Antiestrogens increase protein tyrosine phosphatase activity in human breast cancer cells.

Abstract

Growth of human breast cancer cells is controlled by multiple interacting factors that trigger different intracellular signaling pathways. The nonsteroidal antagonist 4-hydroxytamoxifen (OH-Tam), which acts as an antiestrogen, is also able to inhibit the mitogenic activity of epidermal growth factor (EGF) on hormone-responsive MCF7 cells. To further characterize the mechanism of this antigrowth factor activity, which is accompanied by an increase of high-affinity EGF binding and a drastic decrease in EGF receptor autophosphorylation, we studied the effect of OH-Tam on protein tyrosine phosphatase (PTPase) activity with specific in vitro assays using two different substrates. OH-Tam increased membrane PTPase activity in a time- and dose-dependent fashion whereas cytoplasmic enzyme activity remained unchanged. The increase in PTPase activity was mediated by the estrogen receptor (ER) since it was restricted to ER-positive cells, and the optimal OH-Tam concentration (ED50 = 1 nM) was correlated with the ligand affinity for ER. The increase in enzyme activity was selectively obtained with nuclear receptor ligands (OH-Tam, ICI 164, 384) that inhibited growth factor-induced proliferation, 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 PTPase inhibitor (5 μM sodium orthovanadate) to intact cells in culture prevented OH-Tam inhibition of cell proliferation, suggesting that these 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 rather implied a preferential increase in some specific enzymes as revealed by HPLC. Our results altogether strongly suggest that inhibition of growth factor-stimulated proliferation by OH-Tam involves regulation of membrane PTPase activity. © 1994 by The Endocrine Society.