Effects of low dose rate irradiation on the division potential of cells in vitro. IV. Embryonic and adult human lung fibroblast-like cells

Reduction in Life Span of Normal Human Fibroblasts Exposed to Very Low-Dose-Rate Charged Particles

We studied the effect of chronic low-dose irradiation with heavy ions on the life span of normal human fibroblasts in vitro. Cells were cultured in a CO2 incubator that was placed in the irradiation room for biological studies of heavy ions in the Heavy Ion Medical Accelerator in Chiba (HIMAC) at National Institute of Radiological Sciences (NIRS) and were exposed to scattered radiations produced by heavy-ion beams for the life span of the cell population. The absorbed dose, which was measured using a thermoluminescence dosimeter (TLD) and a silicon semiconductor detector, was 1.4 mGy per day when the HIMAC was operated for biological experiments. The total number of population doublings of the exposed cells as reduced to 79-93% of that of nonexposed control cells. However, the life span of cells exposed to low-dose 137Cs gamma rays (approximately 1 mGy/day) in the CO2 incubator in the gamma-irradiation room in NIRS was prolonged to 104-106% of that of nonexposed control cells. Thus there is evidence that exposure to chronic low-dose heavy-ion radiation reduces the life span of cells.

Growth-inhibiting effect of tumor necrosis factor on human umbilical vein endothelial cells is enhanced with advancing age in vitro

We have examined the effects of in vitro aging on the growth capacity of human umbilical vein endothelial cells (HUVECs) under the influence of tumor necrosis factor (TNF) with or without interferon-gamma (IFN-gamma). The growth and colony-forming abilities of control cells were impaired with advancing age in vitro, especially at later stages (more than 70-80% of life span completed). It was found that treatment with TNF inhibited growth and colony-forming efficiency at any in vitro age. The effects of TNF were shown to increase with increasing in vitro age, as reflected by a more pronounced increase in doubling times, a decrease in saturation density, and a reduction in colony-forming efficiency. However, the characteristics of TNF receptors, including the dissociation constant, and the number of TNF-binding sites per cell-surface area remained rather constant. The effect of TNF was augmented by IFN-gamma at a dose that alone affected growth and colony formation only slightly. The augmentation by IFN-gamma was also found to depend on in vitro age; the synergy with TNF in the deterioration of colony-forming ability was observed only in "aged" cells. These results suggest that the intrinsic responsiveness of HUVECs to growth-inhibiting factors, as well as to growth-stimulating factors, changes during aging in vitro.

Strong effects of 5-azacytidine on the in vitro lifespan of human diploid fibroblasts

Azacytidine (5-aza-CR) and azadeoxycytidine (5-aza-CdR) are known to inhibit the methylation of cytosine (5-mC) in DNA, and their effects on the long-term growth of human fibroblasts, strain MRC-5, have been examined. A single treatment with either analogue initially inhibits growth, but the cells recover to normal morphology, growth rate and cell density at confluence. However, a memory of the treatment is retained, since the cells' subsequent lifespan is considerably reduced in comparison with controls, and the terminal stages of growth are indistinguishable from senescent cultures of untreated cells. The effect of 5-aza-CR or 5-aza-CdR does not appear to be closely related to the concentration used, or to the length of treatment up to about half-way through the total lifespan. Sequential doses have cumulative effects on longevity. There is evidence that the pattern of 5-mC in mammalian DNA is inherited via cell division; therefore, a reduction in 5-mC induced by a pulse treatment of 5-aza-CR or 5-aza-CdR will be transmitted to all descendants. The results are consistent with independent observations that the level of 5-mC declines continually during the serial subculture of human diploid cells. The analogues would be expected to precipitate this decline and thereby advance the physiological age of the culture. The results provide support for the view that the random loss of methyl groups in DNA may eventually have deleterious consequences, such as aberrant epigenetic changes in gene expression.

Genome reorganization during aging of dividing cells

The study of the effect of low dose rate ionizing radiation on the long-term proliferation of fibroblasts led to the observation that radiation accentuated the growth potential of the cells, favoring events which normally take place during division. These events could be related to the genome reorganization taking place during division. Hence, it was hypothesized (Macieira-Coelho, 1979; Macieira-Coelho, 1980; Macieira-Coelho, 1981) that the long-term proliferation of fibroblasts depends upon the potential for reorganization of the genome, the latter being a self-limiting process. At each division residual quantitative and qualitative changes would accumulate in chromatin, limiting the long-term potential for further rearrangements. The hypothesis was checked looking for quantitative and qualitative changes in DNA through the in vitro lifespan of human fibroblast populations. It was found that at each population doubling in 20% of the cells there is unequal distribution of DNA between sister cells. Results show that this could be due to errors in chromosome assembly and segregation, to loss of DNA, to errors during semiconservative DNA synthesis and to multiple rounds of DNA replication at a single origin. An increased alkali- and thermo-lability of chromatin was found during in vitro aging. At the ultrastructural level after mild decondensation, chromatin fibers were spaced and shorter. After Miller's spreading, most of the chromatin of old cells had lost the nucleosome organization and was fragmented. These chromatin changes became apparent only towards the end of the life span of human embryonic fibroblasts but were already present in a significant fraction of low population doubling level (PDL) fibroblasts from human adults. Almost all cells of low-PDL fibroblasts from the Werner syndrome presented these chromatin changes. In addition, short, unbeaded DNA fragments could be seen in these fibroblasts, occasionally forming circles; they could correspond to transposable elements which detach during the division cycle and fail to reintegrate into chromosomes because of the age-related chromatin structural changes.

Late skin damage in rabbits and monkeys after exposure to particulate radiations

Skin biopsies were taken from the central regions of the ears of New Zealand white rabbits following localized exposure of one ear of each rabbit to 530 MeV/amu Ar or 365 MeV/amu Ne ions. The unirradiated ears served as controls. Biopsies were taken also from the chests and inner thighs of rhesus monkeys after whole-body exposure to 32 MeV protons and from unirradiated control animals. The linear energy transfers (LET infinity's) for the radiations were 90 +/- 5, 35 +/- 3, and approximately 1.2 keV/micrometer, respectively. In the rabbit studies, explants were removed with a 2 mm diameter dermal punch at post-irradiation times up to five years after exposure. Similar volumes of monkey tissue were taken from skin samples excised surgically 16-18 years following proton irradiation. Fibroblast cultures were initiated from the explants and were propagated in vitro until terminal senescence (cessation of cell division) occurred. Cultures from irradiated tissue exhibited decreases in doubling potential that were dependent on radiation dose and LET infinity and seemed to reflect damage to stem cell populations. The implications of these results for astronauts exposed to heavy ions and/or protons in space include possible manifestations of residual effects in the skin many years after exposure (e.g. unsatisfactory responses to trauma or surgery).

Effect of growth arrest on the doubling potential of human fibroblasts in vitro: a possible influence of the donor

Population doublings versus time in culture were compared in human postnatal skin fibroblasts from normal donors, a cancer patient, and from donors suffering from Cockayne syndrome, Ataxia telangiectasia, and Fanconi's anemia (FA). Confluent cultures were maintained in a nonproliferating state for 14 to 27 d in 0.5% serum medium. The results show that the ability of cells to resume division after a resting stage can be influenced by pathologic conditions. In arrested FA cell populations an increase of the population doublings and of the calendar time were observed. It is possible that in some cell populations the resting stage favors the expression of growth potentialities related to instability of the cells.

Recent advances in the cell biology of aging

Cultured normal human and animal cells are predestined to undergo irreversible functional decrements that mimic age changes in the whole organism. When normal human embryonic fibroblasts are cultured in vitro, 50 +/- 10 population doublings occur. This maximum potential is diminished in cells derived from older donors and appears to be inversely proportional to their age. The 50 population doubling limit can account for all cells produced during a lifetime. The limitation on doubling potential of cultured normal cells is also expressed in vivo when serial transplants are made. There may be a direct correlation between the mean maximum life spans of several species and the population doubling potential of their cultured cells. A plethora of functional decrements occurs in cultured normal cells as they approach their maximum division capability. Many of these decrements are similar to those occurring in intact animals as they age. We have concluded that these functional decrements expressed in vitro, rather than cessation of cell division, are the essential contributors to age changes in intact animals. Thus, the study of events leading to functional losses in cultured normal cells may provide useful insights into the biology of aging.

Effect of low dose rate irradiation on the division potential of cells in vitro. VII. Human fibroblasts from young and adult donors

Lung and skin fibroblasts from young and adult donors were irradiated with low dose rate ionizing radiation. Experiments show that in fibroblastlike cells from human adult donors there are three main types of response: irradiation can decrease only the total number of cells produced, it can also decrease the number of subcultures, and in some instances it can prolong the growth potential. Previous experiments suggested that these responses are genetically determined. They may have implications concerning the donor's proneness to cancer.

Senescence of cultured human diploid fibroblasts. Are mutations responsible?

Two predictions of the error/mutation hypothesis of cellular senescence (Orgel, '73) namely,a) exponential accumulation of somatic mutations during the replicative lifespan and b) shortening of culture lifespan upon treatment with mutagens have been examined experimentally in a strain of cultured human diploid fibroblasts. Our studies show that as cells traverse the replicative lifespan (from 10 to 75 mean population doublings (MPD); total lifespan congruent to 95 MPD), no rapid and exponential increase occurs in the accumulation of mutations measured by the frequencies of Thgr(thioguanine resistance) and Dipr (diphtheria toxin resistance) mutants. Furthermore, repeated cycles of treatment (from 1- to 14-times) of human fibroblasts with two mutagens, ethyl methane sulfonate (EMS) and N-methyl-N' nitro-nitrosoguanidine, which led to a marked increase in the mutation frequency for the Dipr marker (congruent to 100-fold), failed to shorten the lifespan of cultured fibroblasts. On the contrary, repeated mutagen treatment (12 times with EMS) prolonged the lifespan of one replicative culture (110 MPD versus 94--98 MPD). These results strongly indicate that mutations are unlikely to be the primary event in cellular senescence and suggest instead that senescence is probably controlled by one or more (specific) gene(s) whose expression can be modified by mutations.

Effect of ionizing radiations on the life span of non-transformed human fibroblasts

We studied in vitro the influence of ionizing radiations on the life span of non-transformed HF 19 human fibroblasts. The life span of surviving clones was found to be reduced when the cells had received two or three doses of 6 Gy separated by an interval of 15 doublings. In addition, this reduction in life span was greater when the cells were older at the time of irradiation.