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The mineralocorticoid receptor (MR) plays a central role in salt and water homeostasis via the kidney; however, inappropriate activation of the MR in the heart can lead to heart failure. A selective MR modulator that antagonizes MR signaling in the heart but not the kidney would provide the cardiovascular protection of current MR antagonists but allow for normal electrolyte balance. The development of such a pharmaceutical requires an understanding of coregulators and their tissue-selective interactions with the MR, which is currently limited by the small repertoire of MR coregulators described in the literature. To identify potential novel MR coregulators, we used T7 phage display to screen tissue-selective cDNA libraries for MR-interacting proteins. Thirty MR binding peptides were identified, from which three were chosen for further characterization based on their nuclear localization and their interaction with other MR-interacting proteins, or in the case of XRCC6, its known status as an androgen receptor coregulator. Eukaryotic elongation factor 1A1 (EEF1A1), structure-specific recognition protein 1 (SSRP1), and x-ray repair cross-complementing protein 6 (XRCC6) modulated MR-mediated transcription in a ligand-, cell- and/or promoter-specific manner, and co-localized with the MR upon agonist treatment when imaged using immunofluorescence microscopy. These results highlight the utility of phage display for rapid and sensitive screening of MR binding proteins, and suggest that EEF1A1, SSRP1 and XRCC6 may be potential MR coactivators whose activity is dependent on ligand, cellular context and target gene promoter.
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