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. CONCLUSION 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.

中文翻译：

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Galphas 偶联受体信号主动下调 alpha4beta1-integrin 亲和力：细胞脱粘的可能机制。

背景响应内向外信号的整联蛋白的激活作为白细胞在内皮上停滞和免疫细胞迁移的基础。整联蛋白依赖性粘附受分子构象状态（即对配体和分子伸直（延伸）的亲和力的变化）控制，其受七跨膜鸟嘌呤核苷酸结合蛋白偶联受体（GPCR）调节。alpha4beta1-integrin（CD49d/CD29、Very Late Antigen-4、VLA-4）在白细胞、造血干细胞、造血癌细胞和其他细胞上表达。VLA-4 的亲和力和扩展都通过几个 Galphai 耦合的 GPCR 由内向外信号快速上调。当前报告的目标是研究 Galphas 偶联 GPCR 对整合素激活的影响。结果 使用实时荧光配体结合来评估亲和力和基于 FRET 的测定来探测 alpha4beta1-整合素不弯曲，我们表明两个 Galphas 偶联的 GPCR（H2-组胺受体和 beta2-肾上腺素能受体）以及几种 cAMP 激动剂可以快速下调通过 U937 细胞和原代人外周血单核细胞中的两个 Galphai 偶联受体（CXCR4 和 FPR）激活的 VLA-4 的亲和力。这种下调可以被受体特异性拮抗剂阻断。Galphas 诱导的反应与 Galphai 耦合受体的表达水平的变化无关。相比之下，VLA-4 的分子伸直不受 Galphas 耦合 GPCR 信号的显着影响。在 VLA-4/VCAM-1 特异性骨髓细胞粘附系统中，Galphas 偶联 GPCR 对 VLA-4 亲和力变化的抑制对细胞聚集具有统计学意义。结论我们得出结论，Galphas 偶联的 GPCR 可以通过 cAMP 依赖途径快速下调 VLA-4 结合口袋的亲和力状态。这在调节细胞粘附中起重要作用。我们讨论了这种描述的现象的几个可能的含义。