Studies on the turnover of endogenous choline-containing phospholipids of cultured neuroblastoma cells

Previous studies on neuroblastoma cells in culture showed that the presence of partially purified rat liver phospholipid-transfer protein had a marked differential effect on the uptake and apparent subcellular distribution of radioactively labeled sphingomyelin and phosphatidylcholine (PC) added to the medium as mixed phospholipid (PC/sphingomyelin) liposomes. To determine the effect of phospholipid-transfer protein and exogenous phospholipids on the turnover and subcellular distribution of endogenous phospholipids, neuroblastoma cells were preincubated for 48 h in the presence of [methyl-3H]choline and washed. Aliquots of prelabeled cells were reincubated immediately in medium containing phospholipid-transfer protein mixed phospholipid liposomes, cytochalasin B and 2-deoxyglucose for 45 min at 37 degrees C; additional aliquots were chased first for 2 or 18 h with unlabeled choline before reincubation. The extent of labeled phospholipid degradation and accumulation in the medium, and the subcellular distribution of cell-associated labeled choline-containing phospholipids were determined. During incubation with phospholipid-transfer protein and mixed phospholipid liposomes, 25-35% of the cell-associated radioactive label from prelabeled cells, chased or unchased, was lost to the medium in 45 min. Over 50% of the label appearing in the medium was in water-soluble phospholipid degradation products. The loss of cell-associated label into the medium from unchased cells was stimulated significantly by phospholipid-transfer protein; however, prelabeled cells which had been chased for 18 h with unlabeled choline were unaffected by the presence of transfer protein. Endogenously synthesized radioactively labeled PC and sphingomyelin were distributed throughout all subcellular membranes, but least of all in the crude mitochondrial membrane fraction. Analysis of the subcellular distribution of cell-associated label remaining in chased or unchased cells after 45 min incubation with PC/sphingomyelin liposomes showed proportionate losses from all membrane fractions, except the crude mitochondrial fraction, which showed relative retention of labeled phospholipid. Phospholipid-transfer protein had no effect. The results are in distinct contrast to observations on the turnover, metabolism and subcellular distribution of labeled exogenous phospholipids under the same conditions, indicating that exogenous phospholipids do not intermix freely with any quantitatively major pool of endogenous phospholipid.