Differential tyrosine phosphorylation of phospholipase D isozymes by hydrogen peroxide and the epidermal growth factor in A431 epidermoid carcinoma cells

The regulatory mechanism through which the phospholipase D (PLD) isoforms PLD1 and PLD2 are activated is poorly understood. We investigated the possibility that the PLD isozymes are differentially regulated in response to pharmacologic stimulants in cells. In this report, we demonstrate for the first time that H₂O₂ and EGF differentially induce tyrosine phosphorylation of the PLD isozymes in A431 cells, which express both PLD1 and PLD2. H₂O₂ induced tyrosine phosphorylation of PLD1 and PLD2, whereas EGF only caused the tyrosine phosphorylation of PLD2. Both agents also induced phosphorylation of the EGF receptor. Interestingly, the PLD isozymes were associated with the EGF receptor and PKC-α in a ligand independent manner. Activation of PLD by H₂O₂ and EGF nearly correlated with tyrosine phosphorylation of the protein in PLD1 immune complexes. Activation of PLD by both agents was inhibited by the PKC inhibitor, Ro 31-8220, and by the down-regulation of PKC. Pretreatment of the cells with the tyrosine kinase inhibitor tyrphostin AG1478 resulted in inhibition of the H₂O₂ and EGF-induced tyrosine phosphorylation and PLD activation. These results indicate that H₂O₂ and EGF induce differential tyrosine phosphorylation of PLD isozymes. Also, the activation of PLD by these agonists involves tyrosine phosphorylation and PKC activation.