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

Alexandre Chigaev,Larry A Sklar,Anna Waller,Or Amit

doi:10.1186/1471-2172-9-26

Abstract

BackgroundActivation 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). α4β1-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 Gαi-coupled GPCRs. The goal of the current report was to study the effect of Gαs-coupled GPCRs upon integrin activation.ResultsUsing real-time fluorescent ligand binding to assess affinity and a FRET based assay to probe α4β1-integrin unbending, we show that two Gαs-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 Gαi-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 Gαs-induced responses were not associated with changes in the expression level of the Gαi-coupled receptors. In contrast, the molecular unbending of VLA-4 was not significantly affected by Gαs-coupled GPCR signaling. In a VLA-4/VCAM-1-specific myeloid cell adhesion system, inhibition of the VLA-4 affinity change by Gαs-coupled GPCR had a statistically significant effect upon cell aggregation.ConclusionWe conclude that Gαs-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.

Highlights

Activation of integrins in response to inside-out signaling serves as a basis for leukocyte arrest on endothelium, and migration of immune cells
VLA-4 has the potential to exist in multiple affinity states that can mediate tethering, rolling, and arrest on a ligand (CD106, Vascular Cell Adhesion Molecule-1, VCAM-1) that is upregulated on inflamed endothelia [9,10]
Using a VLA-4/VCAM-1specific myeloid cell adhesion model system, we found that blocking the integrin affinity change by the activation of the Gαs/cyclic adenosine monophosphate (cAMP) signaling pathway had a statistically significant effect upon cell adhesion

Summary

Introduction

Activation of integrins in response to inside-out signaling serves as a basis for leukocyte arrest on endothelium, and migration of immune cells. Α4β1-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 Gαi-coupled GPCRs. The goal of the current report was to study the effect of Gαscoupled GPCRs upon integrin activation. Understanding the molecular mechanisms that regulate rapid changes in cell adhesion avidity is essential, since integrins are known to play roles in many human diseases. They represent an attractive target for several existing and emerging drugs for treatment of inflammatory diseases, anti-angiogenic cancer therapy, anti-thrombotic therapy, and others [2,3,4,5]. The change in the affinity for the ligand as well as the unbending (extension) of the integrin molecule are believed to be a part of this activation process [1214]

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