Karen Bedard and Karl-Heinz Krause

268 KAREN BEDARD AND KARL-HEINZ KRAUSE
in cell proliferation, and the enzyme was therefore even
referred to as “mitogenic oxidase 1” (841). It was suggested
that hydrogen peroxide mediates the cell growth
and transformation caused by the Nox1 (35). Later, however,
the authors of these studies detected the presence of
V12 RAS in their cell lines, suggested that the observed
transformation was probably due to RAS, and cautioned
against the use of these cells (504). Indeed, NOX expression
in other fibroblasts failed to produce transformation
(504). Nevertheless, there is now a significant number of
studies suggesting an involvement of NOX enzymes in cell
proliferation. In vitro studies based on either antisense or
siRNA suppression suggest a role of NOX4 and NOX1 in
smooth muscle cell proliferation (602, 696, 836), a role of
NOX5 in proliferation of esophageal adenocarcinoma
cells (274), and a role for p22 phox in proliferation of endothelial
cells (67). Note, however, that angiotensin IIinduced
aortic smooth muscle proliferation was conserved
in NOX1-deficient mice (291).
Thus a critical review of the literature concerning the
relationship between NOX enzymes and cell proliferation
suggests that 1) there is abundant evidence for a regulation
of cell proliferation in vitro through reactive oxygen
species, 2) in vitro knock-down experiments argue in
favor of NOX enzymes being involved in the regulation of
cell proliferation, and 3) there is so far no convincing data
from knockout mice suggesting that NOX enzymes play a
crucial role for cell proliferation in vivo.
F. Oxygen Sensing
Probably all of our cells are capable of sensing the
ambient oxygen concentration and responding to hypoxia.
However, some organs are specialized in oxygen
sensing, particularly the kidney cortex, the carotid bodies,
and the pulmonary neuroepithelial bodies. At least two
cellular events allow cells to detect hypoxia (Fig. 6):
stabilization of the transcription factor HIF (396) and
activation of redox-sensitive K � channels (519, 713). In
the case of HIF, under normoxic conditions, HIF prolyl
hydroxylases mediate HIF hydroxylation at specific prolines
and thereby promote its rapid degradation (8, 113).
Under hypoxic conditions, this process is inhibited leading
to stabilization of the HIF protein. While the hydroxylase
is undoubtedly a directly oxygen-dependent enzyme,
there is good evidence that increased ROS generation
under hypoxic conditions can also contribute to HIF
stabilization. The ROS effects may be mediated through
oxidation of divalent iron, which is an obligatory cofactor
for the hydroxylase. In the case of K � channels, normoxia
is thought to maintain normal activity, while hypoxia
inactivates K � channels and thereby leads to cellular
depolarization. A well-documented mechanism of K �
channel inactivation during hypoxia involves decreased
Physiol Rev VOL 87 JANUARY 2007 www.prv.org
FIG. 6. Reactive oxygen species (ROS), NOX enzymes, and oxygen
sensing: revised model based on recent findings. At least two cellular
events allow cells to detect hypoxia: stabilization of the transcription
factor HIF and activation of redox-sensitive K � channels. Under normoxic
conditions, HIF is hydroxylated, which leads to its rapid degradation.
Under hypoxic conditions, this process is inhibited leading to
stabilization of the HIF protein. While the HIF hydroxylase is undoubtedly
a directly oxygen-dependent enzyme, there is also good evidence
that increased ROS generation under hypoxic conditions contributes to
HIF stabilization, possibly mediated through oxidation of the hydroxylase
cofactor Fe 2� . Under normoxic conditions redox-sensitive K � channels
are active, maintaining the cellular resting membrane potential.
Hypoxia inactivation of K � channels leads to cellular depolarization.
Pathways leading to K � channel inactivation include hemoxygenasedependent
CO generation, but possibly also ROS. Traditionally, NOX
enzymes were thought to be involved in oxygen sensing through a
decreased ROS generation in hypoxic tissues. However, many recent
results led to a revised model where hypoxia increases ROS generation.
The source of the hypoxia-induced ROS might be mitochondria and/or
NOX enzymes. The physiological effects of ROS, namely, inhibition of
K � channels and stabilization of HIF, are best compatible with this
revised model; however, the reasons why hypoxic cells generate more
ROS are still poorly understood.
carbon monoxide (CO) generation (388). Under normoxic
conditions, CO is generated by hemoxygenase through an
oxygen- and P-450 reductase-dependent breakdown of
heme. However, the hemoxygenase pathway of K � channel
inhibition is not exclusive, and there are good arguments
that ROS-dependent channel inhibition is also involved.
Traditionally NOX enzymes were thought to be involved
in oxygen sensing through a decreased ROS generation
in hypoxic tissues. ROS generation by NOX enzymes
depends on the concentration of its electron acceptor,
i.e., oxygen. Indeed, when NOX2-dependent
respiratory burst is measured at oxygen concentrations
below 1%, there is a steep drop in ROS generation by
NOX2 (281). A reduction in ROS generation during hypoxia
is also observed in isolated perfused lungs (31, 681).
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