Importance of mitochondrial superoxide dismutase expression in insulin-producing cells for the toxicity of reactive oxygen species and proinflammatory cytokines

The pro-radical hydrogen peroxide as a stable hydroxyl radical distributor: lessons from pancreatic beta cells

The toxic potential of H₂O₂ is limited, even if intracellular concentrations of H₂O₂ under conditions of oxidative stress increase to the micromolar concentration range. Its toxicity is mostly restricted to the oxidation of highly reactive thiol groups, some of which are functionally very important. Subsequently, the HO^· radical is generated spontaneously from H₂O₂ in the Fenton reaction. The HO^· radical is extremely toxic and destroys any biological structure. Due to the high reactivity, its action is limited to a locally restricted site of its generation. On the other hand, H₂O₂ with its stability and long half-life can reach virtually any site and distribute its toxic effect all over the cell. Thereby HO^·, in spite of its ultra-short half-life (10^-9 s), can execute its extraordinary toxic action at any target of the cell. In this oxidative stress scenario, H₂O₂ is the pro-radical, that spreads the toxic action of the HO^· radical. It is the longevity of the H₂O₂ molecule allowing it to distribute its toxic action from the site of origin all over the cell and may even mediate intercellular communication. Thus, H₂O₂ acts as a spreader by transporting it to sites where the extremely short-lived toxic HO^· radical can arise in the presence of "free iron". H₂O₂ and HO^· act in concert due to their different complementary chemical properties. They are dependent upon each other while executing the toxic effects in oxidative stress under diabetic metabolic conditions in particular in the highly vulnerable pancreatic beta cell, which in contrast to many other cell types is so badly protected against oxidative stress due to its extremely low H₂O₂ inactivating enzyme capacity.

The role of mitochondrial reactive oxygen species in insulin resistance

Insulin resistance is one of the earliest pathological features of a suite of diseases including type 2 diabetes collectively referred to as metabolic syndrome. There is a growing body of evidence from both pre-clinical studies and human cohorts indicating that reactive oxygen species, such as the superoxide radical anion and hydrogen peroxide are key players in the development of insulin resistance. Here we review the evidence linking mitochondrial reactive oxygen species generated within mitochondria with insulin resistance in adipose tissue and skeletal muscle, two major insulin sensitive tissues. We outline the relevant mitochondria-derived reactive species, how the mitochondrial redox state is regulated, and methodologies available to measure mitochondrial reactive oxygen species. Importantly, we highlight key experimental issues to be considered when studying the role of mitochondrial reactive oxygen species in insulin resistance. Evaluating the available literature on both mitochondrial reactive oxygen species/redox state and insulin resistance in a variety of biological systems, we conclude that the weight of evidence suggests a likely role for mitochondrial reactive oxygen species in the etiology of insulin resistance in adipose tissue and skeletal muscle. However, major limitations in the methods used to study reactive oxygen species in insulin resistance as well as the lack of data linking mitochondrial reactive oxygen species and cytosolic insulin signaling pathways are significant obstacles in proving the mechanistic link between these two processes. We provide a framework to guide future studies to provide stronger mechanistic information on the link between mitochondrial reactive oxygen species and insulin resistance as understanding the source, localization, nature, and quantity of mitochondrial reactive oxygen species, their targets and downstream signaling pathways may pave the way for important new therapeutic strategies.

Effects of vitamin D (VD3) supplementation on the brain mitochondrial function of male rats, in the 6-OHDA-induced model of Parkinson's disease

Mitochondria dysfunction is an important factor involved in PD pathogenesis. We reported neuroprotective actions of vitamin D (VD3) on a PD model, and now we investigated the VD3 effects on the brain mitochondrial function. We focused on oxygen consumption, respiratory control ratio (RCR), ADP/O ratio, mitochondria swelling, H₂O₂ production, and SOD activity. Additionally, immunohistochemistry assays for the dopamine system markers (TH and DAT) and mitochondrial markers (VDAC1 and Hsp60) were also carried out in the striata. Young adult male Wistar rats (250 g, 2.5 months age) were anesthetized and subjected to stereotaxic surgery and injection of saline (SO group) or 6-OHDA, into the right striatum. Brain mitochondria were isolated from the groups: sham-operated (SO), 6-OHDA, 6-OHDA pretreated with VD3 for 7, days before the 6-OHDA lesion (6-OHDA+VD3, pre-) or treated with VD3 for 14 days, after the 6-OHDA lesion (6-OHDA+VD3, post-). VD3 prevented decreases in oxygen consumption, RCR, and ADP/O ratio observed after 6-OHDA injury. Noteworthy, a very low (oxygen consumption and RCR) or no improvement (ADP/O) were observed in the 6-OHDA+VD3 post- group. VD3 also prevented the increased mitochondria swelling and H₂O₂ production and a decrease in SOD activity, respectively, in the 6-OHDA injured mitochondria. Also, VD3 supplementation protected the hemiparkinsonian brain from decreases in TH and DAT expressions and decreased the upregulation of mitochondrial markers, as VDAC 1 and Hsp60. In conclusion, VD3 showed neuroprotective actions on brain mitochondria injured by 6-OHDA and should stimulate translational studies focusing on its use as a therapeutic strategy for the treatment of neurodegenerative diseases as PD.

The importance of aquaporin-8 for cytokine-mediated toxicity in rat insulin-producing cells

Aquaporin-8 (AQP8) is a peroxiporin, a transmembrane water and hydrogen peroxide (H₂O₂) transport protein expressed in the mitochondrial and plasma membranes of pancreatic β-cells. AQP8 protein expression is low under physiological conditions, but it increases after cytokine exposure both, in vitro and in vivo, possibly related to a NF-κB consensus sequence in the promoter. AQP8 knockdown (KD) insulin-producing RINm5F cells are particularly susceptible to cytokine-mediated oxidative stress. Cytokine (a mixture of IL-1β, TNF-α, and IFN-γ) treated AQP8 KD cells exhibited pronounced sensitivity to reactive oxygen and nitrogen species (ROS and RNS), resulting in a significant loss of β-cell viability due to enhanced toxicity of the increased concentrations of H₂O₂ and hydroxyl radicals (^●OH) in mitochondria of AQP8 KD cells. This viability loss went along with increased caspase activities, reduced nitrite concentration (representative of nitric oxide (NO^●) accumulation) and increased lipid peroxidation. The explanation for the increased toxicity of the proinflammatory cytokines in AQP8 KD cells resides in the fact that efflux of the H₂O₂ generated during oxidative stress in the β-cell mitochondria is hampered through the loss of the peroxiporin channels in the mitochondrial membranes of the AQP8 KD cells. The increased proinflammatory cytokine toxicity due to loss of AQP8 expression in the KD β-cell mitochondria is thus the result of increased rates of apoptosis. This decreased cell viability is caused by increased levels of oxidative stress along with a ferroptosis-mediated cell death component due to decreased NO^● generation.

The pancreatic beta cell: an intricate relation between anatomical structure, the signalling mechanism of glucose-induced insulin secretion, the low antioxidative defence, the high vulnerability and sensitivity to diabetic stress

The biosynthesis of insulin takes place in the insulin-producing beta cells that are organized in the form of islets of Langerhans together with a few other islet cell types in the pancreas organ. The signal for glucose-induced insulin secretion is generated in two pathways in the mitochondrial metabolism of the pancreatic beta cells. These pathways are also known as the triggering pathway and the amplifying pathway. Glucokinase, the low-affinity glucose-phosphorylating enzyme in beta cell glycolysis acts as the signal-generating enzyme in this process. ATP ultimately generated is the crucial second messenger in this process. Insulin-producing pancreatic beta cells are badly protected against oxidative stress resulting in a particular vulnerability of this islet cell type due to low expression of H2O2-inactivating enzymes in various subcellular locations, specifically in the cytosol, mitochondria, peroxisomes and endoplasmic reticulum. This is in contrast to the glucagon-producing alpha cells and other islet cell types in the islets that are well equipped with these H2O2-inactivating enzymes. On the other hand the membranes of the pancreatic beta cells are well protected against lipid peroxidation and ferroptosis through high level expression of glutathione peroxidase 4 (GPx4) and this again is at variance from the situation in the non-beta cells of the islets with a low expression level of GPx4. The weak antioxidative defence equipment of the pancreatic beta cells, in particular in states of disease, is very dangerous because the resulting particular vulnerability endangers the functionality of the beta cells, making people prone to the development of a diabetic metabolic state.

Oxidative Stress in Cytokine-Induced Dysfunction of the Pancreatic Beta Cell: Known Knowns and Known Unknowns

Compelling evidence from earlier studies suggests that the pancreatic beta cell is inherently weak in its antioxidant defense mechanisms to face the burden of protecting itself against the increased intracellular oxidative stress following exposure to proinflammatory cytokines. Recent evidence implicates novel roles for nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (Noxs) as contributors to the excessive intracellular oxidative stress and damage under metabolic stress conditions. This review highlights the existing evidence on the regulatory roles of at least three forms of Noxs, namely Nox1, Nox2, and Nox4, in the cascade of events leading to islet beta cell dysfunction, specifically under the duress of chronic exposure to cytokines. Potential crosstalk between key signaling pathways (e.g., inducible nitric oxide synthase [iNOS] and Noxs) in the generation and propagation of reactive molecules and metabolites leading to mitochondrial damage and cell apoptosis is discussed. Available data accrued in investigations involving small-molecule inhibitors and antioxidant protein expression methods as tools toward the prevention of cytokine-induced oxidative damage are reviewed. Lastly, current knowledge gaps in this field, and possible avenues for future research are highlighted.

Oxidative stress pathways in pancreatic β-cells and insulin-sensitive cells and tissues: importance to cell metabolism, function, and dysfunction

It is now accepted that nutrient abundance in the blood, especially glucose, leads to the generation of reactive oxygen species (ROS), ultimately leading to increased oxidative stress in a variety of tissues. In the absence of an appropriate compensatory response from antioxidant mechanisms, the cell, or indeed the tissue, becomes overwhelmed by oxidative stress, leading to the activation of intracellular stress-associated pathways. Activation of the same or similar pathways also appears to play a role in mediating insulin resistance, impaired insulin secretion, and late diabetic complications. The ability of antioxidants to protect against the oxidative stress induced by hyperglycemia and elevated free fatty acid (FFA) levels in vitro suggests a causative role of oxidative stress in mediating the latter clinical conditions. In this review, we describe common biochemical processes associated with oxidative stress driven by hyperglycemia and/or elevated FFA and the resulting clinical outcomes: β-cell dysfunction and peripheral tissue insulin resistance.

Differentiation-Dependent Effects of a New Recombinant Manganese Superoxide Dismutase on Human SK-N-BE Neuron-Like Cells

We have recently isolated a new isoform of recombinant manganese superoxide dismutase (rMnSOD) which provides a potent antitumor activity and strongly counteracts the occurrence of oxidative stress and tissue inflammation. This isoform, in addition to the enzymatic action common to all SODs, also shows special functional and structural properties, essentially due to the presence of a first leader peptide that allows the protein to enter easily into cells. Among endogenous antioxidants, SOD constitutes the first line of natural defence against pathological effects induced by an excess of free radicals. Here, we firstly describe the effects of our rMnSOD administration on the proliferant and malignant undifferentiated human neuroblastoma SK-N-BE cell line. Moreover, we also test the effects of rMnSOD in the all trans retinoic-differentiated SK-N-BE neuron-like cells, a quiescent "not malignant" model. While rMnSOD showed an antitumor activity on proliferating cells, a poor sensitivity to rMnSOD overload in retinoid-differentiated neuron-like cells was observed. However, in the latter case, in presence of experimental-induced oxidative stress, overcharge of rMnSOD enhanced the oxidant effects, through an increase of H₂O₂ due to low activity of both catalase and glutathione peroxidase. In conclusion, our data show that rMnSOD treatment exerts differential effects, which depend upon both cell differentiation and redox balance, addressing attention to the potential use of the recombinant enzyme on differentiated neurons. These facts ultimately pave the way for further preclinical studies aimed at evaluation of rMnSOD effects in models of neurodegenerative diseases.

3,5,4'-Tri-O-acetylresveratrol attenuates seawater inhalation-induced acute respiratory distress syndrome via thioredoxin 1 pathway

The protecting effects of 3,5,4'-tri-O-acetylresveratrol (AC-Res) on seawater inhalation-induced acute respiratory distress syndrome (ARDS) by interfering with the activation of thioredoxin-1 (Trx-1) pathway were evaluated. Seawater inhalation-induced ARDS was assessed by magnitude of pulmonary edema and lung inflammation. Oxidative stress was tested by T-SOD activity and MDA content in lungs and cells. Besides, Trx-1, nuclear factor erythroid 2-related factor 2 (Nrf2) and Txnip expression were measured to explore how seawater induced oxidative stress and the mechanism by which AC-Res attenuated seawater inhalation-induced ARDS. The results showed that seawater inhalation increased wet-to-dry (W/D) ratios of lung tissues, enhanced secretion of tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β), and disturbed the oxidative distress balance probably through interfering the activity of Trx-1 pathway. While treatment of AC-Res in vivo and Res in vitro reduced W/D ratios of lung tissues, decreased cytokines in lungs and maintained the oxidative stress balance through Trx-1 pathway. In conclusion, AC-Res treatment attenuated seawater inhalation induced ARDS via Trx-1 pathway.

ER-resident antioxidative GPx7 and GPx8 enzyme isoforms protect insulin-secreting INS-1E β-cells against lipotoxicity by improving the ER antioxidative capacity

Increased circulating levels of saturated fatty acids (FFAs) and glucose are considered to be major mediators of β-cell dysfunction and death in T2DM. Although it has been proposed that endoplasmic reticulum (ER) and oxidative stress play a crucial role in gluco/lipotoxicity, their interplay and relative contribution to β-cell dysfunction and apoptosis has not been fully elucidated. In addition it is still unclear how palmitate - the physiologically most abundant long-chain saturated FFA - elicits ER stress and which immediate signals commit β-cells to apoptosis. To study the underlying mechanisms of palmitate-mediated ER stress and β-cell toxicity, we exploited the observation that the recently described ER-resident GPx7 and GPx8 are not expressed in rat β-cells. Expression of GPx7 or GPx8 attenuated FFAs-mediated H₂O₂ generation, ER stress, and apoptosis induction. These results could be confirmed by a H₂O₂-specific inactivating ER catalase, indicating that accumulation of H₂O₂ in the ER lumen is critical in FFA-induced ER stress. Furthermore, neither the expression of GPx7 nor of GPx8 increased insulin content or facilitated disulfide bond formation in insulin-secreting INS-1E cells. Hence, reduction of H₂O₂ by ER-GPx isoforms is not rate-limiting in oxidative protein folding in rat β-cells. These data suggest that FFA-mediated ER stress is partially dependent on oxidative stress and selective expression of GPx7 or GPx8 improves the ER antioxidative capacity of rat β-cells without compromising insulin production and the oxidative protein folding machinery.

Molecular mechanisms of ROS production and oxidative stress in diabetes

Oxidative stress and chronic inflammation are known to be associated with the development of metabolic diseases, including diabetes. Oxidative stress, an imbalance between oxidative and antioxidative systems of cells and tissues, is a result of over production of oxidative-free radicals and associated reactive oxygen species (ROS). One outcome of excessive levels of ROS is the modification of the structure and function of cellular proteins and lipids, leading to cellular dysfunction including impaired energy metabolism, altered cell signalling and cell cycle control, impaired cell transport mechanisms and overall dysfunctional biological activity, immune activation and inflammation. Nutritional stress, such as that caused by excess high-fat and/or carbohydrate diets, promotes oxidative stress as evident by increased lipid peroxidation products, protein carbonylation and decreased antioxidant status. In obesity, chronic oxidative stress and associated inflammation are the underlying factors that lead to the development of pathologies such as insulin resistance, dysregulated pathways of metabolism, diabetes and cardiovascular disease through impaired signalling and metabolism resulting in dysfunction to insulin secretion, insulin action and immune responses. However, exercise may counter excessive levels of oxidative stress and thus improve metabolic and inflammatory outcomes. In the present article, we review the cellular and molecular origins and significance of ROS production, the molecular targets and responses describing how oxidative stress affects cell function including mechanisms of insulin secretion and action, from the point of view of possible application of novel diabetic therapies based on redox regulation.

Chemistry and biology of reactive species with special reference to the antioxidative defence status in pancreatic β-cells

BACKGROUND

Diabetes mellitus is a serious metabolic disease. Dysfunction and subsequent loss of the β-cells in the islets of Langerhans through apoptosis ultimately cause a life-threatening insulin deficiency. The underlying reason for the particular vulnerability of the β-cells is an extraordinary sensitivity to the toxicity of reactive oxygen and nitrogen species (ROS and RNS) due to its low antioxidative defense status.

SCOPE REVIEW

This review considers the different aspects of the chemistry and biology of the biologically most important reactive species and their chemico-biological interactions in the β-cell toxicity of proinflammatory cytokines in type 1 diabetes and of lipotoxicity in type 2 diabetes development.

MAJOR CONCLUSION

The weak antioxidative defense equipment in the different subcellular organelles makes the β-cells particularly vulnerable and prone to mitochondrial, peroxisomal and ER stress. Looking upon the enzyme deficiencies which are responsible for the low antioxidative defense status of the pancreatic β-cells it is the lack of enzymatic capacity for H₂O₂ inactivation at all major subcellular sites.

GENERAL SIGNIFICANCE

Diabetes is the most prevalent metabolic disorder with a steadily increasing incidence of both type 1 and type 2 diabetes worldwide. The weak protection of the pancreatic β-cells against oxidative stress is a major reason for their particular vulnerability. Thus, careful protection of the β-cells is required for prevention of the disease.

Molecular Events Linking Oxidative Stress and Inflammation to Insulin Resistance and β-Cell Dysfunction

The prevalence of diabetes mellitus (DM) is increasing worldwide, a consequence of the alarming rise in obesity and metabolic syndrome (MetS). Oxidative stress and inflammation are key physiological and pathological events linking obesity, insulin resistance, and the progression of type 2 DM (T2DM). Unresolved inflammation alongside a “glucolipotoxic” environment of the pancreatic islets, in insulin resistant pathologies, enhances the infiltration of immune cells which through secretory activity cause dysfunction of insulin-secreting β-cells and ultimately cell death. Recent molecular investigations have revealed that mechanisms responsible for insulin resistance associated with T2DM are detected in conditions such as obesity and MetS, including impaired insulin receptor (IR) signalling in insulin responsive tissues, oxidative stress, and endoplasmic reticulum (ER) stress. The aim of the present review is to describe the evidence linking oxidative stress and inflammation with impairment of insulin secretion and action, which result in the progression of T2DM and other conditions associated with metabolic dysregulation.

Cellular Mechanisms of Oxidative Stress and Action in Melanoma

Most melanomas occur on the skin, but a small percentage of these life-threatening cancers affect other parts of the body, such as the eye and mucous membranes, including the mouth. Given that most melanomas are caused by ultraviolet radiation (UV) exposure, close attention has been paid to the impact of oxidative stress on these tumors. The possibility that key epigenetic enzymes cannot act on a DNA altered by oxidative stress has opened new perspectives. Therefore, much attention has been paid to the alteration of DNA methylation by oxidative stress. We review the current evidence about (i) the role of oxidative stress in melanoma initiation and progression; (ii) the mechanisms by which ROS influence the DNA methylation pattern of transformed melanocytes; (iii) the transformative potential of oxidative stress-induced changes in global and/or local gene methylation and expression; (iv) the employment of this epimutation as a biomarker for melanoma diagnosis, prognosis, and drug resistance evaluation; (v) the impact of this new knowledge in clinical practice for melanoma treatment.

Redox homeostasis in pancreatic β cells

We reviewed mechanisms that determine reactive oxygen species (redox) homeostasis, redox information signaling and metabolic/regulatory function of autocrine insulin signaling in pancreatic β cells, and consequences of oxidative stress and dysregulation of redox/information signaling for their dysfunction. We emphasize the role of mitochondrion in β cell molecular physiology and pathology, including the antioxidant role of mitochondrial uncoupling protein UCP2. Since in pancreatic β cells pyruvate cannot be easily diverted towards lactate dehydrogenase for lactate formation, the respiration and oxidative phosphorylation intensity are governed by the availability of glucose, leading to a certain ATP/ADP ratio, whereas in other cell types, cell demand dictates respiration/metabolism rates. Moreover, we examine the possibility that type 2 diabetes mellitus might be considered as an inevitable result of progressive self-accelerating oxidative stress and concomitantly dysregulated information signaling in peripheral tissues as well as in pancreatic β cells. It is because the redox signaling is inherent to the insulin receptor signaling mechanism and its impairment leads to the oxidative and nitrosative stress. Also emerging concepts, admiting participation of redox signaling even in glucose sensing and insulin release in pancreatic β cells, fit in this view. For example, NADPH has been firmly established to be a modulator of glucose-stimulated insulin release.

The molecular mechanisms of pancreatic β-cell glucotoxicity: recent findings and future research directions

It is well established that regular physiological stimulation by glucose plays a crucial role in the maintenance of the β-cell differentiated phenotype. In contrast, prolonged or repeated exposure to elevated glucose concentrations both in vitro and in vivo exerts deleterious or toxic effects on the β-cell phenotype, a concept termed as glucotoxicity. Evidence indicates that the latter may greatly contribute to the pathogenesis of type 2 diabetes. Through the activation of several mechanisms and signaling pathways, high glucose levels exert deleterious effects on β-cell function and survival and thereby, lead to the worsening of the disease over time. While the role of high glucose-induced β-cell overstimulation, oxidative stress, excessive Unfolded Protein Response (UPR) activation, and loss of differentiation in the alteration of the β-cell phenotype is well ascertained, at least in vitro and in animal models of type 2 diabetes, the role of other mechanisms such as inflammation, O-GlcNacylation, PKC activation, and amyloidogenesis requires further confirmation. On the other hand, protein glycation is an emerging mechanism that may play an important role in the glucotoxic deterioration of the β-cell phenotype. Finally, our recent evidence suggests that hypoxia may also be a new mechanism of β-cell glucotoxicity. Deciphering these molecular mechanisms of β-cell glucotoxicity is a mandatory first step toward the development of therapeutic strategies to protect β-cells and improve the functional β-cell mass in type 2 diabetes.

The mitochondrial Atp8 mutation induces mitochondrial ROS generation, secretory dysfunction, and β-cell mass adaptation in conplastic B6-mtFVB mice

Mutations in mitochondrial DNA (mtDNA) cause a variety of pathologic phenotypes. In this study, we used conplastic mouse strains to characterize the impact of a mtDNA mutation in the Atp8 gene on β-cell function, reactive oxygen species (ROS) generation, β-cell mass, and glucose metabolism in response to high-fat diet (HFD). In comparison with B6-mt(AKR) controls, the B6-mt(FVB) strain carries a point mutation of the mtDNA-coded Atp8 gene (ATP synthase), leading to a fragmentated mitochondrial phenotype. Isolated pancreatic islets from 3-month-old B6-mt(FVB) mice showed increased mitochondrial generation of ROS, reduced cellular ATP levels, reduced glucose-induced insulin secretion, higher susceptibility to palmitate stress, and pathological morphology of mitochondria. ROS generation in β-cells was not affected by changes of the ambient glucose concentrations. Feeding a HFD for 3 months resulted in impaired glucose tolerance in B6-mt(FVB) mice but not in B6-mt(AKR) controls. In B6-mt(FVB) animals, glucose intolerance positively correlated with gain of body weight. Serum insulin levels and β-cell mass significantly increased in B6-mt(FVB) mice after a 3-month HFD. The data indicate that the mutation in the Atp8 gene induces mitochondrial dysfunction in β-cells with concomitant impairment of secretory responsiveness. This mitochondrial dysfunction induced a higher susceptibility to metabolic stressors, although this effect appeared not strictly linked to nutrient-induced ROS generation. The Atp8 gene mutation caused mitochondrial dysfunction, apparently stimulating an adaptive increase of β-cell mass in response to HFD, whereas mitochondrial ROS might have had an supportive role.

Effects of maternal protein or energy restriction during late gestation on antioxidant status of plasma and immune tissues in postnatal goats

Maternal malnutrition can have temporary or long-lasting effects on development and physiological function of offspring. Our objective was to investigate whether maternal protein or energy restriction in late gestation affects the antioxidant status of plasma, immune organs (thymus and spleen), and natural barrier organs (jejunum) in neonatal goats and whether the effects could be reversed after nutritional recovery. Forty-five pregnant goats (Liuyang Blacks) of similar age (2.0 ± 0.3 yr) and BW (22.2 ± 1.5 kg at d 90 of gestation) were assigned to 3 dietary treatments during late gestation: control (ME = 9.34 MJ/kg and CP = 12.5%, DM basis), 40% protein restricted (PR), and 40% energy restricted (ER) until parturition, after which offspring received the normal diet for nutritional recovery. Plasma and tissues of kids were sampled to determine antioxidant enzymes [superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), total antioxidant capacity (T-AOC), and catalase (CAT)] and gene expression of antioxidant enzymes (Cu/Zn-SOD [SOD1], CAT, and peroxiredoxin 2 [PRDX2]). Maternal protein or energy restriction decreased (P < 0.05) SOD activities in plasma, liver, thymus, and spleen and SOD1 expression in thymus, and maternal energy restriction also decreased (P < 0.05) plasma GSH-Px activity and expressions of SOD1 and CAT in liver at birth. After nutritional recovery of 6 wk, SOD activities in thymus (both in PR and ER) and spleen (only in PR) were greater (P < 0.05), but CAT activity of thymus (both in PR and ER) and CAT expression (only in ER) were less (P < 0.01) than those in control. After nutritional recovery of 22 wk, SOD1 and PRDX2 expression in thymus (both in PR and ER) and SOD1 expression in liver (only in ER) were greater (P < 0.05) whereas CAT expression in thymus (both in PR and ER) was less (P < 0.001) than in control. The current results indicate that maternal protein or energy restriction can decrease the antioxidant capacity of the neonatal kids and result in an imbalance of SOD and hydrogen peroxide-inactivating systems in thymus, even after 6 or 22 wk of nutritional recovery.

Reactive oxygen and nitrogen species generation, antioxidant defenses, and β-cell function: a critical role for amino acids

Growing evidence indicates that the regulation of intracellular reactive oxygen species (ROS) and reactive nitrogen species (RNS) levels is essential for maintaining normal β-cell glucose responsiveness. While long-term exposure to high glucose induces oxidative stress in β cells, conflicting results have been published regarding the impact of ROS on acute glucose exposure and their role in glucose stimulated insulin secretion (GSIS). Although β cells are considered to be particularly vulnerable to oxidative damage, as they express relatively low levels of some peroxide-metabolizing enzymes such as catalase and glutathione (GSH) peroxidase, other less known GSH-based antioxidant systems are expressed in β cells at higher levels. Herein, we discuss the key mechanisms of ROS/RNS production and their physiological function in pancreatic β cells. We also hypothesize that specific interactions between RNS and ROS may be the cause of the vulnerability of pancreatic β cells to oxidative damage. In addition, using a hypothetical metabolic model based on the data available in the literature, we emphasize the importance of amino acid availability for GSH synthesis and for the maintenance of β-cell function and viability during periods of metabolic disturbance before the clinical onset of diabetes.

Gliclazide may have an antiapoptotic effect related to its antioxidant properties in human normal and cancer cells

Experimental and clinical studies suggest that gliclazide may protect pancreatic β-cells from apoptosis induced by an oxidative stress. However, the precise mechanism(s) of this action are not fully understood and requires further clarification. Therefore, using human normal and cancer cells we examined whether the anti-apoptotic effects of this sulfonylurea is due to its free radical scavenger properties. Hydrogen peroxide (H₂O₂) as a model trigger of oxidative stress was used to induce cell death. Our experiments were performed on human normal cell line (human umbilical vein endothelial cell line, HUVEC-c) and human cancer cell lines (human mammary gland cell line, Hs578T; human pancreatic duct epithelioid carcinoma cell line, PANC-1). To assess the effect of gliclazide the cells were pre-treated with the drug. The 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide assay was employed to measure the impact of gliclazide on cell viability. Generation of reactive oxygen species, mitochondrial membrane potential (∆Ψ_m), and intracellular Ca²⁺ concentration [Ca²⁺] were monitored. Furthermore, the morphological changes associated with apoptosis were determined using double staining with Hoechst 33258-propidium iodide (PI). Gliclazide protects the tested cells from H₂O₂-induced cell death most likely throughout the inhibition of ROS production. Moreover, the drug restored loss of ΔΨ_m and diminished intracellular [Ca²⁺] evoked by H₂O₂. Double staining with Hoechst 33258-PI revealed that pre-treatment with gliclazide diminished the number of apoptotic cells. Our findings indicate that gliclazide may protect both normal and cancer human cells against apoptosis induced by H₂O₂. It appears that the anti-apoptotic effect of the drug is most likely associated with reduction of oxidative stress.

Suppressive effects of natural reduced waters on alloxan-induced apoptosis and type 1 diabetes mellitus

Insulin-producing cells express limited activities of anti-oxidative enzymes. Therefore, reactive oxygen species (ROS) produced in these cells play a crucial role in cytotoxic effects. Furthermore, diabetes mellitus (DM) development is closely linked to higher ROS levels in insulin-producing cells. Hita Tenryosui Water^® (Hita T. W., Hita, Japan) and Nordenau water (Nord. W., Nordenau, Germany), referred to as natural reduced waters (NRWs), scavenge ROS in cultured cells, and therefore, might be a possibility as an alternative to conventional pharmacological agents against DM. Therefore, this study aimed to investigate the role of NRWs in alloxan (ALX)-induced β-cell apoptosis as well as in ALX-induced diabetic mice. NRWs equally suppressed DNA fragmentation levels. Hita T. W. and Nord. W. ameliorated ALX-induced sub-G₁ phase production from approximately 40% of control levels to 8.5 and 11.8%, respectively. NRWs restored serum insulin levels (p < 0.01) and reduced blood glucose levels (p < 0.01) in ALX-induced mice. Hita T. W. restored tissue superoxide dismutase (SOD) (p < 0.05) activity but not tissue catalase activity. Hita T. W. did not elevate SOD or catalase activity in HIT-T15 cells. Nord. W. restored SOD (p < 0.05) and catalase (p < 0.05) activity in both cultured cells and pancreatic tissue to normal levels. Even though variable efficacies were observed between Hita T. W. and Nord. W., both waters suppressed ALX-induced DM development in CD-1 male mice by administering NRWs for 8 weeks. Our results suggest that Hita T. W. and Nord. W. protect against ALX-induced β-cell apoptosis, and prevent the development of ALX-induced DM in experimental animals by regulating ALX-derived ROS generation and elevating anti-oxidative enzymes. Therefore, the two NRWs tested here are promising candidates for the prevention of DM development.

Induction of the intrinsic apoptosis pathway in insulin-secreting cells is dependent on oxidative damage of mitochondria but independent of caspase-12 activation

Pro-inflammatory cytokine-mediated beta cell apoptosis is activated through multiple signaling pathways involving mitochondria and endoplasmic reticulum. Activation of organelle-specific caspases has been implicated in the progression and execution of cell death. This study was therefore performed to elucidate the effects of pro-inflammatory cytokines on a possible cross-talk between the compartment-specific caspases 9 and 12 and their differential contribution to beta cell apoptosis. Moreover, the occurrence of ROS-mediated mitochondrial damage in response to beta cell toxic cytokines has been quantified. ER-specific caspase-12 was strongly activated in response to pro-inflammatory cytokines; however, its inhibition did not abolish cytokine-induced mitochondrial caspase-9 activation and loss of cell viability. In addition, there was a significant induction of oxidative mitochondrial DNA damage and elevated cardiolipin peroxidation in insulin-producing RINm5F cells and rat islet cells. Overexpression of the H(2)O(2) detoxifying enzyme catalase effectively reduced the observed cytokine-induced oxidative damage of mitochondrial structures. Taken together, the results strongly indicate that mitochondrial caspase-9 is not a downstream substrate of ER-specific caspase-12 and that pro-inflammatory cytokines cause apoptotic beta cell death through activation of caspase-9 primarily by hydroxyl radical-mediated mitochondrial damage.

Differential effects of proinflammatory cytokines on cell death and ER stress in insulin-secreting INS1E cells and the involvement of nitric oxide

Proinflammatory cytokines produced by immune cells destroy pancreatic beta cells in type 1 diabetes. The aim of this study was to investigate the cytokine network and its effects in insulin-secreting cells. INS1E cells were exposed to different combinations of proinflammatory cytokines. Cytokine toxicity was estimated by MTT assay and caspase activation measurements. The NFκB-iNOS pathway was analyzed by a SEAP reporter gene assay, Western-blotting and nitrite measurements. Gene expression analyses of ER stress markers, Chop and Bip, were performed by real-time RT-PCR. Cytokines tested in this study, namely IL-1β, TNFα and IFNγ, had deleterious effects on beta cell viability. The most potent toxicity exhibited IL-1β and its combinations with other cytokines. The toxic effects of IL-1β towards cell viability, caspase activation and iNOS activity were dependent on nitric oxide and abolished by an iNOS blocker. IL-1β was the strongest inducer of the NFκB activation. An iNOS blocker inhibited IL-1β-mediated NFκB activation in the first, initial phase of cytokine action, but did not affect significantly NFκB activation after prolonged incubation. Interestingly iNOS protein expression was induced predominantly by IL-1β and decreased in the presence of an iNOS blocker in the case of a short time exposure. The changes in the expression of ER stress markers were also almost exclusively dependent on the IL-1β presence and counteracted by iNOS blockade. Thus cytokine-induced beta cell death is primarily IL-1β mediated with a NO-independent enhancement by TNFα and IFNγ. The deleterious effects on cell viability and function are crucially dependent on IL-1β-induced nitric oxide formation.

Cytokine toxicity in insulin-producing cells is mediated by nitro-oxidative stress-induced hydroxyl radical formation in mitochondria

Although nitric oxide (NO) and oxidative stress both contribute to proinflammatory cytokine toxicity in pancreatic β-cells during type 1 diabetes mellitus (T1DM) development, the interactions between NO and reactive oxygen species (ROS) in cytokine-mediated β-cell death have not been clarified. Exposure of insulin-producing RINm5F cells to IL-1β generated NO, while exposure to a combination of IL-1β, TNF-α, and IFN-γ, which simulates T1DM conditions, generated both NO and ROS. In theory, two reactions between NO and ROS are possible, one with the superoxide radical yielding peroxynitrite, and the other with hydrogen peroxide (H(2)O(2)) yielding hydroxyl radicals. Results of the present work exclude peroxynitrite involvement in cytokine toxicity to β-cells because its generation did not correlate with the toxic action of cytokines. On the other hand, we show that H(2)O(2), produced upon exposure of insulin-producing cell clones and primary rat islet cells to cytokines almost exclusively in the mitochondria, reacted in the presence of trace metal (Fe(++)) with NO forming highly toxic hydroxyl radicals, thus explaining the severe toxicity that causes apoptotic β-cell death. Expression of the H(2)O(2)-inactivating enzyme catalase in mitochondria protected against cytokine toxicity by preventing hydroxyl radical formation. We therefore conclude that proinflammatory cytokine-mediated β-cell death is due to nitro-oxidative stress-mediated hydroxyl radical formation in the mitochondria.

Suppressive effects of electrolyzed reduced water on alloxan-induced apoptosis and type 1 diabetes mellitus

Electrolyzed reduced water, which is capable of scavenging reactive oxygen species, is attracting recent attention because it has shown improved efficacy against several types of diseases including diabetes mellitus. Alloxan produces reactive oxygen species and causes type 1 diabetes mellitus in experimental animals by irreversible oxidative damage to insulin-producing β-cells. Here, we showed that electrolyzed reduced water prevented alloxan-induced DNA fragmentation and the production of cells in sub-G1 phase in HIT-T15 pancreatic β-cells. Blood glucose levels in alloxan-induced type 1 diabetes model mice were also significantly suppressed by feeding the mice with electrolyzed reduced water. These results suggest that electrolyzed reduced water can prevent apoptosis of pancreatic β-cells and the development of symptoms in type 1 diabetes model mice by alleviating the alloxan-derived generation of reactive oxygen species.

Two tales of antioxidant enzymes on β cells and diabetes

Pancreatic islets contain low activities of catalase, selenium-dependent glutathione peroxidase 1 (GPX1), and Cu,Zn-superoxide dismutase 1 (SOD1). Thus, enhancing expression of these enzymes in islets has been unquestionably favored. However, such an attempt has produced variable metabolic outcomes. While β cell-specific overexpression of Sod1 enhanced mouse resistance to streptozotocin-induced diabetes, the same manipulation of catalase aggravated onset of type 1 diabetes in nonobese diabetic mice. Global overexpression of Gpx1 in mice induced type 2 diabetes-like phenotypes. Although knockouts of Gpx1 and Sod1 each alone or together decreased pancreatic β cell mass and plasma insulin concentrations, these knockouts improved body insulin sensitivity to different extents. Pancreatic duodenal homeobox 1, forkhead box A2, and uncoupling protein 2 are three key regulators of β cell mass, insulin synthesis, and glucose-stimulated insulin secretion. Phenotypes resulted from altering GPX1 and/or SOD1 were partly mediated through these factors, along with protein kinase B and c-jun terminal kinase. A shifted reactive oxygen species inhibition of protein tyrosine phosphatases in insulin signaling might be attributed to altered insulin sensitivity. Overall, metabolic roles of antioxidant enzymes in β cells and diabetes depend on body oxidative status and target functions. Revealing regulatory mechanisms for this type of dual role will help prevent potential pro-diabetic risk of antioxidant over-supplementation to humans.

Modulation of Bcl-2-related protein expression in pancreatic beta cells by pro-inflammatory cytokines and its dependence on the antioxidative defense status

Pro-inflammatory cytokines are key mediators in the selective and progressive destruction of insulin-producing beta cells during type 1 diabetes development. However, the mechanisms of cytokine-induced beta cell apoptosis are not fully understood. This study demonstrates that pro-inflammatory cytokines strongly modified the expression of the anti-apoptotic protein Bcl-2 and the pro-apoptotic BH3-only proteins Bad, Bim, and Bid in primary rat islets and insulin-producing RINm5F cells. Overexpression of mitochondrially located catalase (MitoCatalase) specifically increased basal Bcl-2 and decreased basal Bax expression, suppressed cytokine-mediated reduction of Bcl-2, and thereby prevented the release of cytochrome c, Smac/DIABLO and the activation of caspase-9 and -3. Thus, cytokine-mediated decrease of Bcl-2 expression and the sequentially changed Bax/Bcl-2 ratio are responsible for the release of pro-apoptotic mitochondrial factors, activation of caspase-9, and ultimately caspase-3. These results indicate that activation of the intrinsic/mitochondrial apoptosis pathway is essential for cytokine-induced beta cell death and the mitochondrial generation of reactive oxygen species, in particular mitochondrial hydrogen peroxide, differentially regulates the Bax/Bcl-2 ratio.

Peroxisome-generated hydrogen peroxide as important mediator of lipotoxicity in insulin-producing cells

OBJECTIVE

Type 2 diabetes is a complex disease that is accompanied by elevated levels of nonesterified fatty acids (NEFAs), which contribute to β-cell dysfunction and β-cell loss, referred to as lipotoxicity. Experimental evidence suggests that oxidative stress is involved in lipotoxicity. In this study, we analyzed the molecular mechanisms of reactive oxygen species-mediated lipotoxicity in insulin-producing RINm5F cells and INS-1E cells as well as in primary rat islet cells.

RESEARCH DESIGN AND METHODS

The toxicity of saturated NEFAs with different chain lengths upon insulin-producing cells was determined by MTT and propidium iodide (PI) viability assays. Catalase or superoxide dismutase overexpressing cells were used to analyze the nature and the cellular compartment of reactive oxygen species formation. With the new H₂O₂-sensitive fluorescent protein HyPer H₂O₂ formation induced by exposure to palmitic acid was determined.

RESULTS

Only long-chain (>C14) saturated NEFAs were toxic to insulin-producing cells. Overexpression of catalase in the peroxisomes and in the cytosol, but not in the mitochondria, significantly reduced H₂O₂ formation and protected the cells against palmitic acid-induced toxicity. With the HyPer protein, H₂O₂ generation was directly detectable in the peroxisomes of RINm5F and INS-1E insulin-producing cells as well as in primary rat islet cells.

CONCLUSIONS

The results demonstrate that H₂O₂ formation in the peroxisomes rather than in the mitochondria are responsible for NEFA-induced toxicity. Therefore, we propose a new concept of fatty acid-induced β-cell lipotoxicity mediated via reactive oxygen species formation through peroxisomal β- oxidation.

Do insulinotropic glucose-lowering drugs do more harm than good? The hypersecretion hypothesis revisited

Significant progress has been made in recent years in the characterisation of the signal pathways of beta cell dysfunction and death in the pathogenesis of type 2 diabetes. Glucolipotoxicity acts as an exogenous factor whereas oxidative stress and endoplasmic reticulum stress may result from the processes of signal recognition and stimulated secretion within the beta cell. The pharmacological stimulation of secretion may thus appear to be a double-edged sword: it counteracts hyperglycaemia, but may do so at the expense of beta cell mass. So, in the long run, insulinotropic glucose-lowering drugs might do more harm than good. However, much of this logic is derived by analogy from the long-held assumption that beta cell hypersecretion imposed by insulin resistance causes the absolute secretion deficit in the later course of type 2 diabetes. In this concept the beta cell has a secondary role and loss of beta cell mass is necessary for the manifestation of type 2 diabetes. Recent studies have shown that a secretion deficit can exist well before insulin resistance and that major genetic risk factors concern beta cell function. Also, the evidence for a beta cell toxic effect of insulinotropic drugs is currently inconclusive. Assuming that the insulin secretion deficit is of pathogenetic importance in a network with insulin resistance as an aggravating factor, an insulinotropic glucose-lowering drug may do more good than harm if it relieves the beta cell from the stress of glucose overstimulation and does so without inducing hypoglycaemia.

Oxidative stress and beta-cell dysfunction

Diabetes mellitus type 1 and 2 (T1DM and T2DM) are complex multifactorial diseases. Loss of beta-cell function caused by reduced secretory capacity and enhanced apoptosis is a key event in the pathogenesis of both diabetes types. Oxidative stress induced by reactive oxygen and nitrogen species is critically involved in the impairment of beta-cell function during the development of diabetes. Because of their low antioxidant capacity, beta-cells are extremely sensitive towards oxidative stress. In beta-cells, important targets for an oxidant insult are cell metabolism and K(ATP) channels. The oxidant-evoked alterations of K(ATP) channel activity seem to be critical for oxidant-induced dysfunction because genetic ablation of K(ATP) channels attenuates the effects of oxidative stress on beta-cell function. Besides the effects on metabolism, interference of oxidants with mitochondria induces key events in apoptosis. Consequently, increasing antioxidant defence is a promising strategy to delay beta cell failure in (pre)-diabetic patients or during islet transplantation. Knock-out of K(ATP) channels has beneficial effects on oxidant-induced inhibition of insulin secretion and cell death. Interestingly, these effects can be mimicked by sulfonylureas that have been used in the treatment of T2DM for many years. Loss of functional K(ATP) channels leads to up-regulation of antioxidant enzymes, a process that depends on cytosolic Ca(2+). These observations are of great importance for clinical intervention because they show a possibility to protect beta-cells at an early stage before dramatic changes of the secretory capacity and loss of cell mass become manifest and lead to glucose intolerance or even overt diabetes.

Antioxidant protection of Malaysian tualang honey in pancreas of normal and streptozotocin-induced diabetic rats

Glucotoxicity contributes to beta-cell dysfunction through oxidative stress. Our previous study demonstrated that tualang honey ameliorated renal oxidative stress and produced hypoglycemic effect in streptozotocin (STZ)-induced diabetic rats. This present study investigated the hypothesis that hypoglycemic effect of tualang honey might partly be due to protection of pancreas against oxidative stress. Diabetes was induced by a single dose of STZ (60 mg/kg; ip). Diabetic rats were randomly divided into two groups and administered distilled water (0.5 ml/d) and tualang honey (1.0 g/kg/d). Similarly, two groups of non-diabetic rats received distilled water (0.5 ml/d) and tualang honey (1.0 g/kg/d). The animals were treated orally for 28 days. At the end of the treatment period, the honey-treated diabetic rats had significantly (p<0.05) reduced blood glucose levels [8.8 (5.8)mmol/L; median (interquartile range)] compared with the diabetic control rats [17.9 (2.6)mmol/L]. The pancreas of diabetic control rats showed significantly increased levels of malondialdehyde (MDA) and up-regulation of superoxide dismutase (SOD) and glutathione peroxidase (GPx) activities. Catalase (CAT) activity was significantly reduced while glutathione-S-transferase (GST) and glutathione reductase (GR) activities remained unchanged in the pancreas of diabetic rats. Tualang honey significantly (p<0.05) reduced elevated MDA levels. Honey treatment also restored SOD and CAT activities. These results suggest that hypoglycemic effect of tualang honey might be attributed to its antioxidative effect on the pancreas.

Cellular stress responses: cell survival and cell death

Cells can respond to stress in various ways ranging from the activation of survival pathways to the initiation of cell death that eventually eliminates damaged cells. Whether cells mount a protective or destructive stress response depends to a large extent on the nature and duration of the stress as well as the cell type. Also, there is often the interplay between these responses that ultimately determines the fate of the stressed cell. The mechanism by which a cell dies (i.e., apoptosis, necrosis, pyroptosis, or autophagic cell death) depends on various exogenous factors as well as the cell's ability to handle the stress to which it is exposed. The implications of cellular stress responses to human physiology and diseases are manifold and will be discussed in this review in the context of some major world health issues such as diabetes, Parkinson's disease, myocardial infarction, and cancer.

Protection against cytokine toxicity through endoplasmic reticulum and mitochondrial stress prevention by prostacyclin synthase overexpression in insulin-producing cells

Proinflammatory cytokines play a crucial role in the pathogenesis of type 1 diabetes mellitus. One of the cytokine-regulated pathways mediating inflammation in this autoimmune disease is the arachidonic acid metabolism pathway, comprising both the induction of cyclooxygenases and the production of different prostaglandins. Cytokine toxicity is mediated in many cell types, including pancreatic beta cells through this pathway. Interestingly, some cell types have been shown to be insensitive to such toxicity, and this correlated with a high expression of prostacyclin synthase (PGIS). Using insulin-producing RINm5F cells as a model for pancreatic beta cells, PGIS was overexpressed and exhibited a large protective effect against cytokine toxicity. This protective effect of PGIS against cytokine toxicity correlated with a decreased activation of the transcription factor NFkappaB and the inducible NO synthase promoter as well as a reduced inducible NO synthase protein expression and nitrite production. A reduction in the cytokine-stimulated endoplasmic reticulum and mitochondrial stress was also found in the PGIS-overexpressing cells. Moreover, cytokine-induced caspase-3 activation and reduction of glucose oxidation and cell proliferation were suppressed. Thus, PGIS overexpression apparently protects insulin-producing cells against cytokine toxicity via suppression of endoplasmic reticulum and mitochondrial stress-mediated cell death pathways.

Insights into the critical role of NADPH oxidase(s) in the normal and dysregulated pancreatic beta cell

It is now widely accepted that reactive oxygen species (ROS) contribute to cell and tissue dysfunction and damage in diabetes. The source of ROS in the insulin secreting pancreatic beta cells has traditionally been considered to be the mitochondrial electron transport chain. While this source is undoubtedly important, we fully describe in this article recent information and evidence of NADPH oxidase-dependent generation of ROS in pancreatic beta cells and identify the various isoforms that contribute to O(2)(*-) and H(2)O(2) production in various conditions. While glucose-stimulated ROS generation may be important for acute regulation of insulin secretion, at higher levels ROS may disrupt mitochondrial energy metabolism. However, ROS may alter other cellular processes such as signal transduction, ion fluxes and/or cell proliferation/death. The various beta cell isoforms of NADPH oxidase (described in this review) may, via differences in the kinetics and species of ROS generated, positively and negatively regulate insulin secretion and cell survival.

Early low protein diet aggravates unbalance between antioxidant enzymes leading to islet dysfunction

BACKGROUND

Islets from adult rat possess weak antioxidant defense leading to unbalance between superoxide dismutase (SOD) and hydrogen peroxide-inactivating enzymatic activities, catalase (CAT) and glutathione peroxidase (GPX) rending them susceptible to oxidative stress. We have shown that this vulnerability is influenced by maternal diet during gestation and lactation.

METHODOLOGY/PRINCIPAL FINDINGS

The present study investigated if low antioxidant activity in islets is already observed at birth and if maternal protein restriction influences the development of islet antioxidant defenses. Rats were fed a control diet (C group) or a low protein diet during gestation (LP) or until weaning (LPT), after which offspring received the control diet. We found that antioxidant enzymatic activities varied with age. At birth and after weaning, normal islets possessed an efficient GPX activity. However, the antioxidant capacity decreased thereafter increasing the potential vulnerability to oxidative stress. Maternal protein malnutrition changed the antioxidant enzymatic activities in islets of the progeny. At 3 months, SOD activity was increased in LP and LPT islets with no concomitant activation of CAT and GPX. This unbalance could lead to higher hydrogen peroxide production, which may concur to oxidative stress causing defective insulin gene expression due to modification of critical factors that modulate the insulin promoter. We found indeed that insulin mRNA level was reduced in both groups of malnourished offspring compared to controls. Analyzing the expression of such critical factors, we found that c-Myc expression was strongly increased in islets from both protein-restricted groups compared to controls.

CONCLUSION AND SIGNIFICANCE

Modification in antioxidant activity by maternal low protein diet could predispose to pancreatic islet dysfunction later in life and provide new insights to define a molecular mechanism responsible for intrauterine programming of endocrine pancreas.

Oxidative stress: the vulnerable beta-cell

Antioxidative defence mechanisms of pancreatic beta-cells are particularly weak and can be overwhelmed by redox imbalance arising from overproduction of reactive oxygen and reactive nitrogen species. The consequences of this redox imbalance are lipid peroxidation, oxidation of proteins, DNA damage and interference of reactive species with signal transduction pathways, which contribute significantly to beta-cell dysfunction and death in Type 1 and Type 2 diabetes mellitus. Reactive oxygen species, superoxide radicals (O(2)(*-)), hydrogen peroxide (H(2)O(2)) and, in a final iron-catalysed reaction step, the most reactive and toxic hydroxyl radicals (OH(*)) are produced during both pro-inflammatory cytokine-mediated beta-cell attack in Type 1 diabetes and glucolipotoxicity-mediated beta-cell dysfunction in Type 2 diabetes. In combination with NO(*), which is toxic in itself, as well as through its reaction with the O(2)(*-) and subsequent formation of peroxynitrite, reactive species play a central role in beta-cell death during the deterioration of glucose tolerance in the development of diabetes.

Aminoacetone, a putative endogenous source of methylglyoxal, causes oxidative stress and death to insulin-producing RINm5f cells

Aminoacetone (AA), triose phosphates, and acetone are putative endogenous sources of potentially cytotoxic and genotoxic methylglyoxal (MG), which has been reported to be augmented in the plasma of diabetic patients. In these patients, accumulation of MG derived from aminoacetone, a threonine and glycine catabolite, is inferred from the observed concomitant endothelial overexpression of circulating semicarbazide-sensitive amine oxidases. These copper-dependent enzymes catalyze the oxidation of primary amines, such as AA and methylamine, by molecular oxygen, to the corresponding aldehydes, NH4(+) ion and H2O2. We recently reported that AA aerobic oxidation to MG also takes place immediately upon addition of catalytic amounts of copper and iron ions. Taking into account that (i) MG and H2O2 are reportedly cytotoxic to insulin-producing cell lineages such as RINm5f and that (ii) the metal-catalyzed oxidation of AA is propagated by O2(*-) radical anion, we decided to investigate the possible pro-oxidant action of AA on these cells taken here as a reliable model system for pancreatic beta-cells. Indeed, we show that AA (0.10-5.0 mM) administration to RINm5f cultures induces cell death. Ferrous (50-300 microM) and Fe(3+) ion (100 microM) addition to the cell cultures had no effect, whereas Cu(2+) (5.0-100 microM) significantly increased cell death. Supplementation of the AA- and Cu(2+)-containing culture medium with antioxidants, such as catalase (5.0 microM), superoxide dismutase (SOD, 50 U/mL), and N-acetylcysteine (NAC, 5.0 mM) led to partial protection. mRNA expression of MnSOD, CuZnSOD, glutathione peroxidase, and glutathione reductase, but not of catalase, is higher in cells treated with AA (0.50-1.0 mM) plus Cu(2+) ions (10-50 microM) relative to control cultures. This may imply higher activity of antioxidant enzymes in RINm5f AA-treated cells. In addition, we have found that AA (0.50-1.0 mM) plus Cu(2+) (100 microM) (i) increase RINm5f cytosolic calcium; (ii) promote DNA fragmentation; and (iii) increase the pro-apoptotic (Bax)/antiapoptotic (Bcl-2) ratio at the level of mRNA expression. In conclusion, although both normal and pathological concentrations of AA are probably much lower than those used here, it is tempting to propose that excess AA in diabetic patients may drive oxidative damage and eventually the death of pancreatic beta-cells.

Mitochondrial dysfunction is responsible for the intestinal calcium absorption inhibition induced by menadione

Menadione (MEN) inhibits intestinal calcium absorption by a mechanism not completely understood. The aim of this work was to find out the role of mitochondria in this inhibitory mechanism. Hence, normal chicks treated with one i.p. dose of MEN were studied in comparison with controls. Intestinal calcium absorption was measured by the in situ ligated intestinal segment technique. GSH, oxidoreductase activities from the Krebs cycle and enzymes of the antioxidant system were measured in isolated mitochondria. Mitochondrial membrane potential was measured by a flow cytometer technique. DNA fragmentation and cytochrome c localization were determined by immunocytochemistry. Data indicate that in 30 min, MEN decreases intestinal Ca(2+) absorption, which returns to the control values after 10 h. GSH was only decreased for half an hour, while the activity of malate dehydrogenase and alpha-ketoglutarate dehydrogenase was diminished for 48 h. Mn(2+)-superoxide dismutase activity was increased in 30 min, whereas the activity of catalase and glutathione peroxidase remained unaltered. DNA fragmentation and cytochrome c release were maximal in 30 min, but were recovered after 15 h. In conclusion, MEN inhibits intestinal Ca(2+) absorption by mitochondrial dysfunction as revealed by GSH depletion and alteration of the permeability triggering the release of cytochrome c and DNA fragmentation.

Triiodothyronine (T3)-mediated toxicity and induction of apoptosis in insulin-producing INS-1 cells

Thyroid hormones reduce glucose tolerance in humans and animals. This effect is related to a decrease of glucose-induced insulin secretion following a reduction of pancreatic beta cell mass due to beta cell loss. The aim of this study was to analyze in vitro the mechanisms underlying the effects of triiodothyronine (T(3)) on the cell viability and cell cycle caused by changes of cell death or proliferation rate of insulin-producing INS-1 cells. 72-h Exposure of INS-1 cells to increasing T(3) concentrations up to 500 microM resulted in a significant viability reduction. This T(3) toxicity was caused by an increased apoptotic cell death rate, which was accompanied by a decreased proliferation rate. Inhibitory effects of T(3) on glucose-induced insulin secretion were already seen after 24 h of incubation, indicating that the deleterious effects of T(3) were time-dependent, changing from specific cellular dysfunctions to a severe and extended disturbance of the cellular survival program. Only T(3) concentrations higher than 250 microM were able to decrease cell viability and proliferation rate, to increase the rate of apoptosis and to reduce glucose-induced insulin secretion. These micromolar T(3) concentrations were significantly higher than the concentration range of T(3) receptor binding, indicating that other non-receptor-mediated mechanisms beyond the receptor level must be responsible for the observed toxic effects of T(3) in vitro.

Selective susceptibility of different populations of sympathetic neurons to diabetic neuropathy in vivo is reflected by increased vulnerability to oxidative stress in vitro

Diabetes is the major cause of autonomic neuropathy in humans. Sympathetic neurons from the celiac/superior mesenteric ganglia (CG/SMG) develop neuropathic changes in diabetes whereas sympathetic superior cervical ganglion (SCG) neurons do not. Glucose-induced oxidative stress is proposed as a major factor in the development of diabetic neuropathy. The aim of this study was to investigate whether sympathetic neurons that develop neuropathy in diabetes are more susceptible to oxidative stress. Explants of CG/SMG and SCG from control adult rats were cultured in media free of serum and NGF, exposed to menadione for 48 h to induce oxidative stress and assessed for neuronal viability, TUNEL-positive nuclei and tyrosine hydroxylase- (TH)-immunoreactivity. TH-immunoreactivity was also assessed in ganglia from control and 8 week streptozotocin-diabetic rats. Menadione caused a concentration-dependent loss of neuronal viability and increase in TUNEL staining in both ganglia. However, at low concentrations, menadione had a significantly greater effect (p<0.01) on CG/SMG neurons than SCG neurons. At 1 nM, menadione caused a significant increase (p<0.05) in the number of CG/SMG neurons containing intense TH-immunoreactivity without affecting SCG neurons. Similarly, 8 weeks streptozotocin-induced diabetes resulted in a significant increase (p<0.05) in intensely fluorescent TH-containing CG/SMG neurons but not SCG neurons. This is the first demonstration that oxidative stress in vitro causes the same accumulation of TH in CG/SMG neurons as is observed following streptozotocin-induced diabetes in vivo. Furthermore, the selective vulnerability of CG/SMG neurons to diabetes is reflected by increased sensitivity to oxidative stress.

Pioglitazone does not increase cerebral glucose utilisation in a murine model of Alzheimer's disease and decreases it in wild-type mice

AIMS/HYPOTHESIS

Clinical trials are in progress to test thiazolidinediones in neurodegenerative diseases such as Alzheimer's disease that involve deficiencies in brain glucose metabolism. While thiazolidinediones enhance glucose uptake in non-cerebral tissues, their impact on brain energy metabolism has not been investigated in vivo. We thus determined whether the thiazolidinedione pioglitazone reverses the decrease in cerebral glucose utilisation (CGU) in a model of brain metabolic deficiency related to Alzheimer's disease. Results are relevant to diabetes because millions of diabetic patients take pioglitazone as an insulin-sensitising drug, and diabetes increases the risk of developing Alzheimer's disease.

MATERIALS AND METHODS

The regional pattern of CGU was measured with the 2-deoxy [(14)C] glucose autoradiographic technique in adult awake mice overexpressing transforming growth factor beta1 (TGFbeta1), and in wild-type littermates. Mice were treated with pioglitazone for 2 months.

RESULTS

Measurement of CGU in 27 brain regions confirmed that TGFbeta1 overexpression induced hypometabolism across the brain. Pioglitazone did not reverse the effect of TGFbeta1 overexpression and decreased regional CGU in control animals by up to 23%. The extent of the regional CGU decrease induced by pioglitazone, but not that induced by TGFbeta1, correlated strongly with basal CGU, suggesting that the higher the local metabolic rate the greater the reduction of CGU effected by pioglitazone.

CONCLUSIONS/INTERPRETATION

In contrast to its stimulatory effect in non-cerebral tissues, chronic treatment with pioglitazone decreases CGU in vivo. This evidence does not support the hypothesis that pioglitazone could act as a metabolic enhancer in Alzheimer's disease, and raises the question of how thiazolidinediones could be beneficial in neurodegenerative diseases.

The influence of genetic background on the induction of oxidative stress and impaired insulin secretion in mouse islets

AIMS/HYPOTHESIS

We determined whether high-glucose-induced beta cell dysfunction is associated with oxidative stress in the DBA/2 mouse, a mouse strain susceptible to islet failure.

MATERIALS AND METHODS

Glucose- and non-glucose-mediated insulin secretion from the islets of DBA/2 and control C57BL/6 mice was determined following a 48-h exposure to high glucose. Flux via the hexosamine biosynthesis pathway was assessed by determining O-glycosylated protein levels. Oxidative stress was determined by measuring hydrogen peroxide levels and the expression of anti-oxidant enzymes.

RESULTS

Exposure to high glucose levels impaired glucose-stimulated insulin secretion in DBA/2 islets but not C57BL/6 islets, and this was associated with reduced islet insulin content and lower ATP levels than in C57BL/6 islets. Exposure of islets to glucosamine for 48 h mimicked the effects of high glucose on insulin secretion in the DBA/2 islets. High glucose exposure elevated O-glycosylated proteins; however, this occurred in islets from both strains, excluding a role for O-glycosylation in the impairment of DBA/2 insulin secretion. Additionally, both glucosamine and high glucose caused an increase in hydrogen peroxide in DBA/2 islets but not in C57BL/6 islets, an effect prevented by the antioxidant N-acetyl-L: -cysteine. Interestingly, while glutathione peroxidase and catalase expression was comparable between the two strains, the antioxidant enzyme manganese superoxide dismutase, which converts superoxide to hydrogen peroxide, was increased in DBA/2 islets, possibly explaining the increase in hydrogen peroxide levels.

CONCLUSIONS/INTERPRETATION

Chronic high glucose culture caused an impairment in glucose-stimulated insulin secretion in DBA/2 islets, which have a genetic predisposition to failure, and this may be the result of oxidative stress.

Increased vulnerability of newly forming beta cells to cytokine-induced cell death

AIMS/HYPOTHESIS

Beta cell destruction in type 1 diabetes is apparently mediated by the release of cytokines. We questioned whether cytokine-induced apoptosis preferentially kills replicating beta cells.

MATERIALS AND METHODS

In the first experiment, rat insulinoma (RIN) cells were studied for 36 h by time-lapse video microscopy. Cells were exposed to three doses of a cytokine mixture (maximal concentration: IL-1beta 50 U/ml; TNF-alpha 1,000 U/ml; IFN-gamma 1,000 U/ml) or vehicle and analysed for the total cell number (2-h intervals) and timing of each cell death and division. In the second experiment, isolated human islets were incubated with the same cytokine mixture for 24 h and examined for replication and paired (postmitotic) apoptosis.

RESULTS

In the first experiment, after application of cytokines, apoptosis occurred most frequently immediately after the next or subsequent cell mitosis (p<0.05). In the second experiment, cytokines caused increased apoptosis in human islets, with an increase in the proportion of postmitotic apoptotic pairs (p<0.001).

CONCLUSIONS/INTERPRETATION

Cytokine-induced beta cell death preferentially affects newly forming beta cells, which implies that replicating beta cells might be more vulnerable to cytokine destruction. Efforts to expand beta cell mass in type 1 diabetes by fostering beta cell replication are likely to fail unless cytokine-induced apoptosis is concurrently suppressed.