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J. Biol. Chem., Vol. 262, Issue 33, 15894-15899, Nov, 1987

Effects of tamoxifen and 4-hydroxytamoxifen on the pNR-1 and pNR-2 estrogen-regulated RNAs in human breast cancer cells

FE May and BR Westley
Department of Pathology, Royal Victoria Infirmary, Newcastle upon Tyne, United Kingdom.

The estrogenic and antiestrogenic activities of tamoxifen and 4- hydroxytamoxifen have been measured on the expression of two estrogen- regulated RNAs (pNR-1 and pNR-2) in the MCF7 human breast cancer cell line cultured in phenol red-free medium. The two antiestrogens increased the level of the pNR-1 RNA to about 80% of the estradiol- induced level, and the induction by estradiol was not significantly antagonized by either antiestrogen. In contrast, the pNR-2 mRNA was only increased to about 10% of the estradiol-induced level, and its induction by estradiol was antagonized by both tamoxifen and 4- hydroxytamoxifen. Thus, the two RNAs respond in dramatically different ways to these antiestrogens. 4-Hydroxytamoxifen and estradiol have similar affinities for the estrogen receptor; however, the induction of both RNAs by 4-hydroxytamoxifen required a 10-fold higher concentration than estradiol for maximum agonist activity, and a 500-fold molar excess was required to antagonize the induction by estradiol. Tamoxifen has a 20-100-fold lower affinity than estradiol for the estrogen receptor. A 200-fold higher concentration was required for maximum agonist activity and a 10,000-fold molar excess to antagonize the induction by estradiol. These results emphasize the complexity of antiestrogen action in human breast cancer cells.
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