Modulation of PKCδ tyrosine phosphorylation and activity in salivary and PC-12 cells by Src kinases

Protein kinase C (PKC) delta becomes tyrosine phosphorylated in rat parotid acinar cells exposed to muscarinic and substance P receptor agonists, which initiate fluid secretion in this salivary cell. Here we examine the signaling components of PKC delta tyrosine phosphorylation and effects of phosphorylation on PKC delta activity. Carbachol- and substance P-promoted increases in PKC delta tyrosine phosphorylation were blocked by inhibiting phospholipase C (PLC) but not by blocking intracellular Ca2+ concentration elevation, suggesting that diacylglycerol, rather than D-myo-inositol 1,4,5-trisphosphate production, positively modulated this phosphorylation. Stimuli-dependent increases in PKCd activity in parotid and PC-12 cells were blocked in vivo by inhibitors of Src tyrosine kinases. Dephosphorylation of tyrosine residues by PTP1B, a protein tyrosine phosphatase, reduced the enhanced PKC delta activity. Lipid cofactors modified the tyrosine phosphorylation-dependent PKC delta activation. Two PKC delta regulatory sites (Thr-505 and Ser-662) were constitutively phosphorylated in unstimulated parotid cells, and these phosphorylations were not altered by stimuli that increased PKC delta tyrosine phosphorylation. These results demonstrate that PKC delta activity is positively modulated by tyrosine phosphorylation in parotid and PC-12 cells and suggest that PLC-dependent effects of secretagogues on salivary cells involve Src-related kinases.