Cytochrome P450RAI(CYP26) promoter: A distinct composite retinoic acid response element underlies the complex regulation of retinoic acid metabolism

The catabolism of retinoic acid (RA) is an essential mechanism for restrict ing the exposure of specific tissues and cells to RA. We recently reported the identification of a RA-inducible cytochrome P450 [P450RAI(CYP26)], in z ebrafish, mouse, and human, which was shown to be responsible for RA catabo lism. P450RAI exhibits a complex spatiotemporal pattern of expression durin g development and is highly inducible by exogenous RA treatment in certain tissues and cell lines. Sequence analysis of the proximal upstream region o f the P450RAI promoter revealed a high degree of conservation between zebra fish, mouse, and human. This region of the promoter contains a canonical re tinoic acid response element (5'-AGT-TCA-(n)(5)-AGTTCA-3'), embedded within a 32-bp region (designated R1), which is conserved among all three species . Electrophoretic mobility shift assays using this element demonstrated the specific binding of murine retinoic acid receptor-gamma (RAR gamma) and re tinoid X receptor-alpha (RXR alpha) proteins. Transient transfection experi ments with the mouse P450RAI promoter fused to a luciferase reporter gene s howed transcriptional activation in the presence of RA in HeLa, Cos-1, and F9 wild-type cells. This activation, as well as basal promoter activity, wa s abolished upon mutation of the RARE. Deletion and mutational analyses of the P450RAI promoter, as well as DNase I footprinting studies, revealed pot ential binding sites for several other proteins in conserved regions of the promoter. Also, two conserved 5'-TAAT-3' sequences flanking the RARE were investigated for their potential importance in P450RAI promoter activity. M ore-over, these studies revealed an essential requirement for a G-rich elem ent (designated CORE), located just upstream of the RARE, for RA inducibili ty. This element was demonstrated to form complexes with Sp1 and Sp3 using nuclear extracts from either murine F9 or P19 cells. Together, these result s indicate that the P450RAI-RARE is atypical in that conserved flanking seq uences may play a very important role in regulating RA inducibility and exp ression of P450RAI(CYP26).