Kinetics of Syrian Hamster Cells during X-Irradiation Enhancement of Transformation in Vitro by Chemical Carcinogen

Abstract

The X-irradiation of cultured Syrian hamster cells followed by exposure to a chemical carcinogen after seeding for colony formation results in enhancement of transformation ordinarily associated with the chemical. Maximum enhancement occurred when 48 h separated the 2 treatments and when 250 R was used. With different concentrations of benzo(a)pyrene (0.1-5 .mu.g/ml medium) a similar factor of enhancement was obtained with 250 R, which averaged 8.35, implying that irradiation pretreatment caused a constant multiplying factor. These results are consistent with the interpretation that transformation occurs through interactions in keeping with a 1-hit hypothesis. The transitory effect of irradiation and the lack of transformation with irradiation alone argues against the selection of a special radiation-sensitive cell. At the time of maximum enhancement (48 h), the cell cycles of irradiated and nonirradiated cells do not differ. The addition of benzo(a)pyrene to irradiated cells does not alter the number of cells capable of synthesizing DNA or the flow of cells from G1 to S. The number of cells accumulated in mitosis also does not appear to differ between irradiated and nonirradiated cultures treated with 2.5 .mu.g benzo(a)pyrene. Chromosome aberrations or alterations in ploidy cannot account for the significant increase in enhancement observed at 48 h postirradiation. Twice as many cells with chromosome or chromatid gaps were found at 48 h as compared to 24 or 72 h postirradiation.