THE PROXIMAL ELEMENT OF THE HUMAN APOLIPOPROTEIN A-II PROMOTER INCREASES THE ENHANCER ACTIVITY OF THE DISTAL REGION

We have previously shown that human apolipoprotein A-II (apoA-II) tran scription is controlled by a complex set of regulatory elements. In th is study, we demonstrate that the distal region of the apoA-II promote r (-911/-614) acts as an enhancer and results in a 6-fold increase in activity when the proximal AB element is inserted between this enhance r and a TATA box. The AB element alone does not display any transcript ional activity, The combination of the proximal AB element and the enh ancer is sufficient to activate transcription to the same level as tha t achieved with the full-length promoter, DNA binding and competition assays, and binding interference experiments allowed us to identify tw o adjacent binding sites within the AB element. These bind activities designated CIIIB1 and AIIAB3/4, respectively. Mutation on each of thes e sites showed that the binding site of CIIIB1 within the AB element p layed a major role in the interaction with the enhancer. Whereas trans criptional activation of other apolipoprotein genes involves the bindi ng of the liver-enriched hepatocyte nuclear factor 4 (HNF4) on their p roximal promoter, the present study demonstrates that neither HNF4 nor ApoA-I regulatory protein 1, its antagonistic orphan receptor, was ab le to bind the AB element, Instead, apoA-II transcription was driven b y the interaction of apoA-II enhancer with proximal AB element, which involves an unidentified activity, CIIIB1.