Protein kinase C_α mediates phospholipase D activation by nucleotides and phorbol ester in Madin-Darby canine kidney cells: Stimulation of phospholipase D is independent of activation of polyphosphoinositide-specific phospholipase C and phospholipase A₂

Protein kinase C (PKC) has been implicated in the activation of phospholipase D (PLD) in a number of systems. By antisense technology, we have "knocked out" α and β isoforms of PKC to study the role of these isoforms in PLD activation in Madin-Darby canine kidney (MDCK) cells. To this end, we have studied PLD activation by phorbol 12-myristate 13-acetate (PMA), ATP, UTP, and 2-methylthio-ATP in cells labeled with [³H]palmitic acid. [³H]Phosphatidylethanol (PEt) production catalyzed by PLD in the presence of ethanol was time- and concentration-dependent in PMA- and nucleotide-stimulated cells. In Ca²⁺-free medium, [³H]PEt accumulation was diminished for all stimuli assayed. Treatment of cells with chelerythrine, an inhibitor of PKC, and phorbol ester down-regulation of PKC inhibited [³H]PEt production by both PMA and nucleotides. In cells transfected with antisense PKC_α or both PKC_α and PKCβ, PLD activation was inhibited by both PMA and nucleotides, whereas in cells transfected with antisense PKCβ, PLD activation was similar to that of control cells. Moreover, inhibition of polyphosphoinositide-specific PLC (by neomycin) or of release of arachidonic acid and arachidonic acid metabolites (by nordihydroguaiaretic acid or by indomethacin) failed to decrease [³H]PEt accumulation in PMA- and nucleotide-stimulated MDCK-D1 cells. From these data, we conclude that in MDCK-D1 cells PMA and nucleotide receptors utilize PKC_α to regulate PLD activity and that PLD activation is independent of the activation of polyphosphoinositide-specific PLC and phospholipase A₂-mediated release of arachidonic acid or arachidonic acid metabolites.