Estrogen and the growth of breast cancer: new evidence suggests indirect action.

The growth of the MCF-7 human breast cancer cell line is unresponsive to the presence of estrogen in culture media. Paradoxically, in nude mice, growth of these cells and formation of solid tumors are dependent on estrogen. Tumors fail to develop in ovariectomized mice, but do develop in intact mice and in ovariectomized mice given estrogen. Primary cultures derived from MCF-7 tumors revert to unresponsiveness to estrogen. However, when these cultures are again transplanted into nude mice, estrogen is required for tumor formation. The continuous culture, the solid tumor, and the primary cultures therefrom have similar estrogen-binding capacities and affinities. These results indicate that mammary carcinoma cell growth in vivo is subject to inhibition that can be overcome by estrogen.