Effects of fractionated X-irradiation on subsequent response to acute X-irradiation in two human tumour cell lines in vitro.

The exposure of two human tumour cell lines, one derived from a squamous cell carcinoma of the tongue (HN-1) and the other from an adenocarcinoma of the breast (MCF-7), to fractionated X-irradiation in vitro, resulted in altered sensitivity to subsequent acute X-irradiation exposure in the former but not the latter tumour cell type. The X-ray-pretreated HN-1 cells, designated HN-1/DXR11 cells, showed a significantly increased sensitivity to X-irradiation with a D0 of 0.97 Gy, compared with a figure of 1.39 Gy for the parental cells. No significant changes were noted in a number of basic cell kinetic or biological parameters in the X-ray-pretreated cells. However, this enhanced X-ray sensitivity in the HN-1/DXR11 cells was associated with decreased cellular levels of total intracellular glutathione. These findings are consistent with the theory that intracellular thiols are involved in protection from radiation damage. This is one of the first observations that prior exposure to X-irradiation can modify subsequent responses to acute X-irradiation treatment in human tumour cells.