Cloning and characterization of the mouse α1C/A-adrenergic receptor gene and analysis of an α1C promoter in cardiac myocytes: Role of an MCAT element that binds transcriptional enhancer factor-1 (TEF-1)

α1-Adrenergic receptor (AR) subtypes in the heart are expressed by myocytes but not by fibroblasts, a feature that distinguishes α1-ARs from β-ARs. Here we studied myocyte-specific expression of α1-ARs, focusing on the subtype α1C (also called α1A), a subtype implicated in cardiac hypertrophic signaling in rat models. We first cloned the mouse α1C-AR gene, which consisted of two exons with an 18 kb intron, similar to the α1B-AR gene. The receptor coding sequence was >90% homologous to that of rat and human. α1C-AR transcription in mouse heart was initiated from a single Inr consensus sequence at -588 from the ATG; this and a putative polyadenylation sequence 8.5 kb 3′ could account for the predominant 11 kb α1C mRNA in mouse heart. A 5′-nontranscribed fragment of 4.4 kb was active as a promoter in cardiac myocytes but not in fibroblasts. Promoter activity in myocytes required a single muscle CAT (MCT) element, and this MCT bound in vitro to recombinant and endogenous transcriptional enhancer factor-1. Thus, α1C-AR transcription in cardiac myocytes shares MCT dependence with other cardiac-specific genes, including the α- and β-myosin heavy chains, skeletal α-actin, and brain natriuretic peptide. However, the mouse α1C gene was not transcribed in the neonatal heart and was not activated by α1-AR and other hypertrophic agonists in rat myocytes, and thus differed from other MCT-dependent genes and the rat α1C gene.