Estrogen regulation of the apolipoprotein AI gene promoter through transcription cofactor sharing

Estrogen replacement therapy increases plasma concentrations of high density lipoprotein and its major protein constituent, apolipoprotein Al (apoAI), Studies with animal model systems, however, suggest opposite effects. In HepGa cells stably expressing estrogen receptor alpha (ER alpha), 17 beta-estradiol (E2) potently inhibited apoAI mRNA steady state levels. ApoAI promoter deletion mapping experiments indicated that ER alpha plus E2 inhibited apoAI activity through the liver-specific enhancer. Although the ER alpha DNA binding domain was essential but not sufficient for apoAI enhancer inhibition, ER alpha binding to the apoAI enhancer could not be detected by electrophoretic mobility shift assays. Western blotting and cotransfection assays showed that ER alpha plus E2 did not influence the abundance or the activity of the hepatocyte-enriched factors HNF-3 beta and HNF-4, two transcription factors essential for apoAI enhancer function, Expression of the ER alpha 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 ER alpha and RIP140 levels. At low ratios of RIP140 to ER alpha, 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 ER alpha and E2 but also upon the intracellular balance of ER alpha and coactivators utilized by ER alpha and the apoAI enhancer.