We investigated the role of the activation function 1 (AF1) and AF2 domains of estrogen receptor alpha (ER alpha) in mediating dioxin-dependent recruitment of ER alpha to cytochrome P4501A1 (CYP1A1) and CYP1B1 in HuH-7 human hepatoma cells. Dioxin-induced recruitment of ER alpha wildtype (ER alpha-WT) and an ER alpha AF1 deletion mutant (ER alpha-Delta AF1), but not a transcriptional inactive AF2 mutant (ER alpha-AF2mut) to CYP1A1 and CYP1B1. Direct interactions between AHR and the AF1 and AF2 domains of ER alpha were observed, and were independent of mutations in the AF2. Expression of ER alpha-WT increased duoxin-induced CYP1A1 and CYP1B1-regulated reporter activity, and CYP1A1 and CYP1B1 rnRNA levels. However, no increases in gene expression above vector controls were observed in cells transfected with ER alpha-Delta AF1 or ER alpha-AF2mut. Our data show that the AF2 domain Contributes to dioxin-induced recruitment of ER alpha to AHR target genes, but that both the AF1 and AF2 domains are required for ER alpha-dependent increases in AHR activity. (C) 2009 Elsevier Inc. All rights reserved.