E. M. McInerney

Retinoic acid and thyroid hormone receptors can act alternatively as ligand-independent repressors or ligand-dependent activators, based on an exchange of N-CoR or SMRT-containing corepressor complexes for coactivator complexes in response to ligands. We provide evidence that the molecular basis of N-CoR recruitment is similar to that of coactivator recruitment, involving cooperative binding of two helical interaction motifs within the N-CoR carboxyl terminus to both subunits of a RAR-RXR heterodimer. The N-CoR and SMRT nuclear receptor interaction motifs exhibit a consensus sequence of LXX I/H I XXX I/L, representing an extended helix compared to the coactivator LXXLL helix, which is able to interact with specific residues in the same receptor pocket required for coactivator binding. We propose a model in which discrimination of the different lengths of the coactivator and corepressor interaction helices by the nuclear receptor AF2 motif provides the molecular basis for the exchange of coactivators for corepressors, with ligand-dependent formation of the charge clamp that stabilizes LXXLL binding sterically inhibiting interaction of the extended corepressor helix.

We have studied the two estrogen receptor (ER) subtypes, ERα and ERβ, and chimeric constructs with ERα and ERβ to examine the bioactivities of these receptors and their responses to estrogen and antiestrogen ligands. Transcriptional activity of ERβ is highly dependent on cell/promoter context and on the nature of the ligand. ERβ activated significant levels of transcription in response to estrogens in certain cell types, but showed only moderate activity compared with ERα in others. Antiestrogens such as tamoxifen and 2-phenylbenzofuran, which show some agonistic activity with ERα, exhibit no agonistic activity with ERβ. Alteration of the amino-terminal A/B receptor domain can result in a dramatic change in cell type- and ligand-specific transcriptional activity of ERβ. Upon replacing the A/B domain of ERβ with the A/B domain of ERα, this receptor chimera not only exhibits an improved transcriptional response to estrogens, but also is now able to activate transcription upon treatment with these antiestrogens. As antiestrogen agonism was lacking in ERβ and the ERβ/α chimera containing the amino-terminal A/B domain of ERβ fused to domains C through F of ERα, but was restored in an ERα/β chimera containing the A/B domain of ERα, antiestrogen agonism was shown to depend on the A/B domain (activation function-1-containing region) of ERα. Together, these results indicate that the differences in the amino-terminal regions of ERα and ERβ contribute to the cell- and promoter-specific differences in transcriptional activity of these receptors, and their ability to respond to different ligands, thus providing a mechanism for differentially regulated transcription by these two ERs.