Phospholipase C, calcium, and calmodulin are critical for α4β1 integrin affinity up-regulation and monocyte arrest triggered by chemoattractants

During inflammation, monocytes roll on activated endothelium and arrest after stimulation by proteoglycan-bound chemokines and other chemoattractants. We investigated signaling pathways downstream of G protein-coupled receptors (GPCRs) that are relevant to alpha 4 beta 1 integrin affinity up-regulation using formyl peptide receptor-transfected U937 cells stimulated with fMLP or stromal-derived factor-1 alpha and human peripheral blood monocytes stimulated with multiple chemokines or chemoattractants. The upregulation of soluble LDV peptide or vascular cell adhesion molecule-1 (VCAM-1) binding by these stimuli was critically dependent on activation of phospholipase C (PLC), inositol 1,4,5-triphosphate receptors, increased intracellular calcium, influx of extracellular calcium, and calmodulin, suggesting that this signaling pathway is required for alpha 4 integrins to assume a high-affinity conformation. In fact, a rise in intracellular calcium following treatment with thapsigargin or ionomycin was sufficient to induce binding of ligand. Blockade of p44/42 and p38 mitogen-activated protein (MAP) kinases, phosphoinositide 3-kinase, or protein kinase C (PKC) signaling did not inhibit chemoattractant-induced LDV or VCAM-1 binding. However, activation of PKC by phorbol ester up-regulated alpha 4 beta 1 affinity with kinetics distinct from those of GPCR signaling. A critical role for PLC and calmodulin was also established for leukocyte arrest and adhesion strengthening.