Patterns of unscheduled DNA synthesis in mouse embryo cells associated with in vitro aging and with spontaneous transformation to a continuous cell line

Methyl nitrosourea induced unscheduled DNA synthesis in vivo in mice. Effects of background genotype on excision repair during aging

We studied mice from five genetic strains at two ages (young, 10 +/- 0.49 weeks; old, 74.4 +/- 14 weeks) for the induction of unscheduled DNA synthesis (UDS) in bone marrow cells following in vivo challenge by a known mutagen and carcinogen: methyl nitrosourea (MNU). The data, in the form of mean silver grains/cell and the percentage of cells in UDS, were used to evaluate genotype (strain) dependent differences in response to mutagenic challenge and changes in this response with age in otherwise normal individuals. Increasing doses of MNU significantly increased both mean silver grains/cell and the percentage of cells in UDS. The increase was, however, strain specific. Furthermore, the older animals generally had both lower mean silver grains/cell and a smaller percentage of cells in UDS as compared to their younger counterparts. This reduction in UDS patterns during aging (aging response) was variable among mouse genotypes. We attribute these observations to subtle differences in the genetic backgrounds of the mice which affects both their ability to repair damage induced by MNU and the genotype specific decline in this ability with old age.

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.

DNA repair in mouse embryo fibroblasts. II. Responses of nontransformed, preneoplastic and tumorigenic cells to ultraviolet irradiation

Ultraviolet light-induced excision repair, as measured by single-strand DNA-break accumulation in the presence of hydroxyurea and 1-beta-D-arabinofuranosylcytosine, undergoes an apparent decline concomitant with spontaneous transformation of mouse cells in vitro. This decline is seen in preneoplastic transformed cells as well as tumorigenic cells, suggesting that it is an early event in transformation. The difference between nontransformed and transformed mouse cells in apparent incision rates is greatest at short times after irradiation when nontransformed cells show a transient phase of rapid incision. No gross differences in the effects of UV on replicative DNA synthesis, bulk RNA synthesis, cell proliferation or clonal survival in nontransformed and transformed cells were seen, in spite of the reduced incision capacity of the latter. Taken together the results suggest that transformed cells are capable of growth in the presence of significantly increased amounts of DNA damage. A decreased ability of nontumorigenic cells to remove DNA lesions, coupled with unrestricted growth, may be responsible for genetic alterations which increase the probability of a cell becoming tumorigenic.

Excision repair of mouse and human fibroblast cells, and a factor affecting the amount of UV-induced unscheduled DNA synthesis

Excision-repair ability and the amount of unscheduled DNA synthesis (UDS) after UV irradiation of fibroblast cells (in vitro passage 5) from C57BL mouse embryos were compared with those of human skin fibroblast cells. UDS in the mouse cells was approximately 75% of that in the human cells, although the disappearance of T4 endonuclease-V-susceptible sites and the accumulation of single-strand breaks in the mouse cell DNA indicated that the excision-repair capacity of the mouse cells was 20-35% of that in the human cells. This apparent discrepancy was ascribed to the difference in intracellular dTTP pool size, which was approximately twice as large in the human cells as in the mouse cells. UDS may not be suitable as a quantitative measure of excision repair when comparing the cells from different species.

DNA repair ability of cultured cells derived from mouse embryos in comparison with human cells

DNA repair in mouse cells derived from embryos of 3 inbred strains were investigated in comparison with that in human cells. The levels of unscheduled DNA synthesis after UV irradiation appeared to change at different passages, but capacities of host-cell reactivation of UV-irradiated herpes simplex virus were always reduced to the same levels as those in xeroderma pigmentosum cells. This implied that mouse cells are reduced in excision-repair capacities and that the apparently high levels of unscheduled DNA synthesis at certain passages are not quantitatively related to high levels of cell survival. Essentially no differences in DNA repair were noted among 3 strains--BALB/c, C3H/He and C57BL/10.

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.