THE GLUCOCORTICOID RECEPTOR REGULATES THE BINDING OF C EPB-BETA ON THE ALPHA-1-ACID GLYCOPROTEIN PROMOTER IN-VIVO/

A complex interaction between the Glucocorticoid Receptor (GR), C/EBP beta, and other transcription factors activate the Alpha-1 Acid Glycop rotein (AGP) promoter in HTC(JZ-1) rat hepatoma culture cells, This ef fect is mediated by the so-called Steroid Responsive Unit (SRU) of the AGP promoter that contains several binding sites for C/EBP transcript ion factors, some of which overlap with the Glucocorticoid Responsive Element (GRE), Our in vivo footprinting experiments revealed that the GRE- and the C/EBP-binding sites were already occupied glucocorticoid dependently in HTC(JZ-1) cells 10 min after dexamethasone administrati on (10(-6) M). Furthermore, local changes in the chromatine structure shown by the appearance of DNAse I hypersensitive sites (HS sites) als o took place, These changes were probably dependent on a tissue-specif ic organization of the chromatine at the SRU because they were not det ectable in a different glucocorticoid-responsive cell line (PC12) that did not express AGP, Here, we have also shown that withdrawal of dexa methasone or addition of the anti-glucocorticoid RU486 were able to re vert the pattern induced by dexamethasone in viva, The disappearance o f the protected region and the hypersensitive sites, typical of the ho rmone activated promoter, confirmed the necessity of the GR to be boun d by the agonist and the inability of the GR-antagonist complex to bin d the DNA, By functional assays, we showed that the occupancy of the S RU by these transcriptional proteins in vivo correlated with the activ ation of the AGP gene transcription. With these results, we have shown that one of the functions of the GR to activate transcription of the AGP gene is to recruit C/EBP beta and to maintain it bound at its targ et DNA sequences (SRU), This process was not accomplished by RU486.