Natural products as modulators of retinoic acid receptor-related orphan receptors (RORs).

Covering: 1994 to 2020 Retinoic acid receptor-related orphan receptors (RORs) belong to a subfamily of the nuclear receptor superfamily and possess prominent roles in circadian rhythm, metabolism, inflammation, and cancer. They have been subject of research for over two decades and represent attractive but challenging drug targets. Natural products were among the first identified ligands of RORs and continue to be of interest to this day. This review focuses on ligands and indirect modulators of RORs from natural sources and explores their roles in a therapeutic context.

Capsaicin attenuates LPS-induced inflammatory cytokine production by upregulation of LXRα.

Here, we investigated the role of LXRα in capsaicin mediated anti-inflammatory effects. Results revealed that capsaicin inhibits LPS-induced IL-1ß, IL-6 and TNF-α production in a time- and dose-dependent manner. Moreover, capsaicin increases LXRα expression through PPARγ pathway. Inhibition of LXRα activation by siRNA diminished the inhibitory action of capsaicin on LPS-induced IL-1ß, IL-6 and TNF-α production. Additionally, LXRα siRNA abrogated the inhibitory action of capsaicin on p65 NF-κB protein expression. Thus, we propose that the anti-inflammatory effects of capsaicin are LXRα dependent, and LXRα may potentially link the capsaicin mediated PPARγ activation and NF-κB inhibition in LPS-induced inflammatory response.

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.

Nuclear receptors and cholesterol metabolism in the intestine.

Nuclear receptors are involved in many important function and mediate signaling by factors including hormones, vitamins and a number of endogenous ligands and xenobiotics, several of which are involved in lipid metabolism. This review focuses on the liver X receptor (LXR), which is an important regulator of whole-body cholesterol, fatty acid, and glucose homeostasis that binds to LXR response elements as a heterodimer with retinoid X receptors, and the farnesoid X receptor (FXR), which is a bile acid receptor involved in feedback inhibition of bile acid synthesis, and thus cholesterol catabolism. These nuclear receptors regulate gene programs that control intestinal and hepatic lipid homeostasis through their effects on cholesterol transport and catabolism.

Combination of honokiol and magnolol inhibits hepatic steatosis through AMPK-SREBP-1 c pathway.

Honokiol and magnolol, as pharmacological biphenolic compounds of Magnolia officinalis, have been reported to have antioxidant and anti-inflammatory properties. Sterol regulatory element binding protein-1 c (SREBP-1 c) plays an important role in the development and processing of steatosis in the liver. In the present study, we investigated the effects of a combination of honokiol and magnolol on SREBP-1 c-dependent lipogenesis in hepatocytes as well as in mice with fatty liver due to consumption of high-fat diet (HFD). Liver X receptor α (LXRα) agonists induced activation of SREBP-1 c and expression of lipogenic genes, which were blocked by co-treatment of honokiol and magnolol (HM). Moreover, a combination of HM potently increased mRNA of fatty acid oxidation genes. HM induced AMP-activated protein kinase (AMPK), an inhibitory kinase of the LXRα-SREBP-1 c pathway. The role of AMPK activation induced by HM was confirmed using an inhibitor of AMPK, Compound C, which reversed the ability of HM to both inhibit SREBP-1 c induction as well as induce genes for fatty acid oxidation. In mice, HM administration for four weeks ameliorated HFD-induced hepatic steatosis and liver dysfunction, as indicated by plasma parameters and Oil Red O staining. Taken together, our results demonstrated that a combination of HM has beneficial effects on inhibition of fatty liver and SREBP-1 c-mediated hepatic lipogenesis, and these events may be mediated by AMPK activation.

Liver X receptors balance lipid stores in hepatic stellate cells through Rab18, a retinoid responsive lipid droplet protein.

UNLABELLED: Liver X receptors (LXRs) are determinants of hepatic stellate cell (HSC) activation and liver fibrosis. Freshly isolated HSCs from Lxrαß(-/-) mice have increased lipid droplet (LD) size, but the functional consequences of this are unknown. Our aim was to determine whether LXRs link cholesterol to retinoid storage in HSCs and how this impacts activation. Primary HSCs from Lxrαß(-/-) and wild-type mice were profiled by gene array during in vitro activation. Lipid content was quantified by high-performance liquid chromatography and mass spectroscopy. Primary HSCs were treated with nuclear receptor ligands, transfected with small interfering RNA and plasmid constructs, and analyzed by immunocytochemistry. Lxrαß(-/-) HSCs have increased cholesterol and retinyl esters. The retinoid increase drives intrinsic retinoic acid receptor signaling, and activation occurs more rapidly in Lxrαß(-/-) HSCs. We identify Rab18 as a novel retinoic acid-responsive, LD-associated protein that helps mediate stellate cell activation. Rab18 mRNA, protein, and membrane insertion increase during activation. Both Rab18 guanosine triphosphatase activity and isoprenylation are required for stellate cell LD loss and induction of activation markers. These phenomena are accelerated in Lxrαß(-/-) HSCs, where there is greater retinoic acid flux. Conversely, Rab18 knockdown retards LD loss in culture and blocks activation, just like the functional mutants. Rab18 is also induced with acute liver injury in vivo. CONCLUSION: Retinoid and cholesterol metabolism are linked in stellate cells by the LD-associated protein Rab18. Retinoid overload helps explain the profibrotic phenotype of Lxrαß(-/-) mice, and we establish a pivotal role for Rab18 GTPase activity and membrane insertion in wild-type stellate cell activation. Interference with Rab18 may have significant therapeutic benefit in ameliorating liver fibrosis.

Activation of liver X receptor enhances the proliferation and migration of endothelial progenitor cells and promotes vascular repair through PI3K/Akt/eNOS signaling pathway activation.

Vascular endothelial injury is a major cause of many cardiovascular diseases. The proliferation and migration of endothelial progenitor cells (EPCs) play a pivotal role in endothelial regeneration and repair after vascular injury. Recently, liver X receptor (LXR) activation has been suggested as a potential target for novel therapeutic interventions in the treatment of cardiovascular disease. However, the effects of LXR activation on endothelial regeneration and repair, as well as EPC function, have not been investigated. In the present study, we demonstrate that LXRs, including LXRα and LXRß, are expressed and functional in rat bone marrow-derived EPCs. Treatment with an LXR agonist, TO901317 (TO) or GW3965 (GW), significantly increased the proliferation and migration of EPCs, as well as Akt and eNOS phosphorylation in EPCs. Moreover, LXR agonist treatment enhanced the expression and secretion of vascular endothelial growth factor in EPCs. LXR agonists accelerated re-endothelialization in injured mouse carotid arteries in vivo. These data confirm that LXR activation may improve EPC function and endothelial regeneration and repair after vascular injury by activating the PI3K/Akt/eNOS pathway. We conclude that LXRs may be attractive targets for drug development in the treatment of cardiovascular diseases associated with vascular injury.

Liver X Receptor activation delays chondrocyte hypertrophy during endochondral bone growth.

OBJECTIVE: Activation of the Liver X Receptor (LXR) has recently been identified as a therapeutic strategy for osteoarthritis (OA). Human OA articular cartilage explants show decreased LXR expression, and LXRß-null mice display OA-like symptoms. LXR agonist administration to OA articular cartilage explants suppresses proteoglycan degradation and restores LXR-activated transcription. We aimed to investigate the effect of LXR activation on chondrocyte differentiation to elucidate the molecular mechanisms behind its protection against OA. METHOD: The specific LXR agonist, GW3965, was used to examine the effect of LXR activation on chondrocyte differentiation. Tibia organ cultures were used to examine the effect of LXR activation on bone growth and growth plate morphology, followed by immunohistochemical analysis. In ATDC5 and micromass cultures, chondrocyte differentiation was examined through cellular staining and proliferation assays. Various chondrogenic markers were analyzed by real-time reverse-transcription polymerase chain reaction (qRT-PCR) in micromass RNA. RESULTS: Chondrocyte hypertrophy was suppressed by GW3965 treatment, as shown by decreased hypertrophic zone length in the tibial growth plate, decreased alkaline phosphatase staining in ATDC5 and micromass cultures, and down regulation of Col10a1, Mmp13 and Runx2 expression. Increased proliferation in treated ATDC5 cells and up-regulation of Col2a1 expression in treated micromass cultures suggest hypertrophy is suppressed secondary to prolonged proliferation. Decreased p57 levels in treated growth plates suggest this to be due to cell-cycle exit delay. CONCLUSION: Our findings regarding LXR's role in cartilage development provide insight into how LXR activation prevents cartilage breakdown, further solidifying its potential as a therapeutic target of OA.

Activation of liver X receptor alleviates ocular inflammation in experimental autoimmune uveitis.

PURPOSE: To investigate whether a synthetic LXR agonist TO901317 (TO90) ameliorates ocular inflammation in a mouse model of experimental autoimmune uveitis (EAU) and to explore its underlying mechanism. METHODS: EAU was induced with subcutaneous injection of IRBP161-180 peptide (SGIPYIISYLHPGNTILHVD) in B10.RIII mice. TO90 (50 mg/kg/d) or vehicle was administrated orally for successive 16 days or 8 days as prevention or effector phase, respectively. The severity of EAU was evaluated with clinical and histological scores. The levels of LXRs, NF-κB subunit p65, and an LXR target gene ABCA1 in the retina were detected with real-time PCR and Western blotting. The expressions of proinflammatory genes, including TNF-α, IL-1ß, IL-6, MCP-1, IFN-γ, and IL-17, were detected by real-time PCR. IRBP-specific lymphocyte proliferation was detected by MTT. Intracellular IFN-γ and IL-17 in CD4(+) T cells were measured by flow cytometry. RESULTS: We found both LXRα and LXRß were expressed in mouse retina. After administering TO90 orally to B10.RIII mice, the expression of LXRα but not LXRß was upregulated in the naïve mice. Compared with naïve mice, LXRα expression was increased in vehicle and TO90-treated EAU mice, but the LXRß expression was unchanged. The protein level of ABCA1 was enhanced in TO90-treated naïve and EAU mice but was unchanged in vehicle-treated EAU mice, suggesting activation of LXRα by TO90 is ligand dependent. TO90-mediated activation of LXRα improved the clinical and morphological scores in EAU mice. Meanwhile, activation of LXRα decreased the expressions of proinflammatory cytokines, including TNF-α, IL-1ß, IL-6, MCP-1, IFN-γ, and IL-17 in the retina. TO90 treatment inhibited IRBP-specific immune responses. The proportions of Th1 and Th17 expressing IFN-γ and IL-17 were reduced in TO90-treated EAU mice in both prevention and effector phases. Furthermore, TO90 significantly downregulated the expressions of an NF-κB subunit p65 at the protein and mRNA levels. CONCLUSIONS: TO90 activates LXRα and potently attenuates ocular inflammation in EAU. Alleviation of ocular inflammation could partially result from inhibition of the NF-κB signaling pathway. TO90 reduces IFN-γ and IL-17 expression in both prevention and treatment scenarios. Our data suggest that the LXR agonist may become a novel class of therapeutic agent for autoimmune uveitis.

Natural modulators of liver X receptors.

Nuclear receptor transcription factors are ligand-activated proteins that control various biological events from cell growth and development to lipid metabolism, and energy and glucose homeostasis. Nuclear receptors are important drug targets for metabolic diseases. Liver X receptors (LXRs) are nuclear receptor transcription factors that play essential roles in regulation of cholesterol, triglyceride, fatty acid, and glucose homeostasis. LXR-deficient mice have shown the association of LXR-signaling pathway dysfunction with several human pathologies including atherosclerosis, hyperlipidemia, Alzheimer's disease and cancer. Thus, LXRs are promising pharmacological targets for these diseases. Synthetic LXR agonists may lower cholesterol, but increase triglyceride and induce fatty liver. The naturally occurring LXR ligands, with moderate activity, may serve as nutraceuticals for prevention or treatment of the disorders, while minimizing potential side effects. In this review, recent advances in natural LXR modulators are summarized including agonist, antagonist and the modulator of LXR pathway.

Development of new LXR modulators that regulate LXR target genes and reduce lipogenesis in human cell models.

Four new mimics of 22-S-hydroxycholesterol (22SHC) were synthesized and evaluated using molecular modeling and tested in human muscle cells (primary myotubes) and hepatocytes (HepG2 cells). The new compounds (9, 12, 15a and 15b) showed good interrelationship between docking scores, to both LXRα and LXRß, and in vitro results. The LXR agonist T0901317 increased the expressions of genes involved in lipogenesis (SCD1, FAS) and cholesterol efflux (ABCA1), but only 22SHC counteracted the up-regulation of SCD1 and FAS by T0901317. Compound 9 and 12 decreased the expression of SCD1, while 9 also decreased the expression of FAS. Compounds 15a showed a significant antagonistic effect on ABCA1 expression, but neither 15a nor 15b were able to counteract the effect of T0901317 on all genes examined. Lipogenesis was increased after T0901317 treatment and only 22SHC significantly counteracted this effect. Treatment with 22SHC and compound 12 reduced lipogenesis compared to control. An increased glucose uptake was observed for all compounds, except for 15b. In summary, the new synthetic 22SHC mimics showed antagonistic effects similar to that of 22SHC, but the new substances were less potent. The sulfonamide 12 showed similar effects to 22SHC and the best effect on gene expression of the new mimics, however, it was not able to reduce the effect of T0901317 as observed for 22SHC.

Emerging roles of orphan nuclear receptors in cancer.

A growing body of evidence suggests that a subset of orphan nuclear receptors are amplified and prognostic for some human cancers. However, the specific roles of these orphan nuclear receptors in tumor progression and their utility as drug targets are not fully understood. In this review, we summarize recent progress in elucidating the direct and indirect involvement of orphan nuclear receptors in cancer as well as their therapeutic potential in a variety of human cancers. Furthermore, we contrast the role of orphan nuclear receptors in cancer with the known roles of estrogen receptor and androgen receptor in hormone-dependent cancers.

7-Dehydrocholesterol metabolites produced by sterol 27-hydroxylase (CYP27A1) modulate liver X receptor activity.

7-Dehydrocholesterol (7-DHC) is a common precursor of vitamin D3 and cholesterol. Although various oxysterols, oxygenated cholesterol derivatives, have been implicated in cellular signaling pathways, 7-DHC metabolism and potential functions of its metabolites remain poorly understood. We examined 7-DHC metabolism by various P450 enzymes and detected three metabolites produced by sterol 27-hydroxylase (CYP27A1) using high-performance liquid chromatography. Two were further identified as 25-hydroxy-7-DHC and 26/27-hydroxy-7-DHC. These 7-DHC metabolites were detected in serum of a patient with Smith-Lemli-Opitz syndrome. Luciferase reporter assays showed that 25-hydroxy-7-DHC activates liver X receptor (LXR) α, LXRß and vitamin D receptor and that 26/27-hydroxy-7-DHC induces activation of LXRα and LXRß, although the activities of both compounds on LXRs were weak. In a mammalian two-hybrid assay, 25-hydroxy-7-DHC and 26/27-hydroxy-7-DHC induced interaction between LXRα and a coactivator fragment less efficiently than a natural LXR agonist, 22(R)-hydroxycholesterol. These 7-DHC metabolites did not oppose agonist-induced LXR activation and interacted directly to LXRα in a manner distinct from a potent agonist. These findings indicate that the 7-DHC metabolites are partial LXR activators. Interestingly, 25-hydroxy-7-DHC and 26/27-hydroxy-7-DHC suppressed mRNA expression of sterol regulatory element-binding protein 1c, an LXR target gene, in HepG2 cells and HaCaT cells, while they weakly increased mRNA levels of ATP-binding cassette transporter A1, another LXR target, in HaCaT cells. Thus, 7-DHC is catabolized by CYP27A1 to metabolites that act as selective LXR modulators.

Differential interactions of antiretroviral agents with LXR, ER and GR nuclear receptors: potential contributing factors to adverse events.

BACKGROUND AND PURPOSE: Antiretroviral (ARV) drugs activate pregnane X receptors and constitutive androstane receptors, increasing the risk of drug interactions due to altered drug metabolism and disposition. The closely related liver X receptors (LXRα/ß), oestrogen receptors (ERα/ß) and glucocorticoid receptor (GR) regulate many endogenous processes such as lipid/cholesterol homeostasis, cellular differentiation and inflammation. However, ARV drug activation of these nuclear receptors has not been thoroughly investigated. EXPERIMENTAL APPROACH: The ability of an ARV drug library to activate LXRα/ß, ERα/ß and GR was assessed using a combined in silico and in vitro approach encompassing computational docking and molecular descriptor filtering, cell-free time-resolved fluorescence resonance energy transfer co-activator assays to assess direct binding to ligand-binding domains (LBDs), cell-based reporter assays and target gene expression. KEY RESULTS: Direct LBD interactions with LXRα and/or LXRß were predicted in silico and confirmed in vitro for darunavir, efavirenz, flavopiridol, maraviroc and tipranavir. Likewise, efavirenz was also predicted and confirmed as a ligand of ERα-LBD. Interestingly, atazanavir and ritonavir also activated LXRα/ß in reporter assays, while tipranavir enhanced transcriptional activity of ERα. Effects on ER and LXR target gene expression were confirmed for efavirenz and tipranavir. CONCLUSIONS AND IMPLICATIONS: There was good agreement between in silico predictions and in vitro results. However, some nuclear receptor interactions identified in vitro were probably due to allosteric effects or nuclear receptor cross-talk, rather than direct LBD binding. This study indicates that some of the adverse effects associated with ARV use may be mediated through 'off-target' effects involving nuclear receptor activation.

Activation of liver X receptor suppresses the production of the IL-12 family of cytokines by blocking nuclear translocation of NF-κBp50.

There is now convincing evidence that liver X receptor (LXR) is an important modulator of the inflammatory response; however, its mechanism of action remains unclear. This study aimed to examine the effect of LXR on the IL-12 family of cytokines and examined the mechanism by which LXR exerted this effect. We first demonstrated that activation of murine-derived dendritic cells (DC) with a specific agonist to LXR enhanced expression of LXR following activation with LPS, suggesting a role in inflammation. Furthermore, we showed LXR expression to be increased in vivo in dextrane sulphate sodium-induced colitis. LXR activation also suppressed production of IL-12p40, IL-12p70, IL-27 and IL-23 in murine-derived DC following stimulation with LPS, and specifically targeted the p35, p40 and EBI3 subunits of the IL-12 cytokine family, which are under the control of the NF-κB subunit p50 (NF-κBp50). Finally, we demonstrated that LXR can associate with NF-κBp50 in DC and that LXR activation prevents translocation of the p50 subunit into the nucleus. In summary, our study indicates that LXR can specifically suppress the IL-12 family of cytokines though its association with NF-κBp50 and highlights its potential as a therapeutic target for chronic inflammatory diseases.

Evolving pharmacology of orphan GPCRs: IUPHAR Commentary.

The award of the 2012 Nobel Prize in Chemistry to Robert Lefkowitz and Brian Kobilka for their work on the structure and function of GPCRs, spanning a period of more than 20 years from the cloning of the human ß2 -adrenoceptor to determining the crystal structure of the same protein, has earned both researchers a much deserved place in the pantheon of major scientific discoveries. GPCRs comprise one of the largest families of proteins, controlling many major physiological processes and have been a major focus of the International Union of Basic and Clinical Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC-IUPHAR) since its inception in 1987. We report here recent efforts by the British Pharmacological Society and NC-IUPHAR to define the endogenous ligands of 'orphan' GPCRs and to place authoritative and accessible information about these crucial therapeutic targets online.

Liver X receptor modulators: a review of recently patented compounds (2009 - 2012).

INTRODUCTION: The development of small molecule agonists of the liver X receptors (LXRs) has been an area of interest for over a decade, given the critical role of those receptors in cholesterol metabolism, glucose homeostasis, inflammation, innate immunity and lipogenesis. Many potential indications have been characterized over time including atherosclerosis, diabetes, inflammation, Alzheimer's disease and cancer. However, concerns about the lipogenic effects of full LXRα/ß agonists have required extensive efforts aimed at identifying LXRß agonist with limited activity on the LXRα receptor to increase the safety margins. AREAS COVERED: This review includes a summary of the LXR agonists that have reached the clinic and summarizes the patent applications for LXR modulators from September 2009 to December 2012 with emphasis on chemical matters, biological data associated with selected analogs and therapeutic indications. EXPERT OPINION: As LXR agonists have the potential to be useful for many indications, the scientific community, despite setbacks due to on-target side effects, has maintained interest and devised strategies to overcome safety hurdles. While a clinical proof of concept still remains elusive, the recent advancement of compounds into the clinic highlights that acceptable safety margins in preclinical species have been achieved.

Liver x receptor regulates arachidonic acid distribution and eicosanoid release in human macrophages: a key role for lysophosphatidylcholine acyltransferase 3.

OBJECTIVE: Liver X receptors (LXRs) are oxysterol-activated nuclear receptors that are highly expressed in macrophages and regulate lipid homeostasis and inflammation. Among putative LXR target genes, lysophosphatidylcholine acyltransferase 3 (LPCAT3) involved in the Lands cycle controls the fatty acid composition at the sn-2 position of glycerophospholipids and, therefore, the availability of fatty acids, such as arachidonic acid (AA), used for eicosanoid synthesis. The aim of our study was to determine whether LXRs could regulate the Lands cycle in human macrophages, to assess the consequences in terms of lipid composition and inflammatory response, and to work out the relative contribution of LPCAT3 to the observed changes. APPROACH AND RESULTS: Transcriptomic analysis revealed that LPCAT3 was upregulated by LXR agonists in human macrophages. Accordingly, LXR stimulation significantly increased lysophospholipid acyltransferase activity catalyzed by LPCAT3. Lipidomic analysis demonstrated that LXR activation increased the AA content in the polar lipid fraction, specifically in phosphatidylcholines. The LXR-mediated effects on AA distribution were abolished by LPCAT3 silencing, and a redistribution of AA toward the neutral lipid fraction was observed in this context. Finally, we observed that preconditioning of human macrophages by LXR agonist treatment increased the release of arachidonate-derived eicosanoids, such as prostaglandin E2 and thromboxane after lipopolysaccharide stimulation, with a significant attenuation by LPCAT3 silencing. CONCLUSIONS: Altogether, our data demonstrate that the LXR-mediated induction of LPCAT3 primes human macrophages for subsequent eicosanoid secretion by increasing the pool of AA, which can be mobilized from phospholipids.

Thyroid hormone-responsive SPOT 14 homolog promotes hepatic lipogenesis, and its expression is regulated by liver X receptor α through a sterol regulatory element-binding protein 1c-dependent mechanism in mice.

UNLABELLED: The protein, thyroid hormone-responsive SPOT 14 homolog (Thrsp), has been reported to be a lipogenic gene in cultured hepatocytes, implicating an important role of Thrsp in the pathogenesis of nonalcoholic fatty liver disease (NAFLD). Thrsp expression is known to be regulated by a variety of transcription factors, including thyroid hormone receptor, pregnane X receptor, and constitutive androstane receptor. Emerging in vitro evidence also points to a critical role of liver X receptor (LXR) in regulating Thrsp transcription in hepatocytes. In the present study, we showed that Thrsp was up-regulated in livers of db/db mice and high-fat-diet-fed mice, two models of murine NAFLD. Hepatic overexpression of Thrsp increased triglyceride accumulation with enhanced lipogenesis in livers of C57Bl/6 mice, whereas hepatic Thrsp gene silencing attenuated the fatty liver phenotype in db/db mice. LXR activator TO901317 induced Thrsp expression in livers of wild-type (WT) and LXR-ß gene-deficient mice, but not in LXR-α or LXR-α/ß double-knockout mice. TO901317 treatment significantly enhanced hepatic sterol regulatory element-binding protein 1c (SREBP-1c) expression and activity in WT mice, but failed to induce Thrsp expression in SREBP-1c gene-deficient mice. Sequence analysis revealed four LXR response-element-like elements and one sterol regulatory element (SRE)-binding site within a -2,468 ∼+1-base-pair region of the Thrsp promoter. TO901317 treatment and LXR-α overexpression failed to induce, whereas overexpression of SREBP-1c significantly increased Thrsp promoter activity. Moreover, deletion of the SRE site completely abolished SREBP-1c-induced Thrsp transcription. CONCLUSION: Thrsp is a lipogenic gene in the liver that is induced by the LXR agonist through an LXR-α-mediated, SREBP-1c-dependent mechanism. Therefore, Thrsp may represent a potential therapeutic target for the treatment of NAFLD.

Increased effects of ginsenosides on the expression of cholesterol 7α-hydroxylase but not the bile salt export pump are involved in cholesterol metabolism.

An extract from red ginseng [steamed and dried roots of Panax ginseng C.A. Meyer (RGE)] has been shown to have various actions on physiological functions. The mechanisms by which RGE promotes cholesterol metabolism in the liver are unclear, but RGE decreases the plasma levels of cholesterol. We investigated whether RGE affected the mRNA expression of cholesterol metabolism-related proteins such as cytochrome P450 (CYP)7A1 and bile salt export pump (BSEP) in the liver in hypercholesterolemic rats and rat primary hepatocytes. In-vivo studies showed the upregulation of CYP7A1 mRNA in hypercholesterolemic rats treated with RGE. Treatment with RGE exhibited decreased ratios of low-density lipoprotein-cholesterol to high-density lipoprotein-cholesterol compared with hypercholesterolemia without RGE. In-vitro studies also showed the upregulation of CYP7A1 mRNA and protein levels by the addition of RGE to rat primary hepatocytes. The mRNA levels of BSEP exhibited few changes. The sustained levels of the liver X receptor (LXR) in vivo and the increased levels of LXR in vitro on RGE treatment could be involved in the upregulation of CYP7A1. To clarify the effects of 11 ginsenosides including RGE on the mRNA levels of CYP7A1 and BSEP, we performed in-vitro experiments using rat primary hepatocytes. The ginsenosides Ro, Rg3, Re, Rg1, and Rg2 exhibited increased mRNA levels of CYP7A1. These results suggest that several ginsenosides including RGE promoted cholesterol metabolism due to upregulation of CYP7A1.