Effects of prolonged quiescence on neuclei and chromatin of WI-38 fibroblasts

Quiescence-Origin Senescence: A New Paradigm in Cellular Aging

Cellular senescence, traditionally viewed as a consequence of proliferating and growing cells overwhelmed by extensive stresses and damage, has long been recognized as a critical cellular aging mechanism. Recent research, however, has revealed a novel pathway termed "quiescence-origin senescence", where cells directly transition into senescence from the quiescent state, bypassing cell proliferation and growth. This opinion paper presents a framework conceptualizing a continuum between quiescence and senescence with quiescence deepening as a precursor to senescence entry. We explore the triggers and controllers of this process and discuss its biological implications. Given that the majority of cells in the human body are dormant rather than proliferative, understanding quiescence-origin senescence has significant implications for tissue homeostasis, aging, cancer, and various disease processes. The new paradigm in exploring this previously overlooked senescent cell population may reshape our intervention strategies for age-related diseases and tissue regeneration.

Controlling Depth of Cellular Quiescence by an Rb-E2F Network Switch

Quiescence is a non-proliferative cellular state that is critical to tissue repair and regeneration. Although often described as the G0 phase, quiescence is not a single homogeneous state. As cells remain quiescent for longer durations, they move progressively deeper and display a reduced sensitivity to growth signals. Deep quiescent cells, unlike senescent cells, can still re-enter the cell cycle under physiological conditions. Mechanisms controlling quiescence depth are poorly understood, representing a currently underappreciated layer of complexity in growth control. Here, we show that the activation threshold of a Retinoblastoma (Rb)-E2F network switch controls quiescence depth. Particularly, deeper quiescent cells feature a higher E2F-switching threshold and exhibit a delayed traverse through the restriction point (R-point). We further show that different components of the Rb-E2F network can be experimentally perturbed, following computer model predictions, to coarse- or fine-tune the E2F-switching threshold and drive cells into varying quiescence depths.

P2P-R protein localizes to the nucleolus of interphase cells and the periphery of chromosomes in mitotic cells which show maximum P2P-R immunoreactivity

P2P-R is a nuclear protein that can bind both p53 and Rb1. Its functions include roles in the control of RNA metabolism, apoptosis, and p53-dependent transcription. The expression of P2P-R also is repressed in G1 arrested terminally differentiated cells. The current studies therefore evaluated if P2P-R undergoes cell cycle-associated changes in its abundance and/or localization. Western blots show that relative to G0 quiescent cells, P2P-R protein levels are higher in populations of G2/M cells prepared by the physiological parasynchronization technique of serum deprivation followed by serum stimulation. More striking is the > 10-fold enrichment of P2P-R protein in specimens of highly purified mitotic cells prepared by the mitotic shake-select technique, or by synchrony with the mitotic spindle disruption agents nocodazole or vinblastine. These changes in P2P-R protein occur without a concomitant change in P2P-R mRNA expression suggesting that P2P-R immunoreactivity increases during mitosis. Confocal microscopy next established the localization of P2P-R to nucleoli in interphase cells and at the periphery of chromosomes in mitotic cells that lack nucleoli. The high levels of P2P-R localized to the periphery of chromosomes in mitotic cells suggest that P2P-R shares characteristics with other nucleolar proteins that associate with the periphery of chromosomes during mitosis. These include: nucleolin, B23, Ki67, and fibrillarin.

Nuclear proteins of quiescent Xenopus laevis cells inhibit DNA replication in intact and permeabilized nuclei

Quiescent cells from adult vertebrate liver and contact-inhibited or serum-deprived tissue cultures are active metabolically but do not carry out nuclear DNA replication and cell division. Replication of intact nuclei isolated from either quiescent Xenopus liver or cultured Xenopus A6 cells in quiescence was barely detectable in interphase extracts of Xenopus laevis eggs, although Xenopus sperm chromatin was replicated with approximately 100% efficiency in the same extracts. Permeabilization of nuclei from quiescent Xenopus liver or cultured Xenopus epithelial A6 cells did not facilitate efficient replication in egg extracts. Moreover, replication of Xenopus sperm chromatin in egg extracts was strongly inhibited by a soluble extract of isolated Xenopus liver nuclei; in contrast, complementary-strand synthesis on single-stranded DNA templates in egg extracts was not affected. Inhibition was specific to endogenous molecules localized preferentially in quiescent as opposed to proliferating cell nuclei, and was not due to suppression of cdk2 kinase activity. Extracts of Xenopus liver nuclei also inhibited growth of sperm nuclei formed in egg extracts. However, the rate and extent of decondensation of sperm chromatin in egg extracts were not affected. The formation of prereplication centers detected by anti-RP-A antibody was not affected by extracts of liver nuclei, but formation of active replication foci was blocked by the same extracts. Inhibition of DNA replication was alleviated when liver nuclear extracts were added to metaphase egg extracts before or immediately after Ca++ ion-induced transition to interphase. A plausible interpretation of our data is that endogenous inhibitors of DNA replication play an important role in establishing and maintaining a quiescent state in Xenopus cells, both in vivo and in cultured cells, perhaps by negatively regulating positive modulators of the replication machinery.

Confluence-dependent resistance in human colon cancer cells: role of reduced drug accumulation and low intrinsic chemosensitivity of resting cells

In vitro sensitivity of HT29 human colon cancer cells to doxorubicin (DXR), vincristine (VCR), etoposide (VP16), cisplatin (CDDP), melphalan (L-PAM) and 5-fluorouracil (5FU) was markedly reduced when cell-culture density increased. For some drugs, confluence-dependent resistance (CDR) was partly due to decreased intracellular drug accumulation; the ratio of mean intracellular drug content of non confluent to confluent cells (NC/C) was 2.5 for DXR, 4.1 for VCR and 7.4 for VP16. Altered drug penetration with confluence could be related to decrease of plasma membrane fluidity as measured by the fluorescence polarization method. Reduction of drug intracellular accumulation was nil or weak for L-PAM (NC/C = 1.0), CDDP (NC/C = 1.2) and 5 FU (NC/C = 1.8). Even if drug concentration was adjusted in culture medium to produce similar intracellular drug content in confluent and non confluent cells, higher intrinsic resistance of confluent cells was still evidenced for DXR and VP16 but not for VCR, the only agent without direct interaction with DNA. DXR- and VP16-induced DNA breakage was also less important in confluent than in non-confluent cells. CDR appeared closely related to an increased proportion of non-cycling cells at confluence, as demonstrated by flow cytometry, expression of nuclear antigen recognized by Ki67 MAb and expression of topoisomerase II. CDR is probably a major factor in the poor sensitivity of colorectal adenocarcinomas to chemotherapy.

In vitro growth rate of placental fibroblasts is developmentally regulated

Placental cells of mesenchymal origin were used to study the regulation of fetal growth at the cellular level. A significant difference in the in vitro growth rates of placental fibroblasts was observed as a function of gestational age. Cells derived from 10-19-wk placentae exhibited proliferative rates two to three times greater than cells derived from 7-9-wk placentae (16-30 h vs. 30-60 h, P less than 0.001). The proliferation rate remained stable throughout multiple passages in culture. Additionally, these two groups of cell strains exhibited marked differences in their responsiveness to mitogenic stimuli. Using maximal effective concentrations, insulin-like growth factor I interacted synergistically with epidermal growth factor and fibroblast growth factor to stimulate DNA synthesis in cells derived from 10-19-wk placentae. By contrast, the interaction of insulin-like growth factor 1 with epidermal growth factor and fibroblast growth factor exhibited significantly less synergy in 7-9-wk cells. These findings argue that the accelerated growth rate of human fetal cells results primarily from developmental events intrinsic to the cells and is associated with enhanced responsiveness to the mitogenic action of peptide growth factors.

Evidence that density-dependent growth arrest is a two-stage process in WI-38 cells

It was the goal of this study to determine whether during long-term quiescence WI-38 cells gradually lose labile components which then need to be resynthesized before a stimulated cell can progress through G-1 and enter S. The metabolic and molecular status of WI-38 cells was systematically analyzed as they entered and were maintained for an extended period of time in a state of density-dependent growth arrest. Our results indicate that growth arrest in WI-38 cells can be divided into two stages. The first, which we call "early" growth arrest, occurs during the first 7-10 days following cessation of DNA synthesis and mitosis. It is characterized by few biochemical changes compared to actively proliferating cells. During this period of early growth arrest cells do not exhibit a prolongation of the prereplicative stage following serum stimulation. In contrast, WI-38 cells growth arrested for 10-20 days exhibit a number of changes at the molecular and biochemical level (i.e., a twofold decrease in total protein and total RNA content, and decreased levels of most proteins, but an increased amount of fibronectin and collagen). Also, quiescent WI-38 cells stimulated at any time during "later" or "deep" growth arrest do exhibit a prolonged prereplicative phase. Although changes were also observed in the patterns of expression of ten representative growth-associated genes (i.e., histone H-3, p53, c-Ha-ras, 2A9/calcyclin, 4F1/vimentin, LDL-receptor, insulin receptor, collagen, and fibronectin), these occurred mostly at the time when the cells ceased synthesis of DNA and mitosis and became quiescent. No changes in the steady-state levels of the growth-associated transcripts analyzed occurred while the cells were maintained in the growth-arrested state. Thus, these experiments show that although WI-38 cells do cease to incorporate thymidine and divide under crowded culture conditions, the "quiescent" cells continue to undergo changes, are metabolically active, and certainly do not grossly deteriorate.

Analysis of the growth factor requirements for stimulation of WI-38 cells after extended periods of density-dependent growth arrest

When cultures of WI-38 human diploid fibroblasts reach high cell densities, they cease to proliferate and enter a viable state of quiescence. WI-38 cells can remain in this quiescent state for long periods of time; however, the longer the cells remain growth arrested, the more time they require to leave G0, progress through G1, and enter S after stimulation with fresh serum. The experiments presented here compare the response of long-term quiescent WI-38 cells (stimulated 26 days after plating) and short-term quiescent WI-38 cells (stimulated 12 days after plating) to treatment with a variety of individual purified growth factors instead of whole serum. Our results show that the qualitative and quantitative growth factor requirements necessary to stimulate G1 progression and entry into S were the same for both short- and long-term quiescent WI-38 cells, in that the same defined medium (supplemented with epidermal growth factor [EGF], recombinant human insulin-like growth factor 1 [IGF-1], and dexamethasone [DEX]) stimulated both populations of cells to proliferate with the same kinetics and to the same extent as serum. However, the long-term quiescent WI-38 cells were found to exhibit a difference in the time during which either serum or these individual growth factors were required to be present during the prereplicative period. We believe that this difference may be the cause of the prolongation of the prereplicative phase after stimulation of long-term density-arrested WI-38 cells.

A high melting structure in DNA distinguishes phases of the cell cycle

Differential scanning microcalorimetry of the nuclei of dividing CHO cells revealed DNA structures that showed structural transitions at 60, 76, 88, and 105 degrees C (transitions I to IV, respectively). In cultures synchronized by isoleucine deprivation the enthalpies of transitions I and II were rather constant throughout the cell cycle. While the sum of the enthalpies of III and IV was nearly constant, the ratio of IV to III varied substantially from one phase of the cycle to another. A high IV:III ratio of 6 characterized G1 while S phase gave a IV:III ratio of about 2. Cells containing metaphase chromosomes also showed a IV:III ratio near 2. The IV:III ratio for CHO cells showed a progressive decrease as the cells were maintained in isoleucine-free medium from 0 to 6 days.

Expression of c-myc and induction of DNA synthesis by platelet-poor plasma in human diploid fibroblasts

When WI-38 human diploid fibroblasts become confluent, they stop synthesizing DNA and dividing. Addition of serum causes the quiescent cell to reenter the cell cycle. Prolonged quiescence after confluence decreases and delays the response to serum. For a few days after reaching confluence, WI-38 cells also respond to platelet-poor plasma. During this period, although not cycling, WI-38 cells still express c-myc and other growth-regulated genes, as measured by steady-state RNA levels. If the quiescence is prolonged further, c-myc expression (and that of two other growth-regulated genes) is no longer detectable, and its disappearance coincides with a loss of response to platelet-poor plasma. These results suggest that, also under physiological conditions, the expression of c-myc and other growth-regulated genes can cooperate with platelet-poor plasma in inducing cellular DNA synthesis in human diploid fibroblasts.

Specific growth inhibitory sequences in genomic DNA from quiescent human embryo fibroblasts

We used HeLa cells as recipients in a gene transfer assay to characterize DNA sequences that negatively regulate mammalian cell growth. In this assay, genomic DNA from quiescent human embryo fibroblasts was more inhibitory for HeLa replication than was DNA from either Escherichia coli or HeLa cells. Surprisingly, growth inhibitory activity depended on the growth state of the cells from which genomic DNA was prepared; it was strongest in DNA prepared from serum-deprived, quiescent embryo fibroblasts. This latter observation implies a role for DNA modification(s) in regulating the activity of the inhibitory sequences detected in our assay. The level of the observed growth inhibitory activity was sometimes high, suggesting that the relevant sequences may be abundantly represented in the mammalian genome. We speculate that these findings may provide new insights into the molecular mechanisms involved in cellular quiescence and in vitro senescence.

Specific growth inhibitory sequences in genomic DNA from quiescent human embryo fibroblasts

We used HeLa cells as recipients in a gene transfer assay to characterize DNA sequences that negatively regulate mammalian cell growth. In this assay, genomic DNA from quiescent human embryo fibroblasts was more inhibitory for HeLa replication than was DNA from either Escherichia coli or HeLa cells. Surprisingly, growth inhibitory activity depended on the growth state of the cells from which genomic DNA was prepared; it was strongest in DNA prepared from serum-deprived, quiescent embryo fibroblasts. This latter observation implies a role for DNA modification(s) in regulating the activity of the inhibitory sequences detected in our assay. The level of the observed growth inhibitory activity was sometimes high, suggesting that the relevant sequences may be abundantly represented in the mammalian genome. We speculate that these findings may provide new insights into the molecular mechanisms involved in cellular quiescence and in vitro senescence.

The sensitivity of G0-state haemopoietic spleen colony-forming cells to a stimulus for proliferation

Haemopoietic spleen colony-forming units (CFU-s) close to the axis (axial CFU-s) of the long bones have a high probability of self-renewal. They are pluripotent cells and are largely in a G0-State. By contrast, CFU-s close to the bone surface (marginal CFU-s) have a lower probability of self-renewal and are probably more mature, though still pluripotent. Most CFU-s proliferation arises in this zone. As a consequence, marginal CFU-s tend to have shorter G0 histories than do axial CFU-s. Femoral marrow was, therefore, divided into axial and marginal populations and the sensitivity of the CFU-s to an endogenous CFU-s-specific proliferation-stimulating factor was assessed and compared by the tritiated thymidine suicide technique. It was found that axial CFU-s are considerably more resistant to stimulation than are marginal CFU-s in that larger doses for longer periods of exposure are required to increase the proliferative activity of the cells. This behaviour is consistent with the suggestion that cells with a low division probability exist in deeper levels of the quiescent G0-state. Although this hypothesis was developed from the behaviour of cells maintained in culture under sub-optimal physiological conditions, this phenomenon appears, in vivo, to be a characteristic of the stem cell population of haemopoietic tissue; their high resistance to stimulation maintaining the axial CFU-s in a quiescent state.

Methylmercury effects on cell cycle kinetics

Methylmercury (MeHg) effects on cell cycle kinetics were investigated to help identify its mechanisms of action. Flow cytometric analysis of normal human fibroblasts grown in vitro in the presence of BrdU allowed quantitation of the proportion of cells in G1, S, G2 and the next G1 phase. This technique provides a rapid and easily performed method of characterizing phase lengths and transition rates for the complete cell cycle. After first exposure to MeHg the cell cycle time was lengthened due to a prolonged G1. At 3 microM MeHg the G1 phase length was 25% longer than the control. The G1/S transition rate was also decreased in a dose-related manner. Confluent cells exposed to MeHg and replated with MeHg respond in the same way as cells which have not been exposed to MeHg before replating. Cells exposed for long times to MeHg lost a detectable G1 effect, and instead showed an increase in the G2 percentage, which was directly related to MeHg concentration and length of exposure. After 8 days at 5 microM MeHg, 45% of the population was in G2. The G2 accumulation was reversible up to 3 days, but at 6 days the cells remained in G2 when the MeHg was removed. Cell counts and viability indicated that there was not a selective loss of cells from the MeHg. MeHg has multiple effects on the cell cycle which include a lengthened G1 and decreased transition probability after short term exposure of cycling cells, and a G2 accumulation after a longer term exposure. There were no detectable S phase effects. It appears that mitosis (the G2 accumulation) and probably synthesis of some macromolecules in G1 (the lengthened G1 and lowered transition probability) are particularly susceptible to MeHg.

Controls on the cell cycle

The control of cell proliferation, in physiological terms, depends not so much on our understanding of the sequence of biochemical events unfolding as a cell progresses through its proliferation cycle, as upon the recognition by a tissue of the demands for functional cells of a particular type. After considering the modes of control possible, i.e. by recruitment of resting G0-state cells into cycle or by modifying the proliferative behaviour of already proliferating cells, haemopoietic tissue is used as a model to illustrate how the principles of proliferation control in specific cell lineages can be effected. Although the mode of stem cell control is different from that in the maturing populations, all depend on a co-ordination of negative feedback loops for inhibitor and stimulator which are specific to that cell population. The concept of a 'quantal' cell cycle is considered but its application to control in an adult steady-state tissue must be modified to take account of microenvironmental influences which are shown, by their cellular organization, to be an important feature in haemopoietic and probably all other tissues.

Quiescent human diploid fibroblasts. Common mechanism for inhibition of DNA replication in density-inhibited and serum-deprived cells

The mechanism for cessation of proliferation in density-inhibited quiescent human diploid fibroblasts (HDF) and serum-deprived quiescent HDF was compared in two ways. Density-inhibited HDF were fused to either replicating HDF or SV40-transformed HDF and DNA synthesis was measured in the resulting heterokaryons. DNA synthesis was inhibited in the replicating HDF nuclei in heterokaryons in a way that suggested that entry into S phase was blocked, but ongoing DNA synthesis was not inhibited. In contrast, DNA synthesis was induced in the quiescent nuclei in heterokaryons formed with SV40-transformed HDF. Previous experiments had shown that serum-deprived HDF also behave in this way in heterokaryons. To test this similarity further, we examined the inhibitory activity of cell membranes prepared from both types of quiescent HDF. We found that both types of quiescent HDF contain DNA synthesis-inhibitory activity that is (1) effective on replicating HDF; (2) ineffective on SV40-transformed HDF; (3) sensitive to heat and trypsin. Thus, these results support the hypothesis that both density-inhibited HDF and serum-deprived HDF share a common mechanism for arrest in G1 phase. They also suggest that a membrane-bound protein plays a role in the inhibition of DNA synthesis in quiescent HDF.

Regulation of the rate of cell cycle progression in quiescent cytolytic T cells by T cell growth factor: analysis by flow microfluorometry

We have previously shown that greater than 90% of B6.1 cells, a murine cytolytic T lymphocyte (CTL) cloned line which is solely dependent on T cell growth factor (TCGF) for continuous growth in vitro, accumulates in the G1 phase of the cell cycle after transfer into culture medium containing no TCGF. Moreover, when such quiescent cells are exposed again to TCGF, greater than 85% reenter the S phase and subsequently divide in a relatively synchronous fashion. In this study, the regulation of the rate of cell cycle progression of quiescent B6.1 cells after exposure to TCGF was analyzed using two complementary DNA staining techniques, namely, the propodium iodide method (to enumerate cells entering the S phase) and the Hoechst 33342-bromodeoxyuridine substitution technique (to enumerate cells which have gone through mitosis). After TCGF addition, quiescent B6.1 cells resumed DNA synthesis and divided after a lag phase of 10 and 20 h, respectively. The duration of the lag phase was found to be dependent on the length of time during which quiescent B6.1 cells had been deprived of TCGF, but was independent of the concentration of TCGF used for restimulation. In contrast, the proportion of cells responding to TCGF as well as the rate of their first passage through mitosis was dependent on TCGF concentration. The presence of TCGF for at least 6 h was required for a maximal response. Moreover, direct evidence was obtained that TCGF by itself was able to stimulate proliferation of quiescent B6.1 cells in the absence of other growth factors and serum constituents other than bovine serum albumin, transferrin, and lipids.

Advance toward S phase and retreat toward deeper "G0" states in resting 3Y1 cells with environmental changes

To elucidate conditions which affect the lag time for resting cells to enter S phase after serum stimulation, we used a wild-type 3Y1 rat fibroblast line and four temperature-sensitive mutants of 3Y1 (3Y1tsD123, 3Y1tsF121, 3Y1tsG125, and 3Y1tsH203). Among these five lines, in only tsG125 cells was there an obviously prolonged lag time with increase in time in resting state at 33.8 degrees C. The resting wild-type 3Y1 cells, preexposed to 39.8 degrees C, also showed a prolongation of lag time. The prolongation in tsG125 had a certain limit. Preexposure to 39.8 degrees C before serum stimulation accelerated such prolongation in tsG125 to its limit, but did not change the limit, per se. Resting tsG125 cells stimulated by serum at 39.8 degrees C, did not enter S phase, yet they did advance toward S phase. When they were kept at 39.8 degrees C, they retreated toward a deeper resting state ("G0") with time. These retreats correlated with the decrease in stimulating activity in the culture media. About 20% of the resting tsG125 cells stimulated by serum at 39.8 degrees C were committed to enter S phase, when the extent of commitment was examined at 33.8 degrees C. Most of the tsG125 cells committed at 33.8 degrees C did not enter S phase, when the extent of commitment was examined at 39.8 degrees C. More cells were committed after stimulation at 33.8 degrees C than at 39.8 degrees C, when the test was done at 33.8 degrees C. We suggest that resting cells may be reversibly changed within range of resting states, in either direction, that is, advance toward S phase or retreat toward deeper "G0." These changes may be determined by alterations in the balance between synthesis and decay of the preparedness for the initiation of DNA synthesis caused by cellular response to environmental changes (e.g., medium activity, temperature, etc.). The ts defect in tsG125 may affect the cell cycle progression, both before and after commitment by serum.

Isolation of a G0-specific ts mutant from a Fischer rat cell line, 3Y1

A ts mutant clone, tsJT60, was isolated from Fisher rat cell line, 3Y1. During the exponential growth at both 34 and 39.5 degrees C, tsJT60 did not appear as ts mutant cells. However, once entered resting state (G0) under serum deprivation at the confluent state, they could re-enter S phase at 34 degrees C but could not at 39.5 degrees C following the stimulation of cells either by the addition of fetal bovine serum or by trypsinization and replating. These and other results suggested that tsJT60 is a G0-specific ts mutant, i.e., the cells have ts defect(s) in the function which is required for the stimulation from the resting state to S phase but not for the progression of the cell cycle in an exponential growth phase.

Evidence for differences in the mechanism of cell cycle arrest between senescent and serum-deprived human fibroblasts: heterokaryon and metabolic inhibitor studies

It has previously been shown that serum-deprived, early passage quiescent human diploid fibroblastlike (HDFL) cells are able to inhibit cycling cells from entry into DNA synthesis upon cell fusion. We have found that the degree of inhibition of DNA synthesis in the heterokaryon correlates with the duration of serum deprivation, which is consistent with the suggestion that serum-deprived cells may enter progressively deeper stages of G0 as they increase their time in quiescence. In contrast to fusions with senescent cells, in heterokaryons between serum-deprived early passage and cycling young cells transient inhibition of protein synthesis with cycloheximide or inhibition of RNA synthesis with 5-6-dichloro-1-beta-D-ribofuranosyl benzimidazole (DRB) did not stimulate nuclear [3H]-thymidine incorporation. These results suggest that differences may exist in the mechanisms responsible for inhibiting cell cycle progression in senescent vs early passage quiescent HDFL cells.

Cell cycle study on the effect of interferon on synchronized mouse embryo fibroblasts

Mouse embryo fibroblasts synchronized by controlling cultural conditions were used to examine the effects of interferon (IFN) while undergoing a single synchronous cycle of division at the tertiary stage. IFN was added early in G1 and at the G1-S boundary and the duration of specific phases of the cycle were investigated together with biochemical events related to cell cycle progression. Assessment of population distribution by fluorimetric quantitation of DNA content showed that IFN extended G1 and G2 but had no effect on the duration of S phase. Assessment of transport and uptake of exogenous TdR and measurements of specific kinase activity under conditions where DNA synthesis and S phase were not altered showed that IFN had no effect on TdR transport but could markedly reduce TdR uptake, and delay the S phase associated increase of TdR-kinase activity.

On the aging of organisms and their cells

An explanation of organismic ageing based on a limited division capacity of dividing cells is difficult to reconcile with much of the available data. The physiology of cells in ageing organisms tends, on the contrary, to suggest that organisms age as a result of degeneration in their non-dividing cell populations. Senescence in these non-mitotic cells resmebles the ageing of the non-dividing fraction of cell cultures clonally senescing, or maintained in long-term quiescence in vitro. As cultures of diploid human fibroblasts senesce there is an accumulation of non-dividing cells. Alterations in these post-mitotic cells can explain the senescent properties of late passage cultures. It is proposed that during the in vitro senescence of fibroblast cultures, cell ageing results from, as opposed to causes, the absence of mitosis. Cell ageing may primarily result from changes in the chromatin induced by the non-mitotic state.

Evidence that transcription changes in ageing cultures are terminal events occurring after the expression of a reduced replicative potential

There is a progressive decline in replicative capacity with increasing age as expressed in terms of percentage labelled nuclei with 3H-thymidine and altered saturation density at confluency. This expression of ageing in vitro is seen in three different lines of human embryo diploid fibroblasts, although the pattern and rate of decline is different in each case. Generalization about in vitro ageing from studies with one cell line should therefore be made with care or avoided. There was an increase in total cellular RNA content as cultures aged which was more pronounced as cells entered the senescent or terminal phase of their lifespan. This increase appeared to be accompanied by a slightly elevated uptake and incorporation of 3H-uridine per cell. Template activity of isolated nuclei was markedly reduced in very late passage or phase III cells, but did not show a progressive decline with increasing age. These studies show that there is a reduced replicative potential which is not accompanied by a detectable decline in transcription, and suggest that the altered template activity should be regarded as an effect of ageing in vitro.

Metabolic behaviour of nonhistone chromosomal proteins in proliferating and in resting fibroblasts

The metabolism of nonhistone chromosomal proteins was studied in two lines of cells showing a different degree of contact inhibition: human diploid fibroblasts, which are easily contact-inhibited, and Chinese hamster fibroblasts, which had been made to stop proliferating by fasting. By following the 3H414C ratio of [3H]tryptophan-labelled nonhistone chromosomal proteins and [14C]thymidine-labelled DNA in chase experiments three main groups of these proteins could be detected with respect to their metabolic behaviour: (a) a metabolically stable group which is acid-insoluble and represents the bulk of nonhistone chromosomal proteins in proliferating cells; this group is conserved when the cells enter a resting phase; (b) a metabolically labile group which is acid-soluble and is observed as a minor fraction in proliferating cells; (c) a metabolically labile group which is acid-insoluble and accumulates in resting cells; this fraction is much larger in contact-inhibited cells. Stimulation of cell proliferation by trypsinization decreases the amount of nonhistone chromosomal proteins in resting cells to the basic level observed in proliferating cells.

Effects of withdrawal of a mitogenic stimulus on progression of fibroblasts into S phase: differences between serum and purified multiplication-stimulating activity

Multiplication-stimulating activity (MSA) for chicken embryo fibroblasts was purified from serum-free medium conditioned by the growth of a rat liver cell line. A comparison between calf serum and purified MSA was made regarding the regulation of the fibroblast cell cycle. Addition of serum or MSA to stationary, quiescent cells stimulates them to enter the DNA synthetic phase after a characteristic lag period. Exposure to serum for shorter periods of time will irreverisbly commit cells to continue through the cell cycle and initiate DNA replication in the absence of serum. In contrast, the withdrawal of purified MSA from the medium results in an abrupt halt in the progression of cells towards S phase. The results of labeled thymidine incorporation and autoradiographic experiments clearly indicate that the point at which cells become irreversibly committed to enter the DNA synthetic period is at or near the G1-S boundary. The abrupt decay of the stimulation upon withdrawal of purified MSA provides a unique opportunity to investigate the biochemistry of this discrete phase of the cell cycle.