Identification of a retinoid/chicken ovalbumin upstream promoter transcription factor response element in the human retinoid X receptor γ2 gene promoter

To investigate the mechanisms involved in the transcriptional control of retinoid X receptor (RXR) gene expression, the 5'-flanking region of the human RXRγ2 isoform was characterized. An imperfect hexamer repeat (γ retinoid X response element; γRXRE) with a single nucleotide spacer (GGTTGAaAGGTCA) was identified immediately upstream of the RXRγ2 gene transcription start site. Cotransfection studies in CV-1 cells with expression vectors for the retinoid receptors RXRα and retinoic acid receptor β (RARβ) demonstrated that the γRXRE confers retinoid-mediated transcriptional activation with preferential activation by RXR in the presence of its cognate ligand, 9-cis-retinoic acid (RA). Electrophoretic mobility shift assays demonstrated that RXR homodimer binding to γRXRE is markedly enhanced by 9-cis-RA, whereas RAR-RXR heterodimer binding is ligand- independent. DNA binding studies and cell cotransfection experiments also demonstrated that the nuclear receptor, chicken ovalbumin upstream promoter transcription factor (COUP-TF), repressed transcription via the γRXRE. Cotransfection experiments revealed that COUP-TF and RXRα compete at the γRXRE to modulate transcription bidirectionally over a wide range. These results demonstrate that the human RXRγ2 gene promoter contains a novel imperfect repeat element capable of mediating RXR-dependent transcriptional autoactivation and COUP-TF-dependent repression.