Progesterone receptor (PR), a ligand-inducible transcription factor mediates the physiological actions of progesterone (P) through two distinct isoforms PR-A and PR-B and numerous nuclear co-regulators. We previously demonstrated that basic transcription element binding protein-1 (BTEB1), a transcription factor of the Krüppel-like family, is a functional PR-interacting protein, 55 based on the sub-fertility phenotype and reduced P-sensitivity of uterine PR target genes, upon BTEB1 null

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... Here, we examined the role of BTEB1 in PR-mediated signaling in endometrial epithelial cells using Ishikawa human endocarcinoma cells and the P-responsive secretory leukocyte protease inhibitor (SLPI) gene. Treatment of Ishikawa cells with P for 24 h increased SLPI and BTEB1 transcript levels without similar effects on PR expression. P-60 induction was abolished by the PR antagonist RU486, while knockdown of BTEB1 with short interfering RNA reduced P-responsive BTEB1 but not SLPI expression to basal levels. Forced expression of BTEB1, either by stable or transient transfections of BTEB1 expression constructs in endometrial carcinoma cells, enhanced SLPI promoter activity. Chromatin immunoprecipitation with anti-BTEB1 antibody demonstrated BTEB1 recruitment to the 65 proximal GC-rich containing SLPI promoter region (-97 to -86) in human endometrial carcinoma (Hec1A) cells over-expressing BTEB1. In Ishikawa cells, recruitment of BTEB1 to the proximal, GC-rich region and to the distal, PRE-like containing region (-635 to -514) was Pdependent and was accompanied by co-recruitment of PR and the PR coactivator CBP. PR-B, rather than PR-A isoform preferentially associated with BTEB1 in the GC-rich region, whereas 70 both PR isoforms were recruited to the PRE-like site along with BTEB1. Our findings define a novel pathway for BTEB1/PR cross-talk to facilitate P-dependent gene transcription in endometrial epithelial cells.