Function of lipoprotein lipase and endothelial lipase in human macrophages

Abstract

Lipoprotein lipase (LPL) and endothelial lipase (EL) are expressed in atherosclerotic lesions, mainly in macrophages. However, the functional roles of LPL and EL in macrophages are not well characterized. In the present thesis, the effects of these lipases on cholesterol efflux, low density lipoprotein (LDL) catabolism, and proinflammatory cytokine secretion in human macrophages were investigated. Lentivirus transduction successfully induced EL suppression or over-expression in macrophages. LPL suppression was mediated by lentivirus transduction whereas dexamethasone was used to stimulate LPL expression. Apolipoprotein AI- (apoAI-) mediated cholesterol efflux was modestly reduced after LPL and EL suppression, but significantly increased in lipase-overexpressing macrophages as well as transfected 293 cells. This effect was partially inhibited after the elimination of either catalytic or non-catalytic lipase function, but completely abolished when both functions were blocked. The observed effect on cholesterol efflux was mediated partially by an increased apoAI binding, an effect dependent on cell surface lipase. Lipase expression was inversely associated with phosphatidylcholine and sphingomyelin levels, but positively with lysophosphatidylcholine production, the later was shown to promote apoAI-mediated cholesterol efflux dose-dependently. EL expression was positively correlated with both native and oxidized LDL binding and association via non-catalytic function as observed in both EL-suppressed and over-expressed macrophages. By contrast, the catalytic activity of EL did not have a significant role in oxidized LDL metabolism with the exception of a positive correlation with native LDL association, which also partially depended on the LDL receptor. The concentration of interleukin-1β and 6, macrophage chemoattractant protein-1, and tumor necrosis factor-α was reduced after LPL and EL suppression, The lipase suppression also amplified the inhibitory effect of oxidized LDL in macrophages. Microarray analysis indicated that >50 genes, mainly proinflammatory ones, had marked expression changes after lipase suppression. Atorvastatin treatment reduced LPL and EL expression as well as Rho, the liver X receptor (LXR), and nuclear factor-κB (NF-κB) levels. Mechanistic studies identified LXR and NF-κB to be involved in atorvastatin-induced suppression of LPL and EL, respectively. In summary, by promoting apoAI-mediated cholesterol efflux, lipoprotein binding and uptake, and proinflammatory cytokine expression in macrophages, EL and LPL may influence the atherogenic potential of macrophages.