Bioactivities, Estrogen Receptor Interactions, and Plasminogen Activator-inducing Activities of Tamoxifen and Hydroxy-tamoxifen Isomers in MCF-7 Human Breast Cancer Cells

Tamoxifen is used widely in the treatment of endocrine-responsive breast cancers in humans. Studies were undertaken to examine the biological character (estrogenicantiestrogenic properties) and estrogen receptor (ER) interaction of the cis- and trans-isomers of tamoxifen and hydroxytamoxifen in MCF-7 human breast cancer cells. For each compound, the following parameters were monitored: affinity for ER and effects on cellular ER levels; stimulation-inhibition of cell growth, plasminogen activator activity, and cellular progesterone receptor levels; and isomer interconversion and metabolism in vitro. The relative binding affinities of the compounds cis-tamoxifen, trans-tamoxifen, cis-hydroxytamoxifen, and trans-hydroxytamoxifen for cytosol ER were 0.3, 2.5, 1.8, and 310%, respectively, in which the affinity of estradiol is considered 100%. cis-tamoxifen behaved as a weak estrogen agonist in all assays, while transtamoxifen was an effective estrogen antagonist. cis-tamoxifen behaved like estradiol in stimulating MCF-7 cell growth and increasing plasminogen activator activity and cellular progesterone receptor content, although very much higher concentrations of cis-tamoxifen (10-6 m) were needed to achieve the levels of stimulation observed with 10'10 in estradiol. trans-Tamoxifen and trans-hydroxytamoxifen suppressed cell growth, inhibited plasminogen activator activity of control cells, and suppressed estradiot-stimulation of plasminogen activator activity, and they evoked minimal increases in cellular progesterone receptor levels. trans-Hydroxytamoxifen had a 100-fold increased affinity for ER and was approximately 100-times more potent than was trans-tamoxifen in suppressing cell growth and plasminogen activator activity. cis-Hydroxytamoxifen behaved as an estrogen antagonist, suppressing cell growth and plasminogen activator activity, and it elicited submaximal increases in progesterone receptor levels. This apparently paradoxical behavior of cis-hydroxytamoxifen was shown to be due to the fact that the cisand trans-hydroxytamoxifens readily undergo isomeric interconversion upon exposure to our cell culture conditions, resulting in substantial accumulation of the higher-affinity trans-hydroxytamoxifen in the nuclear ER fraction of cells. In contrast to the facile interconversion of the hydroxytamoxifen isomers, there is no metabolism or interconversion of the parent compounds cis-and trans-tamoxifen in vitro. Hence, by the criteria we have used, the biological characters of trans-tamoxifen and trans-hydroxytamoxifen are similar, the major difference being the approximately 100-fold enhanced potency of the hydroxylated form. In contrast, cis-tamoxifen is an estrogen with a biopotency roughly proportional to its ER binding affinity. The apparent estrogenantagonistic character of cis-hydroxytamoxifen appears attributable to isomerization to trans-hydroxytamoxifen in vitro. With all tamoxifen isomers, there was a good correlation between suppression or stimulation of plasminogen activator activity and suppression or stimulation of cell growth, suggesting that cell-associated plasminogen activator activity may serve as a good marker for estrogen action in breast cancer cells.