Galphas-coupled receptor signaling actively down-regulates α4β1-integrin affinity:: A possible mechanism for cell de-adhesion

Background: 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). alpha(4)beta(1)-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 G alpha(i)-coupled GPCRs. The goal of the current report was to study the effect of G alpha(s)-coupled GPCRs upon integrin activation. Results: Using real-time fluorescent ligand binding to assess affinity and a FRET based assay to probe alpha(4)beta(1)-integrin unbending, we show that two G alpha(s)-coupled GPCRs (H2-histamine receptor and beta 2-adrenergic receptor) as well as several cAMP agonists can rapidly down modulate the affinity of VLA-4 activated through two G alpha(i)-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 G alpha(s)-induced responses were not associated with changes in the expression level of the G alpha(i)-coupled receptors. In contrast, the molecular unbending of VLA-4 was not significantly affected by G alpha(s)-coupled GPCR signaling. In a VLA-4/VCAM-1-specific myeloid cell adhesion system, inhibition of the VLA-4 affinity change by Gas-coupled GPCR had a statistically significant effect upon cell aggregation. Conclusion: We conclude that G alpha(s)-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.