ACTIVATION OF THE UNLIGANDED ESTROGEN-RECEPTOR BY EGF INVOLVES THE MAP KINASE PATHWAY AND DIRECT PHOSPHORYLATION

The estrogen receptor (ER) can be activated as a transcription factor either by binding of cognate estrogenic ligand or, indirectly, by a va riety of other extracellular signals, As a first step towards elucidat ing the mechanism of 'steroid-independent activation' of the ER by the epidermal growth factor (EGF), we have mapped the ER target domain an d determined the signaling pathway, We show that the N-terminal transc riptional activation function AF-1, but not the C-terminal AF-2, is ne cessary for the EGF response, Both the EGF-induced hyperphosphorylatio n and the transcriptional activation of the unliganded ER depend on a phosphorylatable serine residue at position 118, However, its phosphor ylation is not sufficient and, hence, there must be other target domai ns or proteins which fulfill an additional requirement for EGF signali ng through the ER, Using dominant-negative Ras and MAP kinase kinase ( MAPK kinase) and constitutively active MAPK kinase mutants, we show th at EGF activates the ER by signaling through the MAPK pathway suggesti ng that MAPK directly phosphorylates the critical serine 118, Our resu lts also imply that the steroid-independent activation of a variety of ER mutants, which arise during the malignant progression of breast tu mors, may contribute to tamoxifen resistance.