Induction of DNA polymerase alpha in senescent cultures of normal and Werner's syndrome cultured skin fibroblasts

Replicative enzymes, DNA polymerase alpha (pol alpha), and in vitro ageing

Normal cells in culture are used to investigate the underlying mechanisms of DNA synthesis because they retain regulatory characteristics of the in vivo replication machinery. During the last few years new studies have identified a number of genetic changes that occur during in vitro ageing, providing insight into the progressive decline in biological function that occurs during ageing. Maintaining genomic integrity in eukaryotic organisms requires precisely coordinated replication of the genome during mitosis, which is the most fundamental aspect of living cells. To achieve this coordinated replication, eukaryotic cells employ an ordered series of steps to form several key protein assemblies at origins of replication. Major progress has recently been made in identifying the enzymes, and other proteins, of DNA replication that are recruited to origin sites and the order in which they are recruited during the process of replication. More than 20 proteins, including DNA polymerases, have been identified as essential components that must be preassembled at replication origins for the initiation of DNA synthesis. Of the polymerases, DNA polymerase alpha-primase (pol alpha) is of particular importance since its function is fundamental to understanding the initiation mechanism of eukaryotic DNA replication. DNA must be replicated with high fidelity to ensure the accurate transfer of genetic information to progeny cells, and decreases in DNA pol alpha activity and fidelity, which are coordinated with cell cycle progression, have been shown to be important facets of a probable intrinsic cause of genetic alterations during in vitro ageing. This has led to the proposal that pol alpha activity and function is one of the crucial determinants in ageing. In this review we summarize the current state of knowledge of DNA pol alpha function in the regulation of DNA replication and focus in particular on its interactive tasks with other proteins during in vitro ageing.

Immortalization in a normal foreskin fibroblast culture following transduction of cyclin A2 or cdk1 genes in retroviral vectors

Human diploid fibroblasts (HDF) rarely, if ever, undergo spontaneous transformation to an immortalized cell type. Here we report the immortalization of an HDF cell line following transduction with cyclin A2 or cdk1 human genes via retroviral vectors. Fluorescence in situ hybridization (FISH) studies using the retroviral vector as a probe indicate that these cell lines are monoclonal. No telomerase activity could be detected in these cell lines, and the telomere length in the immortalized cells was observed to be 10-20 kb longer than that in low-passage cells from the parental fibroblast line. Cytogenetic studies revealed that the immortal lines share common chromosomal aberrations. FISH studies with a probe for p53 revealed loss of one copy of this gene which was associated with reduced steady-state levels of both p53 and p53-regulated p21(WAF1/Sdi1/CIP1) messages in both quiescent and proliferating immortalized cultures relative to the parental cells. Additional FISH studies with probes for p16(INK4a) and Rb, carried out after the immortalized cells proliferated in excess of 100 population doublings, also revealed loss of one copy of these genes in both cell lines. These cell lines, together with the well-characterized parental cells, could provide useful research material for the study of the mechanisms of immortalization and of regulation of proliferative senescence in HDF.

Replicative enzymes and ageing: importance of DNA polymerase alpha function to the events of cellular ageing

A hallmark of cellular ageing is the failure of senescing cells to initiate DNA synthesis and transition from G1 into S phase of the cell cycle. This transition is normally dependent on or concomitant with expression of a set of genes specifying cellular proteins, some of which directly participate in DNA replication. Deregulation of this gene expression may play a pivotal role in the ageing process. The number of known enzymes and co-factors required to maintain integrity of the genome during eukaryotic DNA replication has increased significantly in the past few years, and includes proteins essential for DNA replication and repair, as well as for cell cycle regulation. In eukaryotic cells, ranging from yeast to man, a replicative enzyme essential for initiation of transcription is DNA polymerase alpha (pol alpha), the activity of which is coordinately regulated with the initiation of DNA synthesis. DNA pol alpha, by means of its primase subunit, has the unique ability to initiate de novo DNA synthesis, and as a consequence, is required for the initiation of continuous (leading-strand) DNA synthesis at an origin of replication, as well as for initiation of discontinuous (lagging-strand) DNA synthesis. The dual role of the pol alpha-primase complex makes it a potential interactant with the regulatory mechanisms controlling entry into S phase. The purpose of this review is to address the regulation and/or modulation of DNA pol alpha during ageing that may play a key role in the cascade of events which ultimately leads to the failure of old cells to enter or complete S phase of the cell cycle.

Analysis of the capacity of extracts from normal human young and senescent fibroblasts to support DNA synthesis in vitro

Cytoplasmic extracts from early-passage (young), late-passage (senescent) normal human fibroblast (HF) cultures and immortalized human cell lines (HeLa, HT-1080, and MANCA) were analyzed for their ability to support semiconservative DNA synthesis in an in vitro SV40-ori DNA replication system. Unsupplemented extracts from the three permanent cell lines were demonstrated to be active in this system; whereas young HF extracts were observed to be minimally active, and no activity could be detected in the senescent HF extracts. The activity of these extracts was compared after supplementation with three recombinant human replication factors: (1) the catalytic subunit of DNA polymerase alpha (DNA pol-alpha-cat), (2) the three subunits of replication protein A (RPA), and (3) DNA topoisomerase I (Topo I). The addition of all three recombinant proteins is required for optimum activity in the young and senescent HF extracts; the order of the level of activity is: transformed > young HF > senescent HF. Young HF extracts supplemented with RPA alone are able to support significant replicative activity but not senescent extracts which require both RPA and DNA pol-alpha-cat for any detectable activity. The necessary requirement for these factors is confirmed by the failure of unsupplemented young and senescent extracts to activate MANCA extracts that have been immunodepleted of DNA pol-alpha-cat or RPA. Immunocytochemical studies revealed that RPA, DNA pol-alpha, PCNA, and topo I levels are higher in the immortal cell types used in these studies. In the HF cells, levels of DNA pol-alpha-cat and PCNA are higher (per mg protein) in the low-passage than in the senescent cells. By contrast, RPA levels, as determined by immunocytochemical or Western blot studies, were observed to be similar in both young and senescent cell nuclei. Taken together, these results indicate that the low to undetectable activity of young HF extracts in this system is due mainly to reduced intracellular levels of RPA, while the senescent HF extracts are relatively deficient in DNA polymerase alpha and probably some other essential replication factors, as well as RPA. Moreover, the retention of RPA in the senescent HF nuclei contributes to the low level of this factor in the cytoplasmic extracts from these cells.

Effect of aging on EGF-stimulated replication of specific genes in rat hepatocytes

EGF-stimulated replication of specific genes was examined in primary hepatocyte cultures from mature (6 months) and senescent (24 months) rats. Basal and EGF-stimulated [3H]thymidine incorporation and DNA polymerase alpha activities, as well as total cellular DNA, were also assessed. The genes examined were dihydrofolate reductase (DHFR) and c-myc, as well as total mitochondrial DNA (mt DNA). Although [3H]thymidine incorporation, DNA polymerase alpha activity, total cellular DNA, DHFR, and c-myc gene specific DNA replication stimulated by EGF are reduced with age, mt DNA replication is not affected by either EGF or age. Chromosomal DNA replication is mediated mainly by DNA polymerase alpha while mt DNA replication is mediated by its own DNA polymerase gamma. Thus, the age-related decline in stimulated DNA replication appears to be associated mainly with the DNA polymerase alpha activation pathway.

Molecular mechanisms of impaired stimulation of DNA synthesis in cultured hepatocytes of aged rats

We examined epidermal growth factor (EGF)- and epinephrine-stimulated mitogen-activated protein kinase kinase (MEK) 1 and MEK2 activities, DNA polymerase alpha activity, and EGF-stimulated E2F DNA binding activity in primary cultured hepatocytes from 6- and 24-mo-old rats. MEK stimulation by either EGF or epinephrine was not altered with aging. However, stimulation of DNA polymerase alpha activity by these agents was 70% and 50% lower, respectively, in cells of aged compared with cells of young rats, consistent with a lesser increase in [3H]thymidine incorporation. EGF-stimulated E2F (a transcription factor that regulates expression of the DNA polymerase alpha gene) binding to DNA was reduced with age. PD-098059, a specific inhibitor of MEK, inhibited EGF-stimulated MEK1 and MEK2 activities in hepatocytes from 6- and 24-mo-old rats. Although PD-098059 inhibited EGF-stimulated DNA synthesis in hepatocytes from 6-mo-old rats, it had no effect in 24-mo-old rats. Thus the age-related impairment appears to occur before E2F activation, and signal transduction sequences other than the mitogen-activated protein kinase pathway may be involved in stimulated DNA synthesis in hepatocytes from old rats.

Murine temperature-sensitive DNA polymerase alpha mutant displays a diminished capacity to stimulate DNA synthesis in senescent human fibroblast nuclei in heterokaryons at the nonpermissive condition

We have investigated the capacity of a murine cell line with a temperature-sensitive (ts) mutation in the DNA polymerase alpha (Pola) locus and a series of ts non-Pola mutant cell lines from separate complementation groups to stimulate DNA synthesis, in senescent fibroblast nuclei in heterokaryons. In the Pola mutant x senescent heterodikaryons, both human and murine nuclei display significantly diminished levels of DNA synthesis at the restrictive temperature (39.5 degrees C) as determined by [3H]thymidine labeling in autoradiographs. In contrast, all of the non-Pola mutants, as well as the parental (wild type) murine cells, induced similar levels of DNA synthesis in both parental nuclei at the nonpermissive and permissive temperatures. Similarly, young human fibroblasts are also able to initiate DNA synthesis in heterokaryons with the ts Pola mutant at the two temperatures. In order to determine if complementation of the non-Pola mutants requires induction of serum responsive factors in the senescent cells, fusion studies of similar design were conducted with young and old human fibroblasts incubated in low serum (0.2%) for 48 hr prior to and after cell fusion. Again, a diminished level of DNA synthesis was observed at 39.5 degrees C in the Pola mutant x senescent cell heterokaryons. In these low-serum studies, both parental nuclei in the Pola x young cell heterokaryons and the human nuclei in heterokaryons with one of the non-Pola mutants (FT107) also displayed diminished levels of DNA synthetic activity. All of the other mutants are able to support similar levels of synthetic activity at both temperatures in the presence of reduced serum. The nature of the mutation in three of the non-Pola lines has not been determined but, like the Pola mutant cells, are inhibited in the G1 phase of the cell cycle when incubated at the nonpermissive temperature (39.5 degrees C). The fourth non-Pola mutant line is known to have at least one ts mutation in the cdc2 gene and is inhibited in the G2 phase when exposed to 39.5 degrees C. These results suggest that there may be a functional deficiency of pol alpha in senescent human fibroblasts, and this replication factor may be one of the rate-limiting factors involved in loss of the capacity to initiate DNA synthesis in senescent cells.

Is 'senescence' of diploid cells an ad hoc mechanism suppressing 'replicon' or not?

So-called 'limited life span' of diploid cells does not depend on their unresponsiveness to growth factors from the data obtained by complete protein-free culture of tumor cells. Limited proliferation (called limited life span) of diploid cells is one of the 'ad-hoc' negative regulatory mechanisms in animals, and can not be differentiated from other inhibitory mechanisms called 'tumor suppression' and 'terminal differentiation'. Metabolic imbalance induced by proliferation-dependent time-bomb mechanisms including infidelity of DNA repair is suggested to explain limited proliferation of cells. After maturation, autonomic progression of negative regulators in cells is induced by no prohibition of terminal differentiation, since organisms prepare no programs to stop development and differentiation. It is an attractive hypothesis that a proliferation-dependent time-bomb has been developed to control organogenesis for maturation and to determine body size. It is true that limited proliferation of cells can not explain longevity of individuals, although the so-called longevity genes play roles in ageing. Teleologically, longevity of individuals has been developed to produce genetic heterogeneity according to the selfish gene theory. This inter- and intra-species genetic heterogeneity increases the probability of selfish gene replication in germ cell line. After maturation and reproduction of DNA in germ cell line, individuals as vehicles for the DNA can be discarded by the 'selfish' genes.

Stimulation of DNA synthesis in senescent human cells following incubation with plasma membranes

DNA synthesis and mitosis were increased in mitogen-stimulated senescent WI-38 cells following incubation with plasma membranes prepared from young or senescent WI-38 cells, A431 cells, 3T3 cells, or NR6 cells. The percentage of [3H]thymidine-labeled nuclei in senescent cultures was two- to fivefold greater than that seen in controls in which cells were incubated in the absence of membranes or in the presence of boiled membranes. The effect was trypsin sensitive, suggesting that a protein moiety is necessary for stimulation of DNA synthesis. As the culture age increased, basal levels of DNA synthesis, as well as maximal stimulation of DNA synthesis following incubation with plasma membranes, decreased. These observations are consistent with the hypothesis that different subpopulations exist in senescing cultures and suggest a complex pattern of inhibitory and stimulatory regulation of cell proliferation.

Age related decline in the expression of proliferating cell nuclear antigen in human diploid fibroblasts

Proliferating cell nuclear antigen mRNA and protein levels were determined in human diploid fibroblasts of different in vitro ages as they progressed through the cell cycle. Cells were analyzed at G0; at various stages of G1, including the G1/S interface; and during S. At all in vitro ages, PCNA message levels were low to undetectable at G0, were evident 8 to 12 h following entrance into G1, peaked at G1/S and declined during S phase. Message levels were 2-3-fold lower in older populations at all stages of the cell cycle tested. PCNA protein increased from G0 through S phase in both age groups with 2-3-fold less being found in older cells. The decline in PCNA mRNA in older populations was not the result of changes in mRNA turnover or transcription. The results suggest that the reduction in PCNA expression is due to an age related alteration in a post-transcriptional regulatory function. The decline in the expression of the PCNA gene would contribute to the inability of older cells to initiate replicative DNA synthesis.

EGF- and PDGF-stimulated phosphorylation in young and senescent WI-38 cells

We have examined the ability of epidermal growth factor (EGF) and platelet-derived growth factor (PDGF) to stimulate cultures of young and senescent WI-38 cells to carry out tyrosine-specific phosphorylation of their respective membrane receptors. Previously we reported no reduction in EGF-stimulated phosphorylation in plasma membrane preparations of senescent cells. In this study we found no reduction in PDGF-stimulated phosphorylation in plasma membrane preparations from senescent cells. Furthermore, we found no differences in the EGF- or PDGF-stimulated phosphorylation of their respective receptors in intact cells. These data support the previous findings that although the EGF receptor autokinase activity becomes highly labile during extraction and immunoprecipitation of senescent cells, in situ loss of receptor tyrosine kinase activity is apparently not responsible for the age-associated loss of mitogenic responsiveness.

DNA polymerase alpha and the regulation of entry into S phase in heterokaryons

We have previously reported that the DNA polymerase alpha activity/unit cellular protein is decreased in late-passage (senescent) human diploid fibroblast-like (HDFL) cultures due to the cellular enlargement associated with in vitro aging. In the studies described here, we have used cell fusion technology to investigate the formal kinetic relationship between the concentration of DNA polymerase alpha and the rate of reinitiation of DNA synthesis in nuclei from senescent cells. Heterokaryons were derived from the fusion of senescent cells to a series of actively dividing cell types with inherently different DNA polymerase alpha activities per cell. A kinetic analysis revealed a first-order relationship between the entry into S phase of senescent nuclei and the concentration of DNA polymerase alpha activity calculated to be in heterokaryons. This result suggests that increases in cell volume may be related to the decline in proliferative activity of late-passage HDFL cells, via "dilution" of factors essential for cellular replication.

The relationship between the rate of entry into S phase, concentration of DNA polymerase alpha, and cell volume in human diploid fibroblast-like monokaryon cells

We have examined the kinetic relationship between the rate of entry into the S phase in human diploid fibroblast-like (HDFL) monokaryon cells and (1) the concentration of DNA polymerase alpha activity and (2) the cell volume. In the former studies, a first-order dependence between the rate of entry into the S phase and the concentration of DNA polymerase alpha activity was observed, consistent with the enzyme, or a coregulated factor, being rate limiting for this metabolic process. Examination of the nature of the dependence of the rate of entry into the S phase upon cell volume revealed a more complex relationship. The results obtained in studies with synchronized cultures are consistent with the presence of two to three rate-limiting reactants when cell volume is the independent variable. Studies with asynchronous HDFL cell cultures revealed that the smallest cells in the G1 population, presumably the early G1 cells, enter the S phase at an increasing rate as a function of cell volume up to a certain size, beyond which the cells enter at a decreasing rate similar to that observed in the studies with the synchronized cultures. Similar studies examining the relationship between cell volume and the rate of entry into S phase in three established immortal cell lines revealed positive correlation between the rate of entry into S phase and cell volume throughout the size range of the G1 population. This latter observation suggests that the factors involved in the initiation of the S phase may be present in concentrations that are not rate limiting in immortal cell lines.

Failure to phosphorylate the retinoblastoma gene product in senescent human fibroblasts

Heterokaryon studies suggest that senescent and quiescent human diploid fibroblasts (HDF) contain a common inhibitor of entry into S phase. DNA synthesis can be induced in senescent and quiescent HDF by fusing them with cells containing DNA viral oncogenes such as SV40 T antigen, adenovirus E1A, or human papillomavirus E7. Both senescent and quiescent HDF contained the unphosphorylated form (p110Rb) of the retinoblastoma protein, a putative inhibitor of proliferation. After serum stimulation, senescent HDF did not phosphorylate p110Rb and did not enter S phase, whereas quiescent HDF phosphorylated p110Rb and entered S phase. These findings, combined with the observations that T antigen, E1A, and E7 form complexes with, and presumably inactivate, unphosphorylated p110Rb, suggest that failure to phosphorylate p110Rb may be an immediate cause of failure to enter S phase in senescent HDF.

Cell enlargement: one possible mechanism underlying cellular senescence

We previously demonstrated an inverse relationship between the G1 volume of human diploid fibroblast-like (HDFL) cells obtained from foreskin tissue and clonal replicative potential. On the basis of these results, we suggested that one process underlying in vitro senescence is a progressive increase in the mean cell volume of successive progeny within clonal lineages. We now report that the size of HDFL cells, as well as of chick embryo fibroblasts, can be increased in the virtual absence of cell division by culturing at low density and at low serum concentration (0.1-1.0%). Consequent to an increase in cell size, the replicative potential of the cells is reduced to the level of later-passage cells of similar size. By clonal analysis, the populations of enlarged cells contain up to three times as many nondividing cells as do controls. In the enlarged populations, the proportion of cells producing attenuated clones (four or fewer progeny) increases by about 30%, whereas the proportion of cells yielding greater than 32 cells declines by a similar percentage. These observations lead us to propose that replicative potential may be limited by cell size, which in turn may be regulated by a kinetic relationship between cellular growth and cell division cycles.

Nonterminally differentiated cells express decreased growth factor responsiveness

In 3T3 T mesenchymal stem cells, at least four types of biological states exist that can mediate the control of cell differentiation and/or proliferation. These include the predifferentiation growth arrest state, the nonterminal differentiation state, the terminal differentiation state, and a growth arrest state induced by growth factor/serum deficiency. The current studies were performed to investigate the relative mitogenic responsiveness of cells at these four states and specifically to determine if nonterminally differentiated cells show decreased responsiveness to specific mitogens. Twenty-five different serum, plasma, and growth factor combinations were evaluated. The results show that undifferentiated, growth-arrested cells are highly responsive to numerous mitogens and that by definition terminally differentiated cells are not responsive to any mitogens. In contrast, nonterminally differentiated cells demonstrate a unique pattern of mitogenic responsiveness. Whereas nonterminally differentiated cells can be stimulated to proliferate by high concentrations of serum or plasma supplemented with growth factors, they cannot be stimulated to proliferate by combinations of multiple purified growth factors. These results suggest that the process of nonterminal differentiation is associated with a significant change in factors/cofactors required to stimulate cell proliferation and that these factors/cofactors are present in plasma.

The relationship between cell size, the activity of DNA polymerase alpha and proliferative activity in human diploid fibroblast-like cell cultures

In kinetic studies with human diploid fibroblast-like (HDFL) cells carried out in heterokaryons and in monokaryons, we have observed a first-order relationship between the level or concentration of DNA polymerase alpha and the rate of initiation of new rounds of DNA synthesis. Because cell size is inversely proportional to the concentration of DNA polymerase alpha and presumably other replication factors, it is inversely related to the initiation of new rounds of DNA synthesis. An inverse relationship between cell size and clonogenic activity was also observed in both unsorted HDFL cells and in HDFL cells sorted on the basis of size. Experimental enlargement of cells by serum deprivation at low density resulted in changes in colony-forming ability that would be predicted by these studies. A causal relationship between the observed increase in cell size with advancing passage level and the loss of proliferative activity is suggested by these studies; in addition, cell size may be a useful biophysical marker for cellular aging.

Does genotypic sex have a direct effect on longevity?

Females of the human species live longer than males, and the longevity differential is probably not entirely explained by reasons which are presently obvious. Genotypic sex has long been suspected to affect longevity to the advantage of the female. Several recent findings about the X and Y chromosomes must be reckoned with in considering determinants of longevity which derive from genotypic sex. The advantages of having two X chromosomes are apparent, notwithstanding X-chromosome inactivation. Not only can some cells compensate for biosynthetic deficiencies of others, but also cell selection according to which X chromosome is active can occur during development according to cell viability and proliferative capacity. It has recently been observed that at least some genes on inactive X chromosomes are reactivated late in life. Details of the reactivation process must be studied to determine its significance and the effects of the process on late life survival. The recent mapping of the catalytic polypeptide of DNA-polymerase-alpha to the X chromosome calls attention to a new property of the genotype which could affect the basic ability of cells to proliferate. It is likely that this enzyme, perhaps in concert with DNA-polymerase-delta, is required for DNA replication, suggesting that two alleles for this enzyme and cell selection within the female phenotypic mosaic for DNA replication may provide a sex-linked determinant of cell proliferation which could be advantageous in late life. Much remains to be learned about the Y chromosome, although there are early results consistent with a determinant of longevity on that chromosome which operates to the male disadvantage and probably does not involve sex hormones. The genotype may be a significant determinant of longevity in humans even if it does not appear to be so in non-human animals, because causes of death are different. Determinants of longevity are based on susceptibility or vulnerability to the causes and diseases of mortality, and these differ in different species.

Proliferative potential of human fibroblasts: an inverse dependence on cell size

Human foreskin fibroblast-like cells were separated on the basis of DNA content and cell size by fluorescence-activated cell sorting. Subpopulations of "large" or "small" cells with the same (G1) DNA content were clonally expanded and found to contain predominantly nondividing or highly proliferative cells, respectively. From the rate of clonal growth, we deduce that small cells divide faster than large cells. Intermediate-sized cells were found to yield primarily smaller ("attenuated") clones. The clonal data can be incorporated into a previously reported kinetic model of clonal attenuation. This version of the model postulates that small "stem" cells yield larger daughters which have only a limited proliferative potential. We also postulate that a progressive increase in cell size can account for the decreasing concentration of DNA polymerase alpha, which has been reported in older cultures.

Level of ribosomal RNA required for stimulation from quiescence increases during cellular aging in vitro of mammalian fibroblasts

We have investigated the relation between cell size in terms of cellular ribosomal RNA (rRNA) content and proliferation of diploid human and rat embryo fibroblasts during their aging in vitro. During phase III of the proliferative lifespan in vitro, cellular rRNA content increases by a factor of nearly 3. For very different regimes of stimulation of quiescent cells, a strict correlation was observed, between the proportion of cells stimulated and cellular rRNA content, resembling a steep threshold curve. During aging in vitro, these characteristic curves exhibit an essentially parallel shift to higher values of cellular rRNA content (to higher 'thresholds'). Upon establishment as a permanent cell line, the relation between proliferation stimulation and cellular rRNA ceases to change with further subculturing. It is suggested that the essence of transformation of fibroblasts with a myc-type of oncogenes is a reduction and stabilizing of the critical rRNA content required for proliferation.

Cellular senescence revisited: a review

The field of cellular senescence (cytogerontology) is reviewed. The historical precedence for investigation in this field is summarized, and placed in the context of more recent studies of the regulation of cellular proliferation and differentiation. The now-classical embryonic lung fibroblast model is compared to models utilizing other cell types as well as cells from donors of different ages and phenotypes. Modulation of cellular senescence by growth factors, hormones, and genetic manipulation is contrasted, but newer studies in oncogene involvement are omitted. A current consensus would include the view that the life span of normal diploid cells in culture is limited, is under genetic control, and is capable of being modified. Finally, embryonic cells aging in vitro share certain characteristics with early passage cells derived from donors of increasing age.

Membrane-associated inhibitor of DNA synthesis in senescent human diploid fibroblasts: characterization and comparison to quiescent cell inhibitor

Cell membranes prepared from senescent human diploid fibroblasts (HDF) inhibited entry into S phase by 35% when added to the medium of replicating young HDF. This membrane-associated inhibitory activity was (i) sensitive to trypsin, heat, and periodate, which suggests that the inhibitor is a glycoprotein, and (ii) not able to inhibit DNA synthesis in simian virus 40-transformed HDF, which indicates that not all types of cells are sensitive to this inhibitor. Quiescent young HDF also have a surface membrane-associated inhibitor of DNA synthesis. A comparison of the senescent HDF and quiescent HDF inhibitor activities indicates that they may have the same chemical and physical nature and the same specific activity, but their regulation is different. The inhibitory activity of quiescent young HDF is abolished within 20 hr after refeeding with fresh serum-containing medium, whereas that of senescent HDF remains unchanged. Quiescent old HDF (two or three population doublings remaining) exhibit an intermediate response to serum with approximately two-thirds of the inhibitory activity abolished. The fraction of cells in S phase at 20-24 hr post-stimulation (37% in young HDF, 24% in old HDF, and 0% in senescent HDF) is inversely proportional to inhibitor levels. This suggests that inability to neutralize the inhibitory activity in response to serum stimulation could be involved in the inability of senescent HDF to enter S phase. Disappearance of the inhibitory activity from quiescent young HDF occurs late in G1 phase. Thus, the inhibitor may play a role in determining the length of the G0 to S phase transition in these cells.

Expression of cell cycle-dependent genes in young and senescent WI-38 fibroblasts

We studied the expression of 11 cell cycle-dependent genes in senescent WI-38 fibroblasts and compared the results to those obtained in WI-38 cells from early passages (young cells). Every gene we examined is expressed in the senescent cells at levels similar to those in the young cells, including two genes maximally expressed at the G1/S phase boundary--genes for thymidine kinase and histone H3. The results clearly show that senescent, noncycling WI-38 cells are not similar to quiescent cells. Rather, such senescent WI-38 cells may be blocked just prior to the onset of DNA synthesis.