Senescence of cultured human diploid fibroblasts. Are mutations responsible?

Biology and Evolution of Aging: Implications for Basic Gerontological Health Research

The maximum lifespan of different animal species is genetically determined. Many biochemical and physiological systems which influence longevity have apparently evolved to regulate growth and development in a way which maximizes fitness given the ecological niche and constraints on the species. The diversity of individual genetic effects on aging makes it unlikely that either extrinsic factors such as nutrition and medicine or genetic intervention will have dramatic effects on the maximum lifespan of a species, in spite of significant qualitative effects on individuals. However, understanding the fundamental genetic determinants of senescence may be of particular importance to the treatment and or prevention of age associated problems involving tissue degeneration and/or cancer. Considerable investment in basic biological research on both aging and developmental processes is needed before this understanding can be achieved.

Asymmetric distribution of DNA between daughter cells with final symmetry breaking during aging of human fibroblasts

Human fibroblasts proliferating in vitro go through functional modifications, lose progressively their capacity to divide, and enter finally a post-mitotic state. These events are supposed to reproduce the developmental steps taking place in vivo during aging of the organism. The gradual changes occurring through proliferation are incompatible with an even distribution of the genetic material during cell division. We measured the amount of DNA on pairs of daughter cells at different population doubling levels of human fibroblasts. It was found that at each doubling in a significant fraction of cells, the distribution of DNA between sister cells is asymmetric. The cell system is in a steady state through the different phases of the fibroblast population life span; then during the last mitoses when the cells enter the terminal phase IV there is symmetry breaking with a phase transition, the cells settling into a new state.

Gene targeting in primary fetal fibroblasts from sheep and pig

Nuclear transfer offers a new cell-based route for introducing precise genetic modifications in a range of animal species. However, significant challenges, such as establishment of somatic gene targeting techniques, must be overcome before the technology can be applied routinely. In this report, we describe targeted deletion at the GGTA1 (alpha 1,3-galactosyl transferase) and PrP (prion protein) loci in primary fibroblasts from livestock. We place particular emphasis on the growth characteristics of the primary cell cultures, since these are key to determining success.

Screening for genetic predisposition to mutagens in cancer patients

Both genetic and environmental factors are known to play an important role in the development of cancer. To determine whether, among individuals who develop cancers, some may have been more susceptible to the mutagenic effects of environmental agents, skin biopsies were taken from 79 cancer patients with different common types of cancers (e.g., lung, breast, bladder, colon, cervix, ovary, brain, vocal cord, uterus, skin, testis, stomach, basal cell carcinoma, leukemia, etc.). Fibroblast cultures have been established from skin explants from nearly all of the patients. The sensitivity of some of these cells as well as a number of other fibroblast strains established from "clinically normal" individuals to a battery of mutagenic agents (e.g., ethylmethane sulfonate, methylmethane sulfonate, ethidium bromide, actinomycin D, mitomycin C, bleomycin, camptothecin), which induce different kinds of DNA damage was examined. For the control group of fibroblasts, a normal range of toxicity for all of the above agents have been established. In contrast to other mutagens for which sensitivity of all of the control cell strains lay within a narrow range, large and interesting differences in sensitivity were observed for ethidium bromide. The fibroblast strains established from fetal tissue were found to be highly resistant to ethidium bromide, whereas fibroblasts from two clinically normal persons exhibited greatly enhanced sensitivity to this agent. The genetic or biochemical basis of increased sensitivity or resistance to ethidium bromide remains to be determined. The sensitivity of cells from 28 cancer patients to a number of the mutagenic agents was also examined. Most of these strains exhibited normal range of sensitivity to the mutagens; however, a few showed small but noticeable differences in sensitivity to specific agents. The fibroblast strains from cancer patients provide a useful resource to examine the genetic and metabolic factors that may be important determinants in cancer susceptibility.

DNA damage, mutation and fine structure DNA repair in aging

The primary focus of this review is on correlations found between DNA damage, repair, and aging. New techniques for the measurement of DNA damage and repair at the level of individual genes, in individual DNA strands and in individual nucleotides will allow us to gain information regarding the nature of these correlations. Fine structure studies of DNA damage and repair in specific regions, including active genes, telomeres, and mitochondria have begun. Considerable intragenomic DNA repair heterogeneity has been found, and there have been indications of relationships between aging and repair in specific regions. More studies are necessary, however, particularly studies of the repair of endogenous damage. It is emphasized that the information obtained must be viewed from a perspective that takes into account the total responses of the cell to damaging events and the inter-relationships that exist between DNA repair and transcription.

The cultured diploid fibroblast as a model for the study of cellular aging

The limited proliferative potential of the cultured human diploid fibroblast is now well established. A number of biological correlates suggest that this culture system is a model for the study of aging at the cellular level. The mechanism(s) that causes the loss of proliferative activity is unknown; the results of some recent studies indicate that specific genes may play a pivotal role in cellular aging in vitro. The extent to which changes in proliferative functions are causally related to aging in vivo is currently under investigation.

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.

Tumor necrosis factor reduces lifespan of human endothelial cells in vitro

Tumor necrosis factor (TNF) is known to regulate the proliferation and function of vascular endothelial cells (ECs). We have examined the effects of TNF on the growth and aging of human ECs of different origins and compared them with those in human normal diploid fibroblasts. The results obtained were as follows: (1) TNF reduces the growth rate and in vitro life span of ECs in both dose- and treatment length-dependent fashions; (2) ECs are significantly more sensitive to TNF than fibroblasts; and (3) the life span shortening effect of TNF on ECs increases as a function of in vitro cell age. These results suggest that the aging of ECs is modified by TNF exposure.

Age-related changes in DNA polymerase alpha expression

DNA polymerase alpha isozymes differing in specific activity and affinity of binding to DNA were purified from human fibroblasts derived from donors of different ages. Fetal-derived fibroblasts expressed a single, high-activity enzyme (A2), with high affinity of binding to DNA. Adult-derived fibroblasts exhibited two forms of DNA polymerase alpha, one identical to the fetal enzyme, and a second with about tenfold less activity showing low affinity of binding to DNA (A1). The ratio of DNA polymerase A2/A1 decreased dramatically with age, from 100% A2 in fetal-derived fibroblasts to about 94% A1 in fibroblasts derived from a 66-year-old donor. The DNA binding affinity of polymerase alpha A1 from adult-derived fibroblasts increased concomitant with a significant increase in activity when the enzyme was treated with phosphatidylinositol-4-monophosphate (PIP), or with inositol-1, 4-bisphosphate (I(1,4)P2). The enzyme reverted back to a less active form, with loss of the noncovalently bound I(1,4)P2, as a function of time. When permeabilized human fibroblasts with low DNA excision repair capacity were treated with 7,8-dihydrodiol-9,10-epoxybenzo(a)-pyrene (BPDE) in the presence of 32P-ATP, phosphatidylinositol, and cycloheximide, excision repair was initiated and 32P-labeled DNA polymerase alpha was recovered in the absence of de novo protein synthesis. DNA synthesis associated with either scheduled DNA synthesis or BPDE-initiated excision repair declined as a function of increased age in human cells. The data suggest that the decline in both DNA excision repair-associated and mitogen-activated DNA synthesis may be correlated with decreased total intracellular levels of DNA polymerase and with the decline in polymerase alpha activity as a function of age, that DNA repair-associated initiation of DNA synthesis in adult-derived cells may increase with activation of a pool of low activity DNA polymerase alpha, and that DNA polymerase alpha activity increases as a function of enzyme interaction with a component of the PI phosphorylation cascade.

Strong effects of 5-azacytidine on the in vitro lifespan of human diploid fibroblasts

Azacytidine (5-aza-CR) and azadeoxycytidine (5-aza-CdR) are known to inhibit the methylation of cytosine (5-mC) in DNA, and their effects on the long-term growth of human fibroblasts, strain MRC-5, have been examined. A single treatment with either analogue initially inhibits growth, but the cells recover to normal morphology, growth rate and cell density at confluence. However, a memory of the treatment is retained, since the cells' subsequent lifespan is considerably reduced in comparison with controls, and the terminal stages of growth are indistinguishable from senescent cultures of untreated cells. The effect of 5-aza-CR or 5-aza-CdR does not appear to be closely related to the concentration used, or to the length of treatment up to about half-way through the total lifespan. Sequential doses have cumulative effects on longevity. There is evidence that the pattern of 5-mC in mammalian DNA is inherited via cell division; therefore, a reduction in 5-mC induced by a pulse treatment of 5-aza-CR or 5-aza-CdR will be transmitted to all descendants. The results are consistent with independent observations that the level of 5-mC declines continually during the serial subculture of human diploid cells. The analogues would be expected to precipitate this decline and thereby advance the physiological age of the culture. The results provide support for the view that the random loss of methyl groups in DNA may eventually have deleterious consequences, such as aberrant epigenetic changes in gene expression.

Cellular basis for the species differences in sensitivity to cardiac glycosides (digitalis)

The relative toxicity of numerous cardiotonic steroids (viz. ouabain, digitoxin, digoxin, convallatoxin, SC4453, bufalin, gitaloxin, digoxigenin, actodigin, oleandrin, digitoxigenin, gitoxin, strophanthidin, gitoxigenin, lanatosides A, B and C, alpha- and beta-acetyl digoxin, alpha- and beta-methyl digoxin) and related compounds towards a number of independent cell lines established from human, monkey, mouse, Syrian hamster, and Chinese hamster have been determined. All cardiac glycosides and their genins, as well as the cardiotonic alkaloid cassaine, exhibited greater than 100-fold higher toxicity towards cultured human and monkey cells in comparison to the cell lines of mouse, Syrian hamster, and Chinese hamster origins. These differences are species-related as all cell lines (both normal as well as transformed) from any one species, as well as cells from the closely related species (e.g., man and monkey or mouse, Chinese hamster, and Syrian hamster), showed similar sensitivity towards these drugs. The failure to see any significant differences in cellular toxicity for a larger number of other compounds which either bear limited structural resemblance to cardiac glycosides (viz. estradiol 17-beta-acetate, testosterone propionate, 21-acetoxy pregnenolone, beta-estradiol, digitonin, tigogenin, and tomatine) or interact with the Na+/K+ ATPase in a different manner (viz. veratridine, sanguinarine nitrate, penicillic acid, vanadium pentoxide, harmaline-HCI,5,5'-diphenyl hydantoin, quindonium bromide, and methyl quinolizinum bromide) provides strong evidence that the observed species-related differences are highly specific for cardiotonic steroids. Studies on the binding of [3H]ouabain show that, in comparison to human and monkey cell lines, no significant binding of the drug is observed in cells derived from the resistant species (i.e., mouse and Chinese hamster). The Na+/K+ ATPase from cells of the resistant species is inhibited at much higher concentrations of ouabain and digitoxin in comparison to the enzyme from human cells, and a good correlation is observed between these concentrations and those reported for inhibition of the enzyme from isolated heart muscles of the same species. These results provide strong evidence that the species-related differences in sensitivity to digitalis have a cellular basis and that the cultured cells from various mammalian species provide a useful model system for investigating the mechanism of action of cardiac glycosides.

Autoradiographic detection of diphtheria toxin resistant mutants in human diploid fibroblasts

An autoradiographic procedure for the detection of diphtheria toxin (DT) resistant (DipR) mutants in human diploid fibroblast (HDF) cells has been developed. The assay is based on the observation that when HDFs from confluent cultures are seeded in medium containing 0.01 flocculating units/ml or higher concentration of DT, protein synthesis in sensitive cells is severely inhibited by 4-6 hr. If at this or later time, a radiolabeled protein precursor (eg, 3H-leucine) is added to the culture, it is almost exclusively incorporated into the resistant cells, which are then readily identified by autoradiography. The DipR cells can also be identified by labeling in the presence of 3H-thymidine, although a higher background is observed in these experiments. Reconstruction experiments using DipS and DipR HDFs show that the frequency of heavily labeled cells that are detected by autoradiography show an excellent correlation with the number of DipR cells added and to the number of DipR cells as detected by conventional colony forming assay. These studies provide strong evidence that the labeled cells identified by autoradiography are bona fide DipR mutants. The detection of DipR cells by autoradiography is apparently not affected by the presence of the sensitive cells in the mixtures. The spontaneous frequency of DipR cells in HDFs has been found to be in the range of 1-5 X 10(-6), and this increases in a dose dependent manner upon treatment with the mutagen ethyl methanesulfonate. These results indicate that the autoradiographic assay could be used for quantitative mutagenesis. Since the autoradiographic assay does not depend on cell division, it may prove useful in estimating the incidence of pre-existing mutations in cell populations that either do not divide or have very limited growth potential (eg, lymphocytes, muscle cells, neurons, senescent fibroblasts, etc).

On the importance of deoxyribonucleotide pools in the senescence of cultured human diploid fibroblasts

Previous studies have indicated that ribonucleotide reductase may participate in a mechanism of cellular senescence, which involves modifications in deoxyribonucleotide pools, the products of the reductase reaction. Since very little information about the levels of these pools in senescing cells is currently available, an analysis of deoxyribonucleotide levels was carried out in young, old and very old normal human diploid fibroblasts, as well as in a variant human fibroblast strain with an altered replicative life span. These studies indicated that there are marked age-related perturbations in these pools which imply that they may be fundamentally important in a process determining the nonproliferative or senescent state.

A cloning assay for 6-thioguanine resistance provides evidence against certain somatic mutational theories of aging

The frequencies of 6-thioguanine-resistant primary clones from the kidneys and skeletal muscles of aging male cohorts of two F1 hybrid strains of Mus musculus varied from 0.59 to 10.96 X 10(-5) and did not increase as a function of donor age (up to 40 months). Resistant clones were shown to be severely deficient in the activity of hypoxanthine-guanine phosphoribosyltransferase (EC 2.4.2.8). These deficiencies presumably resulted from molecular alterations at this X-linked locus, including point mutations. No alterations of the X-chromosome were observed at the level of the light microscope. These results are inconsistent with predictions of the intrinsic mutagenesis and protein synthesis error catastrophe theories of aging. They do not rule out, however, somatic mutational theories that invoke comparatively large-scale chromosomal lesions, many of which would be likely to be lethal at the cellular level.

DNA repair deficiencies and cellular senescence are unrelated in xeroderma pigmentosum cell lines

Senescence of skin fibroblast cultures from normal individuals occurred after 23.9 +/- 6.3 (S.D.) passages; senescence in DNA repair-deficient cell lines from xeroderma pigmentosum patients occurred at 22.9 +/- 5.5 passages. Cells from xeroderma pigmentosum variant and Cockayne syndrome patients reached senescence at similar passage numbers. Xeroderma pigmentosum patients contract skin cancer as a consequence of their repair deficiencies but show no symptoms of premature ageing; neither do their cells age prematurely in vitro. The clinical spectrum and the life-span of fibroblasts in culture therefore lend no support for a correlation between ageing and the DNA repair or DNA replication deficiencies found in xeroderma pigmentosum and Cockayne syndrome cells.

Genetic modifications during cellular aging

We review evidence that biological aging is a genetic process related to development and cytodifferentiation and thus may involve alterations of DNA structure and gene expression. We conclude that although determined to a high degree aging also involves stochastic features which lead to progressive somatic cell diversification during the life span. These considerations may help to explain the unevenness of physiological decline and the clonal emergence of certain age-dependent diseases such as cancer.

Species specific differences in the toxicity of mithramycin, chromomycin A3, and olivomycin towards cultured mammalian cells

Three structurally related anticancer drugs, mithramycin, chromomycin A3, and olivomycin, showed large unexpected differences (up to more than 1000 fold) in their toxicity towards cultured cells from various species (human, Chinese hamster, Syrian hamster, and mouse). Among the cell types examined, human cells (both a diploid fibroblast cell strain and HeLa cells) were maximally sensitive to all these drugs, followed by the Syrian hamster kidney cells (BHK 21). The mouse (LMTK- cells) and Chinese hamster (CHO) cells, which were more resistant, showed interesting differences in their sensitivity towards these drugs. For example, whereas the mouse cells were more resistant to mithramycin than CHO cells, the sensitivity pattern was reversed for both chromomycin A3 and olivomycin. In cell extracts derived from human, mouse, and Chinese hamster cells RNA synthesis, which is the cellular target of these drugs, showed identical sensitivity to both mithramycin and chromomycin A3, indicating that the species specific differences in the toxicity to these drugs are at the level of cellular entry of these compounds. Based on the structures of these glycosidic antibiotics and their patterns of toxicity, it is suggested that the intracellular transport of these drugs involves specific interactions between the sugar residues on these compounds and some type of cell surface receptor(s), which differ among different cell types. Some implications of these results for toxicity studies are discussed.

Human fibroblast strains showing increased sensitivity or resistance to ethidium bromide

The sensitivity of a number of human fibroblast cell strains towards the DNA intercalating and mutagenic agent ethidium bromide has been examined. Among the cell strains investigated, 3 were of fetal origin, 6 from clinically normal adult persons, 2 from Lesch-Nyhan individuals and 1 each from persons with 3 genetic disorders, xeroderma pigmentosum, Fanconi anemia and Bloom syndrome, which are known to predispose to cancer. Results of our studies show that cells derived from 2 otherwise normal individuals exhibits a marked sensitivity towards ethidium bromide as compared to the rest of the group. At the same time all 3 cell strains of fetal origin were found to be highly resistant to killing by this agent. These results are discussed in relation to the toxic/mutagenic effects of ethidium bromide.

Podophyllotoxin resistance: a codominant selection system for quantitative mutagenesis studies in mammalian cells

Mutants resistant to the microtubule inhibitor podophyllotoxin (PodR), a codominant marker, can be readily selected in various mammalian cell lines such as, CHO, HeLa, mouse L cells, Syrian hamster cells (BHK21) and a mouse teratocarcinoma cell line OC15. In CHO cells, the recovery of PodR mutants is not affected by cell density (up to 1 X 10(6) cells per 100-mm diameter dish), and after treatment with the mutagen ethyl methanesulfonate maximum mutagenic effect is achieved after a relatively short expression time (40-48 h). The frequency of PodR mutants in various cell lines increased in a dose-dependent manner in response to treatment with the mutagens ethyl methanesulfonate and N-methyl-N'-nitro-N-nitrosoguanidine. The PodR selection system thus provides a new genetic marker which should prove useful in studies of quantitative mutagenesis in mammalian cells.

Diphtheria toxin resistance in human fibroblast cell strains from normal and cancer-prone individuals

Mutants resistant to diphtheria toxin (Dipr) have been selected from a variety of human fibroblast cell strains derived from both normal subjects and individuals with known genetic predisposition to cancer such as xeroderma pigmentosum, Fanconi anemia and Bloom's syndrome. Treatment with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) led to a marked increase in the frequency of Dipr mutants in various cell strains. The increase in the frequency of Dipr mutants occurred in a linear dose-dependent manner in response to MNNG and ethyl methanesulfonate, in one of the cell strains examined. The rate of mutation to diphtheria toxin as determined by fluctuation analysis was very similar in various cell strains (1-3 x 10(-7) mutations/cell/generation), except for the strain GM1492 (8.8 x 10(-7) mutations/cell/generation) which is derived from a Bloom syndrome patient.

A novel synergistic effect of alanosine and guanine on adenine nucleotide synthesis in mammalian cells. Alanosine as a useful probe for investigating purine nucleotide metabolism

A novel synergistic effect of the antitumor agent alanosine (2-amino-3-(hydroxynitrosoamino) propionic acid), which specifically inhibits the enzyme adenylosuccinate synthetase (ASS) and guanine on the growth of Chinese hamster ovary (CHO) cells and human diploid fibroblasts (HDF) has been observed. In the presence of subinhibitory concentrations of alanosine, both CHO cells and the HDF show excessive sensitivity to exogenous guanine--a phenotype which closely resembles that seen with some of the mutants containing reduced enzymatic activity of ASS. The growth inhibitory effects of alanosine, or alanosine and guanine, on CHO cells are completely reverted by the addition of adenine to the culture medium, and the synergistic effect of guanine is not observed in mutants which lack the enzyme hypoxanthine-guanine phosphoribosyl transferase. These resuls suggest that guanine nucleotides exert a regulatory effect on the activity of the enzyme adenylosuccinate synthetase. The ability to confer the guanine-sensitive phenotype and its modulation by subinhibitory concentrations of alanosine in different cell types indicates that alanosine provides a useful probe for investigating the regulation of purine nucleotide metabolism in mammalian cells.