A novel mechanism of CD4 down-modulation induced by monosialoganglioside GM3 - Involvement of serine phosphorylation and protein kinase C delta translocation

In this report the molecular mechanism(s) involved in the rapid and selecti ve endocytosis of cell surface glycoprotein CD4 induced by exogenous monosi aloganglioside GM3 in human peripheral blood lymphocytes have been investig ated. Inhibition of the GM3-induced CD4 down-modulation was observed in the presence of specific protein kinase C (PKC) inhibitors. Scanning confocal microscopy revealed the translocation and clustering on the cell surface of PKC isozymes delta and theta (more evidently than alpha and beta) after GM 3 treatment, suggesting the involvement of these isozymes in the gangliosid e-induced CD4 down-modulation. Exogenous GM3 induced phosphorylation of CD4 molecule, which then dissociated from p56(lck), as early as after 5 min. M oreover, addition of GM3 resulted in a rapid (1 min) cytosolic phospholipas e A(2) activation with consequent arachidonic acid release, whereas no phos phatidylinositol-phospholipase C activity was observed. Both PKC translocat ion and CD4 down-modulation were blocked by the trifluoromethylketone analo g of arachidonic acid, a selective inhibitor of cytosolic phospholipase A(2 ) and by mitogen-activated protein kinase inhibitor PD98059. Taken together , these findings strongly suggest that GM3 may trigger a novel mechanism of modulation of the CD4 surface expression through the activation of enzyme( s) involved in the regulation of cellular functions.