Increased expression of NAD(P)H oxidase in islets of animal models of Type 2 diabetes and its improvement by an AT1 receptor antagonist

MiR-190 ameliorates glucotoxicity-induced dysfunction and apoptosis of pancreatic β-cells by inhibiting NOX2-mediated reactive oxygen species production

Glucotoxicity-induced pancreatic β-cell failure contributes to the development of type 2 diabetes mellitus (T2DM). Accumulating evidence reveals that miRNAs play a critical role in regulating pancreatic β-cell function and survival. In this study, we employed a self-assembled cell microarray (SAMcell)-based functional screening assay to identify miRNAs that are capable of regulating the dysfunction of β-cells induced by glucotoxicity. Among 62 conserved miRNAs we tested, miR-190 was identified as a candidate regulator that could effectively restore insulin expression in NIT-1 cells under high-glucose (HG) stimulation. Further analyses demonstrated that miR-190 was significantly down-regulated in HG-treated NIT-1 cells, as well as in the pancreas of diabetic mice. Mechanistic studies showed that Cybb is the direct target gene of miR-190, which encodes the gp91phox protein, a subunit of the NOX2 complex. Furthermore, both miR-190 overexpression and Cybb knockdown inhibited apoptosis and improved glucose-stimulated insulin secretion (GSIS) in HG-stimulated NIT-1 cells by attenuating the excessive production of reactive oxygen species (ROS). More importantly, a targeted delivery of mPEG-PCL-g-PDMAEMA nanoparticles/miR-190 complexes (PECgD NPs/miR-190) to the pancreas significantly ameliorated hyperglycemia, decreased fasting serum insulin levels, and improved glucose tolerance in diabetic mice. Taken together, our findings suggest that the miR-190/Cybb axis plays an important role in glucotoxicity-induced pancreatic β-cell failure. Restoring miR-190 expression levels may be a possible therapeutic strategy to protect β-cells in T2DM.

HIF-2α Preserves Mitochondrial Activity and Glucose Sensing in Compensating β-Cells in Obesity

In obesity, increased mitochondrial metabolism with the accumulation of oxidative stress leads to mitochondrial damage and β-cell dysfunction. In particular, β-cells express antioxidant enzymes at relatively low levels and are highly vulnerable to oxidative stress. Early in the development of obesity, β-cells exhibit increased glucose-stimulated insulin secretion in order to compensate for insulin resistance. This increase in β-cell function under the condition of enhanced metabolic stress suggests that β-cells possess a defense mechanism against increased oxidative damage, which may become insufficient or decline at the onset of type 2 diabetes. Here, we show that metabolic stress induces β-cell hypoxia inducible factor 2α (HIF-2α), which stimulates antioxidant gene expression (e.g., Sod2 and Cat) and protects against mitochondrial reactive oxygen species (ROS) and subsequent mitochondrial damage. Knockdown of HIF-2α in Min6 cells exaggerated chronic high glucose-induced mitochondrial damage and β-cell dysfunction by increasing mitochondrial ROS levels. Moreover, inducible β-cell HIF-2α knockout mice developed more severe β-cell dysfunction and glucose intolerance on a high-fat diet, along with increased ROS levels and decreased islet mitochondrial mass. Our results provide a previously unknown mechanism through which β-cells defend against increased metabolic stress to promote β-cell compensation in obesity.

Lipotoxicity and β-Cell Failure in Type 2 Diabetes: Oxidative Stress Linked to NADPH Oxidase and ER Stress

A high caloric intake, rich in saturated fats, greatly contributes to the development of obesity, which is the leading risk factor for type 2 diabetes (T2D). A persistent caloric surplus increases plasma levels of fatty acids (FAs), especially saturated ones, which were shown to negatively impact pancreatic β-cell function and survival in a process called lipotoxicity. Lipotoxicity in β-cells activates different stress pathways, culminating in β-cells dysfunction and death. Among all stresses, endoplasmic reticulum (ER) stress and oxidative stress have been shown to be strongly correlated. One main source of oxidative stress in pancreatic β-cells appears to be the reactive oxygen species producer NADPH oxidase (NOX) enzyme, which has a role in the glucose-stimulated insulin secretion and in the β-cell demise during both T1 and T2D. In this review, we focus on the acute and chronic effects of FAs and the lipotoxicity-induced β-cell failure during T2D development, with special emphasis on the oxidative stress induced by NOX, the ER stress, and the crosstalk between NOX and ER stress.

Xanthenone, ACE2 activator, counteracted gentamicin-induced nephrotoxicity in rats: Impact on oxidative stress and ACE2/Ang-(1-7) signaling

Nephrotoxicity is a rapid deterioration of kidney function due to exposure to nephrotoxic drugs as gentamicin. Gentamicin increases the generation of reactive oxygen species (ROS) leading to inflammatory responses and nuclear factor-κB (NF-κB) activation. The renal renin-angiotensin system (RAS) is considered a crucial regulator for physiological homeostasis and disease progression through the classic ACE/Ang-II/AT1 axis and its antagonist, ACE2/Ang-(1-7)/Mas axis which exerts an important role in the kidney. The present study evaluates the protective effects of the angiotensin-converting enzyme 2 (ACE2) activator; xanthenone; against experimental nephrotoxicity induced by gentamicin. Rats were divided into 4 groups, normal control, xanthenone (2 mg/kg, s.c), gentamicin (100 mg/kg, i.p. for one week) and xanthenone + gentamicin groups. Blood urea nitrogen (BUN) and serum creatinine levels were measured. The kidney tissues were used for estimating glutathione (GSH), superoxide dismutase (SOD), malondialdehyde (MDA), tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), NF-κB, Angiotensin II (AngII), and Ang-(1-7). In addition, histopathological examination and Western blot analysis of ACE2 expression were done. Xanthenone significantly restored serum levels of BUN and creatinine. Xanthenone exerted significant antioxidant effect as revealed by increased GSH content and SOD activity together with reduced MDA content. It exerted anti-inflammatory effect by significant reduction in TNF-α, NF-κB and IL-6 expression compared to gentamicin group. Xanthenone increased Ang-(1-7) and ACE2 expression while significantly decreased Ang-II expression. Histopathologically, xanthenone markedly counteracted gentamicin-induced renal aberrations. Activation of ACE2/Ang-(1-7) by xanthenone produced significant antioxidant and anti-inflammatory effects that counteracted gentamicin-induced nephrotoxicity.

Metabolic Consequences of Solid Organ Transplantation

Metabolic complications affect over 50% of solid organ transplant recipients. These include posttransplant diabetes, nonalcoholic fatty liver disease, dyslipidemia, and obesity. Preexisting metabolic disease is further exacerbated with immunosuppression and posttransplant weight gain. Patients transition from a state of cachexia induced by end-organ disease to a pro-anabolic state after transplant due to weight gain, sedentary lifestyle, and suboptimal dietary habits in the setting of immunosuppression. Specific immunosuppressants have different metabolic effects, although all the foundation/maintenance immunosuppressants (calcineurin inhibitors, mTOR inhibitors) increase the risk of metabolic disease. In this comprehensive review, we summarize the emerging knowledge of the molecular pathogenesis of these different metabolic complications, and the potential genetic contribution (recipient +/- donor) to these conditions. These metabolic complications impact both graft and patient survival, particularly increasing the risk of cardiovascular and cancer-associated mortality. The current evidence for prevention and therapeutic management of posttransplant metabolic conditions is provided while highlighting gaps for future avenues in translational research.

NADPH Oxidase Inhibition: Preclinical and Clinical Studies in Diabetic Complications

Significance: Oxidative stress plays a critical role in the development and progression of serious micro- and macrovascular complications of diabetes. Nicotinamide adenine dinucleotide phosphate oxidase (NOX)-derived reactive oxygen species (ROS) significantly contribute to oxidative stress-associated inflammatory pathways that lead to tissue damage of different organs, including the kidneys, retina, brain, nerves, and the cardiovascular system. Recent Advances: Preclinical studies, including genetic-modified mouse models or cell culture models, have revealed the role of specific NOX isoforms in different diabetic complications, and suggested them as a promising target for the treatment of these diseases. Critical Issues: In this review, we provide an overview of the role of ROS and oxidative stress in macrovascular complications, such as stroke, myocardial infarction, coronary artery disease, and peripheral vascular disease that are all mainly driven by atherosclerosis, as well as microvascular complications, such as diabetic retinopathy, nephropathy, and neuropathy. We summarize conducted genetic deletion studies of different Nox isoforms as well as pharmacological intervention studies using NOX inhibitors in the context of preclinical as well as clinical research on diabetic complications. Future Directions: We outline the isoforms that are most promising for future clinical trials in the context of micro- and macrovascular complications of diabetes.

Hv1-deficiency protects β cells from glucotoxicity through regulation of NOX4 level

High glucose (HG)-induced oxidative stress contributes to the dysfunction of pancreatic β cells in diabetes. The voltage-gated proton channel Hv1 has been proposed to support reactive oxygen species (ROS) production during respiratory bursts. However, the effect of Hv1 on glucotoxicity in pancreatic β cells is not clear yet. In this study, we examined the protective effects of Hv1-deficiency in HG cultured β cells. Following 48 h of treatment with 30 mM high glucose, Hv1 KO β cells showed higher cell viability, lower cell apoptosis and a more stable insulin gene expression level compared to WT β cells. In both control and HG cultured β cells, deficiency of Hv1 decreased the glucose- and PMA-induced ROS production. Finally, HG incubation led to NOX4 upregulation in WT β cells, which could be inhibited by HV1 deficiency. In conclusion, Hv1-deficiency prevents the HG treatment-induced NOX4 upregulation and protects β cells from glucotoxicity.

Reactive Oxygen Species and Their Implications on CD4+ T Cells in Type 1 Diabetes

Previous work has indicated that type 1 diabetes (T1D) pathology is highly driven by reactive oxygen species (ROS). One way in which ROS shape the autoimmune response demonstrated in T1D is by promoting CD4⁺ T cell activation and differentiation. As CD4⁺ T cells are a significant contributor to pancreatic β cell destruction in T1D, understanding how ROS impact their development, activation, and differentiation is critical. Recent Advances: CD4⁺ T cells themselves generate ROS via nicotinamide adenine dinucleotide phosphate (NADPH) oxidase expression and electron transport chain activity. Moreover, T cells can also be exposed to exogenous ROS generated by other immune cells (e.g., macrophages and dendritic cells) and β cells. Genetically modified animals and ROS inhibitors have demonstrated that ROS blockade during activation results in CD4⁺ T cell hyporesponsiveness and reduced diabetes incidence. Critical Issues and Future Directions: Although the majority of studies with regard to T1D and CD4⁺ T cells have been done to examine the influence of redox on CD4⁺ T cell activation, this is not the only circumstance in which a T cell can be impacted by redox. ROS and redox have also been shown to play roles in CD4⁺ T cell-related tolerogenic mechanisms, including thymic selection and regulatory T cell-mediated suppression. However, the effect of these mechanisms with respect to T1D pathogenesis remains elusive. Therefore, pursuing these avenues may provide valuable insight into the global role of ROS and redox in autoreactive CD4⁺ T cell formation and function.

Chronic AT1 blockade improves glucose homeostasis in obese OLETF rats

Obesity is associated with the inappropriate activation of the renin-angiotensin system (RAS), which increases arterial pressure, impairs insulin secretion and decreases peripheral tissue insulin sensitivity. RAS blockade reverses these detriments; however, it is not clear whether the disease state of the organism and treatment duration determine the beneficial effects of RAS inhibition on insulin secretion and insulin sensitivity. Therefore, the objective of this study was to compare the benefits of acute vs chronic angiotensin receptor type 1 (AT₁) blockade started after the onset of obesity, hyperglycemia and hypertension on pancreatic function and peripheral insulin resistance. We assessed adipocyte morphology, glucose intolerance, pancreatic redox balance and insulin secretion after 2 and 11 weeks of AT₁ blockade in the following groups of rats: (1) untreated Long-Evans Tokushima Otsuka (lean control; n = 10), (2) untreated Otsuka Long-Evans Tokushima Fatty (OLETF; n = 12) and (3) OLETF + ARB (ARB; 10 mg olmesartan/kg/day by oral gavage; n = 12). Regardless of treatment duration, AT₁ blockade decreased systolic blood pressure and fasting plasma triglycerides, whereas chronic AT₁ blockade decreased fasting plasma glucose, glucose intolerance and the relative abundance of large adipocytes by 22, 36 and 70%, respectively. AT₁ blockade, however, did not improve pancreatic oxidative stress or reverse impaired insulin secretion. Collectively, these data show that AT₁ blockade after the onset of obesity, hyperglycemia and hypertension improves peripheral tissue insulin sensitivity, but cannot completely reverse the metabolic derangement characterized by impaired insulin secretion once it has been compromised.

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.

NMDAR antagonists for the treatment of diabetes mellitus-Current status and future directions

Diabetes mellitus is characterized by chronically elevated blood glucose levels accelerated by a progressive decline of insulin-producing β-cells in the pancreatic islets. Although medications are available to transiently adjust blood glucose to normal levels, the effects of current drugs are limited when it comes to preservation of a critical mass of functional β-cells to sustainably maintain normoglycemia. In this review, we recapitulate recent evidence on the role of pancreatic N-methyl-D-aspartate receptors (NMDARs) in β-cell physiology, and summarize effects of morphinan-based NMDAR antagonists that are beneficial for insulin secretion, glucose tolerance and islet cell survival. We further discuss NMDAR-mediated molecular pathways relevant for neuronal cell survival, which may also be important for the preservation of β-cell function and mass. Finally, we summarize the literature for evidence on the role of NMDARs in the development of diabetic long-term complications, and highlight beneficial pharmacologic aspects of NMDAR antagonists in diabetic nephropathy, retinopathy as well as neuropathy.

4-Hydroxynonenal dependent alteration of TRPV1-mediated coronary microvascular signaling

We demonstrated previously that TRPV1-dependent regulation of coronary blood flow (CBF) is disrupted in diabetes. Further, we have shown that endothelial TRPV1 is differentially regulated, ultimately leading to the inactivation of TRPV1, when exposed to a prolonged pathophysiological oxidative environment. This environment has been shown to increase lipid peroxidation byproducts including 4-Hydroxynonenal (4-HNE). 4-HNE is notorious for producing protein post-translation modification (PTM) via reactions with the amino acids: cysteine, histidine and lysine. Thus, we sought to determine if 4-HNE mediated post-translational modification of TRPV1 could account for dysfunctional TRPV1-mediated signaling observed in diabetes. Our initial studies demonstrate 4-HNE infusion decreases TRPV1-dependent coronary blood flow in C57BKS/J (WT) mice. Further, we found that TRPV1-dependent vasorelaxation was suppressed after 4-HNE treatment in isolated mouse coronary arterioles. Moreover, we demonstrate 4-HNE significantly inhibited TRPV1 currents and Ca²⁺ entry utilizing patch-clamp electrophysiology and calcium imaging respectively. Using molecular modeling, we identified potential pore cysteines residues that, when mutated, could restore TRPV1 function in the presence of 4-HNE. Specifically, complete rescue of capsaicin-mediated activation of TRPV1 was obtained following mutation of pore Cysteine 621. Finally, His tag pull-down of TRPV1 in HEK cells treated with 4-HNE demonstrated a significant increase in 4-HNE binding to TRPV1, which was reduced in the TRPV1 C621G mutant. Taken together these data suggest that 4-HNE decreases TRPV1-mediated responses, at both the in vivo and in vitro levels and this dysfunction can be rescued via mutation of the pore Cysteine 621. Our results show the first evidence of an amino acid specific modification of TRPV1 by 4-HNE suggesting this 4-HNE-dependent modification of TRPV1 may contribute to microvascular dysfunction and tissue perfusion deficits characteristic of diabetes.

Antioxidant effect of angiotensin (1‑7) in the protection of pancreatic β cell function

It is well known that the local renin-angiotensin system (RAS) is activated in the diabetic state, which results in an increase in the level of oxidative stress injury to pancreatic β cells. The angiotensin-converting enzyme 2 (ACE2)/angiotensin (1-7) [Ang (1-7)]/Mas axis is a negative regulator of the classical renin-angiotensin system. In order to investigate the antioxidant effect of Ang (1-7) on pancreatic β cells, INS-1 cells were cultured and oxidative stress was induced by treatment with H₂O₂. Glucose-stimulated insulin secretion (GSIS), the generation of reactive oxygen species (ROS), mitochondrial membrane potential (MMP) and glucose-stimulated calcium (GSCa) responses in β cells were determined following treatment with Ang (1-7). It was observed that H₂O₂ significantly impaired the insulin secreting function, increased the production of ROS, and also decreased the levels of GSCa and MMP. Pre-treatment with Ang (1-7) alleviated these effects and treatment with A779 [antagonist of Ang (1-7)] prevented the effects of Ang (1-7). Based on these findings, it was concluded that Ang (1-7) can protect pancreatic β cells from oxidative injury and such protection can be blocked by its antagonist A779.

Modulation of glucose metabolism by the renin-angiotensin-aldosterone system

The renin-angiotensin-aldosterone system (RAAS) is an enzymatic cascade functioning in a paracrine and autocrine fashion. In animals and humans, RAAS intrinsic to tissues modulates food intake, metabolic rate, adiposity, insulin sensitivity, and insulin secretion. A large array of observations shows that dysregulation of RAAS in the metabolic syndrome favors type 2 diabetes. Remarkably, angiotensin-converting enzyme inhibitors, suppressing the synthesis of angiotensin II (ANG II), and angiotensin receptor blockers, targeting the ANG II type 1 receptor, prevent diabetes in patients with hypertensive or ischemic cardiopathy. These drugs interrupt the negative feedback loop of ANG II on the RAAS cascade, which results in increased production of angiotensins. In addition, they change the tissue expression of RAAS components. Therefore, the concept of a dual axis of RAAS regarding glucose homeostasis has emerged. The RAAS deleterious axis increases the production of inflammatory cytokines and raises oxidative stress, exacerbating the insulin resistance and decreasing insulin secretion. The beneficial axis promotes adipogenesis, blocks the production of inflammatory cytokines, and lowers oxidative stress, thereby improving insulin sensitivity and secretion. Currently, drugs targeting RAAS are not given for the purpose of preventing diabetes in humans. However, we anticipate that in the near future the discovery of novel means to modulate the RAAS beneficial axis will result in a decisive therapeutic breakthrough.

Testosterone reduces AGTR1 expression to prevent β-cell and islet apoptosis from glucotoxicity

Hypogonadism in men is associated with an increased incidence of type 2 diabetes. Supplementation with testosterone has been shown to protect pancreatic β-cell against apoptosis due to toxic substances including streptozotocin and high glucose. One of the pathological mechanisms of glucose-induced pancreatic β-cell apoptosis is the induction of the local rennin-angiotensin-aldosterone system (RAAS). The role of testosterone in regulation of the pancreatic RAAS is still unknown. This study aims to investigate the protective action of testosterone against glucotoxicity-induced pancreatic β-cell apoptosis via alteration of the pancreatic RAAS pathway. Rat insulinoma cell line (INS-1) cells or isolated male mouse islets were cultured in basal and high-glucose media in the presence or absence of testosterone, losartan, and angiotensin II (Ang II), then cell apoptosis, cleaved caspase 3 expression, oxidative stress, and expression of angiotensin II type 1 receptor (AGTR1) and p47(phox) mRNA and protein were measured. Testosterone and losartan showed similar effects in reducing pancreatic β-cell apoptosis. Testosterone significantly reduced expression of AGTR1 protein in INS-1 cells cultured in high-glucose medium or high-glucose medium with Ang II. Testosterone decreased the expression of AGTR1 and p47(phox) mRNA and protein in comparison with levels in cells cultured in high-glucose medium alone. Furthermore, testosterone attenuated superoxide production when co-cultured with high-glucose medium. In contrast, when cultured in basal glucose, supplementation of testosterone did not have any effect on cell apoptosis, oxidative stress, and expression of AGT1R and p47(phox). In addition, high-glucose medium did not increase cleaved caspase 3 in AGTR1 knockdown experiments. Thus, our results indicated that testosterone prevents pancreatic β-cell apoptosis due to glucotoxicity through reduction of the expression of ATGR1 and its signaling pathway.

Inhibition of NADPH oxidase-1 preserves beta cell function

AIMS/HYPOTHESIS

Upregulation of the reactive oxygen species (ROS)-producing enzyme NADPH oxidase (NOX)-1 in islets and beta cells follows acute exposure to inflammatory cytokines and is concomitant with beta cell dysfunction. NOX-1 is a candidate mediator of inflammation-induced beta cell dysfunction. This study aimed to determine whether selective inhibition of NADPH oxidase-1 presents a new strategy to preserve beta cell function.

METHODS

Induced beta cell dysfunction was studied in primary human donor islets, isolated mouse islets and murine beta cell lines. Islets and beta cells were stimulated with inflammatory cytokines (TNF-α, IL-1β, IFN-γ). NOX-1 activity was blocked by the selective inhibitor ML171.

RESULTS

Cytokine induction of intracellular ROS was reduced 80% with 1 μmol/l ML171 in murine beta cell lines (p < 0.01). Cytokine-induced apoptosis, measured by caspase-3 activation or quantified fluorescence microscopy, was prevented in islets and beta cell lines up to 100% with ML171 in a concentration-dependent manner (p < 0.05). Functionally, glucose-stimulated insulin secretion was abolished by cytokine exposure but preserved by ML171 in isolated mouse islets and murine beta cell lines. A feed-forward regulation of NOX-1 in islets and beta cell lines was disrupted by ML171.

CONCLUSIONS/INTERPRETATION

Stimulation of NOX-1 activity is a major component of inflammatory cytokine-induced beta cell dysfunction. Significant protection of beta cells is conferred with selective inhibition of NOX-1. Suppression of NOX-1 activity may present a new therapeutic strategy to preserve and protect beta cell function in diabetes.

Xuezhikang attenuated the functional and morphological impairment of pancreatic islets in diabetic mice via the inhibition of oxidative stress

Xuezhikang, purified from red yeast rice, is a traditional Chinese medicine with pleiotropic effects on the cardiovascular system. Oxidative stress plays a crucial role in the dysfunction of pancreas islet in diabetic condition and represents a promising therapeutical target for diabetes mellitus. Therefore, the purpose of this study was to explore the effects and possible mechanisms of xuezhikang on the microenvironment and insulin secretion by pancreatic islets in db/db diabetic mice. Our results showed that xuezhikang decreased the blood glucose level by improving glucose tolerance and insulin secretion in db/db mice. Xuezhikang protected islets from hyperglycemic injury as illustrated by the conserved β-cell content and microenvironment. Furthermore, xuezhikang potently inhibited the expression of key factors in oxidative stress. In addition, administration of xuezhikang caused an upregulated expression of glucose-sensing apparatus. These observations provide evidence that the influence of xuezhikang on oxidative stress may at least partly account for its protective effects on the microenvironment and insulin secretion function of pancreatic islets in diabetes.

Valsartan slows the progression of diabetic nephropathy in db/db mice via a reduction in podocyte injury, and renal oxidative stress and inflammation

Higher doses of AngII (angiotensin II) blockers are intended to optimize albuminuria reduction rather than for blood pressure control in chronic kidney disease. However, the long-term renoprotection of high-dose AngII blockers has yet to be defined. The present study sought to determine whether doses of ARB (AngII receptor blocker) that maximally reduce proteinuria could slow the progression of glomerulosclerosis in the uninephrectomized db/db mouse, a model of Type 2 diabetes. Untreated uninephrectomized db/db mice had normal blood pressure, but developed progressive albuminuria and mesangial matrix expansion between 18 and 22 weeks of age, which was associated with increased renal expression of TGFβ1 (transforming growth factor β1), PAI-1 (plasminogen-activator inhibitor-1), type IV collagen and FN (fibronectin). Treatment with valsartan in the drinking water of db/db mice from 18 to 22 weeks of age, at a dose that was determined previously to maximally reduce proteinuria, prevented the increases in albuminuria and the markers of renal fibrosis seen in untreated db/db mice. In addition, WT-1 (Wilms tumour protein-1)-immunopositive podocyte numbers were found to be lower in the untreated glomeruli of mice with diabetes. The expression of podocin and nephrin were continually decreased in mice with diabetes between 18 and 22 weeks of age. These changes are indicative of podocyte injury and the administration of valsartan ameliorated them substantially. Renal expression of TNFα (tumour necrosis factor α), MCP-1 (monocyte chemoattractant protein-1), Nox2 (NADPH oxidase 2), p22phox and p47phox and urine TBARS (thiobarbituric acid-reacting substance) levels, the markers of renal inflammation and oxidative stress, were increased during disease progression in mice with diabetes. Valsartan treatment was shown to reduce these markers. Thus high doses of valsartan not only reduce albuminuria maximally, but also halt the progression of the glomerulosclerosis resulting from Type 2 diabetes via a reduction in podocyte injury and renal oxidative stress and inflammation.

Modulation of the action of insulin by angiotensin-(1-7)

The prevalence of Type 2 diabetes mellitus is predicted to increase dramatically over the coming years and the clinical implications and healthcare costs from this disease are overwhelming. In many cases, this pathological condition is linked to a cluster of metabolic disorders, such as obesity, systemic hypertension and dyslipidaemia, defined as the metabolic syndrome. Insulin resistance has been proposed as the key mediator of all of these features and contributes to the associated high cardiovascular morbidity and mortality. Although the molecular mechanisms behind insulin resistance are not completely understood, a negative cross-talk between AngII (angiotensin II) and the insulin signalling pathway has been the focus of great interest in the last decade. Indeed, substantial evidence has shown that anti-hypertensive drugs that block the RAS (renin-angiotensin system) may also act to prevent diabetes. Despite its long history, new components within the RAS continue to be discovered. Among them, Ang-(1-7) [angiotensin-(1-7)] has gained special attention as a counter-regulatory hormone opposing many of the AngII-related deleterious effects. Specifically, we and others have demonstrated that Ang-(1-7) improves the action of insulin and opposes the negative effect that AngII exerts at this level. In the present review, we provide evidence showing that insulin and Ang-(1-7) share a common intracellular signalling pathway. We also address the molecular mechanisms behind the beneficial effects of Ang-(1-7) on AngII-mediated insulin resistance. Finally, we discuss potential therapeutic approaches leading to modulation of the ACE2 (angiotensin-converting enzyme 2)/Ang-(1-7)/Mas receptor axis as a very attractive strategy in the therapy of the metabolic syndrome and diabetes-associated diseases.

Palmitate induces reactive oxygen species production and β-cell dysfunction by activating nicotinamide adenine dinucleotide phosphate oxidase through Src signaling

AIMS/INTRODUCTION

Chronic hyperlipidemia impairs pancreatic β-cell function, referred to as lipotoxicity. We have reported an important role of endogenous reactive oxygen species (ROS) overproduction by activation of Src, a non-receptor tyrosine kinase, in impaired glucose-induced insulin secretion (GIIS) from diabetic rat islets. In the present study, we investigated the role of ROS production by Src signaling in palmitate-induced dysfunction of β-cells.

MATERIALS AND METHODS

After rat insulinoma INS-1D cells were exposed to 0.6 mmol/L palmitate for 24 h (palmitate exposure); GIIS, ROS production and nicotinamide adenine dinucleotide phosphate oxidase (NOX) activity were examined with or without exposure to10 μmol/L 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2), a Src inhibitior, for 30 or 60 min.

RESULTS

Exposure to PP2 recovered impaired GIIS and decreased ROS overproduction as a result of palmitate exposure. Palmitate exposure increased activity of NOX and protein levels of NOX2, a pathological ROS source in β-cells. Palmitate exposure increased the protein level of p47 (phox) , a regulatory protein of NOX2, in membrane fraction compared with control, which was reduced by PP2. Transfection of small interfering ribonucleic acid of p47 (phox) suppressed the augmented p47 (phox) protein level in membrane fraction, decreased augmented ROS production and increased impaired GΙIS by palmitate exposure. In addition, exposure to PP2 ameliorated impaired GIIS and decreased ROS production in isolated islets of KK-A(y) mice, an obese diabetic model with hyperlipidemia.

CONCLUSIONS

Activation of NOX through Src signaling plays an important role in ROS overproduction and impaired GΙIS caused by chronic exposure to palmitate, suggesting a lipotoxic mechanism of β-cell dysfunction of obese mice.

Mild exposure of RIN-5F β-cells to human islet amyloid polypeptide aggregates upregulates antioxidant enzymes via NADPH oxidase-RAGE: an hormetic stimulus

The presence of amyloid aggregates of human islet amyloid polypeptide (hIAPP), a hallmark of type 2 diabetes, contributes to pancreatic β-cell impairment, where oxidative stress plays a key role. A contribution of NADPH oxidase to reactive oxygen species (ROS) generation after cell exposure to micromolar concentrations of hIAPP aggregates has been suggested. However, little is known about β-cells exposure to lower amounts of hIAPP aggregates, similar to those found in human pancreas. Thus, we aimed to investigate the events resulting from RIN-5F cells exposure to nanomolar concentrations of toxic hIAPP aggregates. We found an early and transient rise of NADPH oxidase activity resulting from increased Nox1 expression following the engagement of receptor for advanced glycation end-products (RAGE) by hIAPP aggregates. Unexpectedly, NADPH oxidase activation was not accompanied by a significant ROS increase and the lipoperoxidation level was significantly reduced. Indeed, cell exposure to hIAPP aggregates affected the antioxidant defences, inducing a significant increase of the expression and activity of catalase and glutathione peroxidase. We conclude that exposure of pancreatic β-cells to nanomolar concentrations of hIAPP aggregates for a short time induces an hormetic response via the RAGE-Nox1 axis; the latter stimulates the enzymatic antioxidant defences that preserve the cells against oxidative stress damage.

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Short time exposure of pancreatic β-cells to low hIAPP aggregate amounts is studied.

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NADPH oxidase activity is increased after 3 h treatment with 60 nM hIAPP aggregates.

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RAGE engagement by aggregates increases Nox1 expression.

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Reduced lipoperoxidation and increased antioxidant enzymes were observed.

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A protective hormetic response via RAGE-Nox1 is proposed.

Genetics of new-onset diabetes after transplantation

New-onset diabetes after transplantation is a common complication that reduces recipient survival. Research in renal transplant recipients has suggested that pancreatic β-cell dysfunction, as opposed to insulin resistance, may be the key pathologic process. In this study, clinical and genetic factors associated with new-onset diabetes after transplantation were identified in a white population. A joint analysis approach, with an initial genome-wide association study in a subset of cases followed by de novo genotyping in the complete case cohort, was implemented to identify single-nucleotide polymorphisms (SNPs) associated with the development of new-onset diabetes after transplantation. Clinical variables associated with the development of diabetes after renal transplantation included older recipient age, female sex, and percentage weight gain within 12 months of transplantation. The genome-wide association study identified 26 SNPs associated with new-onset diabetes after transplantation; this association was validated for eight SNPs (rs10484821, rs7533125, rs2861484, rs11580170, rs2020902, rs1836882, rs198372, and rs4394754) by de novo genotyping. These associations remained significant after multivariate adjustment for clinical variables. Seven of these SNPs are associated with genes implicated in β-cell apoptosis. These results corroborate recent clinical evidence implicating β-cell dysfunction in the pathophysiology of new-onset diabetes after transplantation and support the pursuit of therapeutic strategies to protect β cells in the post-transplant period.

Elevated hydrogen peroxide and decreased catalase and glutathione peroxidase protection are associated with aging sarcopenia

Background

Sarcopenia is the progressive loss of skeletal muscle that contributes to the decline in physical function during aging. A higher level of oxidative stress has been implicated in aging sarcopenia. The current study aims to determine if the higher level of oxidative stress is a result of increased superoxide (O₂‾) production by the NADPH oxidase (NOX) enzyme or decrease in endogenous antioxidant enzyme protection.

Methods

Female Balb/c mice were assigned to 4 age groups; 6, 12, 18 and 24 months. Body weight and animal survival rates were recorded over the course of the study. Skeletal muscle tissues were collected and used to measure NOX subunit mRNA, O₂‾ levels and antioxidant enzymes.

Results

Key subunit components of NOX expression were elevated in skeletal muscle at 18 months, when sarcopenia was first evident. Increased superoxide dismutase 1 (SOD1) activity suggests an increase in O₂‾ dismutation and this was further supported by elevated levels of hydrogen peroxide (H₂O₂) and decline in catalase and glutathione peroxidase (GPx) antioxidant protection in skeletal muscle at this time. NOX expression was also higher in skeletal muscle at 24 months, however this was coupled with elevated levels of O₂‾ and a decline in SOD1 activity, compared to 6 and 12 months but was not associated with further loss of muscle mass.

Conclusions

While the source of ROS in sarcopenic muscle remains unknown, this study provides evidence that the NOX enzyme could be involved in ROS production by regulating superoxide in ageing muscles. This study also suggests that H₂O₂ is the key ROS in the onset of sarcopenia and that the decline in antioxidant protection by catalase and GPx is indicative of antioxidant dysfunction and may therefore be a major contributing factor in the development or onset of sarcopenia. Furthermore, the changes in ROS and antioxidant activity after sarcopenia was first evident gives some evidence for a compensatory mechanism, in response to insult, in order to maintain muscle integrity.

Interaction between cytokines and inflammatory cells in islet dysfunction, insulin resistance and vascular disease

Inflammation is an established pathogenic player in insulin resistance, islet demise and atherosclerosis. The complex interactions between cytokines, immune cells and affected tissues result in sustained inflammation in diabetes and atherosclerosis. 12- and 15-lipoxygenase (LO), such as 12/15-LO, produces a variety of metabolites through peroxidation of fatty acids and potentially contributes to the complex molecular crosstalk at the site of inflammation. 12- and 15-LO pathways are frequently activated in tissues affected by diabetes and atherosclerosis including adipose tissue (AT), islets and the vasculature. Moreover, mice with whole body and tissue-specific knockout of 12/15-LO are protected against insulin resistance, hyperglycaemia and atherosclerosis supporting functional contribution of 12- and 15-LO pathways in diabetes and atherosclerosis. Recently, it has emerged that there is a temporal regulation of the particular isoforms of 12- and 15-LO in human AT and islets during the development of type 1 and type 2 diabetes and obesity. Analyses of tissues affected by diabetes and atherosclerosis also implied the roles of interleukin (IL)-12 and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-1 (NOX-1) in islets and IL-17A in atherosclerosis. Future studies should aim to test the efficacy of inhibitions of these mediators for treatment of diabetes and atherosclerosis.

Evidence for the involvement of GPR40 and NADPH oxidase in palmitic acid-induced superoxide production and insulin secretion

G protein coupled receptor 40 (GPR40) and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex have been shown to be involved in the fatty acid amplification of glucose-stimulated insulin secretion (GSIS). The effect of palmitic acid on superoxide production and insulin secretion by INS-1E cells and the possible involvement of GPR40 and NADPH oxidase in these processes were examined in this study. Cells were incubated during 1 h with palmitic acid in low and high glucose concentrations, a GPR40 agonist (GW9508) and inhibitors of NADPH oxidase (diphenyleneiodonium, DPI) and PKC (calphostin C). GW9508 induced superoxide production at 2.8 and 5.6 mM glucose concentrations and stimulated insulin secretion at 16.7 mM glucose concentration involving both PKC and NADPH oxidase activation. Palmitic acid induced superoxide production through NADPH oxidase and GPR40-dependent pathways and the stimulation of insulin secretion in the presence of a high glucose concentration was reduced by knockdown of GPR40 using siRNA. Our results suggest that palmitic acid induces superoxide production and potentiates GSIS through NADPH oxidase and GPR40 pathways in pancreatic ? cells.

The protective effect of telmisartan in Type 2 diabetes rat kidneys is related to the downregulation of thioredoxin-interacting protein

BACKGROUND

Thioredoxin-interacting protein (Txnip), an inhibitor of thioredoxin (Trx), increases in diabetic nephropathy and promotes oxidative stress. The angiotensin II (Ang II) receptor blocker telmisartan may protect renal function in diabetic models and patients via multiple effects including antioxidation. However, its mechanism has not been fully elucidated, and its relationship to Txnip remains unclear.

AIM

This study aimed to investigate whether telmisartan ameliorates oxidative stress by regulating Txnip and Trx expression in Type 2 diabetic rat kidneys and explore the possible relationship between renoprotection by telmisartan and Txnip.

METHODS

Twenty-one rats were equally divided into control (C), streptozotocin-induced diabetic (D), and telmisartan- treated diabetic (T) groups. Txnip and Trx expression in rat kidneys was analyzed by immunohistochemistry, RTPCR, and western blot. Peroxisome proliferator-activated receptor- γ (PPARγ), NADPH oxidase activity, and parameters of renal function and oxidative stress were also measured.

RESULTS

Trx and PPARγ were significantly decreased, and Txnip expression and NADPH oxidase activity markedly increased, in the D and T groups compared to the C group. After telmisartan treatment, Trx and PPARγ were upregulated, while Txnip expression and NADPH oxidase activity were downregulated. Parameters of renal function and oxidative stress were improved by telmisartan.

CONCLUSION

Telmisartan ameliorates oxidative stress and protects renal function in Type 2 diabetic rat kidneys. The downregulation of Txnip by telmisartan may be associated with PPARγ activation.

NADPH oxidase inhibition prevents beta cell dysfunction induced by prolonged elevation of oleate in rodents

AIMS/HYPOTHESIS

The activation of NADPH oxidase has been implicated in NEFA-induced beta cell dysfunction. However, the causal role of this activation in vivo remains unclear. Here, using rodents, we investigated whether pharmacological or genetic inhibition of NADPH oxidase could prevent NEFA-induced beta cell dysfunction in vivo.

METHODS

Normal rats were infused for 48 h with saline or oleate with or without the NADPH oxidase inhibitor apocynin. In addition, NADPH oxidase subunit p47(phox)-null mice and wild-type littermate controls were infused with saline or oleate for 48 h. This was followed by measurement of NADPH oxidase activity, reactive oxygen species (ROS) and superoxide imaging and assessment of beta cell function in isolated islets and hyperglycaemic clamps.

RESULTS

Oleate infusion in rats increased NADPH oxidase activity, consistent with increased total but not mitochondrial superoxide in islets and impaired beta cell function in isolated islets and during hyperglycaemic clamps. Co-infusion of apocynin with oleate normalised NADPH oxidase activity and total superoxide levels and prevented beta cell dysfunction. Similarly, 48 h NEFA elevation in wild-type mice increased total but not mitochondrial superoxide and impaired beta cell function in isolated islets. p47(phox)-null mice were protected against these effects when subjected to 48 h oleate infusion. Finally, oleate increased the levels of total ROS, in both models, whereas inhibition of NADPH oxidase prevented this increase, suggesting that NADPH oxidase is the main source of ROS in this model.

CONCLUSIONS/INTERPRETATION

These data show that NADPH-oxidase-derived cytosolic superoxide is increased in islets upon oleate infusion in vivo; and whole-body NADPH-oxidase inhibition decreases superoxide in concert with restoration of islet function.

Regulation of NOX-1 expression in beta cells: a positive feedback loop involving the Src-kinase signaling pathway

NADPH oxidase-1 (NOX-1) is upregulated in beta cells in response to pro-inflammatory cytokines. Inhibition of NADPH oxidase activity blocked stimulated NOX-1 expression (p<0.05). Regulation of NOX-1 expression in beta cells followed modulation of cellular reactive oxygen species (ROS); pro-oxidants increased NOX-1 (p<0.001) and anti-oxidants decreased NOX-1 (p<0.05). Activation of Src-kinase followed ROS elevation. Inhibition of Src-kinase decreased NOX-1 expression (p<0.01). Beta cell dysfunction, measured by elevated MCP-1 expression, loss of glucose-sensitive insulin secretion or cell death, was induced by pro-inflammatory cytokine stimulation. Importantly, inhibition of Src-kinase or NOX-1 preserved beta cell function and survival. Collectively, these data indicate that expression of NOX-1 in beta cells is regulated in a feed-forward loop mediated by ROS and Src-kinase. Uncoupling of this feed-forward activation could provide new approaches to preserve and protect beta cells in diabetes.

NOX, NOX Who is There? The Contribution of NADPH Oxidase One to Beta Cell Dysfunction

Predictions of diabetes prevalence over the next decades warrant the aggressive discovery of new approaches to stop or reverse loss of functional beta cell mass. Beta cells are recognized to have a relatively high sensitivity to reactive oxygen species (ROS) and become dysfunctional under oxidative stress conditions. New discoveries have identified NADPH oxidases in beta cells as contributors to elevated cellular ROS. Reviewed are recent reports that evidence a role for NADPH oxidase-1 (NOX-1) in beta cell dysfunction. NOX-1 is stimulated by inflammatory cytokines that are elevated in diabetes. First, regulation of cytokine-stimulated NOX-1 expression has been linked to inflammatory lipid mediators derived from 12-lipoxygenase activity. For the first time in beta cells these data integrate distinct pathways associated with beta cell dysfunction. Second, regulation of NOX-1 in beta cells involves feed-forward control linked to elevated ROS and Src-kinase activation. This potentially results in unbridled ROS generation and identifies candidate targets for pharmacologic intervention. Third, consideration is provided of new, first-in-class, selective inhibitors of NOX-1. These compounds could have an important role in assessing a disruption of NOX-1/ROS signaling as a new approach to preserve and protect beta cell mass in diabetes.

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.

NADPH oxidase NOX2 defines a new antagonistic role for reactive oxygen species and cAMP/PKA in the regulation of insulin secretion

In insulin-secreting cells, expression of NADPH oxidase (NOX), a potent source of ROS, has been reported, along with controversial findings regarding its function. Here, the role of NOXs was investigated: first by expression and cellular localization in mouse and human pancreatic islets, and then by functional studies in islets isolated from Nox isoform-specific knockout mice. Both human and mouse β-cells express NOX, in particular NOX2. With use of Nox isoform-specific knockout mice, functional analysis revealed Nox2 as the predominant isoform. In human islets, NOX2 colocalized with both insulin granules and endosome/lysosome membranes. Nox2-deficient islets stimulated with 22.8 mmol/L glucose exhibited potentiation of insulin release compared with controls, an effect confirmed with in vitro knockdown of Nox2. The enhanced secretory function in Nox2-deficient islets was associated with both lower superoxide levels and elevated cAMP concentrations. In control islets, GLP-1 and other cAMP inducers suppressed glucose-induced ROS production similarly to Nox2 deficiency. Inhibiting cAMP-dependent protein kinase reduced the secretory response in Nox2-null islets, although not in control islets. This study ascribes a new role for NOX2 in pancreatic β-cells as negative modulator of the secretory response, reducing cAMP/PKA signaling secondary to ROS generation. Results also show reciprocal inhibition between the cAMP/PKA pathway and ROS.

The role of oxidative stress in the pathogenesis of diabetic vascular complications

Oxidative stress has been paid increasing attention to as an important causative factor for diabetic vascular complications. Among possible various sources, accumulating evidence has indicated that NAD(P)H oxidase may be the most important source for reactive oxygen species production in diabetic vascular tissues. The mechanisms underlying activation and up-regulation of NAD(P)H oxidase has been supposed to be mediated by high glucose-induced protein kinase C (PKC) activation. In this review article, activation of local renin-angiotensin II system induced by chymase activation is also shown to amplify such a PKC-dependent activation of NAD(P)H oxidase. Additionally, human evidence showing the beneficial effect of antioxidants on diabetic vascular complications. Bilirubin has been recognized as a strong endogenous antioxidant. Here markedly lower prevalence of vascular complications is shown in diabetic patients with Gilbert syndrome, a congenital hyperbilirubinemia, as well as reduced markers of oxidative stress and inflammation. Lastly, statin, angiotensin II receptor blocker, chymase inhibitor, bilirubin and biliverdin, PKC β isoform inhibitor, and glucagon-like peptide-1 analog, are shown to serve as antioxidants and have some beneficial effect on diabetic vascular complications, via inhibiting PKC-NAD(P)H oxidase activation, supporting the notion that this mechanism may be an effective therapeutic target for preventing diabetic vascular complications.

Integration of pro-inflammatory cytokines, 12-lipoxygenase and NOX-1 in pancreatic islet beta cell dysfunction

Elevated cellular reactive species, which can be produced by diabetic serum conditions such as elevated inflammatory cytokines, lipotoxicity or glucotoxicity contribute to islet beta cell dysfunction and cell death. Cellular pathways that result in beta cell oxidative stress are poorly resolved. In this study, stimulation of human donor islets, primary mouse islets or homogeneous beta cell lines with a cocktail of inflammatory cytokines (TNFα, IL-1β, and INFγ) significantly induced NADPH oxidase-1 (NOX-1) gene expression (p<0.05). This pro-inflammatory cytokine cocktail concomitantly induced loss of islet glucose stimulated insulin response (p<0.05), elevated expression of MCP-1 (p<0.01), increased cellular reactive oxygen species (ROS) and induced cell death. Inhibitors of NADPH oxidase, apocynin and diphenyleneiodonium, and a dual selective NOX1/4 inhibitor, blocked ROS generation (p<0.01) and induction of MCP-1 (p<0.05) by pro-inflammatory cytokines in beta cells. It has previously been reported that pro-inflammatory cytokine stimulation induces 12-lipoxygenase (12-LO) expression in human islets. 12-Hydroxyeicosatetraenoic acid (12-HETE), a product of 12-LO activity, stimulated NOX-1 expression in human islets (p<0.05). A novel selective inhibitor of 12-LO blocked induction of NOX-1, production of ROS and pro-caspase 3 cleavage by pro-inflammatory cytokines in INS-1 beta cells (p<0.01). Inhibition was not seen with a structurally related but inactive analog. Importantly, islets from human type 2 diabetic donors have an elevated expression of NOX-1 (p<0.05). This study describes an integrated pathway in beta cells that links beta cell dysfunction induced by pro-inflammatory cytokines with 12-lipoxygenase and NADPH oxidase (NOX-1) activation. Inhibitors of this pathway may provide a new therapeutic strategy to preserve beta cell mass in diabetes.

Chronic pulsatile hyperglycemia reduces insulin secretion and increases accumulation of reactive oxygen species in fetal sheep islets

Children from diabetic pregnancies have a greater incidence of type 2 diabetes. Our objective was to determine if exposure to mild-moderate hyperglycemia, by modeling managed diabetic pregnancies, affects fetal β-cell function. In sheep fetuses, β-cell responsiveness was examined after 2 weeks of sustained hyperglycemia with 3 pulses/day, mimicking postprandial excursions, and compared to saline-infused controls (n = 10). Two pulsatile hyperglycemia (PHG) treatments were studied: mild (mPHG, n = 5) with +15% sustained and +55% pulse; and moderate (PHG, n = 10) with +20% sustained and +100% pulse. Fetal glucose-stimulated insulin secretion and glucose-potentiated arginine insulin secretion were lower (P < 0.05) in PHG (0.86 ± 0.13 and 2.91 ± 0.39  ng/ml plasma insulin) but not in mPHG fetuses (1.21 ± 0.08 and 4.25 ± 0.56  ng/ml) compared to controls (1.58 ± 0.25 and 4.51 ± 0.56  ng/ml). Islet insulin content was 35% lower in PHG and 35% higher in mPHG vs controls (P < 0.01). Insulin secretion and maximally stimulated insulin release were also reduced (P < 0.05) in PHG islets due to lower islet insulin content. Isolated PHG islets also had 63% greater (P < 0.01) reactive oxygen species (ROS) accumulation at 11.1  mmol/l glucose than controls (P < 0.01), but oxidative damage was not detected in islet proteins. PHG fetuses showed evidence of oxidative damage to skeletal muscle proteins (P < 0.05) but not insulin resistance. Our findings show that PHG induced dysregulation of islet ROS handling and decreased islet insulin content, but these outcomes are independent. The β-cell outcomes were dependent on the severity of hyperglycemia because mPHG fetuses had no distinguishable impairments in ROS handling or insulin secretion but greater insulin content.

Prophylactic treatment with telmisartan induces tissue-specific gene modulation favoring normal glucose homeostasis in Cohen-Rosenthal diabetic hypertensive rats

The objectives were to assess the potential of long-term prophylactic administration of telmisartan, an angiotensin II receptor antagonist and a partial peroxisome proliferator activator receptor (PPAR)γ agonist, in preventing the development of hypertension and hyperglycemia and to demonstrate the alteration in gene expression associated with the development of hyperglycemia and insulin resistance in Cohen-Rosenthal diabetic hypertensive rat, a unique model of hypertension and type 2 diabetes mellitus comorbidity. Cohen-Rosenthal diabetic hypertensive rats were continuously treated with telmisartan (3 mg/[kg d]) starting at age 6 to 8 weeks before developing hypertension or diabetes. Weight changes, blood pressure, blood insulin, adiponectin, glucose tolerance, and insulin sensitivity were monitored. Fat, liver, and muscle messenger RNAs were examined for the expression of genes potentially involved in the onset of insulin resistance. In addition to the expected antihypertensive effect of prophylactic telmisartan, diabetes was blunted, evidenced at the end of the study by a significantly lower glucose level. This was accompanied by improved glucose tolerance, increased sensitivity to insulin, reduction in fasting insulin levels and homeostasis model assessment index, as well as an increase in serum adiponectin. Telmisartan also prevented the increase in serum triglycerides and the associated appearance of lipid droplets in the liver. Diabetes induced tissue-specific changes in messenger RNAs expression of the following selected genes, which were restored by telmisartan treatment: PPARγ, PPARδ, PPARγ coactivator 1α, adiponectin, adiponectin receptor 1, adiponectin receptor 2, phosphotyrosine binding domain and a pleckstrin homology domain-containing adaptor protein, adenosine monophosphate kinase, and glucose translocator 4. Telmisartan blunted the development of hypertension, insulin resistance, and diabetes in prediabetic Cohen-Rosenthal diabetic hypertensive rats through pleiotropic activity, involving specific gene regulation of target organs.

Increased phagocyte-like NADPH oxidase and ROS generation in type 2 diabetic ZDF rat and human islets: role of Rac1-JNK1/2 signaling pathway in mitochondrial dysregulation in the diabetic islet

OBJECTIVE

To determine the subunit expression and functional activation of phagocyte-like NADPH oxidase (Nox), reactive oxygen species (ROS) generation and caspase-3 activation in the Zucker diabetic fatty (ZDF) rat and diabetic human islets.

RESEARCH DESIGN AND METHODS

Expression of core components of Nox was quantitated by Western blotting and densitometry. ROS levels were quantitated by the 2',7'-dichlorofluorescein diacetate method. Rac1 activation was quantitated using the gold-labeled immunosorbent assay kit.

RESULTS

Levels of phosphorylated p47(phox), active Rac1, Nox activity, ROS generation, Jun NH(2)-terminal kinase (JNK) 1/2 phosphorylation, and caspase-3 activity were significantly higher in the ZDF islets than the lean control rat islets. Chronic exposure of INS 832/13 cells to glucolipotoxic conditions resulted in increased JNK1/2 phosphorylation and caspase-3 activity; such effects were largely reversed by SP600125, a selective inhibitor of JNK. Incubation of normal human islets with high glucose also increased the activation of Rac1 and Nox. Lastly, in a manner akin to the ZDF diabetic rat islets, Rac1 expression, JNK1/2, and caspase-3 activation were also significantly increased in diabetic human islets.

CONCLUSIONS

We provide the first in vitro and in vivo evidence in support of an accelerated Rac1-Nox-ROS-JNK1/2 signaling pathway in the islet β-cell leading to the onset of mitochondrial dysregulation in diabetes.

Review article: pancreatic renin-angiotensin systems in health and disease

BACKGROUND

In addition to the circulating (endocrine) renin-angiotensin system (RAS), local renin-angiotensin systems are now known to exist in diverse cells and tissues. Amongst these, pancreatic renin-angiotensin systems have recently been identified and may play roles in the physiological regulation of pancreatic function, as well as being implicated in the pathogenesis of pancreatic diseases including diabetes, pancreatitis and pancreatic cancer.

AIM

To review and summarise current knowledge of pancreatic renin-angiotensin systems.

METHODS

We performed an extensive PubMed, Medline and online review of all relevant literature.

RESULTS

Pancreatic RAS appear to play various roles in the regulation of pancreatic physiology and pathophysiology. Ang II may play a role in the development of pancreatic ductal adenocarcinoma, via stimulation of angiogenesis and prevention of chemotherapy toxicity, as well as in the initiation and propagation of acute pancreatitis (AP); whereas, RAS antagonism is capable of preventing new-onset diabetes and improving glycaemic control in diabetic patients. Current evidence for the roles of pancreatic RAS is largely based upon cell and animal models, whilst definitive evidence from human studies remains lacking.

CONCLUSIONS

The therapeutic potential for RAS antagonism, using cheap and widely available agents, and may be untapped and such roles are worthy of active investigation in diverse pancreatic disease states.

Role of endogenous ROS production in impaired metabolism-secretion coupling of diabetic pancreatic β cells

One of the characteristics of type 2 diabetes is that the insulin secretory response of β cells is selectively impaired to glucose. In the Goto-Kakizaki (GK) rat, a genetic model of type 2 diabetes mellitus, glucose-induced insulin secretion is selectively impaired due to deficient ATP production derived from impaired glucose metabolism. In addition, islets in GK rat and human type 2 diabetes are oxidatively stressed. In this issue, role of endogenous reactive oxygen species (ROS) production in impaired metabolism-secretion coupling of diabetic pancreatic β cells is reviewed. In β cells, ROS is endogenously produced by activation of Src, a non-receptor tyrosine kinase. Src inhibitors restore the impaired insulin release and impaired ATP elevation by reduction in ROS production in diabetic islets. Src is endogenously activated in diabetic islets, since the level of Src pY416 in GK islets is higher than that in control islets. In addition, exendin-4, a glucagon-like peptide-1 (GLP-1) receptor agonist, decreases Src pY416 and glucose-induced ROS production and ameliorates impaired ATP production dependently on Epac in GK islets. These results indicate that GLP-1 signaling regulates endogenous ROS production due to Src activation and that incretin has unique therapeutic effects on impaired glucose metabolism in diabetic β cells.

Biliverdin protects against the deterioration of glucose tolerance in db/db mice

AIMS/HYPOTHESIS

We have previously shown a negative correlation between serum bilirubin levels and prevalence of type 2 diabetes, suggesting that bilirubin inhibits development of this disease. To confirm this hypothesis, we investigated whether administration of biliverdin, the precursor of bilirubin, protects against the deterioration of glucose tolerance in db/db mice, a rodent model of type 2 diabetes.

METHODS

Biliverdin (20 mg/kg daily) was orally administered to 5-week-old db/db mice for 4 weeks. After 4 weeks of treatment, i.p. glucose tolerance and insulin tolerance tests were performed. Insulin content was evaluated by immunostaining and ELISA. Oxidative stress markers (8-hydroxy-2'-deoxyguansosine and dihydroethidium staining) and expression of NADPH oxidase components Pdx1 and Bax were also evaluated in isolated islets.

RESULTS

Treatment with biliverdin partially prevented worsening of hyperglycaemia and glucose intolerance in db/db mice. This effect was accompanied by a significant increase in insulin content and Pdx1 expression, and a significant decrease of apoptosis and Bax expression in pancreatic islets from db/db mice. At the same time, levels of oxidative stress markers and NADPH oxidase component production in islets were normalised. Biliverdin had little effect on HOMA of insulin resistance or insulin resistance evaluated by insulin tolerance tests.

CONCLUSIONS/INTERPRETATION

Biliverdin may protect against progressive worsening of glucose tolerance in db/db mice, mainly via inhibition of oxidative stress-induced beta cell damage.

Alterations of NADPH oxidase activity in rat pancreatic islets induced by a high-fat diet

OBJECTIVE

The aim of this study was to evaluate the effect of a high-fat diet (HFD) on nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity in rat pancreatic islets. We investigated if changes in NADPH oxidase are connected to beta cell dysfunction reported in obese animals.

METHODS

Male Wistar rats were fed a HFD or control diet for 3 months. DNA fragmentation, insulin secretion, and [U-C]glucose oxidation were examined in isolated pancreatic islets. The oxidative stress markers nitrotyrosine and 4-hydroxy-2-nonenal were assessed by immunohistochemistry. The protein content of gp91 and p47 was evaluated by Western blotting. Production of reactive oxygen species (ROS) was determined by a fluorescence assay using hydroethidine.

RESULTS

Occurrence of DNA fragmentation was reduced in pancreatic islets from HFD rats. There were no differences in oxidative stress markers between the groups. Glucose oxidation and insulin secretion were elevated due to high glucose in pancreatic islets from HFD rats. Protein concentrations of p47 and gp91 subunits were reduced and ROS production was diminished in pancreatic islets from HFD rats.

CONCLUSIONS

The diminished content of NADPH oxidase subunits and ROS concentrations may be associated with increased glucose oxidation and insulin secretion in an attempt to compensate for the peripheral insulin resistance elicited by the HFD.

Targeted expression of islet neogenesis associated protein to beta cells enhances glucose tolerance and confers resistance to streptozotocin-induced hyperglycemia

Islet neogenesis associated protein (INGAP) stimulates experimental pancreatic islet growth, as evidenced by elevated markers of beta cell mass, in rodents, dogs and primates. Previous analyses of mice that have a transgenic expression of INGAP targeted to the exocrine pancreas have indicated additional biological activity attributed to INGAP. In this study we report on mice with a targeted expression of INGAP to the islet beta cell. The beta cell transgenic mice (IP-INGAP) showed enhanced normalization of blood glucose during IPGTT. Further, IP-INGAP mice had a significant delay in development of hyperglycemia following a diabetogenic dose of streptozotocin. INGAP conferred beta cell protection and enhanced islet function. Analysis of oxidative stress genes in IP-INGAP mice revealed a decrease in islet expression of the NADPH oxidase, NOX1, in both basal state and in response to pro-inflammatory cytokine stimulation. These data are consistent with a pleiotropic role for INGAP and reveal new pathways to target in the discovery of improved diabetic therapies.

Friendly, and not so friendly, roles of Rac1 in islet β-cell function: lessons learnt from pharmacological and molecular biological approaches

Glucose-stimulated insulin secretion [GSIS] involves a sequence of metabolic events leading to small G-protein [e.g., Rac1]-mediated cytoskeletal remodeling to promote granule mobilization toward the plasma membrane for fusion and release of insulin. Existing evidence supports a positive modulatory role for Rac1 in GSIS. Specific regulatory factors of Rac1 function, including the guanine nucleotide exchange factors [e.g., Tiam1] have also been identified and studied in the islet. Inhibition of Tiam1/Rac1 signaling axis attenuates GSIS suggesting its pivotal role in insulin secretion. In addition to its positive [i.e., friendly] roles in GSIS, Rac1 also plays "non-friendly" role[s] in the islet function. For example, it up-regulates the intracellular reactive oxygen species [ROS] levels via activation of phagocyte-like NADPH oxidase [Nox]. Despite the emerging evidence that a tonic increase in intracellular ROS is necessary for GSIS, experimental evidence also suggests that chronic exposure of β-cells to high glucose, palmitate or cytokines results in the onset of oxidative stress leading to reduction in mitochondrial membrane potential, cytosolic accumulation of cytochrome C and activation of caspase-3 leading to β-cell apoptosis. Pharmacological and molecular biological inhibition of Rac1 activation affords partial protection against Nox-induced oxidative stress and mitochondrial dysfunction induced by elevated glucose, lipids or cytokines. Herein, we overview the existing evidence to suggest positive as well as negative modulatory roles of Rac1 in islet function. Potential avenues for future research including development of inhibitors to halt the Rac1-Nox activation and generation of oxidative stress leading to the metabolic dysfunction of the β-cell are discussed.

Exendin-4 suppresses SRC activation and reactive oxygen species production in diabetic Goto-Kakizaki rat islets in an Epac-dependent manner

OBJECTIVE

Reactive oxygen species (ROS) is one of most important factors in impaired metabolism secretion coupling in pancreatic β-cells. We recently reported that elevated ROS production and impaired ATP production at high glucose in diabetic Goto-Kakizaki (GK) rat islets are effectively ameliorated by Src inhibition, suggesting that Src activity is upregulated. In the present study, we investigated whether the glucagon-like peptide-1 signal regulates Src activity and ameliorates endogenous ROS production and ATP production in GK islets using exendin-4.

RESEARCH DESIGN AND METHODS

Isolated islets from GK and control Wistar rats were used for immunoblotting analyses and measurements of ROS production and ATP content. Src activity was examined by immunoprecipitation of islet lysates followed by immunoblotting. ROS production was measured with a fluorescent probe using dispersed islet cells.

RESULTS

Exendin-4 significantly decreased phosphorylation of Src Tyr416, which indicates Src activation, in GK islets under 16.7 mmol/l glucose exposure. Glucose-induced ROS production (16.7 mmol/l) in GK islet cells was significantly decreased by coexposure of exendin-4 as well as PP2, a Src inhibitor. The Src kinase-negative mutant expression in GK islets significantly decreased ROS production induced by high glucose. Exendin-4, as well as PP2, significantly increased impaired ATP elevation by high glucose in GK islets. The decrease in ROS production by exendin-4 was not affected by H-89, a PKA inhibitor, and an Epac-specific cAMP analog (8CPT-2Me-cAMP) significantly decreased Src Tyr416 phosphorylation and ROS production.

CONCLUSIONS

Exendin-4 decreases endogenous ROS production and increases ATP production in diabetic GK rat islets through suppression of Src activation, dependently on Epac.

Role of metabolically generated reactive oxygen species for lipotoxicity in pancreatic β-cells

Chronically elevated concentrations of non-esterified fatty acids (NEFAs) in type 2 diabetes may be involved in β-cell dysfunction and apoptosis. It has been shown that long-chain saturated NEFAs exhibit a strong cytotoxic effect upon insulin-producing cells, while short-chain as well as unsaturated NEFAs are well tolerated. Moreover, long-chain unsaturated NEFAs counteract the toxicity of palmitic acid. Reactive oxygen species (ROS) formation and gene expression analyses together with viability assays in different β-cell lines showed that the G-protein-coupled receptors 40 and 120 do not mediate lipotoxicity. This is independent from the role, which these receptors, specifically GPR40, play in the potentiation of glucose-induced insulin secretion by saturated and unsaturated long-chain NEFAs. Long-chain NEFAs are not only metabolized in the mitochondria but also in peroxisomes. In contrast to mitochondrial β-oxidation, the acyl-coenzyme A (CoA) oxidases in the peroxisomes form hydrogen peroxide and not reducing equivalents. As β-cells almost completely lack catalase, they are exceptionally vulnerable to hydrogen peroxide generated in peroxisomes. ROS generation in the respiratory chain is less important because overexpression of catalase and superoxide dismutase in the mitochondria do not provide protection. Thus, peroxisomally generated hydrogen peroxide is the likely ROS that causes pancreatic β-cell dysfunction and ultimately β-cell death.

Candesartan attenuates fatty acid-induced oxidative stress and NAD(P)H oxidase activity in pancreatic beta-cells

Angiotensin II receptor blockers (ARBs) have been shown to decrease insulin resistance in obese diabetic animal models and reduce the risk of new-onset diabetes in hypertensive patients. In the present study, we studied whether candesartan, an ARB, can exert a direct effect against fatty acid-induced oxidative stress in pancreatic beta-cells. The effect of candesartan on lipotoxicity was evaluated using mouse insulin-secreting clonal cell, MIN6 and isolated mouse pancreatic islets. Intracellular insulin and triglyceride content, uncoupling protein-2 (UCP-2) mRNA expression, reactive oxygen species, protein kinase C (PKC) and NAD(P)H oxidase activity were examined. Candesartan recovered decreased insulin content in MIN6 exposed to 25mM glucose with 0.5mM palmitate (P<0.01). Candesartan tended to decrease intracellular triglyceride accumulation in cells exposed to 25mM glucose with 0.5mM palmitate. Palmitate-induced up-regulation of UCP-2 mRNA levels was suppressed by candesartan in a dose-dependent manner. Candesartan decreased palmitate-induced reactive oxygen species accumulation in MIN6 cells by 23% and in mouse islets by 59%. Candesartan also decreased palmitate-induced PKC activity by 21% and NAD(P)H oxidase activity by 37% in MIN6 cells. These findings indicated that candesartan attenuated fatty acid-induced oxidative stress and NAD(P)H oxidase activity in pancreatic beta-cells.

Angiotensin I-converting enzyme type 2 (ACE2) gene therapy improves glycemic control in diabetic mice

OBJECTIVE

Several clinical studies have shown the benefits of renin-angiotensin system (RAS) blockade in the development of diabetes, and a local RAS has been identified in pancreatic islets. Angiotensin I-converting enzyme (ACE)2, a new component of the RAS, has been identified in the pancreas, but its role in β-cell function remains unknown. Using 8- and 16-week-old obese db/db mice, we examined the ability of ACE2 to alter pancreatic β-cell function and thereby modulate hyperglycemia.

RESEARCH DESIGN AND METHODS

Both db/db and nondiabetic lean control (db/m) mice were infected with an adenovirus expressing human ACE2 (Ad-hACE2-eGFP) or the control virus (Ad-eGFP) via injection into the pancreas. Glycemia and β-cell function were assessed 1 week later at the peak of viral expression.

RESULTS

In 8-week-old db/db mice, Ad-hACE2-eGFP significantly improved fasting glycemia, enhanced intraperitoneal glucose tolerance, increased islet insulin content and β-cell proliferation, and reduced β-cell apoptosis compared with Ad-eGFP. ACE2 overexpression had no effect on insulin sensitivity in comparison with Ad-eGFP treatment in diabetic mice. Angiotensin-(1-7) receptor blockade by D-Ala(7)-Ang-(1-7) prevented the ACE2-mediated improvements in intraperitoneal glucose tolerance, glycemia, and islet function and also impaired insulin sensitivity in both Ad-hACE2-eGFP- and Ad-eGFP-treated db/db mice. D-Ala(7)-Ang-(1-7) had no effect on db/m mice. In 16-week-old diabetic mice, Ad-hACE2-eGFP treatment improved fasting blood glucose but had no effect on any of the other parameters.

CONCLUSIONS

These findings identify ACE2 as a novel target for the prevention of β-cell dysfunction and apoptosis occurring in type 2 diabetes.

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.