PEPTIDE GROWTH-FACTOR CROSS-TALK WITH THE ESTROGEN-RECEPTOR REQUIRES THE A B DOMAIN AND OCCURS INDEPENDENTLY OF PROTEIN-KINASE-C OR ESTRADIOL/

Modulation of steroid receptor-dependent transcription by extracellula r ligands represents a novel mechanism of steroid receptor regulation. We have assessed the effects of epidermal growth factor (EGF), transf orming growth factor-alpha (TGF alpha), and insulin-like growth factor I (IGF-I) on transcription from consensus estrogen response elements (ERE) in estrogen receptor (ER)-positive BG-1 human ovarian adenocarci noma cells. EGF, TGF alpha, IGF-I, and estradiol (E(2)) enhanced trans cription in a dose-dependent manner using either a strong or a minimal promoter, and ICI 164,384, a specific ER antagonist, inhibited these responses. Combinations of E(2) with TGF alpha or IGF-I induced synerg istic activation of transcription from an ERE, whereas an additive res ponse was observed with combinations of IGF-I and TGF alpha or EGF. Te tradecanoyl 12-phorbol 13-acetate (TPA), a protein kinase C (PKC) acti vator, stimulated ERE-mediated transcription, and this effect was inhi bited by ICI 164,384. Bisindolyl-maleimide, a relatively specific inhi bitor of PKC, completely antagonized TPA-induced transcription, but di d not affect the response to TGF alpha, IGF-I, or E(2). The combinatio n of TPA with E(2) in transcriptional synergism was inhibited by ICI 1 64,384; conversely, the combination of TPA with either TGF alpha or IG F-I elicited a response only equal to the maximal TPA response. Thus, peptide growth factors elicit ER-dependent transcription independently of PKC; however, there may be a common mechanistic component, as satu ration of response was observed. Finally, activation of ERE-dependent transcription in Chinese hamster ovary cells by IGF-I was observed in the presence of a mutant receptor that lacks estrogen-binding activity . The effects of both IGF-I and E(2) were dependent on the ability of the ER to bind to DNA. IGF-I elicited only weak transcriptional activa tion in the presence of a deletion mutant that lacked the entire A/B d omain; however, synergism between IGF-I and E(2) was observed with thi s mutant. Therefore, ligand-independent activation of ER-dependent tra nscription by IGF-I is predominantly mediated through activation funct ion I by a mechanism distinct from that of E(2).