Activation of apolipoprotein AI gene transcription by the liver-enriched factor HNF-3

Liver-specific expression of the apolipoprotein AI (apoA-I) gene is controlled by the coordinate action of transcription factors bound to three sites (A, B, and C) located within a powerful liver-specific enhancer which spans the -222 to -110 region upstream of the apoA-I gene transcription start site (+1). Sites A and C bind various members of the nuclear receptor superfamily including the liver-enriched factor HNF-4. In the current report, enhancer derivatives with mutagenized protein-binding sites were tested for their ability to stimulate the apoA-I basal promoter in hepatoblastoma HepG2 cells. The results revealed that occupation of both sites A and B, but not C is essential for high level expression. Electrophoretic mobility shift assays showed that in HepG2 cells site B is occupied by the liver-enriched factor HNF-3 beta. Binding of HNF-3 beta to site B transactivates the apoA-I basal promoter in hepatic and nonhepatic cells. HNF-3 beta binding and transactivation were dependent upon the close proximity of two HNF-3 beta binding motifs within site B. Furthermore, HNF-3 beta and HNF-4, bound to their cognate sites within the apoA-I enhancer exhibited strong synergy in transactivation of the apoA-I basal promoter in nonhepatic cells, highlighting the central role of HNF-3 beta in liver-specific transcription of the apoA-I gene. It is concluded that cooperative binding of HNF-3 beta to site B and synergistic interactions between HNF-4 and HNF-3 beta bound to their cognate sites in the apoA-I enhancer may play a fundamental role in apoA-I gene expression in liver.