Structure Basis of Bigelovin as a Selective RXR Agonist with a Distinct Binding Mode

Determination of Molecule Category of Ligands Targeting the Ligand-Binding Pocket of Nuclear Receptors with Structural Elucidation and Machine Learning

The mechanism of transcriptional activation/repression of the nuclear receptors (NRs) involves two main conformations of the NR protein, namely, the active (agonistic) and inactive (antagonistic) conformations. Binding of agonists or antagonists to the ligand-binding pocket (LBP) of NRs can regulate the downstream signaling pathways with different physiological effects. However, it is still hard to determine the molecular type of a LBP-bound ligand because both the agonists and antagonists bind to the same position of the protein. Therefore, it is necessary to develop precise and efficient methods to facilitate the discrimination of agonists and antagonists targeting the LBP of NRs. Here, combining structural and energetic analyses with machine-learning (ML) algorithms, we constructed a series of structure-based ML models to determine the molecular category of the LBP-bound ligands. We show that the proposed models work robustly and with high accuracy (ACC > 0.9) for determining the category of molecules derived from docking-based and crystallized poses. Furthermore, the models are also capable of determining the molecular category of ligands with dual opposite functions on different NRs (i.e., working as an agonist in one NR target, whereas functioning as an antagonist in another) with reasonable accuracy. The proposed method is expected to facilitate the determination of the molecular properties of ligands targeting the LBP of NRs with structural interpretation.

Liver X receptors conserve the therapeutic target potential for the treatment of rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic multi-system autoimmune disease with extremely complex pathogenesis. Significantly altered lipid paradox related to the inflammatory burden is reported in RA patients, inducing 50% higher cardiovascular risks. Recent studies have also demonstrated that lipid metabolism can regulate many functions of immune cells in which metabolic pathways have altered. The nuclear liver X receptors (LXRs), including LXRα and LXRβ, play a central role in regulating lipid homeostasis and inflammatory responses. Undoubtedly, LXRs have been considered as an attractive therapeutic target for the treatment of RA. However, there are some contradictory effects of LXRs agonists observed in previous animal studies where both pro-inflammatory role and anti-inflammatory role were revealed for LXRs activation in RA. Therefore, in addition to updating the knowledge of LXRs as the prominent regulators of lipid homeostasis, the purpose of this review is to summarize the effects of LXRs agonists in RA-associated immune cells, to explore the underlying reasons for the contradictory therapeutic effects of LXRs agonists observed in RA animal models, and to discuss future strategy for the treatment of RA with LXRs modulators.

Sesquiterpene lactone Bigelovin induces apoptosis of colon cancer cells through inducing IKK-β degradation and suppressing nuclear factor kappa B activation

Bigelovin, a sesquiterpene lactone extracted from plant Inula helianthus aquatica, exhibited multiple interesting biological activities, including anti-inflammation, antiangiogenesis and cytotoxic action against cancer cells. In the present study, we found that Bigelovin reduced the viability of human colon cancer cells and induced their apoptosis in a time- and dose-dependent manner, with an IC50-5 μM. RNAseq and luciferase reporter analyses revealed that the nuclear factor kappa B (NF-κB) signaling was one of the most significantly inhibited pathways after Bigelovin treatment. Further systemic examination showed that exposure to Bigelovin resulted in ubiquitination and degradation of inhibitor of kappa-B kinase-beta (IKK-β) and decrease of IκB-α and p65 phosphorylation, which led to the downregulation of NF-κB-regulated genes expression. Moreover, enforced expression of exogenous IKK-β attenuated Bigelovin-induced NF-κB suppression and cell viability reduction. These results indicated that Bigelovin exerts a cytotoxic action against colon cancer cells through the induction of IKK-β degradation and consequently the inhibition of NF-κB signaling. Given the abnormal activation of NF-κB signaling in colorectal cancer (CRC) cells and the critical role of chronic inflammation in CRC development, it is conceivable that at least some colorectal cancer cells are addictive to NF-κB activation and targeting the pathway is an effective anti-CRC strategy.

Stability of the Retinoid X Receptor-α Homodimer in the Presence and Absence of Rexinoid and Coactivator Peptide

Differential scanning calorimetry and differential scanning fluorimetry were used to measure the thermal stability of human retinoid X receptor-α ligand binding domain (RXRα LBD) homodimer in the absence or presence of rexinoid and coactivator peptide, GRIP-1. The apo-RXRα LBD homodimer displayed a single thermal unfolding transition with a T_m of 58.7 °C and an unfolding enthalpy (ΔH) of 673 kJ/mol (12.5 J/g), much lower than average value (35 J/g) of small globular proteins. Using a heat capacity change (ΔC_p) of 15 kJ/(mol K) determined by measurements at different pH values, the free energy of unfolding (ΔG) of the native state was 33 kJ/mol at 37 °C. Rexinoid binding to the apo-homodimer increased T_m by 5 to 9 °C and increased the ΔG of the native homodimer by 12 to 20 kJ/mol at 37 °C, consistent with the nanomolar dissociation constant (K_d) of the rexinoids. GRIP-1 binding to holo-homodimers containing rexinoid resulted in additional increases in ΔG of 14 kJ/mol, a value that was the same for all three rexinoids. Binding of rexinoid and GRIP-1 resulted in a combined 50% increase in unfolding enthalpy, consistent with reduced structural fluidity and more compact folding observed in other published structural studies. The complexes of UAB110 and UAB111 are each more stable than the UAB30 complex by 8 kJ/mol due to enhanced hydrophobic interactions in the binding pocket because of their larger end groups. This increase in thermodynamic stability positively correlates with their improved RXR activation potency. Thermodynamic measurements are thus valuable in predicting agonist potency.

Natural and synthetic retinoid X receptor ligands and their role in selected nuclear receptor action

Important key players in the regulatory machinery within the cells are nuclear retinoid X receptors (RXRs), which compose heterodimers in company with several diverse nuclear receptors, playing a role as ligand inducible transcription factors. In general, nuclear receptors are ligand-activated, transcription-modulating proteins affecting transcriptional responses in target genes. RXR molecules forming permissive heterodimers with disparate nuclear receptors comprise peroxisome proliferator-activated receptors (PPARs), liver X receptors (LXRs), farnesoid X receptor (FXR), pregnane X receptor (PXR) and constitutive androstan receptor (CAR). Retinoid receptors (RARs) and thyroid hormone receptors (TRs) may form conditional heterodimers, and dihydroxyvitamin D₃ receptor (VDR) is believed to form nonpermissive heterodimer. Thus, RXRs are the important molecules that are involved in control of many cellular functions in biological processes and diseases, including cancer or diabetes. This article summarizes both naturally occurring and synthetic ligands for nuclear retinoid X receptors and describes, predominantly in mammals, their role in molecular mechanisms within the cells. A focus is also on triorganotin compounds, which are high affinity RXR ligands, and finally, we present an outlook on human microbiota as a potential source of RXR activators. Nevertheless, new synthetic rexinoids with better retinoid X receptor activity and lesser side effects are highly required.

Alternative retinoid X receptor (RXR) ligands

Retinoid X receptors (RXRs) control a wide variety of functions by virtue of their dimerization with other nuclear hormone receptors (NRs), contributing thereby to activities of different signaling pathways. We review known RXR ligands as transcriptional modulators of specific RXR-dimers and the associated biological processes. We also discuss the physiological relevance of such ligands, which remains frequently a matter of debate and which at present is best met by member(s) of a novel family of retinoids, postulated as Vitamin A5. Through comparison with other natural, but also with synthetic ligands, we discuss high diversity in the modes of ligand binding to RXRs resulting in agonistic or antagonistic profiles and selectivity towards specific subtypes of permissive heterodimers. Despite such diversity, direct ligand binding to the ligand binding pocket resulting in agonistic activity was preferentially preserved in the course of animal evolution pointing to its functional relevance, and potential for existence of other, species-specific endogenous RXR ligands sharing the same mode of function.

A review of the molecular design and biological activities of RXR agonists

An attractive approach to combat disease is to target theregulation of cell function. At the heart of this task are nuclear receptors (NRs); which control functions such as gene transcription. Arguably, the key player in this regulatory machinery is the retinoid X receptor (RXR). This NR associates with a third of the NRs found in humans. Scientists have hypothesized that controlling the activity of RXR is an attractive approach to control cellular functions that modulate diseases such as cancer, diabetes, Alzheimer's disease and Parkinson's disease. In this review, we will describe the key features of the RXR, present a historic perspective of the first RXR agonists, and discuss various templates that have been reported to activate RXR with a focus on their molecular structure, biological activity, and limitations. Finally, we will present an outlook of the field and future directions and considerations to synthesize or modulate RXR agonists to make these compounds a clinical reality.

Preclinical assessment of the ADME, efficacy and drug-drug interaction potential of a novel NAMPT inhibitor

GNE-617 (N-(4-((3,5-difluorophenyl)sulfonyl)benzyl)imidazo[1,2-a]pyridine-6-carboxamide) is a potent, selective nicotinamide phosphoribosyltransferase (NAMPT) inhibitor being explored as a potential treatment for human cancers. Plasma clearance was low in monkeys and dogs (9.14 mL min^-1kg^-1 and 4.62 mL min^-1kg^-1, respectively) and moderate in mice and rats (36.4 mL min^-1kg^-1 and 19.3 mL min^-1kg^-1, respectively). Oral bioavailability in mice, rats, monkeys and dogs was 29.7, 33.9, 29.4 and 65.2%, respectively. Allometric scaling predicted a low clearance of 3.3 mL min^-1kg^-1 and a volume of distribution of 1.3 L kg^-1 in human. Efficacy (57% tumor growth inhibition) in Colo-205 CRC tumor xenograft mice was observed at an oral dose of 15 mg/kg BID (AUC = 10.4 µM h). Plasma protein binding was moderately high. GNE-617 was stable to moderately stable in vitro. Main human metabolites identified in human hepatocytes were formed primarily by CYP3A4/5. Transporter studies suggested that GNE-617 is likely a substrate for MDR1 but not for BCRP. Simcyp^® simulations suggested a low (CYP2C9 and CYP2C8) or moderate (CYP3A4/5) potential for drug-drug interactions. The potential for autoinhibition was low. Overall, GNE-617 exhibited acceptable preclinical properties and projected human PK and dose estimates.

Overview of the structure-based non-genomic effects of the nuclear receptor RXRα

The nuclear receptor RXRα (retinoid X receptor-α) is a transcription factor that regulates the expression of multiple genes. Its non-genomic function is largely related to its structure, polymeric forms and modification. Previous research revealed that some non-genomic activity of RXRα occurs via formation of heterodimers with Nur77. RXRα-Nur77 heterodimers translocate from the nucleus to the mitochondria in response to certain apoptotic stimuli and this activity correlates with cell apoptosis. More recent studies revealed a significant role for truncated RXRα (tRXRα), which interacts with the p85α subunit of the PI3K/AKT signaling pathway, leading to enhanced activation of AKT and promoting cell growth in vitro and in animals. We recently reported on a series of NSAID sulindac analogs that can bind to tRXRα through a unique binding mechanism. We also identified one analog, K-80003, which can inhibit cancer cell growth by inducing tRXRα to form a tetramer, thus disrupting p85α-tRXRα interaction. This review analyzes the non-genomic effects of RXRα in normal and tumor cells, and discusses the functional differences based on RXRα protein structure (structure source: the RCSB Protein Data Bank).

Natural product modulators of human sensations and mood: molecular mechanisms and therapeutic potential

Humans perceive physical information about the surrounding environment through their senses. This physical information is registered by a collection of highly evolved and finely tuned molecular sensory receptors. A multitude of bioactive, structurally diverse ligands have evolved in nature that bind these molecular receptors. The complex, dynamic interactions between the ligands and the receptors lead to changes in our sensory perception or mood. Here, we review our current knowledge of natural products and their derived analogues that interact specifically with human G protein-coupled receptors, ion channels, and nuclear hormone receptors to modulate the sensations of taste, smell, temperature, pain, and itch, as well as mood and its associated behaviour. We discuss the molecular and structural mechanisms underlying such interactions and highlight cases where subtle differences in natural product chemistry produce drastic changes in functional outcome. We also discuss cases where a single compound triggers complex sensory or behavioural changes in humans through multiple mechanistic targets. Finally, we comment on the therapeutic potential of the reviewed area of research and draw attention to recent technological developments in genomics, metabolomics, and metabolic engineering that allow us to tap the medicinal properties of natural product chemistry without taxing nature.

Bigelovin, a sesquiterpene lactone, suppresses tumor growth through inducing apoptosis and autophagy via the inhibition of mTOR pathway regulated by ROS generation in liver cancer

Bigelovin (BigV) is a sesquiterpene lactone, isolated from Inula helianthus aquatica, which has been reported to induce apoptosis and show anti-inflammatory and anti-angiogenic activities. Nevertheless, the effects of BigV on liver cancer and the underlying mechanisms have not been investigated. In the study, we found that BigV exhibited potential anti-tumor activities against human liver cancer in vitro and in vivo. BigV reduced the cell proliferation and colony formation. BigV induced apoptosis through improving the cleavage of Caspase-3 and poly (ADP-ribose) polymerase 1 (PARP-1). The process was along with the activation of autophagy, as proved by the enhanced accumulation of autophagosomes, the microtubule-associated light chain 3B-II (LC3B-II) and Beclin-1, and p62 decrease. Further, the autophagy blockage markedly sensitized BigV-induced cell death, indicating the cytoprotective function of autophagy in liver cancer cell lines. In addition, BigV treatment inactivated the pathway of protein kinase B (AKT)/mammalian target of rapamycin (mTOR)/ribosomal protein S6 kinase (p70S6K). Of note, BigV-induced cell death was abolished by over-expressing the phosphorylation of mTOR. Intriguingly, the induction of apoptosis and autophagy were eliminated by the pretreatment of reactive oxygen species (ROS) scavenger N-acetyl-l-cysteine (NAC), suggesting that ROS played an important role in the regulation of BigV-induced cell death. Finally, in vivo studies demonstrated that BigV significantly suppressed the growth of HepG2 cancer xenograft tumors through the activation of apoptosis and autophagy in a dose-dependent manner with low systemic toxicity. In conclusion, the results revealed that BigV had significant antitumor effects against human liver cancer and it may potentially be used as a novel antitumor agent for the prevention of liver cancer.

Natural products as modulators of the nuclear receptors and metabolic sensors LXR, FXR and RXR

Nuclear receptors (NRs) represent attractive targets for the treatment of metabolic syndrome-related diseases. In addition, natural products are an interesting pool of potential ligands since they have been refined under evolutionary pressure to interact with proteins or other biological targets. This review aims to briefly summarize current basic knowledge regarding the liver X (LXR) and farnesoid X receptors (FXR) that form permissive heterodimers with retinoid X receptors (RXR). Natural product-based ligands for these receptors are summarized and the potential of LXR, FXR and RXR as targets in precision medicine is discussed.

Berberine binds RXRα to suppress β-catenin signaling in colon cancer cells

Berberine, an isoquinoline alkaloid, is a traditional oriental medicine used to treat diarrhea and gastroenteritis. Recently, we reported that it could inhibit the growth of intestinal polyp in animals and in patients with the familial adenomatous polyposis by downregulating β-catenin signaling. However, the intracellular target mediating the effects of berberine remains elusive. Here, we provide evidence that berberine inhibits β-catenin function via directly binding to a unique region comprising residues Gln275, Arg316 and Arg371 in nuclear receptor retinoid X receptor alpha (RXRα), where berberine concomitantly binding to and synergistically activating RXRα with 9-cis-retinoic acid (9-cis-RA), a natural ligand binding to the classical ligand-binding pocket of RXRα. Berberine binding promotes RXRα interaction with nuclear β-catenin, leading to c-Cbl mediated degradation of β-catenin, and consequently inhibits the proliferation of colon cancer cells. Furthermore, berberine suppresses the growth of human colon carcinoma xenograft in nude mice in an RXRα-dependent manner. Together, our study not only identifies RXRα as a direct protein target for berberine but also dissects their binding mode and validates that berberine indeed suppresses β-catenin signaling and cell growth in colon cancer via binding RXRα, which provide new strategies for the design of new RXRα-based antitumor agents and drug combinations.

Anti-inflammatory effects of the petasin phyto drug Ze339 are mediated by inhibition of the STAT pathway

Ze339, an herbal extract from Petasites hybridus leaves is effective in treatment of allergic rhinitis by inhibition of a local production of IL-8 and eicosanoid LTB₄ in allergen-challenged patients. However, the mechanism of action and anti-inflammatory potential in virally induced exacerbation of the upper airways is unknown. This study investigates the anti-inflammatory mechanisms of Ze339 on primary human nasal epithelial cells (HNECs) upon viral, bacterial and pro-inflammatory triggers. To investigate the influence of viral and bacterial infections on the airways, HNECs were stimulated with viral mimics, bacterial toll-like-receptor (TLR)-ligands or cytokines, in presence or absence of Ze339. The study uncovers Ze339 modulated changes in pro-inflammatory mediators and decreased neutrophil chemotaxis as well as a reduction of the nuclear translocation and phosphorylation of STAT molecules. Taken together, this study suggests that phyto drug Ze339 specifically targets STAT-signalling pathways in HNECs and has high potential as a broad anti-inflammatory drug that exceeds current indication. © 2016 BioFactors, 43(3):388-399, 2017.

Bigelovin triggered apoptosis in colorectal cancer in vitro and in vivo via upregulating death receptor 5 and reactive oxidative species

Colorectal cancer (CRC) is the third most prevalent cancer and the third highest cancer-related mortality in the United States. Bigelovin, a sesquiterpene lactone isolated from Inula helianthus aquatica, has been proven to induce apoptosis and exhibit anti-inflammatory and anti-angiogenic activities. However, the effects of bigelovin on CRC and underlying mechanisms have not been explored. The present study demonstrated that bigelovin exhibited potent anti-tumor activities against CRC in vitro and in vivo. Bigelovin suppressed cell proliferation and colony formation and induced apoptosis in human colorectal cancer HT-29 and HCT 116 cells in vitro. Results also revealed that bigelovin activated caspases, caused the G2/M cell cycle arrest and induced DNA damage through up-regulation of death receptor (DR) 5 and increase of ROS. In HCT 116 xenograft model, bigelovin treatment resulted in suppression of tumor growth. Bigelovin at 20 mg/kg showed more significant tumor suppression and less side effects than conventional FOLFOX (containing folinic acid, 5-fluorouracil and oxaliplatin) treatment. In addition, in vivo data confirmed that anti-tumor activity of bigelovin in CRC was through induction of apoptosis by up-regulating DR5 and increasing ROS. In conclusion, these results strongly suggested that bigelovin has potential to be developed as therapeutic agent for CRC patients.

NSC-640358 acts as RXRα ligand to promote TNFα-mediated apoptosis of cancer cell

Retinoid X receptor α (RXRα) and its N-terminally truncated version tRXRα play important roles in tumorigenesis, while some RXRα ligands possess potent anti-cancer activities by targeting and modulating the tumorigenic effects of RXRα and tRXRα. Here we describe NSC-640358 (N-6), a thiazolyl-pyrazole derived compound, acts as a selective RXRα ligand to promote TNFα-mediated apoptosis of cancer cell. N-6 binds to RXRα and inhibits the transactivation of RXRα homodimer and RXRα/TR3 heterodimer. Using mutational analysis and computational study, we determine that Arg316 in RXRα, essential for 9-cis-retinoic acid binding and activating RXRα transactivation, is not required for antagonist effects of N-6, whereas Trp305 and Phe313 are crucial for N-6 binding to RXRα by forming extra π–π stacking interactions with N-6, indicating a distinct RXRα binding mode of N-6. N-6 inhibits TR3-stimulated transactivation of Gal4-DBD-RXRα-LBD by binding to the ligand binding pocket of RXRα-LBD, suggesting a strategy to regulate TR3 activity indirectly by using small molecules to target its interacting partner RXRα. For its physiological activities, we show that N-6 strongly inhibits tumor necrosis factor α (TNFα)-induced AKT activation and stimulates TNFα-mediated apoptosis in cancer cells in an RXRα/tRXRα dependent manner. The inhibition of TNFα-induced tRXRα/p85α complex formation by N-6 implies that N-6 targets tRXRα to inhibit TNFα-induced AKT activation and to induce cancer cell apoptosis. Together, our data illustrate a new RXRα ligand with a unique RXRα binding mode and the abilities to regulate TR3 activity indirectly and to induce TNFα-mediated cancer cell apoptosis by targeting RXRα/tRXRα.

Bigelovin inhibits STAT3 signaling by inactivating JAK2 and induces apoptosis in human cancer cells

AIM

To study the function and mechanism of bigelovin, a sesquiterpene lactone from the flower of Chinese herb Inula hupehensis, in regulating JAK2/STAT3 signaling and cancer cell growth.

METHODS

HepG2 cells stably transfected with the STAT3-responsive firefly luciferase reporter plasmid (HepG2/STAT3 cells), and a panel of human cancer cell lines were used to identify active compounds. Cell viability was measured using MTT assay. Western blotting was used to detect protein expression and phosphorylation. Kinase assays were performed and the reaction between bigelovin and thiol-containing compounds was analyzed with LC-MS.

RESULTS

Bigelovin (1-50 μmol/L) dose-dependently inhibited the IL-6-induced STAT3 activation in HepG2/STAT3 cells (IC50=3.37 μmol/L) and the constitutive STAT3 activation in A549 and MDA-MB-468 cells. Furthermore, bigelovin dose-dependently inhibited JAK2 phosphorylation in HeLa and MDA-MB-468 cells, as well as the enzymatic activity of JAK2 in vitro (IC50=44.24 μmol/L). Pretreatment of the cells with DTT (500 μmol/L) or GSH (500 μmol/L) eliminated the inhibitory effects of bigelovin on the IL-6-induced and the constitutive STAT3 activation. The results in LC-MS analysis suggested that bigelovin might react with cysteine residues of JAK2 leading to inactivation of JAK2. Bigelovin (5 and 20 μmol/L) had no effects on the signaling pathways of growth factors EGF, PDGF or insulin. Finally, bigelovin suppressed the cell viability and induced apoptosis in 10 different human cancer cell lines, particularly those with constitutively activated STAT3.

CONCLUSION

Bigelovin potently inhibits STAT3 signaling by inactivating JAK2, and induces apoptosis of a variety of human cancer cells in vitro.

Nitrostyrene Derivatives Act as RXRα Ligands to Inhibit TNFα Activation of NF-κB

Retinoid X receptor alpha (RXRα) and its N-terminally truncated version, tRXRα, are widely implicated in cancer development and represent intriguing targets for cancer prevention and treatment. Successful manipulation of RXRα and tRXRα requires the identification of their modulators that could produce therapeutic effects. Here, we report that a class of nitrostyrene derivatives bind to RXRα by a unique mechanism, of which the nitro group of nitrostyrene derivatives and Cys432 of RXRα are required for binding. The binding results in the potent activation of Gal4-DBD-RXRα-LBD transactivation. However, the binding inhibits the transactivation of RXRα homodimer, which might be due to the distinct conformation of RXRα homodimer induced by these nitrostyrene derivatives. Two RXRα point mutants with Cys432 substituted with Tyr and Trp, respectively, could mimic the bindings of two nitrostyrene derivatives and have the ability of autotransactivation. In studying the functional consequences of the binding, we show that these nitrostyrene derivatives could potently inhibit the TNFα/NFκB signaling pathway in a tRXRα-dependent manner. tRXRα promotes TNFα-induced NF-κB activation through its interaction with TRAF2 and enhances TNFα-induced ubiquitination of RIP1, which is strongly inhibited by nitrostyrene derivatives. The inhibition of TNFα-induced NF-κB activation results in the synergistic effect of the combination of nitrostyrene derivatives and TNFα on the induction of cancer cell apoptosis. Together, our results show a new class of RXRα modulators that induce apoptosis of cancer cells through their unique binding mode and new mechanism of action.

Botanical compounds and their regulation of nuclear receptor action: the case of traditional Chinese medicine

Nuclear receptors (NRs) are major pharmacological targets that allow an access to the mechanisms controlling gene regulation. As such, some NRs were identified as biological targets of active compounds contained in herbal remedies found in traditional medicines. We aim here to review this expanding literature by focusing on the informative articles regarding the mechanisms of action of traditional Chinese medicines (TCMs). We exemplified well-characterized TCM action mediated by NR such as steroid receptors (ER, GR, AR), metabolic receptors (PPAR, LXR, FXR, PXR, CAR) and RXR. We also provided, when possible, examples from other traditional medicines. From these, we draw a parallel between TCMs and phytoestrogens or endocrine disrupting chemicals also acting via NR. We define common principle of action and highlight the potential and limits of those compounds. TCMs, by finely tuning physiological reactions in positive and negative manners, could act, in a subtle but efficient way, on NR sensors and their transcriptional network.

Rexinoids isolated from Sophora tonkinensis with a gene expression profile distinct from the synthetic rexinoid bexarotene

The retinoid X receptor (RXR) plays a critical role in transcriptional regulation via formation of an RXR homodimer or heterodimers with partner nuclear receptors. Despite the numerous beneficial effects, only a limited number of naturally occurring RXR agonists are known. In this report, two prenylated flavanones (1 and 2) isolated from Sophora tonkinensis were identified as new rexinoids that preferentially activated RXRs, relative to the retinoic acid receptor. The activities of 1 and 2 were the most potent among naturally occurring rexinoids, yet 2 orders of magnitude lower than the synthetic rexinoid bexarotene. Compounds 1 and 2 activated particular RXR heterodimers in a manner similar to bexarotene. A microarray assay followed by quantitative real-time polymerase chain reaction analyses on RNAs isolated from C2C12 myotubes treated with 1 or 2 demonstrated that they significantly increased mRNA levels of lipoprotein lipase, angiopoietin-like protein 4, and heme oxygenase-1. In contrast, bexarotene preferentially potentiated transcription of genes involved in lipogenesis and lipid metabolism such as sterol regulatory element-binding protein-1, fatty acid synthase, and apolipoprotein D by a liver X receptor agonist. In this study, we have demonstrated that two newly identified naturally occurring rexinoids, 1 and 2, possess properties different from bexarotene.

Small compound bigelovin exerts inhibitory effects and triggers proteolysis of E2F1 in multiple myeloma cells

Multiple myeloma (MM) is a currently incurable blood cancer. Here we tested the effects of a small compound bigelovin on MM cells, and reported that it caused cell cycle arrest and subsequently induced apoptosis. Bigelovin triggered proteolysis of E2F1, which could be inhibited by caspase inhibitor. To investigate the clinical relevance, the expression of E2F1 in MM specimens was tested, and the results showed that E2F1 was overexpressed in 25-57% of MM patients and was associated with higher International Staging System (ISS) stage. These results suggest that E2F1 may be important for MM pathogenesis, and bigelovin could serve as a lead compound for the development of E2F1 inhibitor.

Defining the communication between agonist and coactivator binding in the retinoid X receptor α ligand binding domain

Retinoid X receptors (RXRs) are obligate partners for several other nuclear receptors, and they play a key role in several signaling processes. Despite being a promiscuous heterodimer partner, this nuclear receptor is a target of therapeutic intervention through activation using selective RXR agonists (rexinoids). Agonist binding to RXR initiates a large conformational change in the receptor that allows for coactivator recruitment to its surface and enhanced transcription. Here we reveal the structural and dynamical changes produced when a coactivator peptide binds to the human RXRα ligand binding domain containing two clinically relevant rexinoids, Targretin and 9-cis-UAB30. Our results show that the structural changes are very similar for each rexinoid and similar to those for the pan-agonist 9-cis-retinoic acid. The four structural changes involve key residues on helix 3, helix 4, and helix 11 that move from a solvent-exposed environment to one that interacts extensively with helix 12. Hydrogen-deuterium exchange mass spectrometry reveals that the dynamics of helices 3, 11, and 12 are significantly decreased when the two rexinoids are bound to the receptor. When the pan-agonist 9-cis-retinoic acid is bound to the receptor, only the dynamics of helices 3 and 11 are reduced. The four structural changes are conserved in all x-ray structures of the RXR ligand-binding domain in the presence of agonist and coactivator peptide. They serve as hallmarks for how RXR changes conformation and dynamics in the presence of agonist and coactivator to initiate signaling.

Cloning and functional analysis of the ecdysteroid receptor complex in the opossum shrimp Neomysis integer (Leach, 1814)

In this paper, the non-target effects of tebufenozide were evaluated on the estuarine crustacean, the opposum shrimp Neomysis integer (Leach, 1814). Tebufenozide is a synthetic non-steroidal ecdysone agonist insecticide and regarded as potential endocrine-disrupting chemical (EDC). N. integer is the most used crustacean in ecotoxicological research in parallel to Daphnia sp. and has been proposed for the regulatory testing of potential EDCs in the US, Europe and Japan. Major results were: (i) cDNAs encoding the ecdysteroid receptor (EcR) and the retinoid-X-receptor (RXR), were cloned and sequenced, and subsequent molecular phylogenetic analysis (maximum likelihood and neighbor-joining) revealed that the amino acid sequence of the ligand binding domain (LBD) of N. integer EcR (NiEcR) clusters as an outgroup of the Crustacea, while NiRXR-LBD clusters in the Malacostracan clade (bootstrap percentage=75%). (ii) 3D-modeling of ligand binding to NiEcR-LBD demonstrated an incompatibility of the insecticide tebufenozide to fit into the NiEcR-ligand binding pocket. This was in great contrast to ponasterone A (PonA) that is the natural molting hormone in Crustacea and for which efficient docking was demonstrated. In addition, the heterodimerization of NiEcR-LBD with the common shrimp Crangon crangon (Linnaeus, 1758) RXR-LBD (CrcRXR-LBD) was also modeled in silico. (iii) With use of insect Hi5 cells, chimeric constructs of NiEcR-LBD and CrcRXR-LBD fused to either the yeast Gal4-DNA binding domain (DBD) or Gal4-activation domain (AD) were cloned into expression plasmids and co-transfected with a Gal4 reporter to quantify the protein-protein interactions of NiEcR-LBD with CrcRXR-LBD. Investigation of the ligand effect of PonA and tebufenozide revealed that only the presence of PonA could induce dimerization of this heterologous receptor complex. (iv) Finally, in an in vivo toxicity assay, N. integer juveniles were exposed to tebufenozide at a concentration of 100 μg/L, and no effects against the molting process and nymphal development were scored. In conclusion, the in vitro cell reporter assay, based on NiEcR-LBD/CrcRXR-LBD heterodimerization in Hi5 cells and validated with the natural ecdysteroid hormone PonA, represents a useful tool for the screening of putative EDCs. As a test example for non-steroidal ecdysone agonist insecticides, tebufenozide had no negative effects on NiEcR/RXR receptor dimerization in vitro, nor on the molting process and nymphal development of N. integer at the tested concentration (100 μg/L) in vivo.

Design, synthesis, and biological evaluation of indenoisoquinoline rexinoids with chemopreventive potential

Nuclear receptors, such as the retinoid X receptor (RXR), are proteins that regulate a myriad of cellular processes. Molecules that function as RXR agonists are of special interest for the prevention and control of carcinogenesis. The majority of these ligands possess an acidic moiety that is believed to be key for RXR activation. This communication presents the design, synthesis, and biological evaluation of both acidic and nonacidic indenoisoquinolines as new RXR ligands. In addition, a comprehensive structure-activity relationship study is presented that identifies the important features of the indenoisoquinoline rexinoids. The ease of modification of the indenoisoquinoline core and the lack of the necessity of a carboxyl group for activity make them an attractive and unusual family of RXR agonists. This work establishes a structural foundation for the design of new and novel rexinoid cancer chemopreventive agents.

Targeting truncated retinoid X receptor-α by CF31 induces TNF-α-dependent apoptosis

A truncated version of retinoid X receptor-α, tRXR-α, promotes cancer cell survival by activating the phosphoinositide 3-kinase (PI3K)/AKT pathway. However, targeting the tRXR-α-mediated survival pathway for cancer treatment remains to be explored. We report here our identification of a new natural product molecule, CF31, a xanthone isolated from Cratoxylum formosum ssp. pruniflorum, and the biologic evaluation of its regulation of the tRXR-α-mediated PI3K/AKT pathway. CF31 binds RXR-α and its binding results in inhibition of RXR-α transactivation. Through RXR-α mutational analysis and computational studies, we show that Arg316 of RXR-α, known to form salt bridges with certain RXR-α ligands, such as 9-cis-retinoic acid (9-cis-RA), is not required for the antagonist effect of CF31, showing a distinct binding mode. Evaluation of several CF31 analogs suggests that the antagonist effect is mainly attributed to an interference with Leu451 of helix H12 in RXR-α. CF31 is a potent inhibitor of AKT activation in various cancer cell lines. When combined with TNF-α, it suppresses TNF-α activation of AKT by inhibiting TNF-α-induced tRXR-α interaction with the p85α regulatory subunit of PI3K. CF31 inhibition of TNF-α activation of AKT also results in TNF-α-dependent activation of caspase-8 and apoptosis. Together, our results show that CF31 is an effective converter of TNF-α signaling from survival to death by targeting tRXR-α in a unique mode and suggest that identification of a natural product that targets an RXR-mediated cell survival pathway that regulates PI3K/AKT may offer a new therapeutic strategy to kill cancer cells.

Advances in drug design with RXR modulators

INTRODUCTION

Retinoid X receptors (subtypes RXRα or NR2B1, RXRβ or NR2B2 and RXRγ or NR2B3, which originate from three distinct genes) are promiscuous partners with heterodimeric associations to other members of the Nuclear Receptor (NR) superfamily. Some of the heterodimers are "permissive" and transcriptionally active in the presence of either an RXR ligand ("rexinoid") or a NR partner ligand, whereas others are "non-permissive" and unresponsive to rexinoids alone. In rodent models, rexinoids and partner agonists (mainly PPARγ, LXR, FXR) produce beneficial effects on insulin sensitization, diabetes and obesity, but secondary effects have also been noted, such as a raise in tryglyceride levels, supression of the thyroid hormone axis and induction of hepatomegaly.

AREAS COVERED

The authors review recent advances in rexinoid design, including further optimization of known scaffolds, and the discovery of novel RXR modulators by virtual ligand screening or from bioactive natural products. The understanding of rexinoid functions in permissive and non-permissive heterodimers is firmly based on structural knowledge. By strenghtening or disrupting the interaction surface with coregulators rexinoids exert agonist or (partial) antagonist activities. The activity state of the heterodimer can also be fine-tuned by the cellular context and the nature of coregulators.

EXPERT OPINION

The synthetic chemistry toolbox has provided a panel of agonists, partial (ant)agonists and/or heterodimer-selective rexinoids starting from existing, naturally occurring or serendipitously discovered scaffolds. These compounds have an unexplored therapeutic potential that might overcome some of the current limitations of rexinoids in therapy, such as hypertriglyceridemia.

The retinoid X receptors and their ligands

This chapter presents an overview of the current status of studies on the structural and molecular biology of the retinoid X receptor subtypes α, β, and γ (RXRs, NR2B1-3), their nuclear and cytoplasmic functions, post-transcriptional processing, and recently reported ligands. Points of interest are the different changes in the ligand-binding pocket induced by variously shaped agonists, the communication of the ligand-bound pocket with the coactivator binding surface and the heterodimerization interface, and recently identified ligands that are natural products, those that function as environmental toxins or drugs that had been originally designed to interact with other targets, as well as those that were deliberately designed as RXR-selective transcriptional agonists, synergists, or antagonists. Of these synthetic ligands, the general trend in design appears to be away from fully aromatic rigid structures to those containing partial elements of the flexible tetraene side chain of 9-cis-retinoic acid. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Structural basis for retinoic X receptor repression on the tetramer

Retinoic X receptor (RXR) is a master nuclear receptor in the processes of cell development and homeostasis. Unliganded RXR exists in an autorepressed tetramer, and agonists can induce RXR dimerization and coactivator recruitment for activation. However, the molecular mechanisms involving the corepressor recruitment and antagonist-mediated repression of RXR are still elusive. Here we report the crystal structure of RXRα ligand-binding domain (LBD) complexed with silencing mediator for retinoid and thyroid hormone receptors (SMRT) corepressor motif. As the first structural report on the unliganded nuclear receptor bound to the corepressor motif, RXRαLBD-SMRT exhibits a significant structural rearrangement, compared with apoRXRαLBD tetramer. To elucidate further the molecular determinants for RXR repression by its antagonist, we also determine the crystal structure of RXRαLBD-SMRT complexed with the identified antagonist rhein. In the structure, two rhein molecules and two SMRT peptides are in the RXRαLBD tetramer, different from the case in RXRαLBD-SMRT structure, where four SMRT peptides bind to RXRαLBD tetramer. It seems that rhein induces a displacement of SMRT motif by activation function 2 (AF-2) motif binding to the receptor. Combining our current work with the published results, structural superposition of RXRαLBDs in different states reveals that RXR uses an overlapped binding site for coactivator, corepressor, and AF-2 motifs, whereas the AF-2 motif adopts different conformations for agonist or antagonist interaction and coactivator or corepressor recruitment. Taken together, we thus propose a molecular model of RXR repression on the tetramer.