19. Radiation Effects on Phospholipase Signaling Pathways in Cultured Rat Liver Cells

Tetsuo Nakajima and Osami Yukawa

Keywords: diacylglycerol, phospholipase C, phospholipase A₂, tyrosine phosphorylation, rat liver cells

Our previous work demonstrated that radiation induces protein kinase C (PKC) activation due to translocation of PKC molecules from cytosol to membranes in cultured rat hepatocytes, and that the PKC activation is related to lipid peroxidation. In addition, we showed that radiation induces biphasic production of diacylglycerol(DAG), one of the endogenous PKC activators, in rat hepatocytes. On the other hand, it has been reported that the biphasic production of DAG is observed in the cases of treatment of cells with growth factors, H₂O₂ or UV and then DAG is produced by phospholipase C or D. We have already demonstrated that hydroxyl radical induces DAG production through phosphatidylinositol-specific phospholipase C(PI-PLC). Some oxidative stresses are known to induce activation of PLC-1, one of the PI-PLCs, through its tyrosine phosphorylation. In this study, PLC-1 phosphorylation in hydroxyl radical or radiation-induced DAG production was investigated. Additionally, we assessed radiation effects on phospholipase A₂(PLA₂), which is related to PKC activation and lipid peroxidation. Hydroxyl radical and radiation could not induce tyrosine phosphorylation of PLC-1 at the time when DAG was produced by hydroxyl radical or radiation. Therefore, the increase of DAG content by them seems to be through other PI-PLCs, or to be due to phosphorylation-independent PLC-1 activation. As for PLA₂, irradiation with 0.5 Gy to 50 Gy induced no change in PLA₂ activity within 3 hours after irradiation in cultured rat hepatocytes. However, in cultured non-parenchymal cells from rat liver, PLA₂ activity increased 1.5-fold 30 min after irradiation with 5 Gy and the increase was maintained for to 2 hours after irradiation. These results suggest that radiation effects on phospholipase signaling pathways in hepatocytes are different from those in liver non-parenchymal cells. Radiation effects on liver cells might be mediated by cell-to-cell interactions between hapatocytes and non-parenchymal cells, for example, through arachidonic acid.