Diane E. Handy

Glutathione Peroxidase-1 in Health and Disease: From Molecular Mechanisms to Therapeutic Opportunities

Reactive oxygen species, such as superoxide and hydrogen peroxide, are generated in all cells by mitochondrial and enzymatic sources. Left unchecked, these reactive species can cause oxidative damage to DNA, proteins, and membrane lipids. Glutathione peroxidase-1 (GPx-1) is an intracellular antioxidant enzyme that enzymatically reduces hydrogen peroxide to water to limit its harmful effects. Certain reactive oxygen species, such as hydrogen peroxide, are also essential for growth factor-mediated signal transduction, mitochondrial function, and maintenance of normal thiol redox-balance. Thus, by limiting hydrogen peroxide accumulation, GPx-1 also modulates these processes. This review explores the molecular mechanisms involved in regulating the expression and function of GPx-1, with an emphasis on the role of GPx-1 in modulating cellular oxidant stress and redox-mediated responses. As a selenocysteine-containing enzyme, GPx-1 expression is subject to unique forms of regulation involving the trace mineral selenium and selenocysteine incorporation during translation. In addition, GPx-1 has been implicated in the development and prevention of many common and complex diseases, including cancer and cardiovascular disease. This review discusses the role of GPx-1 in these diseases and speculates on potential future therapies to harness the beneficial effects of this ubiquitous antioxidant enzyme.

Aldosterone impairs vascular reactivity by decreasing glucose-6-phosphate dehydrogenase activity

Hyperaldosteronism is associated with impaired vascular reactivity; however, the mechanisms by which aldosterone promotes endothelial dysfunction remain unknown. Glucose-6-phosphate dehydrogenase (G6PD) modulates vascular function by limiting oxidant stress to preserve bioavailable nitric oxide (NO•). Here we show that aldosterone (10−9–;10−7 mol/l) decreased endothelial G6PD expression and activity in vitro, resulting in increased oxidant stress and decreased NO• levels—similar to what is observed in G6PD-deficient endothelial cells. Aldosterone decreased G6PD expression by increasing expression of the cyclic AMP−response element modulator (CREM) to inhibit cyclic AMP−response element binding protein (CREB)-mediated G6PD transcription. In vivo, infusion of aldosterone decreased vascular G6PD expression and impaired vascular reactivity. These effects were abrogated by spironolactone or vascular gene transfer of G6pd. These findings demonstrate that aldosterone induces a G6PD-deficient phenotype to impair endothelial function; aldosterone antagonism or gene transfer of G6pd improves vascular reactivity by restoring G6PD activity.

Epigenetic Modifications: Basic Mechanisms and Role in Cardiovascular Disease

The term epigenetics was first used to refer to the complex interactions between the genome and the environment that are involved in development and differentiation in higher organisms. Today, this term is used to refer to heritable alterations that are not due to changes in DNA sequence. Rather, epigenetic modifications, or tags, such as DNA methylation and histone modification, alter DNA accessibility and chromatin structure, thereby regulating patterns of gene expression. These processes are crucial to normal development and differentiation of distinct cell lineages in the adult organism. They can be modified by exogenous influences, and as such, they can contribute to or be the result of environmental alterations of phenotype or pathophenotype. Importantly, epigenetic programming has a crucial role in the regulation of pluripotency genes, which become inactivated during differentiation. Here, we review the major mechanisms in epigenetic regulation; highlight the role of stable, long-term epigenetic modifications that involve DNA methylation; and discuss those modifications that are more flexible (short-term) and involve histone modifications, such as methylation and acetylation. We will also discuss the role of nutritional and environmental challenges in generational inheritance and epigenetic modifications, concentrating on examples that relate to complex cardiovascular diseases, and specifically dissect the mechanisms by which homocysteine modifies epigenetic tags. Lastly, we will discuss the possibilities of modifying therapeutically acquired epigenetic tags, summarizing currently available agents and speculating on future directions. Chromatin is the complex of chromosomal DNA associated with proteins in the nucleus (for review, see Campos and Reinberg1). DNA in chromatin is packaged around histone proteins, in units referred to as nucleosomes. A nucleosome has 147 base pairs of DNA associated with an octomeric core of histone proteins, which consists of 2 H3-H4 histone dimers surrounded by 2 H2A-H2B dimers. N-terminal histone tails protrude from nucleosomes into the nuclear lumen. …

Redox Regulation of Mitochondrial Function

Redox-dependent processes influence most cellular functions, such as differentiation, proliferation, and apoptosis. Mitochondria are at the center of these processes, as mitochondria both generate reactive oxygen species (ROS) that drive redox-sensitive events and respond to ROS-mediated changes in the cellular redox state. In this review, we examine the regulation of cellular ROS, their modes of production and removal, and the redox-sensitive targets that are modified by their flux. In particular, we focus on the actions of redox-sensitive targets that alter mitochondrial function and the role of these redox modifications on metabolism, mitochondrial biogenesis, receptor-mediated signaling, and apoptotic pathways. We also consider the role of mitochondria in modulating these pathways, and discuss how redox-dependent events may contribute to pathobiology by altering mitochondrial function.

Models of Experimental Hypertension in Mice

Experimental models of hypertension in various animals are useful in the research of vasoactive mechanisms. Recombinant DNA technology has produced genetically engineered animals, mostly mice, useful in hypertension research. However, the development of hypertensive models in mice is fraught with technical difficulties. We describe here the successful development in mice of two common types of experimental hypertension: the renovascular two-kidney, one clip and mineralocorticoid deoxycorticosterone-salt models. By adapting technology previously used in rats, we succeeded in developing hypertension (defined as systolic pressures higher than 140 mm Hg) in more than 50% of mice so treated. We also adapted the methodology for indirect tail-cuff blood pressure measurements as well as for direct intra-arterial monitoring of blood pressure in conscious, freely moving mice. Application of these techniques in transgenic or gene knockout mice with altered vasoactive hormones or receptors should allow elucidation of the role of the target gene products in various types of hypertension.

Glucose-6-Phosphate Dehydrogenase Modulates Cytosolic Redox Status and Contractile Phenotype in Adult Cardiomyocytes

&NA; —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. (Circ Res. 2003;93:e9‐e16.)

Impaired Angiogenesis in Glutathione Peroxidase-1–Deficient Mice Is Associated With Endothelial Progenitor Cell Dysfunction

Several vascular disease are characterized by elevated levels of reactive oxygen species (ROS). Vascular endothelium is protected from oxidant stress by expressing enzymes such as glutathione peroxidase type 1 (GPx-1). In this study, we investigated the effect of vascular oxidant stress on ischemia-induced neovascularization in a murine model of homozygous deficiency of GPx-1. GPx-1–deficient mice showed impaired revascularization following hindlimb ischemic surgery based on laser Doppler measurements of blood flow and capillary density in adductor muscle. GPx-1–deficient mice also showed an impaired ability to increase endothelial progenitor cell (EPC) levels in response to ischemic injury or subcutaneous administration of vascular endothelial growth factor protein. EPCs isolated from GPx-1–deficient mice showed a reduced ability to neutralize oxidative stress in vitro, which was associated with impaired migration toward vascular endothelial growth factor and increased sensitivity to ROS-induced apoptosis. EPCs isolated from GPx-1–deficient mice were impaired in their ability to promote angiogenesis in wild-type mice, whereas wild-type EPCs were effective in stimulating angiogenesis in GPx-1–deficient mice. These data suggest that EPC dysfunction is a mechanism by which elevated levels of ROS can contribute to vascular disease.

Role of the α2B-Adrenergic Receptor in the Development of Salt-Induced Hypertension

Abstract —Salt sensitivity is a common trait in patients with essential hypertension and seems to have both an inherited and an acquired component (eg, is influenced by aging and renal insufficiency). Experimental evidence suggests that salt loading induces hypertension via a neurogenic mechanism mediated by the α2-adrenergic receptors (α2-AR). To explore the α2-AR subtype involved in this mechanism, we studied 2 groups of mice genetically engineered to be deficient in one of the 3 α2-AR subtype genes (either α2B-AR +/− or α2C-AR −/− knockout mice) compared with their wild-type counterparts. The mice (n=10 to 14 in each group) were submitted to subtotal nephrectomy and given 1% saline as drinking water for up to 35 days. Blood pressure (BP) was monitored by tail-cuff readings and confirmed at the end point by direct intra-arterial BP recording. The α2B-AR–deficient mice had an attenuated BP response in this protocol (baseline 101.8±2.7 versus end point 109.9±2.8 mm Hg), whereas the BP of their wild-type counterparts went from a baseline 101.9±2.3 to an end point 141.4±7.1 mm Hg. The other 2 groups had BP increases of 44.6±5.17 and 46.7±7.01 mm Hg, with no difference between the mice deficient in the α2C-AR gene subtype versus their wild-type counterparts. Body weight, renal remnant weight, and residual renal function were no different among groups. These data suggest that a full complement of α2B-AR genes is necessary to raise BP in response to dietary salt loading, whereas complete absence of the α2C-AR subtype does not preclude salt-induced BP elevation. It is unclear whether the mechanism(s) involved in this process are of central origin (inability to increase sympathetic outflow), vascular origin (inability to vasoconstrict), or renal origin (inability to retain excess salt and fluid).

High glucose inhibits glucose-6-phosphate dehydrogenase, leading to increased oxidative stress and β-cell apoptosis

Patients with type 2 diabetes lose β cells, but the underlying mechanisms are incompletely understood. Glucose‐6‐phosphate dehydrogenase (G6PD) is the principal source of the major intracellular reduc‐tant, NADPH, which is required by many enzymes, including enzymes of the antioxidant pathway. Previous work from our laboratory has shown that high glucose impairs G6PD activity in endothelial and kidney cells, which leads to decreased cell survival. Pancreatic β cells are highly sensitive to increased ROS. This study aimed to determine whether G6PD and NADPH play central roles in β‐cell survival. Human and mouse islets, MIN6 cell line, and G6PD deficient mice were studied. High glucose inhibited G6PD expression and activity. Inhibition of G6PD with siRNA led to increased ROS and apoptosis, decreased proliferation, and impaired insulin secretion. High glucose decreased insulin secretion, which was improved by overexpressing G6PD. G6PD‐deficient mice had smaller islets and impaired glucose tolerance compared with control mice, which suggests that G6PD deficiency per se leads to β‐cell dysfunction and death. G6PD plays an important role in β‐cell function and survival. High‐glucose‐mediated decrease in G6PD activity may provide a mechanistic explanation for the gradual loss of β cells in patients with diabetes.—Zhang, Z., Liew, C. W., Handy, D. E., Zhang, Y., Leopold, J. A., Hu, J., Guo, L., Kulkarni, R. N., Loscalzo, J., Stanton, R. C. High glucose inhibits glucose‐6‐phosphate dehydrogenase, leading to increased oxidative stress and β‐cell apoptosis. FASEB J. 24, 1497–1505 (2010). www.fasebj.org

Promoter Polymorphisms in the Plasma Glutathione Peroxidase (GPx-3) Gene: A Novel Risk Factor for Arterial Ischemic Stroke among Young Adults and Children

Background and Purpose— Plasma glutathione peroxidase (GPx-3)–deficiency increases extracellular oxidant stress, decreases bioavailable nitric oxide, and promotes platelet activation. The aim of this study is to identify polymorphisms in the GPx-3 gene, examine their relationship to arterial ischemic stroke (AIS) in a large series of children and young adults, and determine their functional molecular consequences. Methods— We studied the GPx-3 gene promoter from 123 young adults with idiopathic AIS and 123 age- and gender- matched controls by single-stranded conformational polymorphism and sequencing analysis. A second, independent population with childhood stroke was used for a replication study. We identified 8 novel, strongly linked polymorphisms in the GPx-3 gene promoter that formed 2 main haplotypes (H1 and H2). The transcriptional activity of the 2 most prevalent haplotypes was studied with luciferase reporter gene constructs. Results— The H2 haplotype was over-represented in both patient populations and associated with an independent increase in the risk of AIS in young adults (odds ratio=2.07, 95% CI=1.03 to 4.47; P=0.034) and children (odds ratio=2.13, 95% CI=1.23 to 4.90; P=0.027). In adults simultaneously exposed to vascular risk factors, the risk of AIS approximately doubled (odds ratio=5.18, 95% CI=1.82 to 15.03; P<0.001). Transcriptional activity of the H2 haplotype was lower than that of the H1 haplotype, especially after upregulation by hypoxia (normalized relative luminescence: 3.54±0.32 versus 2.47±0.26; P=0.0083). Conclusion— These findings indicate that a novel GPx-3 promoter haplotype is an independent risk factor for AIS in children and young adults. This haplotype reduces the genes transcriptional activity, thereby compromising gene expression and plasma antioxidant and antithrombotic activities.