Beta-adrenergic receptor (βAR) down-regulation and desensitization are hallmarks of heart failure. Agonist occupied βAR undergo desensitization through phosphorylation by G-protein coupled receptor kinases leading to βAR internalization. Phosphorylated βAR becomes resensitized following dephosphorylation by protein phosphatase 2A (PP2A) in the endosomes and we have shown previously shown that PP2A activity is regulated by phosphoinositide3-kinaseγ (PI3Kγ). Traditionally, it has been considered that increased desensitization mechanisms underlie βAR dysfunction in heart failure but it is not known whether resensitization of βARs is altered and an integral contributor to heart failure. To test whether resensitization mechanisms are altered in heart failure and could play a role in recovery of βAR function post-LVAD, we used paired pre- and post-LVAD human heart samples along with non-failing hearts. Since resensitization occurs in endosomes, plasma membranes and endosomes were isolated from these human heart samples and assessed for PI3K activity, PP2A activity, β2AR phosphorylation and adenylyl cyclase (AC) activity as a measure of recovery in βAR function by G-protein coupling. Significant PI3Kγ activity was observed in the endosomes of pre-LVAD compared post-LVAD and non-failing human heart samples while plasma membrane PI3K activity remained similar across all samples. Similarly, AC activity was markedly reduced in plasma membranes and endosomes of pre-LVAD samples compared to post-LVAD and non-failing showing reduced ability of receptors to couple to G-proteins indicating reduced recovery of receptor function in post-LVAD heart samples. Consistent with the reduced recovery in βAR function in pre-LVAD samples, significant β2AR phosphorylation was observed in the endosomes and plasma membranes of pre-LVAD compared to post-LVAD and non-failing. Correspondingly, we observed significant reduction in endosomal PP2A activity in the pre-LVAD samples which was remarkably reversed in post-LVAD samples similar to the activity in non-failing. These studies suggest that resensitization is inhibited in end-stage human heart failure and may critically contribute to cardiac remodeling and hypertrophic response upon cardiac stress.
Expression of beta-arrestins and beta-adrenergic receptor kinases in the failing human heart.