Induction of Two Estrogen-responsive Proteins by Antiestrogens in R27, a Tamoxifen-resistant Clone of MCF7 Cells

In an attempt to approach the mechanism by which estrogen and antiestrogen regulate the growth of estrogen receptor-positive breast cancer, we have studied the effects of the antiestrogens tamoxifen (TAM) and 4-hydroxytamoxifen (OH-TAM) on the induction of two estrogen-specific proteins in a variant of MCF₇ cells termed R₂₇, cloned for its ability to grow in the presence of TAM. We found that, in the R₂₇ variant, antiestrogens as well as estradiol were able to increase specifically the production of a M(r) 52,000 protein which was released into the culture medium. This protein was shown to be identical to the M(r) 52,000 glycoprotein induced by estrogen and released by MCF₇ cells on the basis of its specific inducibility by physiological concentrations of 17β-estradiol and of its resolution in two-dimensional gel analysis. Dose-response analysis showed that, in the R₂₇ variant, TAM and OH-TAM acquired the ability to induce the M(r) 52,000 protein at concentrations compatible with their relative affinities for the estrogen receptor, while these antiestrogens were inefficient in the wild MCF₇ cells. Whereas the relative increase of the M(r) 52,000 protein was similar with TAM, OH-TAM, and 17β-estradiol, the general production of M(r) 52,000 and of total labelled proteins was less with the antiestrogens than with 17β-estradiol. Moreover, OH-TAM displayed a biphasic dose-response curve with inhibitory effects at concentrations above 10 nM, suggesting an additional mechanism. Neither TAM nor OH-TAM had any antiestrogenic effect when added in the presence of 17β-estradiol. In the R₂₇ variant, both estradiol and antiestrogens induced the progesterone receptor sites; however, the extent of the stimulation was lower with antiestrogens than with estradiol. This study shows that, in addition to the classical antiestrogen-resistant breast cancer cells, in which the estrogen receptor is absent or inactive, there is another class of antiestrogen-resistant cells in which the drug becomes a full estrogen agonist as evidenced by the induction of the M(r) 52,000 estrogen-specific protein, which is not induced in the wild-type cells.