Control of apolipoprotein AI gene expression through synergistic interactions between hepatocyte nuclear factors 3 and 4.
Apolipoprotein AI (apoAI) gene expression in liver depends on synergistic interactions between transcription factors bound to three distinct sites (A, B, and C) within a hepatocyte-specific enhancer in the 5'-flanking region of the gene. In this study, we showed that a segment spanning sites A and B retains substantial levels of enhancer activity in hepatoblastoma HepG2 cells and that sites A and B are occupied by the liver-enriched hepatocyte nuclear factors (HNFs) 4 and 3, respectively, in these cells. In non-hepatic CV-1 cells, HNF-4 and HNF-3beta activated this minimal enhancer synergistically. This synergy was dependent upon simultaneous binding of these factors to their cognate sites, but it was not due to cooperativity in DNA binding. Separation of these sites by varying helical turns of DNA did not affect simultaneous binding of HNF-3beta and HNF-4 nor did it influence their functional synergy. The synergy was, however, dependent upon the cell type used for functional analysis. In addition, this synergy was further potentiated by estrogen treatment of cells cotransfected with the estrogen receptor. These data indicate that a cell type-restricted intermediary factor jointly recruited by HNF-4 and HNF-3 participates in activation of the apoAI enhancer in liver cells and suggest that the activity of this factor is regulated by estrogen.
Harnish
D C
DC
Department of Cardiovascular Molecular Biology, Lederle Laboratories, Pearl River, New York 10965, USA.
Malik
S
S
Kilbourne
E
E
Costa
R
R
Karathanasis
S K
SK
D000818:Animals; D016632:Apolipoprotein A-I / Q000235:genetics*; D051778:Basic Helix-Loop-Helix Leucine Zipper Transcription Factors; D001665:Binding Sites / Q000235:genetics; D002460:Cell Line; D004268:DNA-Binding Proteins / Q000008:administration & dosage* / Q000378:metabolism; D004357:Drug Synergism; D004742:Enhancer Elements, Genetic; D004958:Estradiol / Q000494:pharmacology; D005786:Gene Expression Regulation / Q000187:drug effects*; D006367:HeLa Cells; D051542:Hepatocyte Nuclear Factor 3-beta; D051557:Hepatocyte Nuclear Factor 4; D006801:Humans; D007739:L Cells; D008099:Liver / Q000187:drug effects / Q000378:metabolism; D051379:Mice; D009687:Nuclear Proteins / Q000008:administration & dosage* / Q000378:metabolism; D010750:Phosphoproteins / Q000008:administration & dosage* / Q000378:metabolism; D014157:Transcription Factors / Q000008:administration & dosage* / Q000378:metabolism
D016632:Apolipoprotein A-I; D051778:Basic Helix-Loop-Helix Leucine Zipper Transcription Factors; D004268:DNA-Binding Proteins; C496502:FOXA2 protein, human; C496503:Foxa2 protein, mouse; D051557:Hepatocyte Nuclear Factor 4; C403128:MLX protein, human; D009687:Nuclear Proteins; D010750:Phosphoproteins; C501970:Tcfl4 protein, mouse; D014157:Transcription Factors; D051542:Hepatocyte Nuclear Factor 3-beta; D004958:Estradiol