Low- and high-level transgenic expression of β 2 -adrenergic receptors differentially affect cardiac hypertrophy and function in Gαq-overexpressing mice

Transgenic overexpression of Galphaq in the heart triggers events leading to a phenotype of eccentric hypertrophy, depressed ventricular function, marked expression of hypertrophy-associated genes, and depressed beta-adrenergic receptor (betaAR) function. The role of betaAR dysfunction in the development of this failure phenotype was delineated by transgenic coexpression of the carboxyl terminus of the betaAR kinase (betaARK), which acts to inhibit the kinase, or concomitant overexpression of the beta2AR at low (approximately 30-fold, Galphaq/beta2ARL), moderate (approximately 140-fold, Galphaq/beta2ARM), and high (approximately 1,000-fold, Galphaq/beta2ARH) levels above background betaAR density. Expression of the betaARK inhibitor had no effect on the phenotype, consistent with the lack of increased betaARK levels in Galphaq mice. In marked contrast, Galphaq/beta2ARL mice displayed rescue of hypertrophy and resting ventricular function and decreased cardiac expression of atrial natriuretic factor and alpha-skeletal actin mRNA. These effects occurred in the absence of any improvement in basal or agonist-stimulated adenylyl cyclase (AC) activities in crude cardiac membranes, although restoration of a compartmentalized beta2AR/AC signal cannot be excluded. Higher expression of receptors in Galphaq/beta2ARM mice resulted in salvage of AC activity, but hypertrophy, ventricular function, and expression of fetal genes were unaffected or worsened. With approximately 1,000-fold overexpression, the majority of Galphaq/beta2ARH mice died with cardiomegaly at 5 weeks. Thus, although it appears that excessive, uncontrolled, or generalized augmentation of betaAR signaling is deleterious in heart failure, selective enhancement by overexpressing the beta2AR subtype to limited levels restores not only ventricular function but also reverses cardiac hypertrophy.