The mechanism by which the human apolipoprotein B gene reducer operates involves blocking of transcriptional activation by hepatocyte nuclear factor 3.

Previously, we showed that when a DNA fragment extending from -3067 to -2734 of the human apolipoprotein B (apo-B) gene is inserted immediately upstream of an apo-B promoter segment (-139 to +121), transcription from this promoter is reduced by about 10-fold in cultured colon carcinoma cells (CaCo-2) but not in cultured hepatoma cells (HepG2). We postulated that this reducer operates by a mechanism involving active repression of a transcriptional activator that binds to the segment from -111 to -88 of the apo-B promoter (B. Paulweber and B. Levy-Wilson, J. Biol. Chem. 266:24161-24168 1991). In the current study, the reducer element has been localized to a 24-bp sequence from -2801 to -2778 of the apo-B gene that contains a binding site for the negative regulatory protein ARP-1. Furthermore, we have demonstrated that the transcription factor hepatocyte nuclear factor 3 alpha (HNF-3 alpha) binds to the sequence 5'-TGTTTGCTTTTC-3' from -95 to -106 of the apo-B promoter, to stimulate transcription. Transcriptional activation by HNF-3 is repressed when the reducer sequence is inserted immediately upstream of the HNF-3 binding site, suggesting a mechanism by which the reducer-bound protein blocks the activation promoted by HNF-3. Data from cotransfection experiments in which ARP-1 is overexpressed in the absence of its binding site suggest that ARP-1 interacts either directly or via a mediator protein with proteins recognizing the HNF-3 site and that this interaction is sufficient to repress transcriptional activation by HNF-3. Because transcriptional activation by Sp1 is not affected by the reducer, it is unlikely that the reducer interacts directly with basic components of the transcriptional machinery.