Rapid development and a long life: an association expected under a stress theory of aging

Stress and ageing interactions: A paradox in the context of shared etiological and physiopathological processes

Gerontology has made considerable progress in the understanding of the mechanisms underlying the ageing process and age-related neurodegenerative disorders. However, ways to improve quality of life in the elderly remain to be elucidated. It is now clear that stress and the ageing process share a number of underlying mechanisms bound in a very close, if not indissociable, relationship. The ageing process is regulated by the factors underlying the ability to adjust to stress, whilst stress has an influence on the life span and the quality of ageing. In addition, the ability to cope with stress in adulthood predicts life expectancy and quality of life at senescence. The ageing process and stress also share several common mechanisms, particularly in relation to the energy factor. Stress consumes energy and ageing may be considered as a cost of the energy expended to deal with the stressors to which the body is exposed throughout its lifetime. This suggests that the ageing process is associated with and/or a consequence of a long-lasting activation of the major stress responsive systems. However, despite common features, the interaction between stress and the ageing process gives rise to some paradoxes. Stress can either diminish or exacerbate the ageing process just as the ageing process can worsen or counter the effects of stress. There has been little attempt to understand how ageing and stress might interact to promote "successful" or pathological ageing. A key factor in this respect is the individual's ability to adapt to stress. Viewed from this angle, the quality of life of aged subjects may be improved through therapy designed to improve the tolerance to stress.

Energetic efficiency under stress underlies positive genetic correlations between longevity and other fitness traits in natural populations

Evolutionary relationships among fitness traits are considered in terms of the near-to-universal scenario of stressful environments leading to a resource-deficient and hence energy-deficient world. Fitness approximates to energetic (and metabolic) efficiency under this environmental model. When fitness is high, stress resistance (reducible to oxidative-stress resistance) and metabolic stability are maximal, and energy expenditure is minimal. Rapid development should then be favored followed by a long lifespan and high adult survival. Positive associations among diverse fitness or life-history traits are expected, controlled by stress-resistant 'good genotypes'. Heterozygotes tend to show higher energetic efficiency and hence higher fitness than do corresponding homozygotes under extreme environments, and to give parallel associations among life-history traits. Energy budgets under abiotic environments are pivotal for integrative evolutionary studies of life histories in natural populations.

Environments and evolution: interactions between stress, resource inadequacy and energetic efficiency

Evolutionary change is interpreted in terms of the near-universal ecological scenario of stressful environments. Consequently, there is a premium on the energetically efficient exploitation of resources in a resource-inadequate world. Under this environmental model, fitness can be approximated to energetic efficiency especially towards the limits of survival. Furthermore, fitness at one stage of the life-cycle should correlate with fitness at other stages, especially for development time, survival and longevity; 'good genotypes' under stress should therefore be at a premium. Conservation in the wild depends primarily on adaptation to abiotically changing habitats since towards the limits of survival, genomic variation is rarely restrictive. The balance between energetic costs under variable environments and energy from resources provides a model for interpreting evolutionary stasis, punctuational and gradual change, and specialist diversification. Ultimately, a species should be in an equilibrium between the physiology of an organism and its adaptation to the environment. The primary key to understanding evolutionary change should therefore be ecological, highlighting energy availability in a stressed world; this approach is predictive for various patterns of evolutionary change in the living and fossil biota.

Current issues concerning the role of oxidative stress in aging: a perspective

The main tenet of the oxidative stress hypothesis of aging is that accrual of molecular oxidative damage is the principal causal factor in the senescence-related loss of ability to maintain homeostasis. This hypothesis has garnered a considerable amount of supportive correlational evidence, which is now being extended experimentally in transgenic Drosophila over-expressing antioxidative defense enzymes. Some of these studies have reported extensions of life span, while others have not. Interpretation of life spans in poikilotherms is complicated by a number of factors, including the interrelationship between metabolic rate and longevity. The life spans of poikilotherms can be extended multi-fold by reducing the metabolic rate but without affecting the metabolic potential, i.e., the total amount of energy expended during life. A hypometabolic state in poikilotherms also enhances stress resistance and activities of antioxidative enzymes. It is emphasized that extension of life span without simultaneously increasing metabolic potential is of questionable biological significance.

Hormesis: an adaptive expectation with emphasis on ionizing radiation

Non-linear fitness gradients with maxima between extremes are expected for any environmental variable to which free-living populations are exposed. For exceedingly toxic agents, including ionizing radiation, such deviations from linearity are close to zero exposure and are conventionally called hormesis. Accordingly, hormesis is an extreme version of the non-linear fitness gradients for general environmental stresses such as temperature fluctuations, for which maximum fitness occurs at the moderate temperature fluctuations to which free-living populations are most commonly exposed. Some metabolic reserves should occur under moderate temperature stresses because of the need for pre-adaptation enabling survival during exposure to occasional periods of more extreme stress, especially at species borders where selection for stress resistance is likely to be most intense. Because heat shock proteins are induced by all stresses, adaptation to extreme temperatures should translate into adaptation to other stresses. Consequently, metabolic reserves from adaptation to extreme temperatures in the past should translate into protection from correlated abiotic stresses, especially in human populations where modern cultural processes are now ameliorating exposure to extreme stresses. Ambient and man-made radiations of non-catastrophic dimensions should therefore lead to stress-derived radiation hormesis. Other stresses can, in principle, be incorporated into this model. Accordingly, evolutionary and ecological considerations suggest two components of hormesis in relation to ionizing radiation: background radiation hormesis based upon the background exposure to which all organisms on earth are subjected; and stress-derived radiation hormesis. Exposure under stress-derived radiation hormesis is considerably larger than under background radiation hormesis, so significant deleterious effects from non-catastrophic radiation normally may be impossible to detect. Suggestions are provided for testing such postulated deviations from the commonly assumed linear no-threshold (LNT) hypothesis for the biological consequences of exposure to radiation.

Stress-resistance genotypes, metabolic efficiency and interpreting evolutionary change

Assuming stress levels to which free-living populations are normally exposed, an association between rapid development time, a long life, success in mating and size of sexual ornaments can be predicted. Fitness at one stage of the life cycle should therefore correlate with fitness at other stages under this environmental model. Assuming that stress targets energy carriers, high-energy efficiency underlain by stress-resistance genotypes that are likely to be heterozygous is the basis of this prediction. Stress-resistance genotypes therefore have a role in promoting the energy efficiency required for organisms to accommodate a stressed world. Selection for energy efficiency to utilize heterogenous resources implies that the process of speciation should normally occur rapidly and be rarely observed. It follows that the ecological species concept is primary to other species concepts. The intensity of selection for stress resistance goes from an extreme in the highly disturbed and stressful environments of living fossils to relatively stable abiotic habitats, where specialist diversifications and adaptive radiations are likely. Between these extremes, a punctuated pattern of evolutionary change may occur in perturbed environments during a transient phase of increased resources. In abiotically benign tropical habitats where energy constraints are low, specialization of resource utilization by learning appears possible.