Heat shock response and ageing: mechanisms and applications

Ageing is associated with a decrease in the ability of cells to cope with environmental challenges. This is due partly to the attenuation of a primordial stress response, the so-called heat shock (HS) response, which induces the expression of heat shock proteins (HSPs), composed of chaperones and proteases. The attenuation of the HS response during ageing may be responsible for the accumulation of damaged proteins as well as abnormal regulation of cell death. Maintenance of the HS response by repeated mild heat stress causes anti-ageing hormetic effects on cells and organisms. Here, we describe the molecular mechanism and the state of the HS response as well as the role of specific HSPs during ageing, and discuss the possibility of hormetic modulation of ageing and longevity by repeated mild stress.

Reduced levels of oxidized and glycoxidized proteins in human fibroblasts exposed to repeated mild heat shock during serial passaging in vitro

Repeated mild heat shock (RMHS) has beneficial hormesis-like effects on various characteristics of human skin fibroblasts undergoing replicative senescence in vitro. We have tested whether RMHS could reduce the accumulation of oxidized and glycoxidized proteins, which is a major age-related change. Levels of carbonylated proteins, furosine, N(epsilon)-carboxymethyl-lysine-rich proteins and advanced glycation end products increased during serial passaging of fibroblasts in culture. However, the extent of accumulation of oxidized and glycoxidized proteins was significantly reduced in RMHS cells. The basal concentration of reduced glutathione was higher and that of oxidized glutathione was lower in RMHS cells. Whereas the basal level of heat shock protein HSP27 decreased in both RMHS and control cells during serial passaging, the increase of the basal level of HSP70 with increasing passage level was significantly higher in RMHS cells. These results show that the slower accumulation of damaged proteins in fibroblasts exposed to RMHS results partly from the increased ability of these cells to cope with oxidative stress, and to synthesize HSP responsible for protein capping and refolding.

Applying hormesis in aging research and therapy

Biology of aging is well understood at a descriptive level. Based on these data, biogerontological research is now able to develop various possibilities for intervention. A promising approach for the identification of gerontogenes and gerontogenic processes is through the hormetic effects of mild stress on slowing down aging. Although there are several issues remaining to be resolved, specially with regard to the notion of mild stress, application of hormesis in aging research and therapy is a powerful new approach.

Number and proliferative capacity of osteogenic stem cells are maintained during aging and in patients with osteoporosis

Decreased bone formation is an important pathophysiological mechanism responsible for bone loss associated with aging and osteoporosis. Osteoblasts (OBs), originate from mesenchymal stem cells (MSCs) that are present in the bone marrow and form colonies (termed colony-forming units-fibroblastic [CFU-Fs]) when cultured in vitro. To examine the effect of aging and osteoporosis on the MSC population, we quantified the number of MSCs and their proliferative capacity in vitro. Fifty-one individuals were studied: 38 normal volunteers (23 young individuals [age, 22-44 years] and 15 old individuals [age, 66-74 years]) and 13 patients with osteoporosis (age, 58-83 years). Bone marrow was aspirated from iliac crest; mononuclear cells were enriched in MSCs by magnetic activated cell sorting (MACS) using STRO-1 antibody. Total CFU-F number, size distribution, cell density per CFU-F, number of alkaline phosphatase positive (ALP+) CFU-Fs, and the total ALP+ cells were determined. In addition, matrix mineralization as estimated by alizarin red S (AR-S) staining was quantified. No significant difference in colony-forming efficiency between young individuals (mean +/- SEM; 87 +/- 12 CFU-Fs/culture), old individuals (99 +/- 19 CFU-Fs/culture), and patients with osteoporosis (129 +/- 13 CFU-Fs/culture; p = 0.20) was found. Average CFU-F size and cell density per colony were similar in the three groups. Neither the percentage of ALP+ CFU-Fs (66 +/- 6%, 65 +/- 7%, and 72 +/- 4% for young individuals, old individuals, and patients with osteoporosis, respectively) nor the percentage of ALP+ cells per culture (34 +/- 5%, 40 +/- 6%, and 41 +/- 4%) differed between groups. Finally, mineralized matrix formation was similar in young individuals, old individuals, and patients with osteoporosis. Our study shows that the number and proliferative capacity of osteoprogenitor cells are maintained during aging and in patients with osteoporosis and that other mechanisms must be responsible for the defective osteoblast (OB) functions observed in these conditions.

Treatment with 1,25-dihydroxyvitamin D3 reduces impairment of human osteoblast functions during cellular aging in culture

Adequate responses to various hormones, such as 1,25-dihydroxyvitamin D(3) (calcitriol) are a prerequisite for optimal osteoblast functions. We have previously characterized several human diploid osteoblastic cell lines that exhibit typical in vitro aging characteristics during long-term subculturing. In order to study in vitro age-related changes in osteoblast functions, we compared constitutive mRNA levels of osteoblast-specific genes in early-passage (< 50% lifespan completed) with those of late-passage cells (> 90% lifespan completed). We found a significant reduction in mRNA levels of alkaline phosphatase (AP: 68%), osteocalcin (OC: 67%), and collagen type I (ColI: 76%) in in vitro senescent late-passage cells compared to early-passage cells, suggesting an in vitro age-related impairment of osteoblast functions. We hypothesized that decreased osteoblast functions with in vitro aging is due to impaired responsiveness to calcitriol known to be important for the regulation of biological activities of the osteoblasts. Thus, we examined changes in vitamin D receptor (VDR) system and the osteoblastic responses to calcitriol treatment during in vitro osteoblast aging. We found no change in the amount of VDR at either steady state mRNA level or protein level with increasing in vitro osteoblast age and examination of VDR localization, nuclear translocation and DNA binding activity revealed no in vitro age-related changes. Furthermore, calcitriol (10(-8)M) treatment of early-passage osteoblastic cells inhibited their proliferation by 57 +/- 1% and stimulated steady state mRNA levels of AP (1.7 +/- 0.1-fold) and OC (1.8 +/- 0.2-fold). Similarly, calcitriol treatment increased mRNA levels of AP (1.7 +/- 0.2-fold) and OC (3.0 +/- 0.3-fold) in late-passage osteoblastic cells. Thus, in vitro senescent osteoblastic cells maintain their responsiveness to calcitriol and some of the observed in vitro age-related decreases in biological markers of osteoblast functions can be reverted by calcitriol treatment.

Kinetin inhibits protein oxidation and glycoxidation in vitro

We tested the ability of N(6)-furfuryladenine (kinetin) to protect against oxidative and glycoxidative protein damage generated in vitro by sugars and by an iron/ascorbate system. At 50 microM, kinetin was more efficient (82% inhibition) than adenine (49% inhibition) to inhibit the bovine serum albumin (BSA)-pentosidine formation in slow and fast glycation/glycoxidation models. Kinetin also inhibited the formation of BSA-carbonyls after oxidation significantly more than adenine did. However both compounds inhibited the advanced glycation end product (AGE) formation to the same extent (59-68% inhibition). At 200 microM, kinetin but not adenine, limited the aggregation of BSA during glycation. These data suggest that kinetin is a strong inhibitor of oxidative and glycoxidative protein-damage generated in vitro.

Changes in the insulin-like growth factor-system may contribute to in vitro age-related impaired osteoblast functions

Age-related bone loss is thought to be due to impaired osteoblast functions. Insulin-like growth factors (IGFs) have been shown to be important stimulators of bone formation and osteoblast activities in vitro and in vivo. We tested the hypothesis that in vitro osteoblast senescence is associated with changes in components of the IGF-system including IGF-I, IGF-II, IGF-binding proteins (IGFBPs) and IGFBP-specific proteases. We employed a human diploid osteoblast cell line obtained from trabecular bone explants and that exhibit typical characteristics of in vitro senescence during serial subculturing. Using a non-competitive reverse-transcriptase polymerase-chain reaction (RT-PCR) assay, we found that the constitutive level of IGF-I mRNA decreased progressively to 49.9 +/- 4.9% in old osteoblasts as compared to the levels found in the young cells. No age-related change was found in IGF-II steady-state mRNA levels. Changes in IGFBPs gene expression and protein production were assessed using Northern blot analysis and Western ligand blotting (WLB), respectively. IGFBP-3 mRNA levels decreased to 30% and protein production to 16% in aged osteoblasts as compared to levels found in young cells. We also found age-related decreases in mRNA levels of both IGFBP-4 and IGFBP-5 to 70% and 60% in aged osteoblasts, respectively, compared to young cells. While IGFBP-5 protein was not detected by WLB, IGFBP-4 protein production showed a biphasic change with 50% decrease in middle-aged cells and a subsequent increase in aged osteoblasts to levels similar to those in young osteoblasts. We found an age-related increase in the immunoreactive levels of IGFBP-4 protease, however, no detectable IGFBP-4 or IGFBP-3 protease activities in conditioned media from osteoblast cultures were observed. Our findings demonstrate that osteoblast aging is associated with impaired production of the stimulatory components of the IGF-system, that may be a mechanism contributing to age-related decline in osteoblast functions.

Modulating cellular aging in vitro: hormetic effects of repeated mild heat stress on protein oxidation and glycation

Intracellular and extracellular proteins are subject to a variety of spontaneous non-enzymatic modifications which affect their structure, function and stability. Protein oxidation and glycation are tightly linked and are implicated in the development of many pathological consequences of aging. Although multiple endogenous pathways in the cell can prevent the formation of oxidized and glycated proteins, and repair and degrade abnormal proteins, such abnormal proteins do accumulate during aging. The heat shock response involving the family of stress-proteins or the so-called heat shock proteins (HSP), represents the quickest and highly conserved response to proteotoxic insults. Since repeated mild heat stress is able to prevent the onset of various age-related changes during cellular aging in vitro, we suggest that treatments which increase HSP expression should reduce the extent of accumulation of abnormal proteins during aging. Such modulation of aging is an example of hormesis, which is characterized by the beneficial effects resulting from the cellular responses to mild repeated stress.

Biogerontology: the next step

After a long period of collecting empirical data describing the changes in organisms, organs, tissues, cells, and macromolecules, biogerontological research is now able to develop various possibilities for intervention. Because aging is a stochastic and nondeterministic process characterized by a progressive failure of maintenance and repair, it is reasoned that gene involved in homeodynamic repair pathways are the most likely candidate gerontogenes. A promising approach for the identification of critical gerontogenic processes is through the hormesis-like positive effects of mild stress. Stimulation of various repair pathways by mild stress has significant effects on delaying the onset of various age-associated alterations in cells, tissues, and organisms.

Gene-specific DNA repair of pyrimidine dimers does not decline during cellular aging in vitro

A large number of studies have demonstrated that various kinds of DNA damage accumulate during aging and one of the causes for this could be a decrease in DNA repair capacity. However, the level of total genomic repair has not been strongly correlated with aging. DNA repair of certain kinds of damage is known to be closely connected to the transcription process; thus, we chose to investigate the level of gene-specific repair of UV-induced damage using in vitro aging of human diploid skin fibroblasts and trabecular osteoblasts as model systems for aging. We find that the total genomic repair is not significantly affected during cellular aging of cultures of both human skin fibroblasts and trabecular osteoblasts. Gene-specific repair was analyzed during cellular aging in the dihydrofolate reductase housekeeping gene, the p53 tumor suppressor gene, and the inactive region X(754). There was no clear difference in the capacity of young and old cells to repair UV-induced pyrimidine dimers in any of the analyzed genes. Thus, in vitro senescent cells can sustain the ability to repair externally induced damage.

CBFA1 and topoisomerase I mRNA levels decline during cellular aging of human trabecular osteoblasts

In order to understand the reasons for age-related impairment of the function of bone forming osteoblasts, we have examined the steady-state mRNA levels of the transcription factor CBFA1 and topoisomerase I during cellular aging of normal human trabecular osteoblasts, by the use of semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR). There is a progressive and significant reduction of the CBFA1 steady-state mRNA level down to 50% during cellular aging of human osteoblasts. In comparison to the normal cells, human osteosarcoma cell lines SaOS-2 and KHOS/NP, and the SV40-transformed human lung fibroblast cell line MRC5V2 have 20 to 40% higher levels of CBFA1 mRNA. Similar levels of CBFA1 mRNA are detectable in normal human skin fibroblasts, and these cells also exhibit an age-related decline to the same extent. In addition, the expression of topoisomerase I is reduced by 40% in senescent osteoblasts, and the mRNA levels are significantly higher (40-70%) in transformed osteoblasts and fibroblasts. These changes in gene expression may be among the causes of impaired osteoblast functions, resulting in reduced bone formation during aging.

Ageing, gerontogenes, and hormesis

Evolutionary theories of ageing and longevity argue against the existence of specific genes that cause ageing. However, genes whose altered activity influences ageing and longevity, may be termed gerontogenes. Several putative gerontogenes have been identified in various ageing systems, including the Drosophila, budding yeast, nematodes and cells in culture. Since ageing is characterized by a progressive failure of maintenance and repair, it is reasoned that genes involved in homeodynamic repair pathways are the most likely candidate gerontogenes. A promising approach for the identification of critical gerontogenic processes is hormesis-like positive effects of stress. Stimulation of various repair pathways by mild stress has significant effects on delaying the onset of various age-associated alterations in cells, tissues and organisms.

Some unusual nucleic acid bases are products of hydroxyl radical oxidation of DNA and RNA

There are over 100 modified bases and their derivatives found in RNA and DNA. For some of them, data concerning their properties, synthesis and roles in cellular metabolism are available, but for others the knowledge of their functions and biosynthetic pathways is rather limited. We have analysed the chemical structure of modified nucleosides of DNA and RNA considering mainly their putative synthetic routes. On this basis we suggest, that in addition to enzymatic biosynthetic pathways well established for some odd bases, many rare nucleosides can be recognised as products of random chemical reactions. We identify them as primary or secondary products of the reaction of nucleic acids with hydroxyl radicals, the most active oxidising agent in the cell.

N(6)-Furfuryladenine, kinetin, protects against Fenton reaction-mediated oxidative damage to DNA

N(6)-Furfuryladenine (kinetin) has been shown to have anti-ageing effects on several different systems including plants, human cells in culture, and fruitflies. Since most of the experimental data point toward kinetin acting as an antioxidant both in vitro and in vivo, and since much evidence supporting a causal role of oxidative damage in ageing is accumulating, we tested the antioxidant properties of kinetin directly. Using 8-oxo-2'deoxyguanosine (8-oxo-dG) in calf thymus DNA as a marker for oxidative damage, we demonstrate that kinetin significantly (P < 0.005) protects the DNA against oxidative damage mediated by the Fenton reaction. Kinetin inhibited 8-oxo-dG formation in a dose-dependent manner with a maximum of 50% protection observed at 100 microM kinetin.

Telomere shortening during aging of human osteoblasts in vitro and leukocytes in vivo: lack of excessive telomere loss in osteoporotic patients

We have compared the telomere length, as assessed by Southern analysis, of telomere restriction fragments (TRFs) generated by RsaI/HinfI digestion of genomic DNA in: (i) in vitro cultured human trabecular osteoblasts undergoing cellular aging; and (ii) peripheral blood leukocytes (PBL) obtained from three groups of women: young (aged 20-26 years, n = 15), elderly (aged 48-85 years, n = 15) and osteoporotic (aged 52-81 years, n = 14). The mean TRF length in human osteoblasts undergoing aging in vitro decreased from an average of 9.32 kilobasepairs (kb) in middle-aged cells to an average of 7.80 kb in old cells. The rate of TRF shortening was about 100 bp per population doubling, which is similar to what has been reported for other cell types, such as human fibroblasts. Furthermore, there was a 30% decline in the total amount of telomeric DNA in senescent osteoblasts as compared with young cells. In the case of PBL, TRF length in the DNA extracted from young women was slightly longer (6.76 +/- 0.64 kb) than that from a group of elderly women (6.42 +/- 0.71 kb). A comparison of TRFs in the DNA extracted from the PBL from osteoporotic patients and from age-matched controls did not show any significant differences (6.47 +/- 0.94 versus 6.42 +/- 0.71 kb, respectively). Therefore, using TRF length as a marker for cellular aging in vitro and in vivo, our data comparing TRFs from osteoporotic patients and age-matched controls do not support the notion of the occurrence of a generalized premature cellular aging in osteoporotic patients.

The nature of gerontogenes and vitagenes. Antiaging effects of repeated heat shock on human fibroblasts

Our survival and the physical quality of life depends upon an efficient functioning of various maintenance and repair processes. This complex network of the so-called longevity assurance processes is composed of several genes, termed vitagenes. The homeodynamic property of living systems is a function of such a vitagene network. Because aging is characterized by the failure of homeodynamics, a decreased efficiency and accuracy of the vitagene network can transmutate it into a gerontogene network. It is not clear how various components of the vitagene network operate and influence each other in a concordant or a discordant manner. Experimental strategies through which this transmutation of vitagenes into virtual gerontogenes may be elucidated include induction of molecular damage, antisense intervention, and genetic screening for varied efficiencies of the members of the vitagene family. A reversal of this approach by maintaining or recovering the activity of vitagenes will lead to a delay of aging, a decreased occurrence of age-related diseases, and a prolongation of a healthy life span.

Repeated mild heat shock delays ageing in cultured human skin fibroblasts

The effects of repetitive mild heat shock (30 min, 41 degrees C) on growth and various cellular and biochemical characteristics of human skin fibroblasts undergoing ageing in vitro were analysed. Human skin cells not only tolerated more than 30 repeated heat shocks throughout their replicative lifespan, but also maintained several characteristics of young cells until late in life. 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 neutral beta-galactosidase activity were significantly slowed down by repeated mild heat shock. These hormesis-like effects of stress-induced defence processes can be useful to elucidate the role of maintenance and repair mechanisms in ageing.

Cloning and identification of genes that associate with mammalian replicative senescence

Cellular senescence and limited proliferative capacity of normal diploid cells has a dominant phenotype over immortality of cancerous cells, suggesting its regulation by the expression of a set of genes. In order to isolate the genes that associate with senescence, we have employed a clonal system of conditional SV40 T antigen rat embryo fibroblast cell lines which undergo senescence upon T antigen inactivation. Construction of cDNA libraries from two conditional cell lines and application of differential screening and subtractive hybridization techniques have resulted in the cloning of eight senescence-induced genes (SGP-2/Apo J, alpha 1-procollagen, osteonectin, fibronectin, SM22, cytochrome C oxidase, GTP-alpha, and a novel gene) and a senescence-repressed gene (FRS-2). Three of these genes encode for extracellular matrix proteins, others are involved in the calcium-dependent signal transduction pathways, while the SGP-2/Apo J gene may have a cellular protective function. RNA analysis has shown that the senescence-associated genes are overexpressed in both normal rat embryonic fibroblasts and human osteoblasts cell cultures undergoing aging in vitro. In comparison, the expression of these genes in a rat fibroblast immortalized cell line (208F cells) was down-regulated after both its partial and its full transformation by ras oncogenes. Thus, cloning of senescence-associated genes opens up new ways to elucidate and/or to modulate aging and cancer.

Is gene therapy for aging possible?

Throughout human history, search for means to prevent or slow down aging has followed three main lines--(1) removing waste products and cleansing impurities; (2) using products of plants and animals as medicine; and (3) compensating for decrease in various hormones, vitamins and other chemicals in the body. Even in modern times, immense popularity of various spas and water therapies is an example of the first type of anti-aging approach for which there are no real scientific basis. Some preliminary support from laboratory and/or clinical tests is available for various herbal and other medicinal plant products, such as ginseng, ginkgo biloba and garlic, as nutritional supplements. Replacement therapy, especially hormonal replacement therapy as an anti-aging treatment has been used and misused for quite some time.

Furfural, a precursor of the cytokinin hormone kinetin, and base propenals are formed by hydroxyl radical damage of DNA

Recently, we have detected kinetin (N6-furfuryladenine), a well known cytokinin plant hormone, in commercially available DNA, in freshly extracted cellular DNA and in plant cell extracts. We had suggested that the furfuryl moiety of kinetin originates from furfural which is one of the primary oxidation products of deoxyribose in DNA. Here we show that the human cell extracts treated with O-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine (PFBHA) give rise to oxime derivatives of various aldehydes present in the cell. Mass spectrometric analysis of silylated oximes showed several mass signals of different species, one of which was identified as furfural. Furthermore, detailed inspection of the mass spectra of DNA showed the mass signals of 165, 180, 189 and 206 m/z which correspond to cytosine-propenal, thymine-propenal, adenine-propenal and guanine-propenal, respectively. The presence of furfural, along with four base-propenals in the cell extract, as the primary oxidation products of deoxyribose, suggests that degradation of sugar residues in DNA is one of the major routes of cellular damage in addition to the modification of nucleic acid bases.

A mechanism for the in vivo formation of N6-furfuryladenine, kinetin, as a secondary oxidative damage product of DNA

Recently, we have reported the presence of kinetin (N6-furfuryladenine) in commercially available DNA, in freshly extracted cellular DNA and in plant cell extracts. We have also found that kinetin has electrochemical properties which can be used for monitoring the level of this modified base in DNA. Here, for the first time, we propose a mechanism for the formation of kinetin in DNA in vivo, based on the analyses of its mass spectra. Since hydroxy radical oxidation at the carbon 5' of the deoxyribose residue yields furfural, we propose that this aldehyde reacts with the amino group of adenine and, after intramolecular rearrangement, kinetin is formed in vivo. Thus kinetin is the first stable secondary DNA damage product known to date with very well defined cytokinin and anti-aging properties, linked to oxidative processes in the cell. These results also indicate that N6-furfuryladenine or kinetin is an important component of a new salvage pathway of hydroxy radicals constituting a 'free radical sink'. In this way, the cells can neutralize the harmful properties of hydroxyl radical reaction products, such as furfural, and respond to oxidative stress by inducing defence mechanisms of maintenance and repair.

Increased longevity of kinetin-fed Zaprionus fruitflies is accompanied by their reduced fecundity and enhanced catalase activity

Kinetin, a cytokinin plant growth hormone, retards senescence in plants, delays aging in human cells in culture, slows down development of insects and prolongs their lifespan. We have now observed that the increased longevity of Kn-fed Zaprionus fruitflies was accompanied by an increase in the specific activity of catalase during developmental stages and in adult insects. In addition, the egg laying capacity of Kn-fed fruitflies was reduced drastically as compared with those kept on a normal diet. These results support the view that improved maintenance of the soma and prolongation of its life is achieved at the cost of decreased reproductive activity.

Demonstration of cellular aging and senescence in serially passaged long-term cultures of human trabecular osteoblasts

The proliferative capacity and cellular and biochemical characteristics of human trabecular bone osteoblasts were analysed throughout their replicative lifespan in vitro. Like several other cell types, human osteoblasts demonstrated a typical Hayflick phenomenon of cellular aging comprising a period of rapid proliferation until cumulative population doubling level (CPDL) 22 to 24, followed by a phase of slow growth and the final cessation of cell division at CPDL 32 to 34. Comparing young cells (less than 20% lifespan completed) and old cells (more than 90% lifespan completed) revealed a progressive increase in population doubling (PD) time, a decrease in attachment frequency, a decrease in the number of S-phase positive cells, a decrease in the rates of DNA, RNA and protein synthesis, an increase in the protein content per cell and an increased proportion of senescence-specific beta-galactosidase positive cells. While osteoblastic production of collagen type I decreased progressively during aging, alkaline phosphatase activity dropped rapidly after the first few passages and then remained constant during the rest of the proliferative lifespan, Significant morphological changes from thin and spindle-shaped early passage young cells to large, flattened and irregularly shaped late passage old cells full of intracellular debris were observed. In comparison, osteoblasts established from an osteoporotic bone sample showed a maximum CPDL of less than 5, had a longer PD time and exhibited abnormal senescent morphology. Thus, we have demonstrated for the first time that human osteoblasts, like several other diploid cell types, have a limited proliferative capacity in vitro and undergo aging and senescence as measured by various cellular and biochemical markers. In addition, preliminary studies show that cells from osteoporotic bone have a severely reduced proliferative capacity. This model of bone cell aging facilitates study of the molecular mechanisms of osteoblast senescence as well as factors related to osteoblast dysfunction in patients with osteoporosis.

Evidence for the presence of kinetin in DNA and cell extracts

In contrast to the current view that kinetin (N6-furfuryladenine) is an unnatural and synthetic compound, we have detected it in commercially available DNA, in freshly extracted cellular DNA from human cells and in plant cell extracts by two independent methods. First, we discovered that N6-furfuryladenine has electrochemical properties that can be applied for monitoring this modified base by a HPLC/UV/EC method. Second, we have confirmed electrochemical assignments by mass-spectrometric analysis. A pathway of kinetin formation is proposed in which the formation of furfural by oxidative damage of the deoxyribose moiety of DNA is followed by its reaction with adenine residues to form N6-furfuryladenine. Since this modification can lead to mutations, the odd DNA base has to be removed by repair enzymes.

Molecular links between cellular mortality and immortality (review)

Normal diploid cells cultivated in vitro exhibit limited division potential while undergoing ageing during serial passaging. In contrast, cells that have been genetically transformed appear to have lost the regulatory mechanisms of limited growth potential and may continue to divide indefinitely. While cellular mortality is characterised by a progressive cessation of cell growth manifested in cell culture by senescence, immortalisation is the escape from senescence as a result of multiple mechanisms involving the inactivation of tumour suppressor genes, the elevated expression of oncogenes, as well as other genetic and epigenetic changes. The mechanisms governing mortality and immortality are fundamentally linked. The physiological and biochemical features which characterise cellular mortality are examined, herein in the search for markers and timing mechanisms of mortality. The genetic elements involved in the control of mortality and immortality are also discussed, and the fundamental similarities between the molecular and genetic aspects which govern the determination of the phenotypes manifesting the two processes are underlined.

Distinction between differentiation and senescence and the absence of increased apoptosis in human keratinocytes undergoing cellular aging in vitro

We have examined the processes of differentiation and apoptosis in relation to cellular aging by using a culture system based on serial passaging of monolayer cultures of human keratinocytes in a serum-free low calcium medium. Differentiation was analyzed by cellular morphology and by the expression of keratinocyte transglutaminase. Keratinocyte cultures at all passages could be induced to differentiate into mature keratinocytes by raising the calcium concentration in the medium. Differentiation could also be induced in senescent cultures, but the process was significantly slower. Apoptosis was analyzed by cellular morphology and by the expression of tissue transglutaminase. Apoptotic cells constituted a very small proportion of the culture and no detectable change in the incidence of apoptosis occurred during serial passaging.

Cellular and molecular determinants of ageing

The highly complex nature of the process of ageing implicates both genetic and epigenetic causative factors. A progressive failure of maintenance underlines and typifies this process. The instability of the nuclear and the mitochondrial genomes is an important determinant of ageing. Infidelity and misregulation of genetic information transfer, loss of cellular proliferative capacity, altered cellular responsiveness and defective pathways of signal transduction are major aspects of the failure of homeostasis. These are also the basis of age-related diseases and impairments, such as osteoporosis, arthritis, immune deficiency, altered drug clearance and altered functioning of the brain. Studies directed towards understanding the mechanisms of interaction and inter-dependence of various genes involved in maintenance and repair networks are the most promising research strategies for identifying gerontogenes.

Synthesis, modifications, and turnover of proteins during aging

Slowing down of bulk protein synthesis is one of the most commonly observed biochemical changes during aging. The implications and consequences of slower rates of protein synthesis are manifold, including a decrease in the availability of enzymes for the maintenance, repair, and normal metabolic functioning of the cell, an inefficient removal of inactive, abnormal, and damaged macromolecules in the cell, the inefficiency of the intracellular and intercellular signalling pathways, and a decrease in the production and secretion of hormones, antibodies, neurotransmitters, and the components of the extracellular matrix. Age-related changes in the activity, specificity, and stability of a large number of proteins have been reported. However, the molecular mechanisms responsible for such alterations are still poorly understood. Studies on various components of the protein synthetic machinery have revealed a decline in the efficiency and accuracy of ribosomes, an increase in the levels of rRNA and tRNA, and a decrease in the amounts and activities of elongation factors. Because posttranslational modifications of proteins determine their activity and stability, alterations in the extent and level of various modifications such as phosphorylation, methylation, ADP-ribosylation, oxidation, glycation, and conformational changes during aging are being studied. Changes in the regulation of protein synthesis, posttranslational modifications, and protein turnover are crucial determinants of age-related decline in the maintenance, repair, and survival of the organism.

Plant growth hormone kinetin delays ageing, prolongs the lifespan and slows down development of the fruitfly Zaprionus paravittiger

A cytokinin plant growth hormone kinetin (Kn) retards senescence in plants and delays ageing in human cells in culture. We have now observed that Kn also slows down ageing and prolongs the lifespan of the fruitfly Zaprionus paravittiger when these insects are fed with 25-125 ppm Kn added in their diet medium. In addition, 25 ppm Kn also slows down the larval and the pupal stages of the developing insects. However, the anti-ageing effects of Kn are not merely due to the slowing-down of development, but are mainly due to a reduction in the age-specific death rates throughout the adult lifespan.

Dephosphorylation of the phosphorylated elongation factor-2 in the livers of calorie-restricted and freely-fed rats during ageing

Slowing down of the rate of protein synthesis during ageing is accompanied by alterations in the amounts and activities of elongation factors, eEF-1 and eEF-2. Since the activity of eEF-2 is regulated by phosphorylation, we have determined the changes in the activities of eEF-2-specific phosphorylating and dephosphorylating enzymes during ageing. Previously, we have reported an age-related increase in the activity of eEF-2 kinase (BBRC, 192, 1210, 1993). We have now compared the activities of a dephosphorylating enzyme protein phosphatase 2A (PP2A) in young and old liver extracts from freely-fed or calorie-restricted rats. The activity of PP2A remain unaltered during ageing. Furthermore, there was no change in the kinetics and extent of PP2A-dependent and PP2A-independent dephosphorylation of eEF-2 during ageing.

Gerontogenes: real or virtual?

The view that the life span of an organism is intrinsically limited and is largely species-specific necessarily involves certain notions of genetic elements of regulation. The term gerontogenes refers to any such genetic elements that are involved in the regulation of aging and life span. The existence of genes for programmed aging is generally discounted on the basis of evolutionary arguments against the notion of the adaptive nature of aging. It is suggested here that the concept of gerontogenes be linked with the idea of genes involved in homeostasis and longevity assurance, which is not contradictory to the nonadaptive nature of aging. Because these genes were not originally selected as real genes for aging, their involvement in aging is an emergent property making them virtual gerontogenes. Some experimental evidence is available that suggests that sets of genes involved in the maintenance and repair of various cellular functions are the primary candidates qualifying as virtual gerontogenes.

Testing garlic for possible anti-ageing effects on long-term growth characteristics, morphology and macromolecular synthesis of human fibroblasts in culture

The beneficial effects claimed for the use of garlic as a nutritional supplement include detoxification, antioxidation, antifungal activity, antibacterial activity, tumour suppression and, possibly, anti-ageing and rejuvenating effects. We have used the Hayflick system of cellular ageing in culture in order to test garlic for its anti-ageing effects on long-term growth characteristics, morphology and macromolecular synthesis of human skin fibroblasts. Our results show that an addition of garlic extract into the normal cell culture medium can support serial subculturing for over more than 55 population doublings in 475 days, and that this treatment has some youth-preserving, anti-ageing and beneficial effects on human fibroblasts in terms of maximum proliferative capacity and morphological characteristics. In comparison, similar or lesser doses of garlic extracts are growth inhibitory for cancerous cells that could not be grown over longer periods in the presence of garlic. To our knowledge, this is the first report of the effects of garlic on the long-term growth characteristics and macromolecular synthesis of normal human skin cells, the results of which have applications for both anti-ageing and anti-cancer research.

Kinetin delays the onset of ageing characteristics in human fibroblasts

Kinetin (Kn) is a synthetic cytokinin plant growth hormone having some senescence-retarding effects in plants. Its effects on animal cells have been, until now, best considered as growth inhibitory and anti-tumorigenic. However, we have observed that an addition of 40-200 microM Kn in the culture medium of human diploid fibroblasts can both delay the onset and decrease the extent of many of the ageing characteristics that appear during serial passaging of normal cells in culture. Age-related changes that are affected by Kn include morphological alterations, growth rates, cell size, cytoskeletal organisation, macromolecular synthesis and the intensity of autofluorescence due to the oxidative damage product lipofuscin. These anti-ageing effects of Kn are achieved without any increase in the cell culture lifespan in terms of maximum proliferative capacity in vitro.

Phorbol ester PMA stimulates protein synthesis and increases the levels of active elongation factors EF-1 alpha and EF-2 in ageing human fibroblasts

Phorbol esters modulate gene expression, reorganize the cytoskeleton and stimulate bulk protein synthesis and the steps of initiation and elongation. We have observed that a phorbol ester PMA stimulates protein synthesis and increases the amounts of active elongation factors, EF-1 alpha and EF-2 in cultured human fibroblasts MRC-5 undergoing ageing. Although bulk protein synthesis slows down during ageing, the cellular response to the stimulatory effects of PMA is higher in senescent cells. Similarly, despite the age-related decline in the amounts of active EF-1 alpha and EF-2, senescent cells exhibit a higher response to PMA. The results indicate an age-dependent increase of cellular responsiveness to PMA and provide evidence about both the integrity of the translational apparatus and the effectiveness of the signal transduction pathways during cellular ageing. In comparison, the effects of PMA on SV40-transformed MRC-5V2 cells were minimal.

Elongation factor 2-specific calcium and calmodulin dependent protein kinase III activity in rat livers varies with age and calorie restriction

Elongation factors involved in polypeptide chain elongation are considered to be rate limiting for the slowing down of total protein synthesis during ageing. The activities of elongation factors are themselves regulated by various means, including phosphorylation. Here we have compared the activity of a protein kinase, called calcium- and calmodulin-dependent protein kinase III (CaM PK III), specific for the phosphorylation of elongation factor eEF-2, in cell-free extracts prepared from livers isolated from young and old male Fischer 344 rats maintained under freely fed or calorie-restricted dietary regimes. There was a significant increase of more than 70% in the activity of CaM PK III in 24 month old freely fed rat livers as compared with young animals. This age-related increase was found but to a lower extent (46%) in calorie-restricted rats of the same age. Therefore, slowing down of ageing in calorie-restricted animals is also reflected at the level of the regulation of the activity of protein elongation factor eEF-2 by CaM PK III.

Reduction in the amount of 8-hydroxy-2'-deoxyguanosine in the DNA of SV40-transformed human fibroblasts as compared with normal cells in culture

DNA damage due to oxidative free radicals is considered to be a major cause of ageing and age-related diseases including cancer. Of more than 20 modifications formed in DNA by the action of hydroxyl radicals, 8-hydroxy-2'-deoxyguanosine (oh8dG) is potentially highly mutagenic and is known to occur most frequently. Using HPLC combined with electrochemical (HPLC/EC) detection of oh8dG, fivefold higher levels of oh8dG are detected in the DNA of cultured normal human skin fibroblasts as compared with SV40-transformed human fibroblasts MRC-5V2. In comparison, the levels of oh8dG were similar in the growth medium of both types of cells. Applications of this method range from studies on the genomic stability and instability of normal and cancerous cells to the clinical and laboratory testing of toxic substances and drugs.

Specific incorporation of kinetin into eukaryotic and prokaryotic transfer ribonucleic acid molecules

We show that kinetin, a non-natural product with strong cytokinin activity, is incorporated into prokaryotic and eukaryotic tRNAs in the exchange reaction catalysed by a putative tRNA-kinetin transglycosylase. We also show that kinetin is specifically incorporated into E. coli tRNA(Tyr) and most probably at position 37. To our knowledge, this is the first report of a nucleic acid base exchange reaction occurring at this position.

Protein synthesis, posttranslational modifications, and aging

Posttranslational modifications of proteins are involved in determining their activities, stability, and specificity of interaction. More than 140 major and minor modifications of proteins have been reported. Of these, only a few have been studied in relation to the aging of cells, tissues, and organisms. These include phosphorylation, methylation, ADP-ribosylation, oxidation, glycation, and deamidation. Several of these modifications occur on proteins involved in crucial cellular processes, such as DNA synthesis, protein synthesis, protein degradation, signal transduction, cytoskeletal organization, and the components of extracellular matrix. Some of the modifications are the markers of abnormal and altered proteins for rapid degradation. Others make them less susceptible to degradation by normal proteolytic enzymes, and hence these accumulate during aging.

Senescent human fibroblasts are more sensitive to the effects of a phorbol ester on macromolecular synthesis and growth characteristics

4-beta-phorbol-12-beta-myristate-13-alpha-acetate (PMA) alters cellular growth properties by modulating gene expression in a wide variety of cell types. Human diploid fibroblasts MRC-5 were treated with PMA at different phases of their lifespan in vitro and the alterations of their short-term growth characteristics and macromolecular synthesis in response to PMA were analysed. PMA stimulates DNA and RNA synthesis in both Phase II (young) and Phase III (senescent) MRC-5 cells. Treatment of senescent cells with various PMA concentrations results in a greater stimulation of DNA and RNA synthesis than that in young cells. Senescent cells are also more sensitive to the PMA-induced alterations of growth characteristics and higher concentrations of PMA become toxic for them. The age-related alterations of cellular responsiveness are also apparent in the gradual loss of responsiveness to serum, observed in parallel with the increased sensitivity to PMA. Furthermore, serum-induced stimulation of macromolecular synthesis is inhibited by PMA. Since it is known that serum and PMA elicit their effects via different signal transduction pathways, our results point to suggest the differential regulation of the signalling mechanisms during cellular ageing.

Phorbol ester-induced reorganization of the cytoskeleton in human fibroblasts during ageing in vitro

Phorbol esters induce drastic morphological alterations in cells of different origin by altering the conformation and the interrelationship of the elements of the cytoskeletal system. Treatment of early passage (young) and late passage (senescent) human fibroblasts MRC-5 with phorbol-12-myristate-13-acetate (PMA) results in the rearrangement of actin and tubulin filaments. PMA brings about the disorientation and diffusion of the heavily criss-crossed network of actin and microtubulin fibres characteristic of senescent cells suggesting thereby an increased sensitivity of senescent cells to phorbol esters. Since phorbol esters are known to be specific activators of protein kinase C (PKC), the PMA-induced modulation of the cytoskeleton patterns in ageing fibroblasts provides further support for the view that the effectiveness of the signalling mechanisms is retained during cellular ageing.

Altered cellular responsiveness during ageing

The capacity of cells and organisms to respond to external stimuli and to maintain stability in order to survive decreases progressively during ageing. The mitogenic and stimulatory effects of growth factors, hormones and other agents are reduced significantly during cellular ageing. The sensitivity of ageing cells to toxic agents including antibiotics, phorbol esters, radiations and heat shock increases. This failure of homeostasis during cellular ageing does not appear to be due to any quantitative and qualitative defects in the receptor systems. Instead, metabolic defects in the pathways of macromolecular synthesis may be the basis of altered cellular responsiveness during ageing.

Dietary calorie restriction does not affect the levels of protein elongation factors in rat livers during ageing

Dietary calorie restriction of rats has been previously shown to increase protein synthetic rates in liver and kidney cells during ageing. Here we have compared the activity and amounts of active elongation factors EF-1 alpha and EF-2 in cell-free extracts prepared from livers isolated from male Fischer 344 rats of different ages. Although there is some age-related decline in the catalytic activity and amounts of active EF-1 alpha during ageing, no differences between freely-fed and calorie-restricted animals were observed. In the case of EF-2, the amounts of ADP-ribosylatable EF-2 neither declined during ageing nor differed between freely-fed and calorie-restricted animals. Thus differences in the protein synthetic rates in calorie-restricted and freely-fed rats are not reflected at the level of protein elongation factors, and may involve some other mechanisms of regulation.

Homoeostatic imbalance during cellular ageing: altered responsiveness

The inability of normal cells to maintain themselves for ever is a reflection of homoeostatic imbalance and a progressive failure of maintenance. Ageing cells respond less to growth stimulants whereas they show increased sensitivity to toxic agents including antibiotics, phorbol esters, radiation and other physical stresses. No major quantitative and qualitative defects in the receptor systems have been detected that could explain the reasons for altered responsiveness during ageing. Random metabolic defects in the processes involved in maintaining homoeostasis may be critical for causing homoeostatic imbalance, cellular ageing and death.

Protein synthesis and the components of protein synthetic machinery during cellular aging

The slowing down of protein synthesis is a change widely observed during the aging of organisms. It has also been claimed that a decline in the rate of protein synthesis occurs during cellular aging. However, the evidence in favour of this view is not clear-cut, and reliable estimates of rates of protein synthesis during cellular aging have yet to be made. Studies on various components of the protein synthetic machinery during cellular aging have revealed a decline in the efficiency and accuracy of ribosomes, an increase in the levels of rRNA and tRNA, and a decrease in the amounts and activities of elongation factors. Detailed studies on the structure and function of ribosomes, tRNA isoacceptor profiles, activities of aminoacyl-tRNA synthetases, levels and activities of initiation factors, rates of protein elongation, and the accuracy of protein synthesis will be needed before the molecular mechanisms of the regulation of protein synthesis during cellular aging can be understood.

Eukaryotic protein elongation factors

In eukaryotes, peptide chain elongation is mediated by elongation factors EF-1 and EF-2. EF-1 is composed of a nucleotide-binding protein EF-1 alpha, and a nucleotide exchange protein complex, EF-1 beta gamma, while EF-2 catalyses the translocation of peptidyl-tRNA on the ribosome. Elongation factors are highly conserved among different species and may be involved in functions other than protein synthesis, such as organization of the mitotic apparatus, signal transduction, developmental regulation, ageing and transformation. Yeast contains a third factor, EF-3, whose structure and function is not yet well understood.