Modification of the lipidome in RAW264.7 macrophage subjected to stable silencing of oxysterol-binding proteins.

Oxysterol-binding protein homologs (ORPs) are implicated in lipid metabolism, vesicle transport and cell signaling. In this study we generated RAW264.7 cells with ORP1L, ORP3, or ORP8 silenced using shRNA lentiviruses. The lipidome of the cells under basal serum-free culture conditions or as treated with oxidized LDL (oxLDL), enzymatically modified LDL (E-LDL), or lipopolysaccharide (LPS) was analyzed by mass spectrometry. Reduction in each ORP resulted in distinct and complex effects on macrophage lipidome. Under basal conditions, ORP1L silencing had strongest effects on phosphatidylinositols (PI, increase), free cholesterol (FC, increase), and cholesteryl esters (CE, increase). ORP3 silencing affected most the glucosyl ceramides (GluCer, decrease) and PE-plasmalogens (PE-pl, decrease), while ORP8 silencing increased FC and CE, and decreased GluCer and PE-pl. Upon LPS treatment, the ORP effects were modified: under these conditions ORP1L silencing caused increase of Cer, ORP3 silencing decrease of PI, and ORP8 silencing decrease of PI and increase of PE, not detectable under basal conditions. The lipid species data were subjected to multivariate statistical analysis of principal components, revealing numerous specific alterations upon ORP silencing. The cells cultured in basal conditions or treated with LPS showed qualitatively different responses. However, in LPS-stimulated cells silencing of any of the three ORPs decreased the relative amount of arachidonic acid-containing PI species, increased the corresponding PE species, and favored 16-carbon sphingomyelin (SM) species at the expense of the 24-carbon ones. As a conclusion, the present study reveals the distinct and sophisticated roles of different ORP proteins as regulators of macrophage lipid composition, with implications for inflammatory signaling.