RESUMO
In the present study, we analyzed the influence of retinoic acids on the expression of alpha-1 acid glycoprotein (AGP). We show that in rat primary hepatocytes, 9-cis retinoic acid and all-trans retinoic acid increase AGP gene expression at the transcriptional level. Transient transfections of rat primary hepatocytes with a reporter construct driven by the rat AGP gene promoter indicated that retinoids regulate AGP gene expression via the -763/-138 region of the AGP promoter. Furthermore, cotransfection experiments with retinoic acid receptor alpha (RARalpha) and retinoid X receptor alpha (RXRalpha) expression vectors in NIH3T3 cells demonstrated that both RXRalpha/RXRalpha homodimer and RXRalpha/RARalpha heterodimer are competent for ligand-induced transactivation of the AGP promoter. Unilateral deletion and site-directed mutagenesis identified two retinoic-acid responsive elements (RARE), RARE-I and RARE-II, which interestingly correspond to a direct repeat of two TGACCT-related hexanucleotides separated by a single bp only (DR1-type response element). Cotransfection assays showed that RXRalpha and RARalpha activate AGP gene transcription through these two elements either as a homodimer (RXRalpha/RXRalpha) or as a heterodimer (RXRalpha/RARalpha). The RXRalpha/RXRalpha homodimer acts most efficiently through the RARE-I response element to promote AGP transactivation, whereas the RXRalpha/RARalpha heterodimer mediates transactivation better via the RARE-II responsive element.