Involvement of early growth response factor Egr-1 in apolipoprotein AI gene transcription.
Liver-specific expression of the apolipoprotein AI (apoAI) gene is mediated by transcription factors bound to three sites (A, B, and C) in the apoAI enhancer. Sites A and C bind various members of the nuclear receptor superfamily, including the orphan nuclear receptor apolipoprotein regulatory protein-1 (ARP-1); site B binds the liver-enriched factor hepatic nuclear factor-3. The immediate early growth response factor (Egr-1), which is transiently expressed in various pathophysiologic states of the liver, activates the apoAI enhancer and overcomes ARP-1-mediated repression of the enhancer in hepatoblastoma HepG2 cells. Deletion mapping analysis revealed two Egr-1 binding sites, E1 and E2, flanking site A. Erg-1 bound efficiently to both E1 and E2. Sp1 in HepG2 nuclear extracts bound to E2 but not E1. In HepG2 cells, E1 functioned as an Egr-1 response element, whereas E2 had high basal activity and was not further induced by Egr-1. Mutations that prevent Egr-1 binding to the apoAI enhancer abolished its responsiveness to Erg-1, while they had only minor effects on its constitutive activity. These mutations also diminished the ability of Egr-1 to overcome ARP-1-mediated repression. Elimination of transcription factor binding to sites A, B, or C reduced enhancer activity without affecting Egr-1-dependent activation. We argue that Egr-1 is recruited to the apoAI enhancer complex under unusual circumstances, such as those prevailing during liver regeneration, to maintain apoAI transcription levels by overriding prior transcriptional controls.
Kilbourne
E J
EJ
Department of Cardiovascular Molecular Biology, Lederle Laboratories, Pearl River, New York 10965.
Widom
R
R
Harnish
D C
DC
Malik
S
S
Karathanasis
S K
SK
D016632:Apolipoprotein A-I / Q000235:genetics*; D001483:Base Sequence; D004268:DNA-Binding Proteins / Q000502:physiology*; D051766:Early Growth Response Protein 1; D004742:Enhancer Elements, Genetic; D005786:Gene Expression Regulation; D006801:Humans; D017874:Immediate-Early Proteins; D008969:Molecular Sequence Data; D016329:Sp1 Transcription Factor / Q000502:physiology; D014157:Transcription Factors / Q000502:physiology*; D014158:Transcription, Genetic
D016632:Apolipoprotein A-I; D004268:DNA-Binding Proteins; C497325:EGR1 protein, human; D051766:Early Growth Response Protein 1; D017874:Immediate-Early Proteins; D016329:Sp1 Transcription Factor; D014157:Transcription Factors