ABSTRACT
The regulation of ATP-binding cassette transporter 1 (ABC-1) expression by cytokines present within the microenvironment of the atheroma may play an important role in determining the impact of reverse cholesterol transport on the atherosclerotic lesion. We recently reported that the macrophage-activating cytokine interferon (IFN)-gamma inhibited both cholesterol efflux and ABC-1 expression. In the present study, we investigated the effects of transforming growth factor (TGF)-beta, a cytokine also apparent within the atheroma, on cholesterol efflux, ABC-1 expression, and its ability to antagonize the inhibitory effects of IFN-gamma. TGF-beta significantly increased cholesterol efflux in macrophage-derived foam cells from apolipoprotein E (apoE) knockout mice, with maximal effects apparent at 300 pg/ml. The increases in efflux occurred without any effect on the passive diffusion component of efflux mediated by beta-cyclodextrin. Furthermore, the increase in cholesterol efflux occurred without any changes in free or esterified cholesterol pools and was consistent with an increase in both ABC-1 message and protein. Finally, TGF-beta was also demonstrated to inhibit the IFN-gamma-mediated down-regulation of ABC-1. These results further demonstrate the importance of cytokine cross-talk to impact the process of reverse cholesterol transport through a multitude of processes including the regulation of ABC-1.
Subject(s)
ATP-Binding Cassette Transporters/metabolism , Cholesterol/metabolism , Foam Cells/drug effects , Interferon-gamma/metabolism , Transforming Growth Factor beta/pharmacology , beta-Cyclodextrins , ATP-Binding Cassette Transporters/immunology , Animals , Apolipoproteins E/genetics , Cells, Cultured , Cyclodextrins/pharmacology , Dose-Response Relationship, Drug , Foam Cells/metabolism , Immunoblotting , Lipoproteins, HDL/pharmacology , Macrophages, Peritoneal/physiology , Mice , Mice, Inbred BALB C , Mice, KnockoutABSTRACT
Cholesterol efflux is a fundamental process that serves to mitigate cholesterol accumulation and macrophage foam cell formation. Recently, we reported that cholesterol efflux to high density lipoprotein subfraction 3 was reduced by interferon-gamma (IFN-gamma) and that this decrease was associated with an increase in acyl coenzyme A:cholesterol acyltransferase (ACAT) expression. In the present study, although treatment of murine peritoneal macrophages with IFN-gamma resulted in a 2-fold decrease in HDL-mediated cholesterol efflux, efflux to lipid-free apolipoprotein A-I was reduced >4-fold and approached basal levels. This decrease was associated with a 3- to 4-fold reduction in ATP-binding-cassette transporter 1 (ABC1) mRNA content, the gene responsible for the defect in Tangier disease. Consistent with the reduction in cholesterol and phospholipid efflux in Tangier fibroblasts, downregulation of ABC1 expression by IFN-gamma also resulted in reduced phosphatidylcholine and sphingomyelin efflux to apolipoprotein A-I. Whereas foam cells had a 3-fold increase in ABC1 mRNA, the decrease in ABC1 message levels by IFN-gamma was observed in foam cells and control macrophages. This effect of IFN-gamma was independent of general macrophage activation (inasmuch as similar changes were not detected with granulocyte-macrophage colony-stimulating factor) and was not observed with other ABC transporters (inasmuch as the expression of the transporter in antigen processing was upregulated 4-fold in these same cells). Therefore, by decreasing cholesterol efflux through pathways that include the upregulation of ACAT and the downregulation of ABC1, IFN-gamma can shift the equilibrium between macrophages and foam cells and thus impact the progression of an atherosclerotic lesion.
Subject(s)
ATP-Binding Cassette Transporters/genetics , Foam Cells/metabolism , Gene Expression Regulation , Interferon-gamma/pharmacology , Macrophages, Peritoneal/metabolism , Tangier Disease/genetics , Animals , Apolipoprotein A-I/metabolism , Cholesterol/metabolism , Granulocyte-Macrophage Colony-Stimulating Factor/pharmacology , Mice , Mice, Inbred BALB CABSTRACT
The Th1-derived cytokine gamma interferon, IFN-gamma, is present within the microenvironment of an atheromatous lesion and likely contributes to lesion progression through macrophage activation. While the inflammatory effects of IFN-gamma are well known, the role of this cytokine in cholesterol metabolism in macrophage derived foam cells is unclear. In the present study, the incubation of foam cells with IFN-gamma resulted in the reduction of HDL(3)-mediated cholesterol efflux. The decrease in cholesterol efflux was not observed with other macrophage-activating factors as colony-stimulating factors failed to demonstrate a similar effect. The reduction in cholesterol efflux was independent of apoE synthesis or SR-BI expression and was associated with a redistribution of intracellular cholesterol with an increase in cholesteryl ester accumulation. The increase in the esterified pool, primarily in cholesterol eicosapentadenoate, docosapentaenoate, arachidonate, and linoleate was associated with a 2-fold increase in acyl-CoA:cholesterol-O-acyltransferase, ACAT, activity and message without any change in neutral cholesteryl ester hydrolase activity. While CD36 message was reduced in IFN-gamma-treated foam cells, the ability to reverse the decrease in efflux by the ACAT inhibitor A58035 in a dose-dependent manner suggests that the IFN-gamma effect on efflux is primarily through the modulation of ACAT expression. Therefore, in addition to its inflammatory effects, IFN-gamma can contribute to the progression of an atherosclerotic lesion by altering the pathway of intracellular cholesterol trafficking in macrophage derived foam cells.
Subject(s)
Cholesterol, HDL/metabolism , Foam Cells/drug effects , Interferon-gamma/pharmacology , Membrane Proteins , Receptors, Lipoprotein , Base Sequence , Biological Transport , CD36 Antigens , DNA Primers , Enzyme Induction , Foam Cells/enzymology , Foam Cells/metabolism , Humans , Macrophage Activation , Receptors, Immunologic/metabolism , Receptors, Scavenger , Scavenger Receptors, Class B , Sterol O-Acyltransferase/metabolismABSTRACT
The latent membrane protein 2 (LMP2) of Epstein-Barr virus interferes with B-lymphocyte signal transduction through the immunoglobulin (Ig) receptor. Two isoforms of LMP2 exist and differ only in that one isoform (LMP2a) contains an N-terminal cytoplasmic domain that the other isoform does not. LMP2a is a phosphoprotein that is phosphorylated on tyrosines and serines in the cytoplasmic domain. GST1-119, a glutathione S-transferase (GST) fusion protein containing the 119 amino acids of the cytoplasmic domain, affinity precipitated serine kinase activity from BJAB cell extracts. The affinity-precipitated kinase phosphorylated LMP2a sequences, and kinase activity was increased following induction. Probing of Western immunoblots of affinity-precipitated proteins showed that the Erk1 form of mitogen-activated protein kinase (MAPK) was present. Purified MAPK phosphorylated GST fusion proteins containing the cytoplasmic domain of LMP2a and mutational analyses were used to identify S15 and S102 as the sites of in vitro phosphorylation. A polyclonal rabbit antiserum was prepared against a maltose binding protein-LMP2a cytoplasmic domain fusion protein (MBP1-119) and used to immunoprecipitate LMP2a from the in vitro-immortalized lymphoblastoid B-cell line B95-8CR. LMP2a immunoprecipitates from B95-8CR contained MAPK as a coprecipitated protein. Cross-linking surface Ig on B95-8CR cells failed to induce MAPK activity within the cells. Treatment of B95-8CR with phorbol myristate acetate (PMA) was able to bypass the Ig receptor block and activate MAPK activity. Phosphorylation of LMP2a on serine residues increased after PMA induction. The possible role for LMP2a serine phosphorylation by MAPK in the control of latency is discussed.
