Regulation of the nongenomic actions of retinoid X receptor-α by targeting the coregulator-binding sites

Sulindac selectively induces autophagic apoptosis of GABAergic neurons and alters motor behaviour in zebrafish

Nonsteroidal anti-inflammatory drugs compose one of the most widely used classes of medications, but the risks for early development remain controversial, especially in the nervous system. Here, we utilized zebrafish larvae to assess the potentially toxic effects of nonsteroidal anti-inflammatory drugs and found that sulindac can selectively induce apoptosis of GABAergic neurons in the brains of zebrafish larvae brains. Zebrafish larvae exhibit hyperactive behaviour after sulindac exposure. We also found that akt1 is selectively expressed in GABAergic neurons and that SC97 (an Akt1 activator) and exogenous akt1 mRNA can reverse the apoptosis caused by sulindac. Further studies showed that sulindac binds to retinoid X receptor alpha (RXRα) and induces autophagy in GABAergic neurons, leading to activation of the mitochondrial apoptotic pathway. Finally, we verified that sulindac can lead to hyperactivity and selectively induce GABAergic neuron apoptosis in mice. These findings suggest that excessive use of sulindac may lead to early neurodevelopmental toxicity and increase the risk of hyperactivity, which could be associated with damage to GABAergic neurons.

Alpha lipoamide inhibits diabetic kidney fibrosis via improving mitochondrial function and regulating RXRα expression and activation

Previous studies have shown mitochondrial dysfunction in various acute kidney injuries and chronic kidney diseases. Lipoic acid exerts potent effects on oxidant stress and modulation of mitochondrial function in damaged organ. In this study we investigated whether alpha lipoamide (ALM), a derivative of lipoic acid, exerted a renal protective effect in a type 2 diabetes mellitus mouse model. 9-week-old db/db mice were treated with ALM (50 mg·kg-1·d-1, i.g) for 8 weeks. We showed that ALM administration did not affect blood glucose levels in db/db mice, but restored renal function and significantly improved fibrosis of kidneys. We demonstrated that ALM administration significantly ameliorated mitochondrial dysfunction and tubulointerstitial fibrotic lesions, along with increased expression of CDX2 and CFTR and decreased expression of β-catenin and Snail in kidneys of db/db mice. Similar protective effects were observed in rat renal tubular epithelial cell line NRK-52E cultured in high-glucose medium following treatment with ALM (200 μM). The protective mechanisms of ALM in diabetic kidney disease (DKD) were further explored: Autodock Vina software predicted that ALM could activate RXRα protein by forming stable hydrogen bonds. PROMO Database predicted that RXRα could bind the promoter sequences of CDX2 gene. Knockdown of RXRα expression in NRK-52E cells under normal glucose condition suppressed CDX2 expression and promoted phenotypic changes in renal tubular epithelial cells. However, RXRα overexpression increased CDX2 expression which in turn inhibited high glucose-mediated renal tubular epithelial cell injury. Therefore, we reveal the protective effect of ALM on DKD and its possible potential targets: ALM ameliorates mitochondrial dysfunction and regulates the CDX2/CFTR/β-catenin signaling axis through upregulation and activation of RXRα. Schematic figure illustrating that ALM alleviates diabetic kidney disease by improving mitochondrial function and upregulation and activation of RXRα, which in turn upregulated CDX2 to exert an inhibitory effect on β-catenin activation and nuclear translocation. RTEC renal tubular epithelial cell. ROS Reactive oxygen species. RXRα Retinoid X receptor-α. Mfn1 Mitofusin 1. Drp1 dynamic-related protein 1. MDA malondialdehyde. 4-HNE 4-hydroxynonenal. T-SOD Total-superoxide dismutase. CDX2 Caudal-type homeobox transcription factor 2. CFTR Cystic fibrosis transmembrane conductance regulator. EMT epithelial mesenchymal transition. α-SMA Alpha-smooth muscle actin. ECM extracellular matrix. DKD diabetic kidney disease. Schematic figure was drawn by Figdraw ( www.figdraw.com ).

Synergistic Effects of Ginsenoside Rb3 and Ferruginol in Ischemia-Induced Myocardial Infarction

Previous research shows that ginsenoside Rb3 (G-Rb3) exhibit significant protective effects on cardiomyocytes and is considered a promising treatment for myocardial infraction (MI). However, how to improve its oral bioavailability and reduce its dosage remains to be studied. Previous studies suggest that Ferruginol (FGL) may have synergistic effects with G-Rb3. However, the underlying mechanisms remain to be explored. In this study, left anterior descending branch (LAD) coronary artery ligation or oxygen-glucose deprivation-reperfusion (OGD/R) were used to establish MI models in vivo and in vitro. Subsequently, the pharmacological effects and mechanisms of G-Rb3-FGL were explored by in vitro studies. The results showed that the G-Rb3-FGL co-treatment improved heart functions better than the G-Rb3 treatment alone in MI mice models. Meanwhile, the G-Rb3-FGL co-treatment can upregulate fatty acids oxidation (FAO) and suppress oxidative stress in the heart tissues of MI mice. In vitro studies demonstrated that the synergistic effect of G-Rb3-FGL on FAO, oxidation and inflammation was abolished by RXRα inhibitor HX531 in the H9C2 cell model. In summary, we revealed that G-Rb3 and FGL have a synergistic effect against MI. They protected cardiomyocytes by promoting FAO, inhibiting oxidative stress, and suppressing inflammation through the RXRα-Nrf2 signaling pathway.

Sex-differential RXRα gene methylation effects on mRNA and protein expression in umbilical cord of the offspring rat exposed to maternal obesity

Maternal obesity (MO) induces negative consequences in the offspring development. Adiposity phenotype is associated with maternal diet at early pregnancy and DNA methylation marks in the RXRα promotor at birth. Glucocorticoids play an important role in the regulation of metabolism through the activation of nuclear hormone receptors such as the RXRα protein. The aim of the study was to analyze steroid hormone changes at the end of pregnancy in the obese mother and RXRα gene methylation in the umbilical cord. For this purpose, in a well-established MO model, female Wistar rats were fed either standard chow (controls: C) or high-fat obesogenic diet (MO) before and during pregnancy to evaluate at 19 days of gestation (19 dG): 1) maternal concentration of circulating steroid hormones in MO and C groups, 2) maternal and fetal weights, 3) analysis of correlation between hormones concentration and maternal and fetal weights, 4) DNA methylation status of a single locus of RXRα gene near the early growth response (EGR-1) protein DNA binding site, and 5) RXRα mRNA and protein expressions in umbilical cords. Our results demonstrate that at 19 dG, MO body weight before and during pregnancy was higher than C; MO progesterone and corticosterone serum concentrations were higher and estradiol lower than C. There were not differences in fetal weight between male and female per group, therefore averaged data was used; MO fetal weight was lower than C. Positive correlations were found between progesterone and corticosterone with maternal weight, and estradiol with fetal weight, while negative correlation was observed between corticosterone and fetal weight. Additionally, male umbilical cords from MO were hypermethylated in RXRα gene compared to male C group, without differences in the female groups; mRNA and protein expression of RXRα were decreased in F1 male but not in female MO compared to C. In conclusion, MO results in dysregulation of circulating steroid hormones of the obese mothers and low fetal weight in the F1, modifying DNA methylation of RXRα gene as well as RXRα mRNA and protein expression in the umbilical cord in a sex-dependent manner.

Evolution of non-genomic nuclear receptor function

Nuclear receptors (NRs) are responsible for the regulation of diverse developmental and physiological systems in metazoans. NR actions can be the result of genomic and non-genomic mechanisms depending on whether they act inside or outside of the nucleus respectively. While the actions of both mechanisms have been shown to be crucial to NR functions, non-genomic actions are considered less frequently than genomic actions. Furthermore, hypotheses on the origin and evolution of non-genomic NR signaling pathways are rarely discussed in the literature. Here we summarize non-genomic NR signaling mechanisms in the context of NR protein family evolution and animal phyla. We find that NRs across groups and phyla act via calcium flux as well as protein phosphorylation cascades (MAPK/PI3K/PKC). We hypothesize and discuss a possible synapomorphy of NRs in the NR1 and NR3 families, including the thyroid hormone receptor, vitamin D receptor, ecdysone receptor, retinoic acid receptor, steroid receptors, and others. In conclusion, we propose that the advent of non-genomic NR signaling may have been a driving force behind the expansion of NR diversity in Cnidarians, Placozoans, and Bilaterians.

The retinoid X receptor α modulator K-80003 suppresses inflammatory and catabolic responses in a rat model of osteoarthritis

Osteoarthritis (OA), a most common and highly prevalent joint disease, is closely associated with dysregulated expression and modification of RXRα. However, the role of RXRα in the pathophysiology of OA remains unknown. The present study aimed to investigate whether RXRα modulator, such as K-80003 can treat OA. Experimental OA was induced by intra-articular injection of monosodium iodoacetate (MIA) in the knee joint of rats. Articular cartilage degeneration was assessed using Safranin-O and fast green staining. Synovial inflammation was measured using hematoxylin and eosin (H&E) staining and enzyme-linked immunosorbent assay (ELISA). Expressions of MMP-13, ADAMTS-4 and ERα in joints were analyzed by immunofluorescence staining. Western blot, RT-PCR and co-Immunoprecipitation (co-IP) were used to assess the effects of K-80003 on RXRα-ERα interaction. Retinoid X receptor α (RXRα) modulator K-80003 prevented the degeneration of articular cartilage, reduced synovial inflammation, and alleviated osteoarthritic pain in rats. Furthermore, K-80003 markedly inhibited IL-1β-induced p65 nuclear translocation and IκBα degradation, and down-regulate the expression of HIF-2α, proteinases (MMP9, MMP13, ADAMTS-4) and pro-inflammatory factors (IL-6 and TNFα) in primary chondrocytes. Additionally, knockdown of ERα with siRNA blocked these effects of K-80003 in chondrocytes. In conclusion, RXRα modulators K-80003 suppresses inflammatory and catabolic responses in OA, suggesting that targeting RXRα-ERα interaction by RXRα modulators might be a novel therapeutic approach for OA treatment.

Cytoplasmic Localization of RXRα Determines Outcome in Breast Cancer

Simple Summary

Considering the immense development of today’s therapeutic approaches in oncology towards customized therapy, this study aimed to assess the prognostic value of nuclear versus cytoplasmic retinoid X receptor α (RXRα) expression in breast cancer. Our results demonstrate that RXRα expression may have different roles in tumorigenesis according to its subcellular localization. This study strengthens the need for further research on the behavior of RXRα, depending on its intracellular localization.

Abstract

The aim of this retrospective study was to assess the prognostic value of cytoplasmic versus nuclear RXRα expression in breast cancer (BC) tissue samples and to correlate the results with clinicopathological parameters. In 319 BC patients, the expression of RXRα was evaluated via immunohistochemistry. Prognosis-determining aspects were calculated through uni- and multivariate analyses. Correlation analysis revealed a trend association with nuclear RXRα expression regarding an improved overall survival (OS) (p = 0.078), whereas cytoplasmic RXRα expression was significantly correlated with a poor outcomes in terms of both OS (p = 0.038) and disease-free survival (DFS) (p = 0.037). Strengthening these results, cytoplasmic RXRα was found to be an independent marker for DFS (p = 0.023), when adjusted to clinicopathological parameters, whereas nuclear RXRα expression was positively associated with lower TNM-staging, i.e., pT (p = 0.01), pN (p = 0.029) and pM (p = 0.001). Additionally, cytoplasmic RXRα expression was positively associated with a higher histopathological tumor grading (p = 0.02). Cytoplasmic RXRα was also found to be a negative prognosticator for Her-2neu-negative and triple-negative patients. Altogether, these findings support the hypothesis that the subcellular localization of RXRα plays an important role in carcinogenesis and the prognosis of BC. The expression of cytoplasmic RXRα is correlated with a more aggressive course of the disease, whereas nuclear RXRα expression appears to be a protective factor. These data may help to identify high-risk BC subgroups in order to find possible specific options in targeted tumor therapy.

Loss of retinoid X receptor gamma subunit impairs group 1 mGluR mediated electrophysiological responses and group 1 mGluR dependent behaviors

Retinoid X receptors are members of the nuclear receptor family that regulate gene expression in response to retinoic acid and related ligands. Group 1 metabotropic glutamate receptors are G-protein coupled transmembrane receptors that activate intracellular signaling cascades in response to the neurotransmitter, glutamate. These two classes of molecules have been studied independently and found to play important roles in regulating neuronal physiology with potential clinical implications for disorders such as depression, schizophrenia, Parkinson's and Alzheimer's disease. Here we show that mice lacking the retinoid X receptor subunit, RXRγ, exhibit impairments in group 1 mGluR-mediated electrophysiological responses at hippocampal Schaffer collateral-CA1 pyramidal cell synapses, including impaired group 1 mGluR-dependent long-term synaptic depression (LTD), reduced group 1 mGluR-induced calcium release, and loss of group 1 mGluR-activated voltage-sensitive currents. These animals also exhibit impairments in a subset of group 1 mGluR-dependent behaviors, including motor performance, spatial object recognition, and prepulse inhibition. Together, these observations demonstrate convergence between the RXRγ and group 1 mGluR signaling pathways that may function to coordinate their regulation of neuronal activity. They also identify RXRγ as a potential target for the treatment of disorders in which group 1 mGluR signaling has been implicated.

Suppression of TLR4/MyD88/TAK1/NF-κB/COX-2 Signaling Pathway in the Central Nervous System by Bexarotene, a Selective RXR Agonist, Prevents Hyperalgesia in the Lipopolysaccharide-Induced Pain Mouse Model

A selective RXR agonist, bexarotene, has been shown to have anti-inflammatory, anti-nociceptive, and neuroprotective effects in several models of numerous neurological diseases characterized by systemic inflammation. The mechanisms underlying these effects remains unknown. To elucidate these mechanisms, we investigated whether the TLR4/MyD88/TAK1/NF-κB/COX-2 signaling pathway in the CNS mediates the effect of bexarotene to prevent hyperalgesia in the LPS-induced inflammatory pain mouse model. The reaction time to thermal stimuli within 30 s was evaluated by the hot plate test in male mice treated with saline, LPS (10 mg/kg), DMSO, and/or bexarotene (0.1, 1, 3, or 10 mg/kg) after 6 h. The latency to the thermal stimulus (18.11 ± 1.36 s) in the LPS-treated mice was significantly decreased by 30% compared with saline-treated mice (25.84 ± 1.99 s). Treatment with bexarotene only at a dose of 10 mg/kg showed a significant increase in the latency by 22.49 ± 1.00 s compared with LPS-treated mice. Bexarotene also prevented the reduction in RXRα protein expression associated with a rise in the expression of TLR4, MyD88, phosphorylated TAK1, NF-κB p65, phosphorylated NF-κB p65, COX-2, and IL-1β proteins, in addition to COX-2 activity and levels of PGE2 and IL-1β in the brains and spinal cords of the LPS-treated animals. Likely, decreased activity of TLR4/MyD88/TAK1/NF-κB/COX-2 signaling pathway in addition to increased pro-inflammatory cytokine formation in the CNS of mice participates in the protective effect of bexarotene against hyperalgesia induced by LPS.

Serum 25OHD3 of Obese Mice Is Affected by Liver Injury and Correlates with Testosterone Levels and Sperm Motility

INTRODUCTION

The concentration of 25-hydroxycholecalciferol (25OHD3) in the serum of obese people is low and often accompanied by symptoms of low fertility. Therefore, vitamin D is recommended as a potential treatment option. However, after clinical trials, it was found that vitamin D cannot effectively increase the concentration of 25OHD3 in the serum of obese people. How obesity causes low 25OHD3 concentration and low fertility is unclear.

METHODS

We analyzed the physiological and pathological changes in obese mice induced by a high-fat diet (HFD) and the changes in mice after supplementing with 25OHD3.

RESULTS

The concentration of 25OHD3 in the serum of obese mice induced by HFD was significantly reduced, and these mice showed liver hypertrophy accompanied by abnormal liver injury, testicular hypertrophy, low testosterone levels, high leptin levels, and low sperm motility. The mRNA and protein expression of CYP2R1 of hydroxylated vitamin D3 was significantly reduced; CYP11A1 and CYP11A2, which synthesize testosterone, were significantly reduced. After supplementing with 25OHD3, there was an increase in serum 25OHD3 concentration, testosterone level, and sperm motility, but it cannot improve the degree of obesity, CYP2R1 expression, and liver damage.

CONCLUSION

Our research shows that there is a metabolic interference mediated by 25OHD3 and testosterone between obesity and low sperm motility. The results of this study can provide a scientific basis for studying the mechanism of 25OHD3 and hormone regulation and treating obese people with low sperm motility.

Centrosomal Localization of RXRα Promotes PLK1 Activation and Mitotic Progression and Constitutes a Tumor Vulnerability

Retinoid X receptor alpha (RXRα), a nuclear receptor of transcription factor, controls various physiological and pathological pathways including cellular growth, proliferation, differentiation, and apoptosis. Here, we report that RXRα is phosphorylated at its N-terminal A/B domain by cyclin-dependent kinase 1 (Cdk1) at the onset of mitosis, triggering its translocation to the centrosome, where phosphorylated-RXRα (p-RXRα) interacts with polo-like kinase 1 (PLK1) through its N-terminal A/B domain by a unique mechanism. The interaction promotes PLK1 activation, centrosome maturation, and mitotic progression. Levels of p-RXRα are abnormally elevated in cancer cell lines, during carcinogenesis in animals, and in clinical tumor tissues. An RXRα ligand XS060, which specifically inhibits p-RXRα/PLK1 interaction but not RXRα heterodimerization, promotes mitotic arrest and catastrophe in a tumor-specific manner. These findings unravel a transcription-independent action of RXRα at the centrosome during mitosis and identify p-RXRα as a tumor-specific vulnerability for developing mitotic drugs with improved therapeutic index.

Rosiglitazone binds to RXRα to induce RXRα tetramerization and NB4 cell differentiation

Rosiglitazone is a ligand of peroxisome proliferation-activated receptor gamma (PPARγ). However, it exerts biological activities and therapeutic effects through both PPARγ-dependent and independent mechanisms. In this study, we defined that rosiglitazone was also a ligand of retinoid X receptor alpha (RXRα) and displayed RXRα-dependent activities. We found that rosiglitazone directly bound to the ligand binding domain (LBD) of RXRα and induced RXRα/LBD tetramerization. Rosiglitazone inhibited the agonist-induced transcriptional activity of RXRα homodimers and heterodimers likely through inhibiting RXRα homo- and hetero-dimerization. In acute promyelocytic leukemia (APL) NB4 cells, rosiglitazone inhibited cell proliferation and induced cell differentiation, resulting from inhibiting RXRα/PML-RARα complex formation and down-regulating PML-RARα. Together, our study identified RXRα as a novel target of rosiglitazone and RXRα mediating the anti-APL activity of rosiglitazone.

Spp24 is associated with endocytic signalling, lipid metabolism, and discrimination of tissue integrity for 'leaky-gut' in inflammatory bowel disease

Epithelial barrier injury allows contaminants to cross-over into the blood stream and trigger an inflammatory response, contributing to inflammatory bowel disease (IBD). Currently there is no single test that can reliably diagnose intestinal mucosal barrier function or measure impaired epithelial cell integrity associated with increasing permeability. Here, we assess the association between serum proteins and small intestinal permeability as detected by confocal laser endomicroscopy (CLE); in particular the known IBD marker—secreted phosphoprotein 24 (SPP24) and its binding partners; and use developed monoclonal antibodies to assess the role of SPP24 in mucosal healing. Sera were obtained from 28 IBD patients and non-IBD controls undergoing CLE with scores ranging from low to high permeability, as well as active ulcerative colitis from 53 patients undergoing fecal microbiota transplant therapy (FMT). Higher permeability associated with altered lipid metabolism, heightened innate immune response and junctional protein signalling in UC patients. A correlation between increasing leak and SPP24 peptide was observed. There is a strong indication of the novel role of SPP24 in gut barrier dysfunction particularly in ulcerative colitis. Its correlation to the established CLE for monitoring permeability has the potential to provide a blood based parallel to monitor and guide therapy more readily across a broad spectrum of illnesses for which ‘leak’ dominates the pathology.

Retinoid X receptor agonists attenuates cardiomyopathy in streptozotocin-induced type 1 diabetes through LKB1-dependent anti-fibrosis effects

Diabetic cardiac fibrosis increases ventricular stiffness and facilitates the occurrence of diastolic dysfunction. Retinoid X receptor (RXR) plays an important role in cardiac development and has been implicated in cardiovascular diseases. In the present study, we investigated the effects of RXR agonist treatment on streptozotocin (STZ)-induced diabetic cardiomyopathy (DCM) and the underlying mechanism. Sprague-Dawley (SD) rats induced by STZ injection were treated with either RXR agonist bexarotene (Bex) or vehicle alone. Echocardiography was performed to determine cardiac structure and function. Cardiac fibroblasts (CFs) were treated with high glucose (HG) with or without the indicated concentration of Bex or the RXR ligand 9-cis-retinoic acid (9-cis-RA). The protein abundance levels were measured along with collagen, body weight (BW), blood biochemical indexes and transforming growth factor-β (TGF-β) levels. The effects of RXRα down-regulation by RXRα small interfering RNA (siRNA) were examined. The results showed that bexarotene treatment resulted in amelioration of left ventricular dysfunction by inhibiting cardiomyocyte apoptosis and myocardial fibrosis. Immunoblot with heart tissue homogenates from diabetic rats revealed that bexarotene activated liver kinase B1 (LKB1) signaling and inhibited p70 ribosomal protein S6 kinase (p70S6K). The increased collagen levels in the heart tissues of DCM rats were reduced by bexarotene treatment. Treatment of CFs with HG resulted in significantly reduced LKB1 activity and increased p70S6K activity. RXRα mediated the antagonism of 9-cis-RA on HG-induced LKB1/p70S6K activation changes in vitro. Our findings suggest that RXR agonist ameliorates STZ-induced DCM by inhibiting myocardial fibrosis via modulation of the LKB1/p70S6K signaling pathway. RXR agonists may serve as novel therapeutic agents for the treatment of DCM.

Steroid receptor/coactivator binding inhibitors: An update

The purpose of this review is to highlight recent developments in small molecules and peptides that block the binding of coactivators to steroid receptors. These coactivator binding inhibitors bind at the coregulator binding groove, also known as Activation Function-2, rather than at the ligand-binding site of steroid receptors. Steroid receptors that have been targeted with coactivator binding inhibitors include the androgen receptor, estrogen receptor and progesterone receptor. Coactivator binding inhibitors may be useful in some cases of resistance to currently prescribed therapeutics. The scope of the review includes small-molecule and peptide coactivator binding inhibitors for steroid receptors, with a particular focus on recent compounds that have been assayed in cell-based models.

Sulindac-derived retinoid X receptor-α modulator attenuates atherosclerotic plaque progression and destabilization in ApoE-/- mice

BACKGROUND AND PURPOSE

Atherosclerosis is a chronic inflammatory disease, and retinoid X receptor-α (RXRα) is an intriguing anti-atherosclerosis target. This study investigated whether and how an RXRα modulator, K-80003, derived from a non-steroidal anti-inflammatory drug attenuates atherosclerotic plaque progression and destabilization.

EXPERIMENTAL APPROACH

Our previously established ApoE^-/- mouse model of carotid vulnerable plaque progression was treated with K-80003 or vehicle for 4 or 8 weeks. Samples of carotid arteries and serum were collected to determine atherosclerotic lesion size, histological features, expression of related proteins, and lipid profiles. In vitro studies were carried out in 7-ketocholesterol (7-KC)-stimulated macrophages treated with or without K-80003.

KEY RESULTS

K-80003 significantly reduced lesion size, plaque rupture, macrophage infiltration, and inflammatory cytokine levels. Plaque macrophages positive for nuclear p65 (RelA) NF-κB subunit were markedly reduced after K-80003 treatment. Also, K-80003 treatment inhibited 7-KC-induced p65 nuclear translocation, IκBα degradation, and transcription of NF-κB target genes. In addition, K-80003 inhibited NF-κB pathway mainly through the reduction of p62/sequestosome 1 (SQSTM1), probably due to promotion of autophagic flux by K-80003. Mechanistically, cytoplasmic localization of RXRα was associated with decreased autophagic flux. Increasing cytoplasmic RXRα expression by overexpression of RXRα/385 mutant decreased autophagic flux in RAW264.7 cells. Finally, K-80003 strongly inhibited 7-KC-induced RXRα cytoplasmic translocation.

CONCLUSIONS AND IMPLICATIONS

K-80003 suppressed atherosclerotic plaque progression and destabilization by promoting macrophage autophagic flux and consequently inhibited the p62/SQSTM1-mediated NF-κB proinflammatory pathway. Thus, targeting RXRα-mediated autophagy-inflammation axis by its noncanonical modulator may represent a promising strategy to treat atherosclerosis.

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.

Oncogenic potential of truncated RXRα during colitis-associated colorectal tumorigenesis by promoting IL-6-STAT3 signaling

Retinoid X receptor-alpha (RXRα) is a potent regulator of inflammatory responses; however, its therapeutic potential for inflammatory cancer remains to be explored. We previously discovered that RXRα is abnormally cleaved in tumor cells and tissues, producing a truncated RXRα (tRXRα). Here, we show that transgenic expression of tRXRα in mice accelerates the development of colitis-associated colon cancer (CAC). The tumorigenic effect of tRXRα is primarily dependent on its expression in myeloid cells, which results in interleukin-6 (IL-6) induction and STAT3 activation. Mechanistic studies reveal an extensive interaction between tRXRα and TRAF6 in the cytoplasm of macrophages, leading to TRAF6 ubiquitination and subsequent activation of the NF-κB inflammatory pathway. K-80003, a tRXRα modulator derived from nonsteroidal anti-inflammatory drug (NSAID) sulindac, suppresses the growth of tRXRα-mediated colorectal tumor by inhibiting the NF-κB-IL-6-STAT3 signaling cascade. These results provide new insight into tRXRα action and identify a promising tRXRα ligand for treating CAC.

Retinoid X Receptor Alpha Nitro-ligand Z-10 and Its Optimized Derivative Z-36 Reduce β-Amyloid Plaques in Alzheimer's Disease Mouse Model

Bexarotene, an agonist of retinoid X receptor alpha (RXRα), has been shown to increase the expression of apoE, ABCA1, and ABCG1 by activating RXR/LXR and RXR/PPAR heterodimers, resulting in amyloid β (Aβ)-protein clearance in the brain of an Alzheimer's disease (AD) mouse model and reversal of mouse cognitive deficits. Nitrostyrene derivative Z-10 is the first identified nitro-ligand of RXRα. We hypothesized that Z-10 and its derivatives have the similar effect as bexarotene. A series of Z-10 derivatives were synthesized by introducing methoxyl, hydroxyl, and methoxy groups in 2- or 4-position of naphthalene ring, respectively. Our reporter gene assays showed that the derivatives with substituted groups of methyl and methoxyl in position 2 were more potent to activate Gal4-DBD/RXRα-LBD and RXRα homodimer as well as RXRα heterodimers than the corresponding 4-substituted derivatives. The derivatives with hydroxyl substitution in either 2- or 4-position failed to activate RXRα. Consistently, the derivatives with stronger potency of RXRα activation had higher RXRα binding affinity. Z-10 and its 2-ethyoxyl substituted derivative Z-36 reduced Aβ plaques in both hippocampus and cortex of AD mouse model significantly, of which Z-36 had stronger efficacy. This may due to the stronger ability of Z-36 than Z-10 in activating RXR/LXR and RXR/PPAR heterodimers and inducing ABCA1 and ABCG1 expressions. Thus, the 2- rather than 4-position was the better site for Z-10 modification as to RXRα transactivation, and Z-36 is an optimized derivative of Z-10 as to reducing Aβ plaques in AD mouse model.

Bexarotene, a Selective RXRα Agonist, Reverses Hypotension Associated with Inflammation and Tissue Injury in a Rat Model of Septic Shock

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors that can activate or inhibit the expression of many target genes by forming a heterodimer complex with the retinoid X receptor (RXR). The aim of this study was to investigate effects of bexarotene, a selective RXRα agonist, on the changes in renal, cardiac, hepatic, and pulmonary expression/activity of inducible nitric oxide synthase (iNOS) and cytochrome P450 (CYP) 4F6 in relation to PPARα/β/γ-RXRα heterodimer formation in a rat model of septic shock. Rats were injected with dimethyl sulfoxide or bexarotene 1 h after administration of saline or lipopolysaccharide (LPS). Mean arterial pressure (MAP) and heart rate (HR) were recorded from rats, which had received either saline or LPS before and after 1, 2, 3, and 4 h. Serum iNOS, LTB₄, myeloperoxidase (MPO), and lactate dehydrogenase (LDH) levels as well as tissue iNOS and CYP4F6 mRNA expression in addition to PPARα/β/γ and RXRα proteins were measured. LPS-induced decrease in MAP and increase in HR were associated with a decrease in PPARα/β/γ-RXRα heterodimer formation and CYP4F6 mRNA expression. LPS also caused an increase in systemic iNOS, LTB₄, MPO, and LDH levels as well as iNOS mRNA expression. Bexarotene at 0.1 mg/kg (i.p.) prevented the LPS-induced changes, except tachycardia. The results suggest that increased formation of PPARα/β/γ-RXRα heterodimers and CYP4F6 expression/activity in addition to decreased iNOS expression contributes to the beneficial effect of bexarotene to prevent the hypotension associated with inflammation and tissue injury during rat endotoxemia.

Retinoid X Receptor Antagonists

Retinoid X receptor (RXR) antagonists are not only useful as chemical tools for biological research, but are also candidate drugs for the treatment of various diseases, including diabetes and allergies, although no RXR antagonist has yet been approved for clinical use. In this review, we present a brief overview of RXR structure, function, and target genes, and describe currently available RXR antagonists, their structural classification, and their evaluation, focusing on the latest research.

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).

Expression and clinical significance of retinoid X receptor α in esophageal carcinoma

PURPOSE

Esophageal carcinoma (EC) is one of the most aggressive type cancers and dysregulation of retinoid X receptor α (RXRα) involves various tumors. However, the relationship of RXRα with the clinicopathological factors of EC, particularly prognostic characteristics, remains unclear. This present study was to evaluate the effect of RXRα expression in the development of EC.

METHODS

The mRNA and protein expression level of RXRα in EC and normal esophageal tissues using reverse transcription-polymerase chain reaction (RT-PCR) and Western blot, respectively. The subcellular localization was detected by immunohistochemistry (IHC) analysis. The clinicopathological parameters were included age, sex, tumor size, differentiation, TNM stages and lymph node metastasis. Kaplan-Meier method and Cox's regression analyses were performed to evaluate the prognosis of 60 patients with EC.

RESULTS

RXRα was elevated in EC tissues comparing with normal esophageal tissues at both mRNA and protein levels. The overexpression level of RXRα was closely associated to the tumor differentiation, TNM stage and lymph node metastasis of patients with EC. In addition, EC patients with RXRα high expression had significantly lower disease-free survival (DFS) and overall survival (OS). Multivariate analysis showed RXRα expression as an independent predictor for the DFS and OS rate of patients with EC.

CONCLUSIONS

Our results showed that overexpression of RXRα was correlated with unfavorable prognosis, suggesting that RXRα may serve as a potential targeted therapeutic marker in the treatment of EC.

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.

Endocrine-disrupting chemicals and fatty liver disease

A growing epidemic of nonalcoholic fatty liver disease (NAFLD) is paralleling the increase in the incidence of obesity and diabetes mellitus in countries that consume a Western diet. As NAFLD can lead to life-threatening conditions such as cirrhosis and hepatocellular carcinoma, an understanding of the factors that trigger its development and pathological progression is needed. Although by definition this disease is not associated with alcohol consumption, exposure to environmental agents that have been linked to other diseases might have a role in the development of NAFLD. Here, we focus on one class of these agents, endocrine-disrupting chemicals (EDCs), and their potential to influence the initiation and progression of a cascade of pathological conditions associated with hepatic steatosis (fatty liver). Experimental studies have revealed several potential mechanisms by which EDC exposure might contribute to disease pathogenesis, including the modulation of nuclear hormone receptor function and the alteration of the epigenome. However, many questions remain to be addressed about the causal link between acute and chronic EDC exposure and the development of NAFLD in humans. Future studies that address these questions hold promise not only for understanding the linkage between EDC exposure and liver disease but also for elucidating the molecular mechanisms that underpin NAFLD, which in turn could facilitate the development of new prevention and treatment opportunities.

Modulation of nongenomic activation of PI3K signalling by tetramerization of N-terminally-cleaved RXRα

Retinoid X receptor-alpha (RXRα) binds to DNA either as homodimers or heterodimers, but it also forms homotetramers whose function is poorly defined. We previously discovered that an N-terminally-cleaved form of RXRα (tRXRα), produced in tumour cells, activates phosphoinositide 3-kinase (PI3K) signalling by binding to the p85α subunit of PI3K and that K-80003, an anti-cancer agent, inhibits this process. Here, we report through crystallographic and biochemical studies that K-80003 binds to and stabilizes tRXRα tetramers via a ‘three-pronged’ combination of canonical and non-canonical mechanisms. K-80003 binding has no effect on tetramerization of RXRα, owing to the head–tail interaction that is absent in tRXRα. We also identify an LxxLL motif in p85α, which binds to the coactivator-binding groove on tRXRα and dissociates from tRXRα upon tRXRα tetramerization. These results identify conformational selection as the mechanism for inhibiting the nongenomic action of tRXRα and provide molecular insights into the development of RXRα cancer therapeutics.

The transcription factor retinoid X receptor-alpha (RXRα) can also form homotetramers. Here the authors show that the anti-cancer agent K-80003 selectively inhibits the nongenomic action of N-terminally-cleaved RXRα in tumour cells by stabilizing its tetramerization but not that of full-length RXRα.

Colorectal Carcinogenesis, Radiation Quality, and the Ubiquitin-Proteasome Pathway

Adult colorectal epithelium undergoes continuous renewal and maintains homeostatic balance through regulated cellular proliferation, differentiation, and migration. The canonical Wnt signaling pathway involving the transcriptional co-activator β-catenin is important for colorectal development and normal epithelial maintenance, and deregulated Wnt/β-catenin signaling has been implicated in colorectal carcinogenesis. Colorectal carcinogenesis has been linked to radiation exposure, and radiation has been demonstrated to alter Wnt/β-catenin signaling, as well as the proteasomal pathway involved in the degradation of the signaling components and thus regulation of β-catenin. The current review discusses recent progresses in our understanding of colorectal carcinogenesis in relation to different types of radiation and roles that radiation quality plays in deregulating β-catenin and ubiquitin-proteasome pathway (UPP) for colorectal cancer initiation and progression.