| Summary: | <p>Abstract</p> <p>Background</p> <p>Activation of integrins in response to inside-out signaling serves as a basis for leukocyte arrest on endothelium, and migration of immune cells. Integrin-dependent adhesion is controlled by the conformational state of the molecule (i.e. change in the affinity for the ligand and molecular unbending (extension)), which is regulated by seven-transmembrane Guanine nucleotide binding Protein-Coupled Receptors (GPCRs). α<sub>4</sub>β<sub>1</sub>-integrin (CD49d/CD29, Very Late Antigen-4, VLA-4) is expressed on leukocytes, hematopoietic stem cells, hematopoietic cancer cells, and others. Affinity and extension of VLA-4 are both rapidly up-regulated by inside-out signaling through several Gα<sub>i</sub>-coupled GPCRs. The goal of the current report was to study the effect of Gα<sub>s</sub>-coupled GPCRs upon integrin activation.</p> <p>Results</p> <p>Using real-time fluorescent ligand binding to assess affinity and a FRET based assay to probe α<sub>4</sub>β<sub>1</sub>-integrin unbending, we show that two Gα<sub>s</sub>-coupled GPCRs (H2-histamine receptor and β2-adrenergic receptor) as well as several cAMP agonists can rapidly down modulate the affinity of VLA-4 activated through two Gα<sub>i</sub>-coupled receptors (CXCR4 and FPR) in U937 cells and primary human peripheral blood monocytes. This down-modulation can be blocked by receptor-specific antagonists. The Gα<sub>s</sub>-induced responses were not associated with changes in the expression level of the Gα<sub>i</sub>-coupled receptors. In contrast, the molecular unbending of VLA-4 was not significantly affected by Gα<sub>s</sub>-coupled GPCR signaling. In a VLA-4/VCAM-1-specific myeloid cell adhesion system, inhibition of the VLA-4 affinity change by Gα<sub>s</sub>-coupled GPCR had a statistically significant effect upon cell aggregation.</p> <p>Conclusion</p> <p>We conclude that Gα<sub>s</sub>-coupled GPCRs can rapidly down modulate the affinity state of VLA-4 binding pocket through a cAMP dependent pathway. This plays an essential role in the regulation of cell adhesion. We discuss several possible implications of this described phenomenon.</p>
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