10 Hormesis and Aging

Hormesis and Aging Suresh I.S. Rattan and Dino Demirovic Abstract Mild stress-induced hormetic stimulation of protective mechanisms in cells and organisms can result in potential antiaging effects. Detailed molecular mechanisms that bring about the hormetic effects are being increasingly understood and comprise a cascade of stress response and other pathways of maintenance and repair. Although the extent of immediate hormetic effects after exposure to a particu- lar stress may only be moderate, the chain of events following initial hormesis leads to biologically amplified effects that are much larger, synergistic, and pleiotropic. A consequence of hormetic amplification is an increase in the homeodynamic space of a living system in terms of increased defense capacity and reduced load of damaged macromolecules. Hormetic strengthening of the homeodynamic space provides wider margins for metabolic fluctuation, stress tolerance, adaptation, and survival. Hormesis thus counterbalances the progressive shrinkage of the homeody- namic space that is the ultimate cause of aging, diseases, and death. Healthy aging may be achieved by hormesis through mild and periodic but not severe or chronic physical and mental challenges and by the use of nutritional hormesis incorporating mild stress-inducing molecules called hormetins. Keywords Antiaging · Homeostasis · Longevity · Skin · Stress Introduction Because the harmful effects of severe and chronic stress have long overshadowed the beneficial hormetic effects of low-level stress, the application of hormesis in aging research and therapy is a relatively recent development. The paradigm for consid- ering the applicability of hormesis in aging intervention is the well-documented beneficial effect of moderate exercise, which at a biochemical level results in the S.I.S. Rattan (B) Laboratory of Cellular Aging, Department of Molecular Biology, University of Aarhus, DK 8000 Aarhus C, Denmark e-mail: rattan@mb.au.dk 153M.P. Mattson, E.J. Calabrese, Hormesis, DOI 10.1007/978-1-60761-495-1_9, C© Springer Science+Business Media, LLC 2010 154 S.I.S. Rattan and D. Demirovic production of potentially harmful substances such as free radicals, acids, and alde- hydes. Thus, it was hypothesized that if aging systems are deliberately exposed to mild stress, this should lead to achieving beneficial hormetic effects, including health- and longevity-promoting effects. Hormesis in aging is therefore defined as the life-supporting beneficial effects resulting from the cellular responses to single or multiple rounds of mild stress (Rattan, 2001a, b, 2004, 2005). Here we review and analyze the published literature on various physical, chemi- cal, and biological conditions that are known to be potentially harmful at high doses but that at lower doses have the effects of slowing down aging and/or prolonging the lifespan of cells and organisms. Table 1 lists the main stresses that have been shown to have aging- and longevity-modulatory effects in various systems. Table 1 Various Types of Stresses Tested for Their Antiaging Effects Physical stress Thermal Hypergravity Radiation Exercise Biological stress Dietary restriction Dietary components Natural hormetins Chemical stress Minerals Heavy metals Pro-oxidants Synthetic hormetins However, it is important to point out that so far only a few studies have been performed with the specific aim of testing the applicability of hormesis in aging, for example, those using thermal stress and hypergravity as hormetic agents. For most other studies that have been interpreted to involve hormesis as the mode of action of the stressful conditions used in those experiments, these conclusions are generally derived in retrospective analyses. Such studies include the effects of radiation, exer- cise, pro-oxidants, nutritional components, and food restriction. However, to fully appreciate the rationale for using hormesis as a modulator of aging and longevity, we first provide a brief overview of the biological understanding of aging, which is considered to be no longer an unresolved problem in biology (Hayflick, 2007; Holliday, 2006). Recapitulating the Biological Basis of Aging Biogerontology—the study of the biological basis of aging—has unveiled mysteries of aging by describing age-related changes in organisms, organs, tissues, cells, and macromolecules. The large body of descriptive data has led two of the pioneers of Hormesis and Aging 155 modern biogerontology, Leonard Hayflick and Robin Holliday, to declare that aging is no longer an unsolved problem in biology (Hayflick, 2007; Holliday, 2006). This declaration does not mean that there are no remaining descriptive data on aging and that every piece of information about aging in every biological system has been gathered. The bold assertion by Hayflick and Holliday underlines the fact that the biological basis of aging is well understood and a distinctive framework has been established that will not be altered significantly with additional descriptive data. Based on the large body of descriptive data, certain general principles of aging and longevity can be clearly formulated, and these can be the basis for translational research and interventions toward achieving a healthy old age (Table 2). Table 2 General Principles of Aging and Longevity Derived from Modern Biogerontological Research • Life history principle: Aging is an emergent phenomenon seen primarily in protected environments that allows survival beyond the natural lifespan of a species, termed “essential lifespan” (ELS) (Rattan, 2000a, b; Rattan and Clark, 2005). • Differential principle: The progression and rate of aging is different in different species, in organisms within a species, in organs and tissues within an organism, in cell types within a tissue, in subcellular compartments within a cell type, and in macromolecules within a cell. • Mechanistic principle: Aging is characterized by a progressive accumulation of molecular damage in nucleic acids, proteins, and lipids. The inefficiency and failure of maintenance, repair, and turnover pathways is the main cause of age-related accumulation of damage. • Nongenetic principle: There is no fixed and rigid genetic program that determines the exact duration of survival of an organism, and there are no “gerontogenes” whose sole function is to cause aging and to determine precisely the lifespan of an organism. Thus, aging has many facets, and almost all the experimental data suggest that aging is an emergent, epigenetic, meta-phenomenon that is not controlled by a single mechanism. Although individually no tissue, organ, or system becomes functionally exhausted even in very old organisms, it is their combined interaction and interde- pendence that determines the survival of the whole. A combination of genes, milieu, and chance determines the course and consequences of aging and the duration of survival of an individual (Rattan, 2007b). There is much supporting evidence for the theory that the survival and longevity of a species are a function of the ability of its maintenance and repair mechanisms to keep up with damage and wear and tear. All living systems have the intrinsic ability to respond to, counteract, and adapt to external and internal sources of disturbance. The traditional conceptual model to describe this property is homeostasis, which has dominated biology, physiology, and medicine since the 1930s. However, advances in our understanding of the processes of biological growth, development, matura- tion, reproduction, and aging, senescence, and death have led to the realization that the homeostasis model as an explanation is seriously incomplete. The main reason for the incompleteness of the homeostasis model is its defining principle of “sta- bility through constancy,” which does not take into account the new themes, such as cybernetics, control theory, catastrophe theory, chaos theory, information, and interaction networks, that comprise and underlie the modern biology of complexity 156 S.I.S. Rattan and D. Demirovic (Rattan, 2007a). Since the 1990s, the term homeodynamics has been increasingly used to account for the fact that the internal milieu of complex biological systems is not permanently fixed, is not at equilibrium, and is a dynamic regulation and interaction among various levels of organization (Yates, 1994). Aging, senescence, and death are the final manifestations of unsuccessful home- ostasis or failure of homeodynamics (Holliday, 2007; Rattan, 2006). A wide range of molecular, cellular, and physiological pathways of repair are well known, and these range from multiple pathways of nuclear and mitochondrial DNA repair to free radical counteracting mechanisms, protein turnover and repair, detoxification mechanisms, and other processes, including immune and stress responses. All of these processes involve numerous genes whose products and interactions give rise to the “homeodynamic space” or “buffering capacity that is the ultimate determi- nant of an individual’s chance and ability to survive and maintain a healthy state (Holliday, 2007; Rattan, 2006). A progressive shrinking of the homeodynamic space is the hallmark of aging and the cause of age-related diseases. Figure 1 is a pictorial representation of the concept of homeodynamic space and the consequences of its shrinkage during aging. In a normal, healthy, young individ- ual, the complex network of maintenance and repair systems (MRS) constitutes a functional homeodynamic space. Because no MRS can be 100% efficient 100% of the time, even in a young system, there is a probability of incomplete homeodynam- ics, giving rise to a zone of vulnerability, manifested in age-independent diseases and mortality. However, a progressive accumulation of molecular damage and its effects on the interacting molecular networks leads to the reduction in the functional homeodynamic space and effectively increases the vulnerability zone, thus allow- ing for the occurrence and emergence of age-related diseases. Alzheimer’s disease, cancer, cataract, diabetes type 2, osteoporosis, Parkinson’s disease, sarcopenia, and other age-related diseases are the result of an individual’s reduced homeodynamic space. Fig. 1 Pictorial representation of the concept of homeodynamic space, whose progressive shrink- age due to the accumulation of molecular damage leads to an increase in the area of vulnerability zone in the elderly, and hence to the occurrence and emergence of age-related diseases A critical component of the homeodynamic property of living systems is their capacity to respond to stress. In this context, the term “stress” is defined as a signal generated by any physical, chemical, or biological factor (stressor) that in a living Hormesis and Aging 157 system initiates a series of events to enable the organism to counteract, adapt, and survive. Although a successful and over-compensatory response to low doses of stressors improves the overall homeodynamics of cells and organisms, an incom- plete or failed homeodynamic response leads to the damaging and harmful effects of stress, including death. It is this homeodynamic space as a whole or the individ- ual components of the homeodynamic machinery that are the targets of hormetic interventions. Thermal Hormesis in Aging Thermal Hormesis in Organisms Temperature stress, especially high-temperature–induced heat shock (HS), has been widely used with the specific aim of testing and applying hormesis in aging research and interventions. One of the main reasons for choosing HS as a hormetic agent is that HS acts through an evolutionarily highly conserved stress response pathway, known as the heat-shock response, the molecular basis of which is well understood (Sun and MacRae, 2005; Verbeke et al., 2001b). Effects of mild and severe HS have been tested on yeast, nematodes, fruit flies, and rodent and human cells. For example, wild-type and age-1 long-lived mutant hermaphrodite Caenorhabditis ele- gans exposed for 3 to 24 hours to 30◦C exhibited a significant increase in mean lifespan compared to controls (Johnson, 2002; Lithgow et al., 1995). Similarly, a 6-hour exposure at 30◦C of wild-type worms increased their lifespan, but no effect was found after exposures of 2 or 4 hours (Yokoyama et al., 2002). Furthermore, studies of C. elegans subjected to 35◦C HS for different durations showed that HS not longer than 2 hours resulted in an extension of lifespan (Butov et al., 2001; Michalski et al., 2001; Yashin et al., 2001). In a study of multiple stresses in C. elegans an extension of lifespan after 1 and 2 hours of HS at 35◦C was reported (Cypser and Johnson, 2002, 2003). In another study performed on C. elegans it was observed that repeated mild HS throughout life had a larger effect on lifespan than a single mild HS early in life, and the effect was related to the levels of heat-shock protein (HSP) expression (Olsen et al., 2006). In the case of fruit flies, virgin males of inbred lines of Drosophila melanogaster exhibited an increase in mean lifespan and lower mortality rates during several weeks after a heat treatment of 36◦C for 70 min (Khazaeli et al., 1997). It has also been shown that wild-type D. melanogaster exposed to 37◦C for 5 minutes a day, 5 days a week for 1 week lived on average 2 days longer than the control flies (Le Bourg et al., 2001). Longer exposures had either no effect or a negative effect on lifespan. In another study on D. melanogaster, the exposure of young flies to four rounds of mild HS at 34◦C significantly increased the average and maximum lifes- pan of female flies and increased their resistance to potentially lethal HS (Hercus et al., 2003). Of interest, the beneficial effects of HS in Drosophila did not entirely depend on the continuous presence of HSP but were observed long after newly synthesized HSP had disappeared, indicating the involvement of a cascade of poststress events in hormesis (Sørensen et al., 2008). Furthermore, the hormetic 158 S.I.S. Rattan and D. Demirovic effects of HS appear to occur to different extents in male and female Drosophila, which may be due to the fact that females have to trade off stress resistance and reproduction (Sørensen et al., 2008). Studies have also been performed on the effect of subjecting transgenic D. melanogaster overexpressing the inducible HSP70 to 20 minutes at 36◦C (Minois et al., 2001; Minois and Vaynberg, 2002). In the control “parental” line, such expo- sure significantly increased the lifespan of both virgin flies kept in groups and mated flies. In individually kept flies, the same trend was observed but was statistically not significant. No beneficial effect of such HS has been seen in the transgenic lines, which may be suggestive of upper limits of modulating HS responses (Minois and Vaynberg, 2002). In addition to the high temperature, there is some evidence demonstrating that cold shocks at young age increased the longevity and survival of Drosophila at high temperature and increased longevity of Drosophila after cold stress–induced hardening (Le Bourg, 2008; Overgaard et al., 2005). In the case of mammals, irradiated and nonirradiated mice that were given intermittent cold shocks showed lower rates of mortality in the irradiated mice. Longer lifespans were observed in thermally stressed nonirradiated males and irradiated females (Minois, 2000). Similarly, rats kept in water set at 23◦C, 4 hours a day, 5 days a week, had a 5% increase in average lifespan and diminished occurrence of age-related diseases (Holloszy and Smith, 1986). Thermal Hormesis in Human Cells Undergoing Aging in Vitro A series of studies performed in our labs tested the hormesis hypothesis of the ben- eficial effects of mild HS, using the Hayflick system of cellular aging of normal human cells in culture. Employing a mild stress regimen of exposing serially pas- saged human skin fibroblasts to 41◦C for 1 hour twice a week throughout their replicative lifespan, we found several antiaging effects, which are listed in Table 3. The choice of the repeated mild heat shock (RMHS) regimen was based on several pilot experiments performed for selecting conditions in which 30% of the maximal HS response was elicited without affecting cell growth and survival (Kraft et al., 2006; Rattan, 1998). This does not imply that these are the ideal hormetic conditions for these cells. Other combinations of dose and duration may well have similar or even better effects, but that issue remains to be investigated. Furthermore, we also showed that repeated mild HS at 41◦C, but not the relatively severe HS at 42◦C, increased the replicative lifespan and elevated and maintained the basal lev- els of MAP kinases JNK1, JNK2, and p38 in human skin fibroblasts (Nielsen et al., 2006). To confirm the wider applicability of mild HS-induced hormesis in other human cell types, we also performed studies on normal human epidermal keratinocytes (NHEKs) and obtained results that were very similar to those for dermal fibroblasts. NHEK also showed a variety of cellular and biochemical hormetic antiaging effects on repeated exposure to mild HS at 41◦C. These effects included maintenance of youthful cellular morphology, enhanced replicative lifespan, enhanced proteasomal Hormesis and Aging 159 Table 3 A Summary of Results of Studies on the Antiaging Hormetic Effects of Repeated Mild Heat Shock on Human Skin Fibroblasts in Vitro Characteristic Hormetic Effect Reference Cellular phenotype Cell size Reduced enlargement (Rattan, 1998) Cell morphology Reduced irregularization (Rattan, 1998) Proliferative lifespan 20% increase (Nielsen et al., 2006) Wound healing 30% increase (Rattan et al., 2009) Cell physiological phenotype H2O2 decomposing ability 50%–140% increase (Fonager et al., 2002) Survival after H2O2 exposure 10%–18% increase (Fonager et al., 2002) Survival after ethanol exposure 10%–40% increase (Fonager et al., 2002) Survival after ultraviolet A exposure 5%–17% increase (Fonager et al., 2002) Molecular damage Glucation, furasine level 50%–80% reduction (Verbeke et al., 2001a) Glycoxidation level 10%–30% reduction (Verbeke et al., 2001a) Carboxymethyl-lysine–rich protein level 20%–85% reduction (Verbeke et al., 2001a) Lipofuscin pigment level 6%–29% reduction (Verbeke et al., 2001a) Protein carbonyl level 5%–40% reduction (Verbeke et al., 2001a) Reduced glutathione level 3-fold increase (Verbeke et al., 2001a) Oxidized glutathione level 2-fold reduction (Verbeke et al., 2001a) Induction of sugar-induced protein damage 10-fold reduction (Verbeke et al., 2002) Molecular mechanisms HSP27 level 20%–40% increase (Fonager et al., 2002) HSC70 level 20% increase (Fonager et al., 2002) HSP70 level 7- to 20-fold increase (Fonager et al., 2002) HSP90 level 50%–80% reduction (Fonager et al., 2002) Proteasome activities 40%–90% increase (Beedholm et al., 2004) 11S activator content 2-fold increase (Beedholm et al., 2004) 11S activator binding 2-fold increase (Beedholm et al., 2004) JNK1, JNK2 and p38 level 45%–70% increase (Nielsen et al., 2006) activity, and increased levels of HSPs (Rattan and Ali, 2007). In addition, we also studied the effects of HS on Na,K-ATPase and the sodium pump, whose content and activity were increased significantly after mild HS (Rattan and Ali, 2007). However, the molecular mechanisms and interactions that bring about the mild HS-induced increase in the amounts and activity of Na,K-ATPase and their consequences on other biochemical pathways during aging are yet to be elucidated. Notably, com- parable hormetic effects could not be seen in NHEK repeatedly exposed to 43◦C, which underlines the differences between the beneficial effects of mild stress and the harmful effects of severe stress. 160 S.I.S. Rattan and D. Demirovic We also observed that mild HS enhances the ability of serially passaged ker- atinocytes