Abstract
Apolipoprotein E (apoE) has anti-atherosclerotic properties, being involved in the transport and clearance of cholesterol-rich lipoproteins as well as in cholesterol efflux from cells. We hypothesized that glucocorticoids may exert anti-inflammatory properties by increasing the level of macrophage-derived apoE. Our data showed that glucocorticoids increased apoE expression in macrophages in vitro as well as in vivo. Dexamethasone increased ~6 fold apoE mRNA levels in cultured peritoneal macrophages and RAW 264.7 cells. Administered to C57BL/6J mice, dexamethasone induced a two-fold increase in apoE expression in peritoneal macrophages. By contrast, glucocorticoids did not influence apoE expression in hepatocytes, in vitro and in vivo. Moreover, dexamethasone enhanced apoE promoter transcriptional activity in RAW 264.7 macrophages, but not in HepG2 cells, as tested by transient transfections. Analysis of apoE proximal promoter deletion mutants, complemented by protein-DNA interaction assays demonstrated the functionality of a putative glucocorticoid receptors (GR) binding site predicted by in silico analysis in the -111/-104 region of the human apoE promoter. In hepatocytes, GR can bind to their specific site within apoE promoter but are not able to modulate the gene expression. The modulatory blockade in hepatocytes is a consequence of partial involvement of transcription factors and other signaling molecules activated through MEK1/2 and PLA2/PLC pathways. In conclusion, our study indicates that glucocorticoids (1) differentially target apoE gene expression; (2) induce a significant increase in apoE level specifically in macrophages. The local increase of apoE gene expression in macrophages at the level of the atheromatous plaque may have therapeutic implications in atherosclerosis.
Highlights
Apolipoprotein E is a 35 kDa glycoprotein which has an essential contribution to the lipoprotein metabolism [1]
To address the issue of cell-specific Apolipoprotein E (apoE) regulation in response to glucocorticoids, first we evaluated apoE gene expression in macrophages compared to hepatocytes in the presence of dexamethasone, a glucocorticoid receptors (GR) agonist
ApoE gene expression was determined by RealTime PCR in freshly isolated mouse peritoneal macrophages and primary cultures of murine hepatocytes, as well as in established cell lines (RAW 264.7 and HepG2) exposed for 24 hours to various concentrations of dexamethasone
Summary
Apolipoprotein E (apoE) is a 35 kDa glycoprotein which has an essential contribution to the lipoprotein metabolism [1]. Besides the acceleration of the cholesterol efflux, apoE secreted by macrophages at the level of the atherosclerotic plaque plays an important role in changing the macrophage phenotype from proinflammatory M1 to anti-inflammatory M2 [18]. This would lead to major changes in chemokines secretion at the level of the plaque and to stabilization and/or a possible regression of the atheroma. We report that dexamethasone enhanced apoE gene expression in macrophages, but not in hepatocytes This differential effect of glucocorticoids on apoE transcription in the two cell types prompted us to search for the regulatory. Our results reveal a specific glucocorticoid-induced increase in apoE gene expression in macrophages which may have therapeutic implications in atherosclerosis
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