A critical role for PI 3-kinase in cytokine-induced Fca-receptor activation

Abstract

Fc-receptors, such as FcaR and FcgRII, play an important role in leukocyte activation, and rapid modulation of ligand binding (‘‘activation’’) is critical for receptor regulation.We have previously demonstrated that ligand binding to Fc-receptors on human eosinophils is dependent on cytokine stimulation. Utilization of pharmacological inhibitors provided evidence that the phenomenon of interleukin (IL)-5 induced immunoglobulin A (IgA) binding to human eosinophils requires activation of phosphatidylinositol 3-kinase (PI3K). However, eosinophils are refractory to manipulation by molecular techniques such as DNA transfection or viral infection. Here we utilize an IL-3 dependent pre-B cell line to investigate the molecular mechanism of cytokinemediated ligand binding to FcaR. In this system, IgA binding is dependent on IL-3, similarly to the requirement for IL-5 of eosinophils. We show that IL-3-mediated activation of FcaR (CD89) requires the activation of PI3K, independent of p21ras activation. Co-expression of dominant negative (Dp85) and active (p110_K227E) forms of PI3K demonstrate that the affinity switch regulating FcaR activation requires PI3K. Moreover, overexpression of PI3K is both necessary and sufficient for activation of FcaR. Furthermore, we show that IL-3/IL-5/GM-CSF induced inside- out signaling pathways activating FcaR require the involvement of protein kinase C downstream of PI3K. Finally, we show that these inside-out signaling pathways responsible for Fca-receptor modulation require CD89, independent of its association with the FcRg chain.