether these cells might transfer their alloxan resistance to alloxan-sensitive rat or mouse islets. Islets from two species (human-mouse or rat-mouse) were mixed into one graft, which was implanted into the subcapsular kidney space of nude mice. Alloxan or saline was injected intravenously two weeks after implantation and one week thereafter the mice were killed. The number of grafted and endogenous beta-cells were evaluated by a semi-quantitative method after immunohistochemistry. Human islet production of the scavenging enzymes extracellular superoxide dismutase and plasma glutathione peroxidase were analyzed with ELISA-techniques, and mouse and human islet hydrogen peroxide breakdown activity were monitored with a horseradish peroxidase-dependent assay. Mouse beta-cells transplanted together with human islets were protected against alloxan cytotoxicity. Rat islets did not protect mouse beta-cells against alloxan, suggesting that the mixing procedure as such did not impose the protection. Production of extracellular superoxide dismutase and plasma glutathione peroxidase by human islets was very low. Moreover, H2O2 breakdown in vitro, did not differ between human and mouse islets. Alloxan-insensitive human islets protect mouse beta-cells against alloxan-induced lesions, suggesting that yet to be identified extracellular factors are involved in human islet resistance to alloxan toxicity.

Insulin secretion, DNA damage, and apoptosis in human and rat islets of Langerhans following exposure to nitric oxide, peroxynitrite, and cytokines

Cytokine-induced damage may contribute to destruction of insulin-secreting beta-cells in islets of Langerhans during autoimmune diabetes. There is considerable controversy (i) whether human and rat islets respond differently to cytokines, (ii) the extent to which cytokine damage is mediated by induction of nitric oxide formation, and (iii) whether the effects of nitric oxide on islets can be distinguished from those of reactive oxygen species or peroxynitrite. We have analyzed rat and human islet responses in parallel, 48 h after exposure to the nitric oxide donor S-nitrosoglutathione, the mixed donor 3-morpholinosydnonimine, hypoxanthine/xanthine oxidase, peroxynitrite, and combined cytokines (interleukin-1beta, tumor necrosis factor-alpha and interferon-gamma). Insulin secretory response to glucose, insulin content, DNA strand breakage, and early-to-late stage apoptosis were recorded in each experiment. Rat islet insulin secretion was reduced by S-nitrosoglutathione or combined cytokines, but unexpectedly increased by peroxynitrite or hypoxanthine/xanthine oxidase. Effects on human islet insulin secretion were small; cytokines and S-nitrosoglutathione decreased insulin content. Both rat and human islets showed significant and similar levels of DNA damage following all treatments. Apoptosis in neonatal rat islets was increased by every treatment, but was at a low rate in adult rat or human islets and only achieved significance with cytokine treatment of human islets. All cytokine responses were blocked by an arginine analogue. We conclude: (i) Reactive oxygen species increased and nitric oxide decreased insulin secretory responsiveness in rat islets. (ii) Species differences lie mainly in responses to cytokines, applied at a lower dose and shorter time than in most studies of human islets. (iii) Cytokine effects were nitric oxide driven; neither reactive oxygen species nor peroxynitrite reproduced cytokine effects. (iv) Rat and human islets showed equal susceptibility to DNA damage. (v) Apoptosis was not the preferred death pathway in adult islets. (vi) We have found no evidence of human donor variation in the pattern of response to these treatments.

Biological aspects of reactive nitrogen species

Nitric oxide (NO) plays an important role as a cell-signalling molecule, anti-infective agent and, as most recently recognised, an antioxidant. The metabolic fate of NO gives rise to a further series of compounds, collectively known as the reactive nitrogen species (RNS), which possess their own unique characteristics. In this review we discuss this emerging aspect of the NO field in the context of the formation of the RNS and what is known about their effects on biological systems. While much of the insight into the RNS has been gained from the extensive chemical characterisation of these species, to reveal biological consequences this approach must be complemented by direct measures of physiological function. Although we do not know the consequences of many of the dominant chemical reactions of RNS an intriguing aspect is now emerging. This review will illustrate how, when specificity and amplification through cell signalling mechanisms are taken into account, the less significant reactions, in terms of yield or rates, can explain many of the biological responses of exposure of cells or physiological systems to RNS.

Increased oxidative damage to all DNA bases in patients with type II diabetes mellitus

Gas chromatography-mass spectrometry was used to measure the oxidative DNA damage in diabetic subjects and controls. Levels of multiple DNA base oxidation products, but not DNA base de-amination or chlorination products, were found to be elevated in white blood cell DNA from patients with type II diabetes as compared with age-matched controls. The chemical pattern of base damage is characteristic of that caused by an attack on DNA by hydroxyl radical. An increased formation of the highly reactive hydroxyl radical could account for many of the reports of oxidative stress in diabetic subjects. There was no evidence of an increased DNA damage by reactive nitrogen or chlorine species.

Biological tyrosine nitration: a pathophysiological function of nitric oxide and reactive oxygen species

Analytical and immunological methodologies and occasionally both methodologies have been applied to detect and quantify 3-nitrotyrosine in almost every major organ system. In certain diseases increased levels of 3-nitrotyrosine have been correlated with elevated levels of other indices of oxidative stress. Numerous reports have established that nitration is a biological process derived from the biochemical interaction of nitric oxide or nitric oxide-derived secondary products with reactive oxygen species. This article addresses critical issues regarding this biological process, namely the biochemical pathways for nitration of tyrosine residues in vivo, potential protein targets, and pathophysiological consequences of protein tyrosine nitration.

Role of poly(ADP-ribose) synthetase in inflammation and ischaemia-reperfusion

Oxidative and nitrosative stress can trigger DNA strand breakage, which then activates the nuclear enzyme poly(ADP-ribose) synthetase (PARS). This enzyme has also been termed poly(ADP-ribose) polymerase (PARP) or poly(ADP-ribose) transferase (pADPRT). Rapid activation of the enzyme depletes the intracellular concentration of its substrate, nicotinamide adenine dinucleotide, thus slowing the rate of glycolysis, electron transport and subsequently ATP formation. This process can result in cell dysfunction and cell death. In this article, Csaba Szabó and Valina Dawson overview the impact of pharmacological inhibition or genetic inactivation of PARS on the course of oxidant-induced cell death in vitro, and in inflammation and reperfusion injury in vivo. A major trigger for DNA damage in pathophysiological conditions is peroxynitrite, a cytotoxic oxidant formed by the reaction between the free radicals nitric oxide and superoxide. The pharmacological inhibition of poly(ADP-ribose) synthetase is a novel approach for the experimental therapy of various forms of inflammation and shock, stroke, myocardial and intestinal ischaemia-reperfusion, and diabetes mellitus.

Cytokines and their roles in pancreatic islet beta-cell destruction and insulin-dependent diabetes mellitus

Insulin-dependent diabetes mellitus (IDDM) is a disease that results from autoimmune destruction of the insulin-producing beta-cells in the pancreatic islets of Langerhans. The autoimmune response against islet beta-cells is believed to result from a disorder of immunoregulation. According to this concept, a T helper 1 (Th1) subset of T cells and their cytokine products, i.e. Type 1 cytokines--interleukin 2 (IL-2), interferon gamma (IFNgamma), and tumor necrosis factor beta (TNFbeta), dominate over an immunoregulatory (suppressor) Th2 subset of T cells and their cytokine products, i.e. Type 2 cytokines--IL-4 and IL-10. This allows Type 1 cytokines to initiate a cascade of immune/inflammatory processes in the islet (insulitis), culminating in beta-cell destruction. Type 1 cytokines activate (1) cytotoxic T cells that interact specifically with beta-cells and destroy them, and (2) macrophages to produce proinflammatory cytokines (IL-1 and TNFalpha), and oxygen and nitrogen free radicals that are highly toxic to islet beta-cells. Furthermore, the cytokines IL-1, TNFalpha, and IFNgamma are cytotoxic to beta-cells, in large part by inducing the formation of oxygen free radicals, nitric oxide, and peroxynitrite in the beta-cells themselves. Therefore, it would appear that prevention of islet beta-cell destruction and IDDM should be aimed at stimulating the production and/or action of Type 2 cytokines, inhibiting the production and/or action of Type 1 cytokines, and inhibiting the production and/or action of oxygen and nitrogen free radicals in the pancreatic islets.

Scavenging of peroxynitrite by a phenolic/peroxidase system prevents oxidative damage to DNA

We examined the ability of horseradish peroxidase (HRP), an analog of human myeloperoxidase, to protect DNA against oxidative damage caused by peroxynitrite in the presence of chlorogenic acid (CGA), a naturally occurring polyphenol. Chlorogenic acid inhibits the formation of single strand breaks in supercoiled pBR322 DNA by acting as a scavenger of peroxynitrite. Horseradish peroxidase markedly enhances the extent of DNA protection by catalyzing the decomposition of peroxynitrite in the presence of CGA. Horseradish peroxidase alone does not inhibit peroxynitrite-induced DNA strand breaks, indicating that CGA is required as an electron donor to regenerate the active enzyme. The apparent second order rate constant for the HRP-mediated oxidation of CGA in the presence of peroxynitrite at pH 6.9 is 3.4 x 10(7) M(-1) s(-1). This high rate suggests that CGA and other dietary polyphenols might efficiently scavenge peroxynitrite in peroxidase-containing systems in vivo.