TRANSCRIPTIONAL REPRESSION OF APOLIPOPROTEIN AI GENE-EXPRESSION BY ORPHAN RECEPTOR ARP-1

Expression of the apolipoprotein AI (apoAI) gene in the liver is contr olled by a liver specific enhancer. The function of this enhancer depe nds on synergistic interactions between transcription factors bound to at least three sites (designated A, B, and C) located within this enh ancer. We have previously shown that an apoAI gene reporter construct containing the entire enhancer is expressed efficiently in a hepatoma cell line and that its activity is repressed by the orphan receptor AR P-1. Moreover, repression by ARP-1 is overcome by the retinoid X recep tor RXR alpha in the presence of retinoic acid. In this study, we show that ARP-1 represses the apoAI prometer by binding to site A of the a poAI liver specific enhancer, the repression being a promoter context- specific event. Mapping analysis of ARP-1 indicated that its DNA bindi ng domain is essential but not sufficient for repression. Two separate repression domains located at the amino- and carboxyl terminal halves of ARP-1 were found to individually complement the DNA binding domain for efficient repression. We also demonstrate the reversibility of AR P-1 repression by transcription factors C/EBP and Egr-1, which might a lso be involved in apoAI gene expression. Significantly, repression by ARP-1 was found to be a prerequisite for C/EBP-mediated transactivati on. We interpret our results in terms of a model in which ARP-1 repres sion via its interaction with site A is an obligatory intermediate ste p in switching from one activated state of the apoAI gene to another.