Beta adrenergic receptors (βARs) are one of the most powerful regulators of cardiac function, and βAR downregulation and desensitization are hallmarks of heart failure. Agonist occupied βARs undergo desensitization by G-protein coupled receptor kinase phosphorylation leading to βAR internalization. Desensitized βARs are resensitized by dephosphorylation in the endosomes by PP2A; however, less is known about role of resensitization in heart failure. Although we have previously shown that resensitization is regulated by inhibition of PP2A by I2PP2A via PI3Kγ (Vasudevan et. al., 2011), the underlying mechanisms of I2PP2A binding to PP2A are not well understood. We used PyMOL software to find the binding interaction between PP2A and I2PP2A. Based on
in silico
predictions, we generated a mutant PP2A that when expressed would compete out I2PP2A and inhibit I2PP2A from binding to endogenous PP2A. Expression of PP2A mutant in β2AR expressing cells showed preservation of β2AR function following stimulation as measured by reduced β2AR phosphorylation, increased cAMP generation and increased phosphatase function. We also tested whether resensitization as a mechanism is altered in endstage human heart failure samples. Since resensitization occurs in endosomes, plasma membranes and endosomes were isolated from human heart samples and assessed for PI3K activity, PP2A activity, β2AR phosphorylation and adenylyl cyclase (AC) activity as a measure of βAR function. Significant PI3Kγ activity was observed to increase in heart failure samples while AC activity was reduced in plasma membranes. Consistent with the reduced recovery in βAR function in heart failure samples, significant β2AR phosphorylation was observed in the endosomes and plasma membranes of human heart failure samples compared to non-failing. Correspondingly, we observed significant reduction in endosomal PP2A activity in heart failure samples versus controls suggesting inhibited resensitization occurs in human heart failure. Together our studies suggest that resensitization is inhibited in human heart failure and targeting I2PP2A may provide preservation of receptor function and beneficial cardiac remodeling.
Molecular Remodeling in Human Heart Failure⁎⁎Editorials published in the Journal of the American College of Cardiology reflect the views of the authors and do not necessarily represent the views of JACC or the American College of Cardiology.
