Abstract

The β₁-adrenergic receptor (β₁-AR) plays a key role in regulating heart rate and contractility in response to catecholamines. Our studies have focused on defining the factors that regulate the expression of the β₁-AR gene. We determined that a 65-base-pair (bp) region in the β₁-AR promoter between bp −394 and bp −330 directs basal transcription. An element located between −377 and −365 can bind Sp1 and Sp3. InDrosophila melanogaster SL2 cells, Sp1 stimulated the expression of the β₁-AR promoter, whereas Sp3 was unable to activate transcription. Site-directed mutagenesis indicated that an intact Sp1-binding site is essential for maintaining the activity of the basal promoter. In addition to binding Sp family members, the nucleotides between −381 and −367 can bind the zinc-finger transcription factor Egr-1. The Egr-1 and Sp1 binding sites are partially overlapping and their binding sequence is conserved among mammalian β₁-AR genes. The induction of Egr-1 in rat neonatal ventricular myocytes with phorbol-12-myristate-13-acetate or in HeLa S3 cells by regulated expression of Egr-1 in a tetracycline-responsive promoter, suppressed expression from the β₁-AR promoter. Overexpression of Sp1 in SK-N-MC cells increased β₁-AR mRNA by 2.4-fold, whereas overexpression of Egr-1 reduced β₁-AR mRNA by 40%. Coexpression of Egr-1 with Sp1 reduced Sp1-mediated up-regulation of β₁-AR mRNA by 60%. Mutagenesis revealed that an intact Sp1-binding site is essential for observing transcriptional repression by Egr-1 and that Egr-1 suppressed the transcription of the β₁-AR gene by competing with Sp1 for binding to their overlapping sites. These results reveal a novel physiologically relevant transcriptional mechanism for reciprocal regulation of β₁-AR gene expression.