Hormesis : A Quest for Virtuality?
Olivier Toussaint#, José Remacle, Jean-François Dierick, Thierry Pascal, Christophe Frippiat, Joao Pedro Magalhaes, Florence Chainaux
University of Namur (FUNDP), URBC, Belgium
# Correspondence:
University of Namur, Unit of Cellular Biology
Rue de Bruxelles, 61, B-5000 Namur
Belgium
Tel: + 32 8172 4132
Fax: + 32 8172 4135
Email: olivier.toussaint@fundp.ac.be
The birth of new concepts brings constructive confrontations with older concepts. The use of the concept of hormesis in biogerontology is legitimate since the role of stress is fundamental in both aging and hormesis. Are there theoretical restrictions against a generalised role of hormesis in aging? What are the possible side effects? Can hormesis be reconciliated with the life span-prolonging effects of caloric restriction ? The aim of this short paper is to propose clues to answer these important questions.
I. Not at the Right Place at the Right Moment
All cell types are not equal regarding their capability of stress resistance in terms of antioxidant potential, inductibility of heat shock proteins, basal level and induced level of DNA repair enzymes, proteasome activity, etc. At low stress intensities, minimal differences in the cell type-specific regulation of transduction pathways and transcription factors result in huge differences in the stress-induced changes of gene expression. In consequence, all cell types do not adopt the same behavior in response to identical stressful conditions. Some cell types might undergo mitosis in response to low levels of reactive oxygen species (ROS) which might, in other cell types, trigger either apoptosis, stress-induced premature senescence (SIPS) (for reviews on SIPS: (Brack et al., 2000; Toussaint et al., 2000a; Toussaint et al., 2000b; Toussaint et al., 2000c; Toussaint et al., 2000d)), or increase their resistance to further stress by hormesis. Moreover, the border between cellular behaviors such as senescence, activation, hormesis, SIPS, apoptosis or even necrosis is not always clearcut (Toussaint et al., in press). For instance, the signaling pathways activated by Tumor Necrosis Factor-a induce transient increases in the concentration of ROS acting as secondary messengers. Such repeated transient increases in ROS concentration are sufficient for triggerring the appearance of biomarkers of cellular senescence, both in vivo and in vitro (Toussaint et al., 1996; Mendez et al., 1998; Toussaint et al., 1998; Dumont et al., 2000). Therefore it seems difficult to predict which type and intensity of stress to use in order to favor hormesis in all the cell types of an individual.
Whether hormesis will be take place or not might also strongly depends on the global cellular metabolic activity. More cells undergo SIPS in response to a subcytotoxic stress when the availability of substrates of the energy metabolism is decreased (for reviews: (Toussaint et al., 1995; Toussaint & Remacle, 1996; Toussaint et al., 2000a)). In such adverse conditions, less cells resist the stress efficiently, and hormesis takes place in a smaller proportion of cells. In other words, stress conditions which could allow hormesis in a given cell type under certain conditions, could trigger SIPS, apoptosis, or even necrosis, in conditions of shortage of substrates of the energy metabolism. Such shortages are observed in aged patients, who are often afflicted by deficiences in biominerals, vitamins, amino acids, calory intakes, etc. Many physiological changes such as decreased intestinal absorption of aromatic amino acids, deficiences in insulin metabolism, etc, reinforce these deficiencies. Beside shortages of substrates, decreases in the biotransformation of substrates into molecules bearing energy-rich chemical bounds occur in aged cells or tissues. For instance, decreases in the capacity to regenerate ATP by oxidative phosphorylation in mitochondria have been described. In other words, a given stress favoring hormesis in a young tissue could be deleterious in an old tissue.
II. Stimulate me, Don't Stress me!
What is the theoretical difference between hormesis, with possible positive effects on life span and stress-induced premature senescence, with possible adverse effects? In a recent paper, we namely reviewed how the criteria of stability of far-from-equilibrium open systems allow to sort the various global cellular responses to stress into four classes, hormesis included, according to the net result of the balance between the damage generated by the stress, and the efficiency of the stress response (Toussaint et al., in press).
Mild constant ubiquitous stresses do not immediately alter the stability of the biological systems. When repair does not take place rapidly, some of the modifications may become irreversible and accumulate without challeging immediately the cellular stability. Some subsystems could counteract the irreversible errors accumulating in other cellular subsystems. The cells would shift to a new steady state when the level of damage reaches a threshold where these compensatory mechanisms become transiently overwhelmed. This would result in a higher level of damage and a lower global biochemical activity. This is normal aging (Toussaint et al., 1991).
In conditions of moderately elevated stress, the defense systems may be induced enough to prevent further increases in the level of damage. In this sense, stress can be considered as a 'positive' stimulation of the repair systems, e.i. hormesis, as long as it does not involve any accumulation of irreversible modifications (errors). Otherwise, if such modifications accumulate, there might be a trade-off between a direct increase in the capacity of response to further stress, and a premature senescence due to the stress-induced elevation in the level of errors (Toussaint et al., in press).
Stress-induced premature senescence results from exposures to chronic or repeated short stress at subcytotoxic intensities which may overwhelm transiently the defense systems. Damage then spreads in chain to other cell components, requiring other defense systems. If these systems are able to eliminate quickly all the damage, the cell remains stable. If not, increases in the level of damage and decreases in the cellular capacity of storing (ATP, redox potential, etc.) and using (biochemical reactions) free energy will occur. These are the thermodynamical conditions for loss of stability of far-from-equilibrium systems. If a new steady state can ever be found, it will be characterized by two major differences: more damage and a lower global metabolic activity. Depending on the cell type, these conditions can lead to SIPS or trigger the mechanisms of self-destruction by apoptosis. The fourth type of stress intensities covers cytotoxic injuries.
Similar conclusions can be reached from the theory of attractors (Emlen et al., 1998; Toussaint et al., in press).
III. Friends or Foes? Arguments in favor of Life Span-prolonging Effects of Hormesis
The prolongation of life span observed after transgenesis of flies for certain antioxidant enzymes (Orr & Sohal, 1994; Sun & Tower, 1999) or incubation of human fibroblasts or C. elegans with antioxidant chemicals (Chen et al., 1995; Melov et al., 2000) should not be considered as life span-prolonging effects of hormesis. Indeed no extra stress was applied on these systems to obtain a better protection.
The p66shc protein is involved in the triggering of the mitochondrial burst of ROS leading to apoptosis. The prolongation of life span observed in p66shc knock-out mice (Migliaccio et al., 1999) is opposed to hormesis: a decreased stress level prolongs life span.
Only these experiments where a prolongation of life span is observed in conditions of elevated stress level should be considered as experiments really in favor of a positive role of hormesis in longevity. For instance the relationship between increased thermotolerance and life-span of C. elegans was investigated. Pretreatments at sublethal temperatures induced significant increases in thermotolerance and small but statistically highly significant increases in life expectancy (Lithgow et al., 1995). Comparable results were also observed after exposures of flies to heat shocks (Khazaeli et al., 1997). However, it was also observed that, while extra HSP70 provides additional protection against the immediate damage from heat stress, abnormally high concentrations can decrease growth, development and survival to adulthood of Drosophila (Krebs & Feder, 1997). Nevertheless HSP70 induction may also explain the long-lasting resistance to heat of Drosophila melanogaster having lived in hypergravity (Minois et al., 1999). The effects of repetitive mild heat shock on growth and various cellular and biochemical characteristics of human skin fibroblasts undergoing senescence in vitro were analysed. Whereas the growth rates, population doubling rates, and cumulative population doubling levels achieved in vitro remained unaffected, age-related changes in cellular morphology, cell size, cytoskeletal organisation, autofluorescence and senescence-associated ß-galactosidase activity were significantly slowed down by repeated mild heat shocks (Rattan, 1998). Further experiments should be performed to allow definitive conclusions.
IV. Eat less, Stress less. Caloric Restriction and Hormesis.
Caloric restriction is a decrease of the intake of calories, reaching a quantity of calories well under the quantity of calories provided by ad libitum feeding. Caloric restriction must be differentiated from malnutrition, unappropriate diet or deficiencies. Interestingly, caloric restriction prolongs the life span of various species, such as species of rodents, and decreases the level of DNA damage in aged animals (for a review : (Merry, 2000)). The life span prolonging effects of caloric restriction could be opposed to hormesis since, in mice, caloric restriction increases life span concomitantly with a decrease in the expression level of many defense systems (Lee et al., 1999). Yet another possibility is that the rate of error accumulation due to ad libitum feeding is well above the low level of errors permitting hormesis: errors go on accumulating fast despite an overexpression of the defense systems. A decrease of the rate of error accumulation due to caloric restriction would explain the decrease of the expression level of the defence systems. Then the question would be to know whether the relationship existing between the decrease of error accumulation and the decrease of the expression level of the defense systems observed in caloric restriction, allows hormesis or not.
Conclusions
A life span-prolonging effect of hormesis can take place only if the stress-induced increase in some defense systems due to exposure to abnormal stress is not associated with an accumulation of irreversible errors. If errors accumulate, a trade-off could exist between a direct beneficial effect against a further stress (e.i. acclimatization) and premature senescence due to the stress-induced increase in the level of errors.
The (over)expression of specific genes involved in the elimination or repair of damage generated by a specific stress is indeniably observed in some of the organisms studied. A more general question is however to know whether such specific (over)expression is the key for explaining how hormesis can prolong life span. The stress response also triggers a general stimulation of the whole cellular system, including for instance an increase in the rate of regeneration of ATP, therefore making free energy available for many repair pathways and allowing the repair of many sorts of damages at a time.
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