Department of Internal Medicine, Division of Pulmonary and Critical Care Medicine, University of Michigan, Ann Arbor, Michigan 48109
SINCE GERSCHMAN AND
COLLEAGUES (4) first proposed in 1954 that hyperoxic
injury could be attributed to the formation of oxygen free radicals,
our understanding of the physiology of oxygen and its reduced
metabolites, commonly referred to as reactive oxygen species (ROS), has
evolved to incorporate new conceptual paradigms. Although the injurious
effects of ROS cannot be disputed and may, in fact, contribute to the
pathophysiology of many human diseases as well as aging, it is the
paradoxical concept that these same reactive biomolecules participate
in cell signaling that has become the subject of more recent
investigations (reviewed in Ref. 10). Identification and
characterization of specialized plasma membrane ROS-generating
oxidases, similar to the well-characterized phagocytic NADPH oxidase,
in diverse mammalian cells and tissues further support this
"radical" concept (5).
In the study by Parinandi and colleagues, the current article
in focus (Ref. 7, see p. L26 in this
issue), the authors provide evidence that formation of ROS by
cultured vascular endothelial cells exposed to hyperoxia results from
activation of a flavin-dependent NAD(P)H oxidase similar to the
phagocytic NADPH oxidase. The mitochondrion, a well-recognized cellular
source of ROS under hyperoxic conditions (3), appears to
be less important in this system. All of the protein subunits of the
multicomponent NADPH oxidase except one of the regulatory subunits,
p40PHOX, are expressed in these cells, and
antisense targeting of the transmembrane p22PHOX
subunit suppresses hyperoxia-induced ROS production. Moreover, these
investigators provide evidence that the activity of this vascular
NAD(P)H oxidase is regulated by the members of the MAPK family of
protein kinases.
Exposure of cells to hyperoxia can result in ROS production by both
enzymatic and nonenzymatic mechanisms (2). Generation of
ROS in mitochondria is generally regarded as a "side effect" of
cellular respiration related to "leaks" in the electron transport chain. Such unintended reactions may also potentially occur with other
electron-transferring enzymatic systems, particularly when steady-state
concentrations of O2 are elevated. Moreover, it is conceivable that specific ROS-generating enzymes, depending on their
respective Km, may generate higher
concentrations of ROS under hyperoxic conditions. ROS production by
such mechanisms are generally regarded as nonpurposeful and merely a
reflection of cellular metabolic activity in the setting of supranormal
levels of O2. The generation of ROS by these mechanisms is
also likely to be indiscriminant in nature with the potential to damage
cells, as suggested by Gerschman et al. (4) in their
seminal paper on "oxygen poisoning." It is difficult to envision
that such unregulated ROS production could function in cell signaling.
The findings of Parinandi and colleagues (7), however,
suggest that generation of ROS by a plasma membrane-associated NAD(P)H oxidase(s) may have a signaling/regulatory role(s) in hyperoxic responses of the vascular endothelium. This postulate is based on two
important differences between this and other potential mechanisms of
more indiscriminant ROS production in hyperoxia. First, the authors
provide convincing evidence that NAD(P)H oxidase-dependent ROS
generation is regulated. Specifically, hyperoxia-induced
activation of the p38 MAPK is required for oxidase activation. This
type of regulated ROS production by NAD(P)H oxidases occurs in other nonphagocytic cells stimulated with specific growth factors/cytokines (9, 10). Second, the generation of ROS is localized to the plasma membrane where its close proximity to, or association with, other signaling molecules may serve to target specific redox-sensitive proteins that function as part of a large submembranous signaling complex (6, 8). The specific signaling pathways and the cellular responses mediated by hyperoxia-induced NAD(P)H oxidase activation and ROS production are currently unknown. Although speculative, such responses might include an adaptive program that
would favor either cell survival by upregulation of antioxidant defenses or, perhaps under more stressful conditions, apoptotic cell death.
One might question why a cell that is already under hyperoxic oxidative
stress would want to make more ROS. If cell signaling to mediate an
adaptive or apoptotic response is required, could this not be
accomplished by nonredox-regulated mechanisms? And, if redox signaling
pathways are required, why are the ROS already present (in hyperoxia)
not sufficient to activate them? Although the answers to these
questions may not be readily apparent, there may be some logical
explanations. First, the utility of ROS to mediate signaling responses
in hyperoxia may simply be a way of using the toxic molecules
themselves to mediate adaptive or protective responses, as in the
activation of the hydrogen peroxide-sensitive transcription factor OxyR
in bacteria (1). Moreover, this might allow for a more
graded physiological response to the severity of oxidative stress.
Second, although the cellular production of ROS is expected to be high
in hyperoxia, precise localization and regulated generation may be
essential for signaling. This concept of compartmentalized signaling
has largely been disregarded by investigators in the field of redox
signaling but is a well-established principle in conventional signal
transduction (8). Thus specificity conferred by spatial
and temporal restraints on the highly reactive nature of ROS will have
to be better understood before their roles in cell signaling (vs.
injury) can be fully appreciated. Although we await more studies to
address these issues, it is not inconceivable that generation of ROS,
albeit from different sources and by different mechanisms, might
simultaneously mediate both injury and signaling in cells exposed to an
oxidative stress such as hyperoxia.
ARTICLE
TOP
ARTICLE
REFERENCES
![]() |
ACKNOWLEDGEMENTS |
---|
Supported by National Heart, Lung, and Blood Institute Grant HL-67967.
![]() |
FOOTNOTES |
---|
Address for reprint requests and other correspondence: V. J. Thannickal, Univ. of Michigan Medical School, Div. of Pulmonary and Critical Care Medicine, 6301 MSRB III, 1150 W. Medical Center Dr., Ann Arbor, MI 48109 (E-mail: vjt{at}umich.edu).
10.1152/ajplung.00279.2002
![]() |
REFERENCES |
---|
![]() ![]() ![]() |
---|
1.
Christman, MF,
Morgan RW,
Jacobson FS,
and
Ames BN.
Positive control of a regulon for defenses against oxidative stress and some heat-shock proteins in Salmonella typhimurium.
Cell
41:
753-762,
1985[ISI][Medline].
2.
Freeman, BA,
and
Crapo JD.
Biology of disease: free radicals and tissue injury.
Lab Invest
47:
412-426,
1982[ISI][Medline].
3.
Freeman, BA,
and
Crapo JD.
Hyperoxia increases oxygen radical production in rat lungs and lung mitochondria.
J Biol Chem
256:
10986-10992,
1981
4.
Gerschman, R,
Gilbert D,
Nye SW,
Dwyer P,
and
Fenn WO.
Oxygen poisoning and X-irradiation: a mechanism in common.
Science
119:
236-239,
1954.
5.
Lambeth, JD,
Cheng G,
Arnold RS,
and
Edens WA.
Novel homologs of gp91phox.
Trends Biochem Sci
25:
459-461,
2000[ISI][Medline].
6.
Meng, TC,
Fukada T,
and
Tonks NK.
Reversible oxidation and inactivation of protein tyrosine phosphatases in vivo.
Mol Cell
9:
387-399,
2002[ISI][Medline].
7.
Parinandi, NL,
Kleinberg MA,
Usatyuk PV,
Cummings RJ,
Pennathur A,
Cardounel AJ,
Zweier JL,
Garcia JGN,
and
Natarajan V.
Hyperoxia-induced NAD(P)H oxidase activation and regulation by MAP kinases in human lung endothelial cells.
Am J Physiol Lung Cell Mol Physiol
284:
L26-L38,
2003.
8.
Smith, FD,
and
Scott JD.
Signaling complexes: junctions on the intracellular information super highway.
Curr Biol
12:
R32-R40,
2002[ISI][Medline].
9.
Thannickal, VJ,
Aldweib KD,
and
Fanburg BL.
Tyrosine phosphorylation regulates H2O2 production in lung fibroblasts stimulated by transforming growth factor beta1.
J Biol Chem
273:
23611-23615,
1998
10.
Thannickal, VJ,
and
Fanburg BL.
Reactive oxygen species in cell signaling.
Am J Physiol Lung Cell Mol Physiol
279:
L1005-L1028,
2000