ACTIVATION OF APOLIPOPROTEIN AI GENE-TRANSCRIPTION BY THE LIVER-ENRICHED FACTOR HNF-3

Liver-specific expression of the apolipoprotein AI (apoA-I) gene is co ntrolled by the coordinate action of transcription factors bound to th ree sites (A, B, and C) located within a powerful liver-specific enhan cer which spans the -222 to -110 region upstream of the apoA-I gene tr anscription 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 protei n-binding sites were tested for their ability to stimulate the apoA-I basal promoter in hepatoblastoma HepG2 cells. The results revealed tha t occupation of both sites A and B, but not C is essential for high le vel expression. Electrophoretic mobility shift assays showed that in H epG2 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 promo ter in hepatic and nonhepatic cells. HNF-3 beta binding and transactiv ation were dependent upon the close proximity of two HNF-3 beta bindin g motifs within site B. Furthermore, HNF-3 beta and HNF-4, bound to th eir cognate sites within the apoA-I enhancer exhibited strong synergy in transactivation of the apoA-I basal promoter in nonhepatic cells, h ighlighting the central role of HNF-3 beta in liver-specific transcrip tion of the apoA-I gene. It is concluded that cooperative binding of H NF-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.