Phosphatidylcholine metabolism in nuclei of phorbol ester-activated LA-N-1 neuroblastoma cells

The agonist stimulation of a variety of cells results in the induction of specific lipid metabolism in nuclear membranes, supporting the hypothesis of an important role of the lipids in nuclear signal transduction. While the existence of a phosphatidylinositol cycle has been reported in cellular nuclei, little attention has been given to the metabolism of phosphatidylcholine in nuclear signaling. In the present study the metabolism of phosphatidylcholine in the nuclei of neuroblastoma cells LA-N-1 was investigated. The incubation of LA-N-1 nuclei with radioactive choline, phosphocholine or CDP-choline led to the production of labelled phosphatidylcholine. The incorporation of choline and phosphocholine but not CDP-choline was enhanced in nuclei of TPA treated cells. Moreover the presence of choline kinase, phosphocholine cytidylyltransferase and phosphocholine transferase activities were detected in the nuclei and the TPA treatment of the cells stimulated the activity of the phosphocholine cytidylyltransferase. When cells prelabelled with [3H]palmitic acid were stimulated with TPA in the presence of ethanol, an increase of labelled diacylglycerol and phosphatidylethanol in the nuclei was observed. Similarly, an increase of labelled diacylglycerol and phosphatidic acid but not of phosphatidylethanol occurred in [3H]palmitic acid prelabelled nuclei stimulated with TPA in the presence of ethanol. However the production of phosphatidylethanol was observed when the nuclei were treated with TPA in the presence of ATP and GTPgammaS. The stimulation of [3H]choline prelabelled nuclei with TPA also generated the release of free choline and phosphocholine. The results indicate the presence of PLD and probably PLC activities in LA-N-1 nuclei and the involvement of phosphatidylcholine in the production of nuclear lipid second messengers upon TPA stimulation of LA-N-1 cells. The correlation of the disappearance of phosphatidylcholine, the production of diacylglycerol and phosphatidic acid with the stimulation of phosphatidylcholine synthesis in nuclei of TPA treated LA-N-1 suggests the existence of a phosphatidylcholine cycle in these nuclei.