The biological effects of estrogens are mediated by the estrogen receptors ER␣ and ER␤. These receptors regulate gene expression through binding to DNA enhancer elements and subsequently recruiting factors such as coactivators that modulate their transcriptional activity. Here we show that ARNT (aryl hydrocarbon receptor nuclear translocator), the obligatory heterodimerization partner for the aryl hydrocarbon receptor and hypoxia inducible factor 1␣, functions as a potent coactivator of ER␣-and ER␤-dependent transcription. The coactivating effect of ARNT depends on physical interaction with the ERs and involves the C-terminal domain of ARNT and not the structurally conserved basic helixloop-helix and PAS (Per-ARNT-Sim) motifs. Moreover, we show that ARNT͞ER interaction requires the E2-activated ligand binding domain of ER␣ or ER␤. These observations, together with the previous role of ARNT as an obligatory partner protein for conditionally regulated basic helix-loop-helix-PAS proteins like the aryl hydrocarbon receptor or hypoxia inducible factor 1␣, expand the cellular functions of ARNT to include regulation of ER␣ and ER␤ transcriptional activity. ARNT was furthermore recruited to a natural ER target gene promoter in a estrogen-dependent manner, supporting a physiological role for ARNT as an ER coactivator.cross-talk ͉ chromatin immunoprecipitation E strogens regulate important physiological processes, such as development and function of the male and female reproductive system and maintenance of bone mass in women, and represent a protective factor against cardiovascular disease (1). The physiological effects of estrogens are mediated by estrogen receptors ␣ (ER␣) and ␤ (ER␤), which belong to the nuclear receptor superfamily (1, 2). Nuclear receptors are liganddependent transcription factors characterized by a conserved structural arrangement composed of a centrally located DNA binding domain (DBD) containing two highly conserved Zn finger motifs. This domain is flanked in the N terminus by a variable A͞B region, which contains an activation function (AF-1). The ligand binding domain (LBD), located C-terminally of the DBD, contains a second AF (AF-2) and is also responsible for ligand binding, receptor dimerization, and cofactor interaction (3).The ERs are, in the absence of ligand, present in the nucleus in a nonactivated form. The latent ERs interact with corepressors such as N-Cor, SHP, or SMRT, which inhibit constitutive transcriptional activity (1). Binding of agonists induces release of repressor proteins and allows interaction with coactivators of the p160 class like SRC-1 or transcription intermediary factor 2 (TIF-2). These proteins have been shown to increase access to chromatin through acetylation of histones, mediate contact with general transcription factors, and enhance receptor AF-1͞AF-2 synergy (4, 5).Interestingly, p160 coactivators share considerable sequence homology to basic helix-loop-helix (bHLH)-PAS (Per-ARNTSim) transcription factors. This family also includes factors such as the aryl h...

No abstract