Galphas-coupled receptor signaling actively down-regulates alpha4beta1-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). alpha4beta1-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 Galphai-coupled GPCRs. The goal of the current report was to study the effect of Galphas-coupled GPCRs upon integrin activation.


RESULTS
Using real-time fluorescent ligand binding to assess affinity and a FRET based assay to probe alpha4beta1-integrin unbending, we show that two Galphas-coupled GPCRs (H2-histamine receptor and beta2-adrenergic receptor) as well as several cAMP agonists can rapidly down modulate the affinity of VLA-4 activated through two Galphai-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 Galphas-induced responses were not associated with changes in the expression level of the Galphai-coupled receptors. In contrast, the molecular unbending of VLA-4 was not significantly affected by Galphas-coupled GPCR signaling. In a VLA-4/VCAM-1-specific myeloid cell adhesion system, inhibition of the VLA-4 affinity change by Galphas-coupled GPCR had a statistically significant effect upon cell aggregation.


CONCLUSIONS
We conclude that Galphas-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.
Chigaev
Alexandre
A
Department of Pathology, University of New Mexico Health Sciences Center, Albuquerque, NM 87131, USA. achigaev@salud.unm.edu
Waller
Anna
A
Amit
Or
O
Sklar
Larry A
LA
D000318:Adrenergic beta-Agonists / Q000494:pharmacology; D002448:Cell Adhesion / Q000187:drug effects / Q000276:immunology*; D054377:Chemokine CXCL12 / Q000819:agonists / Q000494:pharmacology; D015536:Down-Regulation; D017442:Histamine Agonists / Q000494:pharmacology; D006801:Humans; D039041:Integrin alpha4beta1 / Q000737:chemistry* / Q000276:immunology / Q000378:metabolism*; D007545:Isoproterenol / Q000494:pharmacology; D007963:Leukocytes, Mononuclear; D009842:Oligopeptides / Q000494:pharmacology; D010671:Phenylurea Compounds / Q000494:pharmacology; D011485:Protein Binding / Q000187:drug effects; D011487:Protein Conformation; D020239:Receptor Cross-Talk / Q000276:immunology*; D018343:Receptors, Adrenergic, beta-2 / Q000276:immunology / Q000378:metabolism; D019718:Receptors, CXCR4 / Q000819:agonists; D011970:Receptors, Histamine H2 / Q000276:immunology / Q000378:metabolism; D011994:Recombinant Proteins / Q000819:agonists; D015398:Signal Transduction; D013844:Thiazoles / Q000494:pharmacology; D020298:U937 Cells
C517048:4-((N'-2-methylphenyl)ureido)phenylalanyl-leucyl-alpha-aspartyl-valyl-prolyl-alanyl-alanyl-lysine; D000318:Adrenergic beta-Agonists; C515255:CXCL12 protein, human; C527276:CXCR4 protein, human; D054377:Chemokine CXCL12; D017442:Histamine Agonists; D039041:Integrin alpha4beta1; D009842:Oligopeptides; D010671:Phenylurea Compounds; D018343:Receptors, Adrenergic, beta-2; D019718:Receptors, CXCR4; D011970:Receptors, Histamine H2; D011994:Recombinant Proteins; D013844:Thiazoles; C076207:amthamine; D007545:Isoproterenol