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/promo ter is regulated by estrogens only when it is bound by both ER and the pitu itary-specific transcription factor, Pit-1. Cooperative ER/Pit-1 activation of the dormant PRL enhancer/promoter in pituitary progenitor cells require s the estrogen-dependent activation function-2 (AF-2) of ER, but is inhibit ed by one AF-2-interacting cofactor, RIP140. Here, the complex actions of R IP140 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 i n the absence of AF-1 and RIP140 inhibition of both ER alpha and ERP cooper ative 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 glucocortic oid receptor interacting protein-1 (GRIP1), had negligible effects on ER al pha/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 wi th 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-la an d GRIP1 binding to AF-2 counteracted the inhibition of ER/Pit-1 activation by another AF-2-interacting protein, RIP140. Complex, sometimes antagonisti c, actions of different classes of AF-2-interacting proteins may play an im portant role in the cell- and gene-specific estrogen regulation of PRL and other genes.