Decreased inducibility of TNF expression in lipid-loaded macrophages

In macrophages incubated with acetylated low density lipoprotein (ac-LDL) for 2 days, mRNA expression of TNF in cells stimulated with TNF decreased by 75%. In cell cultures stimulated over night with IL-1β, lipid loading decreased secretion of TNF into culture medium by 48%. These results suggest that lipid accumulation in macrophages makes them less responsive to inflammatory stimuli. Decreased basal activity and inducibility of transcription factor AP-1 was observed in lipid-loaded cells, suggesting a mechanism for the suppression of cytokine expression. NF-κB binding activity and inducibility were only marginally affected by ac-LDL. LDL and ac-LDL did not activate PPARγ. In contrast, oxidized LDL stimulated AP-1 and PPARγ but inhibited NF-κB, indicating that the effects of lipid loading with ac-LDL were not due to oxidation of lipids.Accumulation of lipid, mainly cholesterol, results in down-regulation of TNF expression in macrophages. Since monocytes are known to be activated by cell adhesion, these results suggest that foam cells in atherosclerotic plaques may contribute less potently to an inflammatory reaction than newly arrived monocytes/macrophages.Lipid accumulation in macrophages leads to formation of foam cells, a distinct cell type found in fatty streaks and more advanced atherosclerotic lesions. Apart from effects on cellular lipid metabolism, the effects of intracellular lipid accumulation on gene expression have not been extensively studied. Since uptake of low density lipoprotein (LDL) is subject to negative feedback control of the LDL receptor pathway, we chose to incubate cells with acetylated LDL (ac-LDL), which is taken up by the scavenger receptors. In this way, a more efficient lipid loading is achieved.Lipid loading with ac-LDL in phorbol myristate acetate (PMA)-pretreated THP-1 macrophages has been reported to increase production of interleukin-8 (IL-8) and monocyte chemotactic protein-1 (MCP-1) [1]. It was further shown that IL-8 mRNA is ex