TYROSINE KINASE P21(RAS)/MAP-KINASE PATHWAY ACTIVATION BY ESTRADIOL-RECEPTOR COMPLEX IN MCF-7 CELLS/

The mechanism by which estradiol acts on cell multiplication is still unclear, Under conditions of estradiol-dependent growth, estradiol tre atment of human mammary cancer MCF-7 cells triggers rapid and transien t activation of the mitogen-activated (MAP) kinases, erk-1 and erk-2, increases the active form of p21(ras), tyrosine phosphorylation of Shc and p190 protein and induces association of p190 to p21(ras)-GAP, Bot h Shc and p190 are substrates of activated src and once phosphorylated , they interact with other proteins and upregulate p21(ras), Estradiol activates the tyrosine kinase/p21(ras)/MAP-kinase pathway in MCF-7 ce lls with kinetics which are similar to those of peptide mitogens, It i s only after introduction of the human wild-type 67 kDa estradiol rece ptor cDNA that Cos cells become estradiol-responsive in terms of erk-2 activity, This finding, together with the inhibition by the pure anti -estrogen ICI 182 780 of the stimulatory effect of estradiol on each s tep of the pathway in MCF-7 cells proves that the classic estradiol re ceptor is responsible for the transduction pathway activation, Transfe ction experiments of Cos cells with the estradiol receptor cDNA and in vitro experiments with c-src show that the estradiol receptor activat es c-src and this activation requires occupancy of the receptor by hor mone. Our experiments suggest that c-src is an initial and integral pa rt of the signaling events mediated by the estradiol receptor.