Diminished in vitro tyrosine kinase activity of the EGF receptor of senescent human fibroblasts

Survive or thrive: tradeoff strategy for cellular senescence

Aging-dependent cellular behaviors toward extrinsic stress are characterized by the confined localization of certain molecules to either nuclear or perinuclear regions. Although most growth factors can activate downstream signaling in aging cells, they do not in fact have any impact on the cells because the signals cannot reach their genetic targets in the nucleus. For the same reason, varying apoptotic stress factors cannot stimulate the apoptotic pathway in senescent cells. Thus, the operation of a functional nuclear barrier in an aging-dependent manner has been investigated. To elucidate the mechanism for this process, the housekeeping transcription factor Sp1 was identified as a general regulator of nucleocytoplasmic trafficking (NCT) genes, including various nucleoporins, importins, exportins and Ran GTPase cycle-related genes. Interestingly, the posttranslational modification of Sp1 is readily influenced by extrinsic stress, including oxidative and metabolic stress. The decrease in SP1 O-GlcNAcylation under oxidative stress or during replicative senescence makes it susceptible to proteosomal degradation, resulting in defective NCT functions and leading to nuclear barrier formation. The operation of the nuclear barrier in aging provides a fundamental mechanism for cellular protection against stress and promotes survival at the expense of growth via stress-sensitive transcriptional control.

Nuclear barrier hypothesis of aging as mechanism for trade-off growth to survival

When the aging-dependent cellular behaviors toward growth factors and toxic stress have been analyzed, the perinuclear accumulation of the activated signals, either mitogenic or apoptotic, has been observed, suggesting the aging-dependent inefficiency of the nucleocytoplasmic trafficking of the signals. Thereby, it would be natural to assume the operation of the functional nuclear barrier in aging-dependent manner, which would be designated as "Park and Lim's Barrier." And for the ultimate transcriptional factor for these aging-dependent changes of the functional nuclear barrier, Sp1 transcriptional factor has been suggested to be the most probable candidate. This novel mechanism of aging-dependent operation of the functional nuclear barrier is proposed as the ultimate checking mechanism for cellular protection against toxic environment and the general mechanism for the trade-off growth to survival in aging.

Increased levels of a particular phosphatidylcholine species in senescent human dermal fibroblasts in vitro

Plasma membranes are essential components of living cells, and phospholipids are major components of cellular membranes. Here, we used liquid chromatography/mass spectrometry to investigate changes in the membrane phospholipid content that occur in association with aging. Our results indicate that the levels of a particular species of phosphatidylcholine comprised of stearic acid and arachidonic acid increased with age. To determine the reason for the increased levels of this particular phosphatidylcholine, we examined the effect of highly unsaturated fatty acids, such as arachidonic acid and eicosapentaenoic acid, on cellular aging. Applied arachidonic acid was incorporated into phosphatidylcholine molecules, but neither arachidonic acid nor other related unsaturated fatty acids had any effect. We conclude that increased levels of this distinctive phosphatidylcholine are a result of in vitro senescence.

Caveolin-1 as a prime modulator of aging: a new modality for phenotypic restoration?

Aging can be characterized by structural changes and functional deterioration during the lifetime, for which hundreds of explanations have been put forward. Recently, we have proposed the gate theory of aging, in which gatekeeper molecules at the membrane level would play the prime role in determining the senescent phenotype. Caveolin-1 would be a prime candidate for such a role as a major determinant of the aging process. Caveolin-1 can associate with a variety of molecules, involved in signal transduction, endocytosis and transcytosis, cytoskeletal arrangement, etc. The level of caveolin-1 is strictly regulated to maintain cellular integrity, leading to cellular transformation if depleted, and to the senescent phenotype if overexpressed. In case of senescent cells, the functional and physiological responses to the mitogenic stimuli can be restored and the morphological shape can be resumed by simple adjustment of caveolin-1 status. Therefore, it is suggested that prime modulator molecules, represented by caveolin-1, play a key role in determining the senescent phenotype, either as a physiological response or altered morphology.

Molecular mechanisms involved in muscle wasting in cancer and ageing: cachexia versus sarcopenia

The aim of the present review is to summarize and evaluate the different mechanisms and catabolic mediators involved in cancer cachexia and ageing sarcopenia since they may represent targets for future promising clinical investigations. Cancer cachexia is a syndrome characterized by a marked weight loss, anorexia, asthenia and anemia. In fact, many patients who die with advanced cancer suffer from cachexia. The degree of cachexia is inversely correlated with the survival time of the patient and it always implies a poor prognosis. Unfortunately, at the clinical level, cachexia is not treated until the patient suffers from a considerable weight loss and wasting. At this point, the cachectic syndrome is almost irreversible. The cachectic state is often associated with the presence and growth of the tumour and leads to a malnutrition status due to the induction of anorexia. In recent years, age-related diseases and disabilities have become of major health interest and importance. This holds particularly for muscle wasting, also known as sarcopenia, that decreases the quality of life of the geriatric population, increasing morbidity and decreasing life expectancy. The cachectic factors (associated with both depletion of fat stores and muscular tissue) can be divided into two categories: of tumour origin and humoural factors. In conclusion, more research should be devoted to the understanding of muscle wasting mediators, both in cancer and ageing, in particular the identification of common mediators may prove as a good therapeutic strategies for both prevention and treatment of wasting both in disease and during healthy ageing.

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.

Coordinating epidermal growth factor-induced motility promotes efficient wound closure

Wound healing is a response to injury that is initiated to reconstruct damaged tissue. In skin, reepithelialization involves both epithelial cells and fibroblasts and contributes to the reformation of a barrier between the external environment and internal milieu. Growth factors including epidermal growth factor (EGF) play important roles in promoting this process. In the present studies we employed CV-1 fibroblasts in a tissue culture model of reepithelialization to develop strategies for optimizing wound closure stimulated by EGF. We found that EGF enhanced cell motility within 6-8 h of EGF treatment in serum-free medium but wounds failed to close within 24 h. However, if medium on these cultures was exchanged for medium containing serum, cells pretreated with EGF closed new scrape wounds more rapidly than did cells that were not pretreated. These results indicate that serum factors work in concert with EGF to coordinate cell motility for efficient wound closure. Indeed, EGF enhanced the rate of wound closure in the presence of serum, and this effect also persisted for at least 24 h after EGF was removed. This coordination of EGF-induced cell motility was accompanied by an increase in the transient phosphorylation of ERK1 and ERK2. The persistent effects of EGF were blocked by transient exposure to reversible inhibitors of transcription and translation, indicating that the expression of new proteins mediated this response. We propose that EGF-stimulated CV-1 fibroblast motility is coordinated by a serum component that induces cell-cell adhesive properties consistent with an epithelial phenotype, thereby enhancing the reepithelialization process.

Aging-related attenuation of EGF receptor signaling is mediated in part by increased protein tyrosine phosphatase activity

As fibroblasts near senescence, their responsiveness to external signals diminishes. This well-documented phenomenon likely underlies physiological deterioration and limited tissue regeneration in aging individuals. Understanding the underlying molecular mechanisms would provide opportunities to ameliorate these situations. A key stimulus for human dermal fibroblasts are ligands for the epidermal growth factor receptor (EGFR). We have shown earlier that EGFR expression decreases by about half in near senescent fibroblasts (Shiraha et al., 2000, J. Biol. Chem. 275 (25), 19343-19351). However, as the cell responses are nearly absent near senescence, other aging-related signal attenuation changes must also occur. Herein, we show that EGFR signaling as determined by receptor autophosphorylation is diminished over 80%, with a corresponding decrease in the phosphorylation of the immediate postreceptor adaptor Shc. Interestingly, we found that this was due at least in part to increased dephosphorylation of EGFR. The global cell phosphotyrosine phosphatase activity increased some threefold in near senescent cells. An initial survey of EGFR-associated protein tyrosine phosphatases (PTPases) showed that SHP-1 (PTPIC, HCP, SHPTP-1) and PTPIB levels are increased in parallel in these cells. Concomitantly, we also discovered an increase in expression of receptor protein tyrosine phosphatase alpha (RPTPalpha). Last, inhibition of protein tyrosine phosphatases by sodium orthovanadate in near senescent cells resulted in increased EGFR phosphorylation. These data support a model in which, near senescence, dermal fibroblasts become resistant to EGFR-mediated stimuli by a combination of receptor downregulation and increased signal attenuation.

Expression of epidermal growth factor and surfactant proteins during postnatal and compensatory lung growth

We examined whether lung growth after pneumonectomy (PNX) invokes normal signaling pathways of postnatal development. We qualitatively and quantitatively assessed the immunoexpression of epidermal growth factor (EGF), its receptor (EGFR), surfactant proteins (SP) [SP-A and -D and surfactant proproteins (proSP)-B and -C] and proliferating cell nuclear antigen (PCNA) in immature and mature dog lung. We also assayed these proteins in lungs of immature dogs 3 wk or 10 mo after they underwent right PNX compared with simultaneous matched sham controls. During maturation, alveolar cell proliferation is regionally regulated in parallel with EGF and EGFR levels and inversely correlated with SP-A and proSP-C levels. In contrast, post-PNX lung growth is not associated with EGF or EGFR upregulation but with markedly increased SP-A level and moderately increased SP-D level; proSP-B and proSP-C levels did not change. We conclude that 1) signaling of EGF axis and differential regulation of SPs persist during postnatal lung development, 2) post-PNX lung growth is not a simple recapitulation of maturational responses, and 3) SP-A and SP-D may modulate post-PNX lung growth.

Aging fibroblasts present reduced epidermal growth factor (EGF) responsiveness due to preferential loss of EGF receptors

Wound healing is compromised in aging adults in part due to decreased responsiveness of fibroblasts to extracellular signals. However, the cellular mechanisms underlying this phenomenon are not known. Aged dermal fibroblasts with reduced remaining replicative capacities demonstrated decreased epidermal growth factor (EGF)-induced cell migrative and cell proliferative capacities, as reported previously. Thus, as cells approach senescence, programmed in vivo or in vitro, EGF responsiveness is preferentially lost. To define the rate-limiting signaling event, we found that the activity of two different EGF receptor (EGFR)-signaling pathways to cell migration (phospholipase-C gamma) and/or mitogenesis (extracellular signal/regulated-mitogen-activated kinases) were decreased in near senescent cells despite unchanged levels of effector molecules. Aged cells presented decreased levels of EGFR, although insulin receptor and transferrin receptor levels were relatively unchanged. EGFR mRNA levels and production of new transcripts decreased during aging, suggesting that this preferential loss of EGFR was due to diminished production, which more than counteracts the reduced ligand-induced receptor loss. Since these data suggested that the decrement in EGF was rate-limiting, higher levels of EGFR were established in near senescent cells by electroporation of EGFR cDNA. These cells presented higher levels of EGFR and recovered their EGF-induced migration and proliferation responsiveness. Thus, the defect in EGF responsiveness of aged dermal fibroblasts is secondary to reduced EGFR message transcription. Our experimental model suggests that EGFR gene delivery might be an effective future therapy for compromised wound healing.

Hepatocyte growth factor/scatter factor enhances the thrombopoietin mRNA expression in rat hepatocytes and cirrhotic rat livers

BACKGROUND

Although thrombopoietin (TPO) is mainly produced in the liver, the regulatory mechanism of TPO gene expression in hepatocytes remains unclear. The role of TPO in thrombocytopenia associated with liver cirrhosis has not been identified.

METHODS

We examined the effects of various growth factors and cytokines on TPO mRNA expression in adult rat hepatocytes in primary cultures using a semiquantitative reverse transcription-polymerase chain reaction assay.

RESULTS

Among them, only hepatocyte growth factor/scatter factor (HGF/SF) enhanced TPO mRNA expression; other growth factors (epidermal growth factor and transforming growth factor-beta) and cytokines (erythropoietin, granulocyte-colony stimulating factor, granulocyte-macrophage-colony stimulating factor, interleukin (IL)-3, IL-6 and interferon-gamma) did not. Next, we examined TPO mRNA expression in the livers of rats with CCl4-induced cirrhosis, the effects of HGF/SF on hepatic TPO mRNA expression and peripheral platelet and bone marrow megakaryocyte counts in the cirrhotic rats. In the cirrhotic rats, both the peripheral platelet count and TPO mRNA expression in the livers were markedly decreased compared with those of the normal rats. The administration of HGF/SF to the cirrhotic rats stimulated TPO mRNA expression in the livers and resulted in significant increases of peripheral platelets and bone marrow megakaryocytes.

CONCLUSIONS

These results suggest that HGF/SF is a possible regulatory factor for TPO gene expression and that HGF/SF increases platelet production through an enhancement of TPO mRNA expression in the livers of cirrhotic rats.

Epidermal growth factor receptor-mediated motility in fibroblasts

Cell motility is induced by many growth factors acting through cognate receptors with intrinsic tyrosine kinase activity (RPTK). However, most of the links between receptor activation and the biophysical processes of cell motility remain undeciphered. We have focused on the mechanisms by which the EGF receptor (EGFR) actuates fibroblast cell motility in an attempt to define this integrated process in one system. Our working model is that divergent, but interconnected pathways lead to the biophysical processes necessary for cell motility: cytoskeleton reorganization, membrane extension, formation of new adhesions to substratum, cell contraction, and release of adhesions at the rear. We postulate that for any given growth factor some of the pathways/processes will be actively signaled and rate-limiting, while others will be permissive due to background low-level activation. Certain couplings have been defined, such as PLCgamma and actin modifying proteins being involved in cytoskeletal reorganization and lamellipod extension and MEK being implicated in detachment from substratum. Others are suggested by complementary investigations in integrin-mediated motility, including rac in membrane protrusion, rho in new adhesions, myosin II motors in contraction, and calpain in detachment, but have yet to be placed in growth factor-induced motility. Our model postulates that many biochemical pathways will be shared between chemokinetic and haptokinetic motility but that select pathways will be activated only during RPTK-enhanced motility.

Age-related decrease in the responsiveness of rat articular chondrocytes to EGF is associated with diminished number and affinity for the ligand of cell surface binding sites

The effect of age on the responsiveness of articular chondrocytes (AC) to epidermal growth factor (EGF) was examined. Cells were isolated by digesting cartilage fragments from the humeral and femoral heads of 21-day old, 8- and 14-month old rats with collagenase. The cells were cultured under standard conditions, as monolayers. DNA synthesis was measured by [3H]thymidine incorporation and cell proliferation by the DNA content of subconfluent cultures. [125I]EGF binding and the amounts of EGF and EGF-receptor mRNAs were determined using confluent cells. DNA synthesis was decreased with age of animals. EGF stimulated DNA synthesis in cultures in 1- and 8-month old rats at low serum concentrations (< 5%), and in cultures in 14-month old animals at high serum concentrations. It also increased 5-day DNA content of cultures compared to serum-treated controls but this effect was weak in cultures in 14-month old rats. The number of high affinity binding sites for [125I]EGF decreased from 37,800 in the 1-month old to 1950 in the 14-month old rat AC. The apparent dissociation constant (Kd) also decreased with age: 0.18 nmol/l in the 1-month old; 0.12 nmol/l in the 8-month old; and 0.07 nmol/l in the 14-month old cells. AC in older rats contained more EGF mRNA and less EGF-receptor mRNA. Incubation of the cells with EGF resulted in down regulation of the EGF- and upregulation of EGF-receptor mRNA expressions. These findings show the age-related quantitative and qualitative alterations in EGF and EGF-receptor which may account, at least in part, for the diminished responsiveness of senescent AC to EGF.

The human diploid fibroblast senescence pathway is independent of interleukin-1 alpha mRNA levels and tyrosine phosphorylation of FGFR-1 substrates

In vitro cellular senescence of human umbilical vein endothelial cells (HUVEC) may involve the intracellular activity of the signal peptide-less cytokine interleukin (IL)-1 alpha. To determine whether senescence of other human diploid cells involves the function of IL-1 alpha, we examined the steady-state expression of IL-1 alpha mRNA in IMR-90 fibroblasts. The IL-1 alpha transcript was not elevated in senescent IMR-90 cells. With the exception of the plasminogen activator inhibitor (PAI)-1 transcript, other IL-1 alpha-response gene mRNAs were not induced in senescent IMR-90, although the mRNA for each gene was induced by exogenous IL-1 alpha. The mRNA expression of cell cycle-specific genes demonstrated that Fos and ornithine decarboxylase (ODC) were induced in young and senescent cells in response to both serum and fibroblast growth factor (FGF)-1. Histone (H)3 mRNA was induced by serum in young cells, but not in senescent cells, and FGF-1 failed to induce H3 mRNA in either young or senescent cells. Further, while young IMR-90 populations were able to respond to serum as an initiator of DNA synthesis and cell growth, they did not exhibit a response to exogenous FGF-1. FGF receptor (R)-1 substrates were not tyrosine phosphorylated in either young or senescent IMR-90 cells. These data demonstrate that IL-1 alpha and FGF-1 may have different functions in HUVEC and IMR-90 fibroblast populations including distinct pathways for the regulation of cellular growth and senescence.

FGF-1-dependent proliferative and migratory responses are impaired in senescent human umbilical vein endothelial cells and correlate with the inability to signal tyrosine phosphorylation of fibroblast growth factor receptor-1 substrates

Senescent cells do not proliferate in response to exogenous growth factors, yet the number and affinity of growth factor receptors on the cell surface appear to be similar to presenescent cell populations. To determine whether a defect in receptor signaling exists, we analyzed human umbilical vein endothelial cells (HUVEC) since HUVEC growth is absolutely dependent upon the presence of FGF. We report that in both presenescent and senescent HUVEC populations, FGF-1 induces the expression of cell cycle-specific genes, suggesting that functional FGF receptor (FGFR) may exist on the surface of these cells. However, the tyrosine phosphorylation of FGFR-1 substrates, Src and cortactin, is impaired in senescent HUVEC, and only the presenescent cell populations exhibit a FGF-1-dependent Src tyrosine kinase activity. Moreover, we demonstrate that senescent HUVEC are unable to migrate in response to FGF-1, and these data correlate with an altered organization of focal adhesion sites. These data suggest that the induction of gene expression is insufficient to promote a proliferative or migratory phenotype in senescent HUVEC and that the attenuation of the FGFR-1 signal transduction pathway may be involved in the inability of senescent HUVEC to proliferate and/or migrate.

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.

Role of immune response as determinant of tumor progression in function of host age in the B16 melanoma

Aging constitutes the major cause for the development of most neoplastic diseases. However, tumors in aged people present with a lower degree of aggressiveness than in young patients. The reasons for this paradoxical behavior are not clear. We attempted to verify whether the immune system has a role in the relation between host age, immune response and tumor progression. We compared the growth rate of B16 melanoma and a highly malignant variant, the B16/Col/R, in young and aged mice that have or have not undergone splenectomy. The following results were obtained: (1) Splenectomy stimulated growth in the parental melanoma in both young and aged mice, indicating a protective role of the spleen against this tumor at all ages; (2) Spleen ablation provoked inhibition of the highly-metastatic variant growth in young mice, suggesting a stimulatory role of the spleen in this case; (3) By contrast, in aged mice inoculated with the B16/Col/R variant, splenectomy enhanced tumor growth, indicating a defensive role of the spleen. Age favors a positive host response against the aggressive clone of the melanoma. Differential host response in young versus aged mice can explain, in this tumor system, the difference in tumor progression rate as a function of age.

Analysis of EPC-1 growth state-dependent expression, specificity, and conservation of related sequences

The transcript for EPC-1 (early population doubling level (PDL) cDNA-1) is induced under conditions of growth arrest due to density-dependent contact inhibition and/or serum deprivation in early-passage but not in senescent WI-38 fibroblasts. We have characterized the EPC-1 transcript with respect to its cell-cycle regulation, tissue specificity, and interspecies conservation of related genomic sequences. In low density, quiescent (serum-deprived), early-passage fibroblasts that are stimulated to proliferate with fresh serum, steady-state EPC-1 transcript levels are steadily reduced until they reach a basal level approximately 24 h after stimulation. However, when early-passage fibroblasts are made quiescent by both serum deprivation and density-dependent contact inhibition and then stimulated with serum, steady-state EPC-1 transcript levels remain relatively constant throughout a 36 h period following serum stimulation. Senescent WI-38 cells (> 95% life span completed) do not express EPC-1 under these conditions. We show that differences in the regulation of EPC-1 transcript levels in early-passage cells are due to differences in growth state rather than changes in cell density or contact. We also show that expression of the EPC-1 transcript is limited to specific cell types and that related genomic sequences are found in all mammalian species examined as well as in the chicken.

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.

Bradykinin-stimulated transient modulation of epidermal growth factor receptors in A-431 human epidermoid carcinoma cells

Of nine biological factors (ATP, bradykinin, vasopressin, substance P, angiotensin II, norepinephrine, epinephrine, 12-tetradecanoylphorbol 13-acetate (TPA), and A23187 calcium ionophore) examined, bradykinin, as well as ATP, TPA, and A23187, significantly increased the phosphorylation of epidermal growth factor (EGF) receptors and reduced the binding of EGF to their high-affinity site. The reduction in EGF binding by bradykinin, ATP, and TPA was similarly reversed by concomitant incubation with staurosporine, a protein kinase C inhibitor, implying that the phosphorylation of EGF receptors was catalyzed probably by a protein kinase C of the same or similar type in each case. This possibility was confirmed by the fact that the major phosphorylation site of EGF receptors by the stimulation with either bradykinin, ATP, or TPA was the same (Thr-654). Different from the stimulations with ATP and TPA, the effect of bradykinin of decreasing the high-affinity EGF binding was transient (a minimum binding at 2.5 min); the reduced EGF binding was, however, sustained for up to 30 min in the presence of calyculin A, a phosphoprotein phosphatase inhibitor. Moreover, the homogenate prepared from bradykinin-stimulated A-431 cells had stronger dephosphorylation activity for phosphorylated EGF receptors than that from control cells. These results suggest that bradykinin stimulates both the protein kinase C system and a phosphoprotein phosphatase(s) activity in A-431 cells. Such biphasic effects of bradykinin to phosphorylate and dephosphorylate EGF receptors via protein kinase C and a phosphoprotein phosphatase, respectively, imply a homeostatic control of receptor function in regulating phosphorylation level by the same bioactive factor.

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.

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

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

Occurrence of epidermal growth factor receptors in benign and malignant ovarian tumors and normal ovarian tissues: an immunohistochemical study

Epidermal growth factor receptor (EGF-R) was studied with monoclonal antibody 2E9 on 50 ovarian tumors of various histological types and 10 non-tumorous ovarian tissues by immunohistochemistry. Enhanced expression was observed in 26/50 (52%) of the tumors. Only 25 out of 46 epithelial tumors (54%) showed positivity in epithelial tumor cells. Staining was cytoplasmic in all cases. No correlation was established between EGF-R expression and the histological type of the epithelial tumor. Apart from EGF-R expression in tumor cells, low immunoreactivity was also observed in stromal and endothelial cells in both normal and tumorous ovarian tissues. Furthermore in 8/9 specimens containing necrotic areas, EGF-R was noticed in these areas as well. Both of the latter observations may have impact on the evaluation of the prognostic value of EGF-R activity in tumors, when based on EGF-R measurements using biochemical binding studies. We therefore recommend that EGF-R is measured with both methods in studies regarding its clinical value.

Altered cellular responsiveness during ageing

The capacity of cells and organisms to respond to external stimuli and to maintain stability in order to survive decreases progressively during ageing. The mitogenic and stimulatory effects of growth factors, hormones and other agents are reduced significantly during cellular ageing. The sensitivity of ageing cells to toxic agents including antibiotics, phorbol esters, radiations and heat shock increases. This failure of homeostasis during cellular ageing does not appear to be due to any quantitative and qualitative defects in the receptor systems. Instead, metabolic defects in the pathways of macromolecular synthesis may be the basis of altered cellular responsiveness during ageing.

Homoeostatic imbalance during cellular ageing: altered responsiveness

The inability of normal cells to maintain themselves for ever is a reflection of homoeostatic imbalance and a progressive failure of maintenance. Ageing cells respond less to growth stimulants whereas they show increased sensitivity to toxic agents including antibiotics, phorbol esters, radiation and other physical stresses. No major quantitative and qualitative defects in the receptor systems have been detected that could explain the reasons for altered responsiveness during ageing. Random metabolic defects in the processes involved in maintaining homoeostasis may be critical for causing homoeostatic imbalance, cellular ageing and death.

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.

Characterization of MBT-2 tumour cell "variant" resistant to tumour necrosis factor

Previously we reported sensitivity of MBT-2 murine bladder tumour to tumour necrosis factor (TNF) in vivo and in vitro [8]. We showed that with prolonged exposure of cultured MBT-2 tumour cells to TNF, a resistant MBT-2 "variant" tumour cell population emerged in vitro. This concurred with the finding of transient in vivo cytotoxic effect of TNF against MBT-2 tumour. Herein, we delineate phenotypic changes in MBT-2 cells associated with TNF resistance. Parent MBT-2 (MBT-2P) and the TNF-resistant "variant" MBT-2R cells were compared in terms of in vitro sensitivity to TNF, DNA profile, karyotype and in vitro growth kinetics. We conclude that acquisition of resistance to TNF may be due to cell cycle derangement and differences in in vitro growth characteristics. DNA indices and karyotype of "variant" MBT-2R cells were not altered, indicating the anti-tumour action of TNF is not-mutagenic.

Early kinase C dependent events in aging human diploid fibroblasts

Two early kinase C dependent events, the induction of c-fos transcription and the phosphorylation of an 87 kDa protein, were investigated in aging IMR 90 human diploid fibroblasts. C-fos induction and 87-kDa protein phosphorylation were similarly regulated in aging cells and their young counterparts. The data suggests that G0 exit or the competence phenomenon is intact in aging cells and that the proliferative block occurs later in G0/S.

Net increase of platelet membrane tyrosine specific-protein kinase activity by phorbol myristate acetate

Tyrosine protein kinase (TPK) activity in rabbit platelets after stimulation by phorbol myristate acetate (PMA) or thrombin was directly estimated by 32P incorporation from (gamma-32P)ATP (adenosine triphosphate) into synthetic peptide angiotensin II. By PMA-treatment a net increase of TPK activity was obtained, while thrombin acted on the TPK quickly but stimulation was limited within the range attained by the control after lengthy incubation. The responsive TPK to these stimulators was localized mainly in membrane but much less in cytosol. The specific activity of the particulate TPK was low in the sonicate of control ice cold platelets but increased about 6-fold when the platelets were incubated at 37 degrees C. On a brief contact of platelets with PMA at 37 degrees C the TPK was fully activated and reached a maximum value about 130% of the control. Determination of phosphotyrosine phosphatase in the stimulated platelet sonicate revealed that its participation in the above described increase of 32P-incorporation was meagre. The quick response suggested a possible role of TPK in the signal transduction through the platelet cell membrane.

Stimulation by transforming growth factor-beta of epidermal growth factor-dependent growth of aged human fibro-blasts: recovery of high affinity EGE receptors and growth stimulation by EGF

The stimulatory effects of transforming growth factor beta (TGF-beta) on epidermal growth factor (EGF)-dependent growth of adult and newborn human fibroblasts were investigated. EGF-stimulated growth in low serum of dermal fibroblasts from a 41 year-old adult (HSF-41) was less than half that of newborn foreskin fibroblasts (HFF). The EGF-stimulated growth of HFF after 55 population doublings (HFF-55) was similarly reduced. The decreased growth response to EGF of fibroblasts aged in vivo and in vitro appeared to result principally from a decreased sensitivity to EGF due to a decreased number and affinity of high affinity EGF receptors (H-EGFR). Pre-incubation of HSF-41 and HFF-55 with 25 pM TGF-beta enhanced the growth responses of these cells to EGF and increased the levels of high affinity EGF-binding by these cells. Thus, the stimulation by TGF-beta of EGF-dependent growth of human fibroblasts aged in vivo or in vitro is mediated by increased levels of high affinity EGF binding.

Quantitation of the frequency of immortalization of normal human diploid fibroblasts by SV40 large T-antigen

The mechanism by which SV40 large T-antigen immortalizes human lung fibroblasts is not yet understood, and the frequency with which immortalization occurs is unknown. Here we report detailed studies of the kinetics of immortalization. Approximately 20-50% of individual T-antigen transfected clones of IMR-90 human lung fibroblasts are able to immortalize. The failure of some clones to immortalize is consistent with the hypothesis that in some cells the aneuploidy induced by T-antigen produces extra copies of the chromosome containing a factor that must be inactivated before immortalization occurs. Within immortalization-competent clones, the frequency of immortalization is about 3 X 10(-7). This frequency is consistent with a mutational mechanism for the inactivation of the factor that causes crisis in T-antigen expressing cells.

Exogenous ATP and other nucleoside phosphates modulate epidermal growth factor receptors of A-431 epidermoid carcinoma cells

The binding of epidermal growth factor (EGF) by A-431 human epidermoid carcinoma cells was reduced after exposure of the cells to low concentrations (0.01-1 mM) of ATP and other nucleoside 5'-triphosphates at 37 degrees C, but not at 0 degree C. This was due to loss of high-affinity EGF binding sites. The modulation was associated with transient increases in inositol phosphate synthesis and intracellular Ca2+ and with phosphorylation of the EGF receptor on serine and threonine. There was no evidence for entry of labeled ATP into the cells. ATP appeared to bind to specific cell surface receptors. Such binding was demonstrated directly with the nonmetabolizable ATP analogue adenosine 5'-[beta,gamma-imido]triphosphate.

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.

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.

Collagenase production by early and late passage cultures of human fibroblasts

Comparison of the proteins secreted by early and late passage cell cultures of human fibroblasts revealed a high level of immunoreactive collagenase (Mr = 55,000 Da and 58,000 Da) in the late passage cell culture conditioned medium. Both molecular weight species reacted with a monoclonal anticollagenase antibody and were apparently glycosylation varaents of the same protein. The question of whether the apparent age-dependent differences in collagenase synthesis reflected changes in protein synthesis or secretion was addressed by assaying immunoreactive collagenase and collagenase mRNA. Immunofluorescence microscopy of cellular collagenase revealed that the percentage of collagenase positive cells ranged from 1 to 6% (early passage) to 35 to 46% (late passage) indicating that the late passage cells had higher basal levels of collagenase synthesis. Later passage cultures also secreted higher levels of immunoprecipitable collagenase into the culture medium and Northern analysis established that the basal level of collagenase mRNA was also 10 times greater in late passage cells. High basal levels of collagenase were also observed in fibroblasts cultured from an in vivo aged donor and from donors with Werner's syndrome. Collagenase production was induced in both early and late passage cell cultures by exposure to fibroblast extracellular matrix, fibroblast conditioned media, polypeptide growth factors, or phorbol esters. The induced levels were always greater in the late passage cell cultures than in the early passage cell cultures.

Increased epidermal growth factor receptor in multidrug-resistant human neuroblastoma cells

Multidrug-resistant human neuroblastoma cell lines obtained by selection with vincristine or actinomycin D from two independent clonal lines, SH-SY5Y and MC-IXC, have 3- to 30-fold more cell surface epidermal growth factor (EGF) receptors than the drug-sensitive parental cells as indicated by EGF binding assays and immunoprecipitation, affinity-labeling, and phosphorylation studies. Reversion to drug sensitivity in one line was accompanied by a return to the parental level of EGF receptor. SH-EP cells, a clone derived from the same neuroblastoma cell line as SH-SY5Y but which displays melanocyte rather than neuronal lineage markers, also express significantly more EGF receptor than SH-SY5Y cells. By nucleic acid hybridization analysis with a molecularly cloned probe, increased receptor level in multidrug-resistant cells was shown to be the result of higher levels of EGF receptor mRNA in drug-resistant than in drug-sensitive cells. The increased steady state amount of specific RNA did not result from amplification of receptor-encoding genes. A small difference was observed in the electrophoretic mobility under denaturing conditions of EGF receptor immunoprecipitated from drug-resistant and drug-sensitive cells. Quantitative and qualitative modulation of the EGF receptor might reflect alterations in the transformation and/or differentiation phenotype of the resistant cells or might result from unknown selective pressures associated with the development of multidrug resistance.

Expression and biosynthetic variation of the epidermal growth factor receptor in human hepatocellular carcinoma-derived cell lines

Expression of the epidermal growth factor (EGF) was analyzed in six human hepatocellular carcinoma-derived and one human hepatoblastoma-derived cell line, each of which retained the differentiated phenotype and functions of the parenchymal hepatocyte. The level of receptor expression of each hepatoma cell line was similar to that of the normal human fibroblast, approximately 10(5) molecules per cell. However, NPLC/PRF/5, a subline of the PLC/PRF/5 cell line obtained following reestablishment of a xenograft tumor in vitro, was found to express 4 x 10(6) high-affinity EGF receptor molecules per cell. Proliferation of the NPLC/PRF/5 cell line was inhibited in the presence of nanomolar quantities of ligand. Receptor overexpression was found to result from EGF receptor gene amplification without apparent rearrangement of the EGF receptor coding sequences. Although cell-specific variability in posttranslational processing of EGF receptor N-linked oligosaccharides in the hepatoma cell lines was found, no difference between the receptors in PLC/PRF/5 and NPLC/PRF/5 was observed and no aberrant receptor-related species were detected. EGF receptor gene amplification in the NPLC/PRF/5 cell line is probably a reflection of genome instability and selection of variants with augmented growth potential in limiting concentrations of EGF in vivo. When viewed in this light, EGF receptor overexpression could represent a manifestation of tumor progression in the EGF-responsive hepatocyte.

Expression and biosynthetic variation of the epidermal growth factor receptor in human hepatocellular carcinoma-derived cell lines

Expression of the epidermal growth factor (EGF) was analyzed in six human hepatocellular carcinoma-derived and one human hepatoblastoma-derived cell line, each of which retained the differentiated phenotype and functions of the parenchymal hepatocyte. The level of receptor expression of each hepatoma cell line was similar to that of the normal human fibroblast, approximately 10(5) molecules per cell. However, NPLC/PRF/5, a subline of the PLC/PRF/5 cell line obtained following reestablishment of a xenograft tumor in vitro, was found to express 4 x 10(6) high-affinity EGF receptor molecules per cell. Proliferation of the NPLC/PRF/5 cell line was inhibited in the presence of nanomolar quantities of ligand. Receptor overexpression was found to result from EGF receptor gene amplification without apparent rearrangement of the EGF receptor coding sequences. Although cell-specific variability in posttranslational processing of EGF receptor N-linked oligosaccharides in the hepatoma cell lines was found, no difference between the receptors in PLC/PRF/5 and NPLC/PRF/5 was observed and no aberrant receptor-related species were detected. EGF receptor gene amplification in the NPLC/PRF/5 cell line is probably a reflection of genome instability and selection of variants with augmented growth potential in limiting concentrations of EGF in vivo. When viewed in this light, EGF receptor overexpression could represent a manifestation of tumor progression in the EGF-responsive hepatocyte.

Golgi galactosyltransferase contains serine-linked phosphate

In HeLa and HepG2 cells the Golgi complex enzyme galactosyltransferase became phosphorylated following incubation with 32Pi-Analysis on sodium dodecyl sulphate/polyacrylamide gel electrophoresis revealed incorporation of 32P into the mature 54-kDa form. This phosphorylation was independent of protein synthesis. Serine was identified as the sole phosphorylated amino acid; no radioactive phosphate was detected on N-linked oligosaccharide. The phosphate-labelled galactosyltransferase has the same turnover as [35S]methionine-labelled polypeptides (t1/2 = 20 h). Soluble enzyme, released by the cells, contained very little phosphate relative to that which remained cell-associated. Charge heterogeneity arising from phosphorylation contributes in part to the heterodispersed appearance of the enzyme on two-dimensional gels, as the degree of radioactive phosphate differs among the different iso-enzymes.

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

Tyrosine-specific phosphorylation of the receptor for epidermal growth factor (EGF) in plasma membranes isolated from WI-38 cells is EGF-dependent and occurs to an equivalent extent and on identical tryptic peptides in preparations from cells of various in vitro ages. There is a marked reduction, however, in phosphorylation of receptor molecules from senescent as compared with young WI-38 cells, if enzyme activity is assayed in an immune complex following solubilization of plasma membranes with Nonidet P-40 (NP-40). Differences in the level of receptor phosphorylation in young vs. senescent NP-40 extracts are not resolved by changing the temperature at which the assay is performed, or the length of incubation. Moreover, addition of NP-40 or chloroform-methanol extracts of young cells to assays measuring receptor phosphorylation in senescent cell NP-40 preparations does not augment the senescent enzyme activity. The immunopurified senescent receptor is, however, capable of catalyzing phosphorylation of exogenous substrates. These results indicate that the loss of receptor autophosphorylation in solubilized preparations may result from a differential sensitivity of the senescent cell receptor to the detergent. This finding provides a marker for senescence and suggests subtle changes in protein structure, conformation, or regulation of the EGF receptor in senescent cells.