A general method for determining the replicative age of normal animal cell cultures

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.

Comparison of Colony-Formation Efficiency of Bovine Fetal Fibroblast Cell Lines Cultured with Low Oxygen, Hydrocortisone, L-Carnosine, bFGF, or Different Levels of FBS

A comparison of colony-formation efficiency (CFE) was made between six independent bovine fetal fibroblast (BFF) cell lines used in somatic cell nuclear transfer. Variation in CFE was assessed under different culture conditions. The conditions examined were ambient atmosphere (approximately 20% oxygen) culture versus 5% oxygen culture, three levels of fetal bovine serum (FBS) in the medium (5%, 10% or 20%), and the amendment of 10% FBS medium with basic fibroblast growth factor (1 ng/mL), L-carnosine (20 mM), or hydrocortisone (1 microM). The six BFF cell lines showed significant differences from one another in CFE. No significant difference in CFE was found with reduced oxygen culture. L-Carnosine also had no significant effect on CFE. A FBS concentration of 10% was found to produce the best overall CFE. Hydrocortisone treatment reduced the size of colonies although the number of colonies formed was not affected. Basic FGF increased the size of colonies but the number of colonies formed was not affected. The results showed that different BFF cell lines varied significantly in their CFE. Also, some medium supplements or culture conditions that have shown positive CFE effects on the fibroblasts of other species failed to show significant positive CFE effects on the BFF cell lines tested.

Relationship between in vivo age and in vitro aging: assessment of 669 cell cultures derived from members of the Baltimore Longitudinal Study of Aging

We examined the in vitro proliferative potential of 669 cell cultures established from skin biopsies of members of the Baltimore Longitudinal Study of Aging. The colony size distribution was used to estimate the proliferative life span of the cultures. A significant decline in proliferative potential with donor age was observed for female but not male donors. For both male and female donors, the proliferative potential was significantly greater for donors under the age of 30 years compared with all donors over the age of 30 years. In an attempt to reduce genetic heterogeneity, we examined the proliferative potential of cultures derived at different ages from the same donor. These studies revealed a trend (approaching statistical significance) toward low proliferative potential as donors aged. Interestingly, samples obtained from donors who had a history of skin cancer at the time of biopsy had a significantly lower doubling potential than those from donors who did not. The implications of these results for the use of cells derived from donors of different ages for aging research are discussed.

Insulin-like growth factor-I extends in vitro replicative life span of skeletal muscle satellite cells by enhancing G1/S cell cycle progression via the activation of phosphatidylinositol 3'-kinase/Akt signaling pathway

Interest is growing in methods to extend replicative life span of non-immortalized stem cells. Using the insulin-like growth factor I (IGF-I) transgenic mouse in which the IGF-I transgene is expressed during skeletal muscle development and maturation prior to isolation and during culture of satellite cells (the myogenic stem cells of mature skeletal muscle fibers) as a model system, we elucidated the underlying molecular mechanisms of IGF-I-mediated enhancement of proliferative potential of these cells. Satellite cells from IGF-I transgenic muscles achieved at least five additional population doublings above the maximum that was attained by wild type satellite cells. This IGF-I-induced increase in proliferative potential was mediated via activation of the phosphatidylinositol 3'-kinase/Akt pathway, independent of mitogen-activated protein kinase activity, facilitating G(1)/S cell cycle progression via a down-regulation of p27(Kip1). Adenovirally mediated ectopic overexpression of p27(Kip1) in exponentially growing IGF-I transgenic satellite cells reversed the increase in cyclin E-cdk2 kinase activity, pRb phosphorylation, and cyclin A protein abundance, thereby implicating an important role for p27(Kip1) in promoting satellite cell senescence. These observations provide a more complete dissection of molecular events by which increased local expression of a growth factor in mature skeletal muscle fibers extends replicative life span of primary stem cells than previously known.

A mutation in mouse rad51 results in an early embryonic lethal that is suppressed by a mutation in p53

RecA in Escherichia coli and its homolog, ScRad51 in Saccharomyces cerevisiae, are known to be essential for recombinational repair. The homolog of RecA and ScRad51 in mice, MmRad51, was mutated to determine its function. Mutant embryos arrested early during development. A decrease in cell proliferation, followed by programmed cell death and chromosome loss, was observed. Radiation sensitivity was demonstrated in trophectoderm-derived cells. Interestingly, embryonic development progressed further in a p53 null background; however, fibroblasts derived from double-mutant embryos failed to proliferate in tissue culture.

Replicative potential and the duration of the cell cycle in human fibroblasts: coordinate stimulation by epidermal growth factor

The in vitro replicative potential of human diploid fibroblasts can be increased by polypeptide growth factors such as epidermal growth factor (EGF). Also, the cycle time of EGF-stimulated cells is, on average, decreased and their mitotic cell volume is reduced. Therefore, the regulation of cell size by the duration of the cell cycle may be one process which determines replicative potential. The growth response to a continuous presence of EGF, however, appears to be limited by eventual desensitization to the growth factor.

Tumor suppression by chromosome 11 is not due to cellular senescence

Previous hybrid studies involving fusion of normal with immortal human cells indicated that the phenotype of cellular senescence is dominant and that immortality results from recessive changes in normal growth regulatory genes. We have further assigned 28 different immortal human cell lines to at least four complementation groups for indefinite division. In order to identify the chromosomes involved in regulating cell proliferation, we have introduced single human chromosomes by microcell fusion into immortal human cells representative of the different complementation groups. Our results demonstrate that the introduction of chromosome 11, implicated in tumor suppression, does not cause cellular senescence in three different immortal human cell lines tested.

Independent evidence for a commitment model of clonal attenuation

We previously reported a model of clonal attenuation which assumes three classes of cells: small highly replicative cells; intermediate size cells of limited replicative potential and large non-diving cells. Computer simulations carried out with the model lead to predictions of how the relative proportion of each cell type varies throughout the in vitro replicative life span of a mass population. These predictions appear to be broadly confirmed by independent data recently reported by another laboratory.

Proliferative potential of human fibroblasts: an inverse dependence on cell size

Human foreskin fibroblast-like cells were separated on the basis of DNA content and cell size by fluorescence-activated cell sorting. Subpopulations of "large" or "small" cells with the same (G1) DNA content were clonally expanded and found to contain predominantly nondividing or highly proliferative cells, respectively. From the rate of clonal growth, we deduce that small cells divide faster than large cells. Intermediate-sized cells were found to yield primarily smaller ("attenuated") clones. The clonal data can be incorporated into a previously reported kinetic model of clonal attenuation. This version of the model postulates that small "stem" cells yield larger daughters which have only a limited proliferative potential. We also postulate that a progressive increase in cell size can account for the decreasing concentration of DNA polymerase alpha, which has been reported in older cultures.

Evidence for a universal process underlying clonal attenuation

It is shown by computer simulation that an established commitment model of clonal attenuation can account for clone size distribution data obtained from three vertebrate species--chick, hamster and human--from two evolutionarily divergent classes. The different in vitro replicative lifespans of each cell strain can be explained by differences in cell kinetics. These results suggest that the process of clonal attenuation is qualitatively similar in fibroblasts from all vertebrate species.

Neurotropin increases in vitro life span of human fibroblasts

Fibroblasts isolated from human fetal tissues (skin, lung and heart) exhibited the following population doubling levels (PDL): about 40 PDL for skin; 60 PDL for lung; 10 PDL for heart. Neurotropin extract from vaccinia-virus infected skin tissues of rabbits increases the growth rate of the fibroblast when added to the old lung cell culture (PDL 40, 70% of the maximum life span) but not when added to the young lung (PDL 5, 8% of the maximum life span). Neurotropin (40 or 80 micrograms/ml) increased the replicative life span of the three lines of fibroblasts (skin, lung and heart) when added to the young cell cultures. The increase of the maximum PDL of skin, lung and heart fibroblasts with the Neurotropin treatment was by 19%, 5% and 17%, respectively. When Neurotropin was added to the old lung cell culture (PDL 44, 73% of the maximum life span), the maximum PDL was increased by 9%. Since the existence of hydrocortisone, known to extend the in vitro life span, was negligible in the fluorimetric test of the medium containing Neurotropin, this agent may belong to the class other than hydrocortisone, for increasing in vitro life span.

Effects of skeletal muscle regeneration on the proliferation potential of satellite cells

Skeletal muscle satellite cells are myogenic stem cells that function to repair damaged muscle fibers. Participation of satellite cells in a regeneration response following muscle injury results in a significant reduction in their cumulative proliferation potential. The magnitude of the reduction is proportional to the number of regeneration responses in which the cells participate.

Clonal attenuation in chick embryo fibroblasts. Experimental data, a model and computer simulations

When cells from mass cultures of chick embryo fibroblasts are grown at very low density, some cells yield large clones while others produce smaller clones, and some cells fail to divide at all. The distribution of clone sizes is related to the number of population doublings which the donor mass culture has undergone: the more doublings which have occurred, the smaller the average clone size. In this report we describe a model which analyses this phenomenon, referred to as 'clonal attenuation', in detail. The model is based on the concept that a cell with hypothetically unlimited replicative potential--i.e. a 'stem' cell--can become 'committed' to a programme of limited replicative potential. This event is assumed to be stochastic and to have a fixed probability per stem cell division. The parameters of the model are: Pc, the probability of commitment; N, the number of differentiative divisions; and Tc, the cell-cycle times. By computer simulation, it is shown that Pc increases roughly exponentially at each successive stem cell division. According to the model, when the daughter of a stem cell becomes committed, its progeny proceed through N obligatory divisions before becoming terminally differentiated (post-mitotic). The best-fit value of N was found to be seven. The simulations also reveal that the absolute number of stem cells in the total population increases for most of the lifespan of the culture. When Pc becomes much greater than 0.5, the number of stem cells declines rapidly to zero, and the culture nears senescence. Sensitivity analysis shows that Pc can assume only a limited range of values at each stem-cell division.

A long-lived human diploid fibroblast line for cellular aging studies: applications in cell hybridization

We have isolated a diploid fibroblast culture from human fetal lung with an in vitro lifespan of about 100 population doublings. The culture grows very well at clonal densities and long-lived clones can be isolated for use in cellular aging studies. The longer in vitro lifespan of the culture has allowed us to isolate from it a clone, containing a dominant and recessive mutation, having significant remaining proliferative potential. The nature of the mutations will allow for hybrid selection, after fusion of the mutant clone with wild type human cells. The mass culture and clones derived from it provide a valuable resource for cell aging studies.

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.