EDITORIAL FOCUS
The paradox of reactive oxygen species: injury, signaling, or both?

Victor J. Thannickal

Department of Internal Medicine, Division of Pulmonary and Critical Care Medicine, University of Michigan, Ann Arbor, Michigan 48109


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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.


    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


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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.

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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[Abstract/Free Full Text].

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Am J Physiol Lung Cell Mol Physiol 284(1):L24-L25
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