A cloned mouse embryo-derived fibroblast cell line was used to study morphological transformation induced by X-rays and 254-nm ultraviolet light (UV). The transformation frequency increased exponentially with increasing dose from 10 to 400 rads for X-rays and 1.0 to 7.5 J/sq m for UV exposure. Splitdose X-ray exposures led to an enhancement in transformation at total doses below 100 rads and a reduction at doses of 300 to 400 rads. The induced transformation frequency varied among serum lots and was very dependent upon the initial cell density. Spontaneous transformants were observed in 10 of 22 consecutive experiments; the spontaneous transformation frequency was generally about 1 to 2 × 10−5 as compared to induced frequencies which ranged up to 3 × 10−3 for X-rays and 7.5 × 10−4 for UV exposure. Further results indicate that this cell line has several potential advantages over the mouse 10T½ line for studies with relatively weak in vitro carcinogens such as radiation. These include (a) a reduced overall expression time for the appearance of transformed foci (4 weeks); (b) a high cloning efficiency (50 to 60%); and (c) the fact that about 20 times as many viable cells may be plated per dish for optimal results, allowing transformation frequencies as low as 10−5 to be measured easily. On the other hand, there was more variability in the results among experiments with the 3T3 cell line.

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Supported by Department of Energy Contract EE-77-S-02-4322 and NIH Grants CA-11751 and ES-00002.

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