REPRESSION OF ESTROGEN-DEPENDENT STIMULATION OF THE OXYTOCIN GENE BY CHICKEN OVALBUMIN UPSTREAM PROMOTER TRANSCRIPTION FACTOR-I

The orphan receptor chicken ovalbumin upstream promoter transcription factor I (COUP-TF I) fully prevented not only the activation of the ox ytocin gene by retinoic acid and thyroid hormone but also completely r epressed the estrogen dependent stimulation in transfected P19 EC cell s. DNase I footprinting showed that the COUP-TF I protein bound to the 5'-flanking region of the oxytocin gene at the site of the distal com posite hormone response element, which mediates the responses to estro gen, retinoic acid, and thyroid hormone. Electrophoretic mobility shif t assay using this composite hormone response element as probe showed that COUP-TF I and the estrogen receptor competed for binding but did not form a heterodimer. The binding by COUP-TF I was stronger than the binding of the estrogen receptor. Thus, the mechanism of repression i nvolves occupancy of integrated binding sites. By mutagenesis of the c omposite hormone response element, the COUP-TF I binding site and the estrogen response element could be separated, resulting in functional dissociation of the repressive action of COUP-TF I and the induction b y estrogen. The results show that repression of gene expression by COU P-TF I is not limited to receptors that act through heterodimerization but also extends to the homodimer-forming estrogen receptor in a cont ext-dependent manner. This interaction between COUP-TF I and the estro gen receptor may provide a physiological mechanism of selective antago nism of gene regulation by estrogens.