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Steven J Bensinger,
Michelle N Bradley, Sean B Joseph,
Noam Zelcer,
Edith M Janssen,
Mary Ann Hausner,
Roger Shih,
John S Parks,
Peter A Edwards,
Beth D Jamieson,
Peter Tontonoz
[show abstract]
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ABSTRACT: Cholesterol is essential for membrane synthesis; however, the mechanisms that link cellular lipid metabolism to proliferation are incompletely understood. We demonstrate here that cellular cholesterol levels in dividing T cells are maintained in part through reciprocal regulation of the LXR and SREBP transcriptional programs. T cell activation triggers induction of the oxysterol-metabolizing enzyme SULT2B1, consequent suppression of the LXR pathway for cholesterol transport, and promotion of the SREBP pathway for cholesterol synthesis. Ligation of LXR during T cell activation inhibits mitogen-driven expansion, whereas loss of LXRbeta confers a proliferative advantage. Inactivation of the sterol transporter ABCG1 uncouples LXR signaling from proliferation, directly linking sterol homeostasis to the antiproliferative action of LXR. Mice lacking LXRbeta exhibit lymphoid hyperplasia and enhanced responses to antigenic challenge, indicating that proper regulation of LXR-dependent sterol metabolism is important for immune responses. These results implicate LXR signaling in a metabolic checkpoint that modulates cell proliferation and immunity.
Cell 08/2008; 134(1):97-111. · 32.40 Impact Factor
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Michelle N Bradley,
Cynthia Hong,
Mingyi Chen, Sean B Joseph,
Damien C Wilpitz,
Xuping Wang,
Aldons J Lusis,
Allan Collins,
Willa A Hseuh,
Jon L Collins,
Rajendra K Tangirala,
Peter Tontonoz
[show abstract]
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ABSTRACT: Liver X receptors (LXRs) alpha and beta are transcriptional regulators of cholesterol homeostasis and potential targets for the development of antiatherosclerosis drugs. However, the specific roles of individual LXR isotypes in atherosclerosis and the pharmacological effects of synthetic agonists remain unclear. Previous work has shown that mice lacking LXRalpha accumulate cholesterol in the liver but not in peripheral tissues. In striking contrast, we demonstrate here that LXRalpha(-/-)apoE(-/-) mice exhibit extreme cholesterol accumulation in peripheral tissues, a dramatic increase in whole-body cholesterol burden, and accelerated atherosclerosis. The phenotype of these mice suggests that the level of LXR pathway activation in macrophages achieved by LXRbeta and endogenous ligand is unable to maintain homeostasis in the setting of hypercholesterolemia. Surprisingly, however, a highly efficacious synthetic agonist was able to compensate for the loss of LXRalpha. Treatment of LXRalpha(-/-)apoE(-/-) mice with synthetic LXR ligand ameliorates the cholesterol overload phenotype and reduces atherosclerosis. These observations indicate that LXRalpha has an essential role in maintaining peripheral cholesterol homeostasis in the context of hypercholesterolemia and provide in vivo support for drug development strategies targeting LXRbeta.
Journal of Clinical Investigation 09/2007; 117(8):2337-46. · 15.39 Impact Factor
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Sean B Joseph,
Michelle N Bradley,
Antonio Castrillo,
Kevin W Bruhn,
Puiying A Mak,
Liming Pei,
John Hogenesch,
Ryan M O'connell,
Genhong Cheng,
Enrique Saez,
Jeffery F Miller,
Peter Tontonoz
[show abstract]
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ABSTRACT: The liver X receptors (LXRs) are nuclear receptors with established roles in the regulation of lipid metabolism. We now show that LXR signaling not only regulates macrophage cholesterol metabolism but also impacts antimicrobial responses. Mice lacking LXRs are highly susceptible to infection with the intracellular bacteria Listeria monocytogenes (LM). Bone marrow transplant studies point to altered macrophage function as the major determinant of susceptibility. LXR-null macrophages undergo accelerated apoptosis when challenged with LM and exhibit defective bacterial clearance in vivo. These defects result, at least in part, from loss of regulation of the antiapoptotic factor SPalpha, a direct target for regulation by LXRalpha. Expression of LXRalpha or SPalpha in macrophages inhibits apoptosis in the setting of LM infection. Our results demonstrate that LXR-dependent gene expression plays an unexpected role in innate immunity and suggest that common nuclear receptor pathways mediate macrophage responses to modified lipoproteins and intracellular pathogens.
Cell 11/2004; 119(2):299-309. · 32.40 Impact Factor
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ABSTRACT: Macrophages are an important source of angiogenic activity in wound healing, cancer, and chronic inflammation. Vascular endothelial growth factor (VEGF), a cytokine produced by macrophages, is a primary inducer of angiogenesis and neovascularization in these contexts. VEGF expression by macrophages is known to be stimulated by low oxygen tension as well as by inflammatory signals. In this study, we provide evidence that Vegfa gene expression is also regulated by activation of liver X receptors (LXRs). VEGF mRNA was induced in response to synthetic LXR agonists in murine and human primary macrophages as well as in murine adipose tissue in vivo. The effects of LXR ligands on VEGF expression were independent of hypoxia-inducible factor HIF-1alpha activation and did not require the previously characterized hypoxia response element in the VEGF promoter. Rather, LXR/retinoid X receptor heterodimers bound directly to a conserved hormone response element (LXRE) in the promoter of the murine and human Vegfa genes. Both LXRalpha and LXRbeta transactivated the VEGF promoter in transient transfection assays. Finally, we show that induction of VEGF expression by inflammatory stimuli was independent of LXRs, because these effects were preserved in LXR null macrophages. These observations identify VEGF as an LXR target gene and point to a previously unrecognized role for LXRs in vascular biology.
Journal of Biological Chemistry 04/2004; 279(11):9905-11. · 4.77 Impact Factor
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ABSTRACT: Our previous work led to the hypothesis that peroxisomal proliferator-activated receptor alpha (PPAR alpha) modulates insulin action in a compensatory fashion for hepatic glucose balance vs. peripheral glucose disposal. Therefore, we have examined the expression of insulin-dependent gluconeogenic/glycolytic/pentose cycle enzymes and compared these to insulin responsiveness for peripheral vs. hepatic substrate flux and futile cycling in the PPAR alpha knockout mouse. Hepatic gluconeogenic flux, glucose absorption, clearance and recycling, as well as in vivo glucose disposal were evaluated using new mass isotopomer methods. Insulin-dependent gluconeogenic/glycolytic/pentose cycle enzyme expression and glucose futile cycling were diminished; however, glucose disappearance was increased. This supports the hypothesis of hepatic insulin resistance and increased peripheral glucose uptake as compensatory events secondary to the decrease in fatty acid oxidation characteristic of the PPAR alpha knockout. We conclude that 1) the loss of PPAR alpha results in lower expression levels and diminished response to meal regulation for gluconeogenic/glycolytic enzyme expression; and 2) consequently, substrate/futile cycling of glucose is decreased when PPAR alpha is absent despite increased gluconeogenesis. The compensatory changes in liver and peripheral tissue substrate flux and the resultant adaptation for enzyme expression in the liver to have a diminished insulin dependence reflect the loosely linked correlation between phenotype and genotype in hepatic glucose metabolism.
Endocrinology 04/2004; 145(3):1087-95. · 4.46 Impact Factor
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ABSTRACT: The liver X receptors (LXR) alpha and beta are regulators of cholesterol metabolism and determinants of atherosclerosis susceptibility. Viral and bacterial pathogens have long been suspected to be modulators of atherogenesis; however, mechanisms linking innate immunity to cholesterol metabolism are poorly defined. We demonstrate here that pathogens interfere with macrophage cholesterol metabolism through inhibition of the LXR signaling pathway. Activation of Toll-like receptors (TLR) 3 and 4 by microbial ligands blocks the induction of LXR target genes including ABCA1 in cultured macrophages as well as in aortic tissue in vivo. As a consequence of these transcriptional effects, TLR3/4 ligands strongly inhibit cholesterol efflux from macrophages. Crosstalk between LXR and TLR signaling is mediated by IRF3, a specific effector of TLR3/4 that inhibits the transcriptional activity of LXR on its target promoters. These findings highlight a common mechanism whereby bacterial and viral pathogens may modulate macrophage cholesterol metabolism and cardiovascular disease.
Molecular Cell 11/2003; 12(4):805-16. · 14.18 Impact Factor
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ABSTRACT: The liver X receptors (LXRs) are nuclear receptors activated by oxysterols that are now recognized to play an important role in the control of lipid homeostasis. LXRs have been implicated in the regulation of cholesterol and fatty acid metabolism in multiple tissues, including liver and intestine, as well as in macrophages. The importance of these receptors in physiological lipid metabolism suggests that they may also influence the development of metabolic disorders such as hyperlipidemia and atherosclerosis. Strong support for this idea has been provided by recent studies that directly linked LXR activity to the pathogenesis of atherosclerosis. These observations identify the LXR pathway as an attractive target for intervention in cardiovascular disease.
Current Opinion in Pharmacology 05/2003; 3(2):192-7. · 6.86 Impact Factor
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Bryan A Laffitte,
Lily C Chao,
Jing Li,
Robert Walczak,
Sarah Hummasti, Sean B Joseph,
Antonio Castrillo,
Damien C Wilpitz,
David J Mangelsdorf,
Jon L Collins,
Enrique Saez,
Peter Tontonoz
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ABSTRACT: The control of lipid and glucose metabolism is closely linked. The nuclear receptors liver X receptor (LXR)alpha and LXR beta have been implicated in gene expression linked to lipid homeostasis; however, their role in glucose metabolism is not clear. We demonstrate here that the synthetic LXR agonist GW3965 improves glucose tolerance in a murine model of diet-induced obesity and insulin resistance. Analysis of gene expression in LXR agonist-treated mice reveals coordinate regulation of genes involved in glucose metabolism in liver and adipose tissue. In the liver, activation of LXR led to the suppression of the gluconeogenic program including down-regulation of peroxisome proliferator-activated receptor gamma coactivator-1 alpha (PGC-1), phosphoenolpyruvate carboxykinase (PEPCK), and glucose-6-phosphatase expression. Inhibition of gluconeogenic genes was accompanied by an induction in expression of glucokinase, which promotes hepatic glucose utilization. In adipose tissue, activation of LXR led to the transcriptional induction of the insulin-sensitive glucose transporter, GLUT4. We show that the GLUT4 promoter is a direct transcriptional target for the LXR/retinoid X receptor heterodimer and that the ability of LXR ligands to induce GLUT4 expression is abolished in LXR null cells and animals. Consistent with their effects on GLUT4 expression, LXR agonists promote glucose uptake in 3T3-L1 adipocytes in vitro. Thus, activation of LXR alters the expression of genes in liver and adipose tissue that collectively would be expected to limit hepatic glucose output and improve peripheral glucose uptake. These results outline a role for LXRs in the coordination of lipid and glucose metabolism.
Proceedings of the National Academy of Sciences 05/2003; 100(9):5419-24. · 9.68 Impact Factor
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ABSTRACT: The liver X receptors (LXRs) are members of the nuclear receptor superfamily that are activated by oxysterols. In response to ligand binding, LXRs regulate a variety of genes involved in the catabolism, transport, and uptake of cholesterol and its metabolites. Here we demonstrate that LXRs also regulate plasma lipoprotein metabolism through control of the phospholipid transfer protein (PLTP) gene. LXR ligands induce the expression of PLTP in cultured HepG2 cells and mouse liver in vivo in a coordinate manner with known LXR target genes. Moreover, plasma phospholipid transfer activity is increased in mice treated with the synthetic LXR ligand GW3965. Unexpectedly, PLTP expression was also highly inducible by LXR in macrophages, a cell type not previously recognized to express this enzyme. The ability of synthetic and oxysterol ligands to regulate PLTP mRNA in macrophages and liver is lost in animals lacking both LXRalpha and LXRbeta, 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.
Molecular and Cellular Biology 04/2003; 23(6):2182-91. · 5.53 Impact Factor
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ABSTRACT: Matrix metalloproteinases (MMPs) are zinc endopeptidases that degrade extracellular matrix (ECM) components during normal and pathogenic tissue remodeling. Inappropriate expression of these enzymes contributes to the development of vascular pathology, including atherosclerosis. MMP-9 is expressed in its active form in atherosclerotic lesions and is believed to play an important role in vascular remodeling, smooth muscle cell migration, and plaque instability. We demonstrate here that the liver X receptors (LXRs) LXRalpha and LXRbeta inhibit basal and cytokine-inducible expression of MMP-9. Treatment of murine peritoneal macrophages with the synthetic LXR agonists GW3965 or T1317 reduces MMP-9 mRNA expression and blunts its induction by pro-inflammatory stimuli including lipopolysaccharide, interleukin-1beta, and tumor necrosis factor alpha. In contrast, macrophage expression of MMP-12 and MMP-13 is not altered by LXR ligands. We further show that the ability of LXR ligands to regulate MMP-9 expression is strictly receptor-dependent and is not observed in macrophages obtained from LXRalphabeta null mice. Analysis of the 5'-flanking region of the MMP-9 gene indicates that LXR/RXR heterodimers do not bind directly to the MMP-9 promoter. Rather, activation of LXRs represses MMP-9 expression, at least in part through antagonism of the NFkappaB signaling pathway. These observations identify the regulation of macrophage MMP-9 expression as a mechanism whereby activation of LXRs may impact macrophage inflammatory responses.
Journal of Biological Chemistry 04/2003; 278(12):10443-9. · 4.77 Impact Factor
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ABSTRACT: Macrophages have important roles in both lipid metabolism and inflammation and are central to the pathogenesis of atherosclerosis. The liver X receptors (LXRs) are established mediators of lipid-inducible gene expression, but their role in inflammation and immunity is unknown. We demonstrate here that LXRs and their ligands are negative regulators of macrophage inflammatory gene expression. Transcriptional profiling of lipopolysaccharide (LPS)-induced macrophages reveals reciprocal LXR-dependent regulation of genes involved in lipid metabolism and the innate immune response. In vitro, LXR ligands inhibit the expression of inflammatory mediators such as inducible nitric oxide synthase, cyclooxygenase (COX)-2 and interleukin-6 (IL-6) in response to bacterial infection or LPS stimulation. In vivo, LXR agonists reduce inflammation in a model of contact dermatitis and inhibit inflammatory gene expression in the aortas of atherosclerotic mice. These findings identify LXRs as lipid-dependent regulators of inflammatory gene expression that may serve to link lipid metabolism and immune functions in macrophages.
Nature Medicine 03/2003; 9(2):213-9. · 22.46 Impact Factor
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Jun Xu,
Gary Xiao,
Chuck Trujillo,
Vicky Chang,
Lilia Blanco, Sean B. Joseph,
Sara Bassilian,
Mohammed F. Saad,
Peter Tontonoz,
W. N. Paul Lee,
Irwin J. Kurland
[show abstract]
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ABSTRACT: The hypoglycemia seen in the fasting PPARα null mouse is thought to be due to impaired liver fatty acid β-oxidation. The etiology
of hypoglycemia in the PPARα null mouse was determined via stable isotope studies. Glucose, lactate, and glycerol flux was
assessed in the fasted and fed states in 4-month-old PPARα null mice and in C57BL/6 WT maintained on standard chow using a
new protocol for flux assessment in the fasted and fed states. Hepatic glucose production (HGP) and glucose carbon recycling
were estimated using [U-13C6]glucose, and HGP, lactate, and glycerol turnover was estimated utilizing either [U-13C3]lactate or [2-13C]glycerol infused subcutaneously via Alza miniosmotic pumps. At the end of a 17-h fast, HGP was higher in the PPARα null
mice than in WT by 37% (p < 0.01). However, recycling of glucose carbon from lactate back to glucose was lower in the PPARα null than in WT (39% versus 51%,p < 0.02). The lack of conversion of lactate to glucose was confirmed using an [U-13C3]lactate infusion. In the fasted state, HGP from lactate and lactate production were decreased by 65 and 55%, respectively
(p < 0.05) in PPARα null mice. In contrast, when [2-13C]glycerol was infused, glycerol production and HGP from glycerol increased by 80 and 250%, respectively (p < 0.01), in the fasted state of PPARα null mice. The increased HGP from glycerol was not suppressed in the fed state. While
little change was evident for phosphoenolpyruvate carboxykinase (PEPCK) expression, pyruvate kinase expression was decreased
16-fold in fasted PPARα null mice as compared with the wild-type control. The fasted and fed insulin levels were comparable,
but blood glucose levels were lower in the PPARα null mice than in controls. In conclusion, PPARα receptor function creates
a setpoint for a metabolic network that regulates the rate and route of HGP in the fasted and fed states, in part, by controlling
the flux of glycerol and lactate between the triose-phosphate and pyruvate/lactate pools.
Journal of Biological Chemistry 12/2002; 277(52):50237-50244. · 4.77 Impact Factor
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Jun Xu,
Gary Xiao,
Chuck Trujillo,
Vicky Chang,
Lilia Blanco, Sean B Joseph,
Sara Bassilian,
Mohammed F Saad,
Peter Tontonoz,
W N Paul Lee,
Irwin J Kurland
[show abstract]
[hide abstract]
ABSTRACT: The hypoglycemia seen in the fasting PPARalpha null mouse is thought to be due to impaired liver fatty acid beta-oxidation. The etiology of hypoglycemia in the PPARalpha null mouse was determined via stable isotope studies. Glucose, lactate, and glycerol flux was assessed in the fasted and fed states in 4-month-old PPARalpha null mice and in C57BL/6 WT maintained on standard chow using a new protocol for flux assessment in the fasted and fed states. Hepatic glucose production (HGP) and glucose carbon recycling were estimated using [U-(13)C(6)]glucose, and HGP, lactate, and glycerol turnover was estimated utilizing either [U-(13)C(3)]lactate or [2-(13)C]glycerol infused subcutaneously via Alza miniosmotic pumps. At the end of a 17-h fast, HGP was higher in the PPARalpha null mice than in WT by 37% (p < 0.01). However, recycling of glucose carbon from lactate back to glucose was lower in the PPARalpha null than in WT (39% versus 51%, p < 0.02). The lack of conversion of lactate to glucose was confirmed using an [U-(13)C(3)]lactate infusion. In the fasted state, HGP from lactate and lactate production were decreased by 65 and 55%, respectively (p < 0.05) in PPARalpha null mice. In contrast, when [2-(13)C]glycerol was infused, glycerol production and HGP from glycerol increased by 80 and 250%, respectively (p < 0.01), in the fasted state of PPARalpha null mice. The increased HGP from glycerol was not suppressed in the fed state. While little change was evident for phosphoenolpyruvate carboxykinase (PEPCK) expression, pyruvate kinase expression was decreased 16-fold in fasted PPARalpha null mice as compared with the wild-type control. The fasted and fed insulin levels were comparable, but blood glucose levels were lower in the PPARalpha null mice than in controls. In conclusion, PPARalpha receptor function creates a setpoint for a metabolic network that regulates the rate and route of HGP in the fasted and fed states, in part, by controlling the flux of glycerol and lactate between the triose-phosphate and pyruvate/lactate pools.
Journal of Biological Chemistry 12/2002; 277(52):50237-44. · 4.77 Impact Factor
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Rajendra K Tangirala,
Eric D Bischoff, Sean B Joseph,
Brandee L Wagner,
Robert Walczak,
Bryan A Laffitte,
Chris L Daige,
Diane Thomas,
Richard A Heyman,
David J Mangelsdorf,
Xuping Wang,
Aldons J Lusis,
Peter Tontonoz,
Ira G Schulman
[show abstract]
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ABSTRACT: Recent studies have identified the liver X receptors (LXR alpha and LXR beta) as important regulators of cholesterol metabolism and transport. LXRs control transcription of genes critical to a range of biological functions including regulation of high density lipoprotein cholesterol metabolism, hepatic cholesterol catabolism, and intestinal sterol absorption. Although LXR activity has been proposed to be critical for physiologic lipid metabolism and transport, direct evidence linking LXR signaling pathways to the pathogenesis of cardiovascular disease has yet to be established. In this study bone marrow transplantations were used to selectively eliminate macrophage LXR expression in the context of murine models of atherosclerosis. Our results demonstrate that LXRs are endogenous inhibitors of atherogenesis. Additionally, elimination of LXR activity in bone marrow-derived cells mimics many aspects of Tangier disease, a human high density lipoprotein deficiency, including aberrant regulation of cholesterol transporter expression, lipid accumulation in macrophages, splenomegaly, and increased atherosclerosis. These results identify LXRs as targets for intervention in cardiovascular disease.
Proceedings of the National Academy of Sciences 10/2002; 99(18):11896-901. · 9.68 Impact Factor
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ABSTRACT: Lipid-loaded macrophage "foam cells" accumulate in the subendothelial space during the development of fatty streaks and atherosclerotic lesions. To better understand the consequences of such lipid loading, murine peritoneal macrophages were isolated and incubated with ligands for two nuclear receptors, liver X receptor (LXR) and retinoic acid receptor (RXR). Analysis of the expressed mRNAs using microarray technology led to the identification of four highly induced genes that encode apolipoproteins E, C-I, C-IV, and C-II. Northern blot analysis confirmed that the mRNA levels of these four genes were induced 2-14-fold in response to natural or synthetic ligands for LXR and/or RXR. The induction of all four mRNAs was greatly attenuated in peritoneal macrophages derived from LXRalpha/beta null mice. The two LXR response elements located within the multienhancers ME.1 and ME.2 were shown to be essential for the induction of apoC-II promoter-reporter genes by ligands for LXR and/or RXR. Finally, immunohistochemical studies demonstrate that apoC-II protein co-localizes with macrophages within murine arterial lesions. Taken together, these studies demonstrate that activated LXR induces the expression of the apoE/C-I/C-IV/C-II gene cluster in both human and murine macrophages. These results suggest an alternative mechanism by which lipids are removed from macrophage foam cells.
Journal of Biological Chemistry 09/2002; 277(35):31900-8. · 4.77 Impact Factor
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[show abstract]
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ABSTRACT: Lipid-loaded macrophage “foam cells” accumulate in the subendothelial space during the development of fatty streaks and atherosclerotic
lesions. To better understand the consequences of such lipid loading, murine peritoneal macrophages were isolated and incubated
with ligands for two nuclear receptors, liver X receptor (LXR) and retinoic acid receptor (RXR). Analysis of the expressed
mRNAs using microarray technology led to the identification of four highly induced genes that encode apolipoproteins E, C-I,
C-IV, and C-II. Northern blot analysis confirmed that the mRNA levels of these four genes were induced 2–14-fold in response
to natural or synthetic ligands for LXR and/or RXR. The induction of all four mRNAs was greatly attenuated in peritoneal macrophages
derived from LXRα/β null mice. The two LXR response elements located within the multienhancers ME.1 and ME.2 were shown to
be essential for the induction of apoC-II promoter-reporter genes by ligands for LXR and/or RXR. Finally, immunohistochemical
studies demonstrate that apoC-II protein co-localizes with macrophages within murine arterial lesions. Taken together, these
studies demonstrate that activated LXR induces the expression of the apoE/C-I/C-IV/C-II gene cluster in both human and murine
macrophages. These results suggest an alternative mechanism by which lipids are removed from macrophage foam cells.
Journal of Biological Chemistry 08/2002; 277(35):31900-31908. · 4.77 Impact Factor
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Sean B Joseph,
Elaine McKilligin,
Liming Pei,
Michael A Watson,
Alan R Collins,
Bryan A Laffitte,
Mingyi Chen,
Grace Noh,
Joanne Goodman,
Graham N Hagger,
Jonathan Tran,
Tim K Tippin,
Xuping Wang,
Aldons J Lusis,
Willa A Hsueh,
Ronald E Law,
Jon L Collins,
Timothy M Willson,
Peter Tontonoz
[show abstract]
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ABSTRACT: The nuclear receptors LXRalpha and LXRbeta have been implicated in the control of cholesterol and fatty acid metabolism in multiple cell types. Activation of these receptors stimulates cholesterol efflux in macrophages, promotes bile acid synthesis in liver, and inhibits intestinal cholesterol absorption, actions that would collectively be expected to reduce atherosclerotic risk. However, synthetic LXR ligands have also been shown to induce lipogenesis and hypertriglyceridemia in mice, raising questions as to the net effects of these compounds on the development of cardiovascular disease. We demonstrate here that the nonsteroidal LXR agonist GW3965 has potent antiatherogenic activity in two different murine models. In LDLR(-/-) mice, GW3965 reduced lesion area by 53% in males and 34% in females. A similar reduction of 47% was observed in male apoE(-/-) mice. Long-term (12-week) treatment with LXR agonist had differential effects on plasma lipid profiles in LDLR(-/-) and apoE(-/-) mice. GW3965 induced expression of ATP-binding cassettes A1 and G1 in modified low-density lipoprotein-loaded macrophages in vitro as well as in the aortas of hyperlipidemic mice, suggesting that direct actions of LXR ligands on vascular gene expression are likely to contribute to their antiatherogenic effects. These observations provide direct evidence for an atheroprotective effect of LXR agonists and support their further evaluation as potential modulators of human cardiovascular disease.
Proceedings of the National Academy of Sciences 06/2002; 99(11):7604-9. · 9.68 Impact Factor
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ABSTRACT: The nuclear receptors LXRalpha and LXRbeta have been implicated in the control of lipogenesis and cholesterol homeostasis. Ligand activation of these receptors in vivo induces expression of the LXR target gene SREBP-1c and increases plasma triglyceride levels. Expression of fatty acid synthase (FAS), a central enzyme in de novo lipogenesis and an established target of the SREBP-1 pathway, is also induced by LXR ligands. The effects of LXR ligands on FAS expression have been proposed to be entirely secondary to the induction of SREBP-1c. We demonstrate here that LXRs regulate FAS expression through direct interaction with the FAS promoter as well as through activation of SREBP-1c expression. Induction of FAS expression in HepG2 cells by LXR ligands is reduced, but not abolished, under conditions where SREBP processing is suppressed. Moreover, LXR ligands induce FAS expression in CHO-7 cells without altering expression of SREBP-1. We demonstrate that in addition to tandem SREBP sites, the FAS promoter contains a high affinity binding site for the LXR/RXR heterodimer that is conserved in diverse animal species including birds, rodents, and humans. The LXR and SREBP binding sites independently confer LXR responsiveness on the FAS promoter, and maximal induction requires both transcription factors. Transient elevation of plasma triglyceride levels in mice treated with a synthetic LXR agonist correlates with transient induction of hepatic FAS expression. These results indicate that the LXR signaling pathway modulates FAS expression through distinct but complementary mechanisms and suggest that the FAS gene may be a critical target in the control of lipogenesis by LXRs.
Journal of Biological Chemistry 04/2002; 277(13):11019-25. · 4.77 Impact Factor
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[show abstract]
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ABSTRACT: Apolipoprotein E (apoE) secreted by macrophages in the artery
wall exerts an important protective effect against the development of
atherosclerosis, presumably through its ability to promote lipid
efflux. Previous studies have shown that increases in cellular free
cholesterol levels stimulate apoE transcription in macrophages and
adipocytes; however, the molecular basis for this regulation is
unknown. Recently, Taylor and colleagues [Shih, S. J., Allan, C.,
Grehan, S., Tse, E., Moran, C. & Taylor, J. M. (2000)
J. Biol. Chem. 275, 31567–31572] identified two
enhancers from the human apoE gene, termed multienhancer 1 (ME.1) and
multienhancer 2 (ME.2), that direct macrophage- and adipose-specific
expression in transgenic mice. We demonstrate here that the nuclear
receptors LXRα and LXRβ and their oxysterol ligands are key
regulators of apoE expression in both macrophages and adipose tissue.
We show that LXR/RXR heterodimers regulate apoE transcription
directly, through interaction with a conserved LXR response element
present in both ME.1 and ME.2. Moreover, we demonstrate that the
ability of oxysterols and synthetic ligands to regulate apoE expression
in adipose tissue and peritoneal macrophages is reduced in
Lxrα−/− or Lxrβ−/− mice and
abolished in double knockouts. Basal expression of apoE is not
compromised in Lxr null mice, however, indicating that
LXRs mediate lipid-inducible rather than tissue-specific expression of
this gene. Together with our previous work, these findings support a
central role for LXR signaling pathways in the control of macrophage
cholesterol efflux through the coordinate regulation of apoE, ABCA1,
and ABCG1 expression.
Proceedings of the National Academy of Sciences 01/2001; 98(2):507-512. · 9.68 Impact Factor
