Endocytosis and vesicular trafficking of immune complexes and activation of phospholipase D by the human high-affinity IgG receptor requires distinct phosphoinositide 3-kinase activities

Abstract

FcγRI, the human high-affinity IgG receptor, is responsible for the internalization of immune complexes and their subsequent targetting to the lysosomes for degradation. We show here that aggregation of FcγRI by surface immune complexes in interferon-γ-primed U937 cells causes the transient appearance of swollen vacuolar structures, probably swollen late endosomes, which disappear as the immune complexes are degraded. Wortmannin and LY294002, specific inhibitors of phosphoinositide 3-kinases (PI 3-kinases), delay the disappearance of these structures and also correspondingly inhibit degradation of FcγRI-mediated immune complexes. In addition these inhibitors delay the initial phase of FcγRI-mediated endocytosis of immune complexes and block the activity of FcγRI-stimulated phospholipase D, an enzyme that has previously been implicated in membrane-trafficking events. p85 is the regulatory subunit of PI 3-kinase. A p85-dependent PI 3-kinase was shown to be involved in the initial phase of FcγRI-mediated endocytosis, but not in the trafficking of immune complexes for degradation or the activation of phospholipase D. The results presented here show a role for a p85-independent PI 3-kinase in regulating the trafficking of FcγRI-mediated immune complexes, either directly or as a result of the activation of phospholipase D, and a distinct role for a p85-dependent PI 3-kinase isoform in the initial phases of FcγRI-mediated internalization of immune complexes.

Signalling roles of mammalian phospholipase D1 and D2

“…Aggregation of the high-affinity receptor for IgG, FcgRI, by immune complexes results in the internalisation of these complexes and their trafficking to lysosomes for degradation. Activation of PLD is necessary for the efficient trafficking of the internalised immune complexes to lysosomes using butanol as a suppressor of PA production [87,88]. Aggregation of the receptor stimulates the association of PLD1 with ARF6 and PKCa.…”

Phospholipase D—Structure, regulation and function

“…Neither does it or LY294002 inhibit the activation of the enzyme by angiotensin II in vascular smooth muscle ceils (Andresen et al 2001). On the other hand, the inhibitors block the stimulation of PLD by the high affinity IgG receptor FcyRI in U937 cells (Gillooly et al 1999) and the activation of the enzyme in endothelial cells by stem cell factor (Kozawa et al 1997). Using a different approach, namely using cells expressing wild type or mutant PDGF receptors, no evidence could be obtained for a role of PI 3-kinase in PDGF activation of PLD ).…”