Effects of γ-quanta in the 0.1-50 cGy dose range on the conformation of chromatin in mammalian cells

The effects of 137Cs γ-quanta in mammalian cells were studied by the method of anomalous viscosity time dependence (AVTD). Several different cell types were exposed: VH-10 human fibroblasts, BALB/c mouse splenocytes, and Sprague-Dowley rat thymocytes. The cells were irradiated within the 0.1-50 cGy dose range and then lysed for viscosity measurements. It was established for all types of cells that exposure to a low dose of 0.5 cGy resulted in a statistically significant reduction in viscosity peaks. This reduction reached a maximum value approximately 40-80 min after irradiation. The reduction of viscosity was revealed at doses up to 4 cGy for human fibroblasts with the maximum effect observed at about 2 cGy. The opposite response, an increase in viscosity, was observed after exposing the cells to 10-50 cGy. From the linear approximation of this dose dependence, the increase in viscosity started at doses above 4 cGy. The effect of increased viscosity disappeared with time after irradiation, with kinetics similar to that of DNA repair. Repair of this AVTD effect depended strongly on temperature in the 0-37 deg. C range. To the contrary, kinetics of the 0.5 cGy effect did not depend on temperature. The specific inhibitor of DNA topoisomerase II, etoposide VP-16, was shown to increase the AVTD peaks significantly. The AVTD technique showed the typical biphasic behaviour of chromatin due to intercalation of ethidium bromide to DNA as was previously obtained by other methods such as the halo assay or sedimentation of nucleoids. These data provide further evidence that the AVTD effects correlate with changes in chromatin conformation. Thus, two different types of the chromatin response were observed in mammalian cells after low and high doses. The reasons for observed changes in the chromatin conformation can possibly be identified as: DNA-protein interactions at low doses; DNA breaks at high doses. Main part of this data was published in: Radiat. Res., 145, 687-693, 1996. (authors)