Abstract P193: Nuclear Localization of the α1B-Adrenergic Receptor Subtype Is Required for Hypertrophic Signaling in Cardiac Myocytes

Abstract

We previously demonstrated that α1-adrenergic receptors (α1-ARs) in the heart are required for physiologic hypertrophy during development and prevent a maladaptive response to pathologic stress. We have also shown that the major subtypes, α1A and α1B, both localize to the nucleus in adult cardiac myocytes. Importantly, we have defined a nuclear α1A-ERK survival and an α1A-PKCδ-cTnI inotropic signaling pathway that both originate at the nucleus and are transduced to cytosolic targets, suggesting that the α1A is required for cardiac myocyte survival and contractility. However, less is known about the molecular function of the α1B. In the current study, we examined the role of the α1B-subtype in hypertrophic signaling. First, we identified a bi-partite nuclear localization sequence (NLS) in the carboxy-terminal tail of the receptor. Mutation of the NLS (α1B-NLSmut) disrupted its localization to the nucleus when expressed in adult cardiac myocytes. We then compared hypertrophic signaling of the wild-type α1B- to the mutated receptor by reconstitution in cardiac myocytes lacking endogenous α1B (α1BKO) receptors. Activation of the wild-type receptor by the α1-agonist phenylephrine in α1BKO myocytes restored hypertrophic signaling, as we observed increased phosphorylation of protein kinase C (PKC) isoforms δ and ε, and histone deacetylases (HDAC) 4 and 5. We also observed increased expression of the hypertrophic gene marker, atrial natriuretic factor (ANF). Expression of the α1B-NLSmut failed to activate hypertrophic signaling in α1BKO cardiac myocytes despite phenylephrine stimulation. Together, our data show that nuclear localization of the α1B-subtype is required for hypertrophic signaling and overall further suggest that α1-adrenergic receptors are functional only at the nucleus in cardiac myocytes.