The fate of tritiated 17β-estradiol in MCF-7 cells was followed at various times during incubation at 0 and 37°. in the cold incubations 17β-estradiol (2 × 10-8m) was rapidly taken into the cells and oxidized to estrone, and both estrogens were bound to high-affinity, low-capacity sites in the cytosol. Also at 0° 17β-estradiol was found bound to receptor in the 0.4 m KCI extract of nuclei centrifuged through 2.2 m sucrose. In addition, 17β-estradiol and estrone comprise the non-salt-extractable estrogens in these purified nuclei.

Removing the exogenous tritiated 17β-estradiol and warming the cells to 37° resulted in the migration of cytosolic 17β-estradiol:receptor complex into the nucleus and in the formation of additional receptor-bound estrone in both the cytosol and nucleus. These time-related increases in bound estrone were not associated with the total cytoplasmic estrone, which remained a consistent 7.2 ± 2.8% (S.D.) of the cellular estrogen during the 1 hr at 37°.

The data indicate that, in MCF-7 cells, estrone is readily formed from 17β-estradiol even at 0° and estrone is bound with high affinity to a cytosolic macromolecule in the presence of a 10-fold intracellular excess of 17β-estradiol. Furthermore, after elevating the temperature to 37°, the bound estrone increases in both the cytosol and nucleus, whereas the bound 17β-estradiol decreases in the cytosol and peaks at 20 min in the nucleus although it decreases thereafter.

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Presented at the John E. Fogarty International Center Conference on Hormones and Cancer, March 29 to 31, 1978, Bethesda, Md. This investigation was supported in part by USPHS Research Grants CA-07177 and CA-22828 from the National Cancer Institute.

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