STIMULATION OF PHOSPHATIDYLCHOLINE BIOSYNTHESIS BY HEMICHOLINIUM-3, APOTENT INHIBITOR OF CHOLINE UPTAKE IN HUMAN LEUKEMIC MONOCYTE-LIKE U937 CELLS

The effect of hemicholinium-3 (HC-3) on choline uptake and phosphatidy lcholine (PC) biosynthesis was examined in human leukemic monocyte-lik e U937 cells. HC-3 inhibited [H-3]choline uptake in a dose- and time-d ependent manner. After a 3h treatment, HC-3 (100 mu M) decreased choli ne 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 m anner; the extent of inhibition, however, was always 10-20 per cent le ss than that observed in the total uptake. At 3h HC-3 decreased the in corporation into PC by 65 per cent (p < 0.0001; n = 5). Kinetic studie s in vivo showed that HC-3 inhibited total uptake and incorporation in to PC differently, suggesting that the labelling of PC is not simply d ictated by [H-3]choline uptake. In separate experiments, cells were pr etreated with 100 mu M HC-3 for 3 h. After washing, the inhibitory eff ect on total uptake was no longer observed, while a 20 per cent stimul ation of the incorporation into PC was obtained in these pretreated ce lls. In pulse-chase studies, the cells were prelabelled with [H-3]chol ine 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 w ith 100 mu M HC-3. After a 3 h treatment, the cytosolic CTP:cholinepho sphate cytidylyltransferase (CT) was activated by 56 per cent, while c holine kinase (CK) was inhibited slightly. The stimulation of CT was n ot simply due to the intact HC-3 molecule, and there was no redistribu tion 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.