Reciprocal relationships between the resistance to stresses and cellular aging

Age-related differences in oxidative protein-damage in young and senescent fibroblasts

Aging is accompanied by an accumulation of oxidized proteins and cross-linked modified protein material. The intracellular formation and accumulation of highly oxidized and cross-linked proteins, the so-called lipofuscin, is a typical sign of senescence. However, little is known whether the lipofuscin accumulation during aging is related to environmental conditions, as oxidative stress, and whether the accumulation of oxidized proteins and lipofuscin is preferentially taking place in the cytosol or the nucleus and finally, what is the role of lysosomes in this process. Therefore, we investigated human skin fibroblasts in an early stage of proliferation ("young cells") and in a late stage ("senescent cells"). Such cells were compared for the amount of protein carbonyls and lipofuscin and their distribution within the cytosol and the nucleus. Furthermore, cells were exposed to single and repeated doses of hydrogen peroxide and paraquat, measuring the same set of parameters. In addition to that the role of the proteasome to degrade oxidized proteins in young and senescent cells was tested. Furthermore, detailed microscopic analysis was performed testing the intracellular distribution of lipofuscin. The results clearly demonstrated that repeated/chronic oxidative stress induces a senescence-like phenotype of the distribution of oxidized proteins as well as of lipofuscin. It could be demonstrated that most of the lipofuscin is located in lysosomes and that senescent cells contain less lysosomes not lipofuscin-laden in comparison to young cells.

Telomeres: hallmarks of radiosensitivity

Telomeres are the very ends of the chromosomes. They can be seen as natural double-strand breaks (DSB), specialized structures which prevent DSB repair and activation of DNA damage checkpoints. In somatic cells, attrition of telomeres occurs after each cell division until replicative senescence. In the absence of telomerase, telomeres shorten due to incomplete replication of the lagging strand at the very end of chromosome termini. Moreover, oxidative stress and accumulating reactive oxygen species (ROS) lead to an increased telomere shortening due to a less efficient repair of SSB in telomeres. The specialized structures at telomeres include proteins involved in both telomere maintenance and DNA repair. However when a telomere is damaged and has to be repaired, those proteins might fail to perform an accurate repair of the damage. This is the starting point of this article in which we first summarize the well-established relationships between DNA repair processes and maintenance of functional telomeres. We then examine how damaged telomeres would be processed, and show that irradiation alters telomere maintenance leading to possibly dramatic consequences. Our point is to suggest that those consequences are not restricted to the short term effects such as increased radiation-induced cell death. On the contrary, we postulate that the major impact of the loss of telomere integrity might occur in the long term, during multistep carcinogenesis. Its major role would be to act as an amplificator event unmasking in one single step recessive radiation-induced mutations among thousands of genes and providing cellular proliferative advantage. Moreover, the chromosomal instability generated by damaged telomeres will favour each step of the transformation from normal to fully transformed cells.

Hypoxia reoxygenation induces premature senescence in neonatal SD rat cardiomyocytes

AIM

To investigate whether hypoxia reoxygenation induces premature senescence in neonatal Sprague-Dawley (SD) rat cardiomyocytes.

METHODS

Cardiomyocytes were isolated from neonatal SD rat heart and identified by immunohistochemistry. The control cultures were incubated at 37 degree centigrade in a humidified atmosphere of 5% CO(2) and 95% air. The hypoxic cultures were incubated in a modular incubator chamber filled with 1% O(2), 5% CO(2), and balance N2 for 6 h. The reoxygenated cultures were subjected to 1% O(2) and 5% CO(2) for 6 h, then 21% oxygen for 4, 8, 12, 24, and 48 h, respectively. Cell proliferation was determined using bromodeoxyuridine labeling. The ultrastructure of cardiomyocytes was observed by using an electron microscope. beta-Galactosidase activity was determined by using a senescence beta-galactosidase Staining Kit. p16( INK4a ) and telomerase reverse transcriptase (TERT) mRNA levels were measured by real time quantitative PCR. TERT protein expression was determined by immunohistochemistry. Telomerase activities were assayed by using the Telo TAGGG Telomerase PCR ELISAplus kit.

RESULTS

The initial cultures consisted of pure cardiomyocytes identified by immunohistochemistry. The proportion of BrdU positive cells was reduced significantly in the hypoxia reoxygenation-treated group (P< 0.01). Under the condition of hypoxia reoxygenation, mitochondrial dehydration appeared; p16( INK4a ) and TERT mRNA levels, beta-galactosidase activity, TERT protein expression and telomerase activities were all significantly increased (P< 0.01 or P< 0.05).

CONCLUSION

These data indicate that premature senescence could be induced in neonatal SD rat cardiomyocytes exposed to hypoxia reoxygenation. Although TERT significantly increased, it could not block senescence.

Telomere shortening and mood disorders: preliminary support for a chronic stress model of accelerated aging

BACKGROUND

Little is known about the biological mechanisms underlying the excess medical morbidity and mortality associated with mood disorders. Substantial evidence supports abnormalities in stress-related biological systems in depression. Accelerated telomere shortening may reflect stress-related oxidative damage to cells and accelerated aging, and severe psychosocial stress has been linked to telomere shortening. We propose that chronic stress associated with mood disorders may contribute to excess vulnerability for diseases of aging such as cardiovascular disease and possibly some cancers through accelerated organismal aging.

METHODS

Telomere length was measured by Southern Analysis in 44 individuals with chronic mood disorders and 44 nonpsychiatrically ill age-matched control subjects.

RESULTS

Telomere length was significantly shorter in those with mood disorders, representing as much as 10 years of accelerated aging.

CONCLUSIONS

These results provide preliminary evidence that mood disorders are associated with accelerated aging and may suggest a novel mechanism for mood disorder-associated morbidity and mortality.

Involvement of Rel/NF-kappa B transcription factors in senescence

Senescence is now established as a genetically controlled phenomenon that alters different cell functions, including proliferation, apoptosis, resistance to stress, and energetic metabolism. Underlying changes in gene expression are governed by some transcription factors, whose expression or activity must change with senescence as well. Transcription factors of the Rel/NF-kappa B family are good candidates to participate in the establishment of senescence. Arguments range from correlation between cell functions controlled by these factors and cell functions altered during senescence, to phenotypes resulting from in vitro manipulations of Rel/NF-kappa B activity.

From the Hayflick mosaic to the mosaics of ageing. Role of stress-induced premature senescence in human ageing

The Hayflick limit-senescence of proliferative cell types-is a fundamental feature of proliferative cells in vitro. Various human proliferative cell types exposed in vitro to many types of subcytotoxic stresses undergo stress-induced premature senescence (SIPS) (also called stress-induced premature senescence-like phenotype, according to the definition of senescence). The known mechanisms of appearance the main features of SIPS are reviewed: senescent-like morphology, growth arrest, senescence-related changes in gene expression, telomere shortening. Long before telomere-shortening induces senescence, other factors such as culture conditions or lack of 'feeder cells' can trigger either SIPS or prolonged reversible G(0) phase of the cell cycle. In vivo, 'proliferative' cell types of aged individuals are likely to compose a mosaic made of cells irreversibly growth arrested or not. The higher level of stress to which these cells have been exposed throughout their life span, the higher proportion of the cells of this mosaic will be in SIPS rather than in telomere-shortening dependent senescence. All cell types undergoing SIPS in vivo, most notably the ones in stressful conditions, are likely to participate in the tissular changes observed along ageing. For instance, human diploid fibroblasts (HDFs) exposed in vivo and in vitro to pro-inflammatory cytokines display biomarkers of senescence and might participate in the degradation of the extracellular matrix observed in ageing.

Approach of evolutionary theories of ageing, stress, senescence-like phenotypes, calorie restriction and hormesis from the view point of far-from-equilibrium thermodynamics

B. L. Strehler wrote that "Any system that is not in thermodynamic equilibrium will approach that state at a rate that is a function of absolute temperature and the energy barriers to the rearrangements of components". Far-from-equilibrium thermodynamics allows a global systemic description of the cellular behaviour. This approach transcends the genetic and stochastic considerations on ageing as well as some evolutionary questions about ageing. The fundamental difference between the processes of development and ageing could reflect the intrinsic differences existing between biological systems where an increase in specific entropy production (SEP) is, respectively, still possible or not. The increase of the potential of SEP which probably occurred with evolution might explain in part why life span could increase. However, this SEP-driven increase in life span was possible only in those species which did not take advantage of their increased potential of SEP to ameliorate their reproductive capacity at the expense of possible increases in repair capacity. The criteria of stability of far-from-equilibrium open systems and the theory of attractors also help to sort the possible types of cellular stress responses: normal ageing, hormesis, stress-induced premature senescence, apoptosis or necrosis.

Acute and chronic adaptation to hemodynamic overload and ischemia in the aged heart

Congestive heart failure occurs more frequently in older individuals. This higher incidence of heart failure may be caused by the diminished capacity of aged hearts to adapt to increased hemodynamic overload and ischemia which are the most important triggers for heart failure in the aged. In the immediate early phase after the imposition of ascending aortic banding, the mRNA expression of proto-oncogenes (c-fos, c-jun and c-myc) was diminished in aged rat hearts compared with young adult hearts. In the later phase, the pattern of expression of contractile protein genes in aged hearts differed quantitatively from that in adult hearts. The hypertrophic responses to the imposition of not only pressure-overload but also volume-overload were also diminished at the organ and cellular levels. In addition, this diminution was observed both in the left and right ventricles. Against ischemic insults, aged hearts responded with a diminished expression of proto-oncogenes and heat shock proteins. Thus, aged hearts are characterized by poor adaptation to hemodynamic overload and by a poor self-protective mechanism against cell death through necrosis and apoptosis. Of interest, more severe hemodynamic overload elevated the diminished responses to a level similar to that in adult hearts, suggesting that the threshold for the heart to respond to hemodynamic overload or ischemia is elevated in aged hearts. In addition, even in aged hearts ischemic preconditioning upregulated the diminished gene expression in a gene-dependent manner. Thus, the capacity for adaptation to hemodynamic overload and ischemia is diminished in aged hearts, but aged hearts preserve the ability to respond to these under some conditions.

Hypothesis. Bystanders or bad seeds? Many autoimmune-target cells may be transforming to cancer and signalling "danger" to the immune system

Autoimmune-target cells in autoimmune disease (AID) are usually construed as constitutionally normal healthy cells. A related assumption is that other cells in the body of AID patients, except for certain immunocytes, are healthy cells. An implication of that view is that any systemic pathology in organ-specific AID is related to metabolic derangements secondary to tissue destruction. However, much data on target and other cells in AID suggest widespread primary cellular defects. In insulin-dependent diabetes mellitus (IDDM), for example, many "complications" such as atherosclerosis, premature arterial stiffening, senescence of fibroblasts in vitro, and exuberant growth of smooth muscle and mesangial cells in vivo are not strictly attributable to glucose elevation. Also unexplained is the similar appearance of IDDM beta-cells and cells from insulinoma and why the prodromal phase of IDDM has many insulinoma-like features. While AID target cells have often been likened to neoplastic cells, investigators have rarely explored the possibility that autoimmunity in AID is fundamentally antineoplastic. This is likely because the dominant ideas in oncology and immunology-somatic mutation and clonal deletion, respectively-have prevented explanations for how normal immunity could detect transforming cells not expressing non-self antigens. New and less conventional theories of cancer and immunity have facilitated such an explanation. I use Rubin's "epigenetic" aging model of carcinogenesis and Matzinger's "danger" model of immunity to integrate the immunological and oncological sides of AID. In particular, I postulate that individuals suffering from AID have inherited many foci of prematurely aging cells. Those inherently damaged cells adapt to in vivo challenges by beginning to transform into cancer cells. However, as long as those stressed cells have not fully transformed, they will continue to signal "danger" to the innate immune system. The clinical outcome of that struggle between incipient neoplasia and immunity will vary depending upon the degree of tumor-proneness and resistance of the individual. Borrowing from cancer geneticist Henry Lynch, I postulate that tumor-resistance is inherited as a quantitative polygenic trait in direct proportion to tumor-proneness. I further contend that tumor-proneness and immunity are linked polygenic traits such that the greater one's tumor-proneness, the more powerful his/her antitumor immunity. I point to the shared DNA repair deficiency of certain cancer-prone syndromes and HLA-linked AID, their occasional co-occurrence, and their demonstrably exceptional immunity against solid tumors. I propose that HLA-linked AID constitute "chronic hypersensitivity syndromes" due to immunity's largely hidden battle to suppress multiple incipient neoplastic microfoci. Much of the physiopathology of AID is explicable as a sustained systemic response to threatened neoplastic transformation.

Influence of preculture on the prefreeze and postthaw characteristics of hepatocytes

Recent studies performed in our laboratory have shown that a brief period of preculture prior to cryopreservation improves the postthaw viability of hepatocytes. The purpose of this investigation is to characterize specific metabolic and biochemical characteristics of the hepatocytes (both frozen and nonfrozen) to help elucidate the role of preculture on the postthaw viability. Fresh and thawed hepatocytes were cultured in a bioartificial liver (BAL) to determine albumin secretion as a function of time in culture. In addition, cell extracts were analyzed using nuclear magnetic resonance (NMR) spectroscopy to quantify changes in cell membrane composition and energetics as a function of time in culture prefreeze and postthaw. The results of these studies showed an increase in albumin concentration in the culture medium with time in culture for the period tested for both fresh and frozen and thawed hepatocytes. NMR spectroscopy of lipid extracts indicates that in vitro culture of hepatocytes results in an increase in cholesterol relative to membrane phospholipid. Moreover, the NMR results also indicate phospholipid interconversion, via specific lipases in cultured hepatocytes, and these changes are consistent with water permeability measurements performed previously. Significant changes in phosphoenergetics were also observed, with the net energy charge for the cells increasing significantly with time in culture. In addition, NMR spectra show increased levels of 6-phosphogluconate, another indicator of the cellular response to the stresses of isolation and ex vivo culture. These results suggest that energetic considerations may be a significant factor in the ability of hepatocytes to survive the stresses of freezing and thawing. Significant shifts in membrane phospholipids may also influence membrane permeability and postthaw survival.

A system approach modelling of the three-stage non-linear kinetics in biological ageing

There is a widely observed non-linear kinetics in the ageing of biological systems, which is characterised by three successive stages, (1) the ageing rate is firstly high, but decreases quickly to a minimum, from which (2) it remains nearly constant during the major part of the process until (3) it starts increasing again up to the final collapse of the system. Such kinetics are also encountered in the ageing of inert systems. It is shown that a model useful for the follow-up of operating inert systems allows to find back typical curves and laws related to the ageing of biological systems (mortality rate curves, survival curves, growth curves, Gompertz law, ...). In this model, ageing is seen as a multifactorial process. The classical concepts of lifespan, longevity and life expectancy are given new light using the model, which also gives clues to explain both the discrepancy in the age of death of individuals in a given population and the wide range of lifespans of species encountered in nature. Finally, the model shows in which directions accelerated senescence testing protocols should be orientated for a better understanding of the underlying phenomena and for life prediction purposes.

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.

Role of oxidative stress in telomere length regulation and replicative senescence

Replicative senescence is tied into organismal aging processes in more than one respect, and telomeres appear to be the major trigger of replicative senescence under many conditions in vitro and in vivo. However, the structure-function relationships in telomeres, the mechanisms of telomere shortening with advancing replicative age, and the regulation of senescence by telomeres are far from understood. Combining recent data on telomere structure, function of telomere-binding proteins, and sensitivity of telomeres to oxidative damage, an integrative model of telomere shortening and signaling is developed. The model suggests that t-loop formation hinders access of repair proteins to telomeres, leading to accumulation of a basic sites and single-strand breaks. These might contribute to accelerated telomere shortening by transient stalling of replication as well as, if present in high concentrations, to a relief of torsional tension which might destabilize the telomeric loop structure. As a result, the single-stranded G-rich overhang, which is present at the very ends of telomeres but is normally protected at the base of the telomeric loop, will be exposed to the nucleoplasm. Free G-rich telomeric single strands are a strong inductor of the p53 pathway, and exposure of the overhangs seems to be the first step in the signal transduction cascade to replicative senescence.

Altered ischemic induction of immediate early gene and heat shock protein 70 mRNAs after preconditioning in rat hearts

Immediate early genes and heat shock protein (HSP) 70s, which may play a role in adaptation and cellular protection, respectively, are induced by ischemia in hearts. We examined if the induction of immediate early gene (c-fos, c-myc, c-jun, and junB) and HSP70 mRNAs by ischemia is affected by ischemic preconditioning. Transient ischemia (5 or 10 minute) was applied to Wistar rat (n=75) hearts, by tightening a snare placed around left coronary arterial branches 7 days before applying ischemia. Rats without earlier ischemia (control group, C) and rats with 5-minute ischemia 12 or 24 hours earlier (EI12 or 24 group) were given 10-minute ischemia and sacrificed at 0, 0.5, 1, 2, and 4 hour. RNA was extracted from the ischemic region and Northern blot analysis was performed. The induction of c-fos and c-myc mRNAs was significantly increased in EI12 but not in EI24 compared with that in C. The induction of c-jun and junB mRNAs showed no change in both EI12 and EI24 compared with that in C. The induction of HSP72 and 73 mRNAs was decreased in EI12 and decreased further in EI24. Thus, ischemic preconditioning altered the induction of immediate early gene and HSP70 mRNAs by ischemia. The effect of preconditioning differed among genes studied and changed with time after preconditioning. Ischemic preconditioning alters protective and adaptive responses to ischemia at the gene level.

Induction of replicative senescence biomarkers by sublethal oxidative stresses in normal human fibroblast

We tested the long-term effects of sublethal oxidative stresses on replicative senescence. WI-38 human diploid fibroblasts (HDFs) at early cumulative population doublings (CPDs) were exposed to five stresses with 30 microM tert-butylhydroperoxide (t-BHP). After at least 2 d of recovery, the cells developed biomarkers of replicative senescence: loss of replicative potential, increase in senescence-associated beta-galactosidase activity, overexpression of p21(Waf-1/SDI-1/Cip1), and inability to hyperphosphorylate pRb. The level of mRNAs overexpressed in senescent WI-38 or IMR-90 HDFs increased after five stresses with 30 microM t-BHP or a single stress under 450 microM H(2)O(2). These corresponding genes include fibronectin, osteonectin, alpha1(I)-procollagen, apolipoprotein J, SM22, SS9, and GTP-alpha binding protein. The common 4977 bp mitochondrial DNA deletion was detected in WI-38 HDFs at late CPDs and at early CPDs after t-BHP stresses. In conclusion, sublethal oxidative stresses lead HDFs to a state close to replicative senescence.

Ceroid/lipofuscin-loaded human fibroblasts show increased susceptibility to oxidative stress

To test whether the possibly enhanced sensitivity of aged cells to oxidative stress may depend on their content of ceroid/lipofuscin, AG-1518 human fibroblasts with various amounts of the pigment accumulated due to prolonged cultivation under normobaric hyperoxia were exposed to acute oxidative stress (2.5 microM naphthazarin, 15 min) and then returned to standard culture conditions. Twenty-four hours after the naphthazarin treatment, 37% of the cells were still vital, whereas others had undergone oxidative stress-induced apoptosis with ensuing postapoptotic necrosis. The average amount of ceroid/lipofuscin within the surviving cells was only about half of that of the initial population of cells, as measured before the naphthazarin exposure. This finding suggests that ceroid/lipofuscin-rich cells have an increased sensitivity to oxidative stress. The ceroid/lipofuscin quantity strongly positively correlated with the size of the acidic compartment (as evaluated by uptake of the weakly basic lysosomotropic fluorochrome acridine orange) and with its content of the lysosomal protease cathepsin D, as assayed by immunocytochemistry. We hypothesize that the enhanced sensitivity of ceroid/lipofuscin-loaded cells to oxidative stress may be caused by the increased amounts of lysosomal enzymes, known as mediators of oxidative damage, and/or by catalysis of intralysosomal oxidative reactions by lipofuscin-associated iron.