Studies on chromosome aberrations induced by incorporated tritium: effect of post-treatment with hydroxyurea and caffeine in G2

Evaluation of cell death in EBV-transformed lymphocytes using agarose gel electrophoresis, light microscopy and electron microscopy. II. Induction of non-classic apoptosis ("para-apoptosis") by tritiated thymidine

There is an extensive literature dating back to the late 1950's, on the damaging biological effects of radiolabeling DNA in vivo. Nonetheless, tritiated thymidine has often been used to label DNA in studies of programmed cell death (apoptosis). In the present study, we have investigated the effects of incorporation of tritiated thymidine into the DNA of an Epstein-Barr virus-transformed cell line (NC-37) in the absence of any other apoptosis-inducing agent. Cells were incubated in media containing 1-20 microCi/ml [methyl-3H]-thymidine ([3H]-TdR). At each concentration of tritiated thymidine used, cell proliferation ceased within 12 hours of incubation. The mode of cell death caused by tritiated thymidine incorporation was evaluated using DNA degradation patterns and cellular morphology. DNA degradation, in the absence of a "ladder" pattern, was shown by agarose gel electrophoresis. Electron microscopy was used as the "gold standard" to evaluate the specific morphologic type of cell death that accompanied the DNA degradation. Although some of the features of apoptosis were present, the cells lacked the early margination of the chromatin within an intact nucleus and surface blebbing leading to apoptotic body formation, two characteristic morphological features of apoptosis. We, therefore, coined the term "para-apoptosis" to be more precise about the morphologic type of cell death. The percent of para-apoptotic cells was quantitated by light microscopy using whole mount preparations (cytospins). The morphologic criteria of chromatin condensation, nuclear fragmentation, increase in cell density and cytoplasmic vacuolization were used for the evaluation of para-apoptosis by light microscopy of cytospin preparations. In the absence of tritiated thymidine, < 2% of the cells became apoptotic/para-apoptotic after 43 hours of incubation. However, at all concentrations of tritiated thymidine used in the incubation medium (1-20 microCi/ml), the number of para-apoptotic cells increased. In addition, we detected perturbations in the timing of the cell cycle of the surviving cells and an increase in the number of micronuclei after only one division cycle. The induction of para-apoptosis and micronuclei formation represent two distinct modes of cell death caused by tritiated thymidine incorporation. These studies emphasize the necessity for morphological examination in characterizing the induction of cell death in a new experimental system.

Potentiation by caffeine of the frequencies of micronuclei induced by mitomycin C and cyclophosphamide in young mice

Employing the micronucleus test in mouse bone marrow and in fetal mouse liver, the possible clastogenicity of caffeine as well as its influence on MMC- and CP-induced micronucleus levels were studied. The treatment of male and female C57Bl or BDF1 (C57Bl x DBA2) mice with caffeine (1 or 3 x 50 mg/kg and 100 mg/kg, s.c.) had no clastogenic effect in mouse bone marrow or in the fetal livers and maternal bone marrow when pregnant mice were injected with caffeine on day 16-17 of gestation. MMC (2.0 mg/kg, i.p.) increased up to 10-30-fold the number of MNPCEs in bone marrow compared to a 3-7 fold elevation of MNPCEs in fetal liver. A similar effect was also established in pregnant mice treated with CP (30 mg/kg, i.p.). No significant sex differences in spontaneous and MMC- or CP-induced MNPCEs levels were established in C57Bl and BDF1 mice. However, a significantly higher spontaneous rate of MNPCEs as well as a better-expressed responsiveness to the clastogenic activity of MMC and CP were established in C57Bl compared to BDF1 mice. The pregnancy had no effect on MMC- or CP-induced clastogenicity although a tendency to a decreased sensitivity to the damaging activity of MMC seemed to be detected in pregnant C57Bl mice compared to virgin female animals. The combined treatment of mice with caffeine (3 x 100 mg/kg) and MMC or CP caused an up to 45-49% potentiation of clastogenesis in the bone marrow of male, female and pregnant female C57Bl and BDF1 mice but not in fetal mouse livers.

The effects of tritium on embryo development: the embryotoxic effects of [3H]tryptophan

The effects of tritiated amino acids on the development of the early mouse embryo in vitro are described. [3H]tryptophan was extremely toxic, compared with [3H]glutamic acid, and inhibited growth of the two-cell embryo to the blastocyst stage. The LD50 values for the two amino acids were, respectively, 1 and 296 kBq/ml of medium. The two-cell embryo was more sensitive to the irradiation from incorporated [3H]tryptophan is selectively incorporated into two chromosomal proteins in the two-cell embryo, and that those proteins were stage-specific markers of development.