Inflammatory stress increases unmodified LDL uptake via LDL receptor: an alternative pathway for macrophage foam-cell formation

Abstract

To investigate if inflammatory stress increases intracellular accumulation of unmodified low-density lipoprotein (LDL) in human monocyte cell line (THP-1) macrophages by disrupting the sterol regulatory element binding proteins (SREBPs) cleavage-activating protein (SCAP)-SREBP2-mediated feedback regulation of LDL receptor. THP-1 macrophages were incubated in serum-free medium in the absence or presence of LDL alone, LDL plus lipopolysaccharide (LPS) and LPS alone, then intracellular cholesterol content, tumor necrosis factor alpha level in the supernatants, mRNA and protein expression of LDL receptor, and SREBP2 and SCAP in the treated cells were assessed by Oil Red O staining, cholesterol enzymatic assay, enzyme-linked immunosorbent assay, real-time quantitative polymerase chain reaction, and Western blotting analysis, respectively. We demonstrated that LPS enhanced transformation of THP-1 macrophages into foam cells by increased uptake of unmodified LDL as evidenced by Oil Red O staining and direct assay of intracellular cholesterol. In the absence of LPS, 25 microg/ml LDL decreased LDL receptor mRNA and protein expression (p < 0.05). However, LPS enhanced LDL receptor expression, overcoming the suppression of LDL receptor induced by 25 microg/ml LDL and inappropriately increasing LDL uptake (p < 0.05). Exposure to LPS also caused overexpression of mRNA and protein of SCAP and SREBP2 (p < 0.05). These observations indicate that LPS disrupts cholesterol-mediated LDL receptor feedback regulation, permitting intracellular accumulation of unmodified LDL and causing foam-cell formation. The implication of these findings is that inflammatory stress may contribute to intracellular LDL accumulation in THP-1 macrophages without previous modification of LDL.