Journal of the American College of Cardiology
© 2008 by the American College of Cardiology Foundation
Published by Elsevier Inc.
EDITORIAL COMMENT
NOX5, a New “Radical” Player
in Human Atherosclerosis?*
Eberhard Schulz, MD,
Thomas Münzel, MD, FAHA
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, uncouSee page 1803
pled 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
*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.
From II. Medizinische Klinik und Poliklinik, Kardiologie und Angiologie, Mainz,
Germany. This work was supported by a grant from the Deutsche Forschungsgemeinschaft SCHU 1486/2-1 to Dr. Schulz.
Vol. 52, No. 22, 2008
ISSN 0735-1097/08/$34.00
doi:10.1016/j.jacc.2008.08.040
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
JACC Vol. 52, No. 22, 2008
November 25, 2008:1810–2
Figure 1
Schulz and Münzel
NOX5 and Human Atherosclerosis
1811
Structure of Vascular NADPH Oxidases
The nicotinamide dinucleotide phosphate (NADPH) oxidase (NOX) isoforms being expressed in vascular tissue include NOX1, NOX2, NOX4, and NOX5. The NOX1, NOX2,
and NOX4 isoforms require the subunit p22phox to form functional reactive oxygen species– generating NADPH oxidase enzymes; this is, however, not the case for
NOX5. The NOX1 NADPH oxidase associates with 2 cytosolic factors (p47phox and NOXA1) as well as the small-molecular-weight molecule Rac; NOX2 is regulated by
p47phox and p67phox, whereas NOX4 has at present no regulatory subunits; NOX5 is unique because its activity is regulated by binding to calcium. The oxidizing superoxide (O2·⫺) is generated via an electron shuttle from NADPH to oxygen, which is why one calls it the NADPH oxidase. ER ⫽ endoplasmatic reticulum; VEGF ⫽ vascular
endothelial growth factor; VSMC ⫽ vascular smooth muscle cells.
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 ROSdependent decrease of NO bioavailability; NOX4 is located
in the endoplasmatic reticulum and influences ROSdependent 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 calciumdependent protein kinases.
Taken together, the current study by Guzik et al. (10)
opens a new chapter in NOX5 research by identifying this
1812
Schulz and Münzel
NOX5 and Human Atherosclerosis
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.
Reprint requests and correspondence: Dr. Thomas Münzel, II.
Medizinische Klinik und Poliklinik, Kardiologie und Angiologie,
Langenbeckstrasse 1, 55131 Mainz, Germany. E-mail: tmuenzel@
uni-mainz.de.
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Key Words: superoxide y NADPH oxidase y NOX5.