Extracellular matrix regulates apoptosis in human neutrophils

Abstract

Background. During inflammation, polymorphonuclear neutrophils (PMNs) migrate into the affected tissue interacting with extracellular matrix (ECM) proteins. We tested the hypothesis that PMN-matrix interaction affects PMN apoptosis. Methods. Apoptosis of human PMNs was detected by DNA- fragmentation assay and was quantitated by flow cytometry, ultraviolet and light microscopy. Cell adhesion was assessed by a toluidine blue assay, and cell spreading was detected by phase contrast microscopy. Protein tyrosine phosphorylation was studied using Western blotting and confocal microscopy. Results. PMN apoptosis was not different in unstimulated cultures on either surface-adherent fibronectin or on PolyHema, a surface that prevents cell adherence. However, tumor necrosis factor-α (TNFα) treatment significantly increased apoptosis on fibronectin (37 ± 4%) compared with PolyHema (20 ± 3%). Tests on other matrix substances revealed that the percentage of apoptotic PMNs in the presence of TNFα was 8 ± 1% on PolyHema, 26 2+ 4% on fibronectin, 17 ± 2% on collagen I, 16 ± 2% on collagen IV, and 16 ± 3% on laminin (P < 0.05 for all matrices compared with PolyHema). Preincubation with genistein (50 μM) significantly inhibited TNFα-mediated apoptosis on fibronectin (39 ± 4% to 21 ± 4%) but not on PolyHema (21 ± 4% to 16 ± 4%). Genistein also reduced PMN spreading on fibronectin. In contrast inhibitors of mitogen-activated protein kinase and protein kinase C showed no effect on PMN apoptosis. Fibronectin strongly increased tyrosine phosphorylation of three 102, 63, and 54 kDa proteins. Five newly tyrosine- phosphorylated 185, 85, 66, 56, and 42 kDa bands were also visible. Using confocal microscopy, highest tyrosine phosphorylation was localized to sites of cell-matrix interaction. Conclusions. ECM influences apoptosis in TNFα- activated, adherent, spreading PMNs. The process is regulated by tyrosine phosphorylation. Acceleration of apoptosis may shorten the PMN lifespan and thereby locally regulate inflammation.