Uncoupling Nuclear Receptor LXR and Cholesterol Metabolism in Cancer

Liver X receptors (LXRs) are members of the nuclear receptor superfamily of DNA-binding transcription factors and act as sensors of cholesterol homeostasis. Under normal conditions, when intracellular cholesterol concentration increases, cells synthesize oxysterols and activate the LXR transcriptional network to drive cholesterol efflux and reduce cholesterol influx and synthesis. During normal and cancer cell proliferation, there is a net uncoupling between intracellular cholesterol increase and LXR activation resulting from the reduced intracellular oxysterol concentration. This review dissects the novel mechanisms of a previously unrecognized metabolic uncoupling, supporting the activation of the LXR axis as a bona fide therapeutic approach in cancer. Liver X receptors (LXRs) are members of the nuclear receptor superfamily of DNA-binding transcription factors and act as sensors of cholesterol homeostasis. Under normal conditions, when intracellular cholesterol concentration increases, cells synthesize oxysterols and activate the LXR transcriptional network to drive cholesterol efflux and reduce cholesterol influx and synthesis. During normal and cancer cell proliferation, there is a net uncoupling between intracellular cholesterol increase and LXR activation resulting from the reduced intracellular oxysterol concentration. This review dissects the novel mechanisms of a previously unrecognized metabolic uncoupling, supporting the activation of the LXR axis as a bona fide therapeutic approach in cancer. Cholesterol is an essential component of mammalian cell membranes as well as a precursor of bile acids and steroid hormones (Simons and Ikonen, 2000Simons K. Ikonen E. How cells handle cholesterol.Science. 2000; 290: 1721-1726Crossref PubMed Scopus (1066) Google Scholar). Because of its importance, cells have evolved complex mechanisms to closely regulate the abundance and distribution of sterols. The circulating levels of cholesterol are regulated by a balance among intracellular synthesis, dietary cholesterol absorption, and removal of the excess cholesterol from peripheral tissues. A large body of clinical and experimental evidence suggests the hypothesis that these finely tuned mechanisms become altered during cell division and membrane synthesis both in physiological hyperproliferative conditions and in carcinogenesis (Clendening et al., 2010Clendening J.W. Pandyra A. Boutros P.C. El Ghamrasni S. Khosravi F. Trentin G.A. Martirosyan A. Hakem A. Hakem R. Jurisica I. Penn L.Z. Dysregulation of the mevalonate pathway promotes transformation.Proc. Natl. Acad. Sci. USA. 2010; 107: 15051-15056Crossref PubMed Scopus (276) Google Scholar, Dang, 2012Dang C.V. Links between metabolism and cancer.Genes Dev. 2012; 26: 877-890Crossref PubMed Scopus (753) Google Scholar). Epidemiological studies have shown that patients with cancer have high plasma levels of lipoprotein carriers of cholesterol and that cancer cells exhibit deregulated transcriptional levels of several genes implicated in cholesterol regulation and metabolism, such as low-density lipoprotein receptor (Ldlr), hydroxyl-methyl glutaryl-coenzyme A reductase (Hmgcr), and sterol regulatory element-binding proteins (Srebps) (Llaverias et al., 2011Llaverias G. Danilo C. Mercier I. Daumer K. Capozza F. Williams T.M. Sotgia F. Lisanti M.P. Frank P.G. Role of cholesterol in the development and progression of breast cancer.Am. J. Pathol. 2011; 178: 402-412Abstract Full Text Full Text PDF PubMed Scopus (212) Google Scholar, Scheinman et al., 2013Scheinman E.J. Rostoker R. Leroith D. Cholesterol affects gene expression of the Jun family in colon carcinoma cells using different signaling pathways.Mol. Cell. Endocrinol. 2013; 374: 101-107Crossref PubMed Scopus (14) Google Scholar). Cholesterol is synthesized via the mevalonate pathway, an enzymatic cascade in which the rate-limiting reduction of hydroxyl-methyl glutaryl-coenzyme A (HMG-CoA) to mevalonate is catalyzed by HMGCR. The expression of all of the enzymes acting in the cholesterol biosynthetic pathway is regulated by SREBP transcription factor family (Sato, 2010Sato R. Sterol metabolism and SREBP activation.Arch. Biochem. Biophys. 2010; 501: 177-181Crossref PubMed Scopus (226) Google Scholar). In sterol-rich conditions, SREBP resides in the endoplasmic reticulum as a high-molecular-weight precursor in a repressor complex represented by the SREBP-cleavage activation protein (SCAP), which acts as the cholesterol sensor changing conformation in a cholesterol-dependent fashion, and the anchor insulin-induced gene (INSIG). Moreover, the cholesterol is esterified by acyl-CoA cholesterol acyl transferase (ACAT) and stored in the cytosol in the form of lipid droplets (Figure 1). In sterol-poor conditions, SCAP escorts SREBP into the Golgi complex, where it is cleaved by a series of proteolytic steps (Goldstein et al., 2006Goldstein J.L. DeBose-Boyd R.A. Brown M.S. Protein sensors for membrane sterols.Cell. 2006; 124: 35-46Abstract Full Text Full Text PDF PubMed Scopus (1230) Google Scholar). The released mature transcription factor migrates into the nucleus, where it recognizes its DNA binding sequence and activates gene target transcription (Figure 1). Studies performed on different types of tumors have revealed that cancer cells present deficient feedback control of HMGCR or increased HMGCR expression compared to untransformed cells, suggesting that the dysregulation of the mevalonate pathway may have the oncogenic power to drive malignant transformation (Clendening et al., 2010Clendening J.W. Pandyra A. Boutros P.C. El Ghamrasni S. Khosravi F. Trentin G.A. Martirosyan A. Hakem A. Hakem R. Jurisica I. Penn L.Z. Dysregulation of the mevalonate pathway promotes transformation.Proc. Natl. Acad. Sci. USA. 2010; 107: 15051-15056Crossref PubMed Scopus (276) Google Scholar) and sustain tumor growth (Sorrentino et al., 2014Sorrentino G. Ruggeri N. Specchia V. Cordenonsi M. Mano M. Dupont S. Manfrin A. Ingallina E. Sommaggio R. Piazza S. et al.Metabolic control of YAP and TAZ by the mevalonate pathway.Nat. Cell Biol. 2014; 16: 357-366Crossref PubMed Scopus (528) Google Scholar). Moreover, mevalonate is a precursor of several major products regulating the cell cycle, including dolichol, geranylpyro-phosphate (GPP), and farnesyl-pyrophospate (FPP) (Goldstein and Brown, 1990Goldstein J.L. Brown M.S. Regulation of the mevalonate pathway.Nature. 1990; 343: 425-430Crossref PubMed Scopus (4537) Google Scholar). Dolichol has a stimulatory effect on DNA synthesis and is linked to several tumor cell proteins (Wejde et al., 1998Wejde J. Hjertman M. Carlberg M. Egestad B. Griffiths W.J. Sjövall J. Larsson O. Dolichol-like lipids with stimulatory effect on DNA synthesis: substrates for protein dolichylation?.J. Cell. Biochem. 1998; 71: 502-514Crossref PubMed Scopus (19) Google Scholar). GPP and FPP cause isoprenylation of the intracellular G proteins Ras and Rho, which in turn regulate the signal transduction of several membrane receptors crucial for the transcription of genes involved in cell proliferation, differentiation, and apoptosis (Goldstein and Brown, 1990Goldstein J.L. Brown M.S. Regulation of the mevalonate pathway.Nature. 1990; 343: 425-430Crossref PubMed Scopus (4537) Google Scholar). To sustain the whole-body cholesterol homeostatic state and reduce the high ATP consumption of cholesterol de novo biosynthesis, lipoproteins mediate the processing and delivery of dietary cholesterol to peripheral tissues via circulation. Low-density lipoprotein (LDL) particles deliver cholesterol to most peripheral tissues through a receptor-mediated mechanism (Brown and Goldstein, 1986Brown M.S. Goldstein J.L. A receptor-mediated pathway for cholesterol homeostasis.Science. 1986; 232: 34-47Crossref PubMed Scopus (4350) Google Scholar). LDLRs are present on the plasma membrane of most cells and mediate the selective capture of LDL macromolecules, which are completely and rapidly degraded in the lysosome. The LDL-derived cholesterol acts in the cell at several levels, including suppression of transcription of the Hmgcr gene through the SREBP pathway and activation of ACAT to storage esterified cholesterol (Goldstein and Brown, 2009Goldstein J.L. Brown M.S. The LDL receptor.Arterioscler. Thromb. Vasc. Biol. 2009; 29: 431-438Crossref PubMed Scopus (871) Google Scholar, Brown and Goldstein, 1986Brown M.S. Goldstein J.L. A receptor-mediated pathway for cholesterol homeostasis.Science. 1986; 232: 34-47Crossref PubMed Scopus (4350) Google Scholar). By inhibiting the SREBP pathway, LDL also suppresses transcription of the Ldlr gene (Brown and Goldstein, 1999Brown M.S. Goldstein J.L. A proteolytic pathway that controls the cholesterol content of membranes, cells, and blood.Proc. Natl. Acad. Sci. USA. 1999; 96: 11041-11048Crossref PubMed Scopus (1103) Google Scholar). Lipid profiles of cancer patients display reduction of plasma lipoprotein levels and their restoral to normal values upon successful remission, suggesting the importance of lipoprotein carriers in tumor growth and development, but this feature needs to be more investigated with the aim of considering lipoprotein levels a biomarker in cancer (Solomon and Freeman, 2011Solomon K.R. Freeman M.R. The complex interplay between cholesterol and prostate malignancy.Urol. Clin. North Am. 2011; 38: 243-259Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar). In addition, the intestinal epithelium is equipped with take-up dietary and biliary cholesterol mechanisms from the gut lumen into the enterocytes. Niemann Pick C1-like 1 (NPC1L1) is a protein localized at the brush border membrane of the enterocytes that mediates free cholesterol absorption into the enterocytes. In humans and nonhuman primates, it is also significantly expressed in the liver, where it transports newly secreted biliary cholesterol back into hepatocytes, preventing excessive loss of endogenous cholesterol. Ezetimibe, a drug that inhibits cholesterol absorption by blocking NPC1L1 gut transporters, was administered in a prostate cancer in vivo model showing antitumor effects (Solomon and Freeman, 2011Solomon K.R. Freeman M.R. The complex interplay between cholesterol and prostate malignancy.Urol. Clin. North Am. 2011; 38: 243-259Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar). Cholesterol elimination from the cells is a key process necessary to prevent cholesterol retention and atherosclerotic lesion formation. ATP-binding cassette (ABC) transporters are key mediators of reverse cholesterol transport (RCT), a process by which excess cholesterol from peripheral tissues is returned to the liver by high-density lipoprotein (HDL) for ultimate excretion in bile. The early steps of cholesterol efflux are controlled by ABCA1, which is ubiquitously expressed and promotes net cholesterol efflux to lipid-poor apoA-I leading to discoid HDL formation (Vedhachalam et al., 2007Vedhachalam C. Duong P.T. Nickel M. Nguyen D. Dhanasekaran P. Saito H. Rothblat G.H. Lund-Katz S. Phillips M.C. Mechanism of ATP-binding cassette transporter A1-mediated cellular lipid efflux to apolipoprotein A-I and formation of high density lipoprotein particles.J. Biol. Chem. 2007; 282: 25123-25130Crossref PubMed Scopus (285) Google Scholar). Moreover, ABCG1 cooperates with ABCA1 in macrophages by further adding cellular lipids to the nascent particle, which results in the maturation of HDL (Phillips, 2014Phillips M.C. Molecular mechanisms of cellular cholesterol efflux.J. Biol. Chem. 2014; 289: 24020-24029Crossref PubMed Scopus (397) Google Scholar). Additionally, the ABCG5/G8 transporters, localized on the basal membrane of hepatocytes and enterocytes, inhibit the absorption of cholesterol and plant sterols from the diet by mediating the efflux of these sterols from enterocytes back into the gut lumen and by promoting efficient secretion of cholesterol and plant sterols from hepatocytes into the bile (Wang et al., 2015Wang J. Mitsche M.A. Luetjohann D. Cohen J.C. Xie X.S. Hobbs H.H. Relative Roles of ABCG5/ABCG8 in Liver and Intestine.J. Lipid Res. 2015; 56: 319-330Crossref PubMed Scopus (109) Google Scholar). The cell membrane contains micro-domains known as lipid rafts, which are characterized by high cholesterol content and a strict compartmentalization from the rest of the membrane. These specific regions are associated with several proteins that regulate pro-oncogenic and apoptotic pathways and are involved in the initial stages of cancer development, tumor growth and the potential progression to a migratory and metastatic phenotype (Yang et al., 2014Yang Y.F. Jan Y.H. Liu Y.P. Yang C.J. Su C.Y. Chang Y.C. Lai T.C. Chiou J. Tsai H.Y. Lu J. et al.Squalene synthase induces tumor necrosis factor receptor 1 enrichment in lipid rafts to promote lung cancer metastasis.Am. J. Respir. Crit. Care Med. 2014; 190: 675-687Crossref PubMed Scopus (40) Google Scholar). Among key regulator proteins anchored to cholesterol rafts, death receptors are notably important because, once activated, they trigger apoptotic signal transduction. Avoiding apoptosis is one of the main strategies adopted by cancer cells to improve their rapid proliferative behavior, and they achieve this by modulating the cholesterol composition of lipid rafts, thus disrupting death receptor folding and function. Two classes of death receptors, the Fas receptor and the TNF-related apoptosis-inducing ligand (TRAIL), have been extensively investigated, given their central role in apoptosis activation. Cholesterol depletion studies show a significant inhibition of the cell death program associated with Fas and TRAIL (Li et al., 2006Li Y.C. Park M.J. Ye S.K. Kim C.W. Kim Y.N. Elevated levels of cholesterol-rich lipid rafts in cancer cells are correlated with apoptosis sensitivity induced by cholesterol-depleting agents.Am. J. Pathol. 2006; 168 (quiz 1404–1405): 1107-1118Abstract Full Text Full Text PDF PubMed Scopus (399) Google Scholar, Song et al., 2007Song J.H. Tse M.C. Bellail A. Phuphanich S. Khuri F. Kneteman N.M. Hao C. Lipid rafts and nonrafts mediate tumor necrosis factor related apoptosis-inducing ligand induced apoptotic and nonapoptotic signals in non small cell lung carcinoma cells.Cancer Res. 2007; 67: 6946-6955Crossref PubMed Scopus (125) Google Scholar). Several studies have focused on oncogenic signals transmitted by the Rac protein kinase (AKT) protein, a serine-threonine protein kinase that mediates cell survival and growth. Since AKT is closely associated with cholesterol rafts, many studies indicate that changes in membrane cholesterol content determine alterations in the proliferation rate of tumor cells through AKT signal modulation (Li et al., 2006Li Y.C. Park M.J. Ye S.K. Kim C.W. Kim Y.N. Elevated levels of cholesterol-rich lipid rafts in cancer cells are correlated with apoptosis sensitivity induced by cholesterol-depleting agents.Am. J. Pathol. 2006; 168 (quiz 1404–1405): 1107-1118Abstract Full Text Full Text PDF PubMed Scopus (399) Google Scholar). It was recently shown that in prostate tumor cells, reduction of membrane cholesterol levels causes the rearrangement of lipid rafts with strong effects on AKT. The cholesterol-driven disruption of lipid rafts is characterized by downregulation of the AKT signal and thus the renovation of the apoptotic impulse (Pommier et al., 2010Pommier A.J. Alves G. Viennois E. Bernard S. Communal Y. Sion B. Marceau G. Damon C. Mouzat K. Caira F. et al.Liver X Receptor activation downregulates AKT survival signaling in lipid rafts and induces apoptosis of prostate cancer cells.Oncogene. 2010; 29: 2712-2723Crossref PubMed Scopus (148) Google Scholar). Furthermore, the cell cycle seems to be sensitive to intracellular cholesterol. Inhibition of cholesterol synthesis mediated by statin treatment results in breast cancer cell line arrest in the G1 phase of the cell cycle, given the SREBP pathway activation and then the increase of cyclin-dependent kinase inhibitors p21 and p27 (Rao et al., 1998Rao S. Lowe M. Herliczek T.W. Keyomarsi K. Lovastatin mediated G1 arrest in normal and tumor breast cells is through inhibition of CDK2 activity and redistribution of p21 and p27, independent of p53.Oncogene. 1998; 17: 2393-2402Crossref PubMed Scopus (171) Google Scholar). Moreover, the antitumoral effects of cholesterol biosynthesis inhibition in breast carcinoma cells have been associated with the suppression of the mitogen-activated protein kinase (MEK/Erk) pathway and the dramatic decreases in nuclear factor kappa-B (NF-κB) and activator protein-1 (AP-1) DNA binding activities (Campbell et al., 2006Campbell M.J. Esserman L.J. Zhou Y. Shoemaker M. Lobo M. Borman E. Baehner F. Kumar A.S. Adduci K. Marx C. et al.Breast cancer growth prevention by statins.Cancer Res. 2006; 66: 8707-8714Crossref PubMed Scopus (291) Google Scholar). LXRs are members of the nuclear receptor (NR) superfamily of ligand-activated transcription factors and exist as two isoforms, LXRα and LXRβ. They act as whole-body cholesterol sensors, and their activation results in a net elimination of cholesterol from the body and amelioration of the plasma lipoprotein profile by mobilizing cholesterol from the periphery (Venkateswaran et al., 2000Venkateswaran A. Laffitte B.A. Joseph S.B. Mak P.A. Wilpitz D.C. Edwards P.A. Tontonoz P. Control of cellular cholesterol efflux by the nuclear oxysterol receptor LXR alpha.Proc. Natl. Acad. Sci. USA. 2000; 97: 12097-12102Crossref PubMed Scopus (838) Google Scholar, Repa et al., 2000bRepa J.J. Turley S.D. Lobaccaro J.A. Medina J. Li L. Lustig K. Shan B. Heyman R.A. Dietschy J.M. Mangelsdorf D.J. Regulation of absorption and ABC1-mediated efflux of cholesterol by RXR heterodimers.Science. 2000; 289: 1524-1529Crossref PubMed Scopus (1148) Google Scholar), promoting its hepatic excretion and limiting its absorption (Repa et al., 2002Repa J.J. Berge K.E. Pomajzl C. Richardson J.A. Hobbs H. Mangelsdorf D.J. Regulation of ATP-binding cassette sterol transporters ABCG5 and ABCG8 by the liver X receptors alpha and beta.J. Biol. Chem. 2002; 277: 18793-18800Crossref PubMed Scopus (682) Google Scholar, Yu et al., 2014Yu X.H. Qian K. Jiang N. Zheng X.L. Cayabyab F.S. Tang C.K. ABCG5/ABCG8 in cholesterol excretion and atherosclerosis.Clin. Chim. Acta. 2014; 428: 82-88Crossref PubMed Scopus (113) Google Scholar, Duval et al., 2006Duval C. Touche V. Tailleux A. Fruchart J.C. Fievet C. Clavey V. Staels B. Lestavel S. Niemann-Pick C1 like 1 gene expression is down-regulated by LXR activators in the intestine.Biochem. Biophys. Res. Commun. 2006; 340: 1259-1263Crossref PubMed Scopus (146) Google Scholar), reducing its cellular uptake (Zelcer et al., 2009Zelcer N. Hong C. Boyadjian R. Tontonoz P. LXR regulates cholesterol uptake through Idol-dependent ubiquitination of the LDL receptor.Science. 2009; 325: 100-104Crossref PubMed Scopus (553) Google Scholar), and enhancing its conversion to bile acids in mice (Peet et al., 1998Peet D.J. Turley S.D. Ma W. Janowski B.A. Lobaccaro J.M. Hammer R.E. Mangelsdorf D.J. Cholesterol and bile acid metabolism are impaired in mice lacking the nuclear oxysterol receptor LXR alpha.Cell. 1998; 93: 693-704Abstract Full Text Full Text PDF PubMed Scopus (1237) Google Scholar). The identification of 24S, 25, and 27 hydroxycholesterol as in vivo ligands of LXR endorsed these receptors as sterol sensors. Originally considered orphan receptors, LXRs were identified between 1994 and 1996 as RXR heterodimers permissive to activation by both LXR and RXR ligands. LXRα and LXRβ bind to the LXR responsive element (LXRE), a specific DNA sequence represented by two 5′-AGGTCA-3′ hexameric half-sites separated by a four-nucleotide spacer (DR4 motif) (Willy et al., 1995Willy P.J. Umesono K. Ong E.S. Evans R.M. Heyman R.A. Mangelsdorf D.J. LXR, a nuclear receptor that defines a distinct retinoid response pathway.Genes Dev. 1995; 9: 1033-1045Crossref PubMed Scopus (916) Google Scholar, Apfel et al., 1994Apfel R. Benbrook D. Lernhardt E. Ortiz M.A. Salbert G. Pfahl M. A novel orphan receptor specific for a subset of thyroid hormone-responsive elements and its interaction with the retinoid/thyroid hormone receptor subfamily.Mol. Cell. Biol. 1994; 14: 7025-7035Crossref PubMed Scopus (292) Google Scholar). Following ligand binding to LXR or RXR, corepressors are released and coactivators are recruited, resulting in gene transcription. Studies of sequence comparison revealed a 77% sequence homology in Lxrα and Lxrβ genes, while northern blot analysis showed a different tissue distribution, thus identifying LXRβ as the ubiquitous isoform (Apfel et al., 1994Apfel R. Benbrook D. Lernhardt E. Ortiz M.A. Salbert G. Pfahl M. A novel orphan receptor specific for a subset of thyroid hormone-responsive elements and its interaction with the retinoid/thyroid hormone receptor subfamily.Mol. Cell. Biol. 1994; 14: 7025-7035Crossref PubMed Scopus (292) Google Scholar) and LXRα as selectively expressed in metabolically active tissues such as the liver, adipose tissue, adrenal glands, intestine, and macrophages (Chen et al., 2007Chen W. Chen G. Head D.L. Mangelsdorf D.J. Russell D.W. Enzymatic reduction of oxysterols impairs LXR signaling in cultured cells and the livers of mice.Cell Metab. 2007; 5: 73-79Abstract Full Text Full Text PDF PubMed Scopus (246) Google Scholar). In 1996, Janowski and colleagues relocated LXRs as a specific class of nuclear receptor by discovering oxysterols, monooxygenated derivates of cholesterol, as LXR-specific ligands (Janowski et al., 1996Janowski B.A. Willy P.J. Devi T.R. Falck J.R. Mangelsdorf D.J. An oxysterol signalling pathway mediated by the nuclear receptor LXR alpha.Nature. 1996; 383: 728-731Crossref PubMed Scopus (1458) Google Scholar). Similarly to the assay used for ligand identification of other receptors, concentrated lipid extracts from different tissues were screened in a high-throughput co-transfection assay, revealing a cholesterol-derived compound able to transactivate LXRα (Janowski et al., 1996Janowski B.A. Willy P.J. Devi T.R. Falck J.R. Mangelsdorf D.J. An oxysterol signalling pathway mediated by the nuclear receptor LXR alpha.Nature. 1996; 383: 728-731Crossref PubMed Scopus (1458) Google Scholar). Structural-activity relationship studies (SAR) disclosed that the position of the hydroxyl group on the cholesterol backbone is determinant for LXR high-affinity binding and activation at concentrations occurring in vivo (Janowski et al., 1999Janowski B.A. Grogan M.J. Jones S.A. Wisely G.B. Kliewer S.A. Corey E.J. Mangelsdorf D.J. Structural requirements of ligands for the oxysterol liver X receptors LXRalpha and LXRbeta.Proc. Natl. Acad. Sci. USA. 1999; 96: 266-271Crossref PubMed Scopus (786) Google Scholar). In general, biological oxysterols are classified in two main categories: those oxygenated on the sterol ring on the 7 position (e.g., 7α/β-hydroperoxycholesterol [7OOHC], 7-ketocholesterol [7KC], and 7α/β-hydroxy-cholesterol [7HC]) with a non-enzymatic origin, and those oxygenated on the side chain (e.g., 24S-hydroxycholesterol [24HC], 25-hydroxycholesterol [25HC] and 27-hydroxycholesterol [27HC]), generally produced in enzymatic reactions. In physiological conditions, oxysterols act as regulators of cholesterol excess via both LXR activation and SREBP direct regulation in an LXR-independent way. In order to prevent SREBP maturation in Golgi apparatus and, consequently, suppress the expression of genes involved in sterol metabolism, oxysterols seem to bind to INSIG, retaining the SREBP-SCAP complex in the endoplasmic reticulum (Radhakrishnan et al., 2007Radhakrishnan A. Ikeda Y. Kwon H.J. Brown M.S. Goldstein J.L. Sterol-regulated transport of SREBPs from endoplasmic reticulum to Golgi: oxysterols block transport by binding to Insig.Proc. Natl. Acad. Sci. USA. 2007; 104: 6511-6518Crossref PubMed Scopus (432) Google Scholar, Janowski et al., 2001Janowski B.A. Shan B. Russell D.W. The hypocholesterolemic agent LY295427 reverses suppression of sterol regulatory element-binding protein processing mediated by oxysterols.J. Biol. Chem. 2001; 276: 45408-45416Crossref PubMed Scopus (50) Google Scholar). Moreover, intracellular oxysterol storage triggers HMGCR binding to INSIG, inducing the recruitment of a membrane-associated ubiquitin ligase called GP78 and thus the ubiquitination and degradation of reductase by cytosolic 26S proteasomes (DeBose-Boyd, 2008DeBose-Boyd R.A. Feedback regulation of cholesterol synthesis: sterol-accelerated ubiquitination and degradation of HMG CoA reductase.Cell Res. 2008; 18: 609-621Crossref PubMed Scopus (248) Google Scholar). Finally, oxysterols and in particular 25HC are thought to activate intracellular cholesterol transport to the endoplasmic reticulum and thus intracellular storage in the form of esterified cholesterol (Du et al., 2004Du X. Pham Y.H. Brown A.J. Effects of 25-hydroxycholesterol on cholesterol esterification and sterol regulatory element-binding protein processing are dissociable: implications for cholesterol movement to the regulatory pool in the endoplasmic reticulum.J. Biol. Chem. 2004; 279: 47010-47016Crossref PubMed Scopus (55) Google Scholar). Earlier evidence of the physiological role of LXR in lipid metabolism came from LXRα knock-out mice, which, after being fed a high-cholesterol diet, accumulated large amounts of cholesteryl ester in their livers (Peet et al., 1998Peet D.J. Turley S.D. Ma W. Janowski B.A. Lobaccaro J.M. Hammer R.E. Mangelsdorf D.J. Cholesterol and bile acid metabolism are impaired in mice lacking the nuclear oxysterol receptor LXR alpha.Cell. 1998; 93: 693-704Abstract Full Text Full Text PDF PubMed Scopus (1237) Google Scholar) and showed an altered plasma lipoprotein profile, characterized by increased LDL and reduced HDL cholesterol levels (Janowski et al., 1996Janowski B.A. Willy P.J. Devi T.R. Falck J.R. Mangelsdorf D.J. An oxysterol signalling pathway mediated by the nuclear receptor LXR alpha.Nature. 1996; 383: 728-731Crossref PubMed Scopus (1458) Google Scholar). Cytochrome P450 7A1 (CYP7A1), an enzyme that catalyzes the rate-limiting step in bile acid synthesis and promotes the excretion of fecal sterols (Peet et al., 1998Peet D.J. Turley S.D. Ma W. Janowski B.A. Lobaccaro J.M. Hammer R.E. Mangelsdorf D.J. Cholesterol and bile acid metabolism are impaired in mice lacking the nuclear oxysterol receptor LXR alpha.Cell. 1998; 93: 693-704Abstract Full Text Full Text PDF PubMed Scopus (1237) Google Scholar), was identified as the first LXR target gene in the mouse. Subsequent studies of LXR activation in vivo revealed the unique role of LXR in promoting RCT by induction of ABCA1 (Venkateswaran et al., 2000Venkateswaran A. Laffitte B.A. Joseph S.B. Mak P.A. Wilpitz D.C. Edwards P.A. Tontonoz P. Control of cellular cholesterol efflux by the nuclear oxysterol receptor LXR alpha.Proc. Natl. Acad. Sci. USA. 2000; 97: 12097-12102Crossref PubMed Scopus (838) Google Scholar, Repa et al., 2000bRepa J.J. Turley S.D. Lobaccaro J.A. Medina J. Li L. Lustig K. Shan B. Heyman R.A. Dietschy J.M. Mangelsdorf D.J. Regulation of absorption and ABC1-mediated efflux of cholesterol by RXR heterodimers.Science. 2000; 289: 1524-1529Crossref PubMed Scopus (1148) Google Scholar) and ABCG1 expression (Kennedy et al., 2005Kennedy M.A. Barrera G.C. Nakamura K. Baldán A. Tarr P. Fishbein M.C. Frank J. Francone O.L. Edwards P.A. ABCG1 has a critical role in mediating cholesterol efflux to HDL and preventing cellular lipid accumulation.Cell Metab. 2005; 1: 121-131Abstract Full Text Full Text PDF PubMed Scopus (680) Google Scholar). In addition, LXRs modulate intestinal cholesterol excretion via the induction of ABCG5 and ABCG8 transporters (Repa et al., 2002Repa J.J. Berge K.E. Pomajzl C. Richardson J.A. Hobbs H. Mangelsdorf D.J. Regulation of ATP-binding cassette sterol transporters ABCG5 and ABCG8 by the liver X receptors alpha and beta.J. Biol. Chem. 2002; 277: 18793-18800Crossref PubMed Scopus (682) Google Scholar) and regulate a cluster of lipoprotein genes as well as lipid-remodeling genes (Laffitte et al., 2003Laffitte B.A. Joseph S.B. Chen M. Castrillo A. Repa J. Wilpitz D. Mangelsdorf D. Tontonoz P. The phospholipid transfer protein gene is a liver X receptor target expressed by macrophages in atherosclerotic lesions.Mol. Cell. Biol. 2003; 23: 2182-2191Crossref PubMed Scopus (133) Google Scholar, Zhang et al., 2001Zhang Y. Repa J.J. Gauthier K. Mangelsdorf D.J. Regulation of lipoprotein lipase by the oxysterol receptors, LXRalpha and LXRbeta.J. Biol. Chem. 2001; 276: 43018-43024Crossref PubMed Scopus (244) Google Scholar), thus increasing fecal sterol excretion. Furthermore, the contribution of LXRs to the lowering of whole-body cholesterol levels is given by their ability to shift acetyl-CoA units from cholesterol de novo biosynthesis to fatty acid synthesis. The lipogenic power of LXRs is mediated by direct upregulation of SREBP-1c, the main regulator of hepatic lipogenesis (Repa et al., 2000aRepa J.J. Liang G. Ou J. Bashmakov Y. Lobaccaro J.M. Shimomura I. Shan B. Brown M.S. Goldstein J.L. Mangelsdorf D.J. Regulation of mouse sterol regulatory element-binding protein-1c gene (SREBP-1c) by oxysterol receptors, LXRalpha and LXRbeta.Genes Dev. 2000; 14: 2819-2830Crossref PubMed Scopus (1414) Google Scholar), as well as direct induction of other lipogenic genes such as fatty acid synthase (Fas) (Joseph et al., 2002Joseph S.B. Laffitte B.A. Patel P.H. Watson M.A. Matsukuma K.E. Walczak R. Collins J.L. Osborne T.F. Tontonoz P. Direct and indirect mechanisms for regulation of fatty acid synthase gene expression by liver X receptors.J. Biol. Chem. 2002; 277: 11019-11025Crossref PubMed Scopus (619) Google Scholar) and stearoyl-CoA desaturase 1 (Scd1) (Chu et al., 2006Chu K. Miyazaki M. Man W.C. Ntambi J.M. 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