The Hormonal Response of Estrogen Receptor β Is Decreased by the Phosphatidylinositol 3-Kinase/Akt Pathway via a Phosphorylation-dependent Release of CREB-binding Protein

The hormonal response of estrogen receptors (ER) α and ERβ is controlled by a number of cofactors, including the general transcriptional coactivator CREB-binding protein (CBP). Growing evidence suggests that specific kinase signaling events also modulate the formation and activity of the ER coactivation complex. Here we show that ERβ activity and target gene expression are decreased upon activation of ErbB2/ErbB3 receptors despite the presence of CBP. This inhibition of ERβ involved activation of the phosphatidylinositol 3-kinase/Akt pathway, abrogating the potential of CBP to facilitate ERβ response to estrogen. Such reduced activity was associated with an impaired ability of ERβ to recruit CBP upon activation of Akt. Mutation of serine 255, an Akt consensus site contained in the hinge region of ERβ, prevented the release of CBP and rendered ERβ transcriptionally more responsive to CBP coactivation, suggesting that Ser-255 may serve as a regulatory site to restrain ERβ activity in Akt-activated cells. In contrast, we found that CBP intrinsic activity was increased by Akt through threonine 1872, a consensus site for Akt in the cysteine- and histidine-rich 3 domain of CBP, indicating that such enhanced transcriptional potential of CBP did not serve to activate ERβ. Interestingly, nuclear receptors sharing a conserved Akt consensus site with ERβ also exhibit a reduced ability to be coactivated by CBP, whereas others missing that site were able to benefit from the activation of CBP by Akt. These results therefore outline a regulatory mechanism by which the phosphatidylinositol 3-kinase/Akt pathway may discriminate nuclear receptor response through coactivator transcriptional competence.