Establishment of rat embryonic cells in vitro. Relationship of DNA synthesis, senescence, and acquisition of unlimited growth potential

Can ends justify the means?: telomeres and the mechanisms of replicative senescence and immortalization in mammalian cells

Finite replicative lifespan, or senescence, of mammalian cells in culture is a phenomenon that has generated much curiosity since its description. The obvious significance of senescence to organismal aging and the development of cancer has engendered a long-lasting and lively debate about its mechanisms. Recent discoveries concerning the phenotypes of telomerase knockout mice, the consequences of telomerase reexpression in somatic cells, and genes that regulate senescence have provided striking molecular insights but also have uncovered important new questions. The objective of this review is to reconcile old observations with new molecular details and to focus attention on the key remaining puzzles.

Skin fibroblasts from aged Fischer 344 rats undergo similar changes in replicative life span but not immortalization with caloric restriction of donors

We have compared the in vitro replicative life span and characteristics of immortalization of skin fibroblast cultures derived from ad libitum-fed and caloric-restricted Fischer 344 rats of 6, 24, and 29 months of age. Cells from all 6-, 24-, and 29-month-old animals showed a gradual decline in proliferative potential as evidenced by decreases in harvest density, in the fraction of cells initiating DNA synthesis, and in the number of population doublings per passage. These declines were accompanied by morphological changes including cell enlargement. The replicative life span prior to immortalization decreased significantly with donor age (P less than 0.0001), while caloric restriction had no effect on the cumulative population doubling level. Prior to immortalization mitotic cells from all cultures showed a normal rat karyotype. Postcrisis cultures tended to have more polyploid cells but there were no characteristic or specific chromosomal changes found in the cells with an immortalized phenotype. Interestingly, fibroblasts derived from caloric-restricted animals had a significantly slower growth rate through the tenth week after immortalization (P less than 0.005). When these cultures were seeded at one-quarter the normal seeding density, to favor the outgrowth of the fastest growing cells, a population with a more "transformed" phenotype emerged.

Two classes of continuous cell lines established from Syrian hamster 9 day gestation embryos: preneoplastic cells and progenitor cells

Primary cultures of 9-d-gestation Syrian hamster embryo (E9) cells are distinct from primary cultures of later gestational age in terms of their growth and differentiation. First, primary E9 cell cultures express multiple mesenchymal differentiation lineages (e.g., adipocyte, myoblast) only rarely seen in cultures of 13-d-gestation fetal (F13) cells. Second, although most primary E9 cultures have a limited in vitro proliferative life span and exhibit cellular senescence similar to primary cultures of F13 cells, E9 cultures seem to have higher frequency of escape from senescence and conversion to continuous cell lines compared to F13 cells. Moreover, this frequency can be further increased 4- to 5-fold by continuous exposure of the E9 cells to tumor promoters or epidermal growth factor. Eleven continuous cell lines have been isolated from untreated, promoter-treated, or epidermal growth factor-treated primary E9 cultures. Seven of these are neoplastic or preneoplastic. However, the remaining four do not show any evidence of being in neoplastic progression and three of these continue to express the same differentiated phenotype observed in ther parental primary cell cultures.

Mouse embryo fibroblast proliferation and prostaglandin production in medium supplemented with fetal bovine serum or serum substitutes (Ultroser SF and G): role of glucocorticoids

The growth of DBA/2 mouse embryo fibroblasts, as well as their prostaglandin (PG) production, was compared under 3 different culture conditions: RPMI 1640 supplemented with 10% fetal bovine serum (FBS), 2% Ultroser SF (steroid-free) or with 2% Ultroser G (containing steroids). The effect of the absence or presence of glucocorticoids on both parameters was more precisely investigated. In FBS-supplemented cultures, dexamethasone had a stimulatory effect on cells characterized by a slow growth rate, whereas it markedly inhibited proliferation in rapidly growing fibroblasts. The experiments carried out with serum substitutes (Ultroser SF and G) strongly corroborated the role of the absence or presence of glucocorticoids on fibroblast proliferation. Manipulations of glucocorticoid concentrations in Ultroser SF by adding 5 x 10(-8) M dexamethasone or in Ultroser G by adding 10(-6) M RU 486 reversed the effect of the absence of glucocorticoid in the first case, or in the latter case the effect of the presence of glucocorticoid on both cell growth and PG production. Progesterone had no effect by itself. Our results emphasize the importance of performing complete kinetic studies to investigate the effect of a given factor on cell proliferation in vitro, since glucocorticoids may have opposite effects on fibroblast proliferation according to their cell growth pattern in vitro.

UV-induced DNA excision repair in rat fibroblasts during immortalization and terminal differentiation in vitro

UV-induced DNA excision repair was studied as DNA repair synthesis and dimer removal in rat fibroblast cultures, initiated from either dense or sparse inocula of primary cells grown from skin biopsies. During passaging in vitro an initial increase in DNA repair synthesis, determined both autoradiographically as unscheduled DNA synthesis (UDS) and by means of the BrdU photolysis assay as the number and average size of repair patches, was found to be associated with a morphological shift from small spindle-shaped to large pleiomorphic cells observed over the first twenty generations. In cell populations in growth crisis, a situation exclusively associated with thin-inoculum cultures in which the population predominantly consisted of large pleiomorphic cells, UDS was found to occur at a low level. After development of secondary cultures into immortal cell lines, both repair synthesis and morphology appeared to be the same as in the original primary spindle-shaped cells. At all passages the capacity to remove UV-induced pyrimidine dimers was found to be low, as indicated by the persistence of Micrococcus luteus UV endonuclease-sensitive sites. These results are discussed in the context of terminal differentiation and immortalization of rat fibroblasts upon establishment in vitro.

Phenotypic and karyotypic transitions in the spontaneous transformation of a rat cell line

After 20-50 transfers, a rat myofibroblast line, Hmf-n, 'spontaneously' transforms to an established (immortalized) line of smaller, rapidly cycling fibroblastoid cells (tHmf-f). From these 1 degree transformants, colonies of larger, slower growing anchorage-independent (tHmf-e) cells of epithelioid phenotype emerge. Both transformants grow in low serum and low calcium media, but the tHmf-f cells are highly tumorigenic in nude mice, have diminished substrate adhesivity, and limited anchorage independence, whereas tHmf-e are less tumorigenic, firmly substrate adherent, and markedly anchorage independent. Most tHmf-f are trisomic; most tHmf-e transformants are hypodiploid, a third are tetraploid, and all have chromosomal abnormalities, but no trisomy. Hmf-n cells have polar stress fiber arrays terminating in vinculin adhesion plaques, colinear extracellular fibronectin matrices, and linear non-coincident deposits of fodrin. Microtubules (mt) and vimentin-intermediate filaments (IF) parallel the actin cables. Stress fibers of the tHmf-f are moderately reduced, their vinculin adhesion plaques and fibronectin matrices intact; fodrin is diffuse. Mts and IFs are normal and axial. Most epithelioid tHmf-e have no stress fibers, adhesion plaques, or extracellular fibronectin; instead, dense actin microfilament meshworks are attached to plasma membrane, as is fodrin. Mt and IF are radial. Both transformed phenotypes are stable over greater than 300 continuous passages. The differentiation-inducing agents DMSO, cyclic AMP, 5-azacytidine, and mezerein, were ineffective in normalizing shape or cytoskeleton of transformed Hmf, and butyrate was selectively toxic to 50% of tHmf-e. But hydrocortisone induced striking polarization, and increase in number, and alignment of stress fibers of both tHmf-f and tHmf-e. Growth, anchorage, cytoskeletal arrangements, and tumorigenic potential are not closely correlated in these stable, spontaneously transformed lines of distinct pheno- and karyotype originating from the same normal parental cell, suggesting independent acquisition of properties associated with transformation.

Long-term cultivation of amphibian melanophores. In vitro ageing and spontaneous transformation to a continuous cell line

Pure melanophore populations isolated from the tail skin of the tadpole, Rana catesbeiana, were mass cultured for a period of 2-3 years. All cell lines of amphibian melanophores studied exhibited growth crisis (in vitro ageing) followed by spontaneous transformation to a continuous cell line, as shown by changes in growth characteristics in mass culture and in clone culture, by the appearance of the cells, and by measurements of cell volumes. Even after becoming a continuous cell line, amphibian melanophores continued to have a diploid chromosome number (2n = 26) in three of four cell lines examined. The chromosome mode in one cell line, however, changed to thirty. Measurement of melanin dispersion after the addition of alpha-melanocyte-stimulating hormone suggested that the mechanism for melanin dispersion in melanophores changed during in vitro ageing.

DNA and cholesterol biosynthesis in synchronized embryonic rat fibroblasts. I. Temporal relationships between HMG-CoA reductase activity, sterol biosynthesis and thymidine incorporation into DNA

Temporal relationships between hydroxymethylglutaryl-CoA reductase activity, biosynthesis of C27 sterols, and [3H]thymidine incorporation into DNA were studied in a rat embryo fibroblast cell line synchronized by double thymidine block and cultured in cholesterol-containing medium. Cyclic variations of HMG-CoA reductase activity and C27 sterols occurred, with two maxima in S and G2M phases; the relative shortness of the G1 phase (3 h) in these cells could be responsible for the shift of sterol synthesis in the S phase. No noticeable variation of the individual C27 sterols was observed during the entire cell cycle. In each experiment, there was a good linear correlation between HMG-CoA reductase activity and C27 sterol synthesis, but from one experiment to another, a given level of enzymatic activity led to varying levels of [2-14C]acetate incorporation into sterols. In our experimental conditions, total HMG-CoA reductase activity is measured, and the preceding observation could be explained by a varying degree of phosphorylation of the enzyme depending on the metabolic state of the cells at the start of the experiment. The cyclic variations of the enzyme activity seem to be due more to increased synthesis at given times of the cycle than to periodic dephosphorylation. We question the existence of a relationship between cell division and cyclic sterol synthesis occurring in cells cultured in cholesterol-containing medium.

Establishment and characterization of an epithelial intestinal cell line from rat fetus

An epithelial intestinal cell line has been established from explants of fetal rat small intestine. After the 9th passage (approx. 25 population doublings) epithelial-like cells acquired the properties of a permanent cell line. The epithelial nature of this cell line, and of clone IRD 98 subsequently isolated, is supported by morphological and ultrastructural criteria, and also by the presence of enzymes characteristic of enterocytes, such as aminopeptidase, alkaline phosphatase, gamma-glutamyl transferase, lactase and maltase. The occurrence of the triglyceride pathway enzyme monoacylglycerol acyltransferase and of apoproteins (Apo A1 and Apo E) can also be demonstrated. Taken together, the results presented here provide evidence that clone IRD 98 is an epithelial cell line, most likely originating from the relatively differentiated cell layer of fetal rat small intestine.

Ageing of hamster embryo fibroblasts as the result of both differentiation and stochastic mechanisms

Fibroblasts from hamster embryos were serially cultivated in vitro and their evolution followed from a morphological, physiological and biochemical point of view. After an exponential growth for about 20 passages, cells entered the ageing phase which ended up after 29-34 passages. From our observations, it seems that the arrest of growth results from two different phenomena: first, the typically fibroblastic cells may undergo a stochastic ageing process; second, some of these cells evolve into a terminal differentiation process, characterized by a different non-fibroblastic phenotype.

Reactivation of DNA synthesis in aging diploid human skin fibroblasts by fusion with mouse L karyoplasts cytoplasts and whole L cells

Diploid human skin fibroblasts derived from an 82-year-old donor with a 21-28 cell population doubling (CPD) range (where 28 CPD marked the end of the in vitro life span of the cells) were fused with whole L cells, L karyoplasts and L cytoplasts. The proportion of human nuclei incorporating tritiated thymidine after fusion was measured autoradiographically. Statistically significant increases in the labeling indices were found in the human nuclei in hybrid, heterodikaryon and cybrid cells when compared to control unfused human cells. Fusion of human diploid fibroblasts with human cytoplast derived from cells of the same CPD showed no significant changes in the labeling indices of the human nuclei.