Radiat Environ Biophys (1996) 35: 267 – 271
© Springer-Verlag 1996
O R I G I N A L PA P E R
W.-U. Müller · N. Heckeley · C. Streffer
Effects of cell cycle specific exposure to 3H-thymidine or 3H-arginine on development and cell proliferation of mouse embryos
Received: 12 July 1996 / Accepted in revised form: 30 September 1996
Abstract One-cell mouse embryos were exposed to either 3H-thymidine (100 or 200 kBq/ml) or 3H-arginine (2.5 to 50 kBq/ml) for 2 h either in G1, S or G2 phase. 3H-Arginine affected embryonic development and cell proliferation in an activity-dependent way irrespective of the cell cycle stage exposed, whereas 3H-thymidine was effective only at higher activities and only after exposure during S phase.
Introduction
Most people associate the β-ray-emitting hydrogen radionuclide tritium (3H) with a very low radiation risk. This view originates from the short range of β-rays (on average 1 µm and maximum 7 µm in water). Thus, as long as tritium is located outside the organism, no harm is indeed to be expected, and even when located in the cytoplasm, there is comparatively little risk. If, however, tritium enters the cell nucleus, almost all the energy of a decay will affect the genetic material of the cell. That the high risk exerted by tritium after incorporation into the cell is real and not just theoretically to be expected has been shown in numerous studies of preimplantation embryos [1–10]. Particularly dangerous are tritiated agents that are incorporated into nuclear compounds either almost exclusively (3H-thymidine into DNA) or predominantly (3H-arginine into histones). In a previous paper [9], we analysed the effects of a continuous exposure of preimplantation embryos to 3H-arginine and 3H-thymidine. 3H-Arginine turned out to be more harmful to embryonic development, most probably due to a faster and higher uptake of the amino acid when compared with 3H-thymidine. In the present paper, we would
W.-U. Müller (½) · N. Heckeley · C. Streffer Institut für Medizinische Strahlenbiologie, Universitätsklinikum Essen, D-45122 Essen, Germany
like to introduce data obtained after a pulse-labelling of one-cell mouse embryos for 2 h. This short-time exposure permitted application of the tritiated compounds in defined cell-cycle stages of the one-cell embryo in order to analyse the impact of G1, S and G2 phase on the radiation risk posed by tritium. Early preimplantation embryos are very suitable for this type of experiment because they naturally develop in a highly synchronous way without the necessity to force them into synchrony by some agent [11]. Such a cell-cycle specific exposure has not been studied previously. In addition, we looked at the amount of activity incorporated into cellular macromolecules in order to explain the observed differences in the effects exerted by 3H-thymidine and 3H-arginine. With regard to embryonic development and cell proliferation we found that the cell-cycle stage exposed is of crucial importance in the case of 3H-thymidine, but without effect in the case of 3H-arginine.
Materials and methods One male and three spontaneously ovulating female mice (Heiligenberger Stamm, a mouse strain that has meanwhile been registered as HLG/Zte) were mated between 6 and 9 a.m. Females were checked for vaginal plugs (which indicate mating). One-cell embryos were collected by cutting the ampulla 2 h post conception (p.c.); analyses of our mouse strain have shown that conception (determined by the presence of sperm heads in the oocyte) takes place at around 8 o’clock. Cumulus cells were removed by a 5 min treatment with hyaluronidase (50 U/ml medium). Embryos were cultured in plastic dishes (35 × 10 mm; Greiner, Germany) in 0.1 ml medium [12] overlaid with 3.5 ml silicone oil (tegiloxan 50; Kraemer and Martin, Germany). Cultures were incubated at 37°C, 10% CO2 and water-saturated atmosphere in a Cytoperm incubator (Heraeus, Germany). The following radioactive compounds were used (Radiochemical Centre, Amersham): [methyl-3H]-thymidine, specific activity 1.48 TBq/mMol, L-[5-3H]-arginine, specific activity 62 GBq/mMol. The high specific activities avoid problems with the internal pools of both agents. Especially with regard to thymidine, previous experiments have shown that no adverse effects are to be expected through a potential toxicity of the thymidine itself [1, 13]. Both compounds were routinely checked for purity by thin-layer chromatography (cellulose, n-butanol/acetic acid/water 1:1:2) fol-
268 lowed by scintillation counting of the bands that corresponded to thymidine and arginine, respectively. One-cell embryos were pulse-incubated with different activities of the compounds mentioned above for 2 h during G1 (2–4 h p.c.), S (5–7 h p.c.) and G2 (11–13 h p.c.) phases. After incubation the embryos were washed three times in 10 ml medium, distributed to culture dishes and cultivated in Kasai medium. In order to determine the incorporation of 3H-thymidine and 3 H-arginine for each concentration applied, 40 to 100 embryos (obtained from about 5 to 12 dams) were pooled, washed in medium, and transferred to 0.1 ml distilled water in a scintillation vial immediately after washing. Following deep-freezing and re-thawing, 5 ml of scintillation cocktail (Quickszint; Zinsser Analytic, Frankfurt, Germany) were added, and the samples were analysed in a liquid scintillation counter (Beckman LA 1801, USA). The same procedure was applied to determine the residual radioactivities in the embryos after 24 and 48 h of incubation. The following endpoints were used to characterize the effects of 3 H-arginine and 3H-thymidine: 1. Embryonic development (formation of 2-, 4-, 8-cell stage, and of blastocysts). Development was followed under the microscope. 2. Cell proliferation (number of cells 48 and 144 h p.c.). Embryos were spread on glass slides using the method of Mikamo and Kamiguchi [14] in a slightly modified form. The modifications referred to the concentration of the fetal calf serum in the hypotonic solution (between 40% and 25% depending on the stage) and the times of fixation (between 3 and 5 min, again depending on the stage of development). 3. Distribution of radioactivity in the cell nucleus and in the cytoplasm. After spreading the embryos according to the method of Tarkowski [15], the distribution of radioactivity after exposure to either 3 H-thymidine or 3H-arginine was determined by using standard autoradiography methods. Statistical significance of the data of Table 1 was tested by comparison of two binomial distributions after arcsin-transformation according to Sachs [16], p. 263; the cell numbers of Table 2 were tested using Student’s t-test.
Results
Uptake, localization and persistence of 3H-arginine and 3 H-thymidine activity Figure 1 reveals a marked difference between arginine and thymidine when incorporation of both agents is studied in one-cell embryos depending on the cell cycle stage in which the agent is applied. Significant amounts of radioactive material are found in the embryos after incubation in 3H-arginine for 2 h, irrespective of the cell cycle stage incubated. This is clearly different for 3H-thymidine, because no incorporation at all could be demonstrated for G1 and G2 phases (data not shown) and only a very marginal incorporation after exposure during S phase. It is obvious that arginine and thymidine will act differently with regard to the site of accumulation. Whereas it is to be expected that arginine will be incorporated into proteins inside and outside the cell nucleus, thymidine will be concentrated in the nuclear DNA, with only small, hardly detectable amounts in the mitochondrial DNA. This could be confirmed by autoradiographs of the embryos after exposure to both tritiated compounds for 2 h either in G1, S, or G2 phase. Whereas the silver grains were restricted to the cell nucleus exclusively after application of
3
H-thymidine, for 3H-arginine the nucleus/cytoplasm ratio of the silver grains in similarly sized areas was about 2:1 with an increasing tendency up to 7 h after incubation, irrespective of the cell-cycle stage exposed. No sites of accumulated actitivity were detected for 3H-thymidine or 3 H-arginine; in both cases, the distribution of activity, as taken from the autoradiographs, was random. The persistence of both molecules in the embryos also varied. Whereas the amount of activity in the embryos was the same for 3H-thymidine immediately after exposure and 24 and 48 h later, the activity per embryo declined after 3 H-arginine application, with a half-life time of about 24 h.
Effects on embryonic development Table 1 shows that the pattern of damage to embryonic development was the same for all cell-cycle stages, in which 3 H-arginine was added for 2 h to the medium. Starting at 12.5 kBq/ml, significant effects were observed beginning with the four-cell stage. Actually, the activity showing effects was somewhat lower, because even 5 kBq/ml for 2 h during the G1 phase allowed only 60% of the one-cell embryos to reach the blastocyst stage (P<0.01; data not shown in Table 1). When the embryos were incubated with an activity of 25 or 50 kBq/ml 3H-arginine, the number of embryos developing to the two-cell stage was also significantly reduced. Based on the applied activity, 3H-thymidine was markedly less effective than 3H-arginine. As expected (see above), no effects at all were observed after exposure during G1 or G2 phase (data not shown); but also after incubation of the embryos in 3H-thymidine during S phase, the effects were clearly less than after exposure with 3H-arginine. About 20 times the activity was necessary when using 3H-thymidine to achieve effects of a comparable magnitude to 3H-arginine.
Effects on cell proliferation For some exposure conditions, cell numbers were checked after different times of embryo culture. More or less the same pattern of response was obtained (Table 2) as has been described for embryonic development (Table 1). The effects seem to be slightly more pronounced, because in the case of 3H-arginine, a dose of 2.5 kBq/ml already impaired cell proliferation significantly.
Discussion
Tritium can be incorporated into organic molecules when tritiated water is used for metabolic processes by microorganisms or plants [17]. Tritium as part of a specific organic molecule may represent an underestimated hazard for human beings. This derives from the fact that whenever trit-
269 3
Fig. 1 Incorporation of H-arginine and 3H-thymidine by onecell mouse embryos depending on the cell-cycle stage exposed
Table 1 Effects of various exposure conditions on one-cell mouse embryos with 3H-thymidine (Thym) or 3H-arginine (Arg) on embryonic development. (Each number is based on 4 – 8 independent experiments with about 10 embryos per experiment; difference to control significant at *P < 0.05 or **P < 0.01)
Treatment Agent G1 phase Arg
Thym S phase Arg
Thym
G2 phase Arg
Thym
Activity (kBq/ml) 0 2.5 12.5 25 50 0 100 0 2.5 12.5 0 100 200 0 2.5 12.5 0 100
No. of one-cell embryos
Percentage of embryos developing to the: two-cell
four-cell
eight-cell
blastocyst stage
80 80 80 80 80
93.8 91.3 87.5 80.0** 38.8**
92.5 85.0 60.0** 37.5** 16.3**
92.5 83.8 37.5** 8.8** 0**
87.5 82.5 0** 0** 0**
83 74
92.2 95.2
40 38 44
95.0 92.1 88.6
90.0 84.2 61.4**
87.5 78.9 40.9**
85.9 75.5 0**
86 70 60
91.9 85.7 76.7*
87.8 78.6 61.7**
84.9 55.7** 31.7**
83.3 51.7** 8.3**
30 42 39
90.0 90.5 89.7
83.3 81.0 64.1**
80.0 73.8 46.2**
81.6 71.4 0**
83 63
92.2 92.6
ium is incorporated into molecules of the cell nucleus, almost the entire energy of the β-decay is deposited within the genome of the cell. Just one decay of tritium is estimated to result in a dose of 1.7 mGy [18]. As one-cell embryos are particularly threatened by environmental noxes, because there is no possibility to replace a lethally hit cell by another one, we studied in detail the impact of shorttime exposures of 3H-thymidine and 3H-arginine on this early embryonic stage. As expected, incorporation of 3H-thymidine into the cell nucleus was restricted to S-phase. This was different for
3
– –
– –
– –
– –
83.4 86.4
83.4 83.8
H-arginine, because it was integrated into nuclear proteins, mostly histone molecules, irrespective of the cell cycle stage exposed. This is in accordance with data of various authors which show that histone synthesis in embryos is not restricted to a specific time interval of the cell cycle as in most cell types, but occurs throughout the entire cycle [19–21]. There are several reasons which may explain why less activity of 3H-thymidine than 3H-arginine is incorporated during S-phase exposure: faster transport of the amino acid, smaller intracellular pool of the amino acid; higher frequency of incorporation sites for arginine (many
270 Table 2 Effects of various exposure conditions of one-cell mouse embryos with 3H-thymidine (Thym) or 3H-arginine (Arg) on cell proliferation. (Each number is based on 3 – 5 independent experiments with about 10 embryos per experiment; the total number of embryos for each data point is given in parentheses; difference to control significant at **P < 0.01)
Treatment Agent Arg
Thym
Cell cycle stage exposed Activity (kBq/ml) 0 2.5 12.5 0 100 100 100
Cells per embryo 72 h p.c.
144 h p.c.
G1 G1
15.3 ± 1.8 (48) 15.4 ± 2.0 (43) 12.8 ± 3.1** (38)
71.5 ± 7.5 (54) 38.9 ± 9.2** (51) Embryos degenerated
G1 S G2
13.6 ± 3.1 14.3 ± 2.7 13.3 ± 3.2 12.9 ± 2.4
67.8 ± 11.6 (46) 63.5 ± 12.5 (40) 28.9 ± 8.9** (53) 68.5 ± 9.2 (35)
different protein molecules) compared with thymidine (restriction to DNA). The turnover, again, was different for both molecules: incorporated 3H-thymidine did not decline over the course of time, whereas 3H-arginine showed a half-life of about 24 h. This resulted in the fact that based upon the initially incorporated activity, 3H-thymidine was more effective than 3H-arginine. However, due to the comparatively low incorporation of 3H-thymidine even during S phase, 3 H-arginine was more effective when the comparison is related to the activity to which the embryos were exposed. These differences also have to be taken into consideration when comparing the results of different experimental designs. Thus, the continuous exposure chosen by Clerici et al. [6] and by Müller et al. [9] resulted in a markedly higher risk exerted by 3H-arginine when compared with 3H-thymidine based upon the applied activities. In that case, the differences in half-life are irrelevant, because arginine was present all the time. It is much more important in this situation that 3H-arginine is incorporated into histones irrespective of the cell cycle stage exposed, whereas 3H-thymidine incorporation is restricted to cells in S-phase. A rough estimation of the radiation dose delivered by the decay of incorporated 3H-thymidine resulted in about 1.5 Gy per cell nucleus up to the end of the first mitosis and about 6 Gy up to the end of the second mitosis (calculations were based upon the data published by Yamada and Ohyama [18]). Unfortunately, it was not possible to calculate radiation doses from the activity of 3H-arginine applied, because in contrast to 3H-thymidine it is much more difficult to make reasonable assumptions for dose estimation (3H-arginine is not restricted to the cell nucleus; the size of the cell nucleus varies during the cell cycle and at different developmental stages; re-utilization of cytoplasmic activity during S phase, when new histones are incorporated into the chromatin). Thus, at best, the estimated total doses differ by a factor of about 50 depending on the assumptions made. Such a broad range makes any dose estimation under the conditions of the experiments presented here useless. The results of our study show that even very brief exposures of one-cell embryos to certain tritiated organic molecules (e.g. arginine) incorporated into molecules of
(37) (34) (47) (28)
the cell nucleus may have deleterious effects at comparatively low activities. Acknowledgement This work was funded by the Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit.
References 1. Snow MHL (1973) Abnormal development of pre-implantation mouse embryos grown in vitro with 3H-thymidine. J Embryol Exp Morph 29:601–615 2. Horner D, McLaren A (1974) The effect of low concentrations of [3H]-thymidine on pre- and postimplantation mouse embryos. Biol Reprod 11:553–557 3. Kelly SJ, Rossant J (1976) The effect of short-term labelling in [3H] thymidine on the viability of mouse blastomeres: alone and in combination with unlabelled blastomeres. J Embryol Exp Morph 35:95–106 4. Streffer C, Elias S, Beuningen D van (1977) Comparative effects of tritiated water and thymidine on the preimplanted mouse embryo during cultivation in vitro. Curr Top Radiat Res Quart 12:182–193 5. Spindle A, Wu K, Pedersen RA (1982) Sensitivity of early mouse embryos to [3H] thymidine. Exp Cell Res 142:397–405 6. Clerici L, Carroll MJ, Merlini M, Vercellini L, Campagnari F (1984) The toxicity of tritium: the effects of tritiated amino-acids on preimplanted mouse embryos. Int J Radiat Biol 45:245–250 7. Matsuda Y, Yamada T, Tobari I (1986) Chromosome aberrations induced by tritiated water or 60Co gamma-rays at early pronuclear stage in mouse eggs. Mutat Res 160:87–93 8. Müller W-U, Spindle A (1986) Induction of sister chromatid exchange in preimplantation mouse embryos in vitro by 3H-thymidine or ultraviolet light in combination with caffeine. Teratogenesis Carcinog Mutagen 6:107–114 9. Müller W-U, Streffer C, Molls M, Glück L (1987) Radiotoxicities of 3H-thymidine and of 3H-arginine compared in mouse embryos in vitro. Radiat Res 110:192–198 10. Olsen WM, Storeng R (1987) Effects of 3H-thymidine on preimplantation mouse embryos in vitro. Pharm Toxicol 61: 203–208 11. Molls M, Zamboglou N, Streffer C (1983) A comparison of the cell kinetics of pre-implantation mouse embryos from two different mouse strains. Cell Tissue Kinet 16:277–283 12. Kasai K, Minato Y, Toyoda Y (1978) Fertilization and development in vitro of mouse eggs from inbred strains and F1 hybrids. Jpn J Anim Reprod 24:19–22 13. MacQueen HA (1979) Lethality of radioisotopes in early mouse embryos. J Embryol Exp Morph 52:203–208
271 14. Mikamo K, Kamiguchi Y (1983) A new assessment system for chromosomal mutagenicity using oocytes and early zygotes of the Chinese hamster. In: Ishihara T, Sasaki MS (eds) Radiationinduced chromosome damage in man. Alan R. Liss, New York, pp 411–432 15. Tarkowski AK (1966) An air-drying method for chromosome preparations from mouse eggs. Cytogenetics 5:394–400 16. Sachs L (1984) Angewandte Statistik, 6th edn. Springer, Berlin Heidelberg New York, p 263 17. Sweet CW, Murphy CE Jr, Lorenz R (1983) Environmental tritium transport from an atmospheric release of tritiated water. Health Phys 44:13–18 18. Yamada T, Ohyama H (1988) Effects of organically bound tritium (OBT) on pre-implantation mouse embryos in vitro. In: Pro-
ceedings of the 3rd Japan-US Workshop on Tritium Radiobiology, pp 274–279 19. Siracusa G, Coletta M, Monesi V (1975) Duplication of DNA during the first cell cycle in the mouse embryo. J Reprod Fertil 42:395–398 20. Gallwitz D (1981) Zellteilung und Zellzyklus. Naturwissenschaften 68:163–169 21. Kaye PL, Wales RG (1981) Histone synthesis in preimplantation mouse embryos. In: Glasser SR, Bullock DW (eds) Cellular and molecular aspects of implantation. Plenum Press, New York, pp 407–414
