ECTIVATION OF PHOSPHOLIPASE-D IN CHO CELLS TRANSFECTED WITH THE HUMANEPIDERMAL GROWTH-FACTOR (EGF) RECEPTOR - DIFFERENTIAL-EFFECTS OF PROTEIN-KINASE-C ACTIVATION AND EGF

Multiple intracellular signal transduction pathways, including phospho lipases A2 and D, can be activated by epidermal growth factor (EGF) in both a protein kinase C (PKC)-dependent and -independent manner. We i nvestigated the activation of phospholipase D (PLD) by a PKC activator , phorbol myristate acetate (PMA) and by EGF in CHO cells transfected with the full-length EGF receptor. In cells labelled with arachidonic acid or linoleic acid, PMA activated a PLD, determined by formation of the transphosphatidylation product phosphatidylethanol in the presenc e of ethanol. A basal PLD activity was seen in linoleic acid-labelled cells but not in cells labelled with arachidonic acid. This basal acti vity was augmented by the protein phosphotyrosine phosphatase inhibito r vanadate and reduced by tyrosine kinase inhibition and was contribut ed to by PKC, as activity could not be elicited following prolonged ex posure to phorbol ester, known to down-regulate some PKC isoforms. By contrast, EGF failed to stimulate formation of phosphatidylethanol in cells labelled with either fatty acid species. It is proposed that in the basal condition PKC-dependent PLD activation and protein tyrosine kinase phosphorylation are linked (possibly by a phospholipase C (PLC) -mediated formation of diacylglycerol); EGF which activated a phosphol ipase A2 (PLA2) but which failed to elicit PLC activation in these cel ls is without further effect on PLD.