Role of the lipolysis stimulated lipoprotein receptor LSR in the regulation of macrophage lipid homeostasis and metabolic activity
In atherosclerosis, monocyte-derived macrophages in the subendothelial space of the arterial wall, become lipid-laden and pro-inflammatory. Macrophages depend on lipoprotein receptors to regulate lipid influx, thereby shaping macrophage metabolic state and potentially dictating their phenotype. The lipolysis stimulated lipoprotein receptor, LSR, contributes to lipid homeostasis and was detected by immunofluorescence in aortic atherosclerotic plaques from ApoE-/- mice and by immunoblot in macrophages. The aim was to determine the role of LSR in macrophage lipid handling by studying LSR expression and activity before (M0) or after polarization of Raw 264.7 cells using LPS/IFN{gamma} (M1) or IL-4/IL-13 (M2), respectively. LSR mRNA (0.48-fold, p < 0.001), protein levels (0.28-fold, p < 0.001) were significantly lower in M1 (iNOS+/MCP1+) as compared to M2 (CD206+/Arg1+) macrophages. LSR activity, measured as oleate-induced uptake of DiI-labelled LDL, was significantly lower in M1 as compared to M2 macrophages. siRNA-mediated LSR knockdown (87{+/-}4%, p < 0.01) in M0 cells led to significantly decreased expression of genes involved in lipid influx (LDL-R, LRP1) and transport (ABCA1), increased basal glycolysis and decreased glycolytic reserve, but without changes in inflammatory markers. Raw 264.7 macrophages transformation into foam cells by 24h incubation with oxidized LDL also led to reduced LSR activity and expression. We propose that the lower LSR expression and activity observed in M1 and foam cell macrophages contribute to reduced lipid influx and increased glycolytic state that is associated with pro-inflammatory M1 phenotype. This provides new insight towards understanding how these immune cells depend on lipoprotein receptors regulating their metabolic uptake.