Identification of a negative regulatory region 5' of the human apolipoprotein B promoter.

We have identified a negative regulatory region between positions -1802 and -3211 of the human apolipoprotein B gene that reduces expression of the gene. This "reducer" effect was detected in transient transfection experiments performed with hepatic (HepG2 and Hep3B) cells as well as intestinal (CaCo-2) cells. It appears to be specific for the apolipoprotein B promoter because it did not affect expression from the heterologous thymidine kinase promoter. This reducer segment operated in the presence or absence of the transcriptional enhancer from the second intron of the apolipoprotein B gene, suggesting that the protein factors involved in the enhancer and reducer effect can interact with the transcriptional machinery independently of each other. Deletion experiments further localized the positions of the negative regulatory elements in HepG2 cells: sequences between positions -2738 and -2470 together with those from -2118 to -1802 are required for the reducer activity. DNase I footprinting of these two reducer DNA segments identified several DNA sequences that are bound by nuclear proteins from HepG2 cells. Analysis of the nucleotide sequences within these footprints demonstrated a high degree of similarity with sequences within negative regulatory regions of other genes.