Recent insights on the role and regulation of retinoic acid signaling during epicardial development

The vitamin A metabolite, retinoic acid, carries out essential and conserved roles in vertebrate heart development. Retinoic acid signals via retinoic acid receptors (RAR)/retinoid X receptors (RXRs) heterodimers to induce the expression of genes that control cell fate specification, proliferation, and differentiation. Alterations in retinoic acid levels are often associated with congenital heart defects. Therefore, embryonic levels of retinoic acid need to be carefully regulated through the activity of enzymes, binding proteins and transporters involved in vitamin A metabolism. Here, we review evidence of the complex mechanisms that control the fetal uptake and synthesis of retinoic acid from vitamin A precursors. Next, we highlight recent evidence of the role of retinoic acid in orchestrating myocardial compact zone growth and coronary vascular development.     

Retinoid signaling can repress blastula Wnt signaling and impair dorsal development in Xenopus embryo

Vitamin A derivatives (retinoids) are actively involved during vertebrate embryogenesis. However, exogenous retinoids have also long been known as potent teratogens. The defects caused by retinoid treatment are complex. Here, we provided evidence that RAR-mediated retinoid signaling can repress Xenopus blastula Wnt signaling and impair dorsal development. Exogenous retinoic acid (RA) could antagonize the dorsalizing effects of lithium chloride-mediated Wnt activation in blastula embryos. The Wnt-responsive reporter gene transgenesis and luciferase assay showed that excess RA can repress the Wnt signaling in blastula embryos. In addition, the downstream target genes of the Wnt signaling that direct embryonic dorsal development, were also down-regulated in the RA-treated embryos. Mechanically, RA did not interfere with the stability of beta-catenin, but promoted its nuclear accumulation. The inverse agonist of retinoic acid receptors (RAR) rescued the Wnt signaling repression by RA and relieved the RA-induced nuclear accumulation of beta-catenin. Our results explain one of the reasons for the complicated teratogenic effects of retinoids and shed light on the endogenous way of interactions between two developmentally important signaling pathways.     

Regulation of Gdnf expression by retinoic acid in Sertoli cells

Glial cell line‐derived neurotrophic factor (GDNF) and retinoic acid (RA) are two molecules crucial for the regulation of the spermatogonial compartment of the testis. During the cycle of the seminiferous epithelium, their relative concentration oscillates with lower GDNF levels in stages where RA levels are high. It has been recently shown that RA negatively regulates Gdnf expression but the mechanisms behind are so far unknown. Here, we show that RA directly downregulates Gdnf mRNA levels in primary murine Sertoli cells through binding of RARα to a novel DR5‐RARE on Gdnf promoter. Pharmacological inhibition and chromatin immunoprecipitation–quantitative polymerase chain reaction analysis suggested that the underlying mechanism involved histone deacetylase activity and epigenetic repression of Gdnf promoter upon RA treatment.     

Regeneration of the gut requires retinoic acid in the budding ascidian Polyandrocarpa misakiensis

The protochordate ascidian Polyandrocarpa misakiensis has a striking ability to regenerate. When the posterior half of the adult body is amputated, the anterior half completely loses the esophagus, stomach and intestine. These organs are reconstituted in a week. Histological observation revealed that the regeneration involves transdifferentiation of the atrial epithelium near the cut surface. The morphological features of the gut primordium were similar to those observed in the developing bud of this species. Inhibitors of the synthesis of retinoic acid (RA) suppressed the formation of the gut. 13‐cis RA rescued the regenerates from the inhibitor‐induced hypoplasia. These results suggest that RA is required for the regeneration of the gut. A gene encoding the RA receptor (Pm‐RAR) and its target gene, TRAMP, were expressed in and around the regenerating gut. Pm‐RAR‐specific and TRAMP‐specific double‐stranded RNA molecules inhibited the regeneration of the gut, indicating that the RA signal is mediated at least in part by Pm‐RAR and TRAMP. These results suggested that RA triggers the transdifferentiation of the atrial epithelium into the gut in regenerating animals, as it does during asexual reproduction.     

视黄类受体与视黄酸致畸作用关系

视黄酸（维甲酸）可引起包括人在内的多种动物胚胎畸形,其生物活性是由一系列视黄酸受体及其配体介导的。其中视黄类受体RAR起主要作用,RAR的配体为强致畸物,相对致畸活性由强至弱依次为配体α、配体β和配体γ。视黄酸受体RXR的配体无致畸活性,但是可加强RAR激动剂的某些致畸郊应。视黄酸受体还可通过其它基因的表达而影响胚胎发育。对视黄类受体基因突变和不同视黄类受体及其配体与致畸作用的关系,以及此类受体对其它基因表达的调节作简要综述。 Abstract: Retinoic acid can induce teratogenesis of the fetus of many animals including human, and its biological activities are induced by a serious of different retinoic acid accepters and their ligands. The retinoic acid acceptor RAR plays key roles in the teratogenesis, and the legands of RAR are strong teratogens. The intensity sequence of the relative teratogenesis is ligandα、ligandβ and ligandγ. The ligands of the retinoic acid acceptor RXR cannot induce teratogenesis, but they can enhance the teratogenesis of the RAR stimulus. The retinoic acid acceptors can also affect the development of the fetus by adjusting the expression of the other genes. The relations between the gene mutation of the retinoic acid acceptor, various retinoic acid acceptors and their ligands and teratogenesis of retinoic acid are summarized in this article. In addition, the regulations of the retinoic acid acceptors to the other genes are also discussed.     

Zebrafish cerebrospinal fluid mediates cell survival through a retinoid signaling pathway

Cerebrospinal fluid (CSF) includes conserved factors whose function is largely unexplored. To assess the role of CSF during embryonic development, CSF was repeatedly drained from embryonic zebrafish brain ventricles soon after their inflation. Removal of CSF increased cell death in the diencephalon, indicating a survival function. Factors within the CSF are required for neuroepithelial cell survival as injected mouse CSF but not artificial CSF could prevent cell death after CSF depletion. Mass spectrometry analysis of the CSF identified retinol binding protein 4 (Rbp4), which transports retinol, the precursor to retinoic acid (RA). Consistent with a role for Rbp4 in cell survival, inhibition of Rbp4 or RA synthesis increased neuroepithelial cell death. Conversely, ventricle injection of exogenous human RBP4 plus retinol, or RA alone prevented cell death after CSF depletion. Zebrafish rbp4 is highly expressed in the yolk syncytial layer, suggesting Rbp4 protein and retinol/RA precursors can be transported into the CSF from the yolk. In accord with this suggestion, injection of human RBP4 protein into the yolk prevents neuroepithelial cell death in rbp4 loss‐of‐function embryos. Together, these data support the model that Rbp4 and RA precursors are present within the CSF and used for synthesis of RA, which promotes embryonic neuroepithelial survival. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 75–92, 2016     

Retinoic acid reduces migration of human breast cancer cells: role of retinoic acid receptor beta

Breast cancer is the most common malignancy in women and the appearance of distant metastases produces the death in 98% of cases. The retinoic acid receptor β (RARβ) is not expressed in 50% of invasive breast carcinoma compared with normal tissue and it has been associated with lymph node metastasis. Our hypothesis is that RARβ protein participates in the metastatic process. T47D and MCF7 breast cancer cell lines were used to perform viability assay, immunobloting, migration assays, RNA interference and immunofluorescence. Administration of retinoic acid (RA) in breast cancer cells induced RARβ gene expression that was greatest after 72 hrs with a concentration 1 μM. High concentrations of RA increased the expression of RARβ causing an inhibition of the 60% in cell migration and significantly decreased the expression of migration‐related proteins [moesin, c‐Src and focal adhesion kinase (FAK)]. The treatment with RARα and RARγ agonists did not affect the cell migration. On the contrary, the addition of the selective retinoid RARβ‐agonist (BMS453) significantly reduced cell migration comparable to RA inhibition. When RARβ gene silencing was performed, the RA failed to significantly inhibit migration and resulted ineffective to reduce moesin, c‐Src and FAK expressions. RARβ is necessary to inhibit migration induced by RA in breast cancer cells modulating the expression of proteins involved in cell migration.     

Retinoic acid stimulates growth hormone synthesis in human somatotrophic adenoma cells: Characterization of its nuclear receptors

In order to gain a better understanding on the possible role of retinoic acid (RA) on human GH secretion, we have characterized the expression of its nuclear receptors in somatotropic adenoma cell extracts. By immunoblotting with rabbit polyclonal antibodies directed against RARα, β, and γ and RXRα and β, we could only detect the presence of RARα and RXRα proteins. The predominant expression of RXRα was confirmed at the mRNA level by Northern and slot-blot analysis. When then investigated the effect of RA on GH synthesis in cell culture of adenomatous somatotrophs. In cultured cells, RA (1 μM) stimulated GH secretion, increased intracellular GH content and GH mRNA levels within 72 h, suggesting a modulation of GH synthesis by RA. J. Cell. Biochem 65:25–31. © 1997 Wiley-Liss, Inc.     

A role for retinoic acid in regulating the regeneration of deer antlers

Deer antlers are the only mammalian organs that can be repeatedly regenerated; each year, these complex structures are shed and then regrow to be used for display and fighting. To date, the molecular mechanisms controlling antler regeneration are not well understood. Vitamin A and its derivatives, retinoic acids, play important roles in embryonic skeletal development. Here, we provide several lines of evidence consistent with retinoids playing a functional role in controlling cellular differentiation during bone formation in the regenerating antler. Three receptors (alpha, beta, gamma) for both the retinoic acid receptor (RAR) and retinoid X receptor (RXR) families show distinct patterns of expression in the growing antler tip, the site of endochondral ossification. RAR alpha and RXR beta are expressed in skin ("velvet") and the underlying perichondrium. In cartilage, which is vascularised, RXR beta is specifically expressed in chondrocytes, which express type II collagen, and RAR alpha in perivascular cells, which also express type I collagen, a marker of the osteoblast phenotype. High-performance liquid chromatography analysis shows significant amounts of Vitamin A (retinol) in antler tissues at all stages of differentiation. The metabolites all-trans-RA and 4-oxo-RA are found in skin, perichondrium, cartilage, bone, and periosteum. The RXR ligand, 9-cis-RA, is found in perichondrium, mineralised cartilage, and bone. To further define sites of RA synthesis in antler, we immunolocalised retinaldehyde dehydrogenase type 2 (RALDH-2), a major retinoic acid-generating enzyme. RALDH-2 is expressed in the skin and perichondrium and in perivascular cells in cartilage, although chondroprogenitors and chondrocytes express very low levels. At sites of bone formation, differentiated osteoblasts which express the bone-specific protein osteocalcin express high levels of RALDH2. The effect of RA on antler cell differentiation was studied in vitro; all-trans-RA inhibits expression of the chondrocyte phenotype, an effect that is blocked by addition of the RAR antagonist Ro41-5253. In monolayer cultures of mesenchymal progenitor cells, all-trans-RA increases the expression of alkaline phosphatase, a marker of the osteoblastic phenotype. In summary, this study has shown that antler tissues contain endogenous retinoids, including 9-cis RA, and the enzyme RALDH2 that generates RA. Sites of RA synthesis in antler correspond closely with the localisation of cells which express receptors for these ligands and which respond to the effects of RA.     

Nuclear all-trans retinoic acid receptors

The present study was undertaken to investigate the effects of selenite (Se^IV) and selenate (Se^VI) on the all-trans retinoic acid (RA)-nuclear retinoic acid receptor (RAR) complex formation in rat liver. We also present the data on the in vitro effects of Se^IV on the RARα and the type I iodothyronine 5′-deiodinase gene expression in the GH₄C₁ rat pituitary tumor cells. Se^IV at 1.0 μmol/L was found to reduce (p<0.05) the RA specific binding to RAR in rat liver. Dithiothreitol (DTT), a protective agent for sulfhydryl groups, was found to be slightly effective in protecting the RAR binding properties when affected by Se^IV. Se^VI at 0.1 μmol/L reduced (p<0.05) the RA specific binding to RAR in liver, as well. Seleno-l-methionine (Se-^II) when compared tol-methionine did not exert any inhibitory effect on the formation of the RA-RAR complex. Se^IV (up to 2.5 μmol/L) has no inhibitory effect on GH₄C₁ cell proliferation as well as the prolactin secretion. Se^IV at 1.0 μmol/L significantly decreases the rate of mRNA synthesis and/or degradation of the α form of the RAR and causes the enhancement of the type I iodothyronine 5′-deiodinase gene expression in GH₄C₁ cells. The results based on in vitro experiments suggest that inorganic selenium may affect the RA specific binding to their cognate receptor molecules, and it may reduce expression of the gene encoding the RARα, with the cell vitality and the cell growth remaining unchanged.