Heteromerization of chemokine (C-X-C motif) receptor 4 with α1A/B-adrenergic receptors controls α1-adrenergic receptor function

Recent evidence suggests that chemokine (C-X-C motif) receptor 4 (CXCR4) contributes to the regulation of blood pressure through interactions with alpha(1)-adrenergic receptors (ARs) in vascular smooth muscle. The underlying molecular mechanisms, however, are unknown. Using proximity ligation assays to visualize single-molecule interactions, we detected that alpha(1A/B)-ARs associate with CXCR4 on the cell surface of rat and human vascular smooth muscle cells (VSMC). Furthermore, alpha(1A/B)-AR could be coimmunoprecipitated with CXCR4 in a HeLa expression system and in human VSMC. A peptide derived from the second transmembrane helix of CXCR4 induced chemical shift changes in the NMR spectrum of CXCR4 in membranes, disturbed the association between alpha(1A/B)-AR and CXCR4, and inhibited Ca2+ mobilization, myosin light chain (MLC) 2 phosphorylation, and contraction of VSMC upon alpha(1)-AR activation. CXCR4 silencing reduced alpha(1A/B)-AR: CXCR4 heteromeric complexes in VSMC and abolished phenylephrine-induced Ca2+ fluxes and MLC2 phosphorylation. Treatment of rats with CXCR4 agonists (CXCL12, ubiquitin) reduced the EC50 of the phenylephrineinduced blood pressure response three-to fourfold. These observations suggest that disruption of the quaternary structure of alpha(1A/B)-AR: CXCR4 heteromeric complexes by targeting transmembrane helix 2 of CXCR4 and depletion of the heteromeric receptor complexes by CXCR4 knockdown inhibit alpha(1)-AR-mediated function in VSMC and that activation of CXCR4 enhances the potency of alpha(1)-AR agonists. Our findings extend the current understanding of the molecular mechanisms regulating alpha(1)-AR and provide an example of the importance of G protein-coupled receptor (GPCR) heteromerization for GPCR function. Compounds targeting the alpha(1A/B)-AR: CXCR4 interaction could provide an alternative pharmacological approach to modulate blood pressure.