Steroid Receptor Coactivator-1 from Brain Physically Interacts Differentially with Steroid Receptor Subtypes Abbreviated title: SRC-1 from brain interacts with ER and PR

In vitro studies reveal that nuclear receptor coactivators enhance the transcriptional activity of steroid receptors, including estrogen (ER) and progestin receptors (PR), through ligand-dependent interactions. While work from our lab and others shows that steroid receptor coactivator-1 (SRC-1) is essential for efficient ER and PR action in brain, very little is known about receptor-coactivator interactions in brain. In the present studies, pull-down assays were used to test the hypotheses that SRC-1 from hypothalamic and hippocampal tissue physically associate with recombinant PR or ER in a ligand-dependent manner. SRC-1, from hypothalamus or hippocampus, interacted with PR-A and PR-B in the presence of an agonist, but not in the absence of ligand or in the presence of a selective PR modulator, RU486. Interestingly, SRC-1 from brain associated more with PR-B, the stronger transcriptional activator, than with PR-A. In addition, SRC-1 from brain, which was confirmed by mass spectrometry, interacted with ERα and ERβ in the presence of agonist, but not when unliganded or in the presence of the selective ER modulator, tamoxifen. Furthermore, SRC-1 from hypothalamus, but not hippocampus, interacted more with ERα than with ERβ, suggesting distinct expression patterns of other cofactors in these brain regions. These findings suggest that interactions of SRC-1 from brain with PR and ER are dependent on ligand, receptor subtype and brain region to manifest the pleiotropic functional consequences that underlie steroid-regulated behaviors. The present findings reveal distinct contrasts with previous cell culture studies and emphasize the importance of studying receptor-coactivator interactions using biologically-relevant tissue. 3 INTRODUCTION The steroid hormones, estradiol and progesterone, exert many of their effects on reproductive behavior and physiology by binding to their respective intracellular receptors in specific brain regions (1-3). Intracellular estrogen receptors (ER) exist in two forms, α and β, which are transcribed from different genes (4-6). These subtypes differ in their functions (7), abilities to bind different ligands (8-11) and distribution in brain (12-16). In addition, cell culture experiments indicate that ERα is a stronger transcriptional activator than ERβ, due to differences in the activation function-1 (AF-1) region of the amino-terminus (17). In most species, progestin receptors (PR) are expressed in two forms; the full-length PR-B and the truncated PR-A, which are encoded by the same gene but are under the regulation of different promoters (18, 19). In vitro studies indicate that human PR-B is a stronger transcriptional activator than PR-A (20-24), due to an additional AF domain in …