Activation of 5'-AMP-activated kinase is mediated through c-Src and phosphoinositide 3-kinase activity during hypoxia-reoxygenation of bovine aortic endothelial cells. Role of peroxynitrite

AMP-activated kinase (AMPK) is a fuel-sensing enzyme present in most mammalian tissue. In response to a decrease in the energy state of a cell AMPK is phosphorylated and activated by still poorly characterized upstream events. Exposure of bovine aortic endothelial cells (BAEC) to chemically synthesized ONOO- acutely and significantly increased phosphorylation of c-Src, PDK1, AMPK, and its downstream target, acetyl-CoA carboxylase (ACC), without affecting cellular AMP. This novel pathway for AMPK activation was confirmed by the use of pharmacological inhibitors and dominant-negative mutants. Exposure of BAEC to hypoxia-reoxygenation (H/R) caused a biphasic increase in AMPK and ACC phosphorylation, which was prevented by adenoviral overexpression of superoxide dismutase (SOD) or inhibition of nitric-oxide synthase (NOS) implicating a role of ONOO- formed during H/R. Furthermore, dominant-negative mutants of c-Src or kinase-defective PDK1 also blocked H/R-induced AMPK activation indicating that, as with addition of exogenous ONOO-, both c-Src and PI 3-kinase are upstream of AMPK. Moreover, H/R, like ONOO-, significantly increased co-immunoprecipitation of AMPK with c-Src, suggesting that ONOO- favors physical association of AMPK with upstream kinases. Taken together, our results indicate a novel pathway by which H/R via ONOO- activates AMPK in a c-Src-mediated, PI 3-kinase-dependent manner, and suggest that ONOO--induced activation of AMPK might thereby regulate metabolic enzymes, such as ACC.

Biological chemistry of reactive oxygen and nitrogen and radiation-induced signal transduction mechanisms

In the past few years, nuclear DNA damage-sensing mechanisms activated by ionizing radiation have been identified, including ATM/ATR and the DNA-dependent protein kinase. Less is known about sensing mechanisms for cytoplasmic ionization events and how these events influence nuclear processes. Several studies have demonstrated the importance of cytoplasmic signaling pathways in cytoprotection and mutagenesis. For cytoplasmic signaling, radiation-stimulated reactive oxygen species (ROS) and reactive nitrogen species (RNS) are essential activators of these pathways. This review summarizes recent studies on the chemistry of radiation-induced ROS/RNS generation and emphasizes interactions between ROS and RNS and the relative roles of cellular ROS/RNS generators as amplifiers of the initial ionization events. Cellular mechanisms for regulating ROS/RNS levels are discussed. The mechanisms by which cells sense ROS/RNS are examined in terms of how ROS/RNS modify protein structure and function, for example, interactions with metal-thiol clusters, protein tyrosine nitration, protein cysteine oxidation, S-thiolation and S-nitrosylation. We propose that radiation-induced ROS are the initiators and that nitric oxide (NO*) or derivatives are the effectors activating these signal transduction pathways. In responding to cellular ionization events, the cell converts an oxidative signal to a nitrosative one because ROS are too reactive and unspecific in their reactions for regulatory purposes and the cell is equipped to precisely modulate NO* levels.

Interactions of peroxynitrite, tetrahydrobiopterin, ascorbic acid, and thiols: implications for uncoupling endothelial nitric-oxide synthase

Tetrahydrobiopterin (BH4) serves as a critical co-factor for the endothelial nitric-oxide synthase (eNOS). A deficiency of BH4 results in eNOS uncoupling, which is associated with increased superoxide and decreased NO* production. BH4 has been suggested to be a target for oxidation by peroxynitrite (ONOO-), and ascorbate has been shown to preserve BH4 levels and enhance endothelial NO* production; however, the mechanisms underlying these processes remain poorly defined. To gain further insight into these interactions, the reaction of ONOO- with BH4 was studied using electron spin resonance and the spin probe 1-hydroxy-3-carboxy-2,2,5-tetramethyl-pyrrolidine. ONOO- reacted with BH4 6-10 times faster than with ascorbate or thiols. The immediate product of the reaction between ONOO- and BH4 was the trihydrobiopterin radical (BH3.), which was reduced back to BH4 by ascorbate, whereas thiols were not efficient in recycling of BH4. Uncoupling of eNOS caused by peroxynitrite was investigated in cultured bovine aortic endothelial cells (BAECs) by measuring superoxide and NO* using spin probe 1-hydroxy-3-methoxycarbonyl-2,2,5,5-tetramethyl-pyrrolidine and the NO*-spin trap iron-diethyldithiocarbamate. Bolus ONOO-, the ONOO- donor 3-morpholinosydnonimine, and an inhibitor of BH4 synthesis (2,4-diamino-6-hydroxypyrimidine) uncoupled eNOS, increasing superoxide and decreasing NO* production. Exogenous BH4 supplementation restored endothelial NO* production. Treatment of BAECs with both BH4 and ascorbate prior to ONOO- prevented uncoupling of eNOS by ONOO-. This study demonstrates that endothelial BH4 is a crucial target for oxidation by ONOO- and that the BH4 reaction rate constant exceeds those of thiols or ascorbate. We confirmed that ONOO- uncouples eNOS by oxidation of tetrahydrobiopterin and that ascorbate does not fully protect BH4 from oxidation but recycles BH3. radical back to BH4.

Nifedipine upregulates manganese superoxide dismutase expression in vascular smooth muscle cells via endothelial cell-dependent pathways

Calcium antagonists normalize endothelial dysfunction and improve the clinical outcome in patients with hypertension. However, the mechanism underlying these beneficial effects remains to be elucidated. Here, we show that the calcium antagonist nifedipine upregulates the expression of manganese superoxide dismutase (Mn SOD), an endogenous antioxidant enzyme, in vascular smooth muscle cells (VSMC) via cellular interactions between VSMC and endothelial cells (EC). Nifedipine induced upregulation of Mn SOD activity and expression in VSMC when cocultured with EC but not when cultured individually. NG-Monomethyl-L-arginine (L-NMMA), an inhibitor of nitric oxide (NO) synthesis, inhibited the upregulation of Mn SOD expression induced by nifedipine. Additionally, N-ethyl-2-(1-ethyl-2-hydroxy-2-nitrosohydrazino) ethanamine, a NO donor, reversed this inhibition by L-NMMA, indicating that NO may be involved in the mechanism underlying the nifedipine-induced upregulation of Mn SOD in VSMC. Preincubation of VSMC with Mn SOD antisense oligodeoxyribonucleotides (ODN) blocked the suppressive effects of nifedipine on DNA synthesis in VSMC cocultured with EC, whereas sense ODN had no effect. We conclude that the calcium antagonist nifedipine induces upregulation of Mn SOD expression in VSMC via NO derived from EC. This finding may provide some insight into the mechanism underlying the beneficial effects of calcium antagonists in patients with hypertension.

Experimental diabetes causes breakdown of the blood-retina barrier by a mechanism involving tyrosine nitration and increases in expression of vascular endothelial growth factor and urokinase plasminogen activator receptor

The purpose of these experiments was to determine the specific role of reactive oxygen species (ROS) in the blood-retinal barrier (BRB) breakdown that characterizes the early stages of vascular dysfunction in diabetes. Based on our data showing that high glucose increases nitric oxide, superoxide, and nitrotyrosine formation in retinal endothelial cells, we hypothesized that excess formation of ROS causes BRB breakdown in diabetes. Because ROS are known to induce increases in expression of the well-known endothelial mitogen and permeability factor vascular endothelial growth factor (VEGF) we also examined their influence on the expression of VEGF and its downstream target urokinase plasminogen activator receptor (uPAR). After 2 weeks of streptozotocin-induced diabetes, analysis of albumin leakage confirmed a prominent breakdown of the BRB. This permeability defect was correlated with significant increases in the formation of nitric oxide, lipid peroxides, and the peroxynitrite biomarker nitrotyrosine as well as with increases in the expression of VEGF and uPAR. Treatment with a nitric oxide synthase inhibitor (N-omega-nitro-L-arginine methyl ester, 50 mg/kg/day) or peroxynitrite scavenger (uric acid, 160 mg/kg/day) blocked the breakdown in the BRB and prevented the increases in formation of lipid peroxides and tyrosine nitration as well as the increases in expression of VEGF and uPAR. Taken together, these data indicate that early diabetes causes breakdown of the BRB by a mechanism involving the action of reactive nitrogen species in promoting expression of VEGF and uPAR.

Reactive oxygen species are involved in smoking-induced dysfunction of nitric oxide biosynthesis and upregulation of endothelial nitric oxide synthase: an in vitro demonstration in human coronary artery endothelial cells

BACKGROUND

Our group has previously shown that human umbilical vein endothelial cells exposed to smokers' serum decreased nitric oxide (NO) production and endothelial nitric oxide synthase (eNOS) activity in the presence of increased eNOS expression. In the present study, we examined whether these observations extended to human coronary artery endothelial cells (HCAECs). In addition, the role of reactive oxygen species in the observed alterations was examined.

METHODS AND RESULTS

HCAECs were incubated with serum from 10 nonsmokers and 15 smokers for 12 hours with or without the addition of either polyethylene glycol-superoxide dismutase (PEG-SOD, 300 U/mL), PEG-SOD+PEG-catalase (1000 U/mL), chelerythrine (3 micromol/L), or tetrahydrobiopterin (20 micromol/L). At the end of incubation, NO, eNOS protein, and eNOS activity were measured from the same culture. HCAECs incubated with smokers' serum alone showed significantly lower NO production (P<0.05) and eNOS activity (P<0.005) but higher eNOS expression (P<0.005) compared with nonsmokers. In smokers, addition of PEG-SOD, PEG-SOD+PEG-catalase, or tetrahydrobiopterin significantly (P<0.05) improved NO levels and eNOS activity. Interestingly, in the same smokers, a significant decrease in eNOS expression was only seen with the addition of PEG-SOD+PEG-catalase (P<0.05) and treatment with PEG-SOD alone insignificantly increased eNOS expression.

CONCLUSIONS

The present study indicates that in vitro, HCAECs show similar changes in NO biosynthesis as human umbilical vein endothelial cells when exposed to smokers' serum and also confirms that oxidative stress plays a central role in smoking-mediated dysfunction of NO biosynthesis in endothelial cells. Furthermore, these data support other studies suggesting a role for hydrogen peroxide in the upregulation of eNOS.

New insights on oxidative stress and diabetic complications may lead to a "causal" antioxidant therapy

Evidence implicates hyperglycemia-derived oxygen free radicals as mediators of diabetic complications. However, intervention studies with classic antioxidants, such as vitamin E, failed to demonstrate any beneficial effect. Recent studies demonstrate that a single hyperglycemia-induced process of overproduction of superoxide by the mitochondrial electron-transport chain seems to be the first and key event in the activation of all other pathways involved in the pathogenesis of diabetic complications. These include increased polyol pathway flux, increased advanced glycosylation end product formation, activation of protein kinase C, and increased hexosamine pathway flux. Superoxide overproduction is accompanied by increased nitric oxide generation, due to an endothelial NOS and inducible NOS uncoupled state, a phenomenon favoring the formation of the strong oxidant peroxynitrite, which in turn damages DNA. DNA damage is an obligatory stimulus for the activation of the nuclear enzyme poly(ADP-ribose) polymerase. Poly(ADP-ribose) polymerase activation in turn depletes the intracellular concentration of its substrate NAD(+), slowing the rate of glycolysis, electron transport, and ATP formation, and produces an ADP-ribosylation of the GAPDH. These processes result in acute endothelial dysfunction in diabetic blood vessels that, convincingly, also contributes to the development of diabetic complications. These new findings may explain why classic antioxidants, such as vitamin E, which work by scavenging already-formed toxic oxidation products, have failed to show beneficial effects on diabetic complications and may suggest new and attractive "causal" antioxidant therapy. New low-molecular mass compounds that act as SOD or catalase mimetics or L-propionyl-carnitine and lipoic acid, which work as intracellular superoxide scavengers, improving mitochondrial function and reducing DNA damage, may be good candidates for such a strategy, and preliminary studies support this hypothesis. This "causal" therapy would also be associated with other promising tools such as LY 333531, PJ34, and FP15, which block the protein kinase beta isoform, poly(ADP-ribose) polymerase, and peroxynitrite, respectively. While waiting for these focused tools, we may have other options: thiazolinediones, statins, ACE inhibitors, and angiotensin 1 inhibitors can reduce intracellular oxidative stress generation, and it has been suggested that many of their beneficial effects, even in diabetic patients, are due to this property.

Functional consequences of endothelial nitric oxide synthase uncoupling in congestive cardiac failure

BACKGROUND

Impaired endothelium-mediated vasodilatation (EMVD) in congestive cardiac failure (CCF) has been linked to decreased nitric oxide (NO) bioavailability because of its interaction with vascular superoxide (O2*-), derived predominantly from NAD(P)H-dependent oxidases. When uncoupled from essential cofactors, endothelial nitric oxide synthase (eNOS) produces O2*-. We studied the functional consequences of eNOS uncoupling in relation to EMVD in patients with CCF.

METHODS AND RESULTS

We employed the platelet as a compartmentalized ex-vivo model to examine O2*- and NO production. When eNOS is functioning normally, incorporation of Nomega-Nitro-L-Arginine methyl ester (L-NAME, 1 mmol/L), results in increased O2*- detection, as inhibition of NO production prevents NO scavenging of O2*-. This was observed in controls and 9 of the CCF patients, in whom O2*- detection increased by 63% and 101%, respectively. In the remaining 9 CCF patients, incorporation of L-NAME reduced O2*- production by 39%, indicating O2*- production by eNOS uncoupling. Detection of platelet-derived NO was significantly greater in eNOS-coupled platelets compared with the uncoupled group (2.8+/-1.4 versus 0.9+/-0.4 pmol/108 platelets, P=0.04). Endothelium-dependent and -independent vasodilator responses to acetylcholine and sodium nitroprusside recorded using venous occlusion plethysmography were significantly impaired in patients exhibiting eNOS uncoupling.

CONCLUSIONS

This study provides first evidence that platelet eNOS can become uncoupled in human CCF. Impaired endothelium-dependent and -independent vasodilator responses and diminished platelet-derived NO production occurred in association with enzyme uncoupling.

Nebivolol prevents vascular NOS III uncoupling in experimental hyperlipidemia and inhibits NADPH oxidase activity in inflammatory cells

OBJECTIVE

Nebivolol, in contrast to other selective beta1-adrenergic receptor antagonists like atenolol, improves endothelial function in patients with oxidative stress within vascular tissue. With the present studies we sought to determine whether beta receptor blockade with nebivolol may improve endothelial function in hyperlipidemia and whether this is attributable to reductions in vascular oxidative stress.

METHODS AND RESULTS

Watanabe heritable hyperlipidemic rabbits (WHHL) were treated with nebivolol (10 mg/kg per day for 8 weeks). New Zealand white rabbits (NZWR) served as controls. Nebivolol improved endothelial function, reduced vascular superoxide and vascular macrophage infiltration, and prevented NO synthase uncoupling in WHHL. Nebivolol treatment did not modify the expression of sGC or cGK-I but improved cGK-I activity (assessed by the phosphorylation state of the VAsodilator Stimulated Phosphoprotein at serine239, P-VASP). NAD(P)H oxidase activity in whole blood and isolated neutrophils was dose-dependently inhibited by nebivolol, whereas atenolol, metoprolol, and carvedilol were markedly less effective.

CONCLUSIONS

Nebivolol therapy effectively prevents NO synthase III uncoupling and prevents activation of the neutrophil NAD(P)H oxidase and infiltration of inflammatory cells. These novel antioxidative stress actions of this compound may explain partly the beneficial effects on endothelial function in patients with enhanced vascular oxidative stress.

Ceramide-induced activation of NADPH oxidase and endothelial dysfunction in small coronary arteries

We tested the hypothesis that ceramide induces endothelial dysfunction in small coronary arteries via NADPH oxidase-mediated superoxide and resulting peroxynitrite formation. With the use of dihydroethidium as a superoxide indicator, C(2)-ceramide was found to increase superoxide production in the endothelial cells of small coronary arteries, which was inhibited by the NADPH oxidase inhibitors N-vanillylnonanamide, apocynin, and diphenylene iodonium. NADPH oxidase expression was confirmed in endothelial cells, as indicated by the immunoblotting of its subunits gp91(phox) and p47(phox). C(2)-ceramide increased NADPH oxidase activity by 52%, which was blocked by NADPH oxidase inhibitors but not by inhibitors of NO synthase, xanthine oxidase, and mitochondrial electron transport chain enzymes. By Western blot analysis, ceramide-induced NADPH oxidase activation was found to be associated with the translocation of p47(phox) to the membrane. In isolated and pressurized small coronary arteries, N-vanillylnonanamide, apocynin, or uric acid, a peroxynitrite scavenger, largely restored the inhibitory effects of ceramide on bradykinin- and A-23187-induced vasorelaxation. With the use of nitrotyrosine as a marker, C(2)-ceramide was found to increase peroxynitrite in small coronary arteries, which could be blocked by uric acid. We conclude that NADPH oxidase-mediated superoxide production and subsequent peroxynitrite formation mediate ceramide-induced endothelial dysfunction in small coronary arteries.

Long-term vitamin C treatment increases vascular tetrahydrobiopterin levels and nitric oxide synthase activity

In cultured endothelial cells, the antioxidant, L-ascorbic acid (vitamin C), increases nitric oxide synthase (NOS) enzyme activity via chemical stabilization of tetrahydrobiopterin. Our objective was to determine the effect of vitamin C on NOS function and tetrahydrobiopterin metabolism in vivo. Twenty-six to twenty-eight weeks of diet supplementation with vitamin C (1%/kg chow) significantly increased circulating levels of vitamin C in wild-type (C57BL/6J) and apolipoprotein E (apoE)--deficient mice. Measurements of NOS enzymatic activity in aortas of apoE-deficient mice indicated a significant increase in total NOS activity. However, this increase was mainly due to high activity of inducible NOS, whereas eNOS activity was reduced. Significantly higher tetrahydrobiopterin levels were detected in aortas of apoE-deficient mice. Long-term treatment with vitamin C restored endothelial NOS activity in aortas of apoE-deficient mice, but did not affect activity of inducible NOS. In addition, 7,8-dihydrobiopterin levels, an oxidized form of tetrahydrobiopterin, were decreased and vascular endothelial function of aortas was significantly improved in apoE-deficient mice. Interestingly, vitamin C also increased tetrahydrobiopterin and NOS activity in aortas of C57BL/6J mice. In contrast, long-term treatment with vitamin E (2000 U/kg chow) did not affect vascular NOS activity or metabolism of tetrahydrobiopterin. In vivo, beneficial effect of vitamin C on vascular endothelial function appears to be mediated in part by protection of tetrahydrobiopterin and restoration of eNOS enzymatic activity.

Modulation of cardiac contraction, relaxation and rate by the endothelial nitric oxide synthase (eNOS): lessons from genetically modified mice

The modulatory role of endothelial nitric oxide synthase (eNOS) on heart contraction, relaxation and rate is examined in light of recent studies using genetic deletion or overexpression in mice under specific conditions. Unstressed eNOS-/- hearts in basal conditions exhibit a normal inotropic and lusitropic function, with either decreased or unchanged heart rate. Under stimulation with catecholamines, eNOS-/- mice predominantly show a potentiation in their beta-adrenergic inotropic and lusitropic responsiveness. A similar phenotype is observed in beta 3-adrenoceptor deficient mice, pointing to a key role of this receptor subtype for eNOS coupling. The effect of eNOS on the muscarinic cholinergic modulation of cardiac function probably operates in conjunction with other NO-independent mechanisms, the persistence of which may explain the apparent dispensability of this isoform for the effect of acetylcholine in some eNOS-/- mouse strains. eNOS-/- hearts submitted to short term ischaemia-reperfusion exhibit variable alterations in systolic and diastolic function and infarct size, while those submitted to myocardial infarction present a worsened ventricular remodelling, increased 1 month mortality and loss of benefit from ACE inhibitor or angiotensin II type I receptor antagonist therapy. Although non-conditional eNOS gene deletion may engender phenotypic adaptations (e.g. ventricular hypertrophy resulting from chronic hypertension, or upregulation of the other NOS isoforms) potentially confounding the interpretation of comparative studies, the use of eNOS-/- mice has undoubtedly advanced (and will probably continue to improve) our understanding of the complex role of eNOS (in conjunction with the other NOSs) in the regulation of cardiac function. The challenge is now to confirm the emerging paradigms in human cardiac physiology and hopefully translate them into therapy.

Modulation by peroxynitrite of Akt- and AMP-activated kinase-dependent Ser1179 phosphorylation of endothelial nitric oxide synthase

Peroxynitrite (ONOO(-)), a nitric oxide-derived oxidant, uncouples endothelial nitric oxide synthase (eNOS) and increases enzymatic production of superoxide anions (O(2)()) (Zou, M. H., Shi, C., and Cohen, R. A. (2002) J. Clin. Invest. 109, 817-826). Here we studied how ONOO(-) influences eNOS activity. In cultured bovine aortic endothelial cells (BAEC), ONOO(-) increased basal and agonist-stimulated Ser(1179) phosphorylation of eNOS, whereas it decreased nitric oxide production and bioactivity. However, ONOO(-) strongly inhibited the phosphorylation and activity of Akt, which is known to phosphorylate eNOS-Ser(1179). Moreover, expression of an Akt dominant-negative mutant did not prevent ONOO(-)-enhanced eNOS-Ser(1179) phosphorylation. In contrast to Akt, ONOO(-) significantly activated 5'-AMP-activated kinase (AMPK), as evidenced by its increased Thr(172) phosphorylation as well as increased Ser(92) phosphorylation of acetyl-coenzyme A carboxylase, a downstream target of AMPK. Associated with the increased release of O(2)(), ONOO(-) significantly increased the co-immunoprecipitation of eNOS with AMPK. Further, overexpression of the AMPK-constitutive active adenovirus significantly enhanced ONOO(-) up-regulated eNOS-Ser(P)(1179). In contrast, overexpression of a dominant-negative AMPK mutant attenuated the ONOO(-)-enhanced eNOS-Ser(1179) phosphorylation as well as O(2)() release. We conclude that ONOO(-) inhibits Akt and increases AMPK-dependent Ser(1179) phosphorylation of eNOS resulting in enhanced O(2)() release.

Mediators of injury in lupus nephritis

Systemic lupus erythematosus (SLE) is a prototypic autoimmune disease characterized by the production of autoantibodies and the development of immune complex glomerulonephritis. Lupus nephritis (LN) remains a leading cause of morbidity and mortality in SLE. As a result, defining pathogenetic mediators in LN remains a major research effort. Progression to LN in SLE is dependent on the host breaking immune tolerance and forming autoantibodies that deposit in the kidney. A variety of predisposing factors in the host must then be present for this event to result in renal pathology. In this article, the authors review recent reports that advance our understanding of LN disease mediators, from autoantibody production and immune complex deposition to end stage fibrosis.

Neurovascular disease, antioxidants and glycation in diabetes

People with diabetes are ten to fifteen times more likely to have a lower limb amputation (LLA) than non-diabetic individuals. Fifteen percent of people with diabetes will develop a foot ulcer during their lifetime, the rate of major amputation amongst diabetic individuals continues to rise, foot problems remain the commonest reason for diabetes-related hospitalisation and recurrence rates in patients with previous foot ulcers are 50% or more. Hyperglycaemia-induced oxidative stress has been shown to result in decreased nerve conduction velocity, and decreased endoneural blood flow-both precursors for neuropathy. Vitamin antioxidants have been shown to be effective therapy in experimental models in reducing free radical species and inhibiting the oxidative process in diabetes subjects. Little work has been published, however, regarding the dietary use of antioxidants from foods, and their specific effects on neurovascular disease and glycation within the diabetes population. Aetiological and prevention studies with dietary antioxidants from foods aimed at the complex nature of foot problems in diabetes are needed.