Inhibition of macrophage scavenger receptor activity by tumor necrosis factor-alpha is transcriptionally and post-transcriptionally regulated.
Regulation of expression of the scavenger receptor is thought to play a critical role in the accumulation of lipid by macrophages in atherosclerosis. Tumor necrosis factor-alpha (TNF-alpha) has been shown to suppress macrophage scavenger receptor function (van Lenten, B.J., and Fogelman, A.M. (1992) J. Immunol. 148, 112-6). However, the mechanism by which it does so is unknown. We evaluated the mechanism by which TNF-alpha inhibited macrophage scavenger receptor surface expression and binding of acetylated low density lipoprotein (aLDL). Binding of aLDL to phorbol 12-myristate 13-acetate (PMA)-differentiated THP-1 macrophages was suppressed by TNF-alpha in a dose-dependent manner. Inhibition of aLDL binding was paralleled by a reduction of macrophage scavenger receptor protein as detected by the Western blot. TNF-alpha partially decreased macrophage scavenger receptor mRNA steady state levels in PMA-differentiated THP-1 macrophages, a result that was confirmed by reverse transcription-polymerase chain reaction. PMA increased the luciferase activity driven by the macrophage scavenger receptor promoter in the transfected cells, whereas TNF-alpha partially reduced luciferase activity. However, macrophage scavenger receptor mRNA half-life was dramatically reduced in cells treated with TNF-alpha relative to untreated cells. Reduction in macrophage scavenger receptor message in response to TNF-alpha was dependent on new protein synthesis because it was blocked by cycloheximide. These results indicate that TNF-alpha regulates macrophage scavenger receptor expression in PMA-differentiated THP-1 macrophages by transcriptional and post-transcriptional mechanisms but principally by destabilization of macrophage scavenger receptor mRNA.
Hsu
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Department of Medicine, Cornell University Medical College, New York, 10021, USA.
Nicholson
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Hajjar
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D015152:Blotting, Northern; D002454:Cell Differentiation; D002460:Cell Line; D003513:Cycloheximide / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D005786:Gene Expression Regulation / Q000187:drug effects; D006801:Humans; D007700:Kinetics; D008077:Lipoproteins, LDL / Q000378:metabolism; D008156:Luciferases; D008264:Macrophages / Q000187:drug effects / Q000276:immunology*; D008565:Membrane Proteins; D019301:Oleic Acid; D009829:Oleic Acids / Q000378:metabolism; D016133:Polymerase Chain Reaction; D014176:Protein Biosynthesis / Q000187:drug effects*; D012333:RNA, Messenger / Q000096:biosynthesis / Q000187:drug effects / Q000378:metabolism; D011971:Receptors, Immunologic / Q000037:antagonists & inhibitors / Q000096:biosynthesis*; D011973:Receptors, LDL / Q000187:drug effects / Q000378:metabolism*; D018110:Receptors, Lipoprotein; D051116:Receptors, Scavenger; D011994:Recombinant Proteins / Q000494:pharmacology; D051122:Scavenger Receptors, Class B; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D014158:Transcription, Genetic / Q000187:drug effects*; D014162:Transfection; D014409:Tumor Necrosis Factor-alpha / Q000494:pharmacology*
D008077:Lipoproteins, LDL; D008565:Membrane Proteins; D009829:Oleic Acids; D012333:RNA, Messenger; D011971:Receptors, Immunologic; D011973:Receptors, LDL; D018110:Receptors, Lipoprotein; D051116:Receptors, Scavenger; D011994:Recombinant Proteins; C454739:Scarb1 protein, mouse; D051122:Scavenger Receptors, Class B; D014409:Tumor Necrosis Factor-alpha; D019301:Oleic Acid; D003513:Cycloheximide; D008156:Luciferases; D013755:Tetradecanoylphorbol Acetate