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 extracellular ligands represents a novel mechanism of steroid receptor regulation. We have assessed the effects of epidermal growth factor (EGF), transforming 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 adenocarcinoma cells. EGF, TGF alpha, IGF-I, and estradiol (E(2)) enhanced transcription 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 synergistic activation of transcription from an ERE, whereas an additive response was observed with combinations of IGF-I and TGF alpha or EGF. Tetradecanoyl 12-phorbol 13-acetate (TPA), a protein kinase C (PKC) activator, stimulated ERE-mediated transcription, and this effect was inhibited by ICI 164,384. Bisindolyl-maleimide, a relatively specific inhibitor of PKC, completely antagonized TPA-induced transcription, but did not affect the response to TGF alpha, IGF-I, or E(2). The combination of TPA with E(2) in transcriptional synergism was inhibited by ICI 164,384; conversely, the combination of TPA with either TGF alpha or IGF-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 saturation 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 activation in the presence of a deletion mutant that lacked the entire A/B domain; however, synergism between IGF-I and E(2) was observed with this mutant. Therefore, ligand-independent activation of ER-dependent transcription by IGF-I is predominantly mediated through activation function I by a mechanism distinct from that of E(2).