Glucose-6-phosphate dehydrogenase overexpression decreases endothelial cell oxidant stress and increases bioavailable nitric oxide

Glucose-6 Phosphate Dehydrogenase Deficiency Decreases the Vascular Response to Angiotensin II

Background— Glucose-6-phosphate dehydrogenase (G6PD) regulates production of the reduced form of NADPH through the pentose phosphate pathway. G6PD may therefore affect superoxide anion production via vascular NADPH oxidase, which is key in mediating the vascular response to angiotensin II (Ang II). We determined the hypertensive and vascular hypertrophic response to Ang II in G6PD-deficient mice.

Methods and Results— Ang II (0.7 mg/kg per day) was infused via subcutaneous osmotic pumps for 6 days in male hemizygote G6PD mutant (G6PD mut ) and wild-type (WT) C3H mice. (1) Compared with WT, G6PD mut mouse aorta had 10% to 20% of G6PD activity and 50% less NADPH. (2) Basal systolic blood pressure was not significantly different in G6PD mut mice (WT 88±4 mm Hg versus G6PD mut 95±4 mm Hg), but Ang II increased blood pressure to a lower level in G6PD mut mice (WT 139±4 mm Hg versus G6PD mut 123±5 mm Hg; P <0.05). (3) Ang II increased aortic medial thickness less in G6PD mut mice (WT 71±2 μm versus G6PD mut 62±1 μm; P <0.01). (4) 3- o -Nitrotyrosine staining and dihydroethidium oxidation in the aorta was increased by Ang II less in G6PD mut mice. (5) Smooth muscle cells isolated from G6PD mut mice showed less Ang II–induced phosphorylation of Akt and p42/44 ERK.

Conclusions— G6PD deficiency may reduce vascular superoxide anion production by limiting production of the substrate for NADPH oxidase, thereby inhibiting oxidant-mediated Ang II–induced signaling pathways that contribute to hypertension and smooth muscle hypertrophy.

Oxidative Enzymopathies and Vascular Disease

In the vasculature, reactive oxygen species (ROS) generated by both mitochondrial respiration and enzymatic sources serve as integral components of cellular signaling and homeostatic mechanisms. Because ROS are highly reactive biomolecules, the cellular redox milieu is carefully maintained by small-molecule antioxidants and antioxidant enzymes to prevent the deleterious consequences of ROS excess. When this redox balance is perturbed, because of either increased ROS production or decreased antioxidant capacity, oxidant stress is increased in the vessel wall and, if not offset, vascular dysfunction ensues. A number of heritable polymorphisms of pro-oxidant enzymes, including 5-lipoxygenase, cyclooxygenase-2, nitric oxide synthase-3, and NAD(P)H oxidase, have been identified and found to modulate ROS production and, thereby, the risk of atherothrombotic cardiovascular disease in individuals with these genetic polymorphisms. Similarly, heritable deficiency of the antioxidant enzymes catalase, glutathione peroxidases, glutathione- S -transferases, heme oxygenase, and glucose-6-phosphate dehydrogenase favors ROS accumulation, and has been associated with an increased risk of vascular disease. Individually, each of these polymorphisms imposes a state of uncompensated oxidant stress on the vasculature and collectively comprise the oxidative enzymopathies.

Diabetes causes inhibition of glucose-6-phosphate dehydrogenase via activation of PKA, which contributes to oxidative stress in rat kidney cortex

The incidence of diabetic nephropathy has been increasing. Studies have shown that oxidative stress (due to increased oxidant production and/or decreased antioxidant activity) is a critical underlying mechanism. The principal intracellular reductant is NADPH whose production is mainly dependent on glucose-6-phosphate dehydrogenase (G6PD) activity. Our work in cultured cells previously showed that high glucose caused activation of protein kinase A (PKA) and subsequent phosphorylation and inhibition of G6PD activity and hence decreased NADPH (Zhang Z, Apse K, Pang J, and Stanton RC. J Biol Chem 275:40042-40047, 2000). The purpose of this study was to determine whether these findings occur in diabetic rats (induced by streptozotocin) compared with control. G6PD activity and accordingly NADPH levels and glutathione levels were significantly decreased in diabetic kidneys compared with control kidneys. Lipid peroxidation was significantly increased, which correlated with decreased G6PD activity (r = 0.48). G6PD expression was significantly reduced, which correlated with decreased G6PD activity (r = 0.72). PKA activity and serine phosphorylation of G6PD were significantly increased and were closely correlated with decreased G6PD activity (r = 0.51 for PKA activity; r = 0.93 for serine phosphorylation of G6PD). Insulin treatment and/or correction of hyperglycemia ameliorated the changes caused by diabetes. In conclusion, chronic hyperglycemia caused inhibition of G6PD activity via decreased expression and increased phosphorylation of G6PD, which therefore led to increased oxidative stress.

Genetic Determinants of Vascular Oxidant Stress and Endothelial Dysfunction

Reactive oxygen species are products of cellular metabolism and function as normal signaling molecules in the vasculature. Overproduction of reactive oxygen species that outstrips antioxidant defenses generates oxidant stress that can lead to nitric oxide depletion and endothelial cell injury, and contributes to the pathophysiology of cardiovascular disease and myocardial dysfunction. Antioxidant enzymes that are important in limiting vascular oxidant stress include superoxide dismutases, catalase, glutathione peroxidases, and glucose-6-phosphate dehydrogenase. The relevance of deficiencies in glutathione peroxidase and glucose-6-phosphate dehydrogenase on endothelial and myocardial dysfunction will be reviewed.

Role of oxidative modifications in atherosclerosis

This review focuses on the role of oxidative processes in atherosclerosis and its resultant cardiovascular events. There is now a consensus that atherosclerosis represents a state of heightened oxidative stress characterized by lipid and protein oxidation in the vascular wall. The oxidative modification hypothesis of atherosclerosis predicts that low-density lipoprotein (LDL) oxidation is an early event in atherosclerosis and that oxidized LDL contributes to atherogenesis. In support of this hypothesis, oxidized LDL can support foam cell formation in vitro, the lipid in human lesions is substantially oxidized, there is evidence for the presence of oxidized LDL in vivo, oxidized LDL has a number of potentially proatherogenic activities, and several structurally unrelated antioxidants inhibit atherosclerosis in animals. An emerging consensus also underscores the importance in vascular disease of oxidative events in addition to LDL oxidation. These include the production of reactive oxygen and nitrogen species by vascular cells, as well as oxidative modifications contributing to important clinical manifestations of coronary artery disease such as endothelial dysfunction and plaque disruption. Despite these abundant data however, fundamental problems remain with implicating oxidative modification as a (requisite) pathophysiologically important cause for atherosclerosis. These include the poor performance of antioxidant strategies in limiting either atherosclerosis or cardiovascular events from atherosclerosis, and observations in animals that suggest dissociation between atherosclerosis and lipoprotein oxidation. Indeed, it remains to be established that oxidative events are a cause rather than an injurious response to atherogenesis. In this context, inflammation needs to be considered as a primary process of atherosclerosis, and oxidative stress as a secondary event. To address this issue, we have proposed an "oxidative response to inflammation" model as a means of reconciling the response-to-injury and oxidative modification hypotheses of atherosclerosis.

Increased endothelial tetrahydrobiopterin synthesis by targeted transgenic GTP-cyclohydrolase I overexpression reduces endothelial dysfunction and atherosclerosis in ApoE-knockout mice

OBJECTIVE

Increased production of reactive oxygen species and loss of endothelial nitric oxide (NO) bioactivity are key features of vascular disease states such as atherosclerosis. Tetrahydrobiopterin (BH4) is a required cofactor for NO synthesis by endothelial nitric oxide synthase (eNOS); pharmacologic studies suggest that reduced BH4 availability may be an important mediator of endothelial dysfunction in atherosclerosis. We aimed to investigate the importance of endothelial BH4 availability in atherosclerosis using a transgenic mouse model with endothelial-targeted overexpression of the rate-limiting enzyme in BH4 synthesis, GTP-cyclohydrolase I (GTPCH).

METHODS AND RESULTS

Transgenic mice were crossed into an ApoE knockout (ApoE-KO) background and fed a high-fat diet for 16 weeks. Compared with ApoE-KO controls, transgenic mice (ApoE-KO/GCH-Tg) had higher aortic BH4 levels, reduced endothelial superoxide production and eNOS uncoupling, increased cGMP levels, and preserved NO-mediated endothelium dependent vasorelaxations. Furthermore, aortic root atherosclerotic plaque was significantly reduced in ApoE-KO/GCH-Tg mice compared with ApoE-KO controls.

CONCLUSIONS

These findings indicate that BH4 availability is a critical determinant of eNOS regulation in atherosclerosis and is a rational therapeutic target to restore NO-mediated endothelial function and reduce disease progression.

Enhanced survival effect of pyruvate correlates MAPK and NF-kappaB activation in hydrogen peroxide-treated human endothelial cells

We recently reported that pyruvate inhibited translocation and activation of p53 caused by DNA damage due to oxidant injury (Lee YJ, Kang IJ, Bünger R, and Kang YH. Microvasc Res 66: 91-101, 2003); this was associated with increased expression of apoptosis-related bcl-2 and decreased expression of bax gene. This study attempted to delineate possible regulatory sites and mechanisms of antiapoptotic pyruvate, focusing on reactive oxygen species-mediated signaling in a human umbilical vein endothelial cell model. We compared the effects of the cytosolic reductant l-lactate and malate-aspartate shuttle blocker aminooxyacetate, both of which increase cytosolic NADH, on the downstream signaling pathway. Hydrogen peroxide (0.5 mM H2O2) depleted intracellular total glutathione that was prevented by pyruvate but not by l-lactate or aminooxyacetate. Activation of caspase-3 and the cleavage of procaspase-6 and procaspase-7 were strongly inhibited by pyruvate but markedly enhanced by l-lactate and aminooxyacetate, implicating redox-related antiapoptotic mechanisms of pyruvate. Western blot analysis and immunochemical data revealed that H2O2-induced transactivation of nuclear factor-kappaB (NF-kappaB) was also inhibited by pyruvate but not by l-lactate or aminooxyacetate. In addition, H2O2 downregulated extracellular signal-regulated kinase (ERK1/2) and phosphorylated p38 mitogen-activated protein kinase (MAPK), effects that were fully reversed by pyruvate within 2 h. Collectively, these findings indicate that pyruvate can protect cellular glutathione, thus enhancing cellular antioxidant potential, and that enhanced antioxidant potential can desensitize NF-kappaB transactivation due to reactive oxygen species, suggesting possible metabolic redox relations to NF-kappaB. Furthermore, pyruvate blocked the p38 MAPK pathway and activated the ERK pathway in an apparently redox-sensitive manner, which may regulate expression of genes believed to prevent apoptosis and promote cell survival. Thus pyruvate may have therapeutic potential for reducing endothelial dysfunction and improving survival during oxidative stress.

Tetrahydrobiopterin-dependent preservation of nitric oxide-mediated endothelial function in diabetes by targeted transgenic GTP-cyclohydrolase I overexpression

Increased production of reactive oxygen species and loss of endothelial NO bioactivity are key features of vascular disease states such as diabetes mellitus. Tetrahydrobiopterin (BH4) is a required cofactor for eNOS activity; pharmacologic studies suggest that BH4 may mediate some of the adverse effects of diabetes on eNOS function. We have now investigated the importance and mechanisms of BH4 availability in vivo using a novel transgenic mouse model with endothelial-targeted overexpression of the rate-limiting enzyme in BH4 synthesis, guanosine triphosphate-cyclohydrolase I (GTPCH). Transgenic (GCH-Tg) mice demonstrated selective augmentation of endothelial BH4 levels. In WT mice, induction of diabetes with streptozotocin (STZ) increased vascular oxidative stress, resulting in oxidative loss of BH4, forming BH2 and biopterin. Endothelial cell superoxide production in diabetes was increased, and NO-mediated endothelium-dependent vasodilatation was impaired. In diabetic GCH-Tg mice, superoxide production from the endothelium was markedly reduced compared with that of WT mice, endothelial BH4 levels were maintained despite some oxidative loss of BH4, and NO-mediated vasodilatation was preserved. These findings indicate that BH4 is an important mediator of eNOS regulation in diabetes and is a rational therapeutic target to restore NO-mediated endothelial function in diabetes and other vascular disease states.

Glucose-6-phosphate dehydrogenase modulates vascular endothelial growth factor-mediated angiogenesis

Glucose-6-phosphate dehydrogenase (G6PD), the first enzyme of the pentose phosphate pathway, is the principal intracellular source of NADPH. NADPH is utilized as a cofactor by vascular endothelial cell nitric-oxide synthase (eNOS) to generate nitric oxide (NO*). To determine whether G6PD modulates NO*-mediated angiogenesis, we decreased G6PD expression in bovine aortic endothelial cells using an antisense oligodeoxynucleotide to G6PD or increased G6PD expression by adenoviral gene transfer, and we examined vascular endothelial growth factor (VEGF)-stimulated endothelial cell proliferation, migration, and capillary-like tube formation. Deficient G6PD activity was associated with a significant decrease in endothelial cell proliferation, migration, and tube formation, whereas increased G6PD activity promoted these processes. VEGF-stimulated eNOS activity and NO* production were decreased significantly in endothelial cells with deficient G6PD activity and enhanced in G6PD-overexpressing cells. In addition, G6PD-deficient cells demonstrated decreased tyrosine phosphorylation of the VEGF receptor Flk-1/KDR, Akt, and eNOS compared with cells with normal G6PD activity, whereas overexpression of G6PD enhanced phosphorylation of Flk-1/KDR, Akt, and eNOS. In the Pretsch mouse, a murine model of G6PD deficiency, vessel outgrowth from thoracic aorta segments was impaired compared with C3H wild-type mice. In an in vivo Matrigel angiogenesis assay, cell migration into the plugs was inhibited significantly in G6PD-deficient mice compared with wild-type mice, and gene transfer of G6PD restored the wild-type phenotype in G6PD-deficient mice. These findings demonstrate that G6PD modulates angiogenesis and may represent a novel angiogenic regulator.

Glucose-6-phosphate dehydrogenase modulates cytosolic redox status and contractile phenotype in adult cardiomyocytes

Reactive oxygen species (ROS)-mediated cell injury contributes to the pathophysiology of cardiovascular disease and myocardial dysfunction. Protection against ROS requires maintenance of endogenous thiol pools, most importantly, reduced glutathione (GSH), by NADPH. In cardiomyocytes, GSH resides in two separate cellular compartments: the mitochondria and cytosol. Although mitochondrial GSH is maintained largely by transhydrogenase and isocitrate dehydrogenase, the mechanisms responsible for sustaining cytosolic GSH remain unclear. Glucose-6-phosphate dehydrogenase (G6PD) functions as the first and rate-limiting enzyme in the pentose phosphate pathway, responsible for the generation of NADPH in a reaction coupled to the de novo production of cellular ribose. We hypothesized that G6PD is required to maintain cytosolic GSH levels and protect against ROS injury in cardiomyocytes. We found that in adult cardiomyocytes, G6PD activity is rapidly increased in response to cellular oxidative stress, with translocation of G6PD to the cell membrane. Furthermore, inhibition of G6PD depletes cytosolic GSH levels and subsequently results in cardiomyocyte contractile dysfunction through dysregulation of calcium homeostasis. Cardiomyocyte dysfunction was reversed through treatment with either a thiol-repleting agent (L-2-oxothiazolidine-4-carboxylic acid) or antioxidant treatment (Eukarion-134), but not with exogenous ribose. Finally, in a murine model of G6PD deficiency, we demonstrate the development of in vivo adverse structural remodeling and impaired contractile function over time. We, therefore, conclude that G6PD is a critical cytosolic antioxidant enzyme, essential for maintenance of cytosolic redox status in adult cardiomyocytes. Deficiency of G6PD may contribute to cardiac dysfunction through increased susceptibility to free radical injury and impairment of intracellular calcium transport. The full text of this article is available online at http://www.circresaha.org.