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The targets of the p38 MAPK pathway responsible for regulation of neutrophil chemotaxis and exocytosis are unknown. One target of this pathway is the actin-binding protein, heat shock protein 27 (Hsp27). Therefore, we tested the hypothesis that Hsp27 mediates p38 MAPK-dependent chemotaxis and exocytosis in human neutrophils through regulation of actin reorganization. Sequestration of Hsp27 by introduction of anti-Hsp27 Ab, but not an isotype Ab, inhibited fMLP-stimulated chemotaxis, increased cortical F-actin in the absence of fMLP stimulation, and inhibited fMLP-stimulated exocytosis. Pretreatment with latrunculin A prevented actin reorganization and the changes in fMLP-stimulated exocytosis induced by Hsp27 sequestration. To determine the role of Hsp27 phosphorylation, wild-type, phosphorylation-resistant, or phosphorylation-mimicking recombinant Hsp27 was introduced into neutrophils by electroporation. The phosphorylation-resistant mutant significantly reduced migration toward fMLP, whereas none of the Hsp27 proteins affected fMLP-stimulated or TNF-alpha-stimulated exocytosis or actin polymerization. Endogenous Hsp27 colocalized with F-actin in unstimulated and fMLP-stimulated neutrophils, whereas phosphorylated Hsp27 showed cytosolic localization in addition to colocalization with F-actin. Our results suggest that Hsp27 regulates neutrophil chemotaxis and exocytosis in an actin-dependent, phosphorylation-independent manner. Phosphorylation of Hsp27 regulates chemotaxis, but not exocytosis, independent of regulation of actin reorganization.

Heat Shock Protein 27 Regulates Neutrophil Chemotaxis and Exocytosis through Two Independent Mechanisms