Inhibition of renal gluconeogenesis contributes to hypoglycaemic action of NADPH oxidase inhibitor, apocynin

Apocynin Attenuates Diabetes-Induced Skeletal Muscle Dysfunction by Mitigating ROS Generation and Boosting Antioxidant Defenses in Fast-Twitch and Slow-Twitch Muscles

In response to diabetes mellitus, skeletal muscle is negatively affected, as is evident by reduced contractile force production, increased muscle fatigability, and increased levels of oxidative stress biomarkers. Apocynin is a widely used NADPH oxidase inhibitor, with antioxidant and anti-inflammatory potential. It has been effective for amelioration of a variety of disorders, including diabetic complications. Therefore, the present study was conducted to evaluate the effects and action mechanisms of apocynin in slow- and fast-twitch diabetic rat muscles. Male Wistar rats were rendered diabetic by applying intraperitoneally a single dose of streptozotocin (45 mg/kg). Apocynin treatment (3 mg/kg/day) was administered over 8 weeks. Fasting blood glucose (FBG), insulin tolerance and body weight gain were measured. Both slow (soleus) and fast (extensor digitorum longus, EDL) skeletal muscles were used for muscle function evaluation, oxidative stress markers, and evaluating gene expression using qRT-PCR. Treatment with apocynin significantly reduced FBG levels and enhanced insulin tolerance. Apocynin also prevented muscle contractile dysfunction in EDL muscle but had no significant effect on this parameter in soleus muscles. However, in both types of muscles, apocynin mitigated the oxidative stress by decreasing ROS levels and increasing total glutathione levels and redox state. Concomitantly, apocynin also statistically enhanced Nrf-2 and GLU4 mRNA expression and downregulated NOX2, NOX4, and NF-κB mRNA. Collectively, apocynin exhibits properties myoprotective in diabetic animals. These findings indicate that apocynin predominantly acts as an antioxidant in fast-twitch and slow-twitch muscles but has differential impact on contractile function.

Effects of Apocynin on Heart Muscle Oxidative Stress of Rats with Experimental Diabetes: Implications for Mitochondria

Diabetes mellitus (DM) constitutes one of the public health problems today. It is characterized by hyperglycemia through a defect in the β-cells function and/or decreased insulin sensitivity. Apocynin has been tasted acting directly as an NADPH oxidase inhibitor and reactive oxygen species (ROS) scavenger, exhibiting beneficial effects against diabetic complications. Hence, the present study’s goal was to dissect the possible mechanisms by which apocynin could mediate its cardioprotective effect against DM-induced oxidative stress. Male Wistar rats were assigned into 4 groups: Control (C), control + apocynin (C+A), diabetes (D), diabetes + apocynin (D+A). DM was induced with streptozotocin. Apocynin treatment (3 mg/kg/day) was applied for 5 weeks. Treatment significantly decreased blood glucose levels and insulin resistance in diabetic rats. In cardiac tissue, ROS levels were higher, and catalase enzyme activity was reduced in the D group compared to the C group; the apocynin treatment significantly attenuated these responses. In heart mitochondria, Complexes I and II of the electron transport chain (ETC) were significantly enhanced in the D+A group. Total glutathione, the level of reduced glutathione (GSH) and the GSH/ oxidized glutathione (GSSG) ratio were increased in the D+A group. Superoxide dismutase (SOD) and the glutathione peroxidase (GSH-Px) activities were without change. Apocynin enhances glucose uptake and insulin sensitivity, preserving the antioxidant defense and mitochondrial function.

NADPH Oxidase Inhibition in Fibrotic Pathologies

Significance: Fibrosis is a stereotypic, multicellular tissue response to diverse types of injuries that fundamentally result from a failure of cell/tissue regeneration. This complex tissue remodeling response disrupts cellular/matrix composition and homeostatic cell-cell interactions, leading to loss of normal tissue architecture and progressive loss of organ structure/function. Fibrosis is a common feature of chronic diseases that may affect the lung, kidney, liver, and heart. Recent Advances: There is emerging evidence to support a combination of genetic, environmental, and age-related risk factors contributing to susceptibility and/or progression of fibrosis in different organ systems. A core pathway in fibrogenesis involving these organs is the induction and activation of nicotinamide adenine dinucleotide phosphate oxidase (NOX) family enzymes. Critical Issues: We explore current pharmaceutical approaches to targeting NOX enzymes, including repurposing of currently U.S. Food and Drug Administration (FDA)-approved drugs. Specific inhibitors of various NOX homologs will aid establishing roles of NOXs in the various organ fibroses and potential efficacy to impede/halt disease progression. Future Directions: The discovery of novel and highly specific NOX inhibitors will provide opportunities to develop NOX inhibitors for treatment of fibrotic pathologies.

Potential benefits and harms of NADPH oxidase type 4 in the kidneys and cardiovascular system

The main function of NADPH oxidases is to catalyse the formation of reactive oxygen species (ROS). NADPH oxidase 4 (NOX4) is expressed at high levels in kidney tubular cells, and at lower levels in endothelial cells, cardiomyocytes and other cell types under physiological conditions. NOX4 is constitutively active producing hydrogen peroxide (H2O2) as the prevalent ROS detected, whereas other NOX isoforms present in the renal and cardiovascular systems (i.e. NOX1, NOX2 and NOX5) generate superoxide radical anions as main products. Pharmacological inhibition of NOX4 has received enormous attention for its potential therapeutic benefit in fibrotic disease and nephropathologies. Ongoing clinical trials are testing this approach in humans. Diabetes elevates NOX4 expression in podocytes and mesangial cells, which was shown to damage glomeruli leading to podocyte loss, mesangial cell hypertrophy and matrix accumulation. Consequently, NOX4 represents an interesting therapeutic target in diabetic nephropathy. On the contrary, experiments using NOX4-deficient mice have shown that NOX4 is cytoprotective in tubular cells, cardiomyocytes, endothelial cells and vascular smooth muscle cells, and has a metabolism-regulating role when these cells are subjected to injury. Mice with systemic NOX4 deletion are more susceptible to acute and chronic tubular injury, heart failure and atherosclerosis. Overall, the current literature suggests a detrimental role of increased NOX4 expression in mesangial cells and podocytes during diabetic nephropathy, but a cytoprotective role of this enzyme in other cellular types where it is expressed endogenously. We review here the recent evidence on the role of NOX4 in the kidneys and cardiovascular system. With the emergence of pharmacological NOX4 inhibitors in clinical trials, caution should be taken in identifying potential side effects in patients prone to acute kidney injury and cardiovascular disease.

Molecular signaling mechanisms of renal gluconeogenesis in nondiabetic and diabetic conditions

The kidneys are as involved as the liver in gluconeogenesis which can significantly contribute to hyperglycemia in the diabetic condition. Substantial evidence has demonstrated the overexpression of rate-limiting gluconeogenic enzymes, especially phosphoenolpyruvate carboxykinase and glucose 6 phosphatase, and the accelerated glucose release both in the isolated proximal tubular cells and in the kidneys of diabetic animal models and diabetic patients. The aim of this review is to provide an insight into the mechanisms that accelerate renal gluconeogenesis in the diabetic conditions and the therapeutic approaches that could affect this process in the kidney. Increase in gluconeogenic substrates, reduced insulin concentration or insulin resistance, downregulation of insulin receptors and insulin signaling, oxidative stress, and inappropriate activation of the renin-angiotensin system are likely to participate in enhancing renal gluconeogenesis in the diabetic milieu. Several studies have suggested that controlling glucose metabolism at the renal level favors effective overall glycemic control in both type 1 and type 2 diabetes. Therefore, renal gluconeogenesis may be a promising target for effective glycemic control as a therapeutic strategy in diabetes.

Protective effects of (-)-epicatechin and the colonic metabolite 3,4-dihydroxyphenylacetic acid against glucotoxicity-induced insulin signalling blockade and altered glucose uptake and production in renal tubular NRK-52E cells

Glucotoxicity (high levels of glucose) is a major cause in the pathogenesis of diabetes. Evidences indicate that (-)-epicatechin (EC) and colonic metabolites derived from flavonoid intake could possess antidiabetic effects, but the mechanisms for their preventive activities related to glucose homeostasis and insulin signalling in the kidney remain largely unknown. This work is aimed to investigate the effect of EC and main colonic phenolic acids derived from flavonoid intake, i.e. 2,3-dihydroxybenzoic-acid, 3,4-dihydroxyphenylacetic-acid (DHPAA) and 3-hydroxyphenylpropionic-acid, on insulin signalling, and glucose production and uptake in renal tubular proximal NRK-52E cells treated with high glucose. Pre-treatment with EC or DHPAA prevented the decreased tyrosine-phosphorylated and total levels of IR caused by high glucose. EC and DHPAA pre-treatment also avoided the inactivation of the PI3K/AKT pathway and AMPK, and the elevation of PEPCK levels induced by high glucose. Additionally, EC and DHPAA pre-treatment alleviated the altered glucose uptake and production caused by high glucose, although this protective effect was abrogated when AKT and AMPK were inhibited. These results suggest EC and DHPAA prevent or delay a potential dysfunction of NRK-52E cells treated with high glucose through the attenuation of the insulin signalling blockade and the modulation of glucose homeostasis via AKT and AMPK.

(-)-Epicatechin and the Colonic 2,3-Dihydroxybenzoic Acid Metabolite Regulate Glucose Uptake, Glucose Production, and Improve Insulin Signaling in Renal NRK-52E Cells

SCOPE

(-)-Epicatechin (EC) and main colonic phenolic acids derived from flavonoid intake, such as 2,3-dihydroxybenzoic acid (DHBA), 3,4-dihydroxyphenylacetic acid (DHPAA), 3-hydroxyphenylpropionic acid (HPPA), and vanillic acid (VA), have been suggested to exert beneficial effects in diabetes, although the mechanism for their actions remains unknown. In this study, the modulation of glucose homeostasis and insulin signaling by the mentioned compounds on renal proximal tubular NRK-52E cells is investigated.

METHODS AND RESULTS

Levels of the glucose transporters SGLT-2 and GLUT-2, as well as glucose uptake, glucose production, and key proteins of the insulin pathways, namely insulin receptor (IR), insulin receptor substrate-1 (IRS-1), and PI3K/AKT pathway are analyzed. EC (5-20 μm) and DHBA (20 μm) reduced both renal glucose uptake and production. Interestingly, EC and DHBA did not modify the levels of SGLT-2 and GLUT-2, and modulated the expression of phosphoenolpyruvate carboxykinase via AKT leading to a diminished glucose production. EC and DHBA also enhanced the tyrosine phosphorylation and total IR and IRS-1 levels, and activated the PI3K/AKT pathway in NRK-52E cells.

CONCLUSION

EC and DHBA regulate the renal glucose homeostasis by modulating both glucose uptake and production, and strengthen the insulin signaling by activating key proteins of that pathway in NRK-52E cells.

Evaluating the Oxidative Stress in Renal Diseases: What Is the Role for S-Glutathionylation?

SIGNIFICANCE

Reactive oxygen species (ROS) have long been considered as toxic derivatives of aerobic metabolism displaying a harmful effect to living cells. Deregulation of redox homeostasis and production of excessive free radicals may contribute to the pathogenesis of kidney diseases. In line, oxidative stress increases in patients with renal dysfunctions due to a general increase of ROS paralleled by impaired antioxidant ability.

RECENT ADVANCES

Emerging evidence revealed that physiologically, ROS can act as signaling molecules interplaying with several transduction pathways such as proliferation, differentiation, and apoptosis. ROS can exert signaling functions by modulating, at different layers, protein oxidation since proteins have "cysteine switches" that can be reversibly reduced or oxidized, supporting the dynamic signaling regulation function. In this scenario, S-glutathionylation is a posttranslational modification involved in oxidative cellular response.

CRITICAL ISSUES

Although it is widely accepted that renal dysfunctions are often associated with altered redox signaling, the relative role of S-glutathionylation on the pathogenesis of specific renal diseases remains unclear and needs further investigations. In this review, we discuss the impact of ROS in renal health and diseases and the role of selective S-glutathionylation proteins potentially relevant to renal physiology.

FUTURE DIRECTIONS

The paucity of studies linking the reversible protein glutathionylation with specific renal disorders remains unmet. The growing number of S-glutathionylated proteins indicates that this is a fascinating area of research. In this respect, further studies on the association of reversible glutathionylation with renal diseases, characterized by oxidative stress, may be useful to develop new pharmacological molecules targeting protein S-glutathionylation. Antioxid. Redox Signal. 25, 147-164.

Challenging the dogma of mitochondrial reactive oxygen species overproduction in diabetic kidney disease

The paradigm that high glucose drives overproduction of superoxide from mitochondria as a unifying theory to explain end organ damage in diabetes complications has been tightly held for more than a decade. With the recent development of techniques and probes to measure the production of distinct reactive oxygen species (ROS) in vivo, this widely held dogma is now being challenged with the emerging view that specific ROS moieties are essential for the function of specific intracellular signaling pathways and represent normal mitochondrial function. This review will provide a balanced overview of the dual nature of ROS, detailing current evidence for ROS overproduction in diabetic kidney disease, with a focus on cell types and sources of ROS. The technical aspects of measurement of mitochondrial ROS, both in isolated mitochondria and emerging in vivo methods will be discussed. The counterargument, that mitochondrial ROS production is reduced in diabetic complications, is consistent with a growing recognition that stimulation of mitochondrial biogenesis and oxidative phosphorylation activity reduces inflammation and fibrosis. It is clear that there is an urgent need to fully characterize ROS production paying particular attention to spatiotemporal aspects and to factor in the relevance of ROS in the regulation of cellular signaling in the pathogenesis of diabetic kidney disease. With improved tools and real-time imaging capacity, a greater understanding of the complex role of ROS will be able to guide novel therapeutic regimens.

New pathways driving the experimental hepatoprotective action of tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl) against acute hepatotoxicity

PURPOSE

In absence of liver protective drugs, a large number of hepatopathies may arise during drug administration. This study was executed to investigate the possible new pathways underlying the hepatoprotective effect of Tempol (4-hydroxy-2,2,6,6- tetramethylpiperidine-1-oxyl), following oral administration of carbon tetrachloride in mice.

METHODS AND RESULTS

Thirty albino mice were randomized into 3 equal groups. The duration of study was 28 days. The groups were classified as follows: Group I (healthy control): received saline, in the same volume of CCl4 dose, daily, orally, for 14 days, then sacrificed. Group II: received CCl4, as a single oral dose only, of 1 ml/kg body weight, dissolved in olive oil (1:1 v/v), the animals of this group were sacrificed 14 days after CCl4 single dose intoxication. Group III (protective Tempol treated): received a single dose of Tempol, 20mg/kg, orally, daily for 14 days. Two hours after the last Tempol dose, animals of group III received a single oral dose of CCl4. Fourteen days later, animals were scarified to collect blood and liver tissues for analysis. Tempol pretreatment significantly captured elevated levels of ALT and AST activities, lipid peroxidation, total bilirubin and increased total thiol and catalase contents. Notably, it significantly reduced the expression of tumor necrosis factor-alpha (TNF-α), Caspase-3 and endoplasmic reticulum (ER) inositol-requiring enzyme 1(IRE1) mRNAs, which is an ER trans membrane sensor that activates the unfolded protein response (UPR) to maintain the ER and cellular function.

CONCLUSION

Pretreatment with Tempol has potential hepatoprotective effects against acute liver injury, induced by CCl4, through antioxidant and anti-inflammatory activities.

ERK1/2 pathway is involved in renal gluconeogenesis inhibition under conditions of lowered NADPH oxidase activity

The aim of this study was to elucidate the mechanisms involved in the inhibition of renal gluconeogenesis occurring under conditions of lowered activity of NADPH oxidase (Nox), the enzyme considered to be one of the main sources of reactive oxygen species in kidneys. The in vitro experiments were performed on primary cultures of rat renal proximal tubules, with the use of apocynin, a selective Nox inhibitor, and TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl), a potent superoxide radical scavenger. In the in vivo experiments, Zucker diabetic fatty (ZDF) rats, a well established model of diabetes type 2, were treated with apocynin solution in drinking water. The main in vitro findings are the following: (1) both apocynin and TEMPOL attenuate the rate of gluconeogenesis, inhibiting the step catalyzed by phosphoenolpyruvate carboxykinase (PEPCK), a key enzyme of the process; (2) in the presence of the above-noted compounds the expression of PEPCK and the phosphorylation of transcription factor CREB and ERK1/2 kinases are lowered; (3) both U0126 (MEK inhibitor) and 3-(2-aminoethyl)-5-((4-ethoxyphenyl)methylene)-2,4-thiazolidinedione (ERK inhibitor) diminish the rate of glucose synthesis via mechanisms similar to those of apocynin and TEMPOL. The observed apocynin in vivo effects include: (1) slight attenuation of hyperglycemia; (2) inhibition of renal gluconeogenesis; (3) a decrease in renal PEPCK activity and content. In view of the results summarized above, it can be concluded that: (1) the lowered activity of the ERK1/2 pathway is of importance for the inhibition of renal gluconeogenesis found under conditions of lowered superoxide radical production by Nox; (2) the mechanism of this phenomenon includes decreased PEPCK expression, resulting from diminished activity of transcription factor CREB; (3) apocynin-evoked inhibition of renal gluconeogenesis contributes to the hypoglycemic action of this compound observed in diabetic animals. Thus, the study has delivered some new insights into the recently discussed issue of the usefulness of Nox inhibition as a potential antidiabetic strategy.

Uremic toxins, oxidative stress, and renal fibrosis: an interwined complex

The prevalence of end-stage renal diseases is currently on the rise globally, and finding the way to curb this tide is urgently needed. Tubulointerstitial fibrosis is a common pathway for essentially all the nephropathy categories known to date, and the manifestations of renal fibrosis include excessive deposition of extracellular matrix with distortion of renal microstructures and functional deterioration. Uremic toxins have been gradually found to play an important role in the development of progressive renal fibrosis, with protein-bound indoxyl sulfate, p-cresol, and p-cresyl sulfate receiving the most attention. However, the contribution of oxidative stress among the pathogenesis of uremic toxins and renal fibrosis has not been evaluated much until recently. In this review, we will discuss about the nature and sources of oxidative stress in the kidney and how uremic toxins use oxidative stress to orchestrate the processes of renal fibrosis.

NADPH oxidases, reactive oxygen species, and the kidney: friend and foe

Reactive oxygen species (ROS) play an important role in normal cellular physiology. They regulate different biologic processes such as cell defense, hormone synthesis and signaling, activation of G protein-coupled receptors, and ion channels and kinases/phosphatases. ROS are also important regulators of transcription factors and gene expression. On the other hand, in pathologic conditions, a surplus of ROS in tissue results in oxidative stress with various injurious consequences such as inflammation and fibrosis. NADPH oxidases are one of the many sources of ROS in biologic systems, and there are seven isoforms (Nox1-5, Duox1, Duox2). Nox4 is the predominant form in the kidney, although Nox2 is also expressed. Nox4 has been implicated in the basal production of ROS in the kidney and in pathologic conditions such as diabetic nephropathy and CKD; upregulation of Nox4 may be important in renal oxidative stress and kidney injury. Although there is growing evidence indicating the involvement of NADPH oxidase in renal pathology, there is a paucity of information on the role of NADPH oxidase in the regulation of normal renal function. Here we provide an update on the role of NADPH oxidases and ROS in renal physiology and pathology.

Renoprotective effects of a novel Nox1/4 inhibitor in a mouse model of Type 2 diabetes

Nox (NADPH oxidase)-derived ROS (reactive oxygen species) have been implicated in the development of diabetic nephropathy. Of the Nox isoforms in the kidney, Nox4 is important because of its renal abundance. In the present study, we tested the hypothesis that GKT136901, a Nox1/4 inhibitor, prevents the development of nephropathy in db/db (diabetic) mice. Six groups of male mice (8-week-old) were studied: (i) untreated control db/m, (ii) low-dose GKT136901-treated db/m (30 mg/kg of body weight per day), (iii) high-dose GKT136901-treated db/m (90 mg/kg of body weight per day), (iv) untreated db/db; (v) low dose GKT136901-treated db/db; and (vi) high-dose GKT136901-treated db/db. GKT136901, in chow, was administered for 16 weeks. db/db mice developed diabetes and nephropathy as evidenced by hyperglycaemia, albuminuria and renal injury (mesangial expansion, tubular dystrophy and glomerulosclerosis). GKT136901 treatment had no effect on plasma glucose or BP (blood pressure) in any of the groups. Plasma and urine TBARSs (thiobarbituric acid-reacting substances) levels, markers of systemic and renal oxidative stress, respectively, were increased in diabetic mice. Renal mRNA expression of Nox4, but not of Nox2, increased, Nox1 was barely detectable in db/db. Expression of the antioxidant enzyme SOD-1 (superoxide dismutase 1) decreased in db/db mice. Renal content of fibronectin, pro-collagen, TGFβ (transforming growth factor β) and VCAM-1 (vascular cell adhesion molecule 1) and phosphorylation of ERK1/2 (extracellular-signal-regulated kinase 1/2) were augmented in db/db kidneys, with no change in p38 MAPK (mitogen-activated protein kinase) and JNK (c-Jun N-terminal kinase). Treatment reduced albuminuria, TBARS and renal ERK1/2 phosphorylation and preserved renal structure in diabetic mice. Our findings suggest a renoprotective effect of the Nox1/4 inhibitor, possibly through reduced oxidative damage and decreased ERK1/2 activation. These phenomena occur independently of improved glucose control, suggesting GKT136901-sensitive targets are involved in complications of diabetes rather than in the disease process.

Apocynin improves insulin resistance through suppressing inflammation in high-fat diet-induced obese mice

We investigated the effects of apocynin on high-fat diet- (HFD-) induced insulin resistance in C57BL/6 mice. After 12 weeks of HFD, the mice that exhibited insulin resistance then received 5 weeks of apocynin (2.4 g/L, in water). Following apocynin treatment, fasting glucose, insulin, and glucose tolerance test showed significant improvement in insulin sensitivity in HFD-fed mice. We demonstrated that serum levels of tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), and leptin were remarkably reduced with apocynin treatment. We also found that mRNA expression of TNF-α, IL-6, and monocyte chemoattractant protein-1 (MCP-1) in the liver and mRNA expression of TNF-α, IL-6, MCP-1, and leptin in adipose tissue were suppressed by apocynin. Furthermore, the activity of transcription factor NF-κB in the liver was significantly suppressed with apocynin treatment. These results suggest that apocynin may reduce inflammatory factors in the blood, liver, and adipose tissue, resulting in amelioration of insulin resistance in HFD-fed mice.