Abstract 579: Beta-blockers Regulate Hypoxia Sensing of Beta Adrenergic Receptors

Abstract

Beta adrenergic receptors (βARs) are one of the most powerful regulators of cardiac function. Activation of βAR leads to phosphorylation by G-protein coupled receptor kinase 2 and β-arrestin binding resulting in desensitization. The phosphorylated βARs are resensitized by dephosphorylation by PP2A. Besides classical agonist activation, increasing evidence shows that βARs can be regulated by inflammation and oxidative stress. In this regard, we have shown that βARs can be regulated by hypoxia wherein hypoxia increases the β2AR phosphorylation and HIFα accumulation (Cheong et. al., 2016). However, the mechanisms of βARs regulation in hypoxia are not well understood and we postulated that shift in homeostasis existing between kinase and phosphatase driven mechanisms may underlie βAR dysfunction. To test this hypothesis, β2AR HEK 293 cells were exposed to hypoxia (2% O 2 ) and assessed the mechanisms underlying desensitization (kinases) and resensitization (PP2A). Our studies showed that 6 hours of hypoxia treatment resulted in increase of βAR phosphorylation and GRK2 expression. Assessment of βAR phosphorylation in the plasma membrane and endosomal fractions surprisingly, shoed marked increase in β2AR phosphorylation in the endosomal fraction. Furthermore, we also observed that receptor associated PP2A activity was reduced specifically in the endosomes with minimal changes of activity at the plasma membranes. Given that β-blockers confer beneficial effects, we tested whether β-blockers (propranolol or carvedilol) would prevent βARs phosphorylation under hypoxia or normoxia. Consistently, β-blocker treatment in normoxia results in increased β2AR phosphorylation. Remarkably β-blocker treatment in hypoxia results in loss of β2AR phosphorylation and reduction in GRK2 expression. These studies suggest agonist-independent hypoxia-driven β2AR dysfunction can be ameliorated by β-blockers and the underlying mechanisms driving this hypoxia mediated β2AR dysfunction will be discussed in the presentation. These unexpected findings have significant clinical implications given that hypoxia underlies stroke/myocardial infarction and understanding these mechanisms could provide novel insights into the benefits provided by β-blockers.