Retinoid X Receptor Agonists Research Articles

Abstract LB-170: A novel, nontoxic RXR agonist, UAB30, targets Fanconi anemia DNA repair pathway to act as potent cancer chemopreventive agent

Abstract UAB30, a synthetic analog of 9-cis-RA is a potent retinoid X receptors (RXR) agonist. It was found to have significantly low retinoic acid receptor (RAR)-binding activity. This unique property of UAB30 is responsible for its low lipid toxicity. Thus, it does not cause elevation in serum triglycerides both in humans and in experimental animals. These properties with favorable pharmacokinetics make it a suitable candidate for chronic use as chemopreventive agent. Here, we show that UAB30 is highly effective in inhibiting UVB-induced pathogenesis of skin cancers in Ptch1+/-/SKH-1 hairless mice. Similar to humans, this unique genetically engineered murine model develops both basal cell carcinomas (BCCs) and squamous cell carcinomas (SCCs) of the skin following chronic UVB irradiation. Compared to controls, the UAB30-treated group showed a 48% reduction in tumor number and a 78% reduction in tumor volume. The number of papillomas, BCCs, and SCCs were reduced by 33%, 74%, and 76%, respectively. UAB30 also diminished UVB-induced skin inflammation, hyperplasia, and enhanced differentiation of suprabasal epidermal cells. The expression of cyclins A, D1, D2, and D3, and cdc25A were decreased in UAB30 treated SCCs, while pro-apoptotic proteins, Bax and cleaved caspase 3 were increased. UAB30 when topically applied 30 min after each UVB irradiation for 30 weeks, it showed significant reduction in cancer development. A 57% reduction in tumor number and 72% reduction in tumor volume were observed. BCCs and SCCs were reduced by 55% and 60%, respectively. We also demonstrate that UAB30 acts by a unique molecular mechanism. Induction of lethal DNA double strand breaks (DSBs) characterized by the presence of p-γH2AX was observed following UAB30 treatment. These effects were more prominent in cancer cells than normal keratinocytes. Induction of DSBs was associated with the activation of DNA damage response (DDR) signaling leading to increased phosphorylation of ATM, Rad50, and Chk2. These effects paralleled with the reduction in cell cycle proteins and cell cycle arrest in G2/M phase followed by apoptosis of arrested cells. UAB30-induced DSBs and DDR were not dependent on ROS as we did not observe significant induction of ROS in UAB30-treated cells and did not notice any protective effect of N-acetyl cysteine. These effects were similar to DDR and changes in proliferation/apoptosis response observed following treatment with cisplatin which is known inducer of DSBs. Moreover, other RXR agonists were significantly less potent in inducing DSBs and DDR responses. The underlying mechanism for the accumulation of DSBs was the diminished ability of cancer cells to repair these DNA lesions. Reduction in FANCD2 and FANCC, the Fanconi anemia core complex proteins which are known to be involved in DSBs repair appear to be the cause of extensive DNA damage associated tumor cell death. Citation Format: Sandeep C. Chaudhary, Craig A. Elmets, Mohammad Athar. A novel, nontoxic RXR agonist, UAB30, targets Fanconi anemia DNA repair pathway to act as potent cancer chemopreventive agent [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr LB-170. doi:10.1158/1538-7445.AM2017-LB-170

IRX4204 in combination with erlotinib to target distinct pathways in lung cancer cells.

e14095 Background: IRX4204 is a Retinoid X Receptor (RXR) specific agonist (rexinoid). IRX4204 has high potency as a RXR agonist with low binding affinity to Retinoic Acid Receptors (RARs), PPAR, FXR or LXR. Our prior work with the less specific rexinoid bexarotene and erlotinib showed clinical activity in patients with lung cancer cases that harbored KRAS mutations. IRX4204 inhibits lung cancer cell proliferation and can chemoprevent carcinogen-induced lung cancers in mice. We sought to explore the underlying mechanisms engaged by the combination of IRX4204 and erlotinib. Materials and Methods: Human (H1703 and HOP62) and murine (ED1 and LKR13) lung cancer cell lines were treated with IRX4204, erlotinib, IRX4204 plus erlotinib, or vehicle. Reverse phase protein array (RPPA) and mRNA microarray analyses were performed to analyze comprehensively for differentially expressed growth regulatory proteins. Results: Combination of IRX4204 and erlotinib suppressed proliferation of both KRAS mutant (HOP62 and LKR13) and wild-type (H1703 and ED1) lung cancer cell lines. Additive effect was observed as compared to IRX4204 or erlotinib treatment alone. Combining IRX4204 with erlotinib markedly increased inhibition of specific therapeutic targets including Src, phosphorylated Akt and ribosomal S6 proteins. At the mRNA level, Ingenuity Pathway Analysis of species significantly increased or decreased by the combination treatment revealed multiple pathways related to oncogenic signaling. Specifically, we found in H1703 cell line regulation of Granzyme A and AMPK signaling and in HOP62 cell line inhibition of angiogenesis was implicated by altering TSP1 expression. Notably, in ED1 cell line PPARα/RXRα activation, PTEN signaling, PI3K/AKT signaling, TGF-β signaling, and AMPK signaling were each associated with effects of IRX4204 combined with erlotinib. Conclusions: Taken together, these data highlight specific mechanisms and candidate pharmacodynamic biomarkers of response to the combination of this rexinoid and EGFR-TKI in lung cancer. Based on these findings, a clinical trial (NCT02991651) with IRX4204 in combination with erlotinib is underway to treat patients with chemotherapy-refractory non-small cell lung cancer.

Retinoic acid ameliorates photoaged skin through RAR‑mediated pathway in mice.

Retinoic acid (RA), the bioactive metabolite of vitamin A, has demonstrated efficacy in the treatment of photoaged skin; however, the mechanism of action of RA remains unclear. The aim of the present study was to examine whether the therapeutic effects of RA on photoaged skin are mediated by retinoic acid receptor (RAR) and/or retinoid X receptor (RXR) in mice, and to investigate the underlying mechanism. Photoaged skin in Imprinting Control Region mice was induced by repeated exposure to ultraviolet (UV) irradiation. Mice were randomly divided into nine groups: Normal; UV control; all‑trans retinoic acid (ATRA); ATRA+RAR antagonist; ATRA+RXR antagonist; RAR agonist; RAR agonist+RAR antagonist; RXR agonist; and RXR agonist+RXR antagonist. Masson's trichrome staining was used to examine skin collagen fibers. Hydroxyproline assays were used to determine collagen content. The protein expression of matrix metalloproteinase (MMP)‑3, MMP‑13, type I procollagen, c‑Jun and c‑Fos was detected using western blot analysis. The results demonstrated that ATRA and RAR agonist ameliorated the UV‑induced damage to skin collagen fibers, and increased the collagen content in photoaged skin through RAR. Furthermore, ATRA and RAR agonist stimulated type I procollagen protein expression, and inhibited MMP‑3, MMP‑13 and c‑Jun protein expression through RAR in photoaged skin. However, ATRA and RAR agonist exhibited no significant effect on the protein expression of c‑Fos in photoaged skin. These findings suggest that RA ameliorates photoaged skin through a RAR‑mediated signaling pathway in mice.

Retinoids Regulate Adipogenesis Involving the TGFβ/SMAD and Wnt/β-Catenin Pathways in Human Bone Marrow Mesenchymal Stem Cells.

Retinoids may regulate cell differentiation as ligands of retinoic acid receptors (RARs) and/or retinoid X receptors (RXRs). We showed that RAR agonists promoted adipogenesis by upregulating the expression of CCAAT/enhancer-binding protein β (C/EBPβ) in the early stages, but blocked adipogenesis at a later stage in human bone marrow mesenchymal stem cells (hBMSCs). RXR agonists promoted adipogenesis at all time points in hBMSCs. The effect of RAR agonists was mediated mainly by the RARβ subtype. RAR agonists, in contrast to RXR agonists, significantly promoted the expression of RARβ. Knockdown of the RARβ gene via small hairpin RNA (shRNA) attenuated the inhibition of RAR agonists toward adipogenesis. Furthermore, we found that RAR agonists upregulated the transforming growth factor β (TGFβ)/SMAD pathway and Wnt/β-catenin pathway on adipogenesis in hBMSCs, and the stimulating effects were noticeably decreased with the RARβ gene knockdown. Both RAR agonists and RXR agonists inhibited adipogenesis and blocked the promoter activity of C/EBPβ and peroxisome proliferator-activated receptor γ (PPARγ) in SW872 cell. These results indicated the RAR agonists perform dual roles in adipogenesis in hBMSCs, and the TGFβ/SMAD pathway and Wnt/β-catenin pathway may involve the inhibitory effect of RAR agonists. RARβ is the main receptor subtype mediating the effect. The roles of RXR agonists in adipogenesis exhibited cell type-specific differences, and may be based on the integration of signals from different RXR dimers.

Retinoid X Receptor Ligands with Anti-Type 2 Diabetic Activity.

Peroxisome proliferator-activated receptor gamma (PPARγ) is a ligand-dependent transcription factor that plays an important role in regulating glucose metabolism. Agonists of PPARγ, such as thiazolidinediones, have anti-hyperglycemic activity, and are therefore used to treat type 2 diabetes. However, the functional activity of PPARγ is manifested by heterodimers of PPARγ with retinoid X receptors (RXRs). Since RXR/PPARγ heterodimers can be activated not only by PPARγ agonists, but also by RXR agonists, RXR agonists are also attractive candidates for treatment of type 2 diabetes. However, RXR full agonists have side effects, such as triglyceride elevation and hypothyroidism. Therefore, RXR partial agonists have been developed as new anti-type 2 diabetes agent candidates with reduced side effects. In addition, RXR antagonists also show therapeutic potency in type 2 diabetes patients. Here, we review RXR full agonists, RXR antagonists, and RXR modulators (partial agonists) with reported anti-diabetic effects, and we discuss their potential suitability as anti-diabetic agents.

Modeling, Synthesis, and Biological Evaluation of Potential Retinoid X Receptor (RXR)-Selective Agonists: Analogues of 4-[1-(3,5,5,8,8-Pentamethyl-5,6,7,8-tetrahydro-2-naphthyl)ethynyl]benzoic Acid (Bexarotene) and 6-(Ethyl(5,5,8,8-tetrahydronaphthalen-2-yl)amino)nicotinic Acid (NEt-TMN).

Sulfonic acid analogues of 4-[1-(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydro-2-naphthyl)ethynyl]benzoic acid (bexarotene, 1) as well as seven novel and two reported analogues of 6-(ethyl(5,5,8,8-tetramethyl-5,6,7,8-tetrahydronaphthalen-2-yl)amino)nicotinic acid (NEt-TMN) were synthesized and assessed for selective retinoid X receptor (RXR) agonism. Compound 1 is FDA-approved for treatment of cutaneous T-cell lymphoma (CTCL); however, 1 can provoke side effects by impacting RXR-dependent receptor pathways. All of the analogues in this study were evaluated for their potential to bind RXR through modeling and then assayed in an RXR-RXR mammalian-2-hybrid (M2H) system and in RXR-responsive element (RXRE)-mediated transcriptional experiments. The EC50 profiles for these unique analogues and their analogous effectiveness to inhibit proliferation in CTCL cells relative to 1 suggest that these compounds possess similar or even enhanced therapeutic potential. Several compounds also displayed more selective RXR activation with minimal cross-signaling of the retinoic acid receptor. These results suggest that modifications of potent RXR agonists such as NEt-TMN can lead to improved biological selectivity and potency compared with the known therapeutic.

Coadministration of VDR and RXR agonists synergistically alleviates atherosclerosis through inhibition of oxidative stress: An in vivo and in vitro study

Background and aimsDiabetes contributes to atherosclerosis partially through induction of oxidative stress. Both vitamin D receptor (VDR) and retinoid X receptor (RXR) agonists exhibit anti-atherogenic effects. MethodsWe explored the effects of combination treatment with VDR and RXR agonists (represented by calcitriol and bexarotene, respectively) on atherosclerosis progression and the mechanisms involved, using a diabetes model of mice. The animals were intragastrically fed calcitriol (200 ng/kg, twice-a-week), bexarotene (10 mg/kg, once-daily) either alone or in combination for 12 weeks. ResultsVDR and RXR agonists delayed atherosclerosis progression independent of serum lipid and glucose levels, and significantly reduced the protein expression of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase subunit gp91phox and nuclear factor-kappa B (NF-κB) subunit p65, as well as plasma biomarkers of oxidative stress and inflammation. Combination therapy alleviated atherosclerosis and inhibited indexes of oxidative stress and inflammation to a greater extent than either monotherapy. In the in vitro study, naturally occurring VDR ligand 1α,25-dihydroxyvitamin D3 (1,25[OH]2D3) and RXR ligand 9-cis retinoic acid (9-cis-RA), both significantly inhibited high-glucose-induced endothelial cell apoptosis. Co-administration of VDR and RXR ligands produced synergistic protection against endothelial apoptosis by antagonizing the protein kinase C /NADPH oxidase/reactive oxygen species pathway. The inhibitory effects of 9-cis-RA on oxidative stress was attenuated when VDR was downregulated by VDR siRNA; however, downregulation of RXR by RXR siRNA imposed no influence on the effects of 1,25(OH)2D3. ConclusionsCombination treatment with VDR and RXR agonists synergistically alleviated diabetic atherosclerosis through inhibition of oxidative stress, and the preventive effects of RXR agonist may partially depend on VDR activation.

Effect of LXR/RXR agonism on brain and CSF Aβ40 levels in rats.

Alzheimer's disease (AD) is characterized pathologically by the presence of amyloid plaques and neurofibrillary tangles. The amyloid hypothesis contends that the abnormal accumulation of Aβ, the principal component of amyloid plaques, plays an essential role in initiating the disease. Impaired clearance of soluble Aβ from the brain, a process facilitated by apolipoprotein E (APOE), is believed to be a contributing factor in plaque formation. APOE expression is transcriptionally regulated through the action of a family of nuclear receptors including the peroxisome proliferator-activated receptor gamma and liver X receptors (LXRs) in coordination with retinoid X receptors (RXRs). It has been previously reported that various agonists of this receptor family can influence brain Aβ levels in rodents. In this study we investigated the effects of LXR/RXR agonism on brain and cerebrospinal fluid (CSF) levels of Aβ40 in naïve rats. Treatment of rats for 3 days or 7 days with the LXR agonist, T0901317 or the RXR agonist, bexarotene did not result in significant changes in brain or CSF Aβ40 levels.

Effect of LXR/RXR agonism on brain and CSF Aβ40 levels in rats

Alzheimer's disease (AD) is characterized pathologically by the presence of amyloid plaques and neurofibrillary tangles. The amyloid hypothesis contends that the abnormal accumulation of Aβ, the principal component of amyloid plaques, plays an essential role in initiating the disease. Impaired clearance of soluble Aβ from the brain, a process facilitated by apolipoprotein E (APOE), is believed to be a contributing factor in plaque formation. APOE expression is transcriptionally regulated through the action of a family of nuclear receptors including the peroxisome proliferator-activated receptor gamma and liver X receptors (LXRs) in coordination with retinoid X receptors (RXRs). It has been previously reported that various agonists of this receptor family can influence brain Aβ levels in rodents. In this study we investigated the effects of LXR/RXR agonism on brain and cerebrospinal fluid (CSF) levels of Aβ40 in naïve rats. Treatment of rats for 3 days or 7 days with the LXR agonist, T0901317 or the RXR agonist, bexarotene did not result in significant changes in brain or CSF Aβ40 levels.

Effect of LXR/RXR agonism on brain and CSF Aβ40 levels in rats.

Alzheimer's disease (AD) is characterized pathologically by the presence of amyloid plaques and neurofibrillary tangles. The amyloid hypothesis contends that the abnormal accumulation of Aβ, the principal component of amyloid plaques, plays an essential role in initiating the disease. Impaired clearance of soluble Aβ from the brain, a process facilitated by apolipoprotein E (APOE), is believed to be a contributing factor in plaque formation. APOE expression is transcriptionally regulated through the action of a family of nuclear receptors including the peroxisome proliferator-activated receptor gamma and liver X receptors (LXRs) in coordination with retinoid X receptors (RXRs). It has been previously reported that various agonists of this receptor family can influence brain Aβ levels in rodents. In this study we investigated the effects of LXR/RXR agonism on brain and cerebrospinal fluid (CSF) levels of Aβ40 in naïve rats. Treatment of rats for 3 days or 7 days with the LXR agonist, TO901317 or the RXR agonist, Bexarotene did not result in significant changes in brain or CSF Aβ40 levels.

Double-blind, placebo-controlled, proof-of-concept trial of bexarotene Xin moderate Alzheimer's disease.

BackgroundWe assessed the impact of retinoid X receptor (RXR) agonist bexarotene on brain amyloid measured by amyloid imaging in patients with Alzheimer’s disease (AD) in a proof-of-concept trial.MethodsTwenty patients with AD [Mini Mental State Examination (MMSE) score 10–20 inclusive] with positive florbetapir scans were randomized to receive 300 mg of bexarotene or placebo for 4 weeks. The amyloid imaging result was the primary outcome. Whole-population analyses and prespecified analyses by genotype [apolipoprotein E ε4 (ApoE4) carriers and ApoE4 noncarriers] were conducted. Secondary outcomes included scores on the Alzheimer’s Disease Assessment Scale–Cognitive subscale, Alzheimer’s Disease Cooperative Study–Activities of Daily Living scale, MMSE, Clinical Dementia Rating scale, and Neuropsychiatric Inventory. Serum amyloid-β (Aβ) peptide sequences Aβ1–40 and Aβ1–42 measurements were collected as biomarker outcomes.ResultsThere was no change in the composite or regional amyloid burden when all patients were included in the analysis. ApoE4 noncarriers showed a significant reduction in brain amyloid on the composite measure in five of six regional measurements. No change in amyloid burden was observed in ApoE4 carriers. There was a significant association between increased serum Aβ1–42 and reductions in brain amyloid in ApoE4 noncarriers (not in carriers). There were significant elevations in serum triglycerides in bexarotene-treated patients. There was no consistent change in any clinical measure.ConclusionsThe primary outcome of this trial was negative. The data suggest that bexarotene reduced brain amyloid and increased serum Aβ1–42 in ApoE4 noncarriers. Elevated triglycerides could represent a cardiovascular risk, and bexarotene should not be administered outside a research setting. RXR agonists warrant further investigations as AD therapies.Trial registrationClinicalTrials.gov identifier NCT01782742. Registered 29 January 2013.Electronic supplementary materialThe online version of this article (doi:10.1186/s13195-016-0173-2) contains supplementary material, which is available to authorized users.

Preclinical PET Neuroimaging of [11C]Bexarotene.

Activation of retinoid X receptors (RXRs) has been proposed as a therapeutic mechanism for the treatment of neurodegeneration, including Alzheimer's and Parkinson's diseases. We previously reported radiolabeling of a Food and Drug Administration-approved RXR agonist, bexarotene, by copper-mediated [11C]CO2 fixation and preliminary positron emission tomography (PET) neuroimaging that demonstrated brain permeability in nonhuman primate with regional binding distribution consistent with RXRs. In this study, the brain uptake and saturability of [11C]bexarotene were studied in rats and nonhuman primates by PET imaging under baseline and greater target occupancy conditions. [11C]Bexarotene displays a high proportion of nonsaturable uptake in the brain and is unsuitable for RXR occupancy measurements in the central nervous system.

Effects of RXR Agonists on Cell Proliferation/Apoptosis and ACTH Secretion/Pomc Expression.

Various retinoid X receptor (RXR) agonists have recently been developed, and some of them have shown anti-tumor effects both in vivo and in vitro. However, there has been no report showing the effects of RXR agonists on Cushing’s disease, which is caused by excessive ACTH secretion in a corticotroph tumor of the pituitary gland. Therefore, we examined the effects of synthetic RXR pan-agonists HX630 and PA024 on the proliferation, apoptosis, ACTH secretion, and pro-opiomelanocortin (Pomc) gene expression of murine pituitary corticotroph tumor AtT20 cells. We demonstrated that both RXR agonists induced apoptosis dose-dependently in AtT20 cells, and inhibited their proliferation at their higher doses. Microarray analysis identified a significant gene network associated with caspase 3 induced by high dose HX630. On the other hand, HX630, but not PA024, inhibited Pomc transcription, Pomc mRNA expression, and ACTH secretion dose-dependently. Furthermore, we provide new evidence that HX630 negatively regulates the Pomc promoter activity at the transcriptional level due to the suppression of the transcription factor Nur77 and Nurr1 mRNA expression and the reduction of Nur77/Nurr1 heterodimer recruiting to the Pomc promoter region. We also demonstrated that the HX630-mediated suppression of the Pomc gene expression was exerted via RXRα. Furthermore, HX630 inhibited tumor growth and decreased Pomc mRNA expression in corticotroph tumor cells in female nude mice in vivo. Thus, these results indicate that RXR agonists, especially HX630, could be a new therapeutic candidate for Cushing’s disease.

Rescuing effects of RXR agonist bexarotene on aging-related synapse loss depend on neuronal LRP1

Apolipoprotein E (apoE) plays a critical role in maintaining synaptic integrity by transporting cholesterol to neurons through the low-density lipoprotein receptor related protein-1 (LRP1). Bexarotene, a retinoid X receptor (RXR) agonist, has been reported to have potential beneficial effects on cognition by increasing brain apoE levels and lipidation. To investigate the effects of bexarotene on aging-related synapse loss and the contribution of neuronal LRP1 to the pathway, forebrain neuron-specific LRP1 knockout (nLrp1−/−) and littermate control mice were administered with bexarotene-formulated diet (100mg/kg/day) or control diet at the age of 20–24months for 8weeks. Upon bexarotene treatment, levels of brain apoE and ATP-binding cassette sub-family A member 1 (ABCA1) were significantly increased in both mice. While levels of PSD95, glutamate receptor 1 (GluR1), and N-methyl-d-aspartate receptor NR1 subunit (NR1), which are key postsynaptic proteins that regulate synaptic plasticity, were decreased with aging, they were restored by bexarotene treatment in the brains of control but not nLrp1−/− mice. These results indicate that the beneficial effects of bexarotene on synaptic integrity depend on the presence of neuronal LRP1. However, we also found that bexarotene treatment led to the activation of glial cells, weight loss and hepatomegaly, which are likely due to hepatic failure. Taken together, our results demonstrate that apoE-targeted treatment through the RXR pathway has a potential beneficial effect on synapses during aging; however, the therapeutic application of bexarotene requires extreme caution due to its toxic side effects.

Activation of RXR/PPARγ underlies neuroprotection by bexarotene in ischemic stroke

The identification of novel drug targets for the treatment of ischemic stroke is currently an urgent challenge. Recent experimental findings have highlighted the neuroprotective potential of immunomodulatory strategies, based on polarization of myeloid cells toward non-inflammatory, beneficial phenotypes. Given the role of retinoid X receptors (RXR) in myeloid cells differentiation and polarization, here we have explored the neuroprotective potential of the RXR agonist bexarotene in mice subjected to focal cerebral ischemia.Acute administration of bexarotene significantly reduced blood brain barrier leakage, brain infarct damage and neurological deficit produced by transient middle cerebral artery occlusion in mice, without affecting cerebral blood flow. The rexinoid exerted neuroprotection with a wide time-window, being effective when administered up to 4.5h after the insult. The amelioration of histological outcome, as well as the ability of bexarotene to revert middle cerebral artery occlusion (MCAo)-induced spleen atrophy, was antagonised by BR1211, a pan-RXR antagonist, or by the selective peroxisome proliferator-activated receptor (PPAR)γ antagonist bisphenol A diglycidyl ether (BADGE), highlighting the involvement of the RXR/PPARγ heterodimer in the beneficial effects exerted by the drug. Immunofluorescence analysis revealed that bexarotene elevates Ym1-immunopositive N2 neutrophils both in the ipsilateral hemisphere and in the spleen of mice subjected to transient middle cerebral artery occlusion, pointing to a major role for peripheral neutrophil polarization in neuroprotection.Thus, our findings suggest that the RXR agonist bexarotene exerts peripheral immunomodulatory effects under ischemic conditions to be effectively repurposed for the acute therapy of ischemic stroke.

TSLP expression in the skin is mediated via RARγ-RXR pathways

TSLP is an important trigger and initiator for various atopic diseases mainly atopic dermatitis (AD). Activators of nuclear hormone receptors like bioactive vitamin A and D derivatives are known to induce TSLP up-regulation in the skin. In this study, various combinations of synthetic specific agonists and antagonists of the retinoic acid receptors (RARs), retinoid X receptors (RXRs) and vitamin D receptor (VDR) were topically administered to mice. The aim of the study was to elucidate via which nuclear hormone receptor pathways TSLP is regulated and how this regulation is connected to the development and phenotype of atopic dermatitis. TSLP expression was monitored using QRT-PCR and serum TSLP levels using ELISA. Synthetic agonists of the VDR and RARγ as well as the natural agonist all-trans retinoic acid (ATRA) increased TSLP expression in the skin, while an RXR agonist was not active. Treatments with antagonists of RXRs and RARs in addition to RARα-agonists reduced skin TSLP expression. Strong activation was found after a combination of a VDR and an RXR agonist (ca. 5 times induction) and even stronger by an RARγ and an RXR agonist treatment (ca. 48 times induction). We conclude that besides VDR-mediated signaling mainly RARγ-RXR mediated pathways in the skin are important patho-physiological triggers for increased skin TSLP expression. We conclude that topical synthesized retinoids stimulated by internal or external triggers or topically applied induce TSLP production and are thereby important triggers for atopic dermatitis prevalence.

Neuroprotective Effect of Bexarotene in the SOD1(G93A) Mouse Model of Amyotrophic Lateral Sclerosis.

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by progressive weakness and muscle atrophy related to the loss of upper and lower motor neurons (MNs) without a curative treatment. There is experimental evidence suggesting that retinoids may be involved in ALS pathogenesis. Bexarotene (Bxt) is a retinoid-X receptor agonist used in the treatment of cutaneous lymphoma with a favorable safety profile whose effects have been recently investigated in other neurodegenerative diseases. In this study, we analyze the potential therapeutic effect of Bxt in the SOD1G93A mouse model of ALS. Mice were treated with Bxt or vehicle five times per week from day 60 onward. Survival, weight, and neuromuscular function studies together with histological and biochemical analyses were performed. Bxt significantly delayed motor function deterioration, ameliorated the loss of body weight, and extended mice survival up to 30% of the symptomatic period. Histological analyses of the lumbosacral spinal cord revealed that Bxt markedly delayed the early motor-neuron degeneration occurring at presymptomatic stages in ALS-transgenic mice. Bxt treatment contributed to preserve the MN homeostasis in the SOD1G93A mice. Particularly, it reduced the neuronal loss and the chromatolytic response, induced nucleolar hypertrophy, decreased the formation of ubiquitylated inclusions, and modulated the lysosomal response. As an agonist of the retinoic-X receptor (RXR) pathway, Bxt notably increased the nuclear expression of the RXRα throughout transcriptionally active euchromatin domains. Bxt also contributed to protect the MN environment by reducing reactive astrogliosis and preserving perisomatic synapsis. Overall, these neuroprotective effects suggest that treatment with Bxt could be useful in ALS, particularly in those cases related to SOD1 mutations.

The Continuing Failure of Bexarotene in Alzheimer's Disease Mice.

Alzheimer's disease (AD) is the most common form of dementia characterized by synaptic dysfunction, memory loss, neuroinflammation, and neuronal cell death. Amyloid-β (Aβ), recognized as the main culprit of AD, aggregates and accumulates in the extracellular compartment as neuritic plaques, after deregulation of its production or clearance. Apolipoprotein E (ApoE) plays a major role in Aβ clearance and its expression is transcriptionally regulated by the liver X receptor and retinoid X receptors (RXRs) system. Bexarotene (BEXA), an RXR agonist that increases ApoE expression and microglia phagocytosis has been proposed as a promising therapy for AD, resolving both the amyloid pathology and memory loss. Despite the first compelling report, however, multiple failures have been documented, raising concern about whether BEXA could in fact become a novel disease-modifying strategy for AD. To help clarify this, we investigated the effect of BEXA in vivo at multiple levels in TASTPM transgenic mice. Seven-day oral administration of BEXA to these mice did not achieve any significant memory improvement, plaque reduction, or enhancement of microglial cell activation. No differences were found when specifically investigating the microglial phagocytic state in vivo. In addition, a brain structural analysis with magnetic resonance did not detect any BEXA-mediated change in the volume reduction of the main affected brain areas in our mice. These results suggest that BEXA has no beneficial effects on the multi-factorial pathologic phenotype of AD mice.

Vitamin D controls apoptosis and proliferation of cutaneous T‐cell lymphoma cells

Vitamin D controls apoptosis and proliferation of cutaneous T‐cell lymphoma cells

Tributyltin engages multiple nuclear receptor pathways and suppresses osteogenesis in bone marrow multipotent stromal cells.

Organotins are members of the environmental obesogen class of contaminants because they activate peroxisome proliferator-activated receptor γ (PPARγ), the essential regulator of adipogenesis. Exposure to thiazolidinediones (PPARγ ligands used to treat type 2 diabetes) is associated with increased fractures. Diminished bone quality likely results from PPARγ's role in promoting adipogenesis while suppressing osteogenesis of bone marrow multipotent mesenchymal stromal cells (BM-MSC). We hypothesized that tributyltin (TBT) would be a potent modifier of BM-MSC differentiation and a negative regulator of bone formation. Organotins interact with both PPARγ and retinoid X receptors (RXR), suggesting that they activate multiple nuclear receptor pathways. To investigate the role of RXR in the actions of TBT, the effects of PPARγ (rosiglitazone) and RXR (bexarotene, LG100268) agonists were compared to the effects of TBT in BMS2 cells and primary mouse BM-MSC cultures. In BMS2 cells, TBT induced the expression of Fabp4, Abca1, and Tgm2 in an RXR-dependent manner. All agonists suppressed osteogenesis in primary mouse BM-MSC cultures, based on decreased alkaline phosphatase activity, mineralization, and expression of osteoblast-related genes. While rosiglitazone and TBT strongly activated adipogenesis, based on lipid accumulation and expression of adipocyte-related genes, the RXR agonists did not. Extending these analyses to other RXR heterodimers showed that TBT and the RXR agonists activated the liver X receptor pathway, whereas rosiglitazone did not. Application of either a PPARγ antagonist (T0070907) or an RXR antagonist (HX531) significantly reduced rosiglitazone-induced suppression of bone nodule formation. Only the RXR antagonist significantly reduced LG100268- and TBT-induced bone suppression. The RXR antagonist also inhibited LG100268- and TBT-induced expression of Abca1, an LXR target gene, in primary BM-MSC cultures. These results provide novel evidence that TBT activates multiple nuclear receptor pathways in BM-MSCs, activation of RXR is sufficient to suppress osteogenesis, and TBT suppresses osteogenesis largely through its direct interaction with RXR.