Hydrogen/deuterium exchange reveals distinct agonist/partial agonist receptor dynamics within vitamin D receptor/retinoid X receptor heterodimer

Regulation of nuclear receptor (NR) activity is driven by alterations in the conformational dynamics of the receptor upon ligand binding. Previously, we demonstrated that hydrogen/deuterium exchange (HDX) can be applied to determine novel mechanism of action of PPARγ ligands and in predicting tissue specificity of selective estrogen receptor modulators. Here, we applied HDX to probe the conformational dynamics of the ligand binding domain (LBD) of the vitamin D receptor (VDR) upon binding its natural ligand 1α,25-dihydroxyvitamin D3 (1,25D3), and two analogs, alfacalcidol and ED-71. Comparison of HDX profiles from ligands in complex with the LBD with full-length receptor bound to its cognate receptor retinoid X receptor (RXR) revealed unique receptor dynamics that could not be inferred from static crystal structures. These results demonstrate that ligands modulate the dynamics of the heterodimer interface as well as provide insight into the role of AF-2 dynamics in the action of VDR partial agonists.

Liver X receptor-retinoid X receptor (LXR-RXR) heterodimer cistrome reveals coordination of LXR and AP1 signaling in keratinocytes

Liver X receptors (LXRs) play a critical role in regulating lipid synthesis and transport in numerous tissues. In the skin, activation of LXR induces keratinocyte differentiation and improves epidermal permeability barrier homeostasis. To elucidate the mechanism of LXR action in skin, we mapped its cistrome by identifying LXRβ-RXRα binding sites using ChIP-on-chip in normal human epidermal keratinocytes (NHEKs). The cistrome was integrated with transcription data to obtain a global view of LXR action in keratinocyte biology. Here, we identify 2035 LXRβ-RXRα binding sites containing 4794 LXR response elements in NHEKs and show the presence of consensus heterodimer active regions in genes involved in keratinocyte lipid transport/synthesis and terminal differentiation. Bioinformatics analysis of the cistrome revealed an enrichment of AP1 cis-regulatory motifs in the vicinity of the LXRβ-RXRα binding sites. Importantly, we have demonstrated a direct interaction between LXR and Jun/Fos, indicating that the cooperation between LXR and AP1 may orchestrate keratinocyte differentiation. Finally, we corroborated these results by genome-wide mapping of the c-Fos and c-Jun cistromes in NHEKs, demonstrating that 77% of all the LXRβ-RXRα binding regions show the presence of AP1 motifs at adjacent locations. Our findings provide new insight into the mechanism of LXR action in keratinocyte differentiation, lipid production and barrier formation, further strengthening the validation of LXR as a potential therapeutic target for skin disorders including skin aging, psoriasis, and atopic dermatitis.

Identification of SR8278, a synthetic antagonist of the nuclear heme receptor REV-ERB

REV-ERBα is a member of the nuclear receptor superfamily that functions as a receptor for the porphoryin heme. REV-ERBα suppresses transcription of its target genes in a heme-dependent manner. Recently, the first nonporphyrin synthetic ligand for REV-ERBα, GSK4112, was designed, and it mimics the action of heme acting as agonist. Here, we report the identification of the first REV-ERB antagonist, SR8278. SR8278 is structurally similar to the agonist but blocks the ability of the GSK4112 to enhance REV-ERBα-dependent repression in a cotransfection assay. Additionally, whereas GSK4112 suppresses the expression of REV-ERBα target genes involved in gluconeogenesis, SR8278 stimulates the expression of these genes. Thus, SR8278 represents a unique chemical tool for probing REV-ERB function and may serve as a point for initiation of further optimization to develop REV-ERB antagonists with the ability to explore circadian and metabolic functions.

Identification of SR1078, a synthetic agonist for the orphan nuclear receptors RORα and RORγ

The retinoic acid receptor-related receptors (RORs) are members of the nuclear receptor (NR) superfamily of transcription factors. Several NRs are still characterized as orphan receptors because ligands have not yet been identified for these proteins. Here, we describe the identification of a synthetic RORα/RORγ ligand, SR1078. SR1078 modulates the conformation of RORγ in a biochemical assay and activates RORα and RORγ driven transcription. Furthermore, SR1078 stimulates expression of endogenous ROR target genes in HepG2 cells that express both RORα and RORγ. Pharmacokinetic studies indicate that SR1078 displays reasonable exposure following injection into mice, and consistent with SR1078 functioning as a RORα/RORγ agonist, expression of two ROR target genes, glucose-6-phosphatase and fibroblast growth factor 21, were stimulated in the liver. Thus, we have identified the first synthetic RORα/γ agonist, and this compound can be utilized as a chemical tool to probe the function of these receptors both in vitro and in vivo.

Characterization of the core mammalian clock component, NPAS2, as a REV-ERBalpha/RORalpha target gene

The mammalian clock is regulated at the cellular level by a transcriptional/translational feedback loop. BMAL1/clock (or NPAS2) heterodimers activate the expression of the period (PER) and cryptochrome (CRY) genes acting as transcription factors directed to the PER and CRY promoters via E-box elements. PER and CRY proteins form heterodimers and suppress the activity of the BMAL1/clock (or NPAS2) completing the feedback loop. The circadian expression of BMAL1 is influenced by retinoic acid receptor-related orphan receptor α (RORα) and REV-ERBα, two nuclear receptors that target a ROR-response element in the promoter of the BMAL1 gene. Given that BMAL1 functions as an obligate heterodimer with either clock or NPAS2, it is unclear how the expression of the partner is coordinated with BMAL1 expression. Here, we demonstrate that NPAS2 is also a RORα and REV-ERBα target gene. Using a ChIP/microarray screen, we identified both RORα and REV-ERBα occupancy of the NPAS2 promoter. We identified two functional ROREs within the NPAS2 promoter and also demonstrate that both RORα and REV-ERBα regulate the expression of NPAS2 mRNA. These data suggest a mechanism by which RORα and REV-ERBα coordinately regulate the expression of the positive arm of the circadian rhythm feedback loop.

Regulation of adipogenesis by natural and synthetic REV-ERB ligands

The nuclear hormone receptor, REV-ERB, plays an essential role in adipogenesis. Rev-erbalpha expression is induced in 3T3-L1 cells during adipogenesis, and overexpression of this receptor leads to expression of adipogenic genes. We recently demonstrated that the porphyrin heme functions as a ligand for REV-ERB, and binding of heme is required for the receptor's activity. We therefore hypothesized that REV-ERB ligands may play a role in regulation of adipogenesis. We detected an increase intracellular heme levels during 3T3-L1 adipogenesis that correlated with induction of aminolevulinic acid synthase 1 (Alas1) expression, the rate-limiting enzyme in heme biosynthesis. If the increase in Alas1 expression was blocked, adipogenesis was severely attenuated, indicating that induction of expression of Alas1 and the increase in heme synthesis is critical for differentiation. Inhibition of heme synthesis during adipogenesis leads to decreased recruitment of nuclear receptor corepressor to the promoter of a REV-ERB target gene, suggesting alteration of REV-ERB activity. Treatment of 3T3-L1 cells with a synthetic REV-ERB ligand, SR6452, resulted in induction of adipocyte differentiation to a similar extent as treatment with the peroxisomal proliferator-activated receptor-gamma agonist, rosiglitazone. Combination of SR6452 and rosiglitazone had an additive effect on stimulation of adipocyte differentiation. These results suggest that heme, functioning as a REV-ERB ligand, is an important signaling molecule for induction of adipogenesis. Moreover, synthetic small molecule ligands for REV-ERB are effective modulators of adipogenesis and may be useful for treatment of metabolic diseases.

Ligand regulation of retinoic acid receptor-related orphan receptors: implications for development of novel therapeutics

PURPOSE OF REVIEW

In the late 1980s, the cloning of several nuclear receptors led to the intense search and isolation of new members of this superfamily. Despite their identification, many of these receptors were dubbed 'orphan' receptors, as their physiological ligands remained unknown. Recent reports have presented evidence for one family of orphan receptors, the retinoic acid receptor-related orphan receptors (RORs), in several pathologies, including osteoporosis, several autoimmune diseases, asthma, cancer, diabetes and obesity. The present review summarizes the studies identifying ligands for the RORs and evaluates their role as targets for potential therapeutics.

RECENT FINDINGS

Significant progress was made in the initial identification of ligands for the RORs when X-ray crystallographic studies identified several molecules within the ligand-binding pockets of RORalpha and RORbeta. Recently, we identified endogenous and synthetic ligands for RORalpha and RORgamma, thereby solidifying their function as ligand-dependent transcription factors.

SUMMARY

Recent studies have established roles for the RORs in physiological development and the advent of disease. Identification of ligands for the RORs, both endogenous and synthetic, has established these receptors as attractive new therapeutic targets for the treatment of ROR-related diseases.

Regulation of FGF21 expression and secretion by retinoic acid receptor-related orphan receptor alpha

Fibroblast growth factor 21 (FGF21) is a hormone produced by fat and the liver that plays an important role in lipid metabolism. FGF21 expression is induced by peroxisome proliferator-actived receptor alpha in response to physiological conditions requiring increased fatty acid oxidation. Retinoic acid receptor-related receptor alpha (RORalpha) is another nuclear receptor that plays a critical role in lipid metabolism as well as in regulation of the circadian rhythm. In this study we demonstrate that RORalpha directly regulates the expression and secretion of FGF21. A canonical ROR response element was identified in the proximal promoter of the FGF21 gene and shown to exhibit functional activity. Overexpression of RORalpha in HepG2 cells resulted in increased expression and secretion of FGF21. Suppression of RORalpha expression caused a decrease in FGF21 expression and secretion, suggesting that RORalpha contributes to the basal expression of FGF21. These data suggest that one mechanism by which RORalpha regulates lipid metabolism may be by modulation of FGF21 secretion. Furthermore, this study identifies a clear link between RORalpha, a key regulator of the mammalian clock, and FGF21, an important hormone regulating glucose and lipid homeostasis.

Modulation of retinoic acid receptor-related orphan receptor alpha and gamma activity by 7-oxygenated sterol ligands

The retinoic acid receptor-related orphan receptors alpha and gamma (RORalpha (NR1F1) and RORgamma (NR1F3)) are orphan nuclear receptors and perform critical roles in regulation of development, metabolism, and immune function. Cholesterol and cholesterol sulfate have been suggested to be RORalpha ligands, but the physiological significance is unclear. To date, no endogenous RORgamma ligands have been described. Here, we demonstrate that 7-oxygenated sterols function as high affinity ligands for both RORalpha and RORgamma by directly binding to their ligand-binding domains (K(i) approximately 20 nM), modulating coactivator binding, and suppressing the transcriptional activity of the receptors. One of the 7-oxygenated sterols, 7alpha-hydroxycholesterol (7alpha-OHC), serves as a key intermediate in bile acid metabolism, and we show that 7alpha-OHC modulates the expression of ROR target genes, including Glc-6-Pase and phosphoenolpyruvate carboxykinase, in an ROR-dependent manner. Furthermore, glucose output from hepatocytes is suppressed by 7alpha-OHC functioning as an RORalpha/gamma ligand. Thus, RORalpha and RORgamma are ligand-regulated members of the NR superfamily and may serve as sensors for 7-oxygenated sterols.

The benzenesulfoamide T0901317 [N-(2,2,2-trifluoroethyl)-N-[4-[2,2,2-trifluoro-1-hydroxy-1-(trifluoromethyl)ethyl]phenyl]-benzenesulfonamide] is a novel retinoic acid receptor-related orphan receptor-alpha/gamma inverse agonist

Retinoic acid receptor-related orphan receptors (RORs) regulate a variety of physiological processes including hepatic gluconeogenesis, lipid metabolism, circadian rhythm, and immune function. Here we present the first high-affinity synthetic ligand for both RORalpha and RORgamma. In a screen against all 48 human nuclear receptors, the benzenesulfonamide liver X receptor (LXR) agonist N-(2,2,2-trifluoroethyl)-N-[4-[2,2,2-trifluoro-1-hydroxy-1-(trifluoromethyl)ethyl]phenyl]-benzenesulfonamide (T0901317) inhibited transactivation activity of RORalpha and RORgamma but not RORbeta. T0901317 was found to directly bind to RORalpha and RORgamma with high affinity (K(i) = 132 and 51 nM, respectively), resulting in the modulation of the receptor's ability to interact with transcriptional cofactor proteins. T0901317 repressed RORalpha/gamma-dependent transactivation of ROR-responsive reporter genes and in HepG2 cells reduced recruitment of steroid receptor coactivator-2 by RORalpha at an endogenous ROR target gene (G6Pase). Using small interference RNA, we demonstrate that repression of the gluconeogenic enzyme glucose-6-phosphatase in HepG2 cells by T0901317 is ROR-dependent and is not due to the compound's LXR activity. In summary, T0901317 represents a novel chemical probe to examine RORalpha/gamma function and an excellent starting point for the development of ROR selective modulators. More importantly, our results demonstrate that small molecules can be used to target the RORs for therapeutic intervention in metabolic and immune disorders.

Unique ligand binding patterns between estrogen receptor alpha and beta revealed by hydrogen-deuterium exchange

Here we present the use of hydrogen-deuterium exchange (HDX) mass spectrometry in analyzing the estrogen receptor beta ligand binding domain (ERbeta LBD) in the absence and presence of a variety of chemical compounds with different binding modes and pharmacological properties. Previously, we reported the use of HDX as a method for predicting the tissue selectivity of ERalpha ligands. HDX profiles of ERalpha LBD in complex with ligand could differentiate compounds of the same chemotype. In contrast, similar analysis of ERbeta LBD showed correlation to the compound chemical structures but little correlation with compound tissue selectivity. The different HDX patterns observed for ERbeta LBD when compared to those for ERalpha LBD bound to the same chemical compounds serve as an indication that ERbeta LBD undergoes a different structural response to the same ligand when compared to ERalpha LBD. The conformational dynamics revealed by HDX for ERbeta LBD together with those for ERalpha LBD shed light on ER ligand interactions and offer new structural insights. The compound-specific perturbations in HDX kinetics observed for each of the two isoforms should aid the development of subtype-selective ER ligands.

Identification of adropin as a secreted factor linking dietary macronutrient intake with energy homeostasis and lipid metabolism

Obesity and nutrient homeostasis are linked by mechanisms that are not fully elucidated. Here we describe a secreted protein, adropin, encoded by a gene, Energy Homeostasis Associated (Enho), expressed in liver and brain. Liver Enho expression is regulated by nutrition: lean C57BL/6J mice fed high-fat diet (HFD) exhibited a rapid increase, while fasting reduced expression compared to controls. However, liver Enho expression declines with diet-induced obesity (DIO) associated with 3 months of HFD or with genetically induced obesity, suggesting an association with metabolic disorders in the obese state. In DIO mice, transgenic overexpression or systemic adropin treatment attenuated hepatosteatosis and insulin resistance independently of effects on adiposity or food intake. Adropin regulated expression of hepatic lipogenic genes and adipose tissue peroxisome proliferator-activated receptor gamma, a major regulator of lipogenesis. Adropin may therefore be a factor governing glucose and lipid homeostasis, which protects against hepatosteatosis and hyperinsulinemia associated with obesity.

Implications of the binding of tamoxifen to the coactivator recognition site of the estrogen receptor

A number of studies have reported on the unusual pharmacological behavior of type I antiestrogens, such as tamoxifen. These agents display mixed agonist/antagonist activity in a dose-, cell-, and tissue-specific manner. Consequently, many efforts have been made to develop so-called 'pure' antiestrogens that lack mixed agonist/antagonist activity. The recent report of the structure of estrogen receptor (ER) beta with a second molecule of 4-hydroxytamoxifen (HT) bound in the coactivator-binding surface of the ligand-binding domain (LBD) represents the first direct example of a second ER ligand-binding site and provides insight into the possible origin of mixed agonist/antagonist activity of type I antiestrogens. In this review, we summarize the biological reports leading up to the structural conformation of a second ER ligand-binding site, compare the ERbeta LBD structure bound with two HT molecules to other ER structures, and discuss the potential for small molecular inhibitors designed to directly inhibit ER-coactivator and, more generally, nuclear receptor (NR)-coactivator interactions. The studies support a departure from the traditional paradigm of drug targeting to the ligand-binding site, to that of a rational approach targeting a functionally important surface, namely the NR coactivator-binding (activation function-2) surface. Furthermore, we provide evidence supporting a reevaluation of the strict interpretation of the agonist/antagonist state with respect to the position of helix 12 in the NR LBD.

The selective Alzheimer's disease indicator-1 gene (Seladin-1/DHCR24) is a liver X receptor target gene

The nuclear hormone receptors liver X receptor alpha (LXRalpha) and LXRbeta function as physiological receptors for oxidized cholesterol metabolites (oxysterols) and regulate several aspects of cholesterol and lipid metabolism. Seladin-1 was originally identified as a gene whose expression was down-regulated in regions of the brain associated with Alzheimer's disease. Seladin-1 has been demonstrated to be neuroprotective and was later characterized as 3beta-hydroxysterol-Delta24 reductase (DHCR24), a key enzyme in the cholesterologenic pathway. Seladin-1 has also been shown to regulate lipid raft formation. In a whole genome screen for direct LXRalpha target genes, we identified an LXRalpha occupancy site within the second intron of the Seladin-1/DHCR24 gene. We characterized a novel LXR response element within the second intron of this gene that is able to confer LXR-specific ligand responsiveness to reporter gene in both HepG2 and human embryonic kidney 293 cells. Furthermore, we found that Seladin-1/DHCR24 gene expression is significantly decreased in skin isolated from LXRbeta-null mice. Our data suggest that Seladin-1/DHCR24 is an LXR target gene and that LXR may regulate lipid raft formation.

Regulation of cholesterologenesis by the oxysterol receptor, LXRalpha

Cholesterol is required for normal cellular and physiological function, yet dysregulation of cholesterol metabolism is associated with diseases such as atherosclerosis. Cholesterol biosynthesis is regulated by end product negative feedback inhibition where the levels of sterols and oxysterols regulate the expression of cholesterologenic enzymes. Sterol regulatory element-binding protein-2 is responsive to both sterols and oxysterols and has been shown to mediate the transcriptional response of the cholesterologenic enzymes to these lipids. Here, we show that the nuclear hormone receptor for oxysterols, the liver X receptor alpha (LXRalpha), regulates cholesterol biosynthesis by directly silencing the expression of two key cholesterologenic enzymes (lanosterol 14alpha-demethylase (CYP51A1), and squalene synthase (farnesyl diphosphate farnesyl transferase 1)) via novel negative LXR DNA response elements (nLXREs) located in each of these genes. Examination of the CYP51A1 gene revealed that both the SRE and nLXRE are required for normal oxysterol-dependent repression of this gene. Thus, these data suggest that LXRalpha plays an important role in the regulation of cholesterol biosynthesis.

Relationship between circadian oscillations of Rev-erbalpha expression and intracellular levels of its ligand, heme

The nuclear hormone receptors, REV-ERBalpha [NR1D1] and REV-ERBbeta [NR1D1], were recently demonstrated to be receptors for the porphyrin, heme. Heme regulates the ability of these receptors to repress transcription of their target genes via modulation of the affinity of the receptor's ligand binding domain for the corepressor, NCoR. The REV-ERBs function as critical components of the mammalian clock and their expression oscillates in a circadian manner. Here, we show that in NIH3T3 cells intracellular heme levels also oscillate in a circadian fashion. These data are the first to show the temporal relationship of intracellular heme levels to the expression of its receptor, Rev-erbalpha, and suggest that the rapid oscillations in heme levels may an important component regulating REV-ERB transcriptional activity.

Nuclear hormone receptors for heme: REV-ERBalpha and REV-ERBbeta are ligand-regulated components of the mammalian clock

The nuclear hormone receptors (NHRs), REV-ERBalpha and REV-ERBbeta, regulate a number of physiological functions including the circadian rhythm, lipid metabolism, and cellular differentiation. These two receptors lack the activation function-2 region that is associated with the ability of NHRs to recruit coactivators and activate target gene transcription. These NHRs have been characterized as constitutive repressors of transcription due to their lack of an identified ligand and their strong ability to recruit the corepressor, nuclear receptor corepressor. Recently, the porphyrin heme was demonstrated to function as a ligand for both REV-ERBs. Heme binds directly to the ligand-binding domain and regulates the ability of these NHRs to recruit nuclear receptor corepressor to target gene promoters. This review focuses on the physiological roles that these two receptors play and the implications of heme functioning as their ligand. The prospect that these NHRs, now known to be regulated by small molecule ligands, may be targets for development of drugs for treatment of diseases associated with aberrant circadian rhythms including metabolic and psychiatric disorders as well as cancer is also addressed.

Regulation of human 3 alpha-hydroxysteroid dehydrogenase (AKR1C4) expression by the liver X receptor alpha

Type I human hepatic 3alpha-hydroxysteroid dehydrogenase (AKR1C4) plays a significant role in bile acid biosynthesis, steroid hormone metabolism, and xenobiotic metabolism. Utilization of a hidden Markov model for predictive modeling of nuclear hormone receptor response elements coupled with chromatin immunoprecipitation/microarray technology revealed a putative binding site in the AKR1C4 promoter for the nuclear hormone receptor known as liver X receptor alpha, (LXRalpha [NR1H3]), which is the physiological receptor for oxidized cholesterol metabolites. The putative LXRalpha response element (LXRE), identified by chromatin immunoprecipitation, was approximately 1.5 kilobase pairs upstream of the transcription start site. LXRalpha was shown to bind specifically to this LXRE and mediate transcriptional activation of the AKR1C4 gene, leading to increased AKR1C4 protein expression. These data suggest that LXRalpha may modulate the bile acid biosynthetic pathway at a unique site downstream of CYP7A1 and may also modulate the metabolism of steroid hormones and certain xenobiotics.

A second binding site for hydroxytamoxifen within the coactivator-binding groove of estrogen receptor beta

Evidence is presented that the estrogen antagonist 4-hydroxytamoxifen (HT) can occupy not only the core binding pocket within the ligand-binding domain of estrogen receptor (ER) beta but also a second site on its surface. The crystal structure of the ligand-binding domain (LBD) associated with HT was determined to 2.2 A and revealed two molecules of HT bound to the protein. One was located in the consensus ligand-binding pocket, whereas the other bound to a site that overlaps with the hydrophobic groove of the coactivator recognition surface. Relative to the ERalpha-tamoxifen structure, helix 12 has been displaced from the coactivator recognition surface and occupies a unique position. Although it has been demonstrated that association of the antagonist with the core ligand-binding pocket is sufficient to induce an antagonist ligand-binding domain conformation, this structure suggests that small molecules may directly antagonize receptor-coactivator interactions. These results provide a direct demonstration of two binding sites for HT in ERbeta, as has been previously suggested for ERalpha by using biochemical methods, and represent a crystal structure of a small nonpeptide molecule occupying the coactivator recognition site.

Identification and characterization of noncalcemic, tissue-selective, nonsecosteroidal vitamin D receptor modulators

Vitamin D receptor (VDR) ligands are therapeutic agents for the treatment of psoriasis, osteoporosis, and secondary hyperparathyroidism. VDR ligands also show immense potential as therapeutic agents for autoimmune diseases and cancers of skin, prostate, colon, and breast as well as leukemia. However, the major side effect of VDR ligands that limits their expanded use and clinical development is hypercalcemia that develops as a result of the action of these compounds mainly on intestine. In order to discover VDR ligands with less hypercalcemia liability, we sought to identify tissue-selective VDR modulators (VDRMs) that act as agonists in some cell types and lack activity in others. Here, we describe LY2108491 and LY2109866 as nonsecosteroidal VDRMs that function as potent agonists in keratinocytes, osteoblasts, and peripheral blood mononuclear cells but show poor activity in intestinal cells. Finally, these nonsecosteroidal VDRMs were less calcemic in vivo, and LY2108491 exhibited more than 270-fold improved therapeutic index over the naturally occurring VDR ligand 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] in an in vivo preclinical surrogate model of psoriasis.

A 15-ketosterol is a liver X receptor ligand that suppresses sterol-responsive element binding protein-2 activity

Hypercholesterolemia is a major risk factor for coronary artery disease. Oxysterols are known to inhibit cholesterol biosynthesis and have been explored as potential antihypercholesterolemic agents. The ability of 3beta-hydroxy-5alpha-cholest-8(14)-en-15-one (15-ketosterol) to lower non-HDL cholesterol has been demonstrated in rodent and primate models, but the mechanisms of action remain poorly understood. Here we show in a coactivator recruitment assay and cotransfection assays that the 15-ketosterol is a partial agonist for liver X receptor-alpha and -beta (LXRalpha and LXRbeta). The binding affinity for the LXRs was comparable to those of native oxysterols. In a macrophage cell line of human origin, the 15-ketosterol elevated ATP binding cassette transporter ABCA1 mRNA in a concentration-dependent fashion with a potency similar to those of other oxysterols. We further found that in human embryonic kidney HEK 293 cells, the 15-ketosterol suppressed sterol-responsive element binding protein processing activity and thus inhibited mRNA expression of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, LDL receptor, and PCSK9. Our data thus provide a molecular basis for the hypocholesterolemic activity of the 15-ketosterol and further suggest its potential antiatherosclerotic benefit as an LXR agonist.

A Selective Estrogen Receptor Modulator for the Treatment of Hot Flushes

A selective estrogen receptor modulator (SERM) for the potential treatment of hot flushes is described. (R)-(+)-7,9-difluoro-5-[4-(2-piperidin-1-ylethoxy)phenyl]-5H-6-oxachrysen-2-ol, LSN2120310, potently binds ERalpha and ERbeta and is an antagonist in MCF-7 breast adenocarcinoma and Ishikawa uterine cancer cell lines. The compound is a potent estrogen antagonist in the rat uterus. In ovariectomized rats, the compound lowers cholesterol, maintains bone mineral density, and is efficacious in a morphine dependent rat model of hot flush efficacy.

The pharmacology of LXR

Liver X receptors (LXRs) are members of the nuclear hormone receptor superfamily of ligand-activated transcription factors. Two LXRs (LXRalpha and LXRbeta) were initially characterized as orphan members of this superfamily with disparate patterns of tissue expression. These two receptors later were recognized as sterol-responsive with the ability to directly bind several oxysterol metabolites. Many LXR target genes have been identified that implicate these receptors in a variety of physiological processes including cholesterol transport and metabolism, glucose metabolism, and inflammation. Synthetic LXR ligands have been designed with the potential to treat disorders such as atherosclerosis and diabetes. In this review, we describe the potential utility of LXR ligands in the treatment of disease.