NADPH oxidase-dependent redox signalling in cardiac hypertrophy, remodelling and failure

Markers of increased oxidative stress are known to be elevated following acute myocardial infarction and in the context of chronic left ventricular hypertrophy or heart failure, and their levels may correlate with the degree of contractile dysfunction or cardiac deficit. An obvious pathological mechanism that may account for this correlation is the potential deleterious effects of increased oxidative stress through the induction of cellular dysfunction, energetic deficit or cell death. However, reactive oxygen species have several much more subtle effects in the remodelling or failing heart that involve specific redox-regulated modulation of signalling pathways and gene expression. Such redox-sensitive regulation appears to play important roles in the development of several components of the phenotype of the failing heart, for example cardiomyocyte hypertrophy, interstitial fibrosis and chamber remodelling. In this article, we review the evidence supporting the involvement of reactive oxygen species and redox signalling pathways in the development of cardiac hypertrophy and heart failure, with a particular focus on the NADPH oxidase family of superoxide-generating enzymes which appear to be especially important in redox signalling.     

NADPH oxidases and angiotensin II receptor signaling

Over the last decade many studies have demonstrated the importance of reactive oxygen species (ROS) production by NADPH oxidases in angiotensin II (Ang II) signaling, as well as a role for ROS in the development of different diseases in which Ang II is a central component. In this review, we summarize the mechanism of activation of NADPH oxidases by Ang II and describe the molecular targets of ROS in Ang II signaling in the vasculature, kidney and brain. We also discuss the effects of genetic manipulation of NADPH oxidase function on the physiology and pathophysiology of the renin–angiotensin system.     

Mechanisms of activation of NADPH oxidases

The members of the NOX family of enzymes are expressed in a variety of tissues and serve a number of functions. There is a high level of conservation of primary protein sequence, as well as functional features, although specialized responses are beginning to emerge. In this context, our data demonstrate that the NOX1 cytoplasmic domains interact efficiently with the cytoplasmic subunits of the phagocyte NADPH oxidase and identify the second cytoplasmic loop of NOX electron transporters as a crucial domain for enzyme function. Studies of cytosolic co-factors showed that the C-terminal cytoplasmic domain of NOX1 was absolutely required for activation with NOXO1 and NOXA1 and that this activity required interaction of the putative NADPH-binding region of this domain with NOXA1. Finally, we have provided the first example of how alternative splicing of a NOX co-factor may be involved in the regulation of NADPH oxidase function.     

Activation of antibacterial autophagy by NADPH oxidases

Autophagy plays an important role in immunity to microbial pathogens. The autophagy system can target bacteria in phagosomes, promoting phagosome maturation and preventing pathogen escape into the cytosol. Recently, Toll-like receptor (TLR) signaling from phagosomes was found to initiate their targeting by the autophagy system, but the mechanism by which TLR signaling activates autophagy is unclear. Here we show that autophagy targeting of phagosomes is not exclusive to those containing TLR ligands. Engagement of either TLRs or the Fcγ receptors (FcγRs) during phagocytosis induced recruitment of the autophagy protein LC3 to phagosomes with similar kinetics. Both receptors are known to activate the NOX2 NADPH oxidase, which plays a central role in microbial killing by phagocytes through the generation of reactive oxygen species (ROS). We found that NOX2-generated ROS are necessary for LC3 recruitment to phagosomes. Antibacterial autophagy in human epithelial cells, which do not express NOX2, was also dependent on ROS generation. These data reveal a coupling of oxidative and nonoxidative killing activities of the NOX2 NADPH oxidase in phagocytes through autophagy. Furthermore, our results suggest a general role for members of the NOX family in regulating autophagy.     

Vascular NADPH oxidases: molecular mechanisms of activation

Oxygen-derived free radicals are thought to contribute to the initiation and progression of cardiovascular disease via several different mechanisms, such as consumption of nitric oxide, oxidation of proteins and lipids, and activation of redox-sensitive signalling cascades. Vascular NADPH oxidases are important sources of vascular radical formation. The activities of these enzymes, which in some aspects are similar to the leukocyte NADPH oxidase, are controlled on the expression level and complex activation mechanisms. As a plethora of vascular stimuli, such as growth factors, cytokines, physical stimuli, and lipids elicits radical formation by these enzymes, a careful analysis is required for the understanding of the activation of the NADPH oxidases. This article reviews the components of the NADPH oxidases in leukocytes and vascular tissue. Emphasis is put on the activation of the oxidases, including upstream signalling events and molecular modes of interaction between the subunits.     

NADPH oxidases: new kids on the block

Reactive oxygen species (ROS) play a pivotal role in many physiological processes including host defense, hormone biosynthesis, fertilization and cellular signaling. Altered production of ROS has been implicated in the development of immunodeficiency, hypothyroidism and cardiovascular pathologies. In the last few years, several enzymes were identified at the molecular level, which are now thought to be responsible for ROS production observed in diverse tissues. These enzymes show a high degree of homology to the phagocytic NADPH oxidase and are now designated the Nox family of NADPH oxidases. This review updates our knowledge on six new members of the Nox family: Nox1, Nox3, Nox4, Nox5, Duox1 and Duox2.     

Extracellular matrix and cardiac remodelling

Cardiac remodelling associated with primitive and secondary cardiomyopathy is generally associated with changes in the expression in extracellular matrix (ECM) proteins as well as their transmembrane receptors, the integrins. It emerges now that the ECM provides a structural, chemical, and mechanical substrate that is essential in cardiac function and responses to pathophysiological signals. This review will describe the various elements of the ECM, its modifications that are associated with cardiac hypertrophy and heart failure, and the molecular basis bringing a better insight into the dynamics of the ECM.     

Tissue distribution and putative physiological function of NOX family NADPH oxidases

The NOX family of ROS-generating NADPH oxidases consists of 7 members: NOX1 to NOX5, DUOX1 and 2. NOX1 is predominantly found in the colon, where it possibly plays a role in the host defense. NOX2 is the phagocyte NADPH oxidase, a clearly established host defense enzyme. NOX3 is almost exclusively expressed in the inner ear, where it is involved in otoconia morphogenesis, but based on its localization might also play a role in the auditory system. NOX4, widely expressed in kidney, vascular cells, osteoclasts etc.; it might be a constitutively active enzyme, regulated on the level of gene expression but its precise physiological function remains unknown. NOX5, a Ca2+ activated enzyme is predominantly expressed in lymphoid tissues and testis, where it might be involved in signaling processes. DUOX1 is expressed in the thyroid and in respiratory epithelia, and DUOX2 in the thyroid and in gastrointestinal glandular epithelia. Both DUOX enzymes are involved in thyroid hormone synthesis, but possibly also in epithelial host defense.     

NADPH氧化酶在缺血性卒中后氧化应激中的作用

氧化应激在脑缺血再灌注损伤中起着十分重要的作用.NADPH氧化酶(NADPH oxi-dase,NOX)在缺血性卒中氧化应激中的作用日益受到关注,有可能会成为一个新的治疗靶点.文章归纳了NOX与脑缺血再灌注损伤的主要研究进展,讨论了各种同工酶的特点及其与卒中的关系,从NOX角度进一步探讨卒中的发病机制,为缺血性卒中的防治提供了新的思路.     

Regulation of NADPH oxidases: the role of Rac proteins

The role for reactive oxygen species (ROS) in cellular (patho)physiology, in particular in signal transduction, is increasingly recognized. The family of NADPH oxidases (NOXes) plays an important role in the production of ROS in response to receptor agonists such as growth factors or inflammatory cytokines that signal through the Rho-like small GTPases Rac1 or Rac2. The phagocyte oxidase (gp91phox/NOX2) is the best characterized family member, and its mode of activation is relatively well understood. Recent work has uncovered novel and increasingly complex modes of control of the NOX2-related proteins. Some of these, including NOX2, have been implicated in various aspects of (cardio)vascular disease, including vascular smooth muscle and endothelial cell hypertrophy and proliferation, inflammation, and atherosclerosis. This review focuses on the role of the Rac1 and Rac2 GTPases in the activation of the various NOX family members.     

Expression of mRNA for ROS-generating NADPH oxidases in the aging stomach

Oxidative damage is thought to play a key role in the aging of various organ systems. In this study, we have therefore analyzed mRNA expression of ROS-generating NADPH oxidases in the aging stomach. Gastric biopsies of hospitalized geriatric patients were analyzed for histology (Sidney classification), and real-time PCR was used to quantify mRNA expression of the superoxide-generating NADPH oxidases NOX1, NOX2, and NOX5. We found that stomach biopsies of elderly patients expressed NOX5 and NOX2 mRNA, but not NOX1. The mRNA expression of NOX5 (a lymphocyte NADPH oxidase) neither depended on age nor on the results of the stomach histology. In contrast, mRNA expression of NOX2 (phagocyte NADPH oxidase) was a function of two variables. Increased NOX2 mRNA levels were observed in biopsies with signs of chronic inflammation (p=0.01). Interestingly, however, there was also an age-dependent increase in NOX2 mRNA levels (p=0.01). We conclude that in elderly patients the gastric mRNA expression of the ROS-generating enzyme NOX2 increases as a function of age, possibly contributing to stomach aging and gastric vulnerability of the elderly.     

NADPH oxidase signaling and cardiac myocyte function

The NADPH oxidase family of enzymes has emerged as a major source of reactive oxygen species (ROS) that is important in diverse cellular functions including anti-microbial defence, inflammation and redox signaling. Of the five known NADPH oxidase isoforms, several are expressed in cardiovascular cells where they are involved in physiological and pathological processes such as the regulation of vascular tone, cell growth, migration, proliferation, hypertrophy, apoptosis and matrix deposition. This article reviews current knowledge regarding the role of NADPH oxidases in cardiomyocyte function in health and disease.     

Apocynin is not an inhibitor of vascular NADPH oxidases but an antioxidant

A large body of literature suggest that vascular reduced nicotinamide-adenine dinucleotide phosphate (NADPH) oxidases are important sources of reactive oxygen species. Many studies, however, relied on data obtained with the inhibitor apocynin (4'-hydroxy-3'methoxyacetophenone). Because the mode of action of apocynin, however, is elusive, we determined its mechanism of inhibition on vascular NADPH oxidases. In HEK293 cells overexpressing NADPH oxidase isoforms (Nox1, Nox2, or Nox4), apocynin failed to inhibit superoxide anion generation detected by lucigenin chemiluminescence. In contrast, apocynin interfered with the detection of reactive oxygen species in assay systems selective for hydrogen peroxide or hydroxyl radicals. Importantly, apocynin interfered directly with the detection of peroxides but not superoxide, if generated by xanthine/xanthine oxidase or nonenzymatic systems. In leukocytes, apocynin is a prodrug that is activated by myeloperoxidase, a process that results in the formation of apocynin dimers. Endothelial cells and smooth muscle cells failed to form these dimers and, therefore, are not able to activate apocynin. Dimer formation was, however, observed in Nox-overexpressing HEK293 cells when myeloperoxidase was supplemented. As a consequence, apocynin should only inhibit NADPH oxidase in leukocytes, whereas in vascular cells, the compound could act as an antioxidant. Indeed, in vascular smooth muscle cells, the activation of the redox-sensitive kinases p38-mitogen-activate protein kinase, Akt, and extracellular signal-regulated kinase 1/2 by hydrogen peroxide and by the intracellular radical generator menadione was prevented in the presence of apocynin. These observations indicate that apocynin predominantly acts as an antioxidant in endothelial cells and vascular smooth muscle cells and should not be used as an NADPH oxidase inhibitor in vascular systems.     

NADPH oxidases in cardiovascular disease: insights from in vivo models and clinical studies

NADPH oxidase family enzymes (or NOXs) are the major sources of reactive oxygen species (ROS) that are implicated in the pathophysiology of many cardiovascular diseases. These enzymes appear to be especially important in the modulation of redox-sensitive signalling pathways that underlie key cellular functions such as growth, differentiation, migration and proliferation. Seven distinct members of the family have been identified of which four (namely NOX1, 2, 4 and 5) may have cardiovascular functions. In this article, we review our current understanding of the roles of NOX enzymes in several common cardiovascular disease states, with a focus on data from genetic studies and clinical data where available.