β2-adrenergic receptor signaling in the cardiac myocyte is modulated by interactions with CXCR4

Chemokines are small secreted proteins with chemoattractant properties that play a key role in inflammation, metastasis, and embryonic development. We previously demonstrated a nonchemotactic role for one such chemokine pair, stromal cell-derived factor-1α and its G-protein coupled receptor, CXCR4. Stromal cell-derived factor-1/CXCR4 are expressed on cardiac myocytes and have direct consequences on cardiac myocyte physiology by inhibiting contractility in response to the nonselective β-adrenergic receptor (βAR) agonist, isoproterenol. As a result of the importance of β-adrenergic signaling in heart failure pathophysiology, we investigated the underlying mechanism involved in CXCR4 modulation of βAR signaling. Our studies demonstrate activation of CXCR4 by stromal cell-derived factor-1 leads to a decrease in βAR-induced PKA activity as assessed by cAMP accumulation and PKA-dependent phosphorylation of phospholamban, an inhibitor of SERCA2a. We determined CXCR4 regulation of βAR downstream targets is β₂AR-dependent. We demonstrated a physical interaction between CXCR4 and β₂AR as determined by coimmunoprecipitation, confocal microscopy, and BRET techniques. The CXCR4-β₂AR interaction leads to G-protein signal modulation and suggests the interaction is a novel mechanism for regulating cardiac myocyte contractility. Chemokines are physiologically and developmentally relevant to myocardial biology and represent a novel receptor class of cardiac modulators. The CXCR4-β₂AR complex could represent a hitherto unknown target for therapeutic intervention.