Regulation of human macrophage gene expression by oxidized low density lipoprotein. Studies on tumor necrosis factor-a and global gene expression profiles

Abstract

Atherosclerosis is a multifactorial and progressive disease that is characterized by a focal thickening of the arteries caused by accumulation of lipoproteins, extracellular matrix, and the migration and proliferation of various cell types. Among the cells found in atherosclerotic lesions are macrophages, which accumulate lipids and are transformed into foam cells. The atherosclerotic lesion is also characterized as a site of inflammation. In vivo and in vitro data support the role of macrophages in the development of an inflammatory response in the vessel wall. Several pro-inflammatory cytokines have been detected in the atherosclerotic plaque, among those are tumor necrosis factor-a (TNF-a). Major sources of TNF-a are activated macrophages. Macrophages have been connected to expression of several cytokines in the atherosclerotic plaque that can modulate plaque development.Oxidative modification of lipoproteins is regarded as a key event in the development of atherosclerosis. The oxidation of the low density lipoprotein (LDL) particle produces several biologically active molecules. Among the products formed are oxysterols, which are found in atherosclerotic plaques. Oxysterols have been implicated in the development of atherosclerosis. The main aim of this thesis was to investigate the impact of oxidized LDL (oxLDL) and oxysterols on gene expression in human macrophages.The results obtained in this thesis demonstrate that oxLDL and oxysterols, in particular 25-hydroxyxcholesterol (25-OH), modulates expression of TNF-a. OxLDL decreased the lipopolysaccharide (LPS)-induced DNA binding of the transcription factor nuclear factor kappaB (NF-kB) to the TNF-a promoter. This suggests that oxLDL can decrease the inflammatory response in macrophages. Further, it was demonstrated that 25-OH could decrease an LPS-induced TNF-a secretion, but also that 25-OH together with interferon-gamma (IFN-g) could contribute to an increased expression of TNF-a. 25-OH affects both transcriptional and post-transcriptional events including kinase activity, and also intracellular levels of H2O2 that could influence gene expression. Microarray experiments suggest that oxLDL induces a coordinated stress response, activates detoxification enzymes, as well as the more expected changes in lipid and cholesterol metabolism in macrophages.In conclusion, this thesis shows that components of oxLDL can influence expression of genes in macrophages, which could contribute to the development of atherosclerosis.