Hexadecylphosphocholine inhibits phosphatidylcholine synthesis via both the methylation of phosphatidylethanolamine and CDP-choline pathways in HepG2 cells

Abstract

We reported in a recent publication that hexadecylphosphocholine (HePC), a lysophospholipid analogue, reduces cell proliferation in HepG2 cells and at the same time inhibits the biosynthesis of phosphatidylcholine (PC) via CDP-choline by acting upon CTP:phosphocholine cytidylyltransferase (CT). We describe here the results of our study into the influence of HePC on other biosynthetic pathways of glycerolipids. HePC clearly decreased the incorporation of the exogenous precursor [1,2,3- 3 H ]glycerol into PC and phosphatidylserine (PS) whilst increasing that of the neutral lipids diacylglycerol (DAG) and triacylglycerol (TAG). Interestingly, the uptake of l-[3- 3 H ]serine into PS and other phospholipids remained unchanged by HePC and neither was the activity of either PS synthase or PS decarboxylase altered, demonstrating that the biosynthesis of PS is unaffected by HePC. We also analyzed the water-soluble intermediates and final product of the CDP-ethanolamine pathway and found that HePC caused an increase in the incorporation of [1,2- 14 C ]ethanolamine into CDP-ethanolamine and phosphatidylethanolamine (PE) and a decrease in ethanolamine phosphate, which might be interpreted in terms of a stimulation of CTP:phosphoethanolamine cytidylyltransferase activity. Since PE can be methylated to give PC, we studied this process further and observed that HePC decreased the synthesis of PC from PE by inhibiting the PE N-methyltransferase activity. These results constitute the first experimental evidence that the inhibition of the synthesis of PC via CDP-choline by HePC is not counterbalanced by any increase in its formation via methylation. On the contrary, in the presence of HePC both pathways seem to contribute jointly to a decrease in the overall synthesis of PC in HepG2 cells.