Nuclear Retinoic Acid Receptors Research Articles

A Novel Cytoplasmic Adaptor for Retinoic Acid Receptor (RAR) and Thyroid Receptor Functions as a Derepressor of RAR in the Absence of Retinoic Acid*

In most mammalian cells, the retinoic acid receptor (RAR) is nuclear rather than cytoplasmic, regardless of its cognate ligand, retinoic acid (RA). In testis Sertoli cells, however, RAR is retained in the cytoplasm and moves to the nucleus only when RA is supplied. This led us to identify a protein that regulates the translocation of RAR. From yeast two-hybrid screening, we identified a novel RAR-interacting protein called CART1 (cytoplasmic adaptor for RAR and TR). Systematic interaction assays using deletion mutants showed that the C-terminal CoRNR box of CART1 was responsible for the interaction with the NCoR binding region of RAR and TR. Such interaction was impaired in the presence of ligand RA, as further determined by GST pulldown assays in vitro and immunoprecipitation assays in vivo. Fluorescence microscopy showed that unliganded RAR was captured by CART1 in the cytoplasm, whereas liganded RAR was liberated and moved to the nucleus. Overexpression of CART1 blocked the transcriptional repressing activity of unliganded apoRAR, mediated by corepressor NCoR in the nucleus. CART1 siRNA treatment in a mouse Sertoli cell line, TM4, allowed RAR to move to the nucleus and blocked the derepressing function of CART1, suggesting that CART1 might be a cytoplasmic, testis-specific derepressor of RAR.

Retinal and retinol are potential regulators of gene expression in the keratinocyte cell line HaCaT

Vitamin A is a pivotal regulator of differentiation and growth of developing and adult skin. Retinoic acid is the major physiologically active form of vitamin A regulating the expression of different genes through retinoic acid nuclear receptors. Here, we present evidence that other vitamin A derivates - retinol and retinal - are also capable of functioning as regulators of gene expression in the keratinocyte cell line HaCaT. We have shown that all-trans retinol (ATRol) and all-trans retinal (ATRal) are capable of modulating gene expression in keratinocytes, which is not because of vitamin A metabolism in the cells, and retinol and retinal modulate gene expression through nuclear receptors: retinoic acid receptors (RARs) and retinoid X receptors (RXRs). Based on the data, we propose that ATRol and all-trans retinal, in addition to all-trans retinoic acid, can function as important regulators of gene expression manifesting their effect through the nuclear receptors RARs and RXRs.

Altered Retinoid Uptake and Action Contributes to Cell Survival in Endometriosis

Retinoic acid (RA) controls multiple biological processes via exerting opposing effects on cell survival. Retinol uptake into cells is controlled by stimulated by RA 6 (STRA6). RA is then produced from retinol in the cytosol. Partitioning of RA between the nuclear receptors RA receptor α and peroxisome-proliferator-activated receptor β/δ is regulated by cytosol-to-nuclear shuttling proteins cellular RA binding protein 2 (CRABP2) and fatty acid binding protein 5 (FABP5), which induce apoptosis or enhance survival, respectively. The roles of these mechanisms in endometrium or endometriosis remain unknown. The aim was to determine the regulation of retinoid uptake and RA action in primary stromal cells from endometrium (n = 10) or endometriosis (n = 10). Progesterone receptor was necessary for high STRA6 and CRABP2 expression in endometrial stromal cells. STRA6, which was responsible for labeled retinoid uptake, was strikingly lower in endometriotic cells compared to endometrial cells. CRABP2 knockdown in endometrial cells increased survival, and FABP5 knockdown in endometriotic cells decreased survival without altering the expression of downstream nuclear retinoic acid receptor α and peroxisome-proliferator-activated receptor β/δ. In endometrial stromal cells, progesterone receptor up-regulates expression of STRA6 and CRABP2, which control retinol uptake and growth-suppressor actions of RA. In endometriotic stromal cells, decreased expression of these genes leads to decreased retinol uptake and dominant FABP5-mediated prosurvival activity.

Vinexinβ, an atypical “sensor” of retinoic acid receptor γ signaling: union and sequestration, separation, and phosphorylation

The transcriptional activity of nuclear retinoic acid receptors (RARs) relies on the association/dissociation of coregulators at the ligand-binding domain. However, we determined that the N-terminal domain (NTD) also plays a role through its phosphorylation, and we isolated vinexinβ, a cytoskeleton protein with three SH3 domains, as a new partner of the RARγ NTD. Here we deciphered the mechanism of the interaction and its role in RARγ-mediated transcription. By combining molecular and biophysical (surface plasmon resonance, NMR, and fluorescence resonance energy transfer) approaches, we demonstrated that the third SH3 domain of vinexinβ interacts with a proline-rich domain (PRD) located in RARγ NTD and that phosphorylation at a serine located in the PRD abrogates the interaction. The affinity of the interaction was also evaluated. In vivo, vinexinβ represses RARγ-mediated transcription and we dissected the underlying mechanism in chromatin immunoprecipitation experiments performed with F9 cells expressing RARγ wild type or mutated at the phosphorylation site. In the absence of retinoic acid (RA), vinexinβ does not occupy RARγ target gene promoters and sequesters nonphosphorylated RARγ out of promoters. In response to RA, RARγ becomes phosphorylated and dissociates from vinexinβ. This separation allows RARγ to occupy promoters. This is the first report of an RAR corepressor association/dissociation out of promoters and regulated by phosphorylation.

NSAID Sulindac and Its Analog Bind RXRα and Inhibit RXRα-Dependent AKT Signaling

Nonsteroidal anti-inflammatory drugs (NSAIDs) exert their anticancer effects through cyclooxygenase-2 (COX-2)-dependent and independent mechanisms. Here, we report that Sulindac, an NSAID, induces apoptosis by binding to retinoid X receptor-alpha (RXRalpha). We identified an N-terminally truncated RXRalpha (tRXRalpha) in several cancer cell lines and primary tumors, which interacted with the p85alpha subunit of phosphatidylinositol-3-OH kinase (PI3K). Tumor necrosis factor-alpha (TNFalpha) promoted tRXRalpha interaction with the p85alpha, activating PI3K/AKT signaling. When combined with TNFalpha, Sulindac inhibited TNFalpha-induced tRXRalpha/p85alpha interaction, leading to activation of the death receptor-mediated apoptotic pathway. We designed and synthesized a Sulindac analog K-80003, which has increased affinity to RXRalpha but lacks COX inhibitory activity. K-80003 displayed enhanced efficacy in inhibiting tRXRalpha-dependent AKT activation and tRXRalpha tumor growth in animals.

Interference of some environmental contaminants with retinoid signaling in vitro

Retinoids are dietary hormones important during proliferation, differentiation and apoptosis.They act mainly through nuclear receptors for retinoic acid, RARs and RXRs. Disruption of these signaling pathways can lead to several adverse effects especially during embryonic development. Using pluripotent P19/A15 cell we tested whether some polycyclic aromatic hydrocarbons (PAHs), their N-heterocyclic analogs and some personal care products(UV filters, parabens) can interfere with retinoid signaling. Studied compounds were tested for direct activation of RAR/RXR signaling pathway, but also modulation of the response of natural ligand all-trans retinoic acid (ATRA) by studied compounds was tested. No direct activation was found for studied PAHs and N-PAHs, however, some of these compounds modulated the response of ATRA. Downregulation of the response was observed mostly for 2 and 3-ring N-PAHs, while 4 and 5-ring PAHs and N-PAHs mostly up-regulated the response of ATRA. Interference of personal care products with retinoid signaling in P19/A15 assay is further discussed in the second part of this study.

Repression of Cellular Retinoic Acid-binding Protein II during Adipocyte Differentiation

In preadipocytes, retinoic acid (RA) regulates gene expression by activating the nuclear RA receptor (RAR) and its cognate intracellular lipid-binding protein CRABP-II. It was previously reported that RA inhibits adipocyte differentiation but only when administered early during the differentiation program. The data presented here indicate that the diminished ability of RA to activate RAR following induction of differentiation stems from down-regulation of CRABP-II. The observations show that expression of CRABP-II in preadipocytes is repressed by all three components of the classical hormonal mixture that induces adipocyte differentiation, i.e. isobutylmethylxanthine, insulin, and dexamethasone. Isobutylmethylxanthine-dependent activation of protein kinase A triggered the phosphorylation of the transcription factor cAMP-response element-binding protein, which induced the expression of the cAMP-response element-binding protein family repressor cAMP-response element modulator. In turn, cAMP-response element modulator was found to associate with a cognate response element in the CRABP-II promoter and to repress CRABP-II expression. The data further show that CRABP-II is a direct target gene for the glucocorticoid receptor and that it is subjected to dexamethasone-induced glucocorticoid receptor-mediated repression during adipogenesis. Finally, the observations demonstrate that permanent repression of CRABP-II in mature adipocytes is exerted by the master regulator of adipocyte differentiation CCAAT/enhancer-binding protein alpha and is directly mediated through CCAAT/enhancer-binding protein alpha-response elements in the CRABP-II promoter. Taken together, the observations emphasize the important role of CRABP-II in regulating the transcriptional activity of RA through RAR, and they demonstrate that repression of this gene is critical for allowing adipogenesis to proceed.

Effects of the Aminophenol Analogue p-Dodecylaminophenol on Mouse Skin

p-Dodecylaminophenol (p-DDAP) was designed on the basis of structure-activity relationship studies on N-(4-hydroxyphenyl)retinamide (fenretinide, 4-HPR), a synthetic derivative of retinoic acid (RA). p-DDAP exhibits antioxidative activities greater than those of RA and 4-HPR. RA shows biological effects in epidermal cells that include the inhibition of differentiation to the squamous phenotype. In the current study, we examined the effects of topical p-DDAP treatment on the skin of hairless mice as compared with those of RA treatment. p-DDAP caused an increase in epidermal thickness and decreased matrix metalloprotease and hyaluronidase activities in mouse skin tissues to the same extent that RA did. p-DDAP did not induce desquamation, erythema, or inflammatory cytokine expression as observed with RA treatment. Two-dimensional polyacrylamide gel electrophoresis patterns of proteins from skin treated with p-DDAP were distinct from those treated with RA. A protein induced by both p-DDAP and RA was identified as cytokeratin 16. p-DDAP did not elevate transcriptional activities of RA nuclear receptors. These results suggest that p-DDAP improves skin as potently as RA without causing the desquamation and erythema that the latter does. An increase in cytokeratin 16 expression might be essential for the effects of both p-DDAP and RA in skin healing and maintenance.

Abstract 3315: TAC-101 induces the differentiation of cancer stem-like cells

Abstract TAC-101 (4-[3,5-bis(trimethylsilyl)benzamido] benzoic acid), is an orally bioavailable synthetic retinoid, binding to nuclear retinoic acid receptor (RAR), activates selectively RAR alpha transcriptional. And TAC-101 has demonstrated anti-tumor activity and proplongation of survival in primary and metastatic liver cancer pre-clinical models. In Phase I study, the lymph nodes of a non-small cell lung cancer patient with nodal metastasis returned to normal size, constituting a complete radiologic response. In a Phase I/II trial of TAC-101 in patients with advanced hepatocellular carcinoma, TAC-101 showed significant disease stabilization, 2 PRs after discontinuing TAC-101 treatment, and prolonged median survival, suggesting that TAC-101 provides meaningful patient'ss benefit. Recently, many researchers have reported correlation between cancer stem-like cells and tumor malignancy, such as therapy-resistance. In addition, retinoids were reported to reduce the number of cells with the cancer stem cell marker. So, we hypothesize that TAC-101 induces the differentiation of cancer stem-like cells, and contributes life span prolongation of patients of solid tumor. We cultured A549 cancer cells in mediummedium containing 0.1 and 1 μM of TAC-101 for 8 weeks and analyzed the expression of several stem cell markers using flow cytomoetry technique. Detection method of side population was validated by the result that fumitremorgin C, inhibitor of ABCG2, could eliminate the side population. We found that TAC-101 reduced side-population, CD326 (EpCAM) positive population and standard deviation of side scatter. From these results, it is estimated that treatment of TAC-101 for a certain period of time will be enough to induce of differentiation and to reduce cancer stem-like cells, may result in the tumor shrinkage and prolongation of patient's survival. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3315.

Retinoid receptor‐mediated signaling in motor neurons and its implications for amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is defined by progressive degeneration and death of upper and lower motor neurons. Recent work indicates retinoid signaling may contribute to the disease process. Nuclear retinoic acid receptors (RARs) are ligand‐activated transcription factors activated by all‐trans retinoic acid (ATRA); they mediate gene expression by forming heterodimers with retinoid X receptors (RXRs). We investigated the effects of RAR‐mediated signaling on motor neuron survival, morphology and downstream gene expression. Motor neuron‐enriched cultures established from embryonic day 14 rats were treated with pan‐agonists, pan‐antagonists and isotype‐specific agents. Toxicity studies were performed in the presence or absence of either hydrogen peroxide (to model oxidative stress/injury) or glutamate (to model excitotoxicity). In the presence of ATRA, the number of branch points/cell was decreased in motor neurons. RARalpha was localized to proximal and distal processes; RARbeta was localized to the cell body and nucleus. When ATRA was administered in the presence of toxin, motor neuron cell death was delayed. Signaling through RARs appears to maintain long neurites with less branching and delays toxin‐induced cell death. As we have observed increased nuclear RARbeta in sporadic ALS patients, downstream genes modulated by RARbeta are currently being characterized.Research Support: NIH grant ES013469 (RB); NIH T32 EB001026 (CK); NIH T32 05 TL1 RR024155‐02 (CK)

Involvement of Transcription Factor NR2F2 in Human Trophoblast Differentiation

BackgroundDuring the in vitro differentiation of human villous cytotrophoblast (CTB) cells to a syncytiotrophoblast (STB) phenotype, mRNA levels for the nuclear hormone receptor NR2F2 (ARP-1, COUP-TFII) increase rapidly, reaching a peak at day 1 of differentiation that is 8.8-fold greater than that in undifferentiated CTB cells. To examine whether NR2F2 is involved in the regulation of villous CTB cell differentiation, studies were performed to determine whether NR2F2 regulates the expression of TFAP2A (AP-2α), a transcription factor that is critical for the terminal differentiation of these cells to a STB phenotype.Methodology/Primary FindingsOverexpression of NR2F2 in primary cultures of human CTB cells and JEG-3 human choriocarcinoma cells induced dose-dependent increases in TFAP2A promoter activity. Conversely, siRNA mediated silencing of the NR2F2 gene in villous CTB undergoing spontaneous differentiation blocked the induction of the mRNAs for TFAP2A and several STB cell specific marker genes, including human placental lactogen (hPL), pregnancy specific glycoprotein 1 (PSG1) and corticotropin releasing hormone (CRH) by 51–59%. The induction of TFAP2A promoter activity by NR2F2 was potentiated by the nuclear hormone receptors retinoic acid receptor alpha (RARA) and retinoid X receptor alpha (RXRA).Conclusions/SignificanceTaken together, these results strongly suggest that NR2F2 is involved in villous CTB cell differentiation and that NR2F2 acts, at least in part, by directly activating TFAP2A gene expression and by potentiating the transactivation of TFAP2A by RARA and RXRA.

A high-fat diet induces lower expression of retinoid receptors and their target genes GAP-43/neuromodulin and RC3/neurogranin in the rat brain

Numerous studies have reported an association between cognitive impairment in old age and nutritional factors, including dietary fat. Retinoic acid (RA) plays a central role in the maintenance of cognitive processes via its nuclear receptors (NR), retinoic acid receptor (RAR) and retinoid X receptor (RXR), and the control of target genes, e.g. the synaptic plasticity markers GAP-43/neuromodulin and RC3/neurogranin. Given the relationship between RA and the fatty acid signalling pathways mediated by their respective NR (RAR/RXR and PPAR), we investigated the effect of a high-fat diet (HFD) on (1) PUFA status in the plasma and brain, and (2) the expression of RA and fatty acid NR (RARbeta, RXRbetagamma and PPARdelta), and synaptic plasticity genes (GAP-43 and RC3), in young male Wistar rats. In the striatum of rats given a HFD for 8 weeks, real-time PCR (RT-PCR) revealed a decrease in mRNA levels of RARbeta ( - 14 %) and PPARdelta ( - 13 %) along with an increase in RXRbetagamma (+52 %). Concomitantly, RT-PCR and Western blot analysis revealed (1) a clear reduction in striatal mRNA and protein levels of RC3 ( - 24 and - 26 %, respectively) and GAP-43 ( - 10 and - 42 %, respectively), which was confirmed by in situ hybridisation, and (2) decreased hippocampal RC3 and GAP-43 protein levels (approximately 25 %). Additionally, HFD rats exhibited a significant decrease in plasma ( - 59 %) and brain ( - 6 %) n-3 PUFA content, mainly due to the loss of DHA. These results suggest that dietary fat induces neurobiological alterations by modulating the brain RA signalling pathway and n-3 PUFA content, which have been previously correlated with cognitive impairment.

Keratinocyte differentiation induced by calcium, phorbol ester or interferon-γ elicits distinct changes in the retinoid signalling pathways

Retinoids influence keratinocyte proliferation and differentiation via binding to nuclear retinoic acid receptors (RARalpha, -gamma) and retinoid X receptor alpha (RXRalpha). The effect of keratinocyte differentiation on expression of nuclear retinoid receptors and on the conversion of retinol into retinoic acid has not been examined earlier in depth. Our aim was to examine the expression of retinoid receptors and a retinoid-regulated gene CRABPII, as well as the metabolism of exogenous [(3)H]retinol in cultured human keratinocytes induced to differentiate by exposure to either calcium, phorbol 12-myristate 13-acetate (PMA), or interferon-gamma (IFNgamma). Normal human keratinocytes were cultured and exposed to differentiation-inducing agents. The mRNA and protein expression of retinoid receptors were examined using real-time PCR and Western blot. [(3)H]Retinol uptake and metabolism was monitored by HPLC with on-line radioactivity detection. In calcium-exposed cells, increased expression of RARgamma and RXRalpha, enhanced metabolism of [(3)H]retinol to 3,4-didehydro-RA (ddRA), and an induction of CRABPII mRNA and protein was noted. In contrast, treatment with PMA and IFNgamma reduced the RARgamma and RXRalpha protein expression (preventable by the proteasome inhibitor MG132), increased the accumulation of [(3)H]RA and/or [(3)H]ddRA in the cells, and changed the CRABPII transcription. Retinoid signalling is profoundly altered upon differentiation of keratinocytes and the effects depend on how cellular differentiation is initiated.

Wnt/β-Catenin and Retinoic Acid Receptor Signaling Pathways Interact to Regulate Chondrocyte Function and Matrix Turnover

Activation of the Wnt/beta-catenin and retinoid signaling pathways is known to tilt cartilage matrix homeostasis toward catabolism. Here, we investigated possible interactions between these pathways. We found that all-trans-retinoic acid (RA) treatment of mouse epiphyseal chondrocytes in culture did increase Wnt/beta-catenin signaling in the absence or presence of exogenous Wnt3a, as revealed by lymphoid enhancer factor/T-cell factor/beta-catenin reporter activity and beta-catenin nuclear accumulation. This stimulation was accompanied by increased gene expression of Wnt proteins and receptors and was inhibited by co-treatment with Dickkopf-related protein-1, an extracellular inhibitor of Wnt/beta-catenin signaling, suggesting that RA modulates Wnt signaling at Wnt cell surface receptor level. RA also enhanced matrix loss triggered by Wnt/beta-catenin signaling, whereas treatment with a retinoid antagonist reduced it. Interestingly, overexpression of retinoic acid receptor gamma (RARgamma) strongly inhibited Wnt/beta-catenin signaling in retinoid-free cultures, whereas small interfering RNA-mediated silencing of endogenous RARgamma expression strongly increased it. Small interfering RNA-mediated silencing of RARalpha or RARbeta had minimal effects. Co-immunoprecipitation and two-hybrid assays indicated that RARgamma interacts with beta-catenin and induces dissociation of beta-catenin from lymphoid enhancer factor in retinoid-free cultures. The N-terminal domain (AF-1) of RARgamma but not the C-terminal domain (AF-2) was required for association with beta-catenin, whereas both AF-1 and AF-2 were necessary for inhibition of beta-catenin transcriptional activity. Taken together, our data indicate that the Wnt and retinoid signaling pathways do interact in chondrocytes, and their cross-talks and cross-regulation play important roles in the regulation of cartilage matrix homeostasis.

Convection-Enhanced Delivery of a Synthetic Retinoid Am80, Loaded into Polymeric Micelles, Prolongs the Survival of Rats Bearing Intracranial Glioblastoma Xenografts

Prognosis for the patients with glioblastoma, the most common malignant brain tumor, remains dismal. A major barrier to progress in treatment of glioblastoma is the relative inaccessibility of tumors to chemotherapeutic agents. Convection-enhanced delivery (CED) is a direct intracranial drug infusion technique to deliver chemotherapeutic agents to the central nervous system, circumventing the blood-brain barrier and reducing systemic side effects. CED can provide wider distribution of infused agents compared to simple diffusion. We have reported that CED of a polymeric micelle carrier system could yield a clinically relevant distribution of encapsulated agents in the rat brain. Our aim was to evaluate the efficacy of CED of polymeric micellar Am80, a synthetic agonist with high affinity to nuclear retinoic acid receptor, in a rat model of glioblastoma xenografts. We also used systemic administration of temozolomide, a DNA-alkylating agent, which has been established as the standard of care for newly diagnosed malignant glioma. U87MG human glioma cells were injected into the cerebral hemisphere of nude rats. Rats bearing U87MG xenografts were treated with CED of micellar Am80 (2.4 mg/m(2)) on day 7 after tumor implantation. Temozolomide (200 mg/m(2)/day) was intraperitoneally administered daily for 5 days, starting on day 7 after tumor implantation. CED of micellar Am80 provided significantly longer survival than the control. The combination of CED of micellar Am80 and systemic administration of temozolomide provided significantly longer survival than single treatment. In conclusion, temozolomide combined with CED of micellar Am80 may be a promising method for the treatment of malignant gliomas.

Cytokeratins 16 and 10 bind to retinoic acid covalently in skin tissue of mice

Retinoic acid (RA) has various biological effects in mammalian cells and tissues. In epidermal cells, RA is an inhibitor of differentiation to the squamous phenotype. The molecular mechanisms underlying the effects of RA on epidermal cells and other cell types are mediated by RA nuclear receptors and retinoylation (acylation by RA) of proteins. To understand the components responsible for RA effects via RA nuclear receptors and retinoylation. We examined for the first time RA-binding proteins in mouse skin in vivo by immunoblotting using anti-RA monoclonal antibodies and identified by matrix-assisted laser desorption ionization-time of flight mass spectrometry. We identified eight RA-binding proteins in the skin of hairless mice that were increased by topical RA treatment. Three of these proteins were identified as cytokeratin 10, cytokeratin 16 and serum albumin. These results raise the possibility that RA binding to cytokeratins in vivo may be involved in the effect of RA on keratinocytes in mouse skin.

Inhibition of ectopic bone formation by a selective retinoic acid receptor α‐agonist: A new therapy for heterotopic ossification?

Heterotopic ossification (HO) consists of formation of ectopic cartilage followed by endochondral bone and is triggered by major surgeries, large wounds, and other conditions. Current therapies, including low-dose irradiation, are not always effective and do not target the skeletogenic process directly. Because chondrogenesis requires a decrease of nuclear retinoic acid receptor alpha (RARalpha) action, we reasoned that pharmacologic activation of this receptor pathway should inhibit HO. Thus, we selected the synthetic retinoid NRX195183, a potent and highly selective RARalpha-agonist, and found that it did inhibit chondrogenesis in mouse limb micromass cultures. We established a mouse HO model consisting of subcutaneous implantation of Matrigel mixed with rhBMP-2. Control mice receiving daily oral doses of vehicle (peanut oil) or retinol (a natural nonactive retinoid precursor) developed large HO-like masses by days 9-12 that displayed abundant cartilage, endochondral bone, vessels, and marrow. In contrast, formation of HO-like masses was markedly reduced in companion mice receiving daily oral doses of alpha-agonist. These ectopic masses contained sharply reduced amounts of cartilage and bone, blood vessels, and TRAP-positive osteoclasts, and expressed markedly lower levels of master chondrogenic genes including Sox9, cartilage genes such as collagen XI and X, and osteogenic genes including Runx2. The data provide proof-of-principle evidence that a pharmacological strategy involving a selective RARalpha-agonist can indeed counteract an ectopic skeletal-formation process effectively and efficiently, and could thus represent a novel preventive treatment for HO.

All-trans retinoic acid promotes neural lineage entry by pluripotent embryonic stem cells via multiple pathways.

BackgroundAll-trans retinoic acid (RA) is one of the most important morphogens with pleiotropic actions. Its embryonic distribution correlates with neural differentiation in the developing central nervous system. To explore the precise effects of RA on neural differentiation of mouse embryonic stem cells (ESCs), we detected expression of RA nuclear receptors and RA-metabolizing enzymes in mouse ESCs and investigated the roles of RA in adherent monolayer culture.ResultsUpon addition of RA, cell differentiation was directed rapidly and exclusively into the neural lineage. Conversely, pharmacological interference with RA signaling suppressed this neural differentiation. Inhibition of fibroblast growth factor (FGF) signaling did not suppress significantly neural differentiation in RA-treated cultures. Pharmacological interference with extracellular signal-regulated kinase (ERK) pathway or activation of Wnt pathway effectively blocked the RA-promoted neural specification. ERK phosphorylation was enhanced in RA-treated cultures at the early stage of differentiation.ConclusionRA can promote neural lineage entry by ESCs in adherent monolayer culture systems. This effect depends on RA signaling and its crosstalk with the ERK and Wnt pathways.

Identification of a unique nuclear receptor for 9-cis retinoic acid.

Two families of nuclear receptors (RAR and RXR) for retinoic acid have previously been identified, cloned, and shown to be intimately involved with receptor-mediated regulation of gene transcription. All-trans-retinoic acid (t-RA) has been known to be a high-affinity ligand for RAR but only a weak ligand for RXR, while the endogenous ligand for RXR was unknown. Now two laboratories have isolated and chemically characterized 9-cis-retinoic acid (c-RA) and have shown that it is a naturally occurring ligand for RXR. These results suggest that isomerization of t-RA into c-RA may play an important role in retinoic acid biology.

The molecular basis of the inhibition of collagenase by vitamin A.

Retinoic acid inhibits the enzyme collagenase by forming an inactive complex between the liganded nuclear retinoic acid receptors and c-Jun, a protein that is itself an activator of the collagenase gene.