Paxillin selectively associates with constitutive and chemoattractant-induced high-affinity alpha4beta1 integrins: implications for integrin signaling.

Leukocyte alpha4beta1 integrins regulate hematopoietic and lymphoid development, as well as the emigration of circulating cells to sites of inflammation. Because vascular cell adhesion molecule-1 (VCAM-1) binding to high-affinity alpha4beta1 is stable, these integrins can be detected and selectively precipitated from cell lysates using VCAM-1/Fc. With this approach, high-affinity alpha4beta1 integrin expression was demonstrated on lymphocytes in the bone marrow, thymus, spleen, and the peritoneal cavity of normal mice, but not in peripheral lymph nodes. Immature lymphocytes preferentially expressed high-affinity alpha4beta1 in the bone marrow and thymus. Paxillin is a cytoplasmic adaptor molecule that can bind to the alpha4 tail and initiate signaling. Paxillin was associated selectively with high-affinity integrins that were isolated from human Jurkat T cells or from murine tissues, and blotting with a phospho-specific antibody demonstrated that Ser988 in the alpha4 cytoplasmic tail was dephosphorylated in high-affinity but not low-affinity integrins. A rapid and transient alpha4beta1 affinity up-regulation in formyl peptide receptor-transfected U937 cells stimulated with N-formyl-methyonyl-leucyl-phenylalanine (fMLP) correlated temporally with induced paxillin binding to alpha4 integrins. These data suggest that ligand binding to high-affinity alpha4beta1 integrins may initiate outside-in signaling cascades through paxillin that regulate leukocyte maturation and emigration.