LYSOPHOSPHATIDYLCHOLINE AND 1-O-OCTADECYL-2-O-METHYL-RAC-GLYCERO-3-PHOSPHOCHOLINE INHIBIT THE CDP-CHOLINE PATHWAY OF PHOSPHATIDYLCHOLINE SYNTHESIS AT THE CTP-PHOSPHOCHOLINE CYTIDYLYLTRANSFERASE STEP

The regulation of the CDP choline pathway of phosphatidylcholine synthesis at the CTP:phosphocholine cytidylyltransferase (CT) step by lysophosphatidylcholine (LPC) and the nonhydrolyzable LPC analog, 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3), was investigated in a colony stimulating factor 1-dependent murine macrophage cell line. LPC inhibited phosphatidylcholine synthesis in vivo and led to the accumulation of choline and phosphocholine coupled to the disappearance of CDP choline pointing to CT as the intracellular target. LPC neither inhibited cell growth nor decreased the cellular content of CT or altered the distribution of CT between soluble and particulate subcellular fractions. The inhibition of phosphatidylcholine synthesis was specific for LPC since lysophospholipids lacking the choline headgroup were not inhibitors. ET-18-OCH3 was a more potent inhibitor of phosphatidylcholine synthesis than LPC and caused the translocation of CT from the soluble compartment to the particulate compartment. Both LPC and ET-18-OCH3 were inhibitors of CT activity in vitro and kinetic analysis showed competitive inhibition with respect to the lipid activator. These data point to LPC as a negative regulator of de novo phosphatidylcholine synthesis that acts at the CT step and establish the mechanism for the inhibition of phosphatidylcholine biosynthesis by antineoplastic phospholipids.