Functional and structural consequences of chemokine (C-X-C motif) receptor 4 activation with cognate and non-cognate agonists.

Chemokine (C-X-C motif) receptor 4 (CXCR4) regulates cell trafficking and plays important roles in the immune system. Ubiquitin has recently been identified as an endogenous non-cognate agonist of CXCR4, which activates CXCR4 via interaction sites that are distinct from those of the cognate agonist C-X-C motif chemokine ligand 12 (CXCL12). As compared with CXCL12, chemotactic activities of ubiquitin in primary human cells are poorly characterized. Furthermore, evidence for functional selectivity of CXCR4 agonists is lacking, and structural consequences of ubiquitin binding to CXCR4 are unknown. Here, we show that ubiquitin and CXCL12 have comparable chemotactic activities in normal human peripheral blood mononuclear cells, monocytes, vascular smooth muscle, and endothelial cells. Chemotactic activities of the CXCR4 ligands could be inhibited with the selective CXCR4 antagonist AMD3100 and with a peptide analogue of the second transmembrane domain of CXCR4. In human monocytes, ubiquitin- and CXCL12-induced chemotaxis could be inhibited with pertussis toxin and with inhibitors of phospholipase C, phosphatidylinositol 3 kinase, and extracellular signal-regulated kinase 1/2. Both agonists induced inositol trisphosphate production in vascular smooth muscle cells, which could be inhibited with AMD3100. In β-arrestin recruitment assays, ubiquitin did not sufficiently recruit β-arrestin2 to CXCR4 (EC50  > 10 μM), whereas the EC50 for CXCL12 was 4.6 nM (95% confidence interval 3.1-6.1 nM). Both agonists induced similar chemical shift changes in the 13 C-1 H-heteronuclear single quantum correlation (HSQC) spectrum of CXCR4 in membranes, whereas CXCL11 did not significantly alter the 13 C-1 H-HSQC spectrum of CXCR4. Our findings point towards ubiquitin as a biased agonist of CXCR4.