Interaction of ionizing irradiation with steroid receptors in human breast cancer cells.

Human mammary tumor cells in continuous culture (MCF-7 cells) are hormone- and radiosensitive. The interaction of both factors is analyzed. Ionizing irradiation lowers the concentration of both the estradiol and progesterone receptors per cell. The reduction is dose dependent. However, the effects on the cytoplasmic and nuclear forms of the receptors are not similar. For the estradiol receptor, an accumulation in the nuclear fraction is observed 48 hr after irradiation when no appreciable amounts of estrogens are present. After administration of 10(-8) M estradiol, the cytoplasmic clearance is comparable to the unirradiated controls. However, nuclear accumulation is impaired. The processing of the nuclear estrogen receptor remains identical. Nuclear progesterone receptor is not significantly increased due to irradiation in the absence of progestins. Cytoplasmic decrease after incubation with progestins is unaffected. Again, nuclear accumulation is impaired in contrast to the unchanged processing of the nuclear form of the progesterone receptor. A decrease in "nuclear acceptor sites" for both receptors after irradiation may be an explanation for these observations. No significant effects of ionizing irradiation are observed in the initial steps of steroid hormone action.