Filopodia number increases with age and quiescence in populations of normal WI-38 cells, and is correlated with drug-induced changes in proliferation in both normal and transformed populations

The concept of telomeric non-reciprocal recombination (TENOR) applied to human fibroblasts grown in serial cultures: concordance with genealogical data

Since the discovery of the limited life span of human fibroblasts some 50 years ago, many genealogical studies have been undertaken to describe growth kinetics of fibroblasts in serial cultures by their individual division behavior. It is now accepted that proliferation capacities of human fibroblasts strongly depend on their telomere lengths and integrity. Telomeres shorten with each replication round, and there is a direct correlation between cell division capacity and telomere lengths; that is, the consumption of disposable telomeric DNA repeats during cell divisions progresses until critically short telomeres determining the replicative senescence of the cells are present. Recently, we have suggested that telomeres in fibroblasts can also become elongated during DNA replication by telomeric non-reciprocal recombination (TENOR). Here we discuss genealogical data collected over the last decades as well as more recent findings on the telomere-driven replicative senescence process, and we summarize both to give an integrated picture.

Evidence that filopodia outgrowth is a common final pathway for fibroblast growth inhibition in vitro

To identify events associated with fibroblast growth inhibition, the effect of two known inhibitors, interferon-alpha and all-trans retinoic acid, on the growth and surface morphology of cultured fibroblasts was examined. Interferon-alpha administered at seeding reduced both growth rate and saturation density; all-trans retinoic acid reduced only saturation density. However, both negative growth modulators were associated with an increase in filopodia outgrowth and an increase in intracellular filamentous actin in a time course corresponding to onset of growth inhibition by these agents. In combination with earlier findings, these data suggested that, regardless of etiology, cultured fibroblast growth restriction is mediated in part by an actin-dependent outgrowth of filopodia that augment intercellular contact.

Effect of donor age and prior sun exposure on growth inhibition of cultured human dermal fibroblasts by all trans-retinoic acid

The effect of retinoic acid on human fibroblasts was studied in a cell culture model of chronologic aging and photoaging. During early exponential phase, all trans-retinoic acid significantly stimulated growth rate of adult arm-derived dermal fibroblasts but not of newborn or adult foreskin-derived fibroblasts. Retinoic acid also significantly reduced saturation density in most young adult arm-derived lines and all 24 lines derived from old adult arm and foreskin. However, four of ten young adult outer arm (relatively sun-exposed) and one of ten young adult inner arm (relatively sun-protected) fibroblasts lines increased their saturation density in response to retinoic acid. These data suggest that prior sun exposure and/or donor age may influence cellular responsiveness to retinoic acid. Neither the stimulatory nor the inhibitory effect of retinoic acid could be attributed to cell density, to breakdown of retinoic acid in culture, to nutrient depletion or to serum dependency. However, stationary phase fibroblasts from all sites (foreskin, inner and outer arm) showed an increase in filopodia and in intracellular actin after treatment with retinoic acid that was roughly proportional to the degree of growth inhibition, irrespective of donor age. We suggest that retinoic acid induces premature density dependent growth inhibition at least in part by increasing filopodia-mediated cell contact that is in turn directly related to an increase in fibrillar actin.

Potential and limitations of cultivated fibroblasts in the study of senescence in animals. A review on the murine skin fibroblasts system

Senescence is the last period of the life span, leading to death. It happens in all animals, with the exception of a few didermic species (Hydras) having a stock of embryonic cells and being immortal. The causes of animal senescence are badly known. They depend both on genetic characters (maximum life span of a species) and on medium factors (mean expectation of life of the animals of a species). Animal senescence could depend on cell aging: (1) by senescence and death of the differentiated cells, (2) by modified proliferation of the stem cells of differentiated tissues, (3) by alterations in the extracellular matrices, (4) by interactions between factors (1) (2) and (3) in each tissue, and (5) by interactions between the several tissues of an organism. This complexity badly impedes the experimental study of animal senescence. Normal mammal cells are aging when they are cultivated (in vitro aging). Present literature upon in vitro aging of cultivated human fibroblasts consists essentially of papers devoted to proliferation and differentiation characteristics and not to cell senescence. Murine skin fibroblasts have been studied in our laboratory, using different systems: (1) primary cultures isolated from peeled skins of mouse embryos, (2) mouse derms analysed in the animals, (3) cultivated explants of skins, (4) serial sub-cultures of fibroblasts isolated from these explants, (5) cells cultivated comparably on plane substrates (glass, plastic, collagen films) and on three-dimensional matrices (collagen fibres). In primary cultures (system 1) all the cell generations have been analysed, including the last one until death of the culture. We have shown that many characters are varying with cell generation. All the observed variations were: progressive, non-linear and correlated (intracellular feedbacks). We come to the conclusion that the main effects of cell mitotic age are (1) to depress the plasticity of the chromatin, (2) to change the organization of the cytoplasmic filaments, (3) to change the organization of the extracellular matrix. The collagen fibres are also acting upon nucleus and filaments either in the animals or in the cultures. The phenotype of a fibroblastic cell is thus both age- and environment-dependent. Overall data on in vitro cell aging point to the hypothesis that senescent cells are phenotypic variants and not mutant cells. Aging cell cultures are remarkably useful to the studies on cell proliferation decrease and cell cycle lengthening shown by the stem cells in animal tissues. We propose the hypothesis that the fibroblasts of the vertebrates would be homologous to the pluripotent mesenchyme cells of their embryos.

Cellular senescence revisited: a review

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

Hydrocortisone is associated with cell surface blebbing in cultures of human embryonic lung fibroblasts

Treatment with hydrocortisone (HC) is associated with increased cell surface blebbing of proliferatively active WI-38 and IMR-90 fibroblasts at low and high population doubling levels within the first 24 h after seeding. Time course studies show a marked decrease in blebbing after 2 days. In contrast, very old cells near the end of the population life span (greater than 96% life span completed) show blebbing which is independent of HC treatment. Since the time of increased HC-associated blebbing in cycling cell populations coincides with the time during which HC-treated cells condition the medium with a growth-stimulating (autocrine) factor, it is possible that blebbing may represent extrusion of this factor from the cells at these times.