Enhancement of human ACAT1 gene expression to promote the macrophage-derived foam cell formation by dexamethasone

Abstract

In macrophages, the accumulation of cholesteryl esters synthesized by the activated acyl-coenzyme A:cholesterol acyltransferase-1 (ACAT1) results in the foam cell formation, a hallmark of early atherosclerotic lesions. In this study, with the treatment of a glucocorticoid hormone dexamethasone (Dex), lipid staining results clearly showed the large accumulation of lipid droplets containing cholesteryl esters in THP-1-derived macrophages exposed to lower concentration of the oxidized low-density lipoprotein (ox-LDL). More notably, when treated together with specific anti-ACAT inhibitors, the abundant cholesteryl ester accumulation was markedly diminished in THP-1-derived macrophages, confirming that ACAT is the key enzyme responsible for intracellular cholesteryl ester synthesis. RT-PCR and Western blot results indicated that Dex caused up-regulation of human ACAT1 expression at both the mRNA and protein levels in THP-1 and THP-1-derived macrophages. The luciferase activity assay demonstrated that Dex could enhance the activity of human ACAT1 gene P1 promoter, a major factor leading to the ACAT1 activation, in a cell-specific manner. Further experimental evidences showed that a glucocorticoid response element (GRE) located within human ACAT1 gene P1 promoter to response to the elevation of human ACAT1 gene expression by Dex could be functionally bound with glucocorticoid receptor (GR) proteins. These data supported the hypothesis that the clinical treatment with Dex, which increased the incidence of atherosclerosis, may in part due to enhancing the ACAT1 expression to promote the accumulation of cholesteryl esters during the macrophage-derived foam cell formation, an early stage of atherosclerosis.

TNF-alpha stimulates the ACAT1 expression in differentiating monocytes to promote the CE-laden cell formation

“…Human THP-1 monocytes were adhered on cover slips in a 12-well plate with treatment of 1 mM ATRA or ATRA plus TNFa in the RPMI 1640 medium supplemented with 10% FBS for 40 h; for the NF-kB inhibition assay, the inhibitor PGA1 was added to the medium prior to stimulation of TNFa. Then the cells were cultured with oxidized low-density lipoproteins (oxLDL; 40 mg/ml), which are prepared as described (24,25) but without TNFa, for another 48 h in the fresh RPMI 1640 medium containing 10% lipoprotein-deficient serum (LPDS). For the ACAT inhibition assay, the ACAT inhibitor CP-113,818 was added to the fresh RPMI 1640 medium containing 10% LPDS and oxLDL (40 mg/ml) after stimulation of TNFa.…”

Possible Pivotal Role of Latent Chronic T. gondii Infection in the Pathogenesis of Atherosclerosis

“…Moreover, several authors showed that dexamethasone (DXM, 1mM) (a synthetic GC receptor agonist) could promote the macrophage-derived foam cell formation (a hallmark of early atherosclerotic lesions [738,739] ) at lower concentration of ox-LDL in vitro, and that the drug increased the formation of CEs in macrophages and human SMCs in a dosedependent manner [201,740] . The finding that DXM could promote the foam cell formation at lower ox-LDL concentration might be partially caused by the up-regulation of ACAT1 gene expression at the transcriptional level and the enhancement of cholesterol esterification [194] .…”

“…GCs not only down-regulated proinflammatory cytokines, they also generated a functionally active, antiinflammatory phenotype in human monocytes that suppressed inflammatory processes and, thus, induced resolution of inflammation [733] . Many studies provided the experimental and clinical evidence that treatment with GCs accelerated and/or exacerbated development of atherosclerosis [194,734,735] . Accumulating evidence had demonstrated that foam cells were formed from macrophages in vitro at high concentrations (80-160 mg/mL) of oxLDL [736,737] .…”

“…The LCAT derived CE of HDL is removed from plasma primarily by the liver through selective CE uptake [230] and by whole HDL particle uptake [231] . Normally, more than 70% of the cholesterol in LDL and VLDL is esterified, and two enzymes ACAT2 and 619 and other cell types, ACAT1 overexpression was involved in forming CEs as lipid droplets [191,192] , present in the cell cytoplasm [177] , and finally in foam cell formation [193,194] . Rong et al [195] found an increase in ACAT1 activity after cholesterol loading also in both human and rat aortic SMCs, but ACAT2 was not found to be expressed by arterial SMCs [152] .…”

Lipoproteins and Cancer

“…Although ACAT1 is a critical component of intracellular cholesterol homeostasis, its expression is not known to be regulated by the SREBPs (Goldstein, DeBose-Boyd et al 2006). In monocytes and macrophages, ACAT1 expression was up-regulated by interferon γ and all-trans-retinoic acid via STAT1 (Yang, Duan et al 2001) and by dexamethasone via a glucocorticoid response element in its promoter (Yang, Yang et al 2004). ACAT1 has also been shown to have an NFκB binding element in its proximal promoter and to be upregulated in response to TNFα signaling through NFκB (Lei, Xiong et al 2009).…”