Identification of trans-acting factors that interact with cis-acting elements present in the first nontranslated exon of the human apolipoprotein B gene.

Apolipoprotein B is the sole protein of low density lipoprotein and is produced primarily in the liver. Previously, we have identified two cis-acting elements (+20 to +40; +43 to +53) in the non-translated exon of the human apolipoprotein B gene, using DNase I footprint analysis (S. S. Chuang, H. Zhuang, S. R. Reisher, S. I. Feinstein, and H. K. Das, Biochem. Biophys. Res. Com. 215, 394-404, 1995). Wild type and mutated promoter constructs were used as templates in DNase I footprint analysis with rat liver nuclear extracts. These experiments suggest that trans-acting factors BRF-3 and BRF-4 which recognize these two footprint regions (+20 to +40; +43 to +53) respectively, act independently. In vitro-synthesized hepatocyte nuclear factors HNF-1alpha, HNF-lbeta. HNF-3alpha, and HNF-2/HNF-4 showed no specific protein/DNA interaction with these regions. DNase I footprint analysis using other DNA-binding site oligonucleotides as competitors indicated that BRF-3 and BRF-4 could be different hepatocyte nuclear factors and may contribute to the regulation of transcription of the human apolipoprotein B gene.