A coordinated phosphorylation cascade initiated by p38MAPK/MSK1 directs RARalpha to target promoters

Non-canonical retinoid signaling in neural development, regeneration and synaptic function

Canonical retinoid signaling via nuclear receptors and gene regulation is critical for the initiation of developmental processes such as cellular differentiation, patterning and neurite outgrowth, but also mediates nerve regeneration and synaptic functions in adult nervous systems. In addition to canonical transcriptional regulation, retinoids also exert rapid effects, and there are now multiple lines of evidence supporting non-canonical retinoid actions outside of the nucleus, including in dendrites and axons. Together, canonical and non-canonical retinoid signaling provide the precise temporal and spatial control necessary to achieve the fine cellular coordination required for proper nervous system function. Here, we examine and discuss the evidence supporting non-canonical actions of retinoids in neural development and regeneration as well as synaptic function, including a review of the proposed molecular mechanisms involved.

Retinoic acid signaling in development and differentiation commitment and its regulatory topology

Retinoic acid (RA), the derivative of vitamin A/retinol, is a signaling molecule with important implications in health and disease. It is a well-known developmental morphogen that functions mainly through the transcriptional activity of nuclear RA receptors (RARs) and, uncommonly, through other nuclear receptors, including peroxisome proliferator-activated receptors. Intracellular RA is under spatiotemporally fine-tuned regulation by synthesis and degradation processes catalyzed by retinaldehyde dehydrogenases and P450 family enzymes, respectively. In addition to dictating the transcription architecture, RA also impinges on cell functioning through non-genomic mechanisms independent of RAR transcriptional activity. Although RA-based differentiation therapy has achieved impressive success in the treatment of hematologic malignancies, RA also has pro-tumor activity. Here, we highlight the relevance of RA signaling in cell-fate determination, neurogenesis, visual function, inflammatory responses and gametogenesis commitment. Genetic and post-translational modifications of RAR are also discussed. A better understanding of RA signaling will foster the development of precision medicine to improve the defects caused by deregulated RA signaling.

Multidimensional scaling methods can reconstruct genomic DNA loops using Hi-C data properties

This paper proposes multidimensional scaling (MDS) applied to high-throughput chromosome conformation capture (Hi-C) data on genomic interactions to visualize DNA loops. Currently, the mechanisms underlying the regulation of gene expression are poorly understood, and where and when DNA loops are formed remains undetermined. Previous studies have focused on reproducing the entire three-dimensional structure of chromatin; however, identifying DNA loops using these data is time-consuming and difficult. MDS is an unsupervised method for reconstructing the original coordinates from a distance matrix. Here, MDS was applied to high-throughput chromosome conformation capture (Hi-C) data on genomic interactions to visualize DNA loops. Hi-C data were converted to distances by taking the inverse to reproduce loops via MDS, and the missing values were set to zero. Using the converted data, MDS was applied to the log-transformed genomic coordinate distances and this process successfully reproduced the DNA loops in the given structure. Consequently, the reconstructed DNA loops revealed significantly more DNA-transcription factor interactions involved in DNA loop formation than those obtained from previously applied methods. Furthermore, the reconstructed DNA loops were significantly consistent with chromatin immunoprecipitation followed by sequencing (ChIP-seq) peak positions. In conclusion, the proposed method is an improvement over previous methods for identifying DNA loops.

CK2 promotes jasmonic acid signaling response by phosphorylating MYC2 in Arabidopsis

Jasmonic acid (JA) signaling plays a pivotal role in plant development and defense. MYC2 is a master transcription factor in JA signaling, and was found to be phosphorylated and negatively regulated by MAP kinase and receptor-like kinase. However, the kinases that positively regulate MYC2 through phosphorylation and promote MYC2-mediated activation of JA response have not been identified. Here, we identified CK2 as a kinase that phosphorylates MYC2 and thus regulates the JA signaling. CK2 holoenzyme can interact with MYC2 using its regulatory subunits and phosphorylate MYC2 at multiple sites with its catalytic subunits. Inhibition of CK2 activity in a dominant-negative plant line, CK2mut, repressed JA response. On the other hand, increasing CK2 activity by overexpression of CKB4, a regulatory subunit gene of CK2, enhanced JA response in a MYC2-dependent manner. Substitution of the Ser and Thr residues at phosphorylation sites of MYC2 by CK2 with Ala impaired MYC2 function in activating JA response. Further investigations evidenced that CK2 facilitated the JA-induced increase of MYC2 binding to the promoters of JA-responsive genes in vivo. Our study demonstrated that CK2 plays a positive role in JA signaling, and reveals a previously undiscovered mechanism that regulates MYC2 function.

Treponema denticola Induces Interleukin-36γ Expression in Human Oral Gingival Keratinocytes via the Parallel Activation of NF-κB and Mitogen-Activated Protein Kinase Pathways

The oral epithelial barrier acts as both a physical barrier to the abundant oral microbiome and a sentry for the immune system that, in health, constrains the accumulation of the polymicrobial plaque biofilm. The immune homeostasis during gingivitis that is largely protective becomes dysregulated, unproductive, and destructive to gingival tissue as periodontal disease progresses to periodontitis. The progression to periodontitis is associated with the dysbiosis of the oral microbiome, with increasing prevalences and abundances of periodontal pathogens such as Treponema denticola. Despite the association of T. denticola with a chronic inflammatory disease, relatively little is known about gingival epithelial cell responses to T. denticola infection. Here, we characterized the transcriptome of gingival keratinocytes following T. denticola challenge and identified interleukin-36γ (IL-36γ) as the most differentially expressed cytokine. IL-36γ expression is regulated by p65 NF-κB and the activation of both the Jun N-terminal protein kinase (JNK) and p38 mitogen-activated protein kinase (MAPK) pathways downstream of Toll-like receptor 2 (TLR2). Finally, we demonstrate for the first time that mitogen- and stress-activated kinase 1 (MSK1) contributes to IL-36γ expression and may link the activation of MAPK and NF-κB signaling. These findings suggest that the interactions of T. denticola with the gingival epithelium lead to elevated IL-36γ expression, which may be a critical inducer and amplifier of gingival inflammation and subsequent alveolar bone loss.

Mechanisms of Feedback Regulation of Vitamin A Metabolism

Vitamin A is an essential nutrient required throughout life. Through its various metabolites, vitamin A sustains fetal development, immunity, vision, and the maintenance, regulation, and repair of adult tissues. Abnormal tissue levels of the vitamin A metabolite, retinoic acid, can result in detrimental effects which can include congenital defects, immune deficiencies, proliferative defects, and toxicity. For this reason, intricate feedback mechanisms have evolved to allow tissues to generate appropriate levels of active retinoid metabolites despite variations in the level and format, or in the absorption and conversion efficiency of dietary vitamin A precursors. Here, we review basic mechanisms that govern vitamin A signaling and metabolism, and we focus on retinoic acid-controlled feedback mechanisms that contribute to vitamin A homeostasis. Several approaches to investigate mechanistic details of the vitamin A homeostatic regulation using genomic, gene editing, and chromatin capture technologies are also discussed.

Revisiting APP secretases: an overview on the holistic effects of retinoic acid receptor stimulation in APP processing

Alzheimer's disease (AD) is the leading cause of dementia worldwide and is characterized by the accumulation of the β-amyloid peptide (Aβ) in the brain, along with profound alterations in phosphorylation-related events and regulatory pathways. The production of the neurotoxic Aβ peptide via amyloid precursor protein (APP) proteolysis is a crucial step in AD development. APP is highly expressed in the brain and is complexly metabolized by a series of sequential secretases, commonly denoted the α-, β-, and γ-cleavages. The toxicity of resulting fragments is a direct consequence of the first cleaving event. β-secretase (BACE1) induces amyloidogenic cleavages, while α-secretases (ADAM10 and ADAM17) result in less pathological peptides. Hence this first cleavage event is a prime therapeutic target for preventing or reverting initial biochemical events involved in AD. The subsequent cleavage by γ-secretase has a reduced impact on Aβ formation but affects the peptides' aggregating capacity. An array of therapeutic strategies are being explored, among them targeting Retinoic Acid (RA) signalling, which has long been associated with neuronal health. Additionally, several studies have described altered RA levels in AD patients, reinforcing RA Receptor (RAR) signalling as a promising therapeutic strategy. In this review we provide a holistic approach focussing on the effects of isoform-specific RAR modulation with respect to APP secretases and discuss its advantages and drawbacks in subcellular AD related events.

All trans retinoic acid as a host-directed immunotherapy for tuberculosis

Tuberculosis (TB) is the top bacterial infectious disease killer and one of the top ten causes of death worldwide. The emergence of strains of multiple drug-resistant tuberculosis (MDR-TB) has pushed our available stock of anti-TB agents to the limit of effectiveness. This has increased the urgent need to develop novel treatment strategies using currently available resources. An adjunctive, host-directed therapy (HDT) designed to act on the host, instead of the bacteria, by boosting the host immune response through activation of intracellular pathways could be the answer. The integration of multidisciplinary approaches of repurposing currently FDA-approved drugs, with a targeted drug-delivery platform is a very promising option to reduce the long timeline associated with the approval of new drugs - time that cannot be afforded given the current levels of morbidity and mortality associated with TB infection. The deficiency of vitamin A has been reported to be highly associated with the increased susceptibility of TB. All trans retinoic acid (ATRA), the active metabolite of vitamin A, has proven to be very efficacious against TB both in vitro and in vivo. In this review, we discuss and summarise the importance of vitamin A metabolites in the fight against TB and what is known regarding the molecular mechanisms of ATRA as a host-directed therapy for TB including its effect on macrophages cytokine profile and cellular pathways. Furthermore, we focus on the issues behind why previous clinical trials with vitamin A supplementation have failed, and how these issues might be overcome.

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Tuberculosis deaths and resistance are increasing – novel therapies are needed.

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Vitamin A deficiency is a strong risk factor for active tuberculosis in contacts.

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All Trans Retinoic Acid is a promising host-directed therapy for tuberculosis.

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It has pleiotropic effects on macrophages & other immune cells in vitro and in vivo.

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Inhaled rather than systemic All Trans Retinoic Acid therapy may be most effective.

Hippocampal MSK1 regulates the behavioral and biological responses of mice to chronic social defeat stress: Involving of the BDNF-CREB signaling and neurogenesis

Depression is one of the most common psychiatric diseases in the 21st century, while its pathogenesis is not yet fully understood. Currently, besides to the monoaminergic system, the brain-derived neurotrophic factor (BDNF)-cAMP response element-binding protein (CREB) signaling is one of the most attractive signaling pathways for treating depression. Mitogen and stress-activated kinase (MSK) 1 and 2 are nuclear proteins activated downstream of the ERK1/2 or p38 MAPK pathways, and it has been demonstrated that MSKs are involved in the BDNF-CREB signaling. Here we assumed that MSKs may play a role in depression, and various methods including the chronic social defeat stress (CSDS) model of depression, western blotting, immunofluorescence and virus-mediated gene transfer were used together. It was found that CSDS fully enhanced the expression of both phosphorylated MSK1 and total MSK1 in the hippocampus but not the medial prefrontal cortex (mPFC). CSDS did not influence the expression of phosphorylated MSK2 and total MSK2 in the two brain regions. Genetic over-expression of hippocampal MSK1 fully prevented not only the CSDS-induced depressive-like behaviors but also the CSDS-induced dysfunction in the hippocampal BDNF-CREB signaling and neurogenesis in mice, while genetic knockdown of hippocampal MSK1 aggravated the CSDS-induced depressive-like symptomatology in mice. Our results collectively suggest that although CSDS evidently enhances the activity of hippocampal MSK1, it is not a contributor to the CSDS-induced dysfunction in the brain but a defensive feedback regulator which protects against CSDS. Therefore, hippocampal MSK1 participates in the pathogenesis of depression and is a feasible and potential antidepressant target.

The Interactions of Insulin and Vitamin A Signaling Systems for the Regulation of Hepatic Glucose and Lipid Metabolism

The pandemics of obesity and type 2 diabetes have become a concern of public health. Nutrition plays a key role in these concerns. Insulin as an anabolic hormonal was discovered exactly 100 years ago due to its activity in controlling blood glucose level. Vitamin A (VA), a lipophilic micronutrient, has been shown to regulate glucose and fat metabolism. VA's physiological roles are mainly mediated by its metabolite, retinoic acid (RA), which activates retinoic acid receptors (RARs) and retinoid X receptors (RXRs), which are two transcription factors. The VA status and activations of RARs and RXRs by RA and synthetic agonists have shown to affect the glucose and lipid metabolism in animal models. Both insulin and RA signaling systems regulate the expression levels of genes involved in the regulation of hepatic glucose and lipid metabolism. Interactions of insulin and RA signaling systems have been observed. This review is aimed at summarizing the history of diabetes, insulin and VA signaling systems; the effects of VA status and activation of RARs and RXRs on metabolism and RAR and RXR phosphorylation; and possible interactions of insulin and RA in the regulation of hepatic genes for glucose and lipid metabolism. In addition, some future research perspectives for understanding of nutrient and hormone interactions are provided.

Ligands and DNA in the allosteric control of retinoid receptors function

Retinoids are a family of compounds that include both vitamin A (all-trans retinol) and its naturally occurring metabolites such as retinoic acids (e.g. all-trans retinoic acid) as well as synthetic analogs. They are critically involved in the regulation of a wide variety of essential biological processes, such as embryogenesis and organogenesis, apoptosis, reproduction, vision, and the growth and differentiation of normal and neoplastic cells in vertebrates. The ability of these small molecules to control the expression of several hundred genes through binding to nuclear ligand-dependent transcription factors accounts for most of their functions. Three retinoic acid receptor (RARα,β,γ) and three retinoid X receptor (RXRα,β,γ) subtypes form a variety of RXR-RAR heterodimers that have been shown to mediate the pleiotropic effects of retinoids through the recruitment of high-molecular weight co-regulatory complexes to response-element DNA sequences found in the promoter region of their target genes. Hence, heterodimeric retinoid receptors are multidomain entities that respond to various incoming signals, such as ligand and DNA binding, by allosteric structural alterations which are the basis of further signal propagation. Here, we provide an overview of the current state of knowledge with regard to the structural mechanisms by which retinoids and DNA response elements act as allosteric effectors that may combine to finely tune RXR-RAR heterodimers activity.

Retinoids in hematology: a timely revival?

The retinoic acid receptors (RARA, RARB, and RARG) are ligand-regulated nuclear receptors that act as transcriptional switches. These master genes drew significant interest in the 1990s because of their key roles in embryogenesis and involvement in a rare malignancy, acute promyelocytic leukemia (APL), in which the RARA (and very rarely, RARG or RARB) genes are rearranged, underscoring the central role of deregulated retinoid signaling in leukemogenesis. Several recent provocative observations have revived interest in the roles of retinoids in non-APL acute myeloid leukemia (AML), as well as in normal hematopoietic differentiation. We review the role of retinoids in hematopoiesis, as well as in the treatment of non-APL AMLs. From this perspective, broader uses of retinoids in the management of hematopoietic tumors are discussed.

Toll-like receptors in sepsis-associated cytokine storm and their endogenous negative regulators as future immunomodulatory targets

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Sepsis infects more than 48.9 million people world-wide, with 19.7 million deaths.

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Cytokine storm plays a significant role in sepsis, along with severe COVID-19.

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TLR signaling pathways plays a crucial role in generating the cytokine storm.

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Endogenous negative regulators of TLR signaling are crucial to regulate cytokine storm.

Cytokine storm generates during various systemic acute infections, including sepsis and current pandemic called COVID-19 (severe) causing devastating inflammatory conditions, which include multi-organ failure or multi-organ dysfunction syndrome (MODS) and death of the patient. Toll-like receptors (TLRs) are one of the major pattern recognition receptors (PRRs) expressed by immune cells as well as non-immune cells, including neurons, which play a crucial role in generating cytokine storm. They recognize microbial-associated molecular patterns (MAMPs, expressed by pathogens) and damage or death-associate molecular patterns (DAMPs; released and/expressed by damaged/killed host cells). Upon recognition of MAMPs and DAMPs, TLRs activate downstream signaling pathways releasing several pro-inflammatory mediators [cytokines, chemokines, interferons, and reactive oxygen and nitrogen species (ROS or RNS)], which cause acute inflammation meant to control the pathogen and repair the damage. Induction of an exaggerated response due to genetic makeup of the host and/or persistence of the pathogen due to its evasion mechanisms may lead to severe systemic inflammatory condition called sepsis in response to the generation of cytokine storm and organ dysfunction. The activation of TLR-induced inflammatory response is hardwired to the induction of several negative feedback mechanisms that come into play to conclude the response and maintain immune homeostasis. This state-of-the-art review describes the importance of TLR signaling in the onset of the sepsis-associated cytokine storm and discusses various host-derived endogenous negative regulators of TLR signaling pathways. The subject is very important as there is a vast array of genes and processes implicated in these negative feedback mechanisms. These molecules and mechanisms can be targeted for developing novel therapeutic drugs for cytokine storm-associated diseases, including sepsis, severe COVID-19, and other inflammatory diseases, where TLR-signaling plays a significant role.

p38 MAPK signaling mediates retinoic acid-induced CD103 expression in human dendritic cells

Retinoic acid (RA) is an active derivative of vitamin A and a key regulator of immune cell function. In dendritic cells (DCs), RA drives the expression of CD103 (integrin α_E ), a functionally relevant DC subset marker. In this study, we analyzed the cell type specificity and the molecular mechanisms involved in RA-induced CD103 expression. We show that RA treatment caused a significant up-regulation of CD103 in differentiated monocyte-derived DCs and blood DCs, but not in differentiated monocyte-derived macrophages or T cells. DC treatment with an RA receptor α (RARα) agonist led to an increase in CD103 expression similar to that in RA treatment, whereas RARA gene silencing with small interfering RNA blocked RA-induced up-regulation of CD103, pointing to a major role of RARα in the regulation of CD103 expression. To elucidate RA-induced signaling downstream of RARα, we used Western blot analysis of RA-treated DCs and showed a significant increase of p38 mitogen-activated protein kinase (MAPK) phosphorylation. In addition, DCs cultured with RA and a p38 MAPK inhibitor had a significantly reduced expression of CD103 compared with DCs cultured with RA only, indicating that p38 MAPK is involved in CD103 regulation. In summary, these findings suggest that the RA-induced expression of CD103 is specific to DCs, is mediated primarily through RARα and involves p38 MAPK signaling.

Insights into the activation mechanism of human estrogen-related receptor γ by environmental endocrine disruptors

The estrogen-related receptor γ (ERRγ, NR3B3) is a constitutively active nuclear receptor which has been proposed to act as a mediator of the low-dose effects of a number of environmental endocrine-disrupting chemicals (EDCs) such as the xenoestrogen bisphenol-A (BPA). To better characterize the ability of exogenous compounds to bind and activate ERRγ, we used a combination of cell-based, biochemical, structural and computational approaches. A purposely created stable cell line allowed for the determination of the EC50s for over 30 environmental ERRγ ligands, including previously unknown ones. Interestingly, affinity constants (Kds) of the most potent compounds measured by isothermal titration calorimetry were in the 50-500 nM range, in agreement with their receptor activation potencies. Crystallographic analysis of the interaction between the ERRγ ligand-binding domain (LBD) and compounds of the bisphenol, alkylphenol and naphthol families revealed a partially shared binding mode and minimal alterations of the receptor conformation upon ligand binding. Further biophysical characterizations coupled to molecular dynamics simulations suggested a mechanism through which ERRγ ligands would exhibit their agonistic properties by preserving the transcriptionally active form of the receptor while rigidifying some loop regions with associated functions. This unique mechanism contrasts with the classical one involving a ligand-induced repositioning and stabilization of the C-terminal activation helix H12.

Retinoid receptor turnover mediated by sumoylation, ubiquitination and the valosin-containing protein is disrupted in glioblastoma

Resistance to therapeutic use of retinoids in glioma has been observed for over 20 years; however, the exact mechanism of resistance remains unknown. To understand retinoic acid resistance in glioma, we studied the turnover mechanism of retinoid receptor proteins in neural stem cells and glioma stem-like cells. Here, we show that in normal neural stem cells, proteasomal degradation of retinoid receptors involves sumoylation, ubiquitination and recognition by the valosin-containing protein (VCP/p97/Cdc48). We find that Sumo1 modification has a dual role to stabilize the retinoid receptor from unwanted degradation and signal additional modification via ubiquitination. Subsequently, the modified receptor binds to the VCP chaperone and both proteins are degraded by the proteasome. Additionally, we reveal that all trans retinoic acid (ATRA) induces VCP expression, creating a positive feedback loop that enhances degradation. In contrast, the pathway is impaired in the glioma stem-like cells resulting in the accumulation of sumoylated and high molecular weight forms of retinoid receptors that lack transcriptional activity and fail to be recognized by the proteasome. Moreover, modified receptor accumulation occurs before ATRA treatment; therefore, the transcritptional defect in glioma is due to a block in the proteasomal degradation pathway that occurs after the sumo modification step.

Retinoic acid-stimulated ERK1/2 pathway regulates meiotic initiation in cultured fetal germ cells

In murine fetal germ cells, retinoic acid (RA) is an extrinsic cue for meiotic initiation that stimulates transcriptional activation of the Stimulated by retinoic acid gene 8 (Stra8), which is required for entry of germ cells into meiotic prophase I. Canonically, the biological activities of RA are mediated by nuclear RA receptors. Recent studies in somatic cells found that RA noncanonically stimulates intracellular signal transduction pathways to regulate multiple cellular processes. In this study, using a germ cell culture system, we investigated (1) whether RA treatment activates any mitogen-activated protein kinase (MAPK) pathways in fetal germ cells at the time of sex differentiation, and (2) if this is the case, whether the corresponding RA-stimulated signaling pathway regulates Stra8 expression in fetal germ cells and their entry into meiosis. When XX germ cells at embryonic day (E) 12.5 were cultured with RA, the extracellular-signal-regulated kinase (ERK) 1/2 pathway was predominantly activated. MEK1/2 inhibitor (U0126) treatment suppressed the mRNA expressions of RA-induced Stra8 and meiotic marker genes (Rec8, Spo11, Dmc1, and Sycp3) in both XX and XY fetal germ cells. Furthermore, U0126 treatment dramatically reduced STRA8 protein levels and numbers of meiotic cells among cultured XX and XY fetal germ cells even in the presence of RA. Taken together, our results suggest the novel concept that the RA functions by stimulating the ERK1/2 pathway and that this activity is critical for Stra8 expression and meiotic progression in fetal germ cells.

All-trans Retinoic Acid as a Versatile Cytosolic Signal Modulator Mediated by CRABP1

All-trans retinoic acid (AtRA), an active metabolite of vitamin A, is recognized for its classical action as an endocrine hormone that triggers genomic effects mediated through nuclear receptors RA receptors (RARs). New evidence shows that atRA-mediated cellular responses are biphasic with rapid and delayed responses. Most of these rapid atRA responses are the outcome of its binding to cellular retinoic acid binding protein 1 (CRABP1) that is predominantly localized in cytoplasm and binds to atRA with a high affinity. This review summarizes the most recent studies of such non-genomic outcomes of atRA and the role of CRABP1 in mediating such rapid effects in different cell types. In embryonic stem cells (ESCs), atRA-CRABP1 dampens growth factor sensitivity and stemness. In a hippocampal neural stem cell (NSC) population, atRA-CRABP1 negatively modulates NSC proliferation and affects learning and memory. In cardiomyocytes, atRA-CRABP1 prevents over-activation of calcium-calmodulin-dependent protein kinase II (CaMKII), protecting heart function. These are supported by the fact that CRABP1 gene knockout (KO) mice exhibit multiple phenotypes including hippocampal NSC expansion and spontaneous cardiac hypertrophy. This indicates that more potential processes/signaling pathways involving atRA-CRABP1 may exist, which remain to be identified.

Genomic and non-genomic pathways are both crucial for peak induction of neurite outgrowth by retinoids

Retinoic acid (RA) is the active metabolite of vitamin A and essential for many physiological processes, particularly the induction of cell differentiation. In addition to regulating genomic transcriptional activity via RA receptors (RARs) and retinoid X receptors (RXRs), non-genomic mechanisms of RA have been described, including the regulation of ERK1/2 kinase phosphorylation, but are poorly characterised. In this study, we test the hypothesis that genomic and non-genomic mechanisms of RA are regulated independently with respect to the involvement of ligand-dependent RA receptors. A panel of 28 retinoids (compounds with vitamin A-like activity) showed a marked disparity in genomic (gene expression) versus non-genomic (ERK1/2 phosphorylation) assays. These results demonstrate that the capacity of a compound to activate gene transcription does not necessarily correlate with its ability to regulate a non-genomic activity such as ERK 1/2 phosphorylation. Furthermore, a neurite outgrowth assay indicated that retinoids that could only induce either genomic, or non-genomic activities, were not strong promoters of neurite outgrowth, and that activities with respect to both transcriptional regulation and ERK1/2 phosphorylation produced maximum neurite outgrowth. These results suggest that the development of effective retinoids for clinical use will depend on the selection of compounds which have maximal activity in non-genomic as well as genomic assays.

FLT3-ITD impedes retinoic acid, but not arsenic, responses in murine acute promyelocytic leukemias

Acute promyelocytic leukemia (APL) is often associated with activating FLT3 signaling mutations. These are highly related to hyperleukocytosis, a major adverse risk factor with chemotherapy-based regimens. APL is a model for oncogene-targeted therapies: all-trans retinoic acid (ATRA) and arsenic both target and degrade its ProMyelocytic Leukemia/Retinoic Acid Receptor α (PML/RARA) driver. The combined ATRA/arsenic regimen now cures virtually all patients with standard-risk APL. Although FLT3-internal tandem duplication (ITD) was an adverse risk factor for historical ATRA/chemotherapy regimens, the molecular bases for this effect remain unknown. Using mouse APL models, we unexpectedly demonstrate that FLT3-ITD severely blunts ATRA response. Remarkably, although the transcriptional output of initial ATRA response is unaffected, ATRA-induced PML/RARA degradation is blunted, as is PML nuclear body reformation and activation of P53 signaling. Critically, the combination of ATRA and arsenic fully rescues therapeutic response in FLT3-ITD APLs, restoring PML/RARA degradation, PML nuclear body reformation, P53 activation, and APL eradication. Moreover, arsenic targeting of normal PML also contributes to APL response in vivo. These unexpected results explain the less favorable outcome of FLT3-ITD APLs with ATRA-based regimens, and stress the key role of PML nuclear bodies in APL eradication by the ATRA/arsenic combination.

Non-canonical activity of retinoic acid in relation to the activation of protein kinases in transformed cells of different origin

Background.The non-canonical activity of retinoic acid (RA) was discovered relatively recently and consists in the rapid activation of intracellular signaling pathways by the mechanisms not related to the transcriptional activity of the RA nuclear receptors. Separate data suggest that this activity can stimulate the processes of malignancy and contribute to the formation of tumor cell resistance to RA as a therapeutic agent. However, little is known about the mechanisms of this activity. It is also unclear how universal this effect is; does the RA-dependent activation of different signaling protein kinases occur in the same cells, and whether activation of these kinases is interrelated.

Materials and methods: cultivation of non-small cell lung cancer cells and neuroblastoma cells under standard conditions and with incubation with all-trans retinoic acid (ATRA); immunoblotting.

Results. Here we studied the effect of ATRA on the activation of Akt and Erk1/2 protein kinases depending on the incubation time. The analysis revealed RA-dependent activation of both kinases in all studied non-small cell lung cancer and neuroblastoma cell lines. Activation of Akt and Erk1/2 occurred at five minutes of incubation, which corresponds to the non-transcriptional (non-canonical) activity of the RA, however, further activation kinetics of the two kinases differed essentially.

Conclusion.We found that ATRA causes rapid activation of Erk1/2 and Akt protein kinases in both non-small cell lung cancer and neuroblastoma cells. The differences in the kinetics of RA-dependent stimulation of these two kinases suggest that their activation is mediated by independent mechanisms.

MSK1 functions as a transcriptional coactivator of p53 in the regulation of p21 gene expression

Mitogen- and stress-activated kinase 1 (MSK1) is a chromatin kinase that facilitates activator-dependent transcription by altering chromatin structure through histone H3 phosphorylation. The kinase activity of MSK1 is activated by intramolecular autophosphorylation, which is initially triggered by the activation of upstream mitogen-activated protein kinases (MAPKs), such as p38 and ERK1/2. MSK1 has been implicated in the expression of p21, a p53 target gene; however, the precise connection between MSK1 and p53 has not been clearly elucidated. Here, using in vitro and cell-based transcription assays, we show that MSK1 functions as a transcriptional coactivator of p53 in p21 expression, an action associated with MAPK-dependent phosphorylation of MSK1 and elevated kinase activity. Of special significance, we show that MSK1 directly interacts with p53 and is recruited to the p21 promoter, where it phosphorylates histone H3 in a p53-dependent manner. In addition, phosphomimetic mutant analysis demonstrated that negative charges in the hydrophobic motif are critical for serine 212 phosphorylation in the N-terminal kinase domain, which renders MSK1 competent for histone kinase activity. These studies suggest that MSK1 acts through a direct interaction with p53 to function as a transcriptional coactivator and that MSK1 activation by upstream MAPK signaling is important for efficient p21 gene expression.

RARα supports the development of Langerhans cells and langerin-expressing conventional dendritic cells

Langerhans cells (LC) are the prototype langerin-expressing dendritic cells (DC) that reside specifically in the epidermis, but langerin-expressing conventional DCs also reside in the dermis and other tissues, yet the factors that regulate their development are unclear. Because retinoic acid receptor alpha (RARα) is highly expressed by LCs, we investigate the functions of RARα and retinoic acid (RA) in regulating the langerin-expressing DCs. Here we show that the development of LCs from embryonic and bone marrow-derived progenitors and langerin⁺ conventional DCs is profoundly regulated by the RARα-RA axis. During LC differentiation, RARα is required for the expression of a LC-promoting transcription factor Runx3, but suppresses that of LC-inhibiting C/EBPβ. RARα promotes the development of LCs and langerin⁺ conventional DCs only in hypo-RA conditions, a function effectively suppressed at systemic RA levels. Our findings identify positive and negative regulatory mechanisms to tightly regulate the development of the specialized DC populations.

Langerhans cells (LC) and langerin-expressing conventional dendritic cells are made from distinct progenitors and enriched in the distinct microenvironments of the skin. Here the authors show that these immune cells are regulated by retinoic acid receptor alpha (RARα) via simultaneous induction of LC-promoting Runx3 and repression of LC-inhibiting C/EBPβ.

TRIM24 mediates the interaction of the retinoic acid receptor alpha with the proteasome

The nuclear retinoic acid (RA) receptors (RARα, β and γ) are ligand-dependent regulators of transcription. Upon activation by RA, they are recruited at the promoters of target genes together with several coregulators. Then, they are degraded by the ubiquitin proteasome system. Here, we report that the degradation of the RARα subtype involves ubiquitination and the tripartite motif protein TRIM24, which was originally identified as a ligand-dependent corepressor of RARα. We show that in response to RA, TRIM24 serves as an adapter linking RARα to the proteasome for its degradation. In addition, TRIM24 and the proteasome are recruited with RARα to the promoters of target genes and thus are inherently linked to RARα transcriptional activity.

RARα2 and PML-RAR similarities in the control of basal and retinoic acid induced myeloid maturation of acute myeloid leukemia cells

Treatment of acute promyelocytic leukemia (APL) with all-trans retinoic acid (ATRA) is the first example of targeted therapy. In fact, the oncogenic fusion-protein (PML-RAR) typical of this leukemia contains the retinoid-nuclear-receptor RARα. PML-RAR is responsible for the differentiation block of the leukemic blast. Besides PML-RAR, two endogenous RARα proteins are present in APL blasts, i.e. RARα1 and RARα2. We developed different cell populations characterized by PML-RAR, RARα2 and RARα1 knock-down in the APL-derived NB4 cell-line. Unexpectedly, silencing of PML-RAR and RARα2 results in similar increases in the constitutive expression of several granulocytic differentiation markers. This is accompanied by enhanced expression of the same granulocytic markers upon exposure of the NB4 blasts to ATRA. Silencing of PML-RAR and RARα2 causes also similar perturbations in the whole genome gene-expression profiles of vehicle and ATRA treated NB4 cells. Unlike PML-RAR and RARα2, RARα1 knock-down blocks ATRA-dependent induction of several granulocytic differentiation markers. Many of the effects on myeloid differentiation are confirmed by over-expression of RARα2 in NB4 cells. RARα2 action on myeloid differentiation does not require the presence of PML-RAR, as it is recapitulated also upon knock-down in PML-RAR-negative HL-60 cells. Thus, relative to RARα1, PML-RAR and RARα2 exert opposite effects on APL-cell differentiation. These contrasting actions may be related to the fact that both PML-RAR and RARα2 interact with and inhibit the transcriptional activity of RARα1. The interaction surface is located in the carboxy-terminal domain containing the D/E/F regions and it is influenced by phosphorylation of Ser-369 of RARα1.

Retinoic Acid and Retinoic Acid Receptors as Pleiotropic Modulators of the Immune System

Vitamin A is a multifunctional vitamin implicated in a wide range of biological processes. Its control over the immune system and functions are perhaps the most pleiotropic not only for development but also for the functional fate of almost every cell involved in protective or regulatory adaptive or innate immunity. This is especially key at the intestinal border, where dietary vitamin A is first absorbed. Most effects of vitamin A are exerted by its metabolite, retinoic acid (RA), which through ligation of nuclear receptors controls transcriptional expression of RA target genes. In addition to this canonical function, RA and RA receptors (RARs), either as ligand-receptor or separately, play extranuclear, nongenomic roles that greatly expand the multiple mechanisms employed for their numerous and paradoxical functions that ultimately link environmental sensing with immune cell fate. This review discusses RA and RARs and their complex roles in innate and adaptive immunity.

Retinoic acid induces white adipose tissue browning by increasing adipose vascularity and inducing beige adipogenesis of PDGFRα+ adipose progenitors

Formation of beige adipocytes within white adipose tissue enhances energy expenditure, which is a promising strategy to reduce obesity and prevent metabolic symptoms. Vitamin A and its bioactive metabolite, retinoic acid (RA), have regulatory roles in lipid metabolism. Here we report that RA induces white adipose tissue browning via activating vascular endothelial growth factor (VEGF) signaling. RA triggered angiogenesis and elicited de novo generation of platelet-derived growth factor receptor α positive (PDGFRα⁺) adipose precursor cells via VEGFA/VEGFR2 signaling. In addition, RA promoted beige/brown adipocyte formation from capillary networks in vitro. Using PDGFRα tracking mice, we found that the vascular system acted as an adipogenic repository by containing PDGFRα⁺ progenitors which differentiated into beige adipocytes under RA or VEGF164 treatments. Conditional knockout of VEGF receptors blocked RA-stimulated white adipose tissue browning. Moreover, the VEGFA and RA activated p38MAPK to enhance the binding of RA receptor to RA response elements of the Prdm16 promoter and upregulated Prdm16 transcription. In conclusion, RA induces white adipose tissue browning by increasing adipose vascularity and promoting beige adipogenesis of PDGFRα⁺ adipose progenitors.

Transcriptional suppression of microRNA-27a contributes to laryngeal cancer differentiation via GSK-3β-involved Wnt/β-catenin pathway

miR-27a regulates cell differentiation in a variety of diseases. However, whether and how miR-27a participates in laryngeal cancer cell differentiation remains unknown. Therefore, we explored role and molecular mechanism of miR-27a in laryngeal cancer differentiation in the study. We found that miR-27a expression was inversely correlated with laryngeal cancer differentiation degree based on the clinical pathological diagnosis of each patient. miR-27 asignificantly rescued differentiation and inhibited β-catenin, LEF1, OCT4 and SOX2 in Wnt/β-catenin pathway in all-trans-retinoic acid (ATRA)-induced laryngeal cancer cells. Bindings of RARα to miR-27a and miR-27a to GSK-3β were confirmed by ChIP and Luciferase reporter assays, respectively. In conclusion, miR-27a is a negative regulator in laryngeal cancer differentiation. RARα-mediated miR-27a transcriptional inactivation releases the inhibition of miR-27a on GSK-3β leading to laryngeal cancer differentiation through GSK-3β-involved Wnt/β-catenin pathway, suggesting that miR-27a is a usefully therapeutic target at least in ATRA-induced laryngeal cancer differentiation.

Allosteric Regulation in the Ligand Binding Domain of Retinoic Acid Receptorγ

Retinoic acid (RA) plays key roles in cell differentiation and growth arrest through nuclear retinoic acid receptors (RARs), which are ligand-dependent transcription factors. While the main trigger of RAR activation is the binding of RA, phosphorylation of the receptors has also emerged as an important regulatory signal. Phosphorylation of the RARγ N-terminal domain (NTD) is known to play a functional role in neuronal differentiation. In this work, we investigated the phosphorylation of RARγ ligand binding domain (LBD), and present evidence that the phosphorylation status of the LBD affects the phosphorylation of the NTD region. We solved the X-ray structure of a phospho-mimetic mutant of the LBD (RARγ S371E), which we used in molecular dynamics simulations to characterize the consequences of the S371E mutation on the RARγ structural dynamics. Combined with simulations of the wild-type LBD, we show that the conformational equilibria of LBD salt bridges (notably R387-D340) are affected by the S371E mutation, which likely affects the recruitment of the kinase complex that phosphorylates the NTD. The molecular dynamics simulations also showed that a conservative mutation in this salt bridge (R387K) affects the dynamics of the LBD without inducing large conformational changes. Finally, cellular assays showed that the phosphorylation of the NTD of RARγ is differentially regulated by retinoic acid in RARγWT and in the S371N, S371E and R387K mutants. This multidisciplinary work highlights an allosteric coupling between phosphorylations of the LBD and the NTD of RARγ and supports the importance of structural dynamics involving electrostatic interactions in the regulation of RARs activity.

Enzymatic Metabolism of Vitamin A in Developing Vertebrate Embryos

Embryonic development is orchestrated by a small number of signaling pathways, one of which is the retinoic acid (RA) signaling pathway. Vitamin A is essential for vertebrate embryonic development because it is the molecular precursor of the essential signaling molecule RA. The level and distribution of RA signaling within a developing embryo must be tightly regulated; too much, or too little, or abnormal distribution, all disrupt embryonic development. Precise regulation of RA signaling during embryogenesis is achieved by proteins involved in vitamin A metabolism, retinoid transport, nuclear signaling, and RA catabolism. The reversible first step in conversion of the precursor vitamin A to the active retinoid RA is mediated by retinol dehydrogenase 10 (RDH10) and dehydrogenase/reductase (SDR family) member 3 (DHRS3), two related membrane-bound proteins that functionally activate each other to mediate the interconversion of retinol and retinal. Alcohol dehydrogenase (ADH) enzymes do not contribute to RA production under normal conditions during embryogenesis. Genes involved in vitamin A metabolism and RA catabolism are expressed in tissue-specific patterns and are subject to feedback regulation. Mutations in genes encoding these proteins disrupt morphogenesis of many systems in a developing embryo. Together these observations demonstrate the importance of vitamin A metabolism in regulating RA signaling during embryonic development in vertebrates.

Retinoic acid induces nuclear FAK translocation and reduces breast cancer cell adhesion through Moesin, FAK, and Paxillin

Breast cancer is the most common malignancy in women, with metastases being the cause of death in 98%. In previous works we have demonstrated that retinoic acid (RA), the main retinoic acid receptor (RAR) ligand, is involved in the metastatic process by inhibiting migration through a reduced expression of the specific migration-related proteins Moesin, c-Src, and FAK. At present, our hypothesis is that RA also acts for short periods in a non-genomic action to cooperate with motility reduction and morphology of breast cancer cells. Here we identify that the administration of 10(-6) M RA (10-20 min) induces the activation of the migration-related proteins Moesin, FAK, and Paxillin in T-47D breast cancer cells. The phosphorylation exerted by the selective agonists for RARα and RARβ, on Moesin, FAK, and Paxillin was comparable to the activation exerted by RA. The RARγ agonist only led to a weak activation, suggesting the involvement of RARα and RARβ in this pathway. We then treated the cells with different inhibitors that are involved in cell signaling to regulate the mechanisms of cell motility. RA failed to activate Moesin, FAK, and Paxillin in cells treated with Src inhibitor (PP2) and PI3K inhibitor (WM), suggesting the participation of Src-PI3K in this pathway. Treatment with 10(-6) M RA for 20 min significantly decreased cell adhesion. However, when cells were treated with 10(-6) M RA and FAK inhibitor, the RA did not significantly inhibit adhesion, suggesting a role of FAK in the adhesion inhibited by RA. By immunofluorescence and immunoblotting analysis we demonstrated that RA induced nuclear FAK translocation leading to a reduced cellular adhesion. These findings provide new information on the actions of RA for short periods. RA participates in cell adhesion and subsequent migration, modulating the relocation and activation of proteins involved in cell migration.

Therapeutic use of selective synthetic ligands for retinoic acid receptors: a patent review

INTRODUCTION

Differentiation therapy using all-trans retinoic acid (ATRA) revolutionised the treatment of acute promyelocytic leukaemia to such an extent that it is now one of the most curable types of leukaemia, with ATRA and anthracycline-based chemotherapy providing cure rates above 80%. Isotretinoin is used to treat chronic acne. Here, we examine the information described in recent patents and the extent to which new findings are influencing extending retinoid-based differentiation therapy to other cancers, as well as the development of new therapies for other disorders.

AREAS COVERED

A search has been performed on the literature and worldwide patents filed during 2014 to the present time, focusing on synthetic agonists and antagonists of retinoic acid receptors and novel compositions for the delivery of these agents.

EXPERT OPINION

New potential therapeutic applications have been described, including lung, breast and head and neck cancers, T cell lymphoma and neurodegenerative, metabolic, ophthalmic, muscle, and inflammatory disorders. Recent patents have described the means to maximise retinoid activity. Two decades of efforts to extend retinoid-based therapies have been disappointing and new synthetic retinoids, target diseases and modes of delivery may well resolve this long standing issue.

Phosphoproteome and Transcriptome of RA-Responsive and RA-Resistant Breast Cancer Cell Lines

Retinoic acid (RA), the main active vitamin A metabolite, controls multiple biological processes such as cell proliferation and differentiation through genomic programs and kinase cascades activation. Due to these properties, RA has proven anti-cancer capacity. Several breast cancer cells respond to the antiproliferative effects of RA, while others are RA-resistant. However, the overall signaling and transcriptional pathways that are altered in such cells have not been elucidated. Here, in a large-scale analysis of the phosphoproteins and in a genome-wide analysis of the RA-regulated genes, we compared two human breast cancer cell lines, a RA-responsive one, the MCF7 cell line, and a RA-resistant one, the BT474 cell line, which depicts several alterations of the “kinome”. Using high-resolution nano-LC-LTQ-Orbitrap mass spectrometry associated to phosphopeptide enrichment, we found that several proteins involved in signaling and in transcription, are differentially phosphorylated before and after RA addition. The paradigm of these proteins is the RA receptor α (RARα), which was phosphorylated in MCF7 cells but not in BT474 cells after RA addition. The panel of the RA-regulated genes was also different. Overall our results indicate that RA resistance might correlate with the deregulation of the phosphoproteome with consequences on gene expression.

Emerging Roles of the Mitogen and Stress Activated Kinases MSK1 and MSK2

Mitogen- and stress-activated kinases (MSK) 1 and 2 are nuclear proteins activated downstream of the ERK1/2 or p38 MAPK pathways. MSKs phosphorylate multiple substrates, including CREB and Histone H3, and their major role is the regulation of specific subsets of Immediate Early genes (IEG). While MSKs are expressed in multiple tissues, their levels are high in immune and neuronal cells and it is in these systems most is known about their function. In immunity, MSKs have predominantly anti-inflammatory roles and help regulate production of the anti-inflammatory cytokine IL-10. In the CNS they are implicated in neuronal proliferation and synaptic plasticity. In this review we will focus on recent advances in understanding the roles of MSKs in the innate immune system and neuronal function.

Role of mitogen- and stress-activated kinases in inflammatory arthritis

Lysophosphatidic acid (LPA) is a pleiotropic lipid mediator that promotes motility, survival, and the synthesis of chemokines/cytokines in human fibroblast-like synoviocytes (FLS) from patients with rheumatoid arthritis. LPA activates several proteins within the mitogen activated protein (MAP) kinase signaling network, including extracellular signal-regulated kinases (ERK) 1/2 and p38 MAP kinase (MAPK). Upon docking to mitogen- and stress-activated kinases (MSKs), ERK1/2 and p38 MAPK phosphorylate serine and threonine residues within its C-terminal domain and cause autophosphorylation of MSKs. Activated MSKs can then directly phosphorylate cAMP response element-binding protein (CREB) at Ser133 in FLS. Phosphorylation of CREB by MSKs is essential for the production of pro-inflammatory and anti-inflammatory cytokines. However, other downstream effectors of MSK1/2 such as nuclear factor-kappa B, histone H3, and high mobility group nucleosome binding domain 1 may also regulate gene expression in immune cells involved in disease pathogenesis. MSKs are master regulators of cell function that integrate signals induced by growth factors, pro-inflammatory cytokines, and cellular stresses, as well as those induced by LPA.

All-Trans Retinoic Acid-Induced Deficiency of the Wnt/β-Catenin Pathway Enhances Hepatic Carcinoma Stem Cell Differentiation

Retinoic acid (RA) is an important biological signal that directly differentiates cells during embryonic development and tumorigenesis. However, the molecular mechanism of RA-mediated differentiation in hepatic cancer stem cells (hCSCs) is not well understood. In this study, we found that mRNA expressions of RA-biosynthesis-related dehydrogenases were highly expressed in hepatocellular carcinoma. All-trans retinoic acid (ATRA) differentiated hCSCs through inhibiting the function of β-catenin in vitro. ATRA also inhibited the function of PI3K-AKT and enhanced GSK-3β-dependent degradation of phosphorylated β-catenin. Furthermore, ATRA and β-catenin silencing both increased hCSC sensitivity to docetaxel treatment. Our results suggest that targeting β-catenin will provide extra benefits for ATRA-mediated treatment of hepatic cancer patients.

Retinoids and rexinoids in cancer prevention: from laboratory to clinic

Early in the age of modern medicine the consequences of vitamin A deficiency drew attention to the fundamental link between retinoid-dependent homeostatic regulation and malignant hyperproliferative diseases. The term "retinoid" includes a handful of endogenous and a large group of synthetic derivatives of vitamin A. These multifunctional lipid-soluble compounds directly regulate target genes of specific biological functions and critical signaling pathways to orchestrate complex functions from vision to development, metabolism, and inflammation. Many of the retinoid activities on the cellular level have been well characterized and translated to the regulation of processes like differentiation and cell death, which play critical roles in the outcome of malignant transformation of tissues. In fact, retinoid-based differentiation therapy of acute promyelocytic leukemia was one of the first successful examples of molecularly targeted treatment strategies. The selectivity, high receptor binding affinity and the ability of retinoids to directly modulate gene expression programs present a distinct pharmacological opportunity for cancer treatment and prevention. However, to fully exploit their potential, the adverse effects of retinoids must be averted. In this review we provide an overview of the biology of retinoid (activated by nuclear retinoic acid receptors [RARs]) and rexinoid (engaged by nuclear retinoid X receptors [RXRs]) action concluded from a long line of preclinical studies, in relation to normal and transformed states of cells. We will also discuss the past and current uses of retinoids in the treatment of malignancies, the potential of rexinoids in the cancer prevention setting, both as single agents and in combinations.

RAR/RXR binding dynamics distinguish pluripotency from differentiation associated cis-regulatory elements

In mouse embryonic cells, ligand-activated retinoic acid receptors (RARs) play a key role in inhibiting pluripotency-maintaining genes and activating some major actors of cell differentiation. To investigate the mechanism underlying this dual regulation, we performed joint RAR/RXR ChIP-seq and mRNA-seq time series during the first 48 h of the RA-induced Primitive Endoderm (PrE) differentiation process in F9 embryonal carcinoma (EC) cells. We show here that this dual regulation is associated with RAR/RXR genomic redistribution during the differentiation process. In-depth analysis of RAR/RXR binding sites occupancy dynamics and composition show that in undifferentiated cells, RAR/RXR interact with genomic regions characterized by binding of pluripotency-associated factors and high prevalence of the non-canonical DR0-containing RA response element. By contrast, in differentiated cells, RAR/RXR bound regions are enriched in functional Sox17 binding sites and are characterized with a higher frequency of the canonical DR5 motif. Our data offer an unprecedentedly detailed view on the action of RA in triggering pluripotent cell differentiation and demonstrate that RAR/RXR action is mediated via two different sets of regulatory regions tightly associated with cell differentiation status.

Hepatocyte nuclear factor 4α (HNF4α) in coordination with retinoic acid receptors increases all-trans-retinoic acid-dependent CYP26A1 gene expression in HepG2 human hepatocytes

CYP26A1 expression is very highly induced by retinoic acid (RA) in the liver, compared to most other tissues, suggesting that a liver-enriched factor may be required for its physiological transcriptional response. HNF4α is a highly conserved liver-specific/enriched member of nuclear receptor superfamily. In this study, we hypothesized that HNF4α and RARs may cooperate in an RA-dependent manner to induce a high level of CYP26A1 expression in liver cells. Partial inhibition of endogenous HNF4α by siRNA reduced the level of RA-induced CYP26A1 mRNA in HepG2 cells. Cotransfection of HNF4α, with or without RARs, demonstrated RA-dependent activation of a human CYP26A1 promoter-luciferase construct. Analysis of a 2.5-kbp putative CYP26A1 promoter sequence identified five potential HNF4α DNA response elements: H1 located in a proximal region overlapping with an RAR element-1 (RARE1 or R1); H2 and H3 in the distal region, close to RARE2 (R2) and RARE3 (R3); and H4 and H5 in intermediary regions. In EMSA and ChIP analyses HNF4α and RARs binding in the proximal and distal CYP26A1 promoter regions was significantly higher in RA-treated cells. Mutational analysis of the individual HNF4α DNA-response elements identified H1 as the major site for HNF4α binding because mutation of H1 inhibited the promoter activity by ~90%, followed by H2 mutation with less than 40% inhibition. Our results indicate that HNF4α coordinates with RARs in an RA-dependent manner to strongly induce CYP26A1 gene expression in the liver, which may explain the high level of response to RA observed in vivo.

Nuclear and extranuclear effects of vitamin A

Vitamin A or retinol is a multifunctional vitamin that is essential at all stages of life from embryogenesis to adulthood. Up to now, it has been accepted that the effects of vitamin A are exerted by active metabolites, the major ones being 11-cis retinal for vision, and all trans-retinoic acid (RA) for cell growth and differentiation. Basically RA binds nuclear receptors, RARs, which regulate the expression of a battery of target genes in a ligand dependent manner. During the last decade, new scenarios have been discovered, providing a rationale for the understanding of other long-noted but not explained functions of retinol. These novel scenarios involve: (i) other nuclear receptors such as PPAR β/δ, which regulate the expression of other target genes with other functions; (ii) extranuclear and nontranscriptional effects, such as the activation of kinases, which phosphorylate RARs and other transcription factors, thus expanding the list of the RA-activated genes; (iii) finally, vitamin A is active per se and can work as a cytokine that regulates gene transcription by activating STRA6. New effects of vitamin A and RA are continuously being discovered in new fields, revealing new targets and new mechanisms thus improving the understanding the pleiotropicity of their effects.

Retinoic acid receptors: from molecular mechanisms to cancer therapy

Retinoic acid (RA), the major bioactive metabolite of retinol or vitamin A, induces a spectrum of pleiotropic effects in cell growth and differentiation that are relevant for embryonic development and adult physiology. The RA activity is mediated primarily by members of the retinoic acid receptor (RAR) subfamily, namely RARα, RARβ and RARγ, which belong to the nuclear receptor (NR) superfamily of transcription factors. RARs form heterodimers with members of the retinoid X receptor (RXR) subfamily and act as ligand-regulated transcription factors through binding specific RA response elements (RAREs) located in target genes promoters. RARs also have non-genomic effects and activate kinase signaling pathways, which fine-tune the transcription of the RA target genes. The disruption of RA signaling pathways is thought to underlie the etiology of a number of hematological and non-hematological malignancies, including leukemias, skin cancer, head/neck cancer, lung cancer, breast cancer, ovarian cancer, prostate cancer, renal cell carcinoma, pancreatic cancer, liver cancer, glioblastoma and neuroblastoma. Of note, RA and its derivatives (retinoids) are employed as potential chemotherapeutic or chemopreventive agents because of their differentiation, anti-proliferative, pro-apoptotic, and anti-oxidant effects. In humans, retinoids reverse premalignant epithelial lesions, induce the differentiation of myeloid normal and leukemic cells, and prevent lung, liver, and breast cancer. Here, we provide an overview of the biochemical and molecular mechanisms that regulate the RA and retinoid signaling pathways. Moreover, mechanisms through which deregulation of RA signaling pathways ultimately impact on cancer are examined. Finally, the therapeutic effects of retinoids are reported.

Evolution of retinoic acid receptors and retinoic acid signaling

Retinoic acid (RA) is a vitamin A-derived morphogen controlling important developmental processes in vertebrates, and more generally in chordates, including axial patterning and tissue formation and differentiation. In the embryo, endogenous RA levels are controlled by RA synthesizing and degrading enzymes and the RA signal is transduced by two retinoid receptors: the retinoic acid receptor (RAR) and the retinoid X receptor (RXR). Both RAR and RXR are members of the nuclear receptor superfamily of ligand-activated transcription factors and mainly act as heterodimers to activate the transcription of target genes in the presence of their ligand, all-trans RA. This signaling pathway was long thought to be a chordate innovation, however, recent findings of gene homologs involved in RA signaling in the genomes of a wide variety of non-chordate animals, including ambulacrarians (sea urchins and acorn worms) and lophotrochozoans (annelids and mollusks), challenged this traditional view and suggested that the RA signaling pathway might have a more ancient evolutionary origin than previously thought. In this chapter, we discuss the evolutionary history of the RA signaling pathway, and more particularly of the RARs, which might have experienced independent gene losses and duplications in different animal lineages. In sum, the available data reveal novel insights into the origin of the RA signaling pathway as well as into the evolutionary history of the RARs.

Vitamin a deficiency and alterations in the extracellular matrix

Vitamin A or retinol which is the natural precursor of several biologically active metabolites can be considered the most multifunctional vitamin in mammals. Its deficiency is currently, along with protein malnutrition, the most serious and common nutritional disorder worldwide. It is necessary for normal embryonic development and postnatal tissue homeostasis, and exerts important effects on cell proliferation, differentiation and apoptosis. These actions are produced mainly by regulating the expression of a variety of proteins through transcriptional and non-transcriptional mechanisms. Extracellular matrix proteins are among those whose synthesis is known to be modulated by vitamin A. Retinoic acid, the main biologically active form of vitamin A, influences the expression of collagens, laminins, entactin, fibronectin, elastin and proteoglycans, which are the major components of the extracellular matrix. Consequently, the structure and macromolecular composition of this extracellular compartment is profoundly altered as a result of vitamin A deficiency. As cell behavior, differentiation and apoptosis, and tissue mechanics are influenced by the extracellular matrix, its modifications potentially compromise organ function and may lead to disease. This review focuses on the effects of lack of vitamin A in the extracellular matrix of several organs and discusses possible molecular mechanisms and pathologic implications.

Prostaglandin E2 induces retinoic acid receptor-β up-regulation through MSK1

The pharmacological modulation of putative renoprotective factors hypoxia-inducible factor-1α (HIF-1α) and HIF-1α-regulated vascular endothelial growth factor-A (VEGF-A) in the kidney has therapeutic interest. In human renal proximal tubular HK2 cells, prostaglandin E2 (PGE2) up-regulates HIF-1α and VEGF-A through epidermal growth factor receptor (EGFR)-dependent up-regulation of retinoic acid receptor-β (RARβ). Here we studied the role of mitogen-activated protein kinases (MAPKs) ERK1/2 and p38 and their target kinase, mitogen- and stress activated kinase-1 (MSK1), in the signaling cascade. Treatment of HK2 cells with PGE2 resulted in increased phosphorylation of EGFR, the three studied kinases and the histone H3 (Ser10) at the RARβ gene promoter (the latter has been proposed as a molecular signature of the activated RARβ gene promoter). Prevention of the phosphorylation of EGFR, ERK1/2, p38 MAPK or MSK1 is by incubating, respectively, with AG1478, PD98059, SB203580 or H89 allowed to elucidate the precise phosphorylation order in the signaling cascade triggered by PGE2: first, EGFR; then, ERK1/2 and p38 MAPK and, finally, MSK1. Phosphorylation of MSK1 led to that of Ser10 in histone H3 and to activation of RARβ gene transcription (and the consequent increase in the expression of HIF-1α and VEGF-A), which was suppressed by H89 or by transfecting cells with a vector encoding for a dominant-negative mutant of MSK1. These results highlight the relevance of MSK1 in the up-regulation of RARβ by PGE2. They also may contribute to new therapeutic approaches based upon the pharmacological control of HIF-1α/VEGF-A in the proximal tubule through the modulation of the PGE2/EGFR/MAPK/MSK1/RARβ pathway.

Phosphorylation of the retinoic acid receptor RARγ2 is crucial for the neuronal differentiation of mouse embryonic stem cells

Retinoic acid (RA) plays key roles in cell differentiation and growth arrest by activating nuclear RA receptors (RARs) (α, β and γ), which are ligand-dependent transcription factors. RARs are also phosphorylated in response to RA. Here, we investigated the in vivo relevance of the phosphorylation of RARs during RA-induced neuronal differentiation of mouse embryonic stem cells (mESCs). Using ESCs where the genes encoding each RAR subtype had been inactivated, and stable rescue lines expressing RARs mutated in phospho-acceptor sites, we show that RA-induced neuronal differentiation involves RARγ2 and requires RARγ2 phosphorylation. By gene expression profiling, we found that the phosphorylated form of RARγ2 regulates a small subset of genes through binding an unusual RA response element consisting of two direct repeats with a seven-base-pair spacer. These new findings suggest an important role for RARγ phosphorylation during cell differentiation and pave the way for further investigations during embryonic development.

The rhythm of retinoids in the brain

The retinoids are a family of compounds that in nature are derived from vitamin A or pro-vitamin A carotenoids. An essential part of the diet for mammals, vitamin A has long been known to be essential for many organ systems in the adult. More recently, however, they have been shown to be necessary for function of the brain and new discoveries point to a central role in processes ranging from neuroplasticity to neurogenesis. Acting in several regions of the central nervous system including the eye, hippocampus and hypothalamus, one common factor in its action is control of biological rhythms. This review summarizes the role of vitamin A in the brain; its action through the metabolite retinoic acid via specific nuclear receptors, and the regulation of its concentration through controlled synthesis and catabolism. The action of retinoic acid to regulate several rhythms in the brain and body, from circadian to seasonal, is then discussed to finish with the importance of retinoic acid in the regular pattern of sleep. We review the role of vitamin A and retinoic acid (RA) as mediators of rhythm in the brain. In the suprachiasmatic nucleus and hippocampus they control expression of circadian clock genes while in the cortex retinoic acid is required for delta oscillations of sleep. Retinoic acid is also central to a second rhythm that keeps pace with the seasons, regulating function in the hypothalamus and pineal gland.

Retinoic acid signaling and mouse embryonic stem cell differentiation: Cross talk between genomic and non-genomic effects of RA

Retinoic acid (RA), the active derivative of vitamin A, a fat-soluble vitamin, plays key roles in cell growth and differentiation by activating nuclear receptors, RARs (α, β and γ), which are ligand dependent regulators of transcription. The past years highlighted several novelties in the field that increased the complexity of RA effects. Indeed, in addition to its classical genomic effects, RA also has extranuclear and non-transcriptional effects. RA induces the rapid and transient activation of kinase cascades, which are integrated in the nucleus via the phosphorylation of RARs at a conserved serine residue located in the N-terminal domain and their coregulators. In order to investigate the relevance of RARs' phosphorylation in cell differentiation, mouse embryonic stem (mES) cells were used as a model. When treated with RA, these pluripotent cells give rise to neuronal cells. Cells invalidated for each RAR were generated as well as stable rescue lines expressing RARs mutated in phosphor acceptor sites. Such a strategy revealed that RA-induced neuronal differentiation involves the RARγ2 subtype and requires RARγ2 phosphorylation. Moreover, in gene expression profiling experiments, the phosphorylated form of RARγ2 was found to regulate a small subset of genes through binding a novel RA response element consisting of two direct repeats with a 7 base pair spacer. These new findings suggest an important role for RAR phosphorylation during cell differentiation, and pave the way for further investigations with other cell types and during embryonic development. This article is part of a Special Issue entitled Linking transcription to physiology in lipodomics.

Genes involved in cell adhesion and signaling: a new repertoire of retinoic acid receptor target genes in mouse embryonic fibroblasts

Nuclear retinoic acid (RA) receptors (RARα, β and γ) are ligand-dependent transcription factors that regulate the expression of a battery of genes involved in cell differentiation and proliferation. They are also phosphoproteins and we previously showed the importance of their phosphorylation in their transcriptional activity. In the study reported here, we conducted a genome-wide analysis of the genes that are regulated by RARs in mouse embryonic fibroblasts (MEFs) by comparing wild-type MEFs to MEFs lacking the three RARs. We found that in the absence of RA, RARs control the expression of several gene transcripts associated with cell adhesion. Consequently the knockout MEFs are unable to adhere and to spread on substrates and they display a disrupted network of actin filaments, compared with the WT cells. In contrast, in the presence of the ligand, RARs control the expression of other genes involved in signaling and in RA metabolism. Taking advantage of rescue cell lines expressing the RARα or RARγ subtypes (either wild-type or mutated at the N-terminal phosphorylation sites) in the null background, we found that the expression of RA-target genes can be controlled either by a specific single RAR or by a combination of RAR isotypes, depending on the gene. We also selected genes that require the phosphorylation of the receptors for their regulation by RA. Our results increase the repertoire of genes that are regulated by RARs and highlight the complexity and diversity of the transcriptional programs regulated by RARs, depending on the gene.

Control of mammalian germ cell entry into meiosis

Germ cells are unique in undergoing meiosis to generate oocytes and sperm. In mammals, meiosis onset is before birth in females, or at puberty in males, and recent studies have uncovered several regulatory steps involved in initiating meiosis in each sex. Evidence suggests that retinoic acid (RA) induces expression of the critical pre-meiosis gene Stra8 in germ cells of the fetal ovary, pubertal testis and adult testis. In the fetal testis, CYP26B1 degrades RA, while FGF9 further antagonises RA signalling to suppress meiosis. Failsafe mechanisms involving Nanos2 may further suppress meiosis in the fetal testis. Here, we draw together the growing knowledge relating to these meiotic control mechanisms, and present evidence that they are co-ordinately regulated and that additional factors remain to be identified. Understanding this regulatory network will illuminate not only how the foundations of mammalian reproduction are laid, but also how mis-regulation of these steps can result in infertility or germline tumours.

Retinoids and breast cancer: from basic studies to the clinic and back again

All-trans retinoic acid (ATRA) is the most important active metabolite of vitamin A controlling segmentation in the developing organism and the homeostasis of various tissues in the adult. ATRA as well as natural and synthetic derivatives, collectively known as retinoids, are also promising agents in the treatment and chemoprevention of different types of neoplasia including breast cancer. The major aim of the present article is to review the basic knowledge acquired on the anti-tumor activity of classic retinoids, like ATRA, in mammary tumors, focusing on the underlying cellular and molecular mechanisms and the determinants of retinoid sensitivity/resistance. In the first part, an analysis of the large number of pre-clinical studies available is provided, stressing the point that this has resulted in a limited number of clinical trials. This is followed by an overview of the knowledge acquired on the role played by the retinoid nuclear receptors in the anti-tumor responses triggered by retinoids. The body of the article emphasizes the potential of ATRA and derivatives in modulating and in being influenced by some of the most relevant cellular pathways involved in the growth and progression of breast cancer. We review the studies centering on the cross-talk between retinoids and some of the growth-factor pathways which control the homeostasis of the mammary tumor cell. In addition, we consider the cross-talk with relevant intra-cellular second messenger pathways. The information provided lays the foundation for the development of rational and retinoid-based therapeutic strategies to be used for the management of breast cancer.