Titration by estrogen receptor activation function-2 of targets that are downstream from coactivators

Cross-interference (squelching) among nuclear receptors has been proposed t o reflect the titration of coactivators that bind the receptors in a hormon e-dependent manner. We have tested whether the coactivators are the only ta rget titrated during squelching of one receptor by another, or whether prot eins needed for coactivator function are titrated as well. That the coactiv ators are indeed one target of squelching is apparent. The isolated ligand- binding domain of the estrogen receptor (ER-LBD) squelches transcriptional activation by the thyroid hormone receptor (TR) only when the LED is bound to ligands that promote coactivator interactions and only when regions of t he LED that promote coactivator interactions are undisturbed. Furthermore, the ER-LBD and the TR compete in vitro for the related p160 coactivators, S RC1a and GRIP1 (glucocorticoid receptor interacting protein 1), or the puta tive corepressor, RIP140. Finally TR action becomes more potent when coacti vator levels are raised. Nonetheless, supplying excess SRC1a or GRIP1 does not abolish squelching by the ER. In fact, squelching becomes even more sev ere when coactivators are abundant. Supplying combinations of coactivators from the p160 class and the CREB-binding protein (CBP)/p300 class makes squ elching most severe. Elevated RIP140 inhibits TR action, but also protects the residual TR action from squelching by the ER-LBD. We conclude that ER-L BD squelches TR both by titrating p160-CBP coactivators and additionally by cooperating with the coactivators to titrate a second factor. The second f actor would be needed by the TR for coactivator-mediated transcriptional st imulation.