Ceramide inhibition of phospholipase D and its relationship to RhoA and ARF1 translocation in GTPγS-stimulated polymorphonuclear leukocytes

Abstract

AbstractPhospholipase D (PLD) regulates the polymorphonuclear leukocyte (PMN) functions of phagocytosis, degranulation, and oxidant production. Ceramide inhibition of PLD suppresses PMN function. In streptolysin O–permeabilized PMNs, PLD was directly activated by guanosine 5′-[gamma-thio]triphosphate (GTPγS) stimulation of adenosine diphosphate (ADP)–ribosylation factor (ARF) and Rho, stimulating release of lactoferrin from specific granules of permeabilized PMNs; PLD activation and degranulation were inhibited by C2-ceramide but not dihydro-C2-ceramide. To investigate the mechanism of ceramide’s inhibitory effect on PLD, we used a cell-free system to examine PLD activity and translocation from cytosol to plasma membrane of ARF, protein kinase C (PKC)α and β, and RhoA, all of which can activate PLD. GTPγS-activated cytosol stimulated PLD activity and translocation of ARF, PKCα and β, and RhoA when recombined with cell membranes. Prior incubation of PMNs with 10 μM C2-ceramide inhibited PLD activity and RhoA translocation, but not ARF1, ARF6, PKCα, or PKCβ translocation. However, in intact PMNs stimulated with N-formyl-1-methionyl-1-leucyl-1-phenylalamine (FMLP) or permeabilized PMNs stimulated with GTPγS, C2-ceramide did not inhibit RhoA translocation. Exogenous RhoA did not restore ceramide-inhibited PLD activity but bound to membranes despite ceramide treatment. These observations suggest that, although ceramide may affect RhoA in some systems, ceramide inhibits PLD through another mechanism, perhaps related to the ability of ceramide to inhibit phosphatidylinositol-bisphosphate (PIP2) interaction with PLD.