Modulation of THP-1 macrophage and cholesterol-loaded foam cell apolipoprotein E levels by glycosphingolipids

Macrophages synthesize and secrete apolipoprotein E (apoE) constitutively. This process is upregulated under conditions of cholesterol loading. The response to cholesterol is antiatherogenic as it is believed to promote cholesterol efflux from the artery wall. The concentration of lactosyl ceramide (LacCer), a glycosphingolipid recently discovered to regulate cellular signaling, proliferation, and expression of adhesion molecules, is also increased in atherosclerotic tissues. Here we have investigated the effect of exogenous LacCer on macrophage apoE levels. We show that increasing macrophage LacCer levels sevenfold led to reductions in cellular and secreted apoE (15 and 30%, respectively, over a 24-h period) as determined by enzyme-linked immunosorbent assay. A similar effect was also induced by glucosyl ceramide (GlcCer) but not by ganglioside species. When macrophages were converted to cholesterol-loaded foam cells by incubation with acetylated LDL, the resulting increase in cellular apoE levels was inhibited by 26% when the cells were subsequently enriched with LacCer. After metabolic labeling of cellular glycosphingolipids with [14C]palmitate, we also discovered that high-density lipoprotein (HDL) stimulates the efflux of glycosphingolipids from foam cells. These data imply that LacCer and GlcCer may be proatherogenic due to the suppression of macrophage apoE production. Furthermore, the efflux of glycosphingolipids from macrophage foam cells to HDL could indicate a potential pathway for their removal from the artery wall and subsequent delivery to the liver.