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Background— Downregulation of β-adrenergic receptors (βARs) under conditions of heart failure requires receptor targeting of phosphoinositide 3-kinase (PI3K)–γ and redistribution of βARs into endosomal compartments. Because support with a left ventricular assist device (LVAD) results in significant improvement of cardiac function in humans, we investigated the effects of mechanical unloading on regulation of PI3Kγ activity and intracellular distribution of βARs. Additionally, we tested whether displacement of PI3Kγ from activated βARs would restore agonist responsiveness in failing human cardiomyocytes.Methods and Results— To test the role of PI3K on βAR endocytosis in failing human hearts, we assayed for PI3K activity in human left ventricular samples before and after mechanical unloading (LVAD). Before LVAD, failing human hearts displayed a marked increase in βAR kinase 1 (βARK1)–associated PI3K activity that was attributed exclusively to enhanced activity of the PI3Kγ isoform. Increased βARK1-coupled PI3K activity in the failing hearts was associated with downregulation of βARs from the plasma membrane and enhanced sequestration into early and late endosomes compared with unmatched nonfailing controls. Importantly, LVAD support reversed PI3Kγ activation, normalized the levels of agonist-responsive βARs at the plasma membrane, and depleted the βARs from the endosomal compartments without changing the total number of receptors (sum of plasma membrane and early and late endosome receptors). To test whether the competitive displacement of PI3K from the βAR complex restored receptor responsiveness, we overexpressed the phosphoinositide kinase domain of PI3K (which disrupts βARK1/PI3K interaction) in primary cultures of failing human cardiomyocytes. Adenoviral-mediated phosphoinositide kinase overexpression significantly increased basal contractility and rapidly reconstituted responsiveness to β-agonist.Conclusions— These results suggest a novel paradigm in which human βARs undergo a process of intracellular sequestration that is dynamically reversed after LVAD support. Importantly, mechanical unloading leads to complete reversal in PI3Kγ and βARK1-associated PI3K activation. Furthermore, displacement of active PI3K from βARK1 restores βAR responsiveness in failing myocytes.

Dynamic Regulation of Phosphoinositide 3-Kinase-γ Activity and β-Adrenergic Receptor Trafficking in End-Stage Human Heart Failure