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.
Department of Pathology, University of New Mexico Health Sciences Center, Albuquerque, NM 87131, USA. achigaev@salud.unm.edu
Alexandre
A
Chigaev
Anna
A
Waller
Or
O
Amit
Larry A
LA
Sklar
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