α1-Adrenergic Receptor Subtype mRNAs Are Differentially Regulated by α1-Adrenergic and Other Hypertrophic Stimuli in Cardiac Myocytes in Culture and In Vivo

Abstract

The three cloned ␣ 1 -adrenergic receptor (AR) subtypes, ␣ 1B , ␣ 1C , and ␣ 1D , can all couple to the same effector, phospholipase C, and the reason(s) for conservation of multiple subtypes remain uncertain. All three ␣ 1 -ARs are expressed natively in cultured neonatal rat cardiac myocytes, where chronic exposure to the agonist catecholamine norepinephrine (NE) induces hypertrophic growth and gene transcription. We show here, using RNase protection, that the ␣ 1 -AR subtype mRNAs respond in distinctly different ways during prolonged NE exposure (12-72 h). ␣ 1B and ␣ 1D mRNA levels were repressed by NE, whereas ␣ 1C mRNA was induced. Changes in mRNA levels were mediated by an ␣ 1 -AR, were not explained by altered mRNA stability, and were reflected in receptor proteins by [ 3 H]prazosin binding. ␣ 1 -AR-stimulated phosphoinositide hydrolysis and myocyte growth were not desensitized. Three other hypertrophic agonists in culture, endothelin-1, PGF2␣, and phorbol 12-myristate 13-acetate, also induced ␣ 1C mRNA and repressed ␣ 1B mRNA. In myocytes from hearts with pressure overload hypertrophy, ␣ 1 mRNA changes were identical to those produced by NE in culture. These results provide the first example of a difference in regulation among ␣ 1 -AR subtypes expressed natively in the same cell. Transcriptional induction of the ␣ 1C -AR could be a mechanism for sustained growth signaling through this receptor and is a common feature of a hypertrophic phenotype in cardiac myocytes. The natural catecholamines norepinephrine (NE)1 and epinephrine activate adrenergic receptors (ARs) in three families, ␣ 1 , ␣ 2 , and ␤. Multiple subtypes have been cloned within each family: three ␣ 1 -ARs (B, C, and D), 2 three ␣ 2 -ARs (A, B, and C; also called C10, C2, and C4, respectively), and three ␤-ARs (1, 2, and 3) (1). The reason(s) for conservation of multiple subtypes remain uncertain, since all subtypes in each family couple preferentially to the same effector when overexpressed, ␣ 1 -ARs to activation of phospholipase C (PLC), ␣ 2 -ARs to inhibition of adenylyl cyclase, and ␤-ARs to activation of adenylyl cyclase (1). An intriguing difference among ␤-AR and ␣ 2 -AR subtypes has been suggested recently, in the regulation of receptor levels during prolonged agonist exposure. Levels of the ␤ 3 -AR (2-4) and the ␣ 2C -AR (5, 6) are not down-regulated during long term agonist exposure, at least in some cells, in contrast with downregulation of ␤ 1 -and ␤ 2 -ARs, and ␣ 2A -and ␣ 2B -ARs. Downregulation of receptor expression is thought to be a major determinant of desensitization when catecholamine exposure is prolonged (7). Conversely, a receptor that is induced by agonist might be adapted to mediate catecholamine responses when sympathetic activity is increased chronically, a condition that occurs frequently in the intact organism.We have been studying a physiological response that develops over long periods of catecholamine exposure, ␣ 1 -adrenergic induction of hypertrophic growth and gene transcription in primary cultures of neon...

Knockout of the α1A/C-adrenergic receptor subtype: The α1A/C is expressed in resistance arteries and is required to maintain arterial blood pressure

“…Normal cardiovascular structure in ␣1A͞C KO mice was noteworthy, given the results from rat models implicating the ␣1A͞C receptor in cardiac hypertrophy (12). The ␣1A͞C KO did not alter the normal increase in cardiac size with development, in agreement with an ␣1A͞C transgenic model with unaltered heart size (22).…”

“…A 100,000 ϫ g pellet was used in binding (12 Blood Pressure and Heart Rate (HR). Tail cuff systolic blood pressure and HR were measured using a noninvasive computerized tail cuff system (BP-2000, VisiTech Systems, Apex, NC; ref.…”

Cloning and Characterization of the Rat α1a-Adrenergic Receptor Gene Promoter

“…cDNAs encoding three ␣ 1 AR subtypes (␣ 1a , ␣ 1b , and ␣ 1d ) have been cloned and pharmacologically characterized in several expression systems, leading to the surprising finding that ␣ 1 AR subtype tissue distribution is species-dependent, with expression regulated at both gene and protein levels (4). To gain insight into transcriptional pathways governing ␣ 1 AR expression, several laboratories have initiated cloning and characterization of ␣ 1 AR subtype regulatory regions from different species, including the ␣ 1a AR (human (5), mouse (6)), ␣ 1b AR (mouse (7), rat (8), human (9)), and ␣ 1d AR (rat (10)).Recent studies reveal an important and distinct role for ␣ 1a ARs in the heart, including enhanced contractility (2, 3) and hypertrophy (11)(12)(13)(14). Modulation of ␣ 1a AR expression occurs during pathological states, such as myocardial hypertrophy (13, 14) and hypoxia (15), the latter usually secondary to ischemic injury.…”

“…Recent studies reveal an important and distinct role for ␣ 1a ARs in the heart, including enhanced contractility (2, 3) and hypertrophy (11)(12)(13)(14). Modulation of ␣ 1a AR expression occurs during pathological states, such as myocardial hypertrophy (13, 14) and hypoxia (15), the latter usually secondary to ischemic injury.…”

“…Modulation of ␣ 1a AR expression occurs during pathological states, such as myocardial hypertrophy (13,14) and hypoxia (15), the latter usually secondary to ischemic injury. In hypertrophic models, classically performed in cultured neonatal rat cardiomyocytes, direct chronic ␣ 1a AR stimulation results in increased transcription of the ␣ 1a AR gene itself, providing a pathway for overall sustained ␣ 1 AR signaling in the heart (13,14). In contrast, both in vitro cardiomyocyte and whole animal models of hypoxia (1% O 2 , 72 h) reveal decreased ␣ 1 AR signaling, caused in part by reduced levels of ␣ 1a AR mRNA and protein (15).…”