Have human biological functions evolved in support of a life-span?

Longevity and aging. Mechanisms and perspectives

Longevity can mostly be determined with relative accuracy from birth and death registers when available. Aging is a multifactorial process, much more difficult to quantitate. Every measurable physiological function declines with specific speeds over a wide range. The mechanisms involved are also different, genetic factors are of importance for longevity determinations. The best-known genes involved are the Sirtuins, active at the genetic and epigenetic level. Aging is multifactorial, not "coded" in the genome. There are, however, a number of well-studied physical and biological parameters involved in aging, which can be determined and quantitated. We shall try to identify parameters affecting longevity as well as aging and suggest some reasonable predictions for the future.

Longevity and aging. Role of free radicals and xanthine oxidase. A review

Longevity and aging are differently regulated. Longevity has an important part of genetic determinants, aging is essentially post-genetic. Among the genes involved in longevity determination, sirtuins, activated also by calorie restriction and some others as the TOR pathway, attracted special interest after the insulin–IGF pathway first shown to regulate longevity in model organisms. For most of these genes, postponement of life-threatening diseases is the basis of their action which never exceeds about 35% of all determinants, in humans. Among the post-genetic mechanisms responsible for age-related decline of function, free radicals attracted early interest as well as the Maillard reaction, generating also free radicals. Most attempts to remediate to free radical damage failed however, although different scavenger mechanisms and protective substances are present in the organism. Synthetic protectors were also tested without success. The only example of a successful treatment of a free radical mediated pathology is the case of xanthine oxidase, involved in cardiovascular pathology, essentially during the ischemia-reperfusion process. Its inhibition by allopurinol is currently used to fight this deadly syndrome.

Genetic, epigenetic and posttranslational mechanisms of aging

Gerontological experimentation is and was always strongly influenced by "theories". The early decades of molecular genetics inspired deterministic thinking, based on the "Central Dogma" (DNA --> RNA --> Proteins). With the progress of detailed knowledge of gene-function a much more complicated picture emerged. Regulation of gene-expression turned out to be a highly complicated process. Experimental gerontology produced over the last decades several "paradigms" incompatible with simple genetic determinism. The increasing number of such detailed experimental "facts" revealed the importance of epigenetic factors and of posttranslational modifications in the age-dependent decline of physiological functions. We shall present in this review a short but critical analysis of genetic and epigenetic processes applied to the interpretation of the more and more precisely elucidated experimental paradigms of aging followed by some of the most relevant aging-mechanisms at the post-translational level, the posttranslational modifications of proteins such as the Maillard reaction, the proteolytic production of harmful peptides and the molecular mechanisms of the aging of elastin with the role of the age-dependent uncoupling of the elastin receptor, as well as the loss of several other receptors. We insist also on the well documented influence of posttranslational modifications on gene expression and on the role of non-coding RNA-s. Altogether, these data replace the previous simplistic concepts on gene action as related to aging by a much more complicated picture, where epigenetic and posttranslational processes together with environmentally influenced genetic pathways play key-roles in aging and strongly influence gene expression.

Does senescence give rise to disease?

The distinctions between senescence and disease are blurred in the literature of evolutionary biology, biodemography, biogerontology and medicine. Theories of senescence that have emerged over the past several decades are based on the concepts that organisms are a byproduct of imperfect structural designs built with imperfect materials and maintained by imperfect processes. Senescence is a complex mixture of processes rather than a monolithic process. Senescence and disease have overlapping biological consequences. Senescence gives rise to disease, but disease does not give rise to senescence. Current data indicate that treatment of disease can delay the age of death but there are no convincing data that these interventions alter senescence. An understanding of these basic tenets suggests that there are biological limits to duration of life and the life expectancy of populations and reveal biological domains where the development of interventions and/or treatments may modulate senescence.

[Age and management decisions in patients with primary lung cancer]

INTRODUCTION

Therapeutic decisions are difficult in elderly patients because of the heterogeneity of this population. Our objective was to evaluate the role of age in the management of patients suffering from primary lung cancer seen in the department of respiratory diseases of the Limoges regional teaching hospital between 2002 and 2004.

METHODS

A cross sectional study analysed the management of 363 patients suffering from primary lung cancer. The patients were divided into two groups according to their age (less than seventy or seventy and over). A comparison was made between the management of the two groups.

RESULTS

The comparisons according to age produced evidence of reduced activity, greater dependence, an increased Charlson score, less frequently administered radiotherapy and chemotherapy, and more frequent symptomatic treatment in the elderly group (p<0.001).

CONCLUSIONS

The geriatric assessment of patients suffering from primary lung cancer should make allowance for the physiological age of the patient and adapt the management to ensure the best quality of life.

Biomarkers by gender

Regressions were determined for age-related human biological functions containing information for both genders. Their intercept T(0) on the age axis (x) was used as a measure of the aging rate. The peak of the frequency distribution of T(0) was consistent with earlier estimates. The frequency distribution of the ratio R of T(0)(women)/T(0)(men) peaked at unity. However, when the T(0)-values were divided into two groups, namely those relating to functions involving musculature vs. the rest, respectively, the ratio of R for musculature was <1 and that for the latter significantly >1. This suggests that men are the stronger gender when musculature is involved, but, more broadly, women are "biologically stronger".

Biorepair mechanisms and longevity

The purpose of the study was to find out whether a link could be established between hypothetical biological repair mechanisms, their decay, and longevity. Human biological functions (biomarkers) can be classified accordingly as their generally linear age-related decline starts at birth or in adulthood ( approximately 1% per annum), or occurs at a rate of less than approximately 0.5% per annum Sums of exponentially declining functions representing the decline of repair mechanisms are fitted to the averages of each of the above groups. The time constants of the mechanisms are lowest for those ceasing normal function at the age of approximately 35-40 years, i.e., approximately at twice the maximal age at which puberty is reached. An extrapolation of the overall loss of the mechanisms, postulated to account for the declining biomarkers, is, at present, such as to reach zero in the twelfth decade of life.

Biological age--what is it and can it be measured?

Biological age is a concept used loosely and with little objectivity to describe a shortfall between a population cohort average life expectancy and the perceived life expectancy of an individual of the same age. Many biomarkers decline roughly linearly with age with a slope of <1% per annum. The use of a battery of 16 biomarkers is described as a method of calculating an individual biological age. They include: the concentration of prostacyclin in fibroblasts, cell membrane viscosity, the electroretinogram, baroreflex regulation of the heart rate, the concentration of lymphocytes, leucocyte density and velocity, grip strength, cells of the corneal endothelium and the buccal epithelium, neck muscle mobility, and vital capacity. Although not subjected to a prospective validation, the method might provide an objective approach to this widely used concept.

The eye within the framework of human senescence: biological decline and morbidity

An introduction to basic risk theory is applied to an analysis which is used to calculate risks for mortality, age-related maculopathy, and nuclear cataract. A number of ad hoc assumptions are introduced, and the predicted results compared with results culled from the literature. The results for nuclear cataract indicate that the loss of life expectancy associated with this condition may exceed 3 years. Finally, the theory is applied to an estimate of general blindness.

Human biological decline and mortality rates

Intercepts on the x (age)-axis of 107 normalized declining human biological functions were determined and assembled in 3 histograms, being placed in increasing order within each decade (10 year period). The histograms were classed accordingly as they contained properties associated with dividing cells, sensory properties and non-dividing cells respectively. Their cumulants were determined, multiple regressions calculated and compared with current death-rates for women and men respectively, for 10 amongst the longest living populations in the World. An alternative verification based on risk theory led to an estimate of an optimal life expectancy of 96 years. The survival curve turns out to be of the form (See text: Formula) where the inner integral represents the cumulant dimension (t') and the outer one age (t"). The premises underlying this study are compatible with the notion of a probable life-span, rather than a fixed one.

Presbyopia — a maverick of human aging

The aim of this study was to examine the extent to which the age-related variations of properties of the human lens may be able to account for presbyopia. Dimensionless linear regressions were calculated for age-related biological functions with special reference to ocular and lenticular ones. Their intercepts on the x-(age-)axis are compared, and their distribution is analyzed. An analysis was made of the effect of the growth of the lens on the relation between its shape and the proximal zonular anchorages on the one hand and the age-related variation of the angle between the zonule and the equatorial plane of the lens. The lens is not unusual in seeming to have evolved in support of a life-span of about 120 years. Presbyopia, however, fails to fit into the general picture and this is hypothesized to result from lenticular growth and a combination of factors which are not all governed by senescence. The potential involvement of the root of the iris throws an interesting light on the apparently worldwide variation of the condition.

Why does the human visual system age in the way it does?

The tentative observation that the decrease of lenticular glutathione in man and in cattle may be under genetic control is extended to other biological functions which show a systematic reduction with age. Ocular and visual parameters are shown to decline consistently with the view that the human eye has evolved in keeping with other biological functions sustaining a life-span of approximately 120 years. Analysis of the data suggests that presbyopia represents an outlier in the distribution of ocular attributes, and should not be used as a biomarker for ageing.

Human sensory functions and longevity

Age-related variations of human sensory properties have been expressed in a manner which permits them to be validly compared with other biological functions presented on a similar basis. They are found to be representative of much larger data-base, and it is tentatively suggested that they may have evolved in support of a life-span of approximately 120 years. Some theoretical arguments are advanced to explain this.