Chigaev Alexandre, Waller Anna, Amit Or, Sklar Larry A
Department of Pathology, University of New Mexico Health Sciences Center, Albuquerque, NM 87131, USA.
BMC Immunol. 2008 Jun 5;9:26. doi: 10.1186/1471-2172-9-26.
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.
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.
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.
整合素因外向内信号传导而激活，这是白细胞在内皮上滞留以及免疫细胞迁移的基础。整合素依赖性黏附受分子构象状态的控制（即对配体亲和力的变化和分子伸直（伸展）），其由七跨膜鸟嘌呤核苷酸结合蛋白偶联受体（GPCRs）调节。α4β1整合素（CD49d/CD29，极迟抗原-4，VLA-4）在白细胞、造血干细胞、造血癌细胞等细胞上表达。VLA-4的亲和力和伸展均通过几种Gαi偶联的GPCRs经外向内信号传导迅速上调。本报告的目的是研究Gαs偶联的GPCRs对整合素激活的影响。
使用实时荧光配体结合来评估亲和力，并采用基于荧光共振能量转移（FRET）的检测方法来探测α4β1整合素的伸直，我们发现两种Gαs偶联的GPCRs（H2组胺受体和β2肾上腺素能受体）以及几种环磷酸腺苷（cAMP）激动剂可迅速下调通过两种Gαi偶联受体（CXCR4和FPR）激活的U937细胞和原代人外周血单核细胞中VLA-4的亲和力。这种下调可被受体特异性拮抗剂阻断。Gαs诱导的反应与Gαi偶联受体的表达水平变化无关。相反，VLA-分子的伸直未受Gαs偶联的GPCR信号传导的显著影响。在VLA-4/血管细胞黏附分子-1（VCAM-1）特异性髓样细胞黏附系统中，Gαs偶联的GPCR对VLA-4亲和力变化的抑制对细胞聚集具有统计学上的显著影响。
我们得出结论，Gαs偶联的GPCRs可通过cAMP依赖性途径迅速下调VLA-4结合口袋的亲和力状态。这在细胞黏附的调节中起重要作用。我们讨论了这一所述现象的几种可能影响。
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