MECHANISMS OF 4-HYDROXYTAMOXIFEN ANTI-GROWTH FACTOR ACTIVITY IN BREAST-CANCER CELLS - ALTERATIONS OF GROWTH-FACTOR RECEPTOR-BINDING SITES AND TYROSINE KINASE-ACTIVITY

We previously demonstrated that antiestrogen 4-hydroxytamoxifen (OH-Tam) blocks the mitogenic activity of growth factors in breast cancer. We now investigate this mechanism by evaluating how OH-Tam affects growth factor binding and receptor tyrosine kinase activity. We show here that OH-Tam has an opposite effect on epidermal growth factor (EGF) and insulin-like growth factor -I (IGF-I) binding in estrogen receptor (ER) positive cells. A decrease in IGF-I binding sites may explain the reduced IGF-I mitogenic effect, whereas an increase in high affinity EGF binding associated with a decrease in in vitro receptor autophosphorylation rather favors the possibility of an alteration in EGF receptor tyrosine kinase activity. We conclude that OH-Tam may prevent growth factor action in ER+ cells both by modulating the concentration of growth factor binding sites and by altering growth factor receptor functionality. © 1990 Academic Press, Inc.