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

The agonist stimulation of a variety of cells results in the induction of s pecific 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 nuc lei, little attention has been given to the metabolism of phosphatidylcholi ne in nuclear signaling. In the present study the metabolism of phosphatidy lcholine 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 incorpo ration of choline and phosphocholine but not CDP-choline was enhanced in nu clei of TPA treated cells. Moreover the presence of choline kinase, phospho choline cytidylyltransferase and phosphocholine transferase activities were detected in the nuclei and the TPA treatment of the cells stimulated the a ctivity of the phosphocholine cytidylyltransferase. When cells prelabelled with [H-3]palmitic acid were stimulated with TPA in the presence of ethanol , an increase of labelled diacylglycerol and phosphatidylethanol in the nuc lei was observed. Similarly, an increase of labelled diacylglycerol and pho sphatidic acid but not of phosphatidylethanol occurred in [H-3]palmitic aci d prelabelled nuclei stimulated with TPA in the presence of ethanol. Howeve r the production of phosphatidylethanol was observed when the nuclei were t reated with TPA in the presence of ATP and GTP gammaS. The stimulation of [ H-3]choline prelabelled nuclei with TPA also generated the release of free choline and phosphocholine. The results indicate the presence of PLD and pr obably PLC activities in LA-N-1 nuclei and the involvement of phosphatidylc holine in the production of nuclear lipid second messengers upon TPA stimul ation of LA-N-1 cells. The correlation of the disappearance of phosphatidyl choline, the production of diacylglycerol and phosphatidic acid with the st imulation of phosphatidylcholine synthesis in nuclei of TPA treated LA-N-1 suggests the existence of a phosphatidylcholine cycle in these nuclei.