Distinct calcium-dependent pathways of epidermal growth factor receptor transactivation and PYK2 tyrosine phosphorylation in PC12 cells

Recently, we have demonstrated that in PC12 cells activation of the Ras/ext racellular signal-regulated kinase pathway in response to membrane depolari zation or bradykinin is mediated by calcium-dependent transactivation of th e epidermal growth factor receptor (EGFR). Here we address the question whe ther Ca2+- calmodulin-dependent protein kinase (CaM kinase) has a role in t he EGFR transactivation signal. Using compounds that selectively interfere with either CaM kinase activity or calmodulin function, we show that KCl-me diated membrane depolarization-triggered, but not bradykinin-mediated signa ls involve CaM kinase function upstream of the EGFR. Although both depolari zation-induced calcium influx and bradykinin stimulation of PC12 cells were found to induce c-fos transcription through EGFR activation, the former si gnal is CaM kinase-dependent and the latter was shown to be independent. As PYK2 is also activated upon elevation of intracellular calcium, we investi gated the potential involvement of this cytoplasmic tyrosine kinase in EGFR transactivation. Interestingly, we observed that inhibition of CaM kinase activity in PC12 cells abrogated tyrosine phosphorylation of PYK2 upon KCl but not bradykinin treatment. Nevertheless, PYK2 activation in response to both stimuli appeared to be mediated by pathways parallel to EGFR transacti vation, Our data demonstrate the existence of two distinct calcium-dependen t mechanisms leading either to EGFR-mediated extracellular signal-regulated activation or to PYK2 tyrosine phosphorylation. Both pathways either in co ncert or independently might contribute to the definition of biological res ponses in neuronal cell types.