ACTIVATION OF PHOSPHATIDYLINOSITOL 3-KINASE AND PHOSPHATIDYLINOSITOL 4-KINASE DURING RAT PAROTID ACINAR CELL-PROLIFERATION

We have recently shown that beta-adrenergic agonist, isoproterenol-ind uced parotid acinar cell proliferation is in part mediated by elevated levels of surface galactosyltransferase which undergoes interaction w ith the EGF-R. The receptor subsequently undergoes autophosphorylation on the tyrosine residues in a manner similar to its 'receptor-ligand' interaction (Purushotham et al. (1992) Biochem. J. 284, 767-776). In this study, we provide evidence for phosphatidylinositol 3-kinase and 4-kinase as cytoplasmic signalling proteins involved in both the isopr oterenol and EGF-stimulated signal transduction upon in vitro and in-v ivo stimulation of parotid acinar cells. Total cell lysate activity fo r the PtdIns 4-kinase was 2- and 3-fold higher than unstimulated contr ol cells, while the PtdIns 3-kinase was 1.4- and 2.8-fold higher follo wing stimulation by isoproterenol or EGF, respectively. Increases of 6 - and 2-fold in phosphatidylinositol 3-kinase were observed in anti-ph osphotyrosine-antibody-immunoprecipitated cell lysates upon in-vitro g rowth stimulation with isoproterenol or EGF, respectively. There was a n increase in tyrosine phosphorylation of the holoenzyme and associati on of the p85 subunit of phosphatidylinositol 3-kinase with EGF-R in r esponse to both isoproterenol and EGF treatments. This corresponded wi th the mobilization of p85 from the cytoplasm to the plasma membrane u pon growth stimulation. These results further implicate the phosphoino sitide metabolites in the second messenger signalling pathways of isop roterenol-induced rat parotid cell proliferation. The parallel utiliza tion of EGF indicate that the post-transductional mechanisms of isopro terenol-induced acinar cell proliferation are similar to the growth-fa ctor-mediated activation of intracellular signalling pathways for cell growth.