Stimulation of phosphatidylcholine biosynthesis by hemicholinium-3, a potent inhibitor of choline uptake in human leukemic monocyte-like U937 cells

The effect of hemicholinium-3 (HC-3) on choline uptake and phosphatidylcholine (PC) biosynthesis was examined in human leukemic monocyte-like U937 cells. HC-3 inhibited [³H]choline uptake in a dose- and time-dependent manner. After a 3 h treatment, HC-3 (100 μM) decreased choline uptake by as much as 80 per cent (p < 0·0001; n = 4). Reduction of incorporation of label into PC was also detected in a dose-dependent manner; the extent of inhibition, however, was always 10–20 per cent less than that observed in the total uptake. At 3 h HC-3 decreased the incorporation into PC by 65 per cent (p < 0·0001; n = 5). Kinetic studies in vivo showed that HC-3 inhibited total uptake and incorporation into PC differently, suggesting that the labelling of PC is not simply dictated by [³H]choline uptake. In separate experiments, cells were pretreated with 100 μM HC-3 for 3 h. After washing, the inhibitory effect on total uptake was no longer observed, while a 20 per cent stimulation of the incorporation into PC was obtained in these pretreated cells. In pulse-chase studies, the cells were prelabelled with [³H]choline for 30 min and chased with HC-3 for up to 3 h; the results showed a significant stimulation of incorporation into PC in a longer chase with 100 μM HC-3. After a 3 h treatment, the cytosolic CTP:cholinephosphate cytidylytransferase (CT) was activated by 56 per cent, while choline kinase (CK) was inhibited slightly. The stimulation of CT was not simply due to the intact HC-3 molecule, and there was no redistribution of CT between cytosol and microsomes. Taken together, the results suggest that HC-3 activates PC biosynthesis apart from the inhibitory effect on choline uptake.