EXTINCTION OF INSULIN-LIKE GROWTH-FACTOR-I MITOGENIC SIGNALING BY ANTIESTROGEN-STIMULATED FAS-ASSOCIATED PROTEIN-TYROSINE PHOSPHATASE-1 IN HUMAN BREAST-CANCER CELLS

Steroidal (ICI 182, 780) and nonsteroidal hydroxytamoxifen (OH-Tam) an tiestrogens inhibit growth factor-mitogenic activity in MCF 7 estrogen receptor-positive human breast cancer cells. Cell inhibition is corre lated with-an increase in membrane protein tyrosine phosphatase (PTP) activity, and the addition of orthovanadate prevents OH-Tam inhibition . After RT-PCR cloning of PTPs expressed in MCF 7 cells with primers t o their catalytic domains, we have shown, by differential screening, t hat the expression of two enzymes, leukocyte common antigen-related PT P (LAR) and Fas-associated PTP-1 (FAP-1), was modulated by antiestroge ns. By comparative RT-PCR, in situ hybridization, and Northern blot, L AR and FAP-1 mRNAs accumulation was found to be dose-and time-dependen tly increased by antiestrogens. To further demonstrate that PTPs were key mediators of antiestrogen-inhibitory action on the growth factor p athway, a panel of stable FAP-1 transfectants expressing low to high l evels of antisense mRNAs was established. In these clones, the level o f antisense RNA expression was correlated with a reduction in basal le vels and a complete inhibition of antiestrogen-stimulated values of PT P activity. When FAP-1 expression was abolished, OH-Tam was no longer able to block insulin-like growth factor I mitogenic activity even tho ugh it remained strongly antiestrogenic. However, ICI 182,780 was stil l inhibitory, indicating that its effect was not exclusively mediated by PTP. Our data first demonstrate that a specifically regulated phosp hatase (FAP-1) is implicated in the triggering of negative proliferati on signals in breast cancer cells.