Inhibition of NADPH oxidase-1 preserves beta cell function

Redox Status as a Key Driver of Healthy Pancreatic Beta-Cells

Redox status plays a multifaceted role in the intricate physiology and pathology of pancreatic beta-cells, the pivotal regulators of glucose homeostasis through insulin secretion. They are highly responsive to changes in metabolic cues where reactive oxygen species are part of it, all arising from nutritional intake. These molecules not only serve as crucial signaling intermediates for insulin secretion but also participate in the nuanced heterogeneity observed within the beta-cell population. A central aspect of beta-cell redox biology revolves around the localized production of hydrogen peroxide and the activity of NADPH oxidases which are tightly regulated and serve diverse physiological functions. Pancreatic beta-cells possess a remarkable array of antioxidant defense mechanisms although considered relatively modest compared to other cell types, are efficient in preserving redox balance within the cellular milieu. This intrinsic antioxidant machinery operates in concert with redox-sensitive signaling pathways, forming an elaborate redox relay system essential for beta-cell function and adaptation to changing metabolic demands. Perturbations in redox homeostasis can lead to oxidative stress exacerbating insulin secretion defect being a hallmark of type 2 diabetes. Understanding the interplay between redox signaling, oxidative stress, and beta-cell dysfunction is paramount for developing effective therapeutic strategies aimed at preserving beta-cell health and function in individuals with type 2 diabetes. Thus, unraveling the intricate complexities of beta-cell redox biology presents exciting avenues for advancing our understanding and treatment of metabolic disorders.

Redox Balance in Type 2 Diabetes: Therapeutic Potential and the Challenge of Antioxidant-Based Therapy

Oxidative stress is an important factor in the development of type 2 diabetes (T2D) and associated complications. Unfortunately, most clinical studies have failed to provide sufficient evidence regarding the benefits of antioxidants (AOXs) in treating this disease. Based on the known complexity of reactive oxygen species (ROS) functions in both the physiology and pathophysiology of glucose homeostasis, it is suggested that inappropriate dosing leads to the failure of AOXs in T2D treatment. To support this hypothesis, the role of oxidative stress in the pathophysiology of T2D is described, together with a summary of the evidence for the failure of AOXs in the management of diabetes. A comparison of preclinical and clinical studies indicates that suboptimal dosing of AOXs might explain the lack of benefits of AOXs. Conversely, the possibility that glycemic control might be adversely affected by excess AOXs is also considered, based on the role of ROS in insulin signaling. We suggest that AOX therapy should be given in a personalized manner according to the need, which is the presence and severity of oxidative stress. With the development of gold-standard biomarkers for oxidative stress, optimization of AOX therapy may be achieved to maximize the therapeutic potential of these agents.

Pharmacological and mechanistic study of PS1, a Pdia4 inhibitor, in β-cell pathogenesis and diabetes in db/db mice

Pdia4 has been characterized as a key protein that positively regulates β-cell failure and diabetes via ROS regulation. Here, we investigated the function and mechanism of PS1, a Pdia4 inhibitor, in β-cells and diabetes. We found that PS1 had an IC₅₀ of 4 μM for Pdia4. Furthermore, PS1 alone and in combination with metformin significantly reversed diabetes in db/db mice, 6 to 7 mice per group, as evidenced by blood glucose, glycosylated hemoglobin A1c (Hb_A1c), glucose tolerance test, diabetic incidence, survival and longevity (P < 0.05 or less). Accordingly, PS1 reduced cell death and dysfunction in the pancreatic β-islets of db/db mice as exemplified by serum insulin, serum c-peptide, reactive oxygen species (ROS), islet atrophy, and homeostatic model assessment (HOMA) indices (P < 0.05 or less). Moreover, PS1 decreased cell death in the β-islets of db/db mice. Mechanistic studies showed that PS1 significantly increased cell survival and insulin secretion in Min6 cells in response to high glucose (P < 0.05 or less). This increase could be attributed to a reduction in ROS production and the activity of electron transport chain complex 1 (ETC C1) and Nox in Min6 cells by PS1. Further, we found that PS1 inhibited the enzymatic activity of Pdia4 and mitigated the interaction between Pdia4 and Ndufs3 or p22 in Min6 cells (P < 0.01 or less). Taken together, this work demonstrates that PS1 negatively regulated β-cell pathogenesis and diabetes via reduction of ROS production involving the Pdia4/Ndufs3 and Pdia4/p22 cascades.

Pdia4 regulates β-cell pathogenesis in diabetes: molecular mechanism and targeted therapy

Loss of β‐cell number and function is a hallmark of diabetes. β‐cell preservation is emerging as a promising strategy to treat and reverse diabetes. Here, we first found that Pdia4 was primarily expressed in β‐cells. This expression was up‐regulated in β‐cells and blood of mice in response to excess nutrients. Ablation of Pdia4 alleviated diabetes as shown by reduced islet destruction, blood glucose and HbA1c, reactive oxygen species (ROS), and increased insulin secretion in diabetic mice. Strikingly, this ablation alone or in combination with food reduction could fully reverse diabetes. Conversely, overexpression of Pdia4 had the opposite pathophysiological outcomes in the mice. In addition, Pdia4 positively regulated β‐cell death, dysfunction, and ROS production. Mechanistic studies demonstrated that Pdia4 increased ROS content in β‐cells via its action on the pathway of Ndufs3 and p22^phox. Finally, we found that 2‐β‐D‐glucopyranosyloxy1‐hydroxytrideca 5,7,9,11‐tetrayne (GHTT), a Pdia4 inhibitor, suppressed diabetic development in diabetic mice. These findings characterize Pdia4 as a crucial regulator of β‐cell pathogenesis and diabetes, suggesting Pdia4 is a novel therapeutic and diagnostic target of diabetes.

Early Cytokine-Induced Transient NOX2 Activity Is ER Stress-Dependent and Impacts β-Cell Function and Survival

In type 1 diabetes (T1D) development, proinflammatory cytokines (PIC) released by immune cells lead to increased reactive oxygen species (ROS) production in β-cells. Nonetheless, the temporality of the events triggered and the role of different ROS sources remain unclear. Isolated islets from C57BL/6J wild-type (WT), NOX1 KO and NOX2 KO mice were exposed to a PIC combination. We show that cytokines increase O₂^•− production after 2 h in WT and NOX1 KO but not in NOX2 KO islets. Using transgenic mice constitutively expressing a genetically encoded compartment specific H₂O₂ sensor, we show, for the first time, a transient increase of cytosolic/nuclear H₂O₂ in islet cells between 4 and 5 h during cytokine exposure. The H₂O₂ increase coincides with the intracellular NAD(P)H decrease and is absent in NOX2 KO islets. NOX2 KO confers better glucose tolerance and protects against cytokine-induced islet secretory dysfunction and death. However, NOX2 absence does not counteract the cytokine effects in ER Ca²⁺ depletion, Store-Operated Calcium Entry (SOCE) increase and ER stress. Instead, the activation of ER stress precedes H₂O₂ production. As early NOX2-driven ROS production impacts β-cells’ function and survival during insulitis, NOX2 might be a potential target for designing therapies against early β-cell dysfunction in the context of T1D onset.

NADPH Oxidase (NOX) Targeting in Diabetes: A Special Emphasis on Pancreatic β-Cell Dysfunction

In type 2 diabetes, metabolic stress has a negative impact on pancreatic β-cell function and survival (T2D). Although the pathogenesis of metabolic stress is complex, an imbalance in redox homeostasis causes abnormal tissue damage and β-cell death due to low endogenous antioxidant expression levels in β-cells. Under diabetogenic conditions, the susceptibility of β-cells to oxidative damage by NADPH oxidase has been related to contributing to β-cell dysfunction. Here, we consider recent insights into how the redox response becomes deregulated under diabetic conditions by NADPH oxidase, as well as the therapeutic benefits of NOX inhibitors, which may provide clues for understanding the pathomechanisms and developing strategies aimed at the treatment or prevention of metabolic stress associated with β-cell failure.

CD47 and thrombospondin-1 regulation of mitochondria, metabolism, and diabetes

Thrombospondin-1 (TSP1) is the prototypical member of a family of secreted proteins that modulate cell behavior by engaging with molecules in the extracellular matrix and with receptors on the cell surface. CD47 is widely displayed on many, if not all, cell types and is a high-affinity TSP1 receptor. CD47 is a marker of self that limits innate immune cell activities, a feature recently exploited to enhance cancer immunotherapy. Another major role for CD47 in health and disease is to mediate TSP1 signaling. TSP1 acting through CD47 contributes to mitochondrial, metabolic, and endocrine dysfunction. Studies in animal models found that elevated TSP1 expression, acting in part through CD47, causes mitochondrial and metabolic dysfunction. Clinical studies established that abnormal TSP1 expression positively correlates with obesity, fatty liver disease, and diabetes. The unabated increase in these conditions worldwide and the availability of CD47 targeting drugs justify a closer look into how TSP1 and CD47 disrupt metabolic balance and the potential for therapeutic intervention.

Single-Dose Toxicity Study on ML171, a Selective NOX1 Inhibitor, in Mice

Background

ML171 is a potent nicotinamide adenine dinucleotide phosphate oxidase (NOX) inhibitor with isoform selectivity only for NOX1. This study is aimed at investigating the safety of ML171 after a single intraperitoneal (IP) injection in mice.

Methods

The toxicity of a single dose of ML171 was evaluated in 6-week-old Institute of Cancer Research (ICR) mice in a good laboratory practice (GLP) laboratory. Twenty-five mice of each sex were assigned to five groups: negative control, vehicle control, and 125, 250, and 500 mg/kg of ML171. All mice were acclimatized for one week before beginning the study. Mice received an IP injection of ML171 or vehicle. The general condition and mortality of the animals were observed. The mice were sacrificed to evaluate histopathology 14 days after the administration of ML171 or vehicle.

Results

Bodyweights were not significantly different in any group. Three males and one female died due to ML171 administration in the 500 mg/kg dose group. Autopsies of the surviving mice did not reveal any significant abnormalities after the injection of 125 mg/kg of ML171. However, the anterior lobe edge of the liver was thickened and adhesions between the liver and adjacent organs were observed in mice treated with 250 or 500 mg/kg of ML171. In addition, hypertrophy of centrilobular hepatocytes and inflammatory cell infiltration were observed after injection of 250 and 500 mg/kg of ML171.

Conclusion

Our results indicate that the lethal IP injection dose of ML171 is 500 mg/kg for both males and females. Mortality were not observed for lower doses of ML171. The safe dose of single IP ML171 in ICR mice was 250 mg/kg or less. Further studies are needed to confirm the safety of ML171 in the human body.

Pathophysiology and Therapeutic Potential of NADPH Oxidases in Ischemic Stroke-Induced Oxidative Stress

Stroke is a leading cause of death and disability in humans. The excessive production of reactive oxygen species (ROS) is an important contributor to oxidative stress and secondary brain damage after stroke. Nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, an enzyme complex consisting of membrane subunits and cytoplasmic subunits, regulates neuronal maturation and cerebrovascular homeostasis. However, NADPH oxidase overproduction contributes to neurotoxicity and cerebrovascular disease. NADPH oxidase has been implicated as the principal source of ROS in the brain, and numerous studies have shown that the knockout of NADPH exerts a protective effect in the model of ischemic stroke. In this review, we summarize the mechanism of activation of the NADPH oxidase family members, the pathophysiological effects of NADPH oxidase isoforms in ischemic stroke, and the studies of NADPH oxidase inhibitors to explore potential clinical applications.

Protein Kinases Signaling in Pancreatic Beta-cells Death and Type 2 Diabetes

Type 2 diabetes (T2D) is a worldwide serious public health problem. Insulin resistance and β-cell failure are the two major components of T2D pathology. In addition to defective endoplasmic reticulum (ER) stress signaling due to glucolipotoxicity, β-cell dysfunction or β-cell death initiates the deleterious vicious cycle observed in T2D. Although the primary cause is still unknown, overnutrition that contributes to the induction of the state of low-grade inflammation, and the activation of various protein kinases-related metabolic pathways are main factors leading to T2D. In this chapter following subjects, which have critical checkpoints regarding β-cell fate and protein kinases pathways are discussed; hyperglycemia-induced β-cell failure, chronic accumulation of unfolded protein in β-cells, the effect of intracellular reactive oxygen species (ROS) signaling to insulin secretion, excessive saturated free fatty acid-induced β-cell apoptosis, mitophagy dysfunction, proinflammatory responses and insulin resistance, and the reprogramming of β-cell for differentiation or dedifferentiation in T2D. There is much debate about selecting proposed therapeutic strategies to maintain or enhance optimal β-cell viability for adequate insulin secretion in T2D. However, in order to achieve an effective solution in the treatment of T2D, more intensive clinical trials are required on newer therapeutic options based on protein kinases signaling pathways.

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.

GPCRs, G Proteins, and Their Impact on β-cell Function

Glucose-induced (physiological) insulin secretion from the islet β-cell involves interplay between cationic (i.e., changes in intracellular calcium) and metabolic (i.e., generation of hydrophobic and hydrophilic second messengers) events. A large body of evidence affirms support for novel regulation, by G proteins, of specific intracellular signaling events, including actin cytoskeletal remodeling, transport of insulin-containing granules to the plasma membrane for fusion, and secretion of insulin into the circulation. This article highlights the following aspects of GPCR-G protein biology of the islet. First, it overviews our current understanding of the identity of a wide variety of G protein regulators and their modulatory roles in GPCR-G protein-effector coupling, which is requisite for optimal β-cell function under physiological conditions. Second, it describes evidence in support of novel, noncanonical, GPCR-independent mechanisms of activation of G proteins in the islet. Third, it highlights the evidence indicating that abnormalities in G protein function lead to islet β-cell dysregulation and demise under the duress of metabolic stress and diabetes. Fourth, it summarizes observations of potential beneficial effects of GPCR agonists in preventing/halting metabolic defects in the islet β-cell under various pathological conditions (e.g., metabolic stress and inflammation). Lastly, it identifies knowledge gaps and potential avenues for future research in this evolving field of translational islet biology. Published 2020. Compr Physiol 10:453-490, 2020.

Isoform-selective NADPH oxidase inhibitor panel for pharmacological target validation

Dysfunctional reactive oxygen species (ROS) signaling is considered an important disease mechanism. Therapeutically, non-selective scavenging of ROS by antioxidants, however, has failed in multiple clinical trials to provide patient benefit. Instead, pharmacological modulation of disease-relevant, enzymatic sources of ROS appears to be an alternative, more promising and meanwhile successfully validated approach. With respect to targets, the family of NADPH oxidases (NOX) stands out as main and dedicated ROS sources. Validation of the different NOX isoforms has been mainly through genetically modified rodent models and is lagging behind in other species. It is unclear whether the different NOX isoforms are sufficiently distinct to allow selective pharmacological modulation. Here we show for five widely used NOX inhibitors that isoform selectivity can be achieved, although individual compound specificity is as yet insufficient. NOX1 was most potently (IC₅₀) targeted by ML171 (0.1 μM); NOX2, by VAS2870 (0.7 μM); NOX4, by M13 (0.01 μM) and NOX5, by ML090 (0.01 μM). In addition, some non-specific antioxidant and assay artefacts may limit the interpretation of data, which included, surprisingly, the clinically advanced NOX inhibitor, GKT136901. In a human ischemic blood-brain barrier hyperpermeability model where genetic target validation is not an option, we provide proof-of-principle that pharmacological target validation for different NOX isoforms is possible by applying an inhibitor panel at IC₅₀ concentrations. Moreover, our findings encourage further lead optimization and development efforts for isoform-selective NOX inhibitors in different indications.

Nox1 downregulators: A new class of therapeutics

Chronic non-communicable diseases share the pathomechanism of increased reactive oxygen species (ROS) production by nicotinamide adenine dinucleotide phosphate (NADPH) oxidases, known as Nox. The recent discovery that expression of Nox1, a Nox isoform that has been implicated in the pathogenesis of cardiovascular and kidney disease and cancer is regulated by the expression and activity of G protein-coupled estrogen receptor (GPER) led to the identification of orally active small-molecule GPER blockers as selective Nox1 downregulators (NDRs). Preclinical studies using NDRs have demonstrated beneficial effects in vascular disease, hypertension, and glomerular renal injury. These findings suggest the therapeutic potential of NDRs, which reduce Nox1 protein levels, not only for cardiovascular disease conditions including arterial hypertension, pulmonary hypertension, heart failure with preserved ejection fraction (HFpEF), and chronic renal disease, but also for other non-communicable diseases, such as cerebrovascular disease and vascular dementia, Alzheimer's disease, autoimmune diseases and cancer, in which elevated Nox1-derived ROS production plays a causal role.

Role of the 12-lipoxygenase pathway in diabetes pathogenesis and complications

12-lipoxygenase (12-LOX) is one of several enzyme isoforms responsible for the metabolism of arachidonic acid and other poly-unsaturated fatty acids to both pro- and anti-inflammatory lipid mediators. Mounting evidence has shown that 12-LOX plays a critical role in the modulation of inflammation at multiple checkpoints during diabetes development. Due to this, interventions to limit pro-inflammatory 12-LOX metabolites either by isoform-specific 12-LOX inhibition, or by providing specific fatty acid substrates via dietary intervention, has the potential to significantly and positively impact health outcomes of patients living with both type 1 and type 2 diabetes. To date, the development of truly specific and efficacious inhibitors has been hampered by homology of LOX family members; however, improvements in high throughput screening have improved the inhibitor landscape. Here, we describe the function and role of human 12-LOX, and mouse 12-LOX and 12/15-LOX, in the development of diabetes and diabetes-related complications, and describe promise in the development of strategies to limit pro-inflammatory metabolites, primarily via new small molecule 12-LOX inhibitors.

Apoptosis signal regulating kinase-1 and NADPH oxidase mediate human amylin evoked redox stress and apoptosis in pancreatic beta-cells

Misfolded toxic human islet amyloid polypeptide or amylin (hA) and plasma membrane-associated redox complex, NADPH oxidase (NOX), have been implicated in the islet β-cell demise associated with type-2 diabetes mellitus (T2DM). Studies show that hA accumulation is stressful to β-cells and that misfolding of human amylin evokes redox stress and activates mitogen activated protein (MAP) kinases, p38 MAPK and c-Jun N-terminal (JNK) kinase. However, the molecular link and causality between hA-evoked redox stress, NOX activity and MAP kinases signaling in pancreatic β-cells is incompletely understood. Here, we show that in the process of activating JNK, aggregation prone hA also activates an upstream apoptosis signal regulating kinase-1 (ASK1) with concomitant decrease in intracellular levels of reduced glutathione. Inhibition of ASK1 kinase activity, either by specific ASK1 inhibitor, NQDI1 or by thiol antioxidants reduces human amylin-evoked ASK1 and JNK activation and consequently human amylin toxicity in rat insulinoma Rin-m5F cells and human islets. β-cell specific overexpression of human amylin in mouse islets elicited ASK1 phosphorylation and activation in β-cells but not in other rodent's islet or exocrine cells. This ASK1 activation strongly correlated with islet amyloidosis and diabetes progression. Cytotoxic human amylin additionally stimulated pro-oxidative activity and expressions of plasma membrane bound NADPH oxidase (NOX) and its regulatory subunits. siRNA mediated NOX1 knockdown and selective NOX inhibitors, ML171 and apocynin, significantly reduced hA-induced mitochondrial stress in insulinoma beta-cells. However, NOX inhibitors were largely ineffective against hA-evoked redox stress and activation of cytotoxic ASK1/JNK signaling complex. Thus, our studies suggest that NOX1 and ASK1 autonomously mediate human amylin-evoked redox and mitochondrial stress in pancreatic β-cells.

Involvement of NADPH oxidase 1 in UVB-induced cell signaling and cytotoxicity in human keratinocytes

Members of NADPH oxidase (Nox) enzyme family are important sources of reactive oxygen species (ROS) and are known to be involved in several physiological functions in response to various stimuli including UV irradiation. UVB-induced ROS have been associated with inflammation, cytotoxicity, cell death, or DNA damage in human keratinocytes. However, the source and the role of UVB-induced ROS remain undefined.

Here, we show that Nox1 is involved in UVB-induced p38/MAPK activation and cytotoxicity via ROS generation in keratinocytes. Nox1 knockdown or inhibitor decreased UVB-induced ROS production in human keratinocytes. Nox1 knockdown impaired UVB-induced p38 activation, accompanied by reduced IL-6 levels and attenuated cell toxicity. Treatment of cells with N-acetyl-L-cysteine (NAC), a potent ROS scavenger, suppressed p38 activation as well as consequent IL-6 production and cytotoxicity in response to UVB exposure. p38 inhibitor also suppressed UVB-induced IL-6 production and cytotoxicity. Furthermore, the blockade of IL-6 production by IL-6 neutralizing antibody reduced UVB-induced cell toxicity.

In vivo assay using wild-type mice, the intradermal injection of lysates from UVB-irradiated control cells, but not from UVB-irradiated Nox1 knockdown cells, induced inflammatory swelling and IL-6 production in the skin of ears. Moreover, administration of Nox1 inhibitor suppressed UVB-induced increase in IL-6 mRNA expression in mice skin.

Collectively, these data suggest that Nox1-mediated ROS production is required for UVB-induced cytotoxicity and inflammation through p38 activation and inflammatory cytokine production, such as IL-6. Thus, our findings suggest Nox1 as a therapeutic target for cytotoxicity and inflammation in response to UVB exposure.

•

Nox1 knockdown decreased UVB-increased cellular ROS in keratinocytes.

•

Nox1 knockdown suppressed UVB-induced p38 activation, accompanied by reduced in IL-6 levels and attenuated cell toxicity.

•

UVB-induced cytotoxicity is involved in p38/MAPK pathway and IL-6 production, which is partially dependent on Nox1-generated ROS.

Cambogin exerts anti-proliferative and pro-apoptotic effects on breast adenocarcinoma through the induction of NADPH oxidase 1 and the alteration of mitochondrial morphology and dynamics

Cambogin, a bioactive polycyclic polyprenylated acylphoroglucinol (PPAP) derived from the Garcinia genus, possesses proapoptotic effect in medulloblastoma and breast cancer cells. We have previously demonstrated that the proapoptotic effect of cambogin is driven by the production of reactive oxygen species (ROS). Here we have shown that the inhibitory effect of cambogin on cell proliferation is associated with the loss of mitochondrial transmembrane potential (ΔΨ_m) and mitochondrial fragmentation. Cambogin also promotes the mutual complex formation of the membrane-bound subunit p22^phox of NADPH oxidase 1 (NOX1), as well as the phosphorylation of the cytosolic subunit p47^phox, subsequently enhancing membrane-bound NOX1 activity, which leads to increases in intracellular and mitochondrial levels of O₂^.- and H₂O₂. Pharmacological inhibition of NOX1 using apocynin (pan-NOX inhibitor), ML171 (NOX1 inhibitor) or siRNA against NOX1 prevents the increases in O₂^.- and H₂O₂ levels and the anti-proliferative effect of cambogin. Antioxidants, including SOD (superoxide dismutase), CAT (catalase) and EUK-8, are also able to restore cell viability in the presence of cambogin. Besides, cambogin increases the dissociation of thioredoxin-1 (Trx1) from ASK1, switching the inactive form of ASK1 to the active kinase, subsequently leads to the phosphorylation of JNK/SAPK, which is abolished upon ML171 treatment. The proapoptotic effect of cambogin in breast cancer cells is also aggravated upon knocking down Trx1 in MCF-7 cells. Taken in conjunction, these data indicate that the anti-proliferative and pro-apoptotic effect of cambogin is mediated via inducing NOX1-dependent ROS production and the dissociation of ASK1 and Trx1.

Importance of NADPH oxidase-mediated redox signaling in the detrimental effect of CRP on pancreatic insulin secretion

Elevations in C-reactive protein (CRP) levels are positively correlated with the progress of type 2 diabetes mellitus. However, the effect of CRP on pancreatic insulin secretion is unknown. Here, we showed that purified human CRP impaired insulin secretion in isolated mouse islets and NIT-1 insulin-secreting cells in dose- and time-dependent manners. CRP increased NADPH oxidase-mediated ROS (reactive oxygen species) production, which simultaneously promoted the production of nitrotyrosine (an indicator of RNS, reactive nitrogen species) and TNFα, to diminish cell viability, insulin secretion in islets and insulin-secreting cells. These CRP-mediated detrimental effects on cell viability and insulin secretion were significantly reversed by adding NAC (a potent antioxidant), apocynin (a selective NADPH oxidase inhibitor), L-NAME (a non-selective nitric oxide synthase (NOS) inhibitor), aminoguanidine (a selective iNOS inhibitor), PDTC (a selective NFκB inhibitor) or Enbrel (an anti-TNFα fusion protein). However, CRP-induced ROS production failed to change after adding L-NAME, aminoguanidine or PDTC. In isolated islets and NIT-1 cells, the elevated nitrotyrosine contents by CRP pretreatment were significantly suppressed by adding L-NAME but not PDTC. Conversely, CRP-induced increases in TNF-α production were significantly reversed by administration of PDTC but not L-NAME. In addition, wild-type mice treated with purified human CRP showed significant decreases in the insulin secretion index (HOMA-β cells) and the insulin stimulation index in isolated islets that were reversed by the addition of L-NAME, aminoguanidine or NAC. It is suggested that CRP-activated NADPH-oxidase redox signaling triggers iNOS-mediated RNS and NFκB-mediated proinflammatory cytokine production to cause β cell damage in state of inflammation.

Supplementation with Phycocyanobilin, Citrulline, Taurine, and Supranutritional Doses of Folic Acid and Biotin-Potential for Preventing or Slowing the Progression of Diabetic Complications

Oxidative stress, the resulting uncoupling of endothelial nitric oxide synthase (eNOS), and loss of nitric oxide (NO) bioactivity, are key mediators of the vascular and microvascular complications of diabetes. Much of this oxidative stress arises from up-regulated nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity. Phycocyanobilin (PhyCB), the light-harvesting chromophore in edible cyanobacteria such as spirulina, is a biliverdin derivative that shares the ability of free bilirubin to inhibit certain isoforms of NADPH oxidase. Epidemiological studies reveal that diabetics with relatively elevated serum bilirubin are less likely to develop coronary disease or microvascular complications; this may reflect the ability of bilirubin to ward off these complications via inhibition of NADPH oxidase. Oral PhyCB may likewise have potential in this regard, and has been shown to protect diabetic mice from glomerulosclerosis. With respect to oxidant-mediated uncoupling of eNOS, high-dose folate can help to reverse this by modulating the oxidation status of the eNOS cofactor tetrahydrobiopterin (BH4). Oxidation of BH4 yields dihydrobiopterin (BH2), which competes with BH4 for binding to eNOS and promotes its uncoupling. The reduced intracellular metabolites of folate have versatile oxidant-scavenging activity that can prevent oxidation of BH4; concurrently, these metabolites promote induction of dihydrofolate reductase, which functions to reconvert BH2 to BH4, and hence alleviate the uncoupling of eNOS. The arginine metabolite asymmetric dimethylarginine (ADMA), typically elevated in diabetics, also uncouples eNOS by competitively inhibiting binding of arginine to eNOS; this effect is exacerbated by the increased expression of arginase that accompanies diabetes. These effects can be countered via supplementation with citrulline, which efficiently enhances tissue levels of arginine. With respect to the loss of NO bioactivity that contributes to diabetic complications, high dose biotin has the potential to "pinch hit" for diminished NO by direct activation of soluble guanylate cyclase (sGC). High-dose biotin also may aid glycemic control via modulatory effects on enzyme induction in hepatocytes and pancreatic beta cells. Taurine, which suppresses diabetic complications in rodents, has the potential to reverse the inactivating impact of oxidative stress on sGC by boosting synthesis of hydrogen sulfide. Hence, it is proposed that concurrent administration of PhyCB, citrulline, taurine, and supranutritional doses of folate and biotin may have considerable potential for prevention and control of diabetic complications. Such a regimen could also be complemented with antioxidants such as lipoic acid, N-acetylcysteine, and melatonin-that boost cellular expression of antioxidant enzymes and glutathione-as well as astaxanthin, zinc, and glycine. The development of appropriate functional foods might make it feasible for patients to use complex nutraceutical regimens of the sort suggested here.

Relationship of NADPH Oxidase-1 expression to beta cell dysfunction induced by inflammatory cytokines

Redox stress related loss of beta cell function is a feature of diabetes. Exposure of beta cells and islets to inflammatory mediators elevates reactive oxygen species (ROS) and beta cell dysfunction. Direct molecular manipulation of NADPH oxidase-1 (NOX-1) has identified a key role for NOX-1 in cytokine-induced beta cell dysfunction. Plasmid driven elevation of NOX-1 resulted in elevated ROS, loss of glucose-stimulated-insulin-secretion and increased apoptosis. These outcomes on beta cell function are analogous to cytokine treatment. In contrast, reduction of NOX-1 expression, by shRNA, conferred protection to beta cells and islets from the damaging effects of inflammatory cytokines. Collectively, these data support the therapeutic potential for NOX-1 inhibition in diabetes.

Reactive Oxygen Species Link Gonadotropin-Releasing Hormone Receptor Signaling Cascades in the Gonadotrope

Biological rhythms lie at the center of regulatory schemes that control many aspects of living systems. At the cellular level, meaningful responses to external stimuli depend on propagation and quenching of a signal to maintain vigilance for subsequent stimulation or changes that serve to shape and modulate the response. The hypothalamus-pituitary-gonad endocrine axis that controls reproductive development and function relies on control through rhythmic stimulation. Central to this axis is the pulsatile stimulation of the gonadotropes by hypothalamic neurons through episodic release of the neuropeptide gonadotropin-releasing hormone. Alterations in pulsatile stimulation of the gonadotropes result in differential synthesis and secretion of the gonadotropins LH and FSH and changes in the expression of their respective hormone subunit genes. The requirement to amplify signals arising from activation of the gonadotropin-releasing hormone (GnRH) receptor and to rapidly quench the resultant signal to preserve an adaptive response suggests the need for rapid activation and feedback control operating at the level of intracellular signaling. Emerging data suggest that reactive oxygen species (ROS) can fulfill this role in the GnRH receptor signaling through activation of MAP kinase signaling cascades, control of negative feedback, and participation in the secretory process. Results obtained in gonadotrope cell lines or other cell models indicate that ROS can participate in each of these regulatory cascades. We discuss the potential advantage of reactive oxygen signaling for modulating the gonadotrope response to GnRH stimulation and the potential mechanisms for this action. These observations suggest further targets of study for regulation in the gonadotrope.

NSC23766, a Known Inhibitor of Tiam1-Rac1 Signaling Module, Prevents the Onset of Type 1 Diabetes in the NOD Mouse Model

BACKGROUND/AIMS

Type 1 diabetes (T1D) is characterized by absolute insulin deficiency due to destruction of pancreatic β-cells by cytokines (e.g., interleukin-1β; IL-1β) released by invading immune cells. The mechanisms by which these cytokines induce β-cell dysfunction remain poorly understood. Recent evidence suggests that excessive generation of reactive oxygen species (ROS) by the phagocyte-like NADPH oxidase2 (Nox2), along with significantly low levels of antioxidants in β-cells, drive them toward oxidative damage. Rac1, a small G-protein, is one of the members of Nox2 holoenzyme. We recently reported that NSC23766, a known inhibitor of Rac1, significantly attenuated cytokine-induced Nox2 activation and ROS generation in pancreatic islet β-cells in vitro. Herein, we determined the effects of NSC23766 (2.5 mg/kg/day, i.p/daily) on the development of diabetes in the NOD mouse, a model for T1D.

METHODS

Two groups of experimental animals (Balb/c and NOD mice) received NSC23766, while the two control groups received equal volume of saline. Body weights and blood glucose were measured every week for 34 weeks. Rac1 activation in pancreatic islets was measured by GLISA activation assay. Rac1 and CHOP expression was determined by Western Blotting.

RESULTS

Our findings indicate that administration of NSC23766 significantly prevented the development of spontaneous diabetes in the NOD mice. Furthermore, NSC23766 markedly suppressed Rac1 expression and activity and the endoplasmic reticulum stress (CHOP expression) in NOD islets.

CONCLUSIONS

Our findings provide the first evidence implicating the role of Tiam1-Rac1-Nox2 signaling pathway in the onset of spontaneous diabetes in the NOD mouse model.

Control of Insulin Secretion by Production of Reactive Oxygen Species: Study Performed in Pancreatic Islets from Fed and 48-Hour Fasted Wistar Rats

Mitochondria and NADPH oxidase are important sources of reactive oxygen species in particular the superoxide radical (ROS) in pancreatic islets. These molecules derived from molecular oxygen are involved in pancreatic β-cells signaling and control of insulin secretion. We examined the involvement of ROS produced through NADPH oxidase in the leucine- and/or glucose-induced insulin secretion by pancreatic islets from fed or 48-hour fasted rats. Glucose-stimulated insulin secretion (GSIS) in isolated islets was evaluated at low (2.8 mM) or high (16.7 mM) glucose concentrations in the presence or absence of leucine (20 mM) and/or NADPH oxidase inhibitors (VAS2870–20 μM or diphenylene iodonium—DPI—5 μM). ROS production was determined in islets treated with dihydroethidium (DHE) or MitoSOX Red reagent for 20 min and dispersed for fluorescence measurement by flow cytometry. NADPH content variation was examined in INS-1E cells (an insulin secreting cell line) after incubation in the presence of glucose (2.8 or 16.7 mM) and leucine (20 mM). At 2.8 mM glucose, VAS2870 and DPI reduced net ROS production (by 30%) and increased GSIS (by 70%) in a negative correlation manner (r = -0.93). At 16.7 mM glucose or 20 mM leucine, both NADPH oxidase inhibitors did not alter insulin secretion neither net ROS production. Pentose phosphate pathway inhibition by treatment with DHEA (75 μM) at low glucose led to an increase in net ROS production in pancreatic islets from fed rats (by 40%) and induced a marked increase (by 144%) in islets from 48-hour fasted rats. The NADPH/NADP⁺ ratio was increased when INS-1E cells were exposed to high glucose (by 4.3-fold) or leucine (by 3-fold). In conclusion, increased ROS production through NADPH oxidase prevents the occurrence of hypoglycemia in fasting conditions, however, in the presence of high glucose or high leucine levels, the increased production of NADPH and the consequent enhancement of the activity of the antioxidant defenses mitigate the excess of ROS production and allow the secretory process of insulin to take place.

Hyperglycemic Stress and Carbon Stress in Diabetic Glucotoxicity

Diabetes and its complications are caused by chronic glucotoxicity driven by persistent hyperglycemia. In this article, we review the mechanisms of diabetic glucotoxicity by focusing mainly on hyperglycemic stress and carbon stress. Mechanisms of hyperglycemic stress include reductive stress or pseudohypoxic stress caused by redox imbalance between NADH and NAD(+) driven by activation of both the polyol pathway and poly ADP ribose polymerase; the hexosamine pathway; the advanced glycation end products pathway; the protein kinase C activation pathway; and the enediol formation pathway. Mechanisms of carbon stress include excess production of acetyl-CoA that can over-acetylate a proteome and excess production of fumarate that can over-succinate a proteome; both of which can increase glucotoxicity in diabetes. For hyperglycemia stress, we also discuss the possible role of mitochondrial complex I in diabetes as this complex, in charge of NAD(+) regeneration, can make more reactive oxygen species (ROS) in the presence of excess NADH. For carbon stress, we also discuss the role of sirtuins in diabetes as they are deacetylases that can reverse protein acetylation thereby attenuating diabetic glucotoxicity and improving glucose metabolism. It is our belief that targeting some of the stress pathways discussed in this article may provide new therapeutic strategies for treatment of diabetes and its complications.

Sources and implications of NADH/NAD(+) redox imbalance in diabetes and its complications

NAD(+) is a fundamental molecule in metabolism and redox signaling. In diabetes and its complications, the balance between NADH and NAD(+) can be severely perturbed. On one hand, NADH is overproduced due to influx of hyperglycemia to the glycolytic and Krebs cycle pathways and activation of the polyol pathway. On the other hand, NAD(+) can be diminished or depleted by overactivation of poly ADP ribose polymerase that uses NAD(+) as its substrate. Moreover, sirtuins, another class of enzymes that also use NAD(+) as their substrate for catalyzing protein deacetylation reactions, can also affect cellular content of NAD(+). Impairment of NAD(+) regeneration enzymes such as lactate dehydrogenase in erythrocytes and complex I in mitochondria can also contribute to NADH accumulation and NAD(+) deficiency. The consequence of NADH/NAD(+) redox imbalance is initially reductive stress that eventually leads to oxidative stress and oxidative damage to macromolecules, including DNA, lipids, and proteins. Accordingly, redox imbalance-triggered oxidative damage has been thought to be a major factor contributing to the development of diabetes and its complications. Future studies on restoring NADH/NAD(+) redox balance could provide further insights into design of novel antidiabetic strategies.

The regulatory roles of NADPH oxidase, intra- and extra-cellular HSP70 in pancreatic islet function, dysfunction and diabetes

The 70 kDa heat-shock protein (HSP70) family is important for a dynamic range of cellular processes that include protection against cell stress, modulation of cell signalling, gene expression, protein synthesis, protein folding and inflammation. Within this family, the inducible 72 kDa and the cognate 73 kDa forms are found at the highest level. HSP70 has dual functions depending on location. For example, intracellular HSP70 (iHSP70) is anti-inflammatory whereas extracellular HSP70 (eHSP70) has a pro-inflammatory function, resulting in local and systemic inflammation. We have recently identified a divergence in the levels of eHSP70 and iHSP70 in subjects with diabetes compared with healthy subjects and also reported that eHSP70 was correlated with insulin resistance and pancreatic β-cell dysfunction/death. In the present review, we describe possible mechanisms by which HSP70 participates in cell function/dysfunction, including the activation of NADPH oxidase isoforms leading to oxidative stress, focusing on the possible role of HSPs and signalling in pancreatic islet α- and β-cell physiological function in health and Type 2 diabetes mellitus.