Activation of phospholipase D by PKC and GTPγS in human neuroblastoma cells overexpressing MARCKS

Regulation of phospholipase D (PLD) activity participating in signal transduction involves complex interactions with small G-proteins (ARF, Rho) and protein kinase C isoforms (PKCα). In SK-N-MC human neuroblastoma cells, phorbol ester (TPA) activation of PLD was enhanced by overexpressing myristoylated alanine-rich C kinase substrate (MARCKS). To study MARCKS interactions with PLD, we investigated PLD isoform expression and activation by TPA and GTPγS in intact and digitonin-permeabilized clones transfected with MARCKS (M22). PLD2 was in both cytosol and membrane fractions while PLD1 was primarily membrane-associated in both vector control and M22 cells; location or quantities were unaltered by TPA treatment. TPA-stimulated PLD activity was higher in both intact and digitonin-permeabilized M22 cells than in vector controls. In contrast, GTPγS-stimulated PLD activity was independent of MARCKS expression but was additive with MARCKS-PKC-dependent activation in permeabilized cells. Combinations of PKC inhibition and down- regulation in intact and permeabilized (with GTPγS present) cells indicated that a PKC-mediated phosphorylation event was necessary in intact cells without access to GTPγS, stimulation of PLD mediated by GTPγS was independent of PKC, and PLD activation by PKC in permeabilized cells was kinase-independent. Western blot analysis showed that MARCKS, PKCα, PLD1 and PLD2 were present in a detergent-insoluble fraction (DIF); GTPγS increased recovery of PLD2 in DIF. Disruption of cholesterol-rich DIFs with digitonin, cyclodextrin or filipin potentiated activation of PLD by TPA. Our studies suggest that activation of PLD by PKC requires MARCKS and can involve both phosphorylation-independent and -dependent processes. As PLD activation by GTPγS is PKC-MARCKS-independent, MARCKS may provide a fine tuning component in conjunction with G-protein-mediated mechanisms for regulation of PLD.