Cross-Talk between Peptide Growth Factor and Estrogen Receptor Signaling Pathways

The classical receptor for estradiol is a member of a super-family of nuclear receptors that function as hormone-regulated transcription factors. The ability of the estrogen receptor (ER)-alpha to activate target gene transcription is mediated by two transcriptional activation functions (AF): AF-1 located in the amino-terminal domain and AF-2 found in the carboxyl-terminal portion of the molecule. The ligand binding domain overlaps AF-2, and upon estrogen binding this region undergoes a conformational change that enables it to contribute to the receptor's transcriptional activity. ER activation is accompanied by increased phosphorylation, and in the absence of ligand, activators of protein kinase A or inhibitors of protein phosphatases are able to stimulate ER-dependent gene expression. More importantly, polypeptide growth factors, such as epidermal growth factor (EGF) and insulin-like growth factor-I (IGF-I), also stimulate the ER's transcriptional activity in an estrogen-independent manner. The AF-1 domain appears to be required for activation by EGF and IGF-I, and point mutation of a single phosphorylation site located within this domain inhibits the ability of growth factor to activate the ER. Thus, steroid receptor function may be regulated by estrogenic ligands as well as by pathway "cross-talk" from membrane receptors for growth factors.