A novel glucocorticoid regulatory unit mediates the hormone responsiveness of the β1-adrenergic receptor gene

Abstract

The effects of glucocorticoids on expression of the β1-adrenergic receptor (β1AR) gene have been varied. To study the mechanism underling hormonal regulation of the β1AR, transient transfection of progressively deleted ovine β1AR promoter fragments was used to identify a 43-bp region (−1274 to −1232 from the translation start site) that contains a novel glucocorticoid regulatory unit (GRU) and confers glucocorticoid responsiveness. Using DNase I footprinting and electrophoretic mobility shift assays (EMSA), we demonstrated the GRU was composed of a palindrome, 5′-TAATTA-3′, which is a core binding motif for the homeodomain proteins, an E-box (5′-CACGTG-3′), binding site for the Myc/Max family proteins, and an overlapping glucocorticoid response element (GRE) half-site (5′-TGTTCT-3′). EMSA demonstrated that the GRE half-site is critical for GRU–protein interactions, which also require binding of proteins to the E-box and the homeodomain region. Co-transfection of a plasmid expressing a c-myc antisense construct significantly reduced glucocorticoid responsiveness of the ovine β1AR promoter. Furthermore, expression of proteins binding to the GRU was shown to be developmentally regulated, being high in embryonic, reduced in newborn and not detectable in adult heart. We conclude that the ovine β1AR promoter contains a novel, functional GRU and that glucocorticoid receptor (GR) and the Myc/Max family proteins are involved in the cell-specific nuclear factor binding and transactivation via this element. The results suggest an alternative pathway through which glucocorticoids may exert their effects on genes lacking a full consensus GRE.