ANTIESTROGENS INCREASE PROTEIN-TYROSINE-PHOSPHATASE ACTIVITY IN HUMANBREAST-CANCER CELLS

Growth of human breast cancer cells is controlled by multiple interact ing factors that trigger different intracellular signaling pathways. T he nonsteroidal antagonist 4-hydroxytamoxifen (OH-Tam), which acts as an antiestrogen, is also able to inhibit the mitogenic activity of epi dermal growth factor (EGF) on hormone-responsive MCF7 cells. To furthe r characterize the mechanism of this antigrowth factor activity, which is accompanied by an increase of high-affinity EGF binding and a dras tic decrease in EGF receptor autophosphorylation, we studied the effec t of OH-Tam on protein tyrosine phosphatase (PTPase) activity with spe cific in vitro assays using two different substrates. OH-Tam increased membrane PTPase activity in a time- and dose-dependent fashion wherea s cytoplasmic enzyme activity remained unchanged. The increase in PTPa se activity was mediated by the estrogen receptor (ER) since it was re stricted to ER-positive cells, and the optimal OH-Tam concentration (E D(50) = 1 nM) was correlated with the ligand affinity for ER. The incr ease in enzyme activity was selectively obtained with nuclear receptor ligands (OH-Tam, ICI 164,384) that inhibited growth factor-induced pr oliferation, whereas other inhibitors of estrogenic responses such as synthetic progestins and antiprogestins had no effect. The time course of stimulation (maximal stimulation at day 4) was concomitant to the loss of EGF mitogenic response. Moreover, addition of a specific PTPas e inhibitor (5 mu M sodium orthovanadate) to intact cells in culture p revented OH-Tam inhibition of cell proliferation, suggesting that thes e two events are closely associated. Evaluation of PTPase activity in the treated/untreated membrane mix indicated that regulation of enzyme activity by OH-Tam was not due to a general activation process but ra ther implied a preferential increase in some specific enzymes as revea led by HPLC. Our results altogether strongly suggest that inhibition o f growth factor-stimulated proliferation by OH-Tam involves regulation of membrane PTPase activity.