EGF-dependent phosphorylation of the EGF receptor in plasma membranes isolated from young and senescent WI-38 cells

The ING1a tumor suppressor regulates endocytosis to induce cellular senescence via the Rb-E2F pathway

An age-associated isoform of ING1, ING1a, induces cell senescence by altering endocytosis, subsequently activating the retinoblastoma tumor suppressor.

The INhibitor of Growth (ING) proteins act as type II tumor suppressors and epigenetic regulators, being stoichiometric members of histone acetyltransferase and histone deacetylase complexes. Expression of the alternatively spliced ING1a tumor suppressor increases >10-fold during replicative senescence. ING1a overexpression inhibits growth; induces a large flattened cell morphology and the expression of senescence-associated β-galactosidase; increases Rb, p16, and cyclin D1 levels; and results in the accumulation of senescence-associated heterochromatic foci. Here we identify ING1a-regulated genes and find that ING1a induces the expression of a disproportionate number of genes whose products encode proteins involved in endocytosis. Intersectin 2 (ITSN2) is most affected by ING1a, being rapidly induced >25-fold. Overexpression of ITSN2 independently induces expression of the p16 and p57^KIP2 cyclin-dependent kinase inhibitors, which act to block Rb inactivation, acting as downstream effectors of ING1a. ITSN2 is also induced in normally senescing cells, consistent with elevated levels of ING1a inducing ITSN2 as part of a normal senescence program. Inhibition of endocytosis or altering the stoichiometry of endosome components such as Rab family members similarly induces senescence. Knockdown of ITSN2 also blocks the ability of ING1a to induce a senescent phenotype, confirming that ITSN2 is a major transducer of ING1a-induced senescence signaling. These data identify a pathway by which ING1a induces senescence and indicate that altered endocytosis activates the Rb pathway, subsequently effecting a senescent phenotype.

Alternative splicing of several genes including the p16 and p53 tumor suppressors has been reported to increase during replicative senescence of normal diploid cells, but the biological functions of most alternative transcripts are unknown. We have found that a splicing product of the ING1 epigenetic regulator, ING1a, also increases during senescence; moreover, forced expression of ING1a at these levels in otherwise growth-competent cells can induce senescence. In this study we have determined that a major mechanism by which ING1a induces senescence is through inhibiting endocytosis; this subsequently activates the retinoblastoma (Rb) tumor suppressor pathway by increasing Rb levels and preventing its inactivation through multiple mechanisms. Our study also establishes a link between endocytosis and oxidative stress and suggests that multiple mechanisms that induce cellular senescence may do so by inhibiting normal endocytic processes, thereby affecting normal signal transduction pathways including those mitogenic pathways required for cell growth.

Jun and JunD-dependent functions in cell proliferation and stress response

Jun is essential for fetal development, as fetuses lacking Jun die at mid-gestation with multiple cellular defects in liver and heart. Embryos expressing JunD in place of Jun (Jun(d/d)) can develop to term with normal fetal livers, but display cardiac defects as observed in fetuses lacking Jun. Jun(d/d) mouse embryonic fibroblasts (MEFs) exhibit early senescence, which can be rescued by EGF and HB-EGF stimulation, probably through activation of Akt signaling. Thus, JunD cannot functionally replace Jun in regulating fibroblast proliferation. In Jun(-/-) fetal livers, increased hydrogen peroxide levels are detected and expression of Nrf1 and Nrf2 (nuclear erythroid 2-related transcription factors) is downregulated. Importantly, increased oxidative stress as well as expression of Nrf1 and Nrf2 is rescued by JunD in Jun(d/d) fetal livers. These data show that Jun is of critical importance for cellular protection against oxidative stress in fetal livers and fibroblasts, and Jun-dependent cellular senescence can be restored by activation of the epidermal growth factor receptor pathway.

Replicative senescence: a critical review

Human cells in culture have a limited proliferative capacity. After a period of vigorous proliferation, the rate of cell division declines and a number of changes occur in the cells including increases in size, in secondary lysosomes and residual bodies, nuclear changes and a number of changes in gene expression which provide biomarkers for senescence. Although human cells in culture have been used for over 40 years as models for understanding the cellular basis of aging, the relationship of replicative senescence to aging of the organism is still not clear. In this review, we discuss replicative senescence in the light of current information on signal transduction and mitogenesis, cell stress, apoptosis, telomere changes and finally we discuss replicative senescence as a model of aging in vivo.

Molecular markers of senescence in fibroblast-like cultures

The loss of replicative capacity in vitro of normal human diploid fibroblasts is a model for studying molecular changes that accompany both regulated growth control and cellular senescence. We describe the molecular phenotype of senescent fibroblasts in terms of markers that are altered with proliferative decline. We describe these markers by analyzing pathways and associated mechanisms related to the responsiveness of proliferatively competent and senescent cells to growth signals including changes in the extracellular environment, growth factors, growth factor receptors, secondary messengers, cell-cycle progression, transcription factors, and the fidelity of DNA synthesis. There is an abundance of molecular markers for senescence in culture at every level of information transfer. Although it seems clear that some alterations in gene expression with senescence are the result of specific changes in upstream events, more global dysregulation of coordinated growth control point to as yet undefined mechanisms.

Potentiated bradykinin-induced increase of 1,2-diacylglycerol generation and phospholipase D activity in human senescent fibroblasts

1. The comparative study of the effect of bradykinin (BK) in young and old IMR-90 human fibroblasts shows that old cells are characterized by a reduced increase in 1,2-diacylglycerol (1,2-DAG) generation upon stimulation after short-term treatment and a significant higher increase after long-term agonist treatment. BK-induced activation of phospholipase D (PLD), the major enzyme involved in sustained 1,2-DAG generation, was 2.5-fold higher in old cells, strongly suggesting that it is involved in the potentiated increase of 1,2-DAG formation. The increased activation of PLD by BK in old cells was specific, since in parallel experiments the effect of thrombin was not significantly different in young and old cells. PLD activity in old cells was only reduced by down-regulation of protein kinase C (PKC) activity, in contrast to what was observed in young cells where it was completely abolished. This indicates that the enzyme activity in old cells was partially PKC-independent. BK was also able to increase the release of [14C]ethanolamine, a water-soluble product of hydrolysis of phosphatidylethanolamine (PtdEtn), through PLD activation in young and old cells. The BK effect was significantly higher in old cells and, very likely, PKC-independent, since phorbol 12-myristate 13-acetate failed to induce PtdEtn hydrolysis. 2. The present results indicate that the PLD/1,2-DAG pathway is specifically potentiated by BK in old fibroblasts, demonstrating that the formation of positive effectors of PKC activation is not necessarily decreased in cellular senescence. It remains to be established whether the increased generation of DAG upon BK stimulation plays any role in the altered PKC signalling pathway which characterizes old fibroblasts.

Kinetics of 125I-PDGF binding and down-regulation of PDGF receptor in human arterial smooth muscle cell strains during cellular senescence in vitro

Platelet-derived growth factor (PDGF) is one of the major mitogens in serum to stimulate replication of human smooth muscle cells (SMCs) in culture. Previous studies using human fibroblasts failed to demonstrate changes in the receptor systems for growth factors during cellular senescence. We investigated the kinetics of 125I-PDGF(-BB) binding and down-regulation of the PDGF receptor in three human arterial SMC strains during cellular aging. The number of specific 125I-PDGF binding sites per cell increased slightly at a population doubling level (PDL) of 60%-80% of life span and then decreased at the PDL above 90%. The number of receptors per cell-surface area decreased with increasing in vitro age. The apparent Kd for the 125I-PDGF binding decreased with in vitro senescence. The internalization and degradation of 125I-PDGF per receptor were significantly reduced in senescent SMCs and the amount of 125I-PDGF that escaped degradation and was recycled back to the cell surface was significantly greater in senescent SMCs than young cells. Furthermore, down-regulation of the PDGF receptor was significantly greater in senescent SMCs than young cells. Immunoblot studies demonstrated that changes in beta-subunit of the PDGF receptor accounted for those in the studies using 125I-PDGF and that tyrosine phosphorylation of the PDGF receptor was significantly greater in young SMCs than aged cells. Our results suggest that age-related changes in the receptor systems for PDGF may be important contributors to the failure of DNA synthesis in senescent SMCs.

Involvement of protein kinase C and arachidonate signaling pathways in the alteration of proliferative response of senescent IMR-90 human fibroblasts

The proliferative response of IMR-90 fibroblasts at low and high population doubling level (PDL) to protein kinase C activation has been investigated to clarify whether the reduced mitogenic responsiveness of senescent cells can be ascribed to an alteration in protein kinase C signal transduction pathway. The results show that the signaling pathway leading to DNA synthesis through protein kinase C activation, appears to be modified in senescent IMR-90 human fibroblasts. High PDL fibroblasts exhibit a different sensitivity to phorbol 12-myristate 13-acetate (PMA) and dioctanoylglycerol (diC8); high glucose reduced responsiveness to PMA only in these cells. In addition, high PDL fibroblasts are characterized by an increase in diacylglycerol (DAG) cellular mass that could contribute to the different regulatory properties of the signaling pathway. On the other hand, the ability of the cyclooxygenase inhibitor indomethacin to strikingly improve the proliferative response of high PDL cells to PMA indicates that an altered overall metabolism of arachidonate may represent a crucial step in the reduced mitogenic response involving protein kinase C activation.

Cleavage of the epidermal growth factor receptor by a membrane-bound leupeptin-sensitive protease active in nonionic detergent lysates of senescent but not young human diploid fibroblasts

Numerous studies suggest that epidermal growth factor (EGF) signaling is impaired in nonproliferating senescent human diploid fibroblasts downstream of receptor binding. One possible explanation for these results is that senescent cells possess unique enzymatic activities capable of regulating functional levels of the EGF receptor. To test that hypothesis, nonionic detergent lysates of young and senescent cells were compared for proteolytic activity directed towards the EGF receptor, and a protease that cleaves the 170 kDa EGF receptor was identified in lysates from senescent but not young cells. Although studies presented here were carried out with WI-38 cells, our data indicate that other senescent fibroblasts possess a similar activity. The degradation product immunoprecipitated by a monoclonal antibody specific for an EGF receptor exocytosolic epitope had an approximate molecular weight of 100,000. This product was also detected following cell surface labeling with 125I, and by cross-linking 125I-EGF to intact cells with disuccinimidyl suberate. The proteolytic activity in senescent cell lysates was specifically inhibited by leupeptin and did not require divalent cations; it was also inactivated by aprotic solvents such as dimethylsulfoxide (DMSO) or ethylene carbonate. Interestingly, this protease was not active during ligand-induced intracellular processing of the EGF receptor, suggesting that it does not normally function in endocytic or lysosomal compartments. The susceptibility of the protease to inactivation by cell surface trypsinization is consistent with a plasma membrane localization. Since EGF receptor cleavage is not observed unless senescent cells are solubilized with nonionic detergents, it seems likely that the protease is confined to specialized regions of the plasma membrane. Whether or not the EGF receptor is a physiologic target for this protease is unclear. Its expression at the cell surface is nevertheless significant, since it suggests there are mechanisms for regulating membrane-bound proteins, or biologically active peptides in the extracellular space, in senescent cells that are either absent or inactive in young cells.

Molecular biology of aging

Aging is an extremely complex biologic phenomenon of immense importance. Currently we have only a poor and incomplete understanding of the fundamental molecular mechanisms involved. Despite numerous observations and diverse theories, no unifying or proven hypotheses have emerged. It is reasonable to conclude, however, that aging is a multifactorial process composed of both genetic and environmental components. Each physiologic system within an organism, each tissue within a system, and each cell type with a tissue appears to have its own trajectory of aging. Thus, aging must be studied as parts of a whole and understood as the sum of its parts. Cellular "clocks" exist and operate in the absence of higher-order "clocks". However, higher-order clocks are certainly in place in vivo, but their relationship to cellular clocks is not well understood. All aging changes have a cellular basis, and aging is perhaps best studied, fundamentally, at the cellular level under defined and controlled environmental conditions. Aging changes at the cellular level must be viewed, however, as components of a hierarchical, dynamic, and interacting network whose functional integrity progressively deteriorates with time. The powerful tools of molecular biology are now being applied by scientists to evaluate the leading hypotheses. The results of these studies should serve to advance our understanding of aging and to focus future research efforts. This work should provide the scientific foundation to enhance the quality of life for people suffering the failings of age.

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.

Ageing and photoageing of the skin: observations at the cellular and molecular level

It is now well established that ageing occurs at the level of individual cells in the skin and other organ systems. Changes in cell behaviour, protein production and gene expression in response to standardized stimuli are readily observed in cultured cells derived from young vs old donors and from photoaged vs sun-protected body sites. Whether these changes are best viewed as a cause or a consequence of ageing cannot be determined at present. Nevertheless, available data now provide cellular and molecular correlates for the well-known differences in clinical responsiveness between newborn, adult and photoaged skin. From this basis, it will hopefully be possible to develop a more comprehensive understanding of cutaneous ageing processes.

Decreased response to epidermal growth factor during cellular senescence in cultured human microvascular endothelial cells

We have previously demonstrated that epidermal growth factor (EGF) induces cell migration, tissue-type plasminogen activator synthesis, as well as tubular formation in microvascular endothelial cells from human omental tissue. In this study, we compared the responsiveness to EGF of late passaged (senescent) human omental microvascular endothelial (HOME) cells with that of early passaged (young) HOME cells. We have employed HOME cells derived from surgically resected omental samples from 14 patients. EGF-stimulated cell migration significantly more in the young cells than in the senescent cells during serial cultivation (aging) in vitro. Scatchard analysis demonstrated that the number for both high and low affinity receptors for EGF in HOME cells was decreased dramatically during serial cultivation. The expression of EGF receptor mRNA was also decreased in the senescent HOME cells. Treatment of HOME cells with EGF significantly increased cellular mRNA levels of tissue-type plasminogen activator, and two protooncogenes, c-fos and c-myc, in young HOME cells, but not in senescent HOME cells. Thus HOME cells aged in vitro show a decreased responsiveness to EGF, resulting in decreased migration of human endothelial cells. The serial cultivation of human endothelial cells in vitro may downregulate EGF receptor and decrease responsiveness to exogenous EGF, a potent angiogenic factor.

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.

Cellular senescence: a reflection of normal growth control, differentiation, or aging?

Normal cells, with few exceptions, cannot proliferate indefinitely. Cell populations--in vivo and in culture--generally undergo only a limited number of doublings before proliferation invariably and irreversibly ceases. This process has been termed the finite lifespan phenotype or cellular senescence. There is long-standing, albeit indirect, evidence that cellular senescence plays an important role in complex biological processes as diverse as normal growth control, differentiation, development, aging, and tumorigenesis. In recent years, it has been possible to develop a molecular framework for understanding some of the fundamental features of cellular senescence. This framework derives primarily from the physiology, genetics, and molecular biology of cells undergoing senescence in culture. Our understanding of senescence, and the mechanisms that control it, is still in its infancy. Nonetheless, recent data raise some intriguing possibilities regarding potential molecular bases for the links between senescence in culture and normal and abnormal growth control, differentiation, and aging.

Spontaneous immortalization of mouse embryo cells: strain differences and changes in gene expression with particular reference to retroviral gag-pol genes

We have studied the kinetics with which cultures of primary mouse embryo cells pass through the crisis period, escape their terminal differentiation (cellular senescence), and give rise to an immortal cell line. The process is strain-dependent, with cells from the outbred Swiss CD-1 mouse being considerably more adept at forming an immortal 3T3 line than cells from the inbred SWR line; Balb/c cells appeared intermediate in their behavior. The continued presence of the tumor promoter 12-O-tetradecanoylphorbol-13-acetate or the poly(ADPribose)polymerase inhibitor 3-aminobenzamide affected the kinetics but did not seem to alter the outcome. Changes in expression of various genes, including those encoding mitogen-regulated protein (proliferin), endogenous gag-pol retrovirus sequences, insulin-like growth factor II, and a variety of protooncogenes, were monitored during the process of immortalization, and although certain changes were reproducibly characteristic of cells from a given mouse strain passed according to a specific regimen, none of the observed changes were reproducibly characteristic under all conditions of immortalization. In particular, our data indicate the absence of a strict correlation between cellular immortalization and the activation of endogenous gag-pol expression. We conclude from our observations that the establishment of permanent lines from primary mouse embryo cells in serum-containing medium reflects the selection of a variant subpopulation of cells that did not preexist but rather arose in response to the specific culture conditions by a process resembling differentiation. Multiple and complex changes in gene expression occur that are affected by the culture conditions and the strain (genotype) of the mouse.

Response to epidermal growth factor of skin fibroblasts from donors of varying age is modulated by the extracellular matrix

The present study was undertaken to investigate the effect of epidermal growth factor (EGF) on the biosynthetic activity of skin fibroblasts from donors of varying age and the modulation of their response to this growth factor by culture in a three-dimensional extracellular matrix. When cultured in monolayer on plastic or at the surface of a collagen gel, EGF specifically inhibited collagen synthesis whatever the age of the donor (from 17 to 84 years, n = 11). This inhibition was paralleled by a significant decrease in the steady-state level of procollagen type I mRNAs. When embedded in a three-dimensional floating collagen lattice, EGF stimulated the non-collagen protein (NCP) synthesis in fibroblasts from younger donors (5 out of 6) while fibroblasts from the older ones were not affected. Collagen production by fibroblasts from younger donors was not inhibited as in monolayer (some being even stimulated) while that of the older donors was inhibited as observed in monolayer. The steady-state level of procollagen type I mRNA was not modified by EGF in the three-dimensional culture. No significant difference was observed in the affinity and the number of EGF receptors of the fibroblasts on plastic or embedded in a collagen lattice between young and aged donors. Our results suggest that the environment of the cells can modulate the reactivity to EGF and reveal differences related to in vivo aging.

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.

Novel monoclonal antibodies identify antigenic determinants unique to cellular senescence

Normal human diploid fibroblasts exhibit a limited lifespan in vitro and are used as a model to study in vivo aging. Monoclonal antibodies were generated against partially purified surface membranes from human diploid fibroblasts at the end of their lifespan (senescent). Three hybridomas were isolated that secreted antibodies reacting to cellular determinants expressed specifically on senescent human fibroblasts of different origin, including neonatal foreskin, embryonic lung, and adult skin punch biopsy, but not expressed on matched young cells. The antibodies did not bind to immortal human cells and normal young cells made reversibly nondividing, indicating the antigens are not expressed in cells that are not senescent. The antibodies identified senescent cells in a mixed cell population and expression of the senescent cell antigens correlated strongly with the cells inability to synthesize DNA at the onset of senescence. The antigens appeared to be cell surface or extracellular matrix associated, and the epitopes were destroyed by mild trypsin treatment. Western analysis indicated all three antibodies reacted with fibronectin. Though the antigenic determinants on the fibronectin molecule were not accessible in the intact young cell, the epitopes were present in fibronectin extracted from both senescent and young cells, as well as purified human plasma fibronectin. These antibodies and the senescent specific expression of the antigens provide powerful tools to investigate the mechanisms leading to in vitro senescence. This may enable us to investigate directly the relationship between cellular aging and aging of the individual.

Repression of c-fos transcription and an altered genetic program in senescent human fibroblasts

Normal cells in culture invariably undergo senescence, whereby they cease proliferation after a finite number of doublings. Irreversible changes in gene expression occurred in senescent human fetal lung fibroblasts: a non-cell cycle-regulated mRNA was partially repressed; an unusual polyadenylated histone mRNA was expressed; although serum induced c-H-ras, c-myc, and ornithine decarboxylase mRNA normally, ornithine decarboxylase activity was deficient; and serum did not induce mRNA for a replication-dependent histone and for the c-fos proto-oncogene. The loss of c-fos inducibility was the result of a specific, transcriptional block. The results suggest that senescent fibroblasts were unable to proliferate because of, at least in part, selective repression of c-fos; moreover, the multiple changes in gene expression support the view that cellular senescence is a process of terminal differentiation.

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.

Growth factors as probes of cell aging

We present examples of four types of alterations which contribute to the senescence phenotype of WI-38 cells: a) in senescent cells there is an increased lability of the tyrosine autophosphorylation capacity of detergent isolated EGF receptor; b) following serum stimulation, the calmodulin protein level fails to increase in senescent cells, although the calmodulin mRNA level increases as expected; c) following heat shock at 43 degrees C, senescent cells produce both less RNA and less protein for the HSP70 and HSP90 genes; d) we find that membranes isolated in basic buffer from senescent or young cells increase the EGF proliferative response of senescing cells, in contrast to the finding by others that membranes isolated in neutral buffer inhibit cell proliferation (Pereira-Smith et al., Senescent and quiescent cell inhibitors of DNA synthesis Exp. Cell Res. 160, 297-306, 1985; Stein and Atkins, Membrane-associated inhibition of DNA synthesis in senescent human diploid fibroblasts: Characterization and comparison to quiescent cell inhibitor. Proc. Natl. Acad. Sci. USA 83 9030-9034, 1986). We conclude that senescence alterations are complex and occur at many levels, and that senescence changes are not identical to quiescence features.

Aging and defective lymphoid cell activation

Activation of lymphocytes for proliferation is a crucial process in the immune response. Age-related deficiencies in this cellular response strongly correlate with deficiencies in the immune system response, with concomitant increase in disease severity and mortality. Defects associated with the transmission of the initial activation signal and with IL-2 production contribute to the depressed response, but defects in the IL-2 response mechanism also play important roles. A major factor in this area is the inability of the nuclei of these cells to respond to the intracellular factor ADR, which plays a crucial role in the initiation of DNA replication. These cells produce normal levels of ADR; thus, either the nuclei cannot bind ADR in a productive manner or the defect lies beyond the point of ADR binding, perhaps in one of the other proteins of the initiation complex. An interesting contrast to the age-related failure of nuclei to respond to ADR is the failure of neoplastic nuclei to respond to the ADR inhibitor. This inhibitor, found in the cytoplasm of quiescent cells, suppresses both the activation of quiescent nuclei by ADR and the ongoing DNA synthesis in isolated nuclei from activated cells. Nuclei from spontaneous proliferating cell lines were not affected by this inhibitor, which may be an important factor in the uncontrolled growth seen in neoplastic cells. The investigation of ADR has given hints that perhaps two of the fundamental questions in biology, namely why some cells don't proliferate and why some others won't stop proliferating, may be two sides of the same coin.