Inhibition of lipopolysaccharide-induced interleukin-1 beta mRNA expression in mouse macrophages by oxidized low density lipoprotein.

Abstract

Recent studies have demonstrated the expression of messenger RNA (mRNA) for several cytokines within atherosclerotic arteries. Since cytokines have been shown to modulate functions of cultured arterial wall cells in a manner that could influence atherogenesis, this suggests that factors that modulate cytokine production would influence the atherosclerotic process. To examine whether lipoproteins can modulate cytokine production, the effect of lipoproteins on mouse macrophage interleukin-1 beta (IL-1 beta) mRNA expression was examined by dot blot and Northern blot analyses. Low density lipoprotein (LDL), acetylated-LDL, or malondialdehyde-LDL did not induce IL-1 beta mRNA expression or affect the expression in response to lipopolysaccharide (LPS). Similarly, copper ion-oxidized LDL did not stimulate the production of IL-1 beta mRNA. However, oxidized LDL inhibited the LPS-induced expression in a concentration- and time-dependent manner with a maximum inhibition (greater than 90%) observed after a 2.5 h preincubation with 25 micrograms protein/ml. These conditions did not affect protein synthesis or phagocytosis and the inhibition was partially reversible after 24 h, which together suggest that the inhibition was not due to cell death. An inhibition of IL-1 alpha and IL-6 mRNA expression was also observed while there was no change in gamma-actin mRNA levels. The level of inhibition of IL-1 beta mRNA was dependent upon the extent of LDL oxidation, but did not correlate with recognition by the scavenger receptor. A non-receptor pathway was supported by two lines of evidence: 1) the inhibition could be reproduced with a lipid extract, and 2) oxidized LDL also inhibited scavenger receptor negative THP-1 cell IL-1 beta mRNA expression. Finally, oxidized LDL had no effect on the turnover of IL-1 beta mRNA, suggesting that the decreased accumulation of IL-1 beta mRNA is due to a decrease in gene transcription. Together these studies suggest that as macrophages become foam cells their immune responsiveness is attenuated.