Novel pathways of F-actin polymerization in the human neutrophil

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Abstract
Recently we demonstrated the existence of a phosphatidylinositol 3-kinase (PI3K)–independent F-actin polymerization during neutrophil pseudopod extension. Here we examine the use of the PI3K-dependent and PI3K-independent pathways of activation by the N-formyl peptide receptor and the chemokine receptors, and the priming of the 2 pathways by granulocyte-macrophage colony-stimulating factor (GM-CSF) and insulin. The inhibition of PI3K activity with wortmannin showed that rate of pseudopod extension stimulated with N-formyl-Met-Leu-Phe (fMLP was mostly dependent on PI3K, while the rate of interleukin-8 (IL-8)–stimulated pseudopod extension was less dependent on PI3K. The incubation of cells with either GM-CSF or insulin increased the rate of pseudopod extension by 50% when the cells were stimulated with IL-8 but not with fMLP. The stimulation with IL-8 phosphorylated the PI3K regulatory subunit. This phosphorylation was enhanced by GM-CSF, which increased PI3K activity and total phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) production. The effect of GM-CSF was blocked with wortmannin. In contrast, insulin did not increase p85 phosphorylation and did not enhance PI3K activity or PtdIns(3,4,5)P3 production. The effect of insulin was insensitive to wortmannin; however, it was blocked by an Src homology 2 (SH2)–binding peptide. These data indicate that priming of IL-8 activation with GM-CSF was mediated via the PI3Ks of class IA, while priming with insulin used a PI3K-independent pathway.