Patterns of peptide synthesis in senescent and presenescent human fibroblasts

Differential extracellular matrix gene expression by fibroblasts during their proliferative life span in vitro and at senescence

Increasing evidence supports the idea that the finite proliferative life span of normal fibroblasts is a differentiation-like phenomenon. If this were correct, an ordered sequence of differential gene expression should be associated with the in vitro progression of cells from low passage to high passage (senescence). To define the pattern of expression of fibroblast differentiation-associated genes during this in vitro progression, we have determined the temporal pattern of expression of extracellular matrix (ECM) genes in Syrian hamster dermal fibroblasts as a function of passage level and percentage of proliferative life span in vitro. Steady-state mRNA levels were determined by Northern and dot blot analyses of total cellular RNA hybridized with cDNA probes specific for fibronectin, procollagen alpha 1III, and procollagen alpha 1I. Cells were analyzed at 24 hr postconfluence to minimize the presence of actively proliferating cells, and because maximal levels of fibronectin, alpha 1III, and alpha 1I mRNAs were observed 24 hr postconfluence. Unique, multiphasic patterns of expression of each of these ECM components were observed as the cells progressed from low passage to high passage. As the cells reached midhigh passage, fibronectin mRNA levels increased. This midpassage increase in fibronectin was followed by an increase in the level of alpha 1III mRNA as the cells reached the end of their in vitro proliferative life span, and then alpha 1I when the cells entered the postmitotic senescent phase, at which time the level of fibronectin mRNA also declined. A similar overlapping cascade pattern of up-regulation of these genes is seen during development and wound repair. This suggests that as cultured fibroblasts reach the end of their proliferative life span, they reinitiate a gene expression program used in tissue development and repair.

DNA methylation and cellular ageing

Methylated cytosine (m5C) in DNA appears to be an important modulator of the expression of some genes. There are several lines of evidence that gradual loss of m5C is relevant to in vitro cellular ageing: m5C loss occurs during cell culture; m5C loss is detectable at an early stage of culture; m5C loss appears to be related to cell division not just duration in culture; the rate of m5C loss appears to be related to in vitro lifespan of the cell strain in question; and the total loss of m5C during an in vitro lifespan is significant by comparison with induced-changes in m5C levels which effect cell growth, or cause cell-death in culture. Progressive loss of m5C in dividing cells may thus produce the multi-step cell division "clock" which underlies the Hayflick phenomenon.

Altered regulation of platelet-derived growth factor A-chain and c-fos gene expression in senescent progeria fibroblasts

The study of human genetic disorders known as premature aging syndromes may provide insight into the mechanisms of cellular senescence. These diseases are clinically characterized by the premature onset and accelerated progression of numerous features normally associated with human aging. Previous studies have indicated that fibroblasts derived from premature aging syndrome patients have in vitro growth properties similar to senescent fibroblasts from normal individuals. As an initial approach to determine whether gene expression is altered in premature aging syndrome fibroblasts, RNA was prepared from various cell strains and used for gel blot hybridization experiments. Although normal fibroblasts only express platelet-derived growth factor (PDGF) A-chain mRNA for a brief period following mitogenic stimulation, one strain of Hutchinson-Gilford (progeria) syndrome fibroblasts, AG3513, constitutively expresses PDGF A-chain mRNA and PDGF-AA homodimers. The PDGF A-chain gene does not appear to be amplified or rearranged in these fibroblasts. AG3513 progeria fibroblasts have properties characteristic of senescent cells, including an altered morphology and a diminished mitogenic response to growth promoters. The diminished response of AG3513 progeria fibroblasts to PDGF stimulation was examined in some detail. Studies using 125I-PDGF-BB, which binds with high affinity to both A- and B-type PDGF receptors, indicate that normal and AG3513 progeria fibroblasts have a similar number of PDGF receptors. Although receptor autophosphorylation occurs normally in PDGF-stimulated AG3513 progeria fibroblasts, c-fos mRNA induction does not. The senescent phenotype of AG3513 fibroblasts is probably unrelated to their constitutive PDGF A-chain gene expression; further studies are necessary in order to directly address this issue. Also, additional analysis of this progeria fibroblast strain may provide information on the control of mitogen-inducible gene expression in normal cells.

Protein Turnover During Aging of Cultured Human Fibroblasts

Les cellules qui vieillissentin vitroou bien celles qui sont prélevées de donneurs d'àge avancé ou de sujets manifestant certaines des particularités qui s'apparentent à un vieillissement accéléré (progérie ou le syndrome de Werner), peuvent être qualifiées de 'vieilles'. Celles-ci on des taux de croissance ralentis en milieu de culture si on les compare aux cellules nouvelles ou à mi-passage prélevées de jeunes donneurs normaux. Durant la croissance exponentielle, les taux de constantes pour la synthèse protéique dans les jeunes cellules ne sont pas significativement différents de ceux retrouvés dans les vieilles cellules (0.023 ± 0.002h1vs. 0.021 ± 0.002h1respectivement) et pourtant les taux de croissance (i.e. accrétion protéique) sont de seulement 0.013±0.003h1dans les vieilles cellules comparés à 0.022±0.002h1dans les jeunes cellules. Done, le taux ralenti d'accumulation protéique durant la croissance des vieilles cellules comparé aux jeunes cellules est associé à une dégradation protéique accélérée (0.01±0.002h1vs 0.001 ±0.002h1;P<0.05) plutôt qu'à des taux ralentis de synthèse protéique. Lorsque les cellules deviennent quiescentes suivant une période d'inhibition de croissance due à la densité, les taux de synthèse protéique diminuent dans les jeunes et les vieilles cellules pour aboutir à des niveaux comparables (0.013±0.002h1) où les taux de croissance (0.003±0.0003h1) et de dégradation (0.01±0.003h1) ne sont significativement pas differents dans les deux groupes. Done, ce n'est qu'en période de croissance exponentielle qu'une différence dans le turn-over protéique entre jeunes et vieilles cellules est observée, alors que la dégradation est accélérée dans les vieilles cellules. La relation causale entre la dégradation protéique accélérée et les taux de croissance ralentis dans les vieilles cellules demeure inconnue.

Enhanced synthesis of a Mr = 55,000 dalton peptide by senescent human fibroblasts

The secretory protein profiles of early and late passage cultures of human fibroblasts were compared using polyacrylamide gel electrophoresis. In comparison with early passage cell cultures (40-50% lifespan completed), late passage (greater than 80% of lifespan completed) cell cultures exhibited enhanced production of several peptides in the Mr range 55-60,000. One of those peptides had an apparent molecular weight of Mr = 55,000 and was constitutively present in the late passage cell conditioned medium. Late passage cell cultures synthesized the Mr = 55,000 peptide in the presence or absence of fetal bovine serum. Serum did not enhance its production by early passage cells. Further, production of the peptide was not induced in early passage cell cultures whose proliferation was arrested either by serum starvation or by contact inhibition. Pulse chase studies demonstrated that the peptide appears in the culture medium within 60 min of labeling. There was no evidence that it is derived via degradation of other proteins present either in early passage or late passage cell conditioned media. Further, the production of the 55,000 dalton peptide did not appear to be regulated by factors present in conditioned media. The peptide was detected in the conditioned media produced by late passage cultures of several different cell strains.

Changes in the higher order organization of DNA during aging of human fibroblast-like cells

Nucleosome spacing (DNA repeat length) was determined in human diploid fibroblast-like cells (HDF) of different in vitro ages following the electrophoretic separation of micrococcal nuclease digestion products. The results indicate that a heterogeneity of DNA repeat lengths is present in HDF of all in vitro ages. In older cells the organization of part of the DNA is conserved, but a greater proportion of shorter repeats is evident. The shorter repeat lengths are not due to nucleosome sliding, but result from the presence of shorter linker regions which are reduced by as much as 25% in part of the chromatin of high PDL cells.

Receptor for epidermal growth factor retains normal structure and function in aging cells

Cellular responsiveness to epidermal growth factor (EGF) and the structure of the receptor for epidermal growth factor (EGF-R) were compared in young and senescent human fibroblast (HF) cells. Biosynthetic labeling of HF cells with [35S] methionine followed by immunoprecipitation with EGF-R antibody revealed the presence of Mr 170 000 EGF-R in cells from both stages. Autophosphorylation of EGF-R in response to EGF was identical in young and senescent cells. Phosphoamino acid analysis of the autophosphorylated EGF-R indicated that tyrosine residues were phosphorylated in each preparation. Two-dimensional peptide mapping of [125I]EGF-R from young and senescent cells showed essentially the same pattern, indicating that EGF-R does not apparently undergo detectable changes in senescent human fibroblasts. The responsiveness of aging HF cells to EGF for the induction of ornithine decarboxylase activity and for the production of secretory proteins was measured. Young and senescent HF cells showed about a three-fold induction of collagenase activity upon addition of EGF. Ornithine decarboxylase activity was also stimulated by EGF to a comparable level in young and senescent cells. Our results indicate that the responsiveness of HF cells to EGF for these two biochemical parameters does not decline with the loss of proliferative activity.

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.

Effects of beta interferon on human fibroblasts at different population doubling levels. Proliferation, cell volume, thymidine uptake, and DNA synthesis

Cellular aging had no effect on the ability of beta interferon to increase cell volume and population doubling time in 76-109 cells, a line of human skin fibroblasts. However, DNA synthesis in cells at high population doubling levels (PDL 55-70) was inhibited after 72 h of beta interferon treatment (1,000 U/ml) while no inhibition of DNA synthesis was observed in cells at middle population doubling levels (PDL 30-40).

[125I]EGF binding ability is stable throughout the replicative life-span of WI-38 cells

Using a serum-free medium supplemented with hormones and growth factors, which included epidermal growth factor (EGF), we investigated the binding and processing-degradation of [125I]EGF in WI-38 cells of various in vitro ages. The binding and processing-degradation systems of these cells remained essentially unchanged throughout their lifespan. The number of specific [125I]EGF binding sites per cell increased as the cultures senesced, though the number of specific binding sites per micron 2 (surface area) remained constant. The kinetics of ligand degradation as well as the qualitative and quantitative nature of the degradation products also remained essentially unchanged throughout the life-span. The only consistent alteration in any of the binding parameters measured was the slight decrease in the apparent Kd of the ligand-receptor complex, independent of temperature. Quantitation of EGF-stimulated DNA synthesis revealed a decrease in the percentage of cells incorporating [3H]thymidine ([3H]TdR) during a 30-h exposure from 45% in young cells to 0.25% in senescent cells, although [125I]EGF binding or processing-degradation did not differ significantly in young and old cells. Thus, EGF binding does not decrease in senescence.

Senescence of aortic endothelial cells in vitro: influence of culture conditions and preliminary characterization of the senescent phenotype

The replicative lifespan of a cloned strain of adult bovine aortic endothelial cells has been examined by serial passage in culture using conditions where the cells were either dependent on fibroblast growth factor (FGF) for rapid growth or relatively independent on FGF for growth. In FGF-dependent cultures a replicative lifespan of 130 generations was attained with a phase III period which spanned approximately 20 generations. Withdrawal of FGF at generation 55 and repeated passage of such cultures in the absence of the growth factor resulted in the loss of proliferative potential within 15 generations. The morphological changes occurring upon FGF withdrawal were different than those that occurred when cultures senesced in the presence of FGF, FGF withdrawn cells showed a homogeneous increase in cell size and after several passages overgrew one another at confluency. Endothelial cells which senesced in the presence of FGF showed a very heterogeneous distribution of enlarged cells, many of which were binucleated but continued to form a confluent monolayer at high cell densities. Under FGF-independent conditions (begun at generation 48) a replicative lifespan of 105 generations was attained in the presence of the growth factor. FGF withdrawal under these conditions only decreased the replicative lifespan to 95 generations. Under these conditions the morphological changes occurring during phase III were identical in the presence and absence of FGF. Examination of the sensitivity of endothelial cells to FGF as they entered phase III showed that their dose-response characteristics were not qualitatively altered after the onset of phase III, although the number of cells responding to FGF progressively dropped. Comparison of the patterns of proteins synthesized in phase II and phase III cultures showed that phase III cultures even when plated at sparse densities continued to synthesize proteins which were normally observed in phase II confluent cultures. The results indicate that onset of the phase III period in aortic endothelial cells cloned and maintained in the presence of fibroblast growth factor can be delayed to a greated extent if the cells are maintained under conditions where they are made dependent on FGF for rapid growth in culture.

Protein synthetic errors do not increase during aging of cultured human fibroblasts

To test the error catastrophe theory of aging we determined the error frequency of protein synthesis in several strains of cultured human fibroblasts at early and late passage. Error rates were calculated from analysis of native and substituted actins on two-dimensional gels of cellular proteins after induction of mistranslation by histidine starvation in the presence of histidinol. Early-passage cells from fetal, young, and old donors and cells from subjects with the Hutchinson-Gilford and Werner syndromes of accelerated aging had similar error frequencies. Late-passage cells from fetal, young, and old normal donors had similar or lower error frequencies than corresponding early-passage cells. No correlation was observed between error frequency, donor age, or maximal life span in vitro. We also examined an immortal cell line, simian virus 40-transformed W138 fibroblasts. These cells had a significantly elevated rate of mistranslation (2.8 +/- 0.2 x 10(-4))(+/- SEM) compared to their untransformed counterpart WI38 (0.6 +/- 0.1 X 10(-4)) or all diploid cells combined (1.1 +/- 0.1 x 10(-4)). Taken together, the data fail to support the error catastrophe theory of aging.