β1 adrenergic receptors (β1ARs) are central regulators of cardiac function and a drug target for cardiac disease. As a member of the G protein–coupled receptor family, β1ARs activate cellular signaling by primarily coupling to Gs proteins to activate adenylyl cyclase, cAMP-dependent pathways, and the multifunctional adaptor-transducer protein β-arrestin. Carvedilol, a traditional β-blocker widely used in treating high blood pressure and heart failure by blocking β adrenergic receptor–mediated G protein activation, can selectively stimulate Gs-independent β-arrestin signaling of β adrenergic receptors, a process known as β-arrestin–biased agonism. Recently, a DNA-encoded small-molecule library screen against agonist-occupied β2 adrenergic receptors (β2ARs) identified Compound-6 (Cmpd-6) to be a positive allosteric modulator for agonists on β2ARs. Intriguingly, it was further discovered that Cmpd-6 is positively cooperative with the β-arrestin–biased ligand carvedilol at β2ARs. Here we describe the surprising finding that at β1ARs unlike β2ARs, Cmpd-6 is cooperative only with carvedilol and not agonists. Cmpd-6 increases the binding affinity of carvedilol for β1ARs and potentiates carvedilol-stimulated, β-arrestin–dependent β1AR signaling, such as epidermal growth factor receptor transactivation and extracellular signal-regulated kinase activation, whereas it does not have an effect on Gs-mediated cAMP generation. In vivo, Cmpd-6 enhances the antiapoptotic, cardioprotective effect of carvedilol in response to myocardial ischemia/reperfusion injury. This antiapoptotic role of carvedilol is dependent on β-arrestins since it is lost in mice with myocyte-specific deletion of β-arrestins. Our findings demonstrate that Cmpd-6 is a selective β-arrestin–biased allosteric modulator of β1ARs and highlight its potential clinical utility in enhancing carvedilol-mediated cardioprotection against ischemic injury.
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