Enhanced SCAP Glycosylation by Inflammation Induces Macrophage Foam Cell Formation

Chao Zhou,John F Moorhead,Xiong Z Ruan,Yaxi Chen,Zac Varghese,Qing Liu,Han Lei,Lung-Chih Li,Hongyuan Yang

doi:10.1371/journal.pone.0075650

Abstract

Inflammatory stress promotes foam cell formation by disrupting LDL receptor feedback regulation in macrophages. Sterol Regulatory Element Binding Proteins (SREBPs) Cleavage-Activating Protein (SCAP) glycosylation plays crucial roles in regulating LDL receptor and 3-hydroxy-3-methyl-glutaryl-CoA reductase (HMGCoAR) feedback regulation. The present study was to investigate if inflammatory stress disrupts LDL receptor and HMGCoAR feedback regulation by affecting SCAP glycosylation in THP-1 macrophages. Intracellular cholesterol content was assessed by Oil Red O staining and quantitative assay. The expression of molecules controlling cholesterol homeostasis was examined using real-time quantitative RT-PCR and Western blotting. The translocation of SCAP from the endoplasmic reticulum (ER) to the Golgi was detected by confocal microscopy. We demonstrated that exposure to inflammatory cytokines increased lipid accumulation in THP-1 macrophages, accompanying with an increased SCAP expression even in the presence of a high concentration of LDL. These inflammatory cytokines also prolonged the half-life of SCAP by enhancing glycosylation of SCAP due to the elevated expression of the Golgi mannosidase II. This may enhance translocation and recycling of SCAP between the ER and the Golgi, escorting more SREBP2 from the ER to the Golgi for activation by proteolytic cleavages as evidenced by an increased N-terminal of SREBP2 (active form). As a consequence, the LDL receptor and HMGCoAR expression were up-regulated. Interestingly, these effects could be blocked by inhibitors of Golgi mannosidases. Our results indicated that inflammation increased native LDL uptake and endogenous cholesterol de novo synthesis, thereby causing foam cell formation via increasing transcription and protein glycosylation of SCAP in macrophages. These data imply that inhibitors of Golgi processing enzymes might have a potential vascular-protective role in prevention of atherosclerotic foam cell formation.

Highlights

Atherosclerosis, a maladaptive chronic inflammatory response in the vessel wall, is the primary cause of coronary artery disease, stroke and peripheral vascular disease and it represents the most common cause of morbidity and mortality worldwide [1]
Our previous studies showed that the accelerating effects of inflammatory cytokines on lipid droplets accumulation in various peripheral cells such as human mesangial cells (HMCs), vascular smooth muscle cells (VSMCs) and macrophages [6,7,8], were not be inhibited by scavenger receptors blocker, but were blocked by LDL receptor (LDLr) specific antibody (MB47) and heparin, which removes LDL bound to the cell surface [7,8]
The glycosylation inhibitors reduced intracellular cholesterol ester (CE) and total cholesterol (TC) induced by IL-6 and TNF-a (Figure 1D)

Summary

Introduction

Atherosclerosis, a maladaptive chronic inflammatory response in the vessel wall, is the primary cause of coronary artery disease, stroke and peripheral vascular disease and it represents the most common cause of morbidity and mortality worldwide [1]. Our previous studies showed that the accelerating effects of inflammatory cytokines on lipid droplets accumulation in various peripheral cells such as human mesangial cells (HMCs), vascular smooth muscle cells (VSMCs) and macrophages [6,7,8], were not be inhibited by scavenger receptors blocker, but were blocked by LDL receptor (LDLr) specific antibody (MB47) and heparin, which removes LDL bound to the cell surface [7,8]. This suggests LDLr pathway involvement in lipid accumulation under inflammatory stress

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Results

Conclusion