Regulation of estrogen receptor activation of the prolactin enhancer/promoter by antagonistic activation function-2-interacting proteins.

Transcriptional responses to estrogens are controlled by the cell- and gene-specific interactions of the nuclear estrogen receptor (ER) with cofactors and other transcription factors. The pituitary-specific PRL enhancer/promoter is regulated by estrogens only when it is bound by both ER and the pituitary-specific transcription factor, Pit-1. Cooperative ER/Pit-1 activation of the dormant PRL enhancer/promoter in pituitary progenitor cells requires the estrogen-dependent activation function-2 (AF-2) of ER, but is inhibited by one AF-2-interacting cofactor, RIP140. Here, the complex actions of RIP140 and other AF-2-interacting proteins at the PRL enhancer/promoter were shown to operate via ER itself. RIP140 inhibition of ER/Pit-1 activation in the absence of AF-1 and RIP140 inhibition of both ER alpha and ER beta cooperative activation with Pit-1 suggested a conserved ER site for RIP140 action, possibly AF-2. Coexpression of other AF-2-interacting proteins, including the p160 factors, steroid receptor coactivator-1a (SRC-1a) and glucocorticoid receptor interacting protein-1 (GRIP1), had negligible effects on ER alpha/Pit-1 cooperative activation, but partially relieved RIP140 inhibition. Relief of RIP140 inhibition required the AF-2-binding, LXXLL motifs in SRC-1a and GRIP1. An ER AF-2 mutant that selectively blocked ER interaction with p160s, but not RIP140, still cooperated with Pit-1 and was inhibited by RIP140, but was not relieved by SRC-1a or GRIP1 expression. Thus, SRC-1a and GRIP1 binding to AF-2 counteracted the inhibition of ER/Pit-1 activation by another AF-2-interacting protein, RIP140. Complex, sometimes antagonistic, actions of different classes of AF-2-interacting proteins may play an important role in the cell- and gene-specific estrogen regulation of PRL and other genes.