Sci. Aging Knowl. Environ., 29 June 2005
Vol. 2005, Issue 26, p. pe20
[DOI: 10.1126/sageke.2005.26.pe20]

PERSPECTIVES

Reactive Oxygen Species and Aging: Evolving Questions

Laura L. Dugan, and Kevin L. Quick

The authors are in the Division of General Medicine and Geriatrics in the Department of Medicine at the University of California, San Diego, La Jolla, CA 92093, USA. E-mail: ladugan{at}ucsd.edu (L.L.D.)

http://sageke.sciencemag.org/cgi/content/full/2005/26/pe20

Key Words: reactive oxygen species • oxidative damage • mitochondria • superoxide dismutase mimetic • calorie restriction • Sir2

Since Harman first proposed the idea in 1951 that reactive oxygen species (ROS) such as superoxide radical (O2{dotminus}) and hydrogen peroxide (H2O2) might be key components of the aging process (1) (see Harman Classic Paper), a large body of literature has emerged that supports this concept (see "The Two Faces of Oxygen"). In organisms ranging from yeast to man, accumulation of oxidized nuclear and mitochondrial DNA (nDNA, mtDNA), proteins, and lipids has been reported to occur during aging (2). Overexpression of endogenous antioxidant enzymes, including superoxide dismutase (SOD) and/or catalase (3, 4), or treatment with small-molecule antioxidant SOD mimetics (see "Wrinkle Treatment for Worms") successfully increases life span in standard multicellular models of aging, including Caenorhabditis elegans (5). On the other hand, elimination of antioxidant defenses within these organisms decreases life span. For example, targeted deletion of the gene encoding Cu/Zn SOD in Drosophila reduces life span by 80% as compared with wild type (6). Thus, there is compelling evidence that aging in lower organisms is modulated by ROS. In addition, calorie restriction, a robust method for extending life span across species (see Masoro Subfield History), appears to decrease ROS production during aging, not only in lower organisms but also in mammals. However, although increased levels of oxidatively modified cell components (7), significant ROS-mediated mitochondrial dysfunction (8, 9), and activation of redox-sensitive signaling pathways (10) have all been observed in aging mammals, these data have been mostly correlative. In fact, in contrast to the situation in C. elegans and Drosophila, life-span extension produced by overexpression of antioxidant enzymes has not, for a variety of potential reasons, been reproduced successfully in mammals.

Although there have been a number of successful attempts at manipulating oxidative stress in lower organisms (resulting in increased life span), there have been a limited number of studies in mice that overexpress various antioxidant systems to see whether these transgenic mice also display increased life span. To date, none of these mouse lines have displayed an "anti-aging" phenotype or increased longevity (a possible exception is discussed below). However, transgenic approaches might have been unsuccessful because of complex and tissue-specific redox regulation of intra- and intercellular signaling that is adversely affected in the transgenic lines. Finally, it may be that, in fact, ROS are minimally involved in the fundamental process of aging in higher organisms but are crucial to many diseases processes that can shorten life span. This role might not manifest significantly in laboratory mice that live in pathogen-free, unstressed conditions but would be a major contributor to aging and longevity in humans and other mammals in their native environment, where exposure to toxins, pathogens, and stress is a daily occurrence (see "Get Wild"). Thus, as our understanding of the genes and pathways that regulate aging increases, the question of whether (and to what extent) ROS contribute to aging in higher organisms such as mammals has come to the forefront.

Exploring the mechanisms that underlie aging in mammals, as opposed to lower organisms, poses a host of challenges. One such challenge is differentiating the impact of disease processes such as cancer, atherosclerotic disease, and inflammatory and rheumatological disorders--which can contribute to functional decline and death--from the aging process. Many of these diseases are clearly caused, at least in part, by free radical-mediated events such as oxidative damage to DNA, proteins, or lipids. Atherosclerosis, with its adverse association with stroke and cardiac ischemia, has been linked to oxidative modification of lipoprotein particles. DNA oxidation can lead to neoplastic transformation of cells and the development of cancer. However, although many of these diseases or conditions are associated with aging, they are not considered part of the aging process per se. Thus, one question that needs to be answered in aging-related research is the following: In complex organisms such as mammals, do ROS regulate life span by their effects on specific disease processes, by mediating the underlying mechanism of aging, or by both paths? That is, could free radicals affect life span if no specific disease occurred? To investigate the impact that ROS have on the fundamental process of aging, one can try to dissociate age-associated diseases from the intrinsic rate of aging. Differentiating between these possibilities can be accomplished partially by examining the effects of antioxidant treatments on the maximum life span of a species rather than mean life span alone, differences that can be revealed in Kaplan-Meier plots, which graph the surviving population of individuals as a function of time. This approach can determine whether only age-associated diseases or both age-associated diseases and rate of aging are being reduced by a treatment (11). In addition, an accepted method for specifically examining the rate of aging is by the Gompertz function, which represents the mortality rate. Both of these methods have been employed to examine the effect of calorie restriction, a treatment suggested to reduce oxidative stress, on life span in lower organisms and rodents (12). Such analyses have found that calorie restriction both reduces age-associated diseases and slows the intrinsic rate of aging (13).

If free radicals are found to modulate aging in mammals, this raises a second question: How do ROS actually regulate the intrinsic aging program? Two prominent theories include cumulative oxidative damage to macromolecules such as DNA, proteins, and lipids (leading to impaired cellular function and eventually to replicative senescence and death) and dysregulation of redox-sensitive signaling pathways (2, 14). It is plausible that both mechanisms contribute to the process of aging, especially in higher organisms. Progressive accumulation of mitochondria with large deletions of mtDNA is observed in older members of species ranging from birds through mammals, including humans. Other forms of oxidative DNA damage have been found to be increased with age, such as 8-oxo-2-deoxyguanosine (see Skinner Review), and have been shown to occur in increased amounts specifically in mtDNA versus nDNA (15). Mitochondria, therefore, may be central to both "theories" because they are a crucial source of ROS in aging (see Nicholls Perspective and Kristal Perspective) and are, in addition, prime targets for oxidative damage and subsequent impaired bioenergetic and respiratory function. Of note, an intriguing recent report suggested that targeting catalase to mitochondria could extend the median life span of mice (16) (see Schriner et al. Science paper and "Catalase and Mouse"), although, because survival was only measured in early back-crossed generations (F2 to F4), this result will need to be replicated in later generations to fully rule out the contribution of founder-line genes to increased longevity.

In addition to influencing mitochondrial function, ROS are known to modulate redox-sensitive gene expression and intracellular signaling pathways, actions that might underlie the effects of ROS on aging. A number of aging-associated pathways have been identified in C. elegans and yeast that have also been shown to regulate some aspects of aging in mice (see Reznick Perspective for an evolutionary biologist's perspective on the genetics of aging). These pathways and proteins include the insulin/insulin-like growth factor-1 signaling cascade and, more recently, sirtuin proteins (14; see Kaeberlein Perspective). Members of the Sir2 family of deacetylases, which include yeast and Drosophila Sir2, C. elegans Sir-2.1, and human Sirt1, are suspected of mediating the anti-aging effects of calorie restriction. For lower organisms such as yeast and Drosophila, direct evidence exists that shows that the life-span enhancement produced by calorie restriction is dependent on Sir2 (13) (see "UnSIRtainty Principle" and "Calorie Restriction Un-SIR-tainty"). However, it is not yet known whether this is true in mammals. Of interest, Sir2 is regulated by the redox status of NAD/NADH (nicotinamide adenine dinucleotide), with NAD activating Sir2 and NADH inhibiting its activity (13). The functional status of mitochondria, and their level of ROS production, can determine the NAD/NADH ratio, so it is possible that one point at which the genetics of aging and the "mitochondrial theory of aging" converge is at redox regulation of Sir2 or its orthologs. This is a rapidly advancing area that should provide exciting new insights into aging in the near future.

As we begin to more fully define how free radicals regulate the biochemistry of senescence at the cellular level, new approaches to determining their importance to the aging of complex organisms will certainly emerge. These advances will allow for a more definitive examination of the impact of ROS on signaling and macromolecule oxidative damage. The development or description of new antioxidant compounds, such as SOD mimetics, will allow for new investigations into the role of ROS in aging (rate of aging) and age-associated diseases, with a new focus on the importance of tissue distribution and subcellular localization of antioxidants to better delineate the complexities of organismal aging.


June 29, 2005
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Citation: L. L. Dugan, K. L. Quick, Reactive Oxygen Species and Aging: Evolving Questions. Sci. Aging Knowl. Environ. 2005 (26), pe20 (2005).




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