Organization of the regulatory elements and nuclear activities participating in the transcription of the human apolipoprotein B gene.

The human apolipoprotein B (apoB) promoter region (-150 to +124) is transcriptionally active in HepG2 and CaCo-2 but not in HeLa cells. In the present study, we demonstrate that transcription of the human apoB gene in hepatic cells is achieved through the interaction of proteins that bind to five regulatory elements, I (-118 to -98), II (-112 to -94), III (-86 to -62), IV (-72 to -53), and V (-53 to -33). Elements I, IV, and V can bind to bacterially expressed enhancer-binding protein (C/EBP) although with different affinities. In addition, elements I, IV, and V interact with at least two other heat-stable nuclear factors, designated NF-BA2 and NF-BA3, that are distinct from C/EBP. These activities have been separated by ion exchange chromatography and shown to produce footprints in elements I, IV, and V that are similar to those obtained with C/EBP. Element II is recognized by three activities designated NF-BCB1, NF-BCB2, and NF-BCB3, which have been separated by heparin-Sepharose chromatography. Saturation mutagenesis of the region established that they belong to a family of proteins recognizing the same motif, 5'-AAAAGCAAACAG-3'. Substitution mutations in element II that abolish binding of all three activities in vitro also reduce hepatic transcription in vivo to 10% of control, indicating that the binding of NF-BCB1, NF-BCB2, and NF-BCB3 activities is required for efficient transcription of the apoB gene. Furthermore, element III is recognized by the previously characterized nuclear factor NF-BA1, the binding of which is essential for the transcriptional activation of the apoB gene in hepatic and intestinal cells.