Influence of temperature on radiation-induced inhibition of DNA supercoiling

The influence of subnormal temperatures (2, 15 and 28 degrees C) on the effects of radiation in MCF-7 cell cultures was studied using the fluorescent (halo) nucleoid assay. Increasing the propidium iodide (PI) concentration (0.5-7.5 microgram/ml PI) resulted in relaxation, i.e. in increasing nucleoid area; higher concentrations up to 50 microgram/ml caused rewinding that resulted in nucleoid contraction. Rewinding was inhibited by X irradiation (2, 4 and 8 Gy) in a dose-dependent way. Incubation at subnormal temperature did not influence the nucleoid area but did reduce radiation-induced inhibition of rewinding after 4 Gy. The low temperature (2 degrees C) during rather than prior to irradiation appeared to protect from radiation-induced inhibition of nucleoid rewinding. Decreased temperature during irradiation may change the conditions so as to reduce DNA- matrix damage induced by radiation.