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Previous Article | Next Article 
Molecular and Cellular Biology, March 2003, p. 2182-2191, Vol. 23, No. 6
0270-7306/03/$08.00+0 DOI: 10.1128/MCB.23.6.2182-2191.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Howard Hughes Medical Institute,1 Department of Pathology and Laboratory Medicine, University of California, Los Angeles, California 90095-1662,2 Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, Texas 75390-90503
Received 14 August 2002/ Returned for modification 14 October 2002/ Accepted 9 December 2002
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ABSTRACT |
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and LXRß, confirming the critical role of these receptors. We further demonstrate that the PLTP promoter contains a high-affinity LXR response element that is bound by LXR/RXR heterodimers in vitro and is activated by LXR/RXR in transient-transfection studies. Finally, immunohistochemistry studies reveal that PLTP is highly expressed by macrophages within human atherosclerotic lesions, suggesting a potential role for this enzyme in lipid-loaded macrophages. These studies outline a novel pathway whereby LXR and its ligands may modulate lipoprotein metabolism. |
INTRODUCTION |
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HDL displays several antiatherogenic activities. Among these is the ability of HDL to shuttle cholesterol from peripheral tissues to the liver in the so-called reverse cholesterol transport pathway (34). HDL is a heterogenous population of particles that differ in size, composition, and the ability to perform specific functions. For example, large, spherical HDL particles (called -HDL) effectively deliver cholesteryl esters to the liver. In contrast, small discoidal lipid-poor HDL particles (called pre-ß-HDL) are effective acceptors of cholesterol from peripheral tissues. Therefore, plasma proteins that assist in the remodeling of HDL are important to the proper functioning of HDL.
One key modulator of HDL levels, size, and composition is the phospholipid transfer protein (PLTP) (10, 40). PLTP assists in the formation of pre-ß-HDL and -HDL by transferring phospholipids and cholesterol from VLDL and chylomicrons to HDL during lipolysis by lipoprotein lipase. This activity is clearly demonstrated in PLTP-deficient mice, which lack phospholipid transfer activity from VLDL to HDL, display massive reductions in HDL phospholipid and HDL cholesterol levels, and, on a high-fat diet, show increases in VLDL and LDL levels (15). Furthermore, both in vitro and in vivo studies have demonstrated that PLTP acts directly on mature HDL, causing formation of larger -HDL and smaller pre-ß-HDL particles (11, 13-15, 24, 37, 39). Mice expressing moderate levels of human transgenes for both PLTP and apolipoprotein AI (ApoAI) have increased levels of pre-ß-HDL and -HDL (14). However, when PLTP is massively overexpressed, HDL cholesterol levels are decreased due to increased hepatic uptake of HDL cholesterol esters and phospholipids (1, 7, 8). These mice, despite lower HDL cholesterol levels, also displayed marked increases in pre-ß-HDL (11, 39). These findings support a central role for PLTP in HDL metabolism and suggest that PLTP may increase reverse cholesterol transport.
Nuclear receptors are a class of transcription factors that regulate gene expression in response to ligand binding. These receptors typically bind to lipid ligands and regulate the expression of genes that control various aspects of the metabolism of the ligand. The liver X receptor (LXR) subfamily of nuclear receptors are bound and transcriptionally activated by oxidized forms of cholesterol (oxysterols) (12, 23). In response to ligand binding, LXRs regulate genes involved in the uptake, catabolism, and transport of excess cholesterol and oxysterols in the body (22, 25, 32). Thus, LXRs act as molecular sensors of cholesterol levels and respond by inducing processes that reduce cholesterol levels. In peripheral cells such as macrophages, LXRs increase cholesterol efflux by inducing expression of ATP-binding cassette A1 (ABCA1), ABCG1, and ApoE (5, 21, 33, 41, 42). In the intestine, LXR agonists decrease cholesterol absorption through induction of ABCA1, ABCG5, and ABCG8 (2, 30, 33). In murine (but not human) liver, LXRs increase the catabolism of cholesterol by increasing bile acid synthesis through induction of the CYP7A1 gene (23, 29).
Here we demonstrate an additional role of LXRs in the control of plasma HDL metabolism. We show that oxysterol and synthetic LXR ligands regulate expression of the PLTP gene in liver and macrophages in an LXR-dependent manner and that synthetic LXR ligands induce plasma PLTP enzyme activity in mice. Surprisingly, PLTP is highly expressed by macrophages in human atherosclerotic lesions, suggesting an unrecognized role for PLTP in this context. These findings suggest that LXRs may regulate reverse cholesterol transport through multiple mechanisms, including induction of cholesterol efflux, cholesterol catabolism, and lipoprotein remodeling.
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MATERIALS AND METHODS |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionAddendumReferences |
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, LXRß, and retinoid X receptor (RXR) have been described (21). pCMX-VP16-LXR and pCMX-VP16-LXRß were gifts from Ron Evans (Salk Institute). GW3965 and T0901317 were kindly provided by Jon Collins and Tim Willson (GlaxoSmithKline). LG268 was kindly provided by Rich Heyman (Ligand Pharmaceuticals). Ligands were dissolved in ethanol or dimethyl sulfoxide (DMSO) prior to use in cell culture. Reporter plasmids containing two copies of a PLTP direct repeat (DR4; see below) elements (pTK-2xDR4B-Luc) were generated by ligating two copies of oligonucleotides corresponding to sequences from the human PLTP promoter into the BamHI site of pTK-Luciferase. For PLTP DR4A and DR4B, the oligonucleotides used were gatcggtgTGACCCacca TGCCCAgcct and gatcccttTGAACTctag TAACCTtcct, respectively (sense strand only; half-sites shown in capitals). Cell culture and transfections. THP-1 and wild-type (WT) Macs cells were cultured in RPMI medium containing 10% fetal bovine serum (FBS). HepG2 cells were grown in minimal essential medium (MEM) containing 10% FBS. Peritoneal macrophages were obtained from thioglycolate-injected mice as described elsewhere (41) and cultured in Dulbecco's MEM (DMEM) containing 10% FBS. For ligand treatments, cells were cultured in RPMI medium or DMEM, supplemented with 5% lipoprotein-deficient serum (LPDS) (Intracel) and receptor ligands for 24 h unless otherwise indicated. In some experiments, cells were sterol depleted by the inclusion of 5 µM simvastatin and 100 µM mevalonic acid during the treatment period. Transient transfections of HepG2 cells were performed in triplicate in 48-well plates. Cells were transfected with reporter plasmid (100 ng/well), receptor plasmids (5 to 50 ng/well), pCMV-ß-galactosidase (50 ng/well), and pTKCIII (to a total of 205 ng/well) with the MBS mammalian transfection kit (Stratagene). Following transfection, cells were incubated in MEM containing 10% LPDS and the appropriate ligands or vehicle control for 24 h. Luciferase activity was normalized to ß-galactosidase activity.
RNA and protein analysis. Total RNA was isolated by using the Trizol reagent (Life Technologies, Inc.). Northern analysis was performed as described elsewhere (41) using radiolabeled cDNA probes. Blots were normalized using cDNA probes to 36B4 and quantitated by PhosphorImager (Molecular Dynamics) analysis. Real-time quantitative PCR assays were performed with an Applied Biosystems 7700 sequence detector. Briefly, 1 µg of total RNA was reverse transcribed with random hexamers by using a TaqMan reverse transcription reagent kit (Applied Biosystems) according to the manufacturer's protocol. Each amplification mixture (20 µl) contained 20 ng of cDNA, 500 nM forward primer, 500 nM reverse primer, 100 nM dually labeled fluorogenic probe (Applied Biosystems), and 12.5 µl of universal PCR master mix. PCR thermocycling parameters were 50°C for 2 min, 95°C for 10 min, and 40 cycles of 95°C for 15 s and 60°C for 1 min. All samples were analyzed for 36B4 (human or mouse) expression in parallel in the same run for normalization. Quantitative expression values were extrapolated from separate standard curves for 36ß4 or the indicated gene generated with 10-fold dilutions of cDNA (in duplicate). Each sample was normalized to 36ß4, and then replicates were averaged and the relative (-fold) induction was determined. Probe and primer sequences are available upon request. For Western blots, total cell lysates were separated by polyacrylamide gel electrophoresis, transferred to membranes, and blotted for PLTP by using an anti-PLTP polyclonal antibody (Cardiovascular Targets, Inc., New York, N.Y.). Equal loading and transfer of proteins were verified by blotting for ß-actin.
Gel shift assays.
In vitro-translated RXR and LXR were generated from pCMX-RXR and pCMX-hLXR plasmids using the TNT quick coupled transcription-translation system (Promega). Gel shift assays were performed as described elsewhere (20) using in vitro-translated proteins and the following oligonucleotides (only one strand shown): PLTP DR4A, GATCGGTGTGACCC ACCATGCCCAGCCT; PLTP DR4B, GATCCCTTTGAACTCTAGTAACCT TCCT; hLXR DR4C, GATCGCTGAGGTTACTGCTGGTCATTCA; CYP7A1 LXRE, CCTTTGGTCACTCAAGTTCAAGTG.
Animals and diets. ApoE knockout mice on a C57BL6 background or C57BL6 WT mice were maintained on a normal chow diet. Six-week-old male C57BL6 WT mice or 8-month old ApoE-/- mice were treated with GW3965 by gavage for 5 days at 30 mg/kg of mouse weight. LXR WT and LXR double-knockout (DKO) animals were treated with LG268 (30 mg/kg) solubilized in 0.9% carboxy methylcellulose, 9% PEG-400, and 0.05% Tween 80 or T1317 (50 mg/kg) in 1% methylcellulose and 1% Tween 80 for 10 days. Following euthanization, blood was collected from the abdominal vena cava and tissues were harvested for RNA with the Trizol reagent. All animal experiments were approved by the Institution Animal Care and Research Advisory Committees of the University of Southwestern Medical Center and the University of California, Los Angeles.
PLTP activity assay. Plasma PLTP activity was measured by using a fluorescence-based assay (Cardiovascular Targets, Inc.). Briefly, 3 µl of mouse plasma was incubated with donor and acceptor particles. The donor contains a fluorescent phospholipid that is in the quenched state when associated with the donor. PLTP-mediated transfer of the phospholipid to the acceptor results in increased fluorescence intensity. A PLTP-neutralizing antibody (Cardiovascular Targets, Inc.) was used where indicated.
Immunohistochemistry. A segment of human left coronary artery containing atherosclerotic plaques was harvested from a patient at autopsy, snap-frozen, and stored at -80°C. After fixing for 5 min in cold acetone, frozen sections (6 µm) were processed for immunohistochemical staining. For PLTP staining, frozen sections were incubated with purified rabbit anti-human PLTP polyclonal antibody (Cardiovascular Targets, Inc.), followed by biotinylated goat anti-rabbit immunoglobulin G IgG (Dako). Following incubation with an avidin-biotin peroxidase conjugate, samples were visualized with 3,3'-diaminobenzidine (Vector Laboratories). CD68 staining was performed with an anti-human CD68 monoclonal antibody (Dako), followed by incubation with alkaline phosphatase-conjugated anti-mouse IgG (Vector Laboratories) and visualization with vector red alkaline phosphatase substrate solution. Staining with nonimmune rabbit IgG (Dako) served as a negative control.
DNA microarray analysis. Thioglycolate-elicited peritoneal macrophages were obtained as described above, and cells were cultured in DMEM supplemented with 10% LPDS (Intracel), 5 µM simvastatin, and 100 µM mevalonic acid in the presence or absence of T1317 (10 µM) for 48 h. Total RNA was isolated by using the Trizol reagent and further purified with an RNeasy total RNA isolation kit (Qiagen). Total RNA (10 µg) was reverse transcribed by using a T7-(dT)24 primer (Genset Corp.) and the Superscript Choice system (Life Technologies). The resulting cDNA was used to generate biotin-labeled cRNA with a bioarray high-yield transcript labeling kit (Enzo). Fragmentation of cRNA was performed at 94°C using 40 mM Tris-acetate (pH 8.1), 100 mM potassium acetate, and 30 mM magnesium acetate. Samples were hybridized to Affymetrix murine U74Av2 microarrays and visualized by the PAN Facility at Stanford University. The results of the microarrays were analyzed with GeneChip Analysis Suite software (Affymetrix).
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RESULTS |
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mRNA was also not different between the two groups. To further localize the expression of PLTP to atherosclerotic lesions, we performed immunohistochemical staining for PLTP on sections from human atherosclerotic lesions. As shown in Fig. 3C, anti-PLTP antibody showed strong staining within the atherosclerotic lesions. By contrast, no macrophage staining was observed with control nonspecific antibody (Fig. 3B). Moreover, this expression was found to be particularly strong in regions containing large numbers of CD68-positive macrophages (Fig. 3D and F). Expression of PLTP antigen in macrophage-rich regions of the lesion was further confirmed with dual staining (Fig. 3E and G). PLTP expression within lesions was more diffuse than that observed for CD68, consistent with the fact that it is a secreted protein. Taken together, these results demonstrate that PLTP is expressed by macrophages within atherosclerotic lesions and can be regulated by LXR in that context.
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gene (Fig. 5A and B). We also analyzed the murine PLTP gene by searching the Celera database (mCG17532). Both human and murine PLTP DR4 sequences show significant similarity to other known LXREs (Fig. 4B).
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and RXR expression vectors, the pTK-2xDR4B-Luc reporter was activated approximately ninefold by the LXR agonist GW3965, whereas the empty vector pTK-Luc was not responsive (Fig. 5D). Furthermore, cotransfection of expression vectors encoding constitutively active VP16-LXR fusion proteins (VP-LXR or VP-LXRß) resulted in marked activation of the pTK-2xDR4B-Luc reporter (Fig. 5D). These results demonstrate the presence of a functional LXRE within the promoter region of the human PLTP gene.
It has been shown that overexpression of LXR can potentiate expression of LXR target genes to ligand activation (41). Therefore, we generated LXR-overexpressing cells by infecting RAW murine macrophages with a retrovirus encoding LXR (RAW-LXR cells). Cells infected with the empty vector served as a control. RAW-LXR or RAW control cells were exposed to increasing concentrations of the LXR ligand 20(S)-OHC, and PLTP mRNA levels were monitored by TaqMan assays. As we have previously shown for LXR target ABCA1 and ABCG1 genes, overexpression of LXR resulted in a shift of the dose-response curve to the left (Fig. 6). Thus, PLTP mRNA levels were induced in RAW-LXR cells at lower concentrations of 20(S)-OHC than RAW control cells (Fig. 6).
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or LXRß modestly reduced the induction of PLTP by 22R-OHC but had no significant effect on activation by T1317. However, the induction of PLTP by LXR and RXR ligands was completely abolished in macrophages lacking both LXR and LXRß (LXR DKO) (Fig. 7A and B). We also investigated PLTP expression in WT and DKO macrophages. As shown in Fig. 7C, treatment with an LXR agonist resulted in an induction in PLTP expression as measured by Western blotting with anti-PLTP antibody. Moreover, expression of PLTP was markedly reduced in macrophages lacking LXRs and was not inducible by an LXR agonist.
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(PPAR)/RXR, that have been reported (3, 20) to activate PLTP. These finding definitively establish a role for LXRs in the regulation of PLTP mRNA levels in vivo.
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DISCUSSION |
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and LXRß are key regulators of various aspects of cholesterol metabolism. The LXRs have been shown to regulate cholesterol absorption in the intestine, cholesterol catabolism in the liver, and cholesterol efflux from peripheral tissues. We show here that LXRs regulate expression of PLTP mRNA levels in response to LXR ligands in a variety of tissues, including intestine, liver, and fat. We demonstrate that both LXR and RXR ligands regulate PLTP levels in a variety of cell types and tissues in a coordinate fashion with other known LXR target genes. Furthermore, PLTP activity was increased in the plasma of mice treated with a synthetic LXR ligand. Regulation of PLTP by LXR ligands was abolished in LXR-deficient animals or cells, demonstrating a critical role for LXR in the regulation of PLTP. These findings identify an additional role for LXRs in the control of lipoprotein metabolism. The LXRs have been implicated in the control of HDL metabolism through regulation of the cholesterol-phospholipid transporter ABCA1 (22, 25, 32). Loss-of-function mutations of ABCA1 result in extremely low HDL levels in both humans and mice (28). Short-term administration of synthetic LXR ligands to animals results in induction of ABCA1 and increases in plasma HDL cholesterol and phospholipid levels (17, 35). At the same, short-term treatment with LXR ligands also dramatically raises plasma triglyceride levels (17, 35). The mechanism for this effect is likely to involve the transcriptional induction of genes critical for lipogenesis, including SREBP-1c and FAS (17, 31, 35, 43). A complete understanding of the mechanisms underlying LXR effects on systemic lipid levels will be required if LXR agonists are to be optimized as therapeutic agents.
The present work suggests that LXR agonists may also modulate lipid metabolism through control of PLTP expression. The role of PLTP in mammalian physiology is complex and the subject of ongoing research, and the implications of the PLTP-LXR connection are not yet clear. PLTP has been identified as a key modulator of HDL metabolism, and studies have also suggested a role in reverse cholesterol transport (10, 40). The importance of PLTP in HDL metabolism was clearly demonstrated by the massive decrease in HDL levels in mice lacking PLTP (15). In addition, PLTP has recently been shown to regulate VLDL secretion from the liver. PLTP-deficient mice were shown to have decreased levels of VLDL and LDL when placed on an ApoE-deficient or ApoB-transgenic background (16). Therefore, the previously reported actions of LXR agonists on HDL and VLDL levels are consistent with the known roles for PLTP in lipoprotein metabolism. In particular, it seems likely that the ability of LXR agonists to raise plasma VLDL and triglyceride levels may involve PLTP.
Despite the importance of PLTP in various aspects of lipoprotein metabolism, relatively little is known about the mechanisms controlling its expression. In addition to LXR, PLTP expression is regulated by other nuclear receptor RXR heterodimers, including PPAR/RXR and FXR/RXR. In mice, treatment with PPAR ligands such as fibrates results in increased HDL size through induction of hepatic PLTP mRNA and plasma PLTP activity (3). This increase in HDL size by fibrate treatment was completely abolished in PLTP-deficient mice, and the induction of PLTP mRNA and activity was dramatically reduced in PPAR-deficient animals (3). However, no positive response element for PPAR has been identified in the PLTP promoter. PLTP mRNA expression is also regulated by ligands of FXR (bile acids) in both human and rat hepatocytes, and overexpression of FXR increases this response (20). Hepatic PLTP levels are induced in mice fed a diet high in the bile acid cholate, and this effect is lost in FXR-deficient animals (19). The promoter of the human PLTP gene contains a functional FXR response element and is therefore likely to be a direct target of FXR (20, 38). In the present study we show that treatment of mice with the RXR ligand (LG268) results in increased expression of hepatic PLTP mRNA. This increase is reduced, but not abolished, in LXR-deficient mice, consistent with a role for LXR in the control of hepatic PLTP expression. The residual activation by RXR ligands in LXR-deficient liver is likely to be due to the activity of FXR/RXR or PPAR/RXR heterodimers.
It was recently reported that pharmacological activation of LXR inhibits the formation of atherosclerotic lesions in two mouse models (18). Chronic LXR ligand administration reduced lesions by about 50% in both LDL receptor-/- and ApoE-/- mice, despite relatively minor changes in plasma lipoprotein levels. It has further been shown that transplantation of LXR-deficient bone marrow into LDL receptor-/- and ApoE-/- mice results in substantially increased lesion formation compared to controls (36). These findings demonstrate that bone marrow-derived cells, such as macrophages, are involved in the antiatherogenic activities of LXRs in mice. Therefore, understanding gene expression changes within atherosclerotic lesions in response to LXR ligands may provide additional clues as to the prevention of atherosclerosis. Previous work has shown that LXRs regulate cholesterol efflux from macrophages at least in part through the induction of ABC transporter proteins, including those for ABCA1 and possibly ABCG1 (6, 33, 41). LXRs have also been shown to regulate the expression of apolipoproteins, including ApoE, ApoCI, ApoCII, and ApoCIV, in macrophages (21, 27). Each of these lipoproteins can serve as a cholesterol acceptor within the artery wall. In the present work, we show that PLTP is also highly expressed by macrophages within atherosclerotic lesions and is inducible by LXR. The physiologic role PLTP in this context and its connection to atherosclerosis remain to be elucidated. It will be interesting to examine the role of macrophage expression of PLTP by using bone marrow transplants from PLTP-deficient mice or tissue-specific knockouts of PLTP.
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ADDENDUM |
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FOOTNOTES |
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