Author + information
- Eberhard Schulz, MD and
- Thomas Münzel, MD, FAHA⁎ ()
- ↵⁎Reprint requests and correspondence:
Dr. Thomas Münzel, II. Medizinische Klinik und Poliklinik, Kardiologie und Angiologie, Langenbeckstrasse 1, 55131 Mainz, Germany
During the last 20 years, a large body of evidence has defined a causal role for oxidative stress in the development of vascular disease. Reactive oxygen species (ROS) are continuously being produced in the vasculature, and in low doses they even may be essential because they act as important signalling molecules (1). However, known cardiovascular risk factors such as smoking, hypertension, diabetes, and hypercholesterolemia will lead to an increased ROS flux from all layers of the vascular wall, thereby accelerating the atherosclerotic process. So far, numerous ROS sources in the vasculature have been identified, including the reduced nicotinamide dinucleotide phosphate (NADPH) oxidase, mitochondria, xanthine oxidase, uncoupled endothelial nitric oxide synthase (eNOS), and cytochrome P450 (2). Among all potential ROS sources, the NADPH oxidase has, since the early 1990s, attracted particular interest. Historically, this oxidase was discovered as a part of the neutrophil bactericidal response, when it produces large amounts of superoxide anions (3). Its absence is associated with chronic granulomatous disease, a condition that does not allow proper elimination of pathogen bacteria. In the atherosclerotic process, invasion of inflammatory cells is a common feature and contributes to increased ROS flux. However, with the discovery of vascular isoforms of the neutrophil NADPH oxidase (4), a new era in the field of vascular oxidative stress research began.
The extraordinary importance of the NADPH oxidase regarding vascular oxidative stress is underlined by two observations. First, vascular NADPH oxidases were found to contribute largely to the overall vascular ROS production in vivo (5). Second, other potential ROS sources such as the uncoupling of eNOS or xanthine oxidase require a primary ROS generating system, and NADPH oxidases may serve to produce these “kindling radicals” (6,7).
The majority of ROS is produced by single electron transfer from NADPH to molecular oxygen (that is why one calls it NADPH oxidase), resulting in superoxide anion formation. The latter can be transformed into hydrogen peroxide by endogenous superoxide dismutase or spontaneous dismutation. Vascular damage by increased ROS can be caused by several mechanisms: superoxide anions react rapidly with nitric oxide, leading to the degradation of this vascular protective molecule and simultaneously to the formation of the highly reactive intermediate peroxynitrite, which was recently identified as being responsible for eNOS uncoupling (8). Moreover, ROS will lead to increased expression of transcription factors, inflammatory cytokines, and adhesion molecules; they promote smooth muscle proliferation and migration and, therefore, initiate and perpetuate the atherosclerotic process.
Although the NADPH oxidase (NOX) isoform NOX4 is expressed constitutively, all other NOX isoforms require activation by an isoform-specific assembly of different subunits. As the prototype of all NADPH oxidases, NOX2 consists of 2 membrane-associated proteins, NOX2 and p22phox, whereas on activation the cytosolic components p47phox, p67phox, and rac-GTPase are translocated to the membrane to form a complete, superoxide anion generating NADPH oxidase complex. So far, at least 7 different NOX isoforms have been described (NOX1-5, DUOX 1/2), and so far 4 of them have been found in the vascular wall (NOX1, NOX2, NOX4, NOX5), whereas their expression pattern is different in each layer of the vascular wall (Fig. 1) (9).
In this issue of the Journal, Guzik et al. (10) extend our knowledge about the expression and distribution of NOX5 in vascular tissue. So far, the characterization of NOX5 has been hampered by the fact that rodents lack the NOX5 gene. It is known that NOX5 is a calcium-dependent NOX isoform that does not require other subunits for its activation. In their current work, Guzik et al. (10) studied the expression of NOX5 in human coronary arteries from explanted hearts and in endothelial cells from human origin. They found that NOX5 is expressed predominantly in the healthy endothelium, whereas its expression was sharply increased in atherosclerotic vessels. In atherosclerosis, NOX5-dependent ROS production largely contributed to the overall NADPH oxidase activity, and interestingly, its expression is considerably increased in smooth muscle adjacent to atherosclerotic plaques. Recent studies also indicate that NOX5 can directly activate eNOS, all of which may be considered as a compensatory response to increased superoxide production (11), ultimately leading to enhanced peroxynitrite formation and therefore to endothelial dysfunction (12).
Despite these intriguing observations, several issues remain unresolved. First, it is unclear whether the observed increase in NOX5 expression causally contributes to vascular pathology or is just an epiphenomenon. The large contribution to the overall NADPH-oxidase–triggered ROS production in the current work and its localization in the endothelium might support a causal role for NOX5 in the atherosclerotic process. It is tempting to speculate that NOX5 might affect plaque rupture, given its increased expression in areas close to atherosclerotic plaques. We know today that ROS generated by NADPH oxidases participate in diverse signalling events, and therefore, their vascular and subcellular location plays a crucial role regarding the function of different NOX isoforms. For example, endothelial NOX2 is known to account for a ROS-dependent decrease of NO bioavailability; NOX4 is located in the endoplasmatic reticulum and influences ROS-dependent signalling processes such as regulation of phosphatases (1). Also the kind of ROS that are produced by NADPH oxidases, in particular superoxide anion versus hydrogen peroxide, depends on subcellular location and individual NOX protein conformation. It is still a matter of debate whether hydrogen peroxide is directly produced by the NADPH oxidase or is just a consequence of rapid dismutation of superoxide anion (by superoxide dismutase or spontaneously). This dismutation might also relate to positively charged heme groups in the NOX5 protein, which retain the superoxide anion until its dismutation to hydrogen peroxide occurs. Nevertheless, the predominant production of hydrogen peroxide has been described for NOX4 and seems to prevail also in NOX5-dependent ROS generation. To understand the cellular function of NOX5, information about its subcellular expression pattern will be helpful.
The calcium-dependent activation of NOX5 is unique for all vascular NADPH oxidases and might point to a specific function. Also, this property of NOX5 may allow pharmacological interventions, because calcium antagonists are available and might be a suitable tool for modulating NOX5-dependent ROS production. However, this aspect is not as straightforward as it seems because: 1) endothelial cells lack voltage-dependent calcium channels and are therefore not susceptible to a therapy with calcium-antagonists such as amlodipine; and 2) even if NOX5 expression occurs in smooth muscle cells during atherosclerosis, calcium antagonists may also affect the activity of other NOX isoforms, for example, by decreased activation of calcium-dependent protein kinases.
Taken together, the current study by Guzik et al. (10) opens a new chapter in NOX5 research by identifying this oxidase as a new “radical player.” Unfortunately, interventional studies with murine knockout models are not feasible given the natural lack of the NOX5 gene in rodents. Therefore, future investigations will be complicated by the choice of an adequate animal model or the use of human tissue. Because NOX5 is the only calcium-dependent vascular NOX isoform so far, exciting studies with the use of calcium antagonists are warranted and will reveal whether this approach can be used to modify NOX5 activity, with all of the caveats mentioned herein.
This work was supported by a grant from the Deutsche Forschungsgemeinschaft SCHU 1486/2-1 to Dr. Schulz.
↵⁎ Editorials published in the Journal of the American College of Cardiology reflect the views of the authors and do not necessarily represent the views of JACC or the American College of Cardiology.
- American College of Cardiology Foundation
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