Transactivation and repression of the α-fetoprotein gene promoter by retinoid X receptor and chicken ovalbumin upstream promoter transcription factor

Retinoic acid (RA) is widely involved in the control of cell proliferation and differentiation, as well as embryo pattern formation. Transcription of the oncodevelopmental protein, alpha-fetoprotein (AFP), is stimulated by retinoic acid (RA) in neoplastic cells. To study RA regulation of AFP gene expression, the 5'-flanking region of AFP gene was cloned and analyzed. In the present study, transfection of deletion mutants and sequence analysis revealed a retinoid X receptor response element (AFP-RXRE) located at position -139 to -127 of the AFP promoter. Synthetic AFP-RXRE was ligated into a reporter construct with the heterologous promoter and chloramphenicol acetyltransferase (CAT). AFP-RXRE conferred a marked RA responsiveness in the cotransfection with retinoid X receptor (RXR), but not with retinoic acid receptors (RARs). Consistent with these data, only RXR bound to AFP-RXRE with high affinity in the mobility shift assays. Chicken ovalbumin upstream promoter transcription factor (COUP-TF), an orphan member of the steroid/thyroid hormone superfamily, also demonstrated specific binding activity to AFP-RXRE in vitro. In cotransfection assays, COUP-TF dramatically repressed the transactivation of RXR on AFP-RXRE. The mechanism of repression by COUP-TF may involve the mutual occupancy of the AFP-RXRE binding site between RXR and COUP-TF.