Human Aging Process Research Articles

Secondary Causes of Osteoporosis

Secondary Causes of Osteoporosis

Growing Pains: Bioethical Perspectives on Growth Hormone Replacement Research

The debate over growth hormone replacement in older adults reflects a set of contextual issues that are deeper than clinical concerns over the risks and efficacy of the intervention. One question that is central to the public and professional acceptance of growth hormone replacement asks whether the human aging process is an appropriate target for biomedical intervention in the first place. Researchers and clinicians in the field should be prepared to address this question, and this review surveys the major arguments regarding it. Five points of the debate are examined: the relative virtues of human aging, the notion of a natural human life cycle, the pathological status of aging, the fairness of anti-aging medicine, and the problem of the metaphysical misconception in recruiting human subjects for anti-aging research.

In vivo dehydroepiandrosterone restores age-associated defects in the protein kinase C signal transduction pathway and related functional responses.

Elderly subjects are at increased risk of pneumonia, influenza, and tuberculosis. Besides the known age-related decrease in mechanisms for mechanical clearance of the lungs, impaired alveolar macrophage function contributes to the increased risk of illness in the elderly. We have previously shown that age-induced macrophage immunodeficiencies are associated with a defective system for anchoring protein kinase C. Castration of young male rats produces effects on alveolar macrophages similar to those of aging, suggesting a relationship between circulating sex hormones, particularly androgens, and the decreases in the receptor for activated C kinase (RACK-1) and macrophage function observed. The aging process in humans and rats is associated with a decline in the plasma concentrations of dehydroepiandrosterone (DHEA) and its sulfate, among other steroid hormones. We report here that in vitro and in vivo administration of DHEA to rats restores the age-decreased level of RACK-1 and the LPS-stimulated production of TNF-alpha in alveolar macrophages. DHEA in vivo also restores age-decreased spleen mitogenic responses and the level of RACK-1 expression. These findings suggest that the age-related loss in immunological responses, linked to defective pathways of signal transduction, are partially under hormonal control and can be restored by appropriate replacement therapy.

Werner Syndrome.

Werner syndrome is a premature aging disease caused by the mutation in the WRN gene. The cloning and characterization of the WRN gene and its product allows investigators to study the disease and the human aging process at molecular level. This review summarizes the recent progresses on various aspects of the WRN research including functional analysis of the protein, interactive cloning, complexes formation, mouse models, and SNPs (single nucleotide polymorphisms). These in depth investigations have greatly advanced our understanding of the disease and elucidated future research direction for Werner syndrome and the human aging process.

Cell death and aging--a question of cell type.

Replicative senescence of human cells in primary culture is a widely accepted model for studying the molecular mechanisms of human ageing. The standard model used for studying human ageing consists of fibroblasts explanted from the skin and grown into in vitro senescence. From this model, we have learned much about molecular mechanisms underlying the human ageing process; however, the model presents clear limitations. In particular, a long-standing dogma holds that replicative senescence involves resistance to apoptosis, a belief that has led to considerable confusion concerning the role of apoptosis during human ageing. While there are data suggesting that apoptotic cell death plays a key role for ageing in vitro and in the pathogenesis of various age-associated diseases, this is not reflected in the current literature on in vitro senescence. In this article, I summarize key findings concerning the relationship between apoptosis and ageing in vivo and also review the literature concerning the role of apoptosis during in vitro senescence. Recent experimental findings, summarized in this article, suggest that apoptotic cell death (and probably other forms of cell death) are important features of the ageing process that can also be recapitulated in tissue culture systems to some extent. Another important lesson to learn from these studies is that mechanisms of in vivo senescence differ considerably between various histotypes.

In Human Keratinocytes the Common Deletion Reflects Donor Variabilities Rather Than Chronologic Aging and can be Induced by Ultraviolet A Irradiation

Mitochondrial DNA mutations play a major role in human aging processes and degenerative diseases. The most frequently reported marker for mutations of the mitochondrial DNA in human skin is a 4977 bp large-scale deletion, called the Common Deletion. Although this deletion is rarely detectable and constitutes only one example of the multitude of about 50,000 known mutations in mitochondrial DNA, it can represent "the tip of the iceberg" of all types of mitochondrial DNA mutations. We established a quantitative real-time polymerase chain reaction assay to detect the Common Deletion in vitro as well as in vivo/ex vivo. In contrast to previous studies, we were able to demonstrate that the Common Deletion is frequently abundant in keratinocytes isolated from various donors. Quantitative analysis of the mutation indicated interperson variations but obviously no relation to the donors' ages. Prolonged proliferation of keratinocytes led to a distinct reduction in the amount of the Common Deletion. Single ultraviolet A irradiation (12 J per cm2 and 15 J per cm2) neither in vitro nor in vivo increased the incidence of the mutation in keratinocytes, whereas repetitive irradiation resulted in a clear increase in vitro. Again, prolonged cultivation of these irradiated cells caused a significant reduction in the amounts of the deletion. In view of these results, the Common Deletion appears to be a useful marker rather for ultraviolet-A-induced alterations than for chronologic aging in human skin keratinocytes.

Possible protein link to ageing and cancer

Researchers have found both a yeast and human protein that binds telomeres, the very ends of chromosomes, which contain repeating chains of DNA (Peter Baumann and Thomas R. Cech, Science, 11 May). It is thought that the function of telomeres is to act as a biological clock by signalling the cell to stop dividing after a certain point. The telomere end-binding protein is called Protection of Telomeres or POT-1. When the POT-1 gene was knocked out, the yeast could not maintain its normal linear chromosomes and the ends rapidly degraded. The finding could have implications that reach as far as the treatment of cancer (the POT-1 protein might have a role in regulating telomere replication), the human ageing process or the production of immortal cell-lines. DM

Statut en vitamine C de 71 résidents de maisons de retraite au centre hospitalier de Blois

Vitamin E, the most active form is alpha-tocopherol, widely distributed in nature with different biological activities. It is a major lipid-soluble antioxidant responsible for protecting membranes against lipid peroxidation which could slow the aging process in humans or animals. Several roles of vitamin E have been reported such as antioxidant, intermediary in arachidonic acid and prostaglandin metabolism, nucleic acid, protein and lipid metabolism, mitochondrial function, sex hormones production, in maintaining the integrity of membranes, in protection against hemolytic anemia and impaired erythropoiesis, reducing the risks of heart disease, cancer, neurological diseases, cataract, retinopathy of premature infants and arthritis. Vitamin E deficiency results in neurological syndrome in people with chronic malabsorption. It is useful in the neurological diseases such as Parkinson's, Huntington's, epilepsy and tardiv dyskinesia. Several clinical applications of vitamin E are known in diseases such as abetalipoproteinemia, cystic fibrosis, cholestic liver disease, hemolytic anemias, respiratory distress, epilepsy, burns, aging, cancer, ischemic heart disease and cataract. The future study of vitamin E in humans or animal models should provide more definitive evidence of its absorption, transport, utilization and retention in various body organs and tissues as well as in protection and prevention of major neurological diseases.

The Werner Syndrome: A Model for the Study of Human Aging

Human aging is a complex process that leads to the gradual deterioration of body functions with time. Various models to approach the study of aging have been launched over the years such as the genetic analysis of life span in the yeast S. cerevisiae, the worm C. elegans, the fruitfly, and mouse, among others. In human models, there have been extensive efforts using replicative senescence, the study of centenerians, comparisons of young versus old at the organismal, cellular, and molecular levels, and the study of premature aging syndromes to understand the mechanisms leading to aging. One good model for studying human aging is a rare autosomal recessive disorder known as the Werner syndrome (WS), which is characterized by accelerated aging in vivo and in vitro. A genetic defect implicated in WS was mapped to the WRN locus. Mutations in this gene are believed to be associated, early in adulthood, with clinical symptoms normally found in old individuals. WRN functions as a DNA helicase, and recent evidence, summarized in this review, suggests specific biochemical roles for this multifaceted protein. The interaction of WRN protein with RPA (replication protein A) and p53 will undoubtedly direct efforts to further dissect the genetic pathway(s) in which WRN protein functions in DNA metabolism and will help to unravel its contribution to the human aging process.

Vitamin supplements through GM plants

Vitamins A and E are linked by common biosynthetic pathways and functions. Plants make carotenoids and tocopherols (vitamin E) in plastids, including chloroplasts, where they function as pigments and antioxidants in photosynthesis. Extra carotenoids and tocopherols reportedly enhance cardiovascular health, prevent cancer and slow aging processes in humans. An Agrobacterium-mediated transformation has been used to introduce the entire β-carotene biosynthetic pathway into the rice endosperm in a single transformation event using genes for a daffodil (Narcissus pseudonarcissus) phytoene synthase, a bacterial desaturase and a daffodil cyclase, to produce golden yellow rice. Just 300 g of rice each day should prevent vitamin A deficiency, and rice breeders in Asia, Africa and Latin America are now busy transferring the trait to common varieties. Science (2000) 287, 303–305.

Endothelial dysfunction in the klotho mouse and downregulation of klotho gene expression in various animal models of vascular and metabolic diseases.

The human aging process is associated with vascular endothelial dysfunction. However, humoral factors which might protect against endothelial dysfunction during aging have not yet been identified. We recently identified the klotho gene as a possible regulator of human aging. In the present study using the klotho-deficient heterozygous mouse, we examined whether the Klotho protein is a humoral factor protecting against endothelial dysfunction. We further cloned rat klotho cDNA and investigated whether klotho mRNA expression in rat kidney is altered under pathological conditions such as hypertension, hyperlipidemia, renal failure, and inflammatory stress. The Klotho protein itself, or its metabolites, promotes endothelial NO production in aorta as well as arterioles, and klotho mRNA in kidney is downregulated under sustained circulatory stress.

The Biology of Aging

Nothing has been demonstrated to slow or reverse the primary aging process in humans; instead, the factors that are known to affect longevity do so by their influence on disease development, which is part of secondary aging. Preventive strategies against secondary aging are aimed at maintaining health and functional capacity and rectangularizing, rather than extending, the survival curve. Interventions for preventive geriatrics and successful aging include a low-fat, low-energy diet with a high content of fruits and vegetables; exercise; and hormone replacement.

Age-dependent changes of antioxidant activities and markers of free radical damage in human skeletal muscle

This study was conducted in order to provide evidence for the role of reactive oxygen species (ROS) in human skeletal muscle aging. We used human muscle samples obtained from hospitalized patients in an open study with matched pairs of individuals of different ages. The subjects, ranging in age from 17 to 91 years, were grouped as follows: 17–25-, 26–35-, 36–45-, 46–55-, 56–65-, 66–75-, 76–85-, and 86–91-year-old groups. To investigate the relationship between muscle aging and oxidative damage we measured total and Mn-dependent superoxide dismutase (total SOD, MnSOD), glutathione peroxidase (GSHPx), and catalase (CAT) activities; total reduced and oxidized glutathione (GSHtot, GSH, and GSSG) levels; lipid peroxidation (LPO), and protein carbonyl content (PrC). Total SOD activity decreases significantly with age in the 66–75-year-old group, although MnSOD activity increases significantly in the 76–85-year-old group. The activity of the two H 2O 2 detoxifying enzymes (GSHPx and CAT) did not change with age, as do GSHtot and GSH levels. GSSG levels increased significantly (76–85- and 86–91-year-old groups) with age. We observed a significant increase in LPO levels (66–75- and 76–85-year-old groups), although the PrC content shows a trend of increase without gaining the statistical significance. These results support the idea that ROS play an important role in the human muscle aging process.

Dynamic paradigms for human mortality and aging.

Hazard models are often applied to mortality data of humans and other species so that the parameter estimates made for those models can be used to make inferences about the biology, and comparative biology, of aging processes. Enough longitudinal data on physiological and functional changes in humans now exist to know that the age trajectory of the physiological state of individuals is multidimensional, stochastic, and plastic. Thus, to fully assess the biological significance of existing longitudinal data on human aging and mortality processes, multivariate stochastic process models must be developed that are biologically detailed and valid. This requires assessing genetic mechanisms controlling human longevity and rates of aging, developing models of how those traits may have evolved, and developing statistical methods for identifying gene environment interactions. This article examines the theoretical basis for such models and the biological rationale of their parametric structure. Several examples are given.

Oxidative damage elicited by imbalance of free radical scavenging enzymes is associated with large-scale mtDNA deletions in aging human skin

Mitochondrial DNA (mtDNA) mutations and impaired respiratory function have been demonstrated in various tissues of aged individuals. We hypothesized that age-dependent increase of ROS and free radicals production in mitochondria is associated with the accumulation of large-scale mtDNA deletions. In this study, we first confirmed that the proportion of mtDNA with the 4977 bp deletion in human skin tissues increases with age. We then investigated the 8-hydroxy-2′-deoxyguanosine (8-OH-dG) content in skin tissues and lipid peroxides content of the skin fibroblasts from subjects of different ages. The results showed an age-dependent increase of 8-OH-dG level in the total DNA of skin tissues of the subjects above the age of 60 years. The specific content of malondialdehyde, an end product of lipid peroxidation, was also found to increase with age. On the other hand, we examined the enzyme activities of Cu,Zn-superoxide dismutase (Cu,Zn-SOD), Mn-superoxide dismutase (Mn-SOD), catalase, and glutathione peroxidase (GPx) in the skin fibroblasts. The activities of Cu,Zn-SOD, catalase and glutathione peroxidase were found to decrease with age. However, the activity of Mn-SOD was increased with age before 60 years but was decreased thereafter. Moreover, the activity ratios of Mn-SOD/catalase and Mn-SOD/GPx exhibited the same pattern of change with age. This indicates that free radical scavenging enzymes can effectively dispose of ROS and free radicals before 60 years of age. However, elevated oxidative stress caused by an imbalance between the production and removal of ROS and free radicals occurred in skin fibroblasts after 60 years of age. Taken together, we suggest that the functional decline of free radical scavenging enzymes and the elevation of oxidative stress may play an important role in eliciting oxidative damage and mutation of mtDNA during the human aging process.

Mitochondrial events in the life and death of animal cells: a brief overview.

Traditionally, mitochondria have been viewed as the "powerhouse" of the cell, i.e., the site of the oxidative phosphorylation machinery involved in ATP production. Consequently, much of the research conducted on mitochondria over the past 4 decades has focused on elucidating both those molecular events involved in ATP synthesis by oxidative phosphorylation and those involved in the biogenesis of the oxidative phosphorylation machinery. While monumental achievements have been made, and continue to be made, in the study of these remarkable but extremely complex processes essential for the life of most animal cells, it has been only in recent years that a large body of biological and biomedical scientists have come to recognize that mitochondria participate in other important processes. Two of these are cell death and aging which, not surprisingly, are related processes both involving, in part, the oxidative phosphorylation machinery. This new awareness has sparked a new and growing area of mitochondrial research, that has become of great interest to a wide variety of scientists ranging from those involved in elucidating the role of mitochondria in cell death and aging to those interested in either suppressing or facilitating these processes as it relates to identifying new therapies or drugs for human disease. It is the purpose of this brief introductory review to provide an overview of those mitochondrial events involved in the life and death of animal cells and to indicate how these events might relate to the human aging process. Much more is known, much remains controversial, and even more remains to be learned as indicated in the excellent set of minireviews that follow.

Changes in antioxidant enzyme levels and DNA damage during aging.

Multiple mechanisms underlie the human aging process, but interest continues in the role that free radicals and antioxidants may play. The concentrations of lymphocyte free radical generation (O(2) (-)& H(2)O(2)), DNA damage and antioxidant enzyme levels (glutathione Stransferase, superoxide dismutase and catalase) were evaluated in 110 healthy individuals with an age range of 20-80 years. The antioxidant enzyme levels were significantly less in very old age when compared to young. Moreover, the levels of free radical concentration and DNA damage were increased in the same age group with respect to younger group. Cigarette smoking had a positive relation with free radicals and DNA damage, and inverse relation with antioxidants. On the other hand, body mass was found to have positive relation with free radical generation only. The data indicate that depletion of antioxidant enzyme levels would render the older people more susceptible to free radical stress and DNA damage.

Qualitative and Quantitative Changes in Skeletal Muscle mtDNA and Expression of Mitochondrial-Encoded Genes in the Human Aging Process

It has been widely postulated that age-dependent changes in the mitochondrial genetic system may contribute to the human aging process. We recently reported unchanged specific activities of mitochondrial respiratory chain enzymes and a decrease in oxidation capacity of different substrates with aging, due, in part, to some confounding variables such as physical activity or tobacco consumption. The present study deals with age-related changes in muscle mtDNA structure and its biogenesis in humans. We found a low prevalence of mtDNA rearrangements with aging, only detected by PCR. The mtDNA content increased significantly with age (b= 0.0115,P< 0.0001). Also, an unchanged steady-state level of mitochondrial transcripts, a reduced transcription rate (P< 0.0001), and an increase in mitochondrial membrane lipid peroxidation (P< 0.0001) were observed in aging. These data demonstrate that minor structural mtDNA changes appear during the human aging process. By contrast, alterations in mitochondrial homeostasis ultimately producing modifications in mitochondrial biogenesis rates could play a role in the process of human senescence.

Role of Mitochondria in Human Aging.

Mitochondria are the major intracellular source and target sites of reactive oxygen species (ROS) that are continually generated as by-products of aerobic metabolism in animal and human cells. It has been demonstrated that mitochondrial respiratory function declines with age in various human tissues and that a defective respiratory chain results in enhanced production of ROS and free radicals in mitochondria. On the other hand, accumulating evidence now indicates that lipid peroxidation, protein modification and mitochondrial DNA (mtDNA) mutation are concurrently increased during aging. On the basis of these observations and the fact that the rate of cellular production of superoxide anions and hydrogen peroxide increases with age, it has recently been postulated that oxidative stress is a major contributory factor in the aging process. A causal relationship between oxidative modification and mutation of mtDNA, mitochondrial dysfunction and aging has emerged, although some details have remained unsolved. In this article, the role of mitochondria in the human aging process is reviewed on the basis of recent findings gathered from our and other laboratories.

Ageing-associated large-scale deletions of mitochondrial DNA in human hair follicles.

Hair follicles plucked from the bi-temporal region of the scalp of 433 Chinese subjects of different ages were used for the examination of ageing-associated mutations of human mitochondrial DNA (mtDNA). By use of PCR techniques, we detected the 4,977 bp and 7,436 bp deletions of mtDNA in hair follicles from aged individuals. The frequencies of occurrence of both mtDNA deletions were found to increase with age of the subject. Moreover, we employed a semi-quantitative PCR method to determine the proportion of the 4,977 bp deleted mtDNA (dmtDNA) in hair follicles. The results showed that the average proportion of the 4,977 bp dmtDNA in hair follicles were 0.05% +/- 0.01%, 0.00%, 0.55% +/- 0.05%, 0.52% +/- 0.24%, 0.65% +/- 0.17%, 1.33 +/- 0.25%, and 1.89% +/- 0.81% for the subjects in the age groups of 21-30, 31-40, 41-50, 51-60, 61-70, 71-80, and 81-99, respectively. Furthermore, we screened all the subjects harboring the 4,977 bp and/or 7,436 bp deletions for tandem duplications in the D-loop region of mtDNA by PCR with back-to-back primers. The results showed that none of the previously reported tandem duplications were present in all the hair follicles examined. This indicates that tandem duplications do not predispose to large-scale deletions of mtDNA. However, the data suggest that mtDNA deletions occur and accumulate in hair follicles during human ageing. As hair follicles can be easily and non-invasively obtained from the human, we suggest that the aged-dependent accumulation of dmtDNAs in hair follicles may be used for the monitoring of human ageing process.