Estrogen receptor does not directly regulate the murine Muc-1 promoter

Muc-1 is a heavily O-glycosylated, type 1 membrane glycoprotein present on the surface of polarized secretory uterine epithelial cells. Previous studies have shown that treatment of ovariectomized mice with 17-β-estradiol (E₂) strongly induces Muc-1 mRNA expression in an estrogen receptor (ER)-mediated fashion in the uterus. In this study, the 5.4 kb Muc-1 gene promoter has been isolated from a mouse genomic library and the proximal 1.85 kb region has been sequenced. Sequence analysis revealed the presence of one potential full estrogen response element (ERE) (GCTCGCGGTGACC) located at -748 to -735 bp in the Muc-1 promoter and several potential ERE half sites. Electrophoretic mobility shift assays (EMSA) showed that neither ERα nor ERβ bind efficiently to this sequence. Transient cotransfection assays using constructs containing various deletion mutations of the 5' Muc-1 flanking sequences showed that E₂ had no direct stimulation on promoter-driven reporter in NMuMG cells or primary mouse uterine epithelial cells, but did stimulate a consensus ERE CAT-reporter gene activity. In addition, E₂-treatment of Weg-ER cells, a mouse uterine epithelial cell line stably expressing human ERα, did not restore endogenous Muc-1 expression or activate Muc-1 promoter-driven CAT activity. These results indicate that regions of the Muc-1 gene promoter within -1838 to +43 bp do not respond to E₂ and ER stimulation and that ER alone is not sufficient to restore Mud gene expression. Deletion analyses also revealed that the sequence between -73 and +43 bp of the Muc-1 promoter is the minimal promoter region required for maximal Muc-1 promoter activity. Collectively, these results demonstrate that ER does not directly regulate the 1.85 kb murine Muc-1 gene promoter. Therefore, E₂ control of uterine Muc-1 gene expression is likely to be indirect, i.e. mediated by stromal cell-derived factors.