Translocation of glomerular p47phox and p67phox by protein kinase C-beta activation is required for oxidative stress in diabetic nephropathy

Enhanced superoxide release and elevated protein kinase C activity in neutrophils from diabetic patients: association with periodontitis

Inflammation and oxidative stress are important factors in the pathogenesis of diabetes and contribute to the pathogenesis of diabetic complications. Periodontitis is an inflammatory disease that is characterized by increased oxidative stress, and the risk for periodontitis is increased significantly in diabetic subjects. In this study, we examined the superoxide (O(2)(-))-generating reduced nicotinamide adenine dinucleotide phosphate-oxidase complex and protein kinase C (PKC) activity in neutrophils. Fifty diabetic patients were grouped according to glycemic control and the severity of periodontitis. Neutrophils from diabetic patients with moderate [amount of glycated hemoglobin (HbA(1c)) between 7.0% and 8.0%] or poor (HbA(1c) >8.0%) glycemic control released significantly more O(2)(-) than neutrophils from diabetic patients with good glycemic control (HbA(1c) <7.0%) and neutrophils from nondiabetic, healthy individuals upon stimulation with 4beta-phorbol 12-myristate 13-acetate or N-formyl-Met-Leu-Phe. Depending on glycemic status, neutrophils from these patients also exhibited increased activity of the soluble- and membrane-bound forms of PKC, elevated amounts of diglyceride, and enhanced phosphorylation of p47-phox during cell stimulation. In addition, we report a significant correlation between glycemic control (HbA(1c) levels) and the severity of periodontitis in diabetic patients, suggesting that enhanced oxidative stress and increased inflammation exacerbate both diseases. Thus, hyperglycemia can lead to a novel form of neutrophil priming, where elevated PKC activity results in increased phosphorylation of p47-phox and O(2)(-) release.

Glycation of mitochondrial proteins from diabetic rat kidney is associated with excess superoxide formation

Chronic hyperglycemia causes structural alterations of proteins through the Maillard reaction. In diabetes, methylglyoxal (MGO)-induced hydroimidazolones are the predominant modification. In contrast to acute hyperglycemia, mitochondrial respiration is depressed in chronic diabetes. To determine whether MGO-derived protein modifications result in abnormalities in mitochondrial bioenergetics and superoxide formation, proteomics and functional studies were performed in renal cortical mitochondria isolated from rats with 2, 6, and 12 mo of streptozotocin-induced diabetes. MGO-modified proteins belonged to the following two pathways: 1) oxidative phosphorylation and 2) fatty acid β-oxidation. Two of these proteins were identified as components of respiratory complex III, the major site of superoxide production in health and disease. Mitochondria from rats with diabetes exhibited a diminution of oxidative phosphorylation. A decrease in the respiratory complex III activity was significantly correlated with the quantity of MGO-derived hydroimidazolone present on mitochondrial proteins in both diabetic and control animals. In diabetes, isolated renal mitochondria produced significantly increased quantities of superoxide and showed evidence of oxidative damage. Administration of aminoguanidine improved mitochondrial respiration and complex III activity and decreased oxidative damage to mitochondrial proteins. Therefore, posttranslational modifications of mitochondrial proteins by MGO may represent pathogenic events leading to mitochondria-induced oxidative stress in the kidney in chronic diabetes.

Putative roles of kinin receptors in the therapeutic effects of angiotensin 1-converting enzyme inhibitors in diabetes mellitus

The role of endogenous kinins and their receptors in diabetes mellitus is being confirmed with the recent developments of molecular and genetic animal models. Compelling evidence suggests that the kinin B(2) receptor is organ-protective and partakes to the therapeutic effects of angiotensin 1-converting enzyme inhibitors (ACEI) and angiotensin AT(1) receptor antagonists. Benefits derive primarily from vasodilatory, antihypertensive, antiproliferative, antihypertrophic, antifibrotic, antithrombotic and antioxidant properties of kinin B(2) receptor activation. Mechanisms include the formation of nitric oxide and prostacyclin and the inhibition of NAD(P)H oxidase activity involving classical and novel signalling pathways. Kinin B(2) receptor also ameliorates insulin resistance by increasing glucose uptake and supply, and by inducing glucose transporter-4 translocation either directly or through phosphorylation of insulin receptor. The kinin B(1) receptor, which is induced by the cytokine network, growth factors and hyperglycaemia, mediates hyperalgesia, vascular hyperpermeability and leukocytes infiltration in diabetic animals. However, emerging data highlight reno- and cardio-protective effects mediated by kinin B(1) receptor under chronic ACEI therapy in diabetes mellitus. Thus, the Janus-faced of kinin receptors needs to be taken into account in future drug development. For instance, locally acting kinin B(1)/B(2) receptor agonists if used in a safe therapeutic window may represent a more rationale strategy in the prevention and management of diabetic complications. Because kinin B(2) receptor antagonists may further increase insulin resistance, the persisting dogma that restricts the development of kinin receptor analogues to antagonists (that is still relevant to abrogate pain and inflammation) needs to be revisited.

Nitric oxide and superoxide in the renal medulla: a delicate balancing act

PURPOSE OF REVIEW

Endothelial nitric oxide synthase (eNOS) and nicotinamide adenine dinucleotide (phosphate) oxidase [NAD(P)H oxidase] are both expressed in tubular epithelial cells within the renal medulla, particularly the thick ascending limb of the loop of Henle (mTALH). Thick ascending limbs contribute to long-term blood pressure control, both because they reabsorb approximately 30% of filtered sodium, and because they produce paracrine factors like nitric oxide (NO) that control medullary blood flow (MBF), which in turn has a major impact on tubular sodium reabsorption. Herein, we review recent evidence for roles of NO and superoxide (O2*-) in autocrine control of tubular sodium reabsorption, and in paracrine control of MBF.

RECENT FINDINGS

O2*- can have a direct action to reduce MBF, and to enhance sodium reabsorption from mTALH. These actions oppose those of NO produced in mTALH, which inhibits tubular sodium reabsorption (autocrine) and increases MBF (paracrine). NO and O2*- also oppose each other's actions through chemical combination to produce peroxynitrite. Thus, interactions between NO and O2*-, at both the chemical and cellular levels, likely contribute to long-term blood pressure control. This hypothesis is supported by recent data showing that sodium retention and hypertension can develop when the balance of production of these free radicals is tipped towards O2*-, such as in diabetes, atherosclerosis and renin-angiotensin-system activation.

SUMMARY

Interactions between O2*- and NO produced within the mTALH regulate tubular and vascular function in the renal medulla. Dysregulation of these systems in states of oxidative stress likely promotes salt and water retention, and thus hypertension.

Src-mediated Tyrosine Phosphorylation of p47 in Hyperoxia-induced Activation of NADPH Oxidase and Generation of Reactive Oxygen Species in Lung Endothelial Cells

Superoxide (O(2)(-)) production by nonphagocytes, similar to phagocytes, is by activation of the NADPH oxidase multicomponent system. Although activation of neutrophil NADPH oxidase involves extensive serine phosphorylation of p47(phox), the role of tyrosine phosphorylation of p47(phox) in NADPH oxidase-dependent O(2)(-) production is unclear. We have shown recently that hyperoxia-induced NADPH oxidase activation in human pulmonary artery endothelial cells (HPAECs) is regulated by mitogen-activated protein kinase signal transduction. Here we provided evidence on the role of nonreceptor tyrosine kinase, Src, in hyperoxia-induced tyrosine phosphorylation of p47(phox) and NADPH oxidase activation in HPAECs. Exposure of HPAECs to hyperoxia for 1 h resulted in increased O(2)(-) and reactive oxygen species (ROS) production and enhanced tyrosine phosphorylation of Src as determined by Western blotting with phospho-Src antibodies. Pretreatment of HPAECs with the Src kinase inhibitor PP2 (1 mum) or transient expression of a dominant-negative mutant of Src attenuated hyperoxia-induced tyrosine phosphorylation of Src and ROS production. Furthermore, exposure of cells to hyperoxia enhanced tyrosine phosphorylation of p47(phox) and its translocation to cell peripheries that were attenuated by PP2. In vitro, Src phosphorylated recombinant p47(phox) in a time-dependent manner. Src immunoprecipitates of cell lysates from control cells revealed the presence of immunodetectable p47(phox) and p67(phox), suggesting the association of oxidase components with Src under basal conditions. Moreover, exposure of HPAECs to hyperoxia for 1 h enhanced the association of p47(phox), but not p67(phox), with Src. These results indicated that Src-dependent tyrosine phosphorylation of p47(phox) regulates hyperoxia-induced NADPH oxidase activation and ROS production in HPAECs.

Oxidative stress and the use of antioxidants in diabetes: linking basic science to clinical practice

Cardiovascular complications, characterized by endothelial dysfunction and accelerated atherosclerosis, are the leading cause of morbidity and mortality associated with diabetes. There is growing evidence that excess generation of highly reactive free radicals, largely due to hyperglycemia, causes oxidative stress, which further exacerbates the development and progression of diabetes and its complications. Overproduction and/or insufficient removal of these free radicals result in vascular dysfunction, damage to cellular proteins, membrane lipids and nucleic acids. Despite overwhelming evidence on the damaging consequences of oxidative stress and its role in experimental diabetes, large scale clinical trials with classic antioxidants failed to demonstrate any benefit for diabetic patients. As our understanding of the mechanisms of free radical generation evolves, it is becoming clear that rather than merely scavenging reactive radicals, a more comprehensive approach aimed at preventing the generation of these reactive species as well as scavenging may prove more beneficial. Therefore, new strategies with classic as well as new antioxidants should be implemented in the treatment of diabetes.

The next generation of diabetic nephropathy therapies: an update

Although treatments for diabetic kidney disease are available, many patients still have progressive disease. More effective therapies are urgently needed. Novel agents currently under evaluation in clinical trials are described in this review. Sulodexide, a mixture of three glycosaminoglycans, appears to prevent diabetic nephropathy in experimental models by ameliorating abnormalities in the glomerular basement membrane and mesangial matrix. Pyridoxamine is an inhibitor of advanced glycation end-product (AGE) formation derived from vitamin B(6). Alagebrium is an AGE cross-link breaker. AGEs injure the kidneys and other vascular targets by mechanisms such as oxidative stress, inflammation, and protein cross-linking, among others. By inhibiting AGE formation or breaking AGE cross-links, experimental models have demonstrated kidney protection. Ruboxistaurin is an inhibitor of protein kinase C beta (PKC-beta), a mediator of signal transduction that leads to cell growth, fibrosis, and tissue injury. In diabetes, PKC-beta is up-regulated and activated in the kidney. Ruboxistaurin prevents diabetic kidney disease in animal models. These agents have appeared promising (by reduction of albuminuria and preservation of kidney function) in phase II studies. To determine whether clinical outcomes (mortality, renal, and cardiovascular events) are improved beyond the current standard of care, phase III trials are planned.

Metformin prevents glucose-induced protein kinase C-beta2 activation in human umbilical vein endothelial cells through an antioxidant mechanism

Hyperglycemia determines the vascular complications of diabetes through different mechanisms: one of these is excessive activation of the isoform beta2 of protein kinase C (PKC-beta2). Metformin, a widely used antidiabetic agent, is associated with decreased cardiovascular mortality in obese type 2 diabetic patients. Therefore, we assessed the role of metformin in glucose-induced activation of PKC-beta2 and determined the mechanism of its effect in human umbilical venous endothelial cells grown to either normo- (5 mmol/l) or hyperglycemia (10 mmol/l) and moderately and acutely exposed to 25 mmol/l glucose. We studied PKC-beta2 activation by developing adenovirally expressed chimeras encoding fusion protein between green fluorescent protein (GFP) and conventional beta2 isoform (PKC-beta2-GFP). Glucose (25 mmol/l) induced the translocation of PKC-beta2-GFP from the cytosol to the membrane in cells grown to hyperglycemia but not in those grown in normal glucose medium. Metformin (20 micromol/l) prevented hyperglycemia-induced PKC-beta2-GFP translocation. We also assessed oxidative stress under the same conditions with a 4-((9-acridine-carbonyl)amino)-2,2,6,6-tetramethylpiperidin-oxyl,free radical (TEMPO-9-AC) fluorescent probe. We observed significantly increased radical oxygen species production in cells grown in hyperglycemia medium, and this effect was abolished by metformin. We show that in endothelial cells, metformin inhibits hyperglycemia-induced PKC-beta2 translocation because of a direct antioxidant effect. Our data substantiate the findings of previous large intervention studies on the beneficial effect of this drug in type 2 diabetic patients.

Effect of High Glucose on Superoxide in Human Mesangial Cells: Role of Angiotensin II

BACKGROUND/AIMS

Reactive oxygen species, and especially superoxide (O2*),have been implicated in diabetic nephropathy. O2* accumulation in cells is dependent on O2* production (by NADH/NADPH oxidase) as well as scavenging by superoxide dismutase (SOD) activity. This study was designed to investigate the effects of high glucose (HG) on O2* accumulation and SOD activity in human mesangial cells (HMC) and to determine if these effects are mediated by angiotensin II (Ang II).

METHODS

HMC were incubated in media containing 10 mM glucose (control, C), 30 mM glucose (HG), 10 mM glucose + either 20 mM 2-deoxy-D-glucose (2-DG) or 20 mM mannitol (high mannitol, HM) (osmotic controls), or Ang II (10(-5) M). Ang II action was antagonized by employing 10(-4) M of Ang II receptor antagonists (losartan or irbesartan) or 10(-4) M of NADH/NADPH oxidase inhibitors [diphenyleneiodonium chloride (DPI) or apocynin]. Superoxide and total SOD activity were assayed using chemiluminescence of lucigenin.

RESULTS

Incubation of HMC in HG resulted in a 1.6-fold increase in Ang I (p < 0.05) and a 1.4-fold increase in Ang II levels (p < 0.05) in cell lysates. These changes were accompanied by a >2-fold increase in O2* accumulation (p < 0.01), which was inhibited by losartan and irbesartan. Exogenous Ang II increased net O2* accumulation by 2.7-fold (p < 0.01), which was normalized by losartan and irbesartan. DPI and apocynin blocked the HG and Ang II-induced increases in O2* (p < 0.01). HG but not exogenous Ang II inhibited total SOD activity by 30%, which was not affected by losartan.

CONCLUSION

High glucose increases O2* accumulation in HMC primarily by increasing its production via the Ang II-NADH/NADPH oxidase system.

Requirement of aldose reductase for the hyperglycemic activation of protein kinase C and formation of diacylglycerol in vascular smooth muscle cells

Activation of protein kinase C (PKC) has been linked to the development of secondary diabetes complications. However, the underlying molecular mechanisms remain unclear. We examined the contribution of aldose reductase, which catalyzes the first, and the rate-limiting, step of the polyol pathway of glucose metabolism, to PKC activation in vascular smooth muscle cells (VSMCs) isolated from rat aorta and exposed to high glucose in culture. Exposure of VSMCs to high glucose (25 mmol/l), but not iso-osmotic mannitol, led to an increase in total membrane-associated PKC activity, which was prevented by the aldose reductase inhibitors tolrestat or sorbinil or by the ablation of aldose reductase by small interfering RNA (siRNA). The VSMCs were found to express low levels of sorbitol dehydrogenase, and treatment with the sorbitol dehydrogenase inhibitor CP-166572 did not prevent high-glucose-induced PKC activation. Stimulation with high glucose caused membrane translocation of conventional (alpha, beta1, beta2, and gamma) and novel (delta and epsilon) isoforms of PKC. Inhibition of aldose reductase prevented membrane translocation of PKC-beta2 and -delta and delayed the activation of PKC-beta1 and -epsilon, whereas membrane translocation of PKC-alpha and -gamma was not affected. Treatment with tolrestat prevented phosphorylation of PKC-beta2 and -delta. High glucose increased the formation of diacylglycerol (DAG) and enhanced phosphorylation of phospholipase C-gamma1 (PLC-gamma1). Inhibition of aldose reductase prevented high glucose-induced DAG formation and phosphorylation of PLC-gamma1 and PLC-beta2 and -delta. Inhibition of phospholipid hydrolysis by D609, but not by the synthetic alkyl-1-lysophospholipid 1-O-octadecyl-2-O-methyl-rac-glycerophosphocholine, or edelfosine, prevented DAG formation. Treatment with sorbinil decreased the levels of reactive oxygen species in high-glucose-stimulated VSMCs. Hence, inhibition of aldose reductase, independent of sorbitol dehydrogenase, appears to be effective in diminishing oxidative stress and hyperglycemic changes in signaling events upstream to the activation of multiple PKC isoforms and PLC-gamma1 and may represent a useful approach for preventing the development of secondary vascular complications of diabetes.

PKC-delta-dependent activation of oxidative stress in adipocytes of obese and insulin-resistant mice: role for NADPH oxidase

Oxidative stress is thought to be one of the causative factors contributing to insulin resistance and type 2 diabetes. Previously, we showed that reactive oxygen species (ROS) production is significantly increased in adipocytes from high-fat diet-induced obese and insulin-resistant mice (HF). ROS production was also associated with the increased activity of PKC-delta. In the present studies, we hypothesized that PKC-delta contributes to ROS generation and determined their intracellular source. NADPH oxidase inhibitor diphenyleneiodonium chloride (DPI) reduced ROS levels by 50% in HF adipocytes, and inhibitors of NO synthase (L-NAME, 1 mM), xanthine oxidase (allopurinol, 100 microM), AGE formation (aminoguanidine, 10 microM), or the mitochondrial uncoupler (FCCP, 10 microM) had no effect. Rottlerin, a selective PKC-delta inhibitor, suppressed ROS levels by approximately 50%. However, neither GO-6976 nor LY-333531, effective inhibitors toward conventional PKC or PKC-beta, respectively, significantly altered ROS levels in HF adipocytes. Subsequently, adenoviral-mediated expression of wild-type PKC-delta or its dominant negative mutant (DN-PKC-delta) in HF adipocytes resulted in either a twofold increase in ROS levels or their suppression by 20%, respectively. In addition, both ROS levels and PKC-delta activity were sharply reduced by glucose depletion. Taken together, these results suggest that PKC-delta is responsible for elevated intracellular ROS production in HF adipocytes, and this is mediated by high glucose and NADPH oxidase.

Proatherosclerotic mechanisms involving protein kinase C in diabetes and insulin resistance

In diabetes and insulin resistance, activation of protein kinase C (PKC) in vascular cells may be a key link between elevated plasma and tissue concentrations of glucose and nonesterified fatty acids and abnormal vascular cell signaling. Initial studies of PKC activation in diabetes focused on microvascular complications, but increasing evidence supports that PKC plays a role in several mechanisms promoting atherosclerosis. This review explains how PKC is thought to be activated in diabetes and insulin resistance through de novo synthesis of diacylglycerol. Furthermore, the review summarizes studies that implicate PKC in promoting proatherogenic mechanisms or inhibiting antiatherogenic mechanisms, including studies of endothelial dysfunction; gene induction and activation of vascular NAD(P)H oxidase; endothelial nitric oxide synthase expression and function; endothelin-1 expression; growth, migration, and apoptosis of vascular smooth muscle cells; induction of adhesion molecules; and oxidized low-density lipoprotein uptake by monocyte-derived macrophages.

Vascular Dysfunction of Venous Bypass Conduits Is Mediated by Reactive Oxygen Species in Diabetes: Role of Endothelin-1

Diabetes is associated with increased risk for complications following coronary bypass grafting (CABG) surgery. Augmented superoxide (*O2*) production plays an important role in diabetic complications by causing vascular dysfunction. The potent vasoconstrictor endothelin-1 (ET-1) is also elevated in diabetes and following CABG; however, the effect of ET-1 on *O2* generation and/or vascular dysfunction in bypass conduits remain unknown. Accordingly, this study investigated basal and ET-1-stimulated *O2* production in bypass conduits and determined the effect of *O2* on conduit reactivity. Saphenous vein specimens were obtained from nondiabetic (n = 24) and diabetic (n = 24) patients undergoing CABG. Dihydroethidium staining and NAD(P)H oxidase activity assays (5380 +/- 940 versus 16,362 +/- 2550 relative light units/microg) demonstrated increased basal *O2* levels in the diabetes group (p < 0.05). Plasma ET-1 levels were associated with elevated basal *O2* levels, and treatment of conduits with exogenous ET-1 further increased *O2* production and augmented vasoconstriction. Furthermore, vascular relaxation was impaired in the diabetic group (75 versus 40%), which was restored by *O2* scavenger superoxide dismutase. These findings suggest that ET-1 causes bypass conduits dysfunction via stimulation of *O2* production in diabetes. Novel therapies that attenuate *O2* generation in bypass conduits may improve acute and late outcome of CABG in diabetic patients.

Nitric oxide, oxidative excess, and vascular complications of diabetes mellitus

The prevalence of diabetes mellitus is rising worldwide and has reached epidemic dimensions. Diabetes mellitus places patients at high cardiovascular risk. High blood glucose levels, altered insulin signaling, reactive oxygen species (ROS), inflammation, and protein kinase C activation might lead to a decrease in nitric oxide (NO) bioavailability. Diminished NO and enhanced oxidative stress play a central role in several pathophysiologic pathways, leading to vascular damage, such as endothelial dysfunction, vascular inflammation, atherosclerotic plaque formation and vulnerability, and promotion of a prothrombotic state. Possible sources of oxidative excess in diabetes are reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, xanthine oxidase, uncoupled NO synthase, and the mitochondria. Advances in understanding the pathophysiologic mechanisms leading to vascular damage in diabetes will result in discovery of new therapeutic targets, which should help reduce cardiovascular risk in these patients.