Deleterious effects on the heart from chronic stimulation of β-adrenergic receptors (βARs), members of the 7 transmembrane receptor family, have classically been shown to result from G s -dependent adenylyl cyclase activation. Here, we identify a new signaling mechanism using both in vitro and in vivo systems whereby β-arrestins mediate β 1 AR signaling to the EGFR. This β-arrestin-dependent transactivation of the EGFR, which is independent of G protein activation, requires the G protein-coupled receptor kinases 5 and 6. In mice undergoing chronic sympathetic stimulation, this novel signaling pathway is shown to promote activation of cardioprotective pathways that counteract the effects of catecholamine toxicity. These findings suggest that drugs that act as classical antagonists for G protein signaling, but also stimulate signaling via β-arrestin-mediated cytoprotective pathways, would represent a novel class of agents that could be developed for multiple members of the 7 transmembrane receptor family.Introduction β-Adrenergic receptors (βARs) belong to the family of 7 transmembrane receptors (7TMRs) (1) and mediate the powerful regulatory effects on cardiac function of the catecholamine neurotransmitters epinephrine and norepinephrine. β 1 ARs constitute more than 70% of the cardiac βARs. Catecholamine stimulation of β 1 ARs results in activation of heterotrimeric G proteins followed by rapid phosphorylation of the receptor, resulting in desensitization (2). Homologous desensitization of β 1 ARs is brought about by phosphorylation of the receptor by G protein-coupled receptor kinases (GRKs), leading to the recruitment of β-arrestin, which then sterically interdicts further coupling to G proteins (3) and targets the receptor for internalization (3). In addition to β-arrestin's role in terminating G protein signaling, recent studies demonstrate that β-arrestins also function as adapter molecules, allowing for the assembly of multiprotein signaling complexes such as ERKs and tyrosine kinases (4, 5). For the angiotensin II type 1A receptor (AT 1A R), this second wave of β-arrestin-mediated signaling has recently been demonstrated to be independent of G protein signaling (6) and to require the activity of GRKs 5 and 6 (7).The signaling mechanisms that underlie the activation of the mitogenic ERK growth response by 7TMRs are complex and likely result from both classical G protein-regulated effectors such as PKA and PKC and non-G protein-mediated crosstalk, such as EGFR transactivation (8). The current paradigm of transactivation involves agonist stimulation of a 7TMR, which through a number of undefined steps leads to MMP-mediated cleavage and

Recent evidence suggests that binding of agonist to its cognate receptor initiates not only classical G protein-mediated signaling, but also ␤-arrestin-dependent signaling. One such ␤-arrestin-mediated pathway uses the ␤1-adrenergic receptor (␤1AR) to transactivate the EGFR. To determine whether ␤-adrenergic ligands that do not activate G protein signaling (i.e., ␤-blockers) can stabilize the ␤1AR in a signaling conformation, we screened 20 ␤-blockers for their ability to stimulate ␤-arrestin-mediated EGFR transactivation. Here we show that only alprenolol (Alp) and carvedilol (Car) induce ␤1AR-mediated transactivation of the EGFR and downstream ERK activation. By using mutants of the ␤1AR lacking G protein-coupled receptor kinase phosphorylation sites and siRNA directed against ␤-arrestin, we show that Alp-and Car-stimulated EGFR transactivation requires ␤1AR phosphorylation at consensus G protein-coupled receptor kinase sites and ␤-arrestin recruitment to the ligand-occupied receptor. Moreover, pharmacological inhibition of Src and EGFR blocked Alp-and Car-stimulated EGFR transactivation. Our findings demonstrate that Alp and Car are ligands that not only act as classical receptor antagonists, but can also stimulate signaling pathways in a G protein-independent, ␤-arrestin-dependent fashion.␤-adrenergic receptor ͉ G protein-coupled receptor ͉ signaling

G protein-coupled receptors (GPCRs) are widely implicated in human heart disease, making them an important target for cardiac drug therapy. The most commonly studied and clinically targeted cardiac GPCRs include the adrenergic, angiotensin, endothelin, and adenosine receptors. Treatment options focusing on the complex and integrated signaling pathways of these GPCRs are critical for the understanding and amelioration of heart disease. The focus of this review is to highlight the most commonly studied and clinically targeted cardiac GPCRs, placing emphasis on their common signaling components implicated in cardiac disease.

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