Nuclear localization of a1A-adrenergic receptors is required for signaling in cardiac myocytes: an inside-out a1-AR signaling pathway.

Recent studies indicate that a1-adrenergic receptors (a1-ars) are cardioprotective by preventing cardiac myocyte death and augmenting contractility in heart failure. although g-protein-coupled receptors are assumed to localize to and signal at the plasma membrane,we previously demonstrated that endogenous a1-ars localize to the nuclei in adult cardiac myocytes. however,the functional consequence of this nuclear localization remains unclear. here,we attempted to reconcile nuclear localization of a1-ars with their physiologic function by examining a1-ar-induced contractility in adult cardiac myocytes. by measuring shortening in unloaded,cultured adult cardiac myocytes,we found that the a1a-subtype regulated contractility through phosphorylation of cardiac troponin i (ctni) at the protein kinase c (pkc) site,threonine 144. reconstitution of an a1a-subtype nuclear localization mutant in cardiac myocytes lacking a1-ars failed to rescue nuclear a1a-mediated phosphorylation of ctni and myocyte contractility. leptomycin b,the nuclear export inhibitor,also blocked a1a-mediated phosphorylation of ctni. these data indicate that a1-ar signaling originates in the nucleus. consistent with these observations,we localized the a1a-subtype to the inner nuclear membrane,identified pkca,d,and e in the nucleus,and found that a1-ars activate pkcd in nuclei isolated from adult cardiac myocytes. finally,we found that a pkcd nuclear localization mutant blunted a1-induced phosphorylation of ctni. together,our data identify a novel, inside-out nuclear a1a-subtype/pkcd/ctni-signaling pathway that regulates contractile function in adult cardiac myocytes. importantly,these data help resolve the discrepancy between nuclear localization of a1-ars and a1-ar-mediated physiologic function.