IUBMB Life, 52: 7– 16, 2001 Copyright c ° 2001 IUBMB 1521-6543/01 $12.00 + .00 Critical Review Cell Compartmentalization in Redox Signaling Giovambattista Pani, Barbara Bedogni, Renata Colavitti, Rosanna Anzevino, Silvia Borrello, and Tommaso Galeotti Institute of General Pathology, Catholic University Medical School, Rome, Italy Summary From a growing body of evidence on the role of Reactive Oxy- gen Species as intracellular signaling molecules, the concept starts to emerge that cell responses to redox changes are function of the intracellular site where oxidants are produced and/or meet their molecular targets. In particular, a major distinction between oxida- tive events in the cytosolic versus the mitochondrial compartment appears to exist in terms of physiological stimuli, signaling mecha- nisms and functional consequences. Experimental data supporting this view are reviewed here, and the potential implications of this new perspective in redox signaling are discussed. IUBMB Life, 52: 7 – 16, 2001 Keywords Apoptosis; mitochondria; MnSOD; oncogenes; Rac-1; re- active oxygen species; signal transduction. INTRODUCTION For many decades after their discovery in biological systems, Reactive Oxygen Species have been generally considered as harmful by-products of oxidative metabolism, responsible for chronic cell damage and tissue deterioration (1). Along this line of thinking, the concept of “Oxygen paradox” (2) was intro- duced to underline the two-edged effect of this element, neces- sary for the production of energy through mitochondrial respi- ration but paradoxically dangerous for the deleterious effects of its reactive intermediates on nearly all the biological molecules. This widespread point of view, which found its most important expression in the “Free radical theory of aging,” accepted one major exception: oxygen species, although in general toxic and undesirable, turned extremely useful in killing microorganisms, allowing the body to resist and overcome bacterial and fungal infections. Unlike reactive species generated in mitochondria during respiration and likely responsible of tissue aging, oxy- Received 3 August 2001; accepted 4 August 2001. Address correspondence to Tommaso Galeotti, Catholic University, Institute of General Pathology, Largo F. Vito #1, 00168 Rome, Italy. Fax: 39-06-3386446 . E-mail: Tgaleotti@rm.unicatt.it gen radicals involved in immune response were produced ex- clusively in specialized cells, the professional phagocytes, by plasma membrane-associated, NADPH-dependent enzyme complexes, known as NADPH oxidases (3). It was therefore clear, since early in the history of biological oxidations, that oxygen species may exert both deleterious and benecial ef- fects in physiological settings, and that this may depend not only on the properties of highly specialized cells, but also on the intracellular sites where oxygen species are generated. Since the beginning of the ’90s, new scientic interest to- ward reactive oxygen species has been elicited by a number of observations suggesting for these compound a potential role as messenger intermediates involved in signal transduction (4). Accumulating evidence in support of this view has substan- tially modied the way of viewing oxygen-derived species, from randomly toxic compounds to ne modulators of important cell functions, such as protein phosphorylation and gene transcrip- tion, in response to environmental stimuli (5). Again, however, as signaling molecules, ROS turned out to have diverse and po- tentially opposite effects, for instance, being able to promote cell proliferation and survival under some circumstances (6), and to induce programmed cell death and inhibition of tumor growth in others (7 ). Although this apparent contradiction still waits for denitive explanations, recent evidence suggests that cell com- partmentalization may offer a theoretical framework to decode the complex language of redox signaling; and depict a scenario in which the old dualism between mitochondrial (or deleteri- ous) and membrane-derived (or “benecial”) oxygen species emerges again in relation with signal transduction roles of oxy- gen species in both physiological and pathological settings. NADPH Oxidase and Mitochondrial Respiratory Chain as Major Radical Sources Involved in Signal Transduction The idea that reactive oxygen species may serve as signaling molecules raises the question of how and where oxygen species can be generated in a stimulus-dependent fashion. In search of potential intracellular generators of redox signals, researchers’ attention has initially focused, as one would expect, on the two 7