Estrogen Regulation of the Apolipoprotein AI Gene Promoter through Transcription Cofactor Sharing

Abstract Estrogen replacement therapy increases plasma concentrations of high density lipoprotein and its major protein constituent, apolipoprotein AI (apoAI). Studies with animal model systems, however, suggest opposite effects. In HepG2 cells stably expressing estrogen receptor α (ERα), 17β-estradiol (E2) potently inhibited apoAI mRNA steady state levels. ApoAI promoter deletion mapping experiments indicated that ERα plus E2 inhibited apoAI activity through the liver-specific enhancer. Although the ERα DNA binding domain was essential but not sufficient for apoAI enhancer inhibition, ERα binding to the apoAI enhancer could not be detected by electrophoretic mobility shift assays. Western blotting and cotransfection assays showed that ERα plus E2 did not influence the abundance or the activity of the hepatocyte-enriched factors HNF-3β and HNF-4, two transcription factors essential for apoAI enhancer function. Expression of the ERα coactivator RIP140 dramatically repressed apoAI enhancer function in cotransfection experiments, suggesting that RIP140 may also function as a coactivator on the apoAI enhancer. Moreover, estrogen regulation of apoAI enhancer activity was dependent upon the balance between ERα and RIP140 levels. At low ratios of RIP140 to ERα, E2 repressed apoAI enhancer activity, whereas at high ratios this repression was reversed. Regulation of the apoAI gene by estrogen may thus vary in direction and magnitude depending not only on the presence of ERα and E2 but also upon the intracellular balance of ERα and coactivators utilized by ERα and the apoAI enhancer.