Abstract 120: Inhibitory Effects of Cholesterol on Lysophosphatidylcholine Metabolism in Human Monocytes

Abstract

Background and aims: In hypercholesterolemic conditions blood cells are exposed to high concentrations of lipids, particularly cholesterol and lysophosphatidylcholine (lyso-PtdCho). While the metabolism of lyso-PtdCho by monocytes has been studied, the effects of high cholesterol, which complexes with lyso-PtdCho, are not well understood. In this study we used a novel assay developed in our laboratory, based on the ability of lyso-PtdCho to solubilize and deliver cholesterol into the cells, to study whether cholesterol affects the metabolism of lyso-PtdCho. Methods: In THP-1 monocytes 14 C-lyso-PtdCho metabolism was analyzed by TLC and cytotoxicity by WST-1 assay. Results: Monocytes incubated with lyso-PtdCho alone metabolize the incorporated lyso-PtdCho by 4h mainly into phosphatidylcholine, with a small percentage into sphingomyelin and phosphocholine. The latter could represent PLase C reaction that would generate mono- and diacyl-glycerols, important signaling molecules. However in cells incubated with cholesterol or cholesteryl ester (CE)/lyso-PtdCho micelles, at 4h the radioactivity remained mainly in the lyso-PtdCho fraction (p<0.01 and p<0.001). Lyso-PtdCho decreased monocyte viability at concentrations over 30μM, but cholesterol and CE inhibited the lyso-PtdCho-induced cytotoxicity (p<0.001). Lyso-PtdCho and CE/lyso-PtdCho, but not cholesterol/lyso-PtdCho, increased the gene expression of the pro-inflammatory chemokine MCP-1 (p<0.05). Peripheral blood monocytes from LDL-R -/- mice on a high fat diet had higher levels of CE than monocytes from mice in a normal diet (n=10, p<0.05). Monocyte CE content in these mice positively correlated with plasma LDL/HDL ratio (p<0.05, R 2 =0.65). Conclusions: We show that cholesterol and CE inhibit the cytotoxic effects of lyso-PtdCho in monocytes, but only cholesterol counteracted the pro-inflammatory action of lyso-PtdCho. Since lyso-PtdCho concentrations in plasma are increased in hypercholesterolemic conditions, and its content is increased during the oxidation of LDL, the current studies might indicate that cholesterol and lyso-Ptdcho could influence each other’s metabolism. This is important as lyso-PtdCho has been suggested to influence plaque vulnerability.