Involvement of protein kinases and phosphatases in tyrosine phosphorylation of PKCdelta in rat parotid acinar cells exposed to secretory stimuli.

The aim of these studies was to examine the involvement of tyrosine phosphorylation in the signal transduction pathways and secretory events that are promoted by receptor agonists acting on rat parotid acinar cells. Fluid secretion by parotid acinar cells is initiated by the binding of neurotransmitters to GTP(G)-protein-coupled receptors that are linked to phospholipase C, which hydrolyzes phosphatidlyinositol 4,5-bisphosphate to diacylglycerol and inositol 1,4,5-trisphosphate. Although growth factors produce large changes in tyrosine phosphorylation of multiple proteins involved in proliferation and other cellular processes, tyrosine phosphorylation is not considered to be a general phenomenon of G-protein-coupled receptor activation. However, our results shown that carbachol (a muscarinic acetylcholine receptor agonist), and ligands to other phospholipase C-linked receptors, promoted a rapid increase in the tyrosine phosphorylation of protein kinase Cdelta (PKCdelta), a member of the PKC family of proteins. Phorbol 12-myristate 13-acetate, which binds to the site on PKCdelta to which the endogenous activator sn-1,2-diacylglycerol binds, also increased the tyrosine phosphorylation of PKCdelta. Genistein and staurosporine, two protein kinase inhibitors, blocked the tyrosine phosphorylation of this protein. Thus, PKCdelta becomes tyrosine phosphorylated in response to receptor activation, and this event appears to involve both diacylglycerol production and protein tyrosine kinase activity. This may contribute to early physiological events, including alterations in fluid secretion, that are initiated by neurotransmitters acting on the parotid salivary gland.