A bifunctional regulatory element of the human ApoA-I gene responsive to a distal enhancer.

Promoter elements located up to 2 kb upstream of the apolipoprotein A-I (apoA-I) gene are necessary for apoA-I expression in liver and intestine cells in tissue culture. In transgenic mice, a distal enhancer located between the apoA-IV and apoC-III genes is additionally necessary for tissue-specific expression of apoA-I in liver and intestine. We have identified a previously uncharacterized regulatory element between 746 and 856 nucleotides 5' of the apoA-I transcription start site that differentially affects the expression of apoA-I reporter plasmids in intestine cells dependent on the presence of the distal apolipoprotein enhancer. Deletion of the -856/-746 sequence strongly repressed transcription in the presence of the apolipoprotein enhancer, but in the absence of the enhancer, deletion of the -856/-746 element increased transcription. By contrast, in liver cells, deletion of the -856/-746 element strongly repressed transcription in the presence of the distal enhancer but had no detectable effect on transcription in the absence of the distal enhancer. Electrophoretic mobility shift analysis revealed tissue-specific and sequence-specific protein-DNA complexes formed by the -856/-746 element in intestine, liver, and HeLa cell nuclear extracts. The complexes formed by extracts of intestinal cells differed from those of liver and HeLa cells by their sensitivity to DNase digestion and their pattern of protein footprints. Collectively, the data suggest that the -856/-746 sequence is a composite regulatory element that interacts with multiple proteins and the apolipoprotein distal enhancer to achieve tissue-specific expression of apoA-I.