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.
Harnish
D C
DC
Department of Cardiovascular Molecular Biology, Lederle Laboratories, Pearl River, New York 10965.
Malik
S
S
Karathanasis
S K
SK
D000818:Animals; D016632:Apolipoprotein A-I / Q000096:biosynthesis*; D001483:Base Sequence; D001665:Binding Sites; D016466:CHO Cells; D006528:Carcinoma, Hepatocellular; D002460:Cell Line; D002522:Chlorocebus aethiops; D006224:Cricetinae; D004247:DNA / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000378:metabolism*; D005786:Gene Expression Regulation; D006367:HeLa Cells; D051542:Hepatocyte Nuclear Factor 3-beta; D006801:Humans; D007668:Kidney; D008099:Liver / Q000378:metabolism*; D008113:Liver Neoplasms; D008156:Luciferases / Q000096:biosynthesis; D008969:Molecular Sequence Data; D009687:Nuclear Proteins / Q000378:metabolism*; D009838:Oligodeoxyribonucleotides; D009928:Organ Specificity; D011401:Promoter Regions, Genetic; D014157:Transcription Factors / Q000378:metabolism*; D014158:Transcription, Genetic; D014162:Transfection
D016632:Apolipoprotein A-I; D004268:DNA-Binding Proteins; C496502:FOXA2 protein, human; D009687:Nuclear Proteins; D009838:Oligodeoxyribonucleotides; D014157:Transcription Factors; D051542:Hepatocyte Nuclear Factor 3-beta; D004247:DNA; D008156:Luciferases