Using a combination of hormone-binding assays, immunologic techniques, and mRNA hybridizations we have measured the estrogen receptor (ER) content and studied the hormonal regulation of ER mRNA in one estrogen responsive and one estrogen unresponsive breast cancer cell line, MCF-7 and T47D(CO) respectively. Estradiol binding could be detected in cytosol from MCF-7 cells but not in T47D(CO) cells. However, when measured by an enzyme-linked immunosorbent assay, T47D(CO) cells were found to contain approximately 15 fmol ER/mg cytosolic protein or 10% of the ER content in MCF-7 cells. Immunologically reactive ER in T47D(CO) cells was indistinguishable in size (≃68 KD) from the ER in MCF-7 cells, as shown by Western blotting using a monoclonal antihuman ER antibody. Quantification of ER mRNA in MCF-7 and T47D(CO) cells indicated that T47D(CO) cells contained approximately 50% of the ER mRNA levels found in MCF-7 cells. This basal level of ER mRNA in T47D(CO) cells was not decreased by estradiol treatment, as opposed to in MCF-7 cells where estradiol caused 40-60% decrease in the ER mRNA expression. Also, estradiol did not increase the progesterone receptor (PR) mRNA levels in T47D(CO) cells whereas in MCF-7 cells an approximately 5-fold increase of the PR mRNA levels occurred after estradiol treatment. However, incubation of the cells with the synthetic progestin R5020 decreased the ER mRNA levels to approximately the same degree in both cell lines. In conclusion, we have shown that estrogen down-regulates ER mRNA and up-regulates PR mRNA in MCF-7 cells. Neither of these estrogenic effects were seen in T47D(CO) cells. It appears that the steroid-resistance in T47D(CO) cells does not occur as a consequence of a complete absence of ER mRNA or protein. Since T47D(CO) cells do not bind estrogen one may speculate that estrogen resistance in these cells could be due to the production of a defective receptor protein.
ASJC Scopus subject areas
- Molecular Biology