Antagonistic effects of protein kinase C α and δ on both transformation and phospholipase D activity mediated by the epidermal growth factor receptor

Downregulation of protein kinase C δ (PKC δ) by treatment with the tumor-promoting phorbol ester 12-O-tetradecanoylphorbo;-13-acetate (TPA) transforms cells that overexpress the non-receptor class tyrosine kinase c- Src (Z. Lu et al., Mol. Cell. Biol. 17:3418-3428, 1997). We extended these studies to cells overexpressing a receptor class tyrosine kinase, the epidermal growth factor (EGF) receptor (EGFR cells); like c-Src, the EGF receptor is overexpressed in several human tumors. In contrast with expectations, downregulation of PKC isoforms with TPA did not transform the EGFR cells; however, treatment with EGF did transform these cells. Since TPA downregulates all phorbol ester-responsive PKC isoforms, we examined the effects of PKC δ- and PKC α-specific inhibitors and the expression of dominant negative mutants for both PKC δ and α. Consistent with a tumor- suppressing function for PKC δ, the PKC δ-specific inhibitor rottlerin and a dominant negative PKC δ mutant transformed the EGFR cells in the absence of EGF. In contrast, the PKC α-specific inhibitor Go6976 and expression of a dominant negative PKC α mutant blocked the transformed phenotype induced by both EGF and PKC α inhibition. Interestingly, both rottlerin and EGF induced substantial increases in phospholipase D (PLD) activity, which is commonly elevated in response to mitogenic stimuli. The elevation of PLD activity in response to inhibiting PKC δ, like transformation, was dependent upon PKC α and restricted to the EGFR cells. These data demonstrate that PKC isoforms α and δ have antagonistic effects on both transformation and PLD activity and further support a tumor suppressor role for PKC δ that may be mediated by suppression of tyrosine kinase-dependent increases in PLD activity.