Retinoid X Receptor Selective Agonists and their Synthetic Methods

The retinoid X receptor has a critical role in synthetic rexinoid-induced increase in cellular all-trans-retinoic acid

Rexinoids are agonists of nuclear rexinoid X receptors (RXR) that heterodimerize with other nuclear receptors to regulate gene transcription. A number of selective RXR agonists have been developed for clinical use but their application has been hampered by the unwanted side effects associated with the use of rexinoids and a limited understanding of their mechanisms of action across different cell types. Our previous studies showed that treatment of organotypic human epidermis with the low toxicity UAB30 and UAB110 rexinoids resulted in increased steady-state levels of all-trans-retinoic acid (ATRA), the obligatory ligand of the RXR-RAR heterodimers. Here, we investigated the molecular mechanism underlying the increase in ATRA levels using a dominant negative RXRα that lacks the activation function 2 (AF-2) domain. The results demonstrated that overexpression of dnRXRα in human organotypic epidermis markedly reduced signaling by resident ATRA, suggesting the existence of endogenous RXR ligand, diminished the biological effects of UAB30 and UAB110 on epidermis morphology and gene expression, and nearly abolished the rexinoid-induced increase in ATRA levels. Global transcriptome analysis of dnRXRα-rafts in comparison to empty vector-transduced rafts showed that over 95% of the differentially expressed genes in rexinoid-treated rafts constitute direct or indirect ATRA-regulated genes. Thus, the biological effects of UAB30 and UAB110 are mediated through the AF-2 domain of RXRα with minimal side effects in human epidermis. As ATRA levels are known to be reduced in certain epithelial pathologies, treatment with UAB30 and UAB110 may represent a promising therapy for normalizing the endogenous ATRA concentration and signaling in epithelial tissues.

Delineating the role of nuclear receptors in colorectal cancer, a focused review

Colorectal cancer (CRC) stands as one of the most prevalent form of cancer globally, causing a significant number of deaths, surpassing 0.9 million in the year 2020. According to GLOBOCAN 2020, CRC ranks third in incidence and second in mortality in both males and females. Despite extensive studies over the years, there is still a need to establish novel therapeutic targets to enhance the patients' survival rate in CRC. Nuclear receptors (NRs) are ligand-activated transcription factors (TFs) that regulate numerous essential biological processes such as differentiation, development, physiology, reproduction, and cellular metabolism. Dysregulation and anomalous expression of different NRs has led to multiple alterations, such as impaired signaling cascades, mutations, and epigenetic changes, leading to various diseases, including cancer. It has been observed that differential expression of various NRs might lead to the initiation and progression of CRC, and are correlated with poor survival outcomes in CRC patients. Despite numerous studies on the mechanism and role of NRs in this cancer, it remains of significant scientific interest primarily due to the diverse functions that various NRs exhibit in regulating key hallmarks of this cancer. Thus, modulating the expression of NRs with their agonists and antagonists, based on their expression levels, holds an immense prospect in the diagnosis, prognosis, and therapeutical modalities of CRC. In this review, we primarily focus on the role and mechanism of NRs in the pathogenesis of CRC and emphasized the significance of targeting these NRs using a variety of agents, which may represent a novel and effective strategy for the prevention and treatment of this cancer.

FXR Friend-ChIPs in the Enterohepatic System

Chronic liver diseases encompass a wide spectrum of hepatic maladies that often result in cholestasis or altered bile acid secretion and regulation. Incidence and cost of care for many chronic liver diseases are rising in the United States with few Food and Drug Administration-approved drugs available for patient treatment. Farnesoid X receptor (FXR) is the master regulator of bile acid homeostasis with an important role in lipid and glucose metabolism and inflammation. FXR has served as an attractive target for management of cholestasis and fibrosis; however, global FXR agonism results in adverse effects in liver disease patients, severely affecting quality of life. In this review, we highlight seminal studies and recent updates on the FXR proteome and identify gaps in knowledge that are essential for tissue-specific FXR modulation. In conclusion, one of the greatest unmet needs in the field is understanding the underlying mechanism of intestinal versus hepatic FXR function.

Computational refinement identifies functional destructive single nucleotide polymorphisms associated with human retinoid X receptor gene

Alterations in the nuclear retinoid X receptor (RXRs) signalling have been implicated in neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, stroke, multiple sclerosis and glaucoma. Single nucleotide polymorphisms (SNPs) are the main cause underlying single nucleic acid variations which in turn determine heterogeneity within various populations. These genetic polymorphisms have been suggested to associate with various degenerative disorders in population-wide analysis. This bioinformatics study was designed to investigate, search, retrieve and identify deleterious SNPs which may affect the structure and function of various RXR isoforms through a computational and molecular modelling approach. Amongst the 1,813 retrieved SNPs several were found to be deleterious with rs140464195_G139R, rs368400425_R358W and rs368586400_L383F RXRα mutant variants being the most detrimental ones causing changes in the interatomic interactions and decreasing the flexibility of the mutant proteins. Molecular genetics analysis identified seven missense mutations in RXRα/β/γ isoforms. Two novel mutations SNP IDs (rs1588299621 and rs1057519958) were identified in RXRα isoform. We used several in silico prediction tools such as SIFT, PolyPhen, I-Mutant, Protein Variation Effect Analyzer (PROVEAN), PANTHER, SNP&Go, PhD-SNP and SNPeffect to predict pathogenicity and protein stability associated with RXR mutations. The structural assessment by DynaMut tool revealed that hydrogen bonds were affected along with hydrophobic and carbonyl interactions resulting in reduced flexibility at the mutated residue positions but ultimately stabilizing the molecule as a whole. Summarizing, analysis of the missense mutations in RXR isoforms showed a mix of conclusive and inconclusive genotype-phenotype correlations suggesting the use of sophisticated computational analysis tools for studying RXR variants.Communicated by Ramaswamy H. Sarma.

Mutation of two residues converts the ligand-binding domain of RXRα into a uniform monomer without impairing the binding of retinoic acid and cofactors

Retinoid X receptor (RXRα) is a nuclear receptor (NR) for retinoic acid (RA) and regulates various NR signaling pathways. Ligand-binding domain (LBD) of RXRα can bind with its ligand 9-cis-RA and cofactors, and mediate the forming of homodimer and homotetramer of RXRα and its heterodimer with other NRs, conferring RXRα the ability to play complicated roles in development and diseases. Due to the coexistence of monomer, dimer and tetramer, there are difficulties to study the structure and interaction of RXRα-LBD with its ligands and cofactors in solution and to distinguish the roles of different forms of RXRα in cell. Here, through analyzing available structures of RXRα-LBD, we selected two residues, D379 and L420, in the homodimer interface to design three mutants of RXRα-LBD. Recombinant proteins of the three mutants showed decreased proportions of dimer and tetramer but unchanged overall structure and binding affinities to 9-cis-RA, corepressor SMRT, and coactivator SRC2. Especially, the double-site mutant RXRα-LBD^D379A-L420G existed as a uniform monomer. Furthermore, L420 was found to play a similar role in forming RXRα-LBD homodimer and its heterodimer with various NRs, while the role of D379 varies a lot, as it shows almost no interaction with RARα/β, LXRα/β, and THRα/β. This study provides a new insight into the mechanism for forming RXRα-LBD homodimer and its heterodimer with other NRs, and will facilitate the studies on the structure and interaction of RXRα-LBD with ligands, cofactors and drugs in solution, and the broad physiological functions of RXRα cooperating with various NRs in cell.

Sulfuretin exerts diversified functions in the processing of amyloid precursor protein

Sulfuretin is a flavonoid that protects cell from damage induced by reactive oxygen species and inflammation. In this study, we investigated the role of sulfuretin in the processing of amyloid precursor protein (APP), in association with the two catalytic enzymes the α-secretase a disintegrin and metalloproteinase (ADAM10), and the beta-site APP cleaving enzyme 1 (BACE1) that play important roles in the generation of β amyloid protein (Aβ) in Alzheimer's disease (AD). We found that sulfuretin increased the levels of the immature but not the mature form of ADAM10 protein. The enhanced ADAM10 transcription by sulfuretin was mediated by the nucleotides −444 to −300 in the promoter region, and was attenuated by silencing or mutation of transcription factor retinoid X receptor (RXR) and by GW6471, a specific inhibitor of peroxisome proliferator-activated receptor α (PPAR-α). We further found that sulfuretin preferentially increased protein levels of the immature form of APP (im-APP) but significantly reduced those of BACE1, sAPPβ and β-CTF, whereas Aβ1-42 levels were slightly increased. Finally, the effect of sulfuretin on BACE1 and im-APP was selectively attenuated by the translation inhibitor cycloheximide and by lysosomal inhibitor chloroquine, respectively. Taken together, (1) RXR/PPAR-α signaling was involved in sulfuretin-mediated ADAM10 transcription. (2) Alteration of Aβ protein level by sulfuretin was not consistent with that of ADAM10 and BACE1 protein levels, but was consistent with the elevated level of im-APP protein, suggesting that im-APP, an isoform mainly localized to trans-Golgi network, plays an important role in Aβ generation.

Roles of nuclear receptors in hepatic stellate cells

Introduction: Hepatic stellate cells (HSCs) are essential for physiological homeostasis of the liver extracellular matrix (ECM). Excessive transdifferentiation of HSC from a quiescent to an activated phenotype contributes to disrupt this balance and can lead to liver fibrosis. Accumulating evidence has suggested that nuclear receptors (NRs) are involved in the regulation of HSC activation, proliferation, and function. Therefore, these NRs may be therapeutic targets to balance ECM homeostasis and inhibit HSC activation in liver fibrosis.Areas covered: In this review, the authors summarized the recent progress in the understanding of the regulatory role of NRs in HSCs and their potential as drug targets in liver fibrosis.Expert opinion: NRs are still potential therapy targets for inhibiting HSCs activation and liver fibrosis. However, the development of NRs agonists or antagonists to inhibit HSCs requires fully consideration of systemic effects.

Adjuvant effects of vitamin A and vitamin D supplementation on treatment of children with attention-deficit/hyperactivity disorder: a study protocol for a randomised, double-blinded, placebo-controlled, multicentric trial in China

INTRODUCTION

Approximately 7.2% of children in the world suffer from attention-deficit/hyperactivity disorder (ADHD). Due to the availability of the osmotic-release oral-system methylphenidate, ADHD currently has a remission rate of up to 30.72%. Nevertheless, it has been reported that patients with ADHD tend to exhibit vitamin A and vitamin D deficiency, which may aggravate the symptoms of ADHD. This study aims to determine the effect of vitamin A and vitamin D supplementation as adjunctive therapy to methylphenidate on the symptoms of ADHD.

METHODS AND ANALYSIS

This is a parallel, prospective, interventional multicentric study. Patients will be enrolled from the southern, central and northern parts of China. A target of 504 patients will be followed for 8 weeks. They will be allocated into three groups (vitamin AD, vitamin D and placebo) and administered the interventions accordingly. Data on changes in the symptoms of ADHD as well as changes in the serum concentrations of vitamin A and vitamin D will be recorded. Both responders and nonresponders based on the sociodemographic and clinical data will also be described to mitigate selection bias.

ETHICS AND DISSEMINATION

This study is performed in accordance with the Declaration of Helsinki and was approved by the Institutional Review Board of Children's Hospital of Chongqing Medical University, China (approval number: (2019) IRB (STUDY) number 262). The results of the trial will be reported in peer-reviewed scientific journals and academic conferences regardless of the outcomes.

TRIAL REGISTRATION NUMBER

NCT04284059.

Potential therapeutic uses of rexinoids

The discovery of nuclear receptors, particularly retinoid X receptors (RXR), and their involvement in numerous pathways related to development sparked interest in their immunomodulatory properties. Genetic models using deletion or overexpression of RXR and the subsequent development of several small molecules that are agonists or antagonists of this receptor support a promising therapeutic role for these receptors in immunology. Bexarotene was approved in 1999 for the treatment of cutaneous T cell lymphoma. Several other small molecule RXR agonists have since been synthesized with limited preclinical development, but none have yet achieved FDA approval. Cancer treatment has recently been revolutionized with the introduction of immune checkpoint inhibitors, but their success has been restricted to a minority of patients. This review showcases the emerging immunomodulatory effects of RXR and the potential of small molecules that target this receptor as therapies for cancer and other diseases. Here we describe the essential roles that RXR and partner receptors play in T cells, dendritic cells, macrophages and epithelial cells, especially within the tumor microenvironment. Most of these effects are site and cancer type dependent but skew immune cells toward an anti-inflammatory and anti-tumor effect. This beneficial effect on immune cells supports the promise of combining rexinoids with approved checkpoint blockade therapies in order to enhance efficacy of the latter and to delay or potentially eliminate drug resistance. The data compiled in this review strongly suggest that targeting RXR nuclear receptors is a promising new avenue in immunomodulation for cancer and other chronic inflammatory diseases.

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.

Constitutive Androstane Receptor-Mediated Inhibition of Metformin on Phase II Metabolic Enzyme SULT2A1

BACKGROUND

Metformin, as a first-line treatment for diabetes, interacts with many protein kinases and transcription factors which affect the expression of downstream target genes governing drug metabolism. Sulfotransferase, SULT2A1, one phase II metabolic enzyme, sulfonates both xenobiotic and endobiotic compounds to accelerate drug excretion. Herein, we designed experiments to investigate the effects and mechanisms of metformin on SULT2A1 expression in vitro.

METHODS

The hepatocellular carcinoma cell line, HepaRG, was cultured with different concentrations of metformin. The cell viability was measured using CCK8 kit. HepaRG was used to evaluate the protein expression of pregnane X receptor (PXR), the constitutive androstane receptor (CAR), SULT2A1, AMP-activated protein kinase (AMPK), and phosphorylation of AMPK (p-AMPK), respectively, at different concentrations of metformin with or without rifampin (human PXR activator) and CITCO (human CAR activator). The coregulators with CAR on SULT2A1 promoter response elements have also been characterized.

RESULTS

We showed that metformin did not affect the basic expression of SULT2A1 but could suppress the expression of SULT2A1 induced by the activator of human CAR. Investigations revealed that metformin which could block CAR nuclear translocation further suppress SULT2A1. In addition, we found that the prevented CAR transfer into the nucleus by metformin was partially an AMPK-dependent event.

CONCLUSION

The present study indicated that the activation of AMPK-CAR pathway mediated the suppression of SULT2A1 by metformin. Metformin may affect the metabolism and clearance of drugs which are SULT2A1 substrates. The results that emerged from this work provide substantial insights into an appropriate medication in the treatment of diabetes patients.

Natural and synthetic retinoids in preclinical colorectal cancer models

Colorectal cancer (CRC) remains a leading cause of cancer-related morbidity and mortality worldwide. Although targeted therapy in combination with chemotherapy in CRC prolongs the overall survival of patients with metastatic disease, acquired resistance and relapse hinder their clinical benefits. Moreover, patients with some specific genetic profile are unlikely to benefit from targeted therapy, suggesting the need for safe and effective treatment strategies. Retinoids, comprising of natural and synthetic analogs, are a class of chemical compounds that regulate cellular proliferation, differentiation, and cell death. Retinoids have been used in the clinic for several leukemias and solid tumors, either as single agents or in combination therapy. Furthermore, retinoids have shown potent chemotherapeutic and chemopreventive properties in different cancer models, including CRC. In this review, we summarize the major preclinical findings in CRC in which natural and synthetic retinoids showed promising antitumor activities and stress on the proposed mechanisms of action. Understanding of the retinoids' antitumor mechanisms would provide insights to support and warrant their development in the management of CRC.

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.

Widespread epigenetic changes to the enhancer landscape of mouse liver induced by a specific xenobiotic agonist ligand of the nuclear receptor CAR

CAR (Nr1i3), a liver nuclear receptor and xenobiotic sensor, induces drug, steroid and lipid metabolism and dysregulates genes linked to hepatocellular carcinogenesis, but its impact on the liver epigenome is poorly understood. TCPOBOP, a halogenated xenochemical and highly specific CAR agonist ligand, induces localized chromatin opening or closing at several thousand mouse liver genomic regions, discovered as differential DNase-hypersensitive sites (ΔDHS). Active enhancer and promoter histone marks induced by TCPOBOP were enriched at opening DHS and TCPOBOP-inducible genes. Enrichment of CAR binding and CAR motifs was seen at opening DHS and their inducible drug/lipid metabolism gene targets, and at many constitutively open DHS located nearby. TCPOBOP-responsive cell cycle and DNA replication genes co-dependent on MET/EGFR signaling for induction were also enriched for CAR binding. A subset of opening DHS and many closing DHS mapping to TCPOBOP-responsive target genes did not bind CAR, indicating an indirect mechanism for their changes in chromatin accessibility. TCPOBOP-responsive DHS were also enriched for induced binding of RXRA, CEBPA and CEBPB, and for motifs for liver-enriched factors that may contribute to liver-specific transcriptional responses to TCPOBOP exposure. These studies elucidate the enhancer landscape of TCPOBOP-exposed liver and the widespread epigenetic changes that are induced by both direct and indirect mechanisms linked to CAR activation. The global maps of thousands of environmental chemical-induced epigenetic changes described here constitute a rich resource for further research on xenochemical effects on liver chromatin states and the epigenome.

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.

A novel gene expression analytics-based approach to structure aided design of rexinoids for development as next-generation cancer therapeutics

Rexinoids are powerful ligands that bind to retinoid-X-receptors (RXRs) and show great promise as therapeutics for a wide range of diseases, including cancer. However, only one rexinoid, bexarotene (Targretin TM) has been successfully transitioned from the bench to the clinic and used to treat cutaneous T-cell lymphoma (CTCL). Our goal is to develop novel potent rexinoids with a less untoward side effect profile than bexarotene. To this end, we have synthesized a wide array of rexinoids with EC50 values and biological activity similar to bexarotene. In order to determine their suitability for additional downstream analysis, and to identify potential candidate analogs for clinical translation, we treated human CTCL cells in culture and employed microarray technology to assess gene expression profiles. We analyzed twelve rexinoids and found they could be stratified into three distinct categories based on their gene expression: similar to bexarotene, moderately different from bexarotene, and substantially different from bexarotene. Surprisingly, small changes in the structure of the bexarotene parent compound led to marked differences in gene expression profiles. Furthermore, specific analogs diverged markedly from our hypothesis in expression of genes expected to be important for therapeutic promise. However, promoter analysis of genes whose expression was analyzed indicates general regulatory trends along structural frameworks. Our results suggest that certain structural motifs, particularly the basic frameworks found in analog 4 and analog 9, represent important starting points to exploit in generating additional rexinoids for future study and therapeutic applications.

Decreased expression of peroxisome proliferator-activated receptor alpha indicates unfavorable outcomes in hepatocellular carcinoma

Introduction

Hepatocellular carcinoma (HCC) has a close relationship with lipid metabolism. Peroxisome proliferator-activated receptor α (PPARα) plays a crucial role in the regulation of fatty acid oxidation in the liver. However, the role of PPARα in HCC remains unclear.

Methods

A total of 804 HCC specimens were collected to construct a tissue microarray and for immunohistochemical analysis. The relationship between PPARα expression and clinical features of HCC patients was analyzed. Kaplan–Meier analysis was conducted to assess the prognostic value of PPARα expression levels.

Results

The expression of PPARα in HCC was noticeably decreased in HCC tissues. HCC patients with high levels of PPARα expression in cytoplasm had smaller tumors (P=0.027), less vascular invasion (P=0.049), and a higher proportion of complete involucrum (P=0.038). Kaplan–Meier analysis showed that HCC patients with low PPARα expression in the cytoplasm had significantly worse outcomes in terms of overall survival (P<0.001), disease-free survival (P=0.024), and the probability of recurrence (P=0.037). Similarly, overall survival was significantly shorter in HCC patients with negative PPARα expression in the nucleus (P=0.034). Multivariate Cox analyses indicated that tumor size (P=0.001), TNM stage (P<0.001), vascular invasion (P<0.001), and PPARα expression in the cytoplasm (P<0.001) were found to be independent prognostic variables for overall survival.

Conclusion

Our data revealed that PPARα expression was decreased in HCC samples. High PPARα expression was correlated with longer survival times in HCC patients, and served as an independent factor for better outcomes. Our study therefore provides a promising biomarker for prognostic prediction and a potential therapeutic target for HCC.

Effects of retinoic acid signaling on extraocular muscle myogenic precursor cells in vitro

One major difference between limb and extraocular muscles (EOM) is the presence of an enriched population of Pitx2-positive myogenic precursor cells in EOM compared to limb muscle. We hypothesize that retinoic acid regulates Pitx2 expression in EOM myogenic precursor cells and that its effects would differ in leg muscle. The two muscle groups expressed differential retinoic acid receptor (RAR) and retinoid X receptor (RXR) levels. RXR co-localized with the Pitx2-positive cells but not with those expressing Pax7. EOM-derived and LEG-derived EECD34 cells were treated with vehicle, retinoic acid, the RXR agonist bexarotene, the RAR inverse agonist BMS493, or the RXR antagonist UVI 3003. In vitro, fewer EOM-derived EECD34 cells expressed desmin and fused, while more LEG-derived cells expressed desmin and fused when treated with retinoic acid compared to vehicle. Both EOM and LEG-derived EECD34 cells exposed to retinoic acid showed a higher percentage of cells expressing Pitx2 compared to vehicle, supporting the hypothesis that retinoic acid plays a role in maintaining Pitx2 expression. We hypothesize that retinoic acid signaling aids in the maintenance of large numbers of undifferentiated myogenic precursor cells in the EOM, which would be required to maintain EOM normalcy throughout a lifetime of myonuclear turnover.