Molecular Radiobiology of Human Cell Lines: IV. Variation in Ultraviolet Light and X-Ray Sensitivity during the Division Cycle

The recent introduction of quantitative techniques for measuring the lethal effects of radiation on cultured mammalian cells (1, 2) has permitted the study of the finer details of this phenomenon in a manner hitherto possible only for bacteria and a few other unicellular microorganisms. Under standard experimental conditions it was possible to obtain reproducible radiation survival curves, providing no profound genetic changes and population shifts occurred in the cell line under study (3). However, various experimental modifications, including (1) dose fractionation with concomitant more pronounced postirradiation repair (4), (2) incorporation of thymine analogs (5, 6), and (3) cell synchronization (7), resulted in pronounced changes in the slope, extrapolation number, and general shape of the normally sigmoid survival curve. The previous interest of this laboratory in the mechanism of the radiobiological response in human cell lines, in particular radiosensitization with halogenated thymidine analogs, prompted the present study on variation in radiosensitivity during the divisional cycle. Cell synchronization was achieved by temporary blockage of thymidylic acid synthesis, as first introduced for thymineless bacteria by Barner and Cohen (8) and later applied to cultured HeLa cells by Rueckert and Mueller (9). In the present application of this method, 5-fluorodeoxyuridine (FUdR) served as the specific inhibitor of thymidylic acid synthetase, since it had been previously found useful for effecting massive incorporation of the radiosensitizing thymidine analogs into the DNA of human cell cultures (5). It is shown in this paper that the radiation response of human cells varies markedly with the stage in the synchronized division cycle produced by thymidine (TdR) or 5-bromodeoxyuridine (BUdR) reversal of a 20-hour FUdR inhibition.