13. Radiation Induces Diacylglycerol Biphasic Production through Phosphoinoside-specific Phospholipase C and Phosphatidylcholine-specific Phospholipase C in Cultured Rat Hepatocytes
Tetsuo Nakajima and Osami Yukawa
Keywords: rat hepatocytes, diacylglycerol, phosphoinosidespecific phospholipase C, phosphatidylcholine-specific phospholipase C, phospholipase D
We have already reported that radiation induces protein kinase C (PKC) activation through radiation-induced lipid peroxidation. We also demonstrated that radiation induces diacylglycerol (DAG) biphasic production in cultured rat hepatocytes. DAG is a PKC endogenous activator and its DAG production might be related to radiation induced PKC signaling pathways. In the previous study, we suggested that DAG is produced by phosphatidylinositol-specific phospholipase C (PI-PLC) in the first production, using a generating system of hydroxyl radical, the main oxygen radical induced by radiation. But it remains unknown whether radiation induces PI-PLC activation. In addition, as it was reported that radiation induces phospholipase D (PLD) activation, PLD might participate in radiation-induced DAG production. PI-PLC produces inositol trisphosphates (IP3) and DAG. Though inositol phosphates including IP3 are metabolized rapidly the metabolization is inhibited in the presence of LiCL. The inhibition results in accumulation of inositol phosphates. The accumulation can be an indicator to analyze rapid PI-PLC activation after irradiation. In this study, we investigated participation of PI-PLC and PLD in radiation-induced DAG production. Cultured rat hepatocytes were labeled with [3H] inositol, and the labeled hepatocytes were irradiated with 20 Gy in the presence of LiCL. Immediately after irradiation, inositol phosphates were fractionated by an SAX column. Accumulation of inositol phosphates involving IP3 was observed in the fraction from the irradiated hepatocytes. Though the accumulation might include other inositol phosphates, this result suggested participation of PI-PLC activation in radiation-induced DAG production. Measurement of the more purified IP3 fraction is underway. Furthermore, PLD activity in hepatocytes was measured after irradiatton. PLD actnvity was evaluated by measuring phosphatidylethanol produc tion in the presence of ethanol in the labeled hepatocytes with [3Hl myristic acid. However, PLD activity in irradiated hepatocytes showed no change within lh after irradiation with 20 Gy. We have already suggested participation of phosphatidylcholinespecific PLC (PC-PLC) in radiation-induced DAG production. Overall, these results indicated that radiation induces DAG production through PI-PLC and PC-PLC.
Publication:
Nakajima, T. and Yukawa, O.: J. Radiat. Res., 40 135-144, 1999.