A radiation resistance factor in cultured Cloudman S91 mouse melanoma cells

The gamma-ray survival of a radiation-sensitive amelanotic subclone of Cloudman S91 mouse melanoma, S91/amel, is increased by the presence in the tissue culture dishes of heavily irradiated cells from the same cell line (amel-HRCells) and from clonally related melanotic S91/I3 radioresistant cells (I3-HRCells). The D0 of the target S91/amel cells increases from 1.25 to 2.08 Gy in the presence of 60,000 heavily irradiated S91/amel cells per dish. The presence of I3-HRCells in dishes of target S91/I3 cells does not increase their radioresistance. Comparable numbers of I3-HRCells are more effective than amel-HRCells at increasing survival of target S91/amel cells irradiated with 3 Gy of gamma rays. Conditioned medium from the S91 melanoma cells also increases the radioresistance of S91/amel, but is not as effective as the HRCells. Unirradiated cells can condition the medium as effectively as irradiated cells. It is concluded that the radiosensitive mouse melanoma cell line is made significantly more resistant by a diffusible cellular factor(s) elaborated more proficiently by radiation-resistant cells.