INVOLVEMENT OF EARLY GROWTH-RESPONSE FACTOR EGR-1 IN APOLIPOPROTEIN AI GENE-TRANSCRIPTION

Liver-specific expression of the apolipoprotein AI (apoAI) gene is med iated by transcription factors bound to three sites (A, B, and C) in t he apoAI enhancer. Sites A and C bind various members of the nuclear r eceptor superfamily, including the orphan nuclear receptor apolipoprot ein regulatory protein-1 (ARP-1); site B binds the liver enriched fact or hepatic nuclear factor-3. The immediate early growth response facto r (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. Del etion mapping analysis revealed two Egr-1 binding sites, E1 and E2, fl anking site A. Egr-1 bound efficiently to both E1 and E2. Sp1 in HepG2 nuclear extracts bound to E2 but not E1. In HepG2 cells, E1 functione d 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 bindin g to the apoAI enhancer abolished its responsiveness to Egr-1, while t hey had only minor effects on its constitutive activity. These mutatio ns also diminished the ability of Egr-1 to overcome ARP-1-mediated rep ression. Elimination of transcription factor binding to sites A, B, or C reduced enhancer activity without affecting Egr-1 dependent activat ion. We argue that Egr-1 is recruited to the apoAI enhancer complex un der unusual circumstances, such as those prevailing during liver regen eration, to maintain apoAI transcription levels by overriding prior tr anscriptional controls.