Novel retinoic acid receptor ligands in Xenopus embryos

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.

Anterior patterning genes induced by Zic1 are sensitive to retinoic acid and its metabolite, 4-oxo-RA

BACKGROUND

Development of paired sensory organs is a highly complex and coordinated process. These organs arise from ectodermal thickenings in the cephalic region known as cranial placodes. We have previously shown that Zic1 is a critical regulator for the formation of the pre-placodal region (PPR), the common territory for the development of all cranial placodes in Xenopus laevis.

RESULTS

In this study, we have analyzed a number of Zic1 targets for their expression during PPR patterning, as well as their regulation by retinoic acid (RA) and one of its major metabolites, 4-oxo-RA. Our findings show that anteriorly Zic1 regulates several transcription factors, Crx, Fezf2, Nkx3-1, and Xanf1 as well as a serine/threonine/tyrosine kinase, Pkdcc.2. These factors are all expressed in the vicinity of the PPR and as such are candidate regulators of placode formation downstream of Zic1. In addition to their differential regulation by RA, we find that 4-oxo-RA is also capable of modulating the expression of these genes, as well as a broad array of RA-regulated genes.

CONCLUSION

Our data highlight the complexity of retinoid-mediated regulation required for Zic1-activated anterior structure specification in Xenopus, and the potential physiological role of 4-oxo-RA in cranial placode development.

E-liquids and vanillin flavoring disrupts retinoic acid signaling and causes craniofacial defects in Xenopus embryos

Environmental teratogens such as smoking are known risk factors for developmental disorders such as cleft palate. While smoking rates have declined, a new type of smoking, called vaping is on the rise. Vaping is the use of e-cigarettes to vaporize and inhale an e-liquid containing nicotine and food-like flavors. There is the potential that, like smoking, vaping could also pose a danger to the developing human. Rather than waiting for epidemiological and mammalian studies, we have turned to an aquatic developmental model, Xenopus laevis, to more quickly assess whether e-liquids contain teratogens that could lead to craniofacial malformations. Xenopus, like zebrafish, has the benefit of being a well-established developmental model and has also been effective in predicting whether a chemical could be a teratogen. We have determined that embryonic exposure to dessert flavored e-liquids can cause craniofacial abnormalities, including an orofacial cleft in Xenopus. To better understand the underlying mechanisms contributing to these defects, transcriptomic analysis of the facial tissues of embryos exposed to a representative dessert flavored e-liquid vapor extract was performed. Analysis of differentially expressed genes in these embryos revealed several genes associated with retinoic acid metabolism or the signaling pathway. Consistently, retinoic acid receptor inhibition phenocopied the craniofacial defects as those embryos exposed to the vapor extract of the e-liquid. Such malformations also correlated with a group of common differentially expressed genes, two of which are associated with midface birth defects in humans. Further, e-liquid exposure sensitized embryos to forming craniofacial malformations when they already had depressed retinoic acid signaling. Moreover, 13-cis-retinoic acid treatment could significantly reduce the e-liquid induced malformation in the midface. Such results suggest the possibility of an interaction between retinoic acid signaling and e-liquid exposure. One of the most popular and concentrated flavoring chemicals in dessert flavored e-liquids is vanillin. Xenopus embryos exposed to this chemical closely resembled embryos exposed to dessert-like e-liquids and a retinoic acid receptor antagonist. In summary, we determined that e-liquid chemicals, in particular vanillin, can cause craniofacial defects potentially by dysregulating retinoic acid signaling. This work warrants the evaluation of vanillin and other such flavoring additives in e-liquids on mammalian development.

Retinoic acid production, regulation and containment through Zic1, Pitx2c and Cyp26c1 control cranial placode specification

All paired sensory organs arise from a common precursor domain called the pre-placodal region (PPR). In Xenopus, Zic1 non-cell autonomously regulates PPR formation by activating retinoic acid (RA) production. Here, we have identified two Zic1 targets, the RA catabolizing enzyme Cyp26c1 and the transcription factor Pitx2c, expressed in the vicinity of the PPR as being crucially required for maintaining low RA levels in a spatially restricted, PPR-adjacent domain. Morpholino- or CRISPR/Cas9-mediated Cyp26c1 knockdown abrogated PPR gene expression, yielding defective cranial placodes. Direct measurement of RA levels revealed that this is mediated by a mechanism involving excess RA accumulation. Furthermore, we show that pitx2c is activated by RA and required for Cyp26c1 expression in a domain-specific manner through induction of FGF8. We propose that Zic1 anteriorly establishes a program of RA containment and regulation through activation of Cyp26c1 and Pitx2c that cooperates to promote PPR specification in a spatially restricted domain.

Disruption of Nuclear Receptor Signaling Alters Triphenyl Phosphate-Induced Cardiotoxicity in Zebrafish Embryos

Triphenyl phosphate (TPHP) is an unsubstituted aryl phosphate ester used as a flame retardant and plasticizer within the United States. Using zebrafish as a model, the objectives of this study were to rely on (1) mRNA-sequencing to uncover pathways disrupted following embryonic TPHP exposure and (2) high-content screening to identify nuclear receptor ligands that enhance or mitigate TPHP-induced cardiotoxicity. Based on mRNA-sequencing, TPHP exposure from 24 to 72-h postfertilization (hpf) resulted in a concentration-dependent increase in the number of transcripts significantly affected at 72 hpf, and pathway analysis revealed that 5 out of 9 nuclear receptor pathways were associated with the retinoid X receptor (RXR). Based on a screen of 74 unique nuclear receptor ligands as well as follow-up experiments, 2 compounds-ciglitazone (a peroxisome proliferator-activated receptor gamma, or PPARγ, agonist) and fenretinide (a pan-retinoic acid receptor, or RAR, agonist)-reliably mitigated TPHP-induced cardiotoxicity in the absence of effects on TPHP uptake or metabolism. As these data suggested that TPHP may be activating RXR (a heterodimer for both RARs and PPARγ), we coexposed embryos to HX 531-a pan-RXR antagonist-from 24 to 72 hpf and, contrary to our hypothesis, found that coexposure to HX 531 significantly enhanced TPHP-induced cardiotoxicity. Using a luciferase reporter assay, we also found that TPHP did not activate nor inhibit chimeric human RXRα, RXRβ, or RXRγ, suggesting that TPHP does not directly bind nor interact with RXRs. Overall, our data suggest that TPHP may interfere with RXR-dependent pathways involved in cardiac development.

RARγ is required for mesodermal gene expression prior to gastrulation in Xenopus

The developing vertebrate embryo is exquisitely sensitive to retinoic acid (RA) concentration, particularly during anteroposterior patterning. In contrast to Nodal and Wnt signaling, RA was not previously considered to be an instructive signal in mesoderm formation during gastrulation. Here, we show in Xenopus that RARγ is indispensable for the expression of early mesoderm markers and is, therefore, an obligatory factor in mesodermal competence and/or maintenance. We identified several novel targets upregulated by RA receptor signaling in the early gastrula that are expressed in the circumblastoporal ring and linked to mesodermal development. Despite overlapping expression patterns of the genes encoding the RA-synthesizing enzyme Aldh1a2 and the RA-degrading enzyme Cyp26a1, RARγ1 functions as a transcriptional activator in early mesoderm development, suggesting that RA ligand is available to the embryo earlier than previously appreciated. RARγ1 is required for cellular adhesion, as revealed by spontaneous dissociation and depletion of ncam1 mRNA in animal caps harvested from RARγ1 knockdown embryos. RARγ1 knockdown obliterates somite boundaries, and causes loss of Myod protein in the presomitic mesoderm, but ectopic, persistent expression of Myod protein in the trunk. Thus, RARγ1 is required for stabilizing the mesodermal fate, myogenic commitment, somite boundary formation, and terminal skeletal muscle differentiation.

Effects of Perinatal Exposure to Dibutyltin Chloride on Fat and Glucose Metabolism in Mice, and Molecular Mechanisms, in Vitro

BACKGROUND

The organotin dibutyltin (DBT) is used in the manufacture of polyvinyl chloride (PVC) plastics, in construction materials, and in medical devices. Previous animal studies showed detrimental effects of DBT during in utero development at relatively high doses, but little was known about the effects of DBT exposure at environmentally relevant doses on endpoints such as obesity and metabolic disease.

OBJECTIVES

We tested the potential obesogenic effects of DBT using in vitro and in vivo models.

METHODS

We evaluated the effects of DBT on nuclear receptor activation and adipogenic potential using human and mouse multipotent mesenchymal stromal stem cells (MSCs). We also evaluated the effects of perinatal exposure to environmentally relevant doses of DBT in C57BL/6J mice.

RESULTS

DBT activated human and mouse PPARγ and RXRα in transient transfection assays, increased expression of adipogenic genes, promoted adipogenic differentiation and increased lipid accumulation in mouse and human MSCs, in vitro. DBT-induced adipogenic differentiation was abolished by the PPARγ antagonist T0070907, indicating that DBT was acting primarily through PPARγ. Perinatal exposure to low doses of DBT led to increased fat storage, decreased glucose tolerance, and increased circulating leptin levels in male, but not female, mice.

CONCLUSIONS

DBT acted as an obesogen by inducing lipid accumulation in human and mouse MSCs through a PPARγ-dependent pathway. In vivo exposure to biologically relevant doses of DBT during perinatal development led to increased fat storage, elevated leptin levels in plasma, and glucose intolerance in mice. Based on these findings, we posit that monitoring of DBT levels in human samples may aid in understanding and potentially preventing the rising rates of metabolic disorders in human populations. https://doi.org/10.1289/EHP3030.

Generating retinoic acid gradients by local degradation during craniofacial development: One cell's cue is another cell's poison

Retinoic acid (RA) is a vital morphogen for early patterning and organogenesis in the developing embryo. RA is a diffusible, lipophilic molecule that signals via nuclear RA receptor heterodimeric units that regulate gene expression by interacting with RA response elements in promoters of a significant number of genes. For precise RA signaling, a robust gradient of the morphogen is required. The developing embryo contains regions that produce RA, and specific intracellular concentrations of RA are created through local degradation mediated by Cyp26 enzymes. In order to elucidate the mechanisms by which RA executes precise developmental programs, the kinetics of RA metabolism must be clearly understood. Recent advances in techniques for endogenous RA detection and quantification have paved the way for mechanistic studies to shed light on downstream gene expression regulation coordinated by RA. It is increasingly coming to light that RA signaling operates not only at precise concentrations but also employs mechanisms of degradation and feedback inhibition to self-regulate its levels. A global gradient of RA throughout the embryo is often found concurrently with several local gradients, created by juxtaposed domains of RA synthesis and degradation. The existence of such local gradients has been found especially critical for the proper development of craniofacial structures that arise from the neural crest and the cranial placode populations. In this review, we summarize the current understanding of how local gradients of RA are established in the embryo and their impact on craniofacial development.

An Endogenous Mammalian Retinoid X Receptor Ligand, At Last!

9-cis-Retinoic acid was identified and claimed to be the endogenous ligand of the retinoid X receptors (RXRs) in 1992. Since then, the endogenous presence of this compound has never been rigorously confirmed. Instead, concerns have been raised by other groups that have reported that 9-cis-retinoic acid is undetectable or that its presence occurs at very low levels. Furthermore, these low levels could not satisfactorily explain the physiological activation of RXR. Alternative ligands, among them various lipids, have also been identified, but also did not fulfill criteria for rigorous endogenous relevance, and their consideration as bona fide endogenous mammalian RXR ligand has likewise been questioned. Recently, novel studies claim that the saturated analogue 9-cis-13,14-dihydroretinoic acid functions as an endogenous physiologically relevant mammalian RXR ligand.

Quantitation of retinaldehyde in small biological samples using ultrahigh-performance liquid chromatography tandem mass spectrometry

We report an ultrahigh-performance liquid chromatography tandem mass spectrometry (UHPLC-MS/MS) method to quantify all-trans-retinal in biological samples of limited size (15-35mg), which is especially advantageous for use with adipose. To facilitate recovery, retinal and the internal standard 3,4-didehydroretinal were derivatized in situ into their O-ethyloximes. UHPLC resolution combined with high sensitivity and specificity of MS/MS allowed quantification of retinal-O-ethyloximes with a 5-fmol lower limit of detection and a linear range from 5fmol to 1pmol. This assay revealed that extraocular concentrations of retinal range from approximately 2 to 40pmol/g in multiple tissues-the same range as all-trans-retinoic acid. All-trans-retinoic acid has high affinity (kd⩽0.4nM) for its nuclear receptors (RARα, -β, and -γ), whereas retinal has low (if any) affinity for these receptors, making it unlikely that these retinal concentrations would activate RAR. We also show that the copious amount of vitamin A used in chow diets increases retinal in adipose depots 2- to 5-fold relative to levels in adipose of mice fed a vitamin A-sufficient diet, as recommended for laboratory rodents. This assay also is proficient for quantifying conversion of retinol into retinal in vitro and, therefore, provides an efficient method to study metabolism of retinol in vivo and in vitro.

Active repression by RARγ signaling is required for vertebrate axial elongation

Retinoic acid receptor gamma 2 (RARγ2) is the major RAR isoform expressed throughout the caudal axial progenitor domain in vertebrates. During a microarray screen to identify RAR targets, we identified a subset of genes that pattern caudal structures or promote axial elongation and are upregulated by increased RAR-mediated repression. Previous studies have suggested that RAR is present in the caudal domain, but is quiescent until its activation in late stage embryos terminates axial elongation. By contrast, we show here that RARγ2 is engaged in all stages of axial elongation, not solely as a terminator of axial growth. In the absence of RA, RARγ2 represses transcriptional activity in vivo and maintains the pool of caudal progenitor cells and presomitic mesoderm. In the presence of RA, RARγ2 serves as an activator, facilitating somite differentiation. Treatment with an RARγ-selective inverse agonist (NRX205099) or overexpression of dominant-negative RARγ increases the expression of posterior Hox genes and that of marker genes for presomitic mesoderm and the chordoneural hinge. Conversely, when RAR-mediated repression is reduced by overexpressing a dominant-negative co-repressor (c-SMRT), a constitutively active RAR (VP16-RARγ2), or by treatment with an RARγ-selective agonist (NRX204647), expression of caudal genes is diminished and extension of the body axis is prematurely terminated. Hence, gene repression mediated by the unliganded RARγ2-co-repressor complex constitutes a novel mechanism to regulate and facilitate the correct expression levels and spatial restriction of key genes that maintain the caudal progenitor pool during axial elongation in Xenopus embryos.

Chemical activation of RARβ induces post-embryonically bilateral limb duplication during Xenopus limb regeneration

The anuran amphibian Xenopus laevis can regenerate its limbs for a limited time during the larval stage, while limbs are still developing. Using this regeneration model, we evaluated the proximo-distal blastema cell identity when endogenous retinoids were increased by CYP26 inhibition or when RAR-specific agonists altered RA signaling. Simultaneous proximo-distal and antero-posterior limb duplications were generated, and the RAR-specific agonist can modify blastema identity after amputation, because chemical activation of RARβ produced bilateral hindlimb duplications that resulted in a drastic duplication phenotype of regenerating limbs.

Retinyl ester hydrolases and their roles in vitamin A homeostasis

In mammals, dietary vitamin A intake is essential for the maintenance of adequate retinoid (vitamin A and metabolites) supply of tissues and organs. Retinoids are taken up from animal or plant sources and subsequently stored in form of hydrophobic, biologically inactive retinyl esters (REs). Accessibility of these REs in the intestine, the circulation, and their mobilization from intracellular lipid droplets depends on the hydrolytic action of RE hydrolases (REHs). In particular, the mobilization of hepatic RE stores requires REHs to maintain steady plasma retinol levels thereby assuring constant vitamin A supply in times of food deprivation or inadequate vitamin A intake. In this review, we focus on the roles of extracellular and intracellular REHs in vitamin A metabolism. Furthermore, we will discuss the tissue-specific function of REHs and highlight major gaps in the understanding of RE catabolism. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

Retinoids regulate stem cell differentiation

Retinoids are ubiquitous signaling molecules that influence nearly every cell type, exert profound effects on development, and complement cancer chemotherapeutic regimens. All-trans retinoic acid (RA) and other active retinoids are generated from vitamin A (retinol), but key aspects of the signaling pathways required to produce active retinoids remain unclear. Retinoids generated by one cell type can affect nearby cells, so retinoids also function in intercellular communication. RA induces differentiation primarily by binding to RARs, transcription factors that associate with RXRs and bind RAREs in the nucleus. Binding of RA: (1) initiates changes in interactions of RAR/RXRs with co-repressor and co-activator proteins, activating transcription of primary target genes; (2) alters interactions with proteins that induce epigenetic changes; (3) induces transcription of genes encoding transcription factors and signaling proteins that further modify gene expression (e.g., FOX03A, Hoxa1, Sox9, TRAIL, UBE2D3); and (4) results in alterations in estrogen receptor α signaling. Proteins that bind at or near RAREs include Sin3a, N-CoR1, PRAME, Trim24, NRIP1, Ajuba, Zfp423, and MN1/TEL. Interactions among retinoids, RARs/RXRs, and these proteins explain in part the powerful effects of retinoids on stem cell differentiation. Studies of this retinol signaling cascade enhance our ability to understand and regulate stem cell differentiation for therapeutic and scientific purposes. In cancer chemotherapeutic regimens retinoids can promote tumor cell differentiation and/or induce proteins that sensitize tumors to drug combinations. Mechanistic studies of retinoid signaling continue to suggest novel drug targets and will improve therapeutic strategies for cancer and other diseases, such as immune-mediated inflammatory diseases.

Determination and occurrence of retinoic acids and their 4-oxo metabolites in Liaodong Bay, China, and its adjacent rivers

Retinoic acids (RAs) and their metabolites play an important role in abnormal morphological development and are speculated to be a possible cause for the increased rates of deformities in wild frog populations. In the current study, a method using ultra-performance liquid chromatography-electrospray ionization tandem mass spectrometry was developed for simultaneously analyzing all-trans-RA (at-RA), 13-cis-RA (13c-RA), 9-cis-RA (9c-RA), and their 4-oxo metabolites, all-trans-4-oxo-RA (at-4-oxo-RA), 13-cis-4-oxo-RA (13c-4-oxo-RA), and 9-cis-4-oxo-RA (9c-4-oxo-RA) in wastewaters and surface waters. Method detection limits were matrix dependent, ranging from 0.02 to 0.37 ng/L. The method was used to investigate the occurrence of RAs and 4-oxo-RAs in Liaodong Bay and its adjacent rivers. Of these six retinoids, at-RA, 13c-RA, at-4-oxo-RA, and 13c-4-oxo-RA were detected in river waters at detection frequencies of 100%, 92%, 48.6%, and 21.6%, and concentrations of 0.05 to 1.23 ng/L, less than 0.03 to 0.41 ng/L, less than 0.02 to 1.00 ng/L, and less than 0.06 to 0.81 ng/L, respectively. Retinoic acids were detected for the first time in the aquatic environment and were found to be more persistent than 4-oxo-RAs. The hazard quotient for mortality of frog embryos caused by induction by retinoids detected in the current study was then estimated, and the value was calculated to be 0.09. No retinoid was detected in seawaters.

Quantification of endogenous retinoids

Numerous physiological processes require retinoids, including development, nervous system function, immune responsiveness, proliferation, differentiation, and all aspects of reproduction. Reliable retinoid quantification requires suitable handling and, in some cases, resolution of geometric isomers that have different biological activities. Here we describe procedures for reliable and accurate quantification of retinoids, including detailed descriptions for handling retinoids, preparing standard solutions, collecting samples and harvesting tissues, extracting samples, resolving isomers, and detecting with high sensitivity. Sample-specific strategies are provided for optimizing quantification. Approaches to evaluate assay performance also are provided. Retinoid assays described here for mice also are applicable to other organisms including zebrafish, rat, rabbit, and human and for cells in culture. Retinoid quantification, especially that of retinoic acid, should provide insight into many diseases, including Alzheimer's disease, type 2 diabetes, obesity, and cancer.

Metabolism and regulation of gene expression by 4-oxoretinol versus all-trans retinoic acid in normal human mammary epithelial cells

We previously demonstrated that 4-oxoretinol (4-oxo-ROL) activated retinoic acid receptors (RARs) in F9 stem cells. We showed that 4-oxo-ROL inhibited the proliferation of normal human mammary epithelial cells (HMECs). To understand the mechanisms by which 4-oxo-ROL regulates HMEC growth we examined gene expression profiles following 4-oxo-ROL or all-trans retinoic acid (tRA). We also compared growth inhibition by tRA, 4-oxo-ROL, or 4-oxo-RA. All three retinoids inhibited HMEC proliferation. Gene expression analyses indicated that 4-oxo-ROL and tRA modulated gene expression in closely related pathways. The expression of many genes, e.g. ATP-binding cassette G1 (ABCG1); adrenergic receptorbeta2 (ADRB2); ras-related C3 botulinum toxin substrate (RAC2); and short-chain dehydrogenase/reductase 1 gene (SDR1) was changed after 4-oxo-ROL or tRA. Metabolism of these retinoids was analyzed by high-performance liquid chromatography (HPLC). In 1 microM tRA treated HMECs all of the tRA was found intracellularly, and tRA was the predominant intracellular retinoid. In 1 microM 4-oxo-ROL treated HMECs most 4-oxo-ROL was esterified to 4-oxoretinyl esters, no tRA was detected, and 4-oxo-ROL and 4-oxo-RA were observed intracellularly. In 1 microM 4-oxoretinoic acid (4-oxo-RA) treated HMECs little intracellular 4-oxo-RA was detected; most 4-oxo-RA was in the medium. Our results indicate that: (a) 4-oxo-ROL regulates gene expression and inhibits proliferation of HMECs; (b) 4-oxo-ROL and tRA regulate some of the same genes; (c) more tRA is found in cells, as compared to 4-oxoretinoic acid, when each drug is added at the same concentration in the medium; and (d) the mechanism by which 4-oxo-ROL exerts its biological activity does not involve intracellular tRA production.

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.

Locust retinoid X receptors: 9-Cis-retinoic acid in embryos from a primitive insect

The retinoid X receptor (RXR) is activated by its often elusive cognate ligand, 9-cis-retinoic acid (9-cis-RA). In flies and moths, molting is mediated by a heterodimer ecdysone receptor consisting of the ecdysone monomer (EcR) and an RXR homolog, ultraspiracle (USP); the latter is believed to have diverged from its RXR origin. In the more primitive insect, Locusta migratoria (Lm), RXR is more similar to human RXRs than to USPs. LmRXR was detected in early embryos when EcR transcripts were absent, suggesting another role apart from ecdysone signaling. Recombinant LmRXRs bound 9-cis-RA and all-trans-RA with high affinity (IC(50) = 61.2-107.7 nM; K(d) = 3 nM), similar to human RXR. To determine whether specific binding had functional significance, the presence of endogenous retinoids was assessed. Embryos were extracted by using modified Bligh and Dyer and solid-phase protocols to avoid the oily precipitate that makes this material unsuitable for assay. These extracts contained retinoids (5.4 nM) as assessed by RA-inducible Cyp26A1-promoter luciferase reporter cell lines. Furthermore, the use of HPLC and MS confirmed the presence of retinoids and identified in any embryo, 9-cis-RA, in addition to all-trans-RA. We estimate that whole embryos contain 3 nM RA, including 9-cis-RA at a concentration of 1.6 nM. These findings strongly argue for a functional role for retinoids in primitive insects and favor a model where signaling through the binding of 9-cis-RA to its RXR is established relatively early in evolution and embryonic development.

A role of unliganded thyroid hormone receptor in postembryonic development in Xenopus laevis

A fascinating feature of thyroid hormone (T3) receptors (TR) is that they constitutively bind to promoter regions of T3-response genes, providing dual functions. In the presence of T3, TR activates T3-inducible genes, while unliganded TR represses these same genes. Although this dual function model is well demonstrated at the molecular level, few studies have addressed the presence or the role of unliganded TR-induced repression in physiological settings. Here, we analyze the role of unliganded TR in Xenopus laevis development. The total dependence of amphibian metamorphosis upon T3 provides us a valuable opportunity for studying TR function in vivo. First, we designed a dominant negative form of TR-binding corepressor N-CoR (dnN-CoR) consisting of its receptor interacting domain. We confirmed its dominant negative activity by showing that dnN-CoR competes away the binding of endogenous N-CoR to unliganded TR and relieves unliganded TR-induced gene repression in frog oocytes. Next, we overexpressed dnN-CoR in tadpoles through transgenesis and analyzed its effect on gene expression and development. Quantitative RT-PCR revealed significant derepression of T3-response genes in transgenic animals. In addition, transgenic tadpoles developed faster than wild type siblings, with an acceleration of as much as 7 days out of the 30-day experiment. These data thus provide in vivo evidence for the presence and a role of unliganded TR-induced gene repression in physiological settings and strongly support our earlier model that unliganded TR represses T3-response genes in premetamorphic tadpoles to regulate the progress of development.

LIF removal increases CRABPI and CRABPII transcripts in embryonic stem cells cultured in retinol or 4-oxoretinol

Murine embryonic stem (ES) cells cultured without leukemia inhibitory factor (LIF) or with retinoids differentiate and concomitantly metabolize retinol (vitamin A) to 4-oxoretinol. Our objective was to examine the effects of retinol or 4-oxoretinol on cellular retinoic acid binding protein (CRABP) I and II mRNA levels and retinol metabolism. ES cells were cultured with or without LIF, and with various doses of all-trans-retinol, all-trans-4-oxoretinol, or all-trans-retinoic acid (RA). In ES cells treated with retinol or 4-oxoretinol in the absence of LIF the CRABP-I (Crabp1, NM_013496; GI:7304974) and CRABP-II (Crabp2, NM_007759; GI:33469074) mRNA levels at 72h were 66+/-4 and 413+/-6 fold higher, respectively, than the levels in control ES cells cultured without retinoids and in the presence of LIF. The increase in CRABPI mRNA occurred through an increase in CRABPI gene transcription. CRABPI protein was also increased by >50-fold in cells treated with retinol in the absence of LIF. However [(3)H]4-oxoretinol does not bind to murine CRABPI or CRABPII. CYP26A1 mRNA levels and [(3)H]4-oxoretinol production from [(3)H]retinol increased in cells cultured without LIF and with exogenous retinoids. The enormous increases in CRABPI and II transcripts ( approximately 60 and 400-fold, respectively) in the absence of LIF may regulate aspects of the ES cell differentiation program in response to retinol.

The Intersection Between the Aryl Hydrocarbon Receptor (AhR)‐ and Retinoic Acid‐Signaling Pathways

Data from a variety of animal and cell culture model systems have demonstrated an interaction between the aryl hydrocarbon receptor (AhR)- and retinoic acid (RA)-signaling pathways. The AhR(1) was originally identified as the receptor for the polycyclic aromatic hydrocarbon family of environmental contaminants; however, recent data indicate that the AhR binds to a variety of endogenous and exogenous compounds, including some synthetic retinoids. In addition, activation of the AhR pathway alters the function of nuclear hormone-signaling pathways, including the estrogen, thyroid, and RA pathways. Activation of the AhR pathway through exposure to environmental compounds results in significant changes in RA synthesis, catabolism, transport, and excretion. Some effects on retinoid homeostasis mediated by the AhR pathway may result from the interactions of these two pathways at the level of activating or repressing the expression of specific genes. This chapter will review these two pathways, the evidence demonstrating a link between them, and the data indicating the molecular basis of the interactions between these two pathways.

Retinoids in cosmeceuticals

Retinoids are natural and synthetic vitamin A derivatives. They are lipophilic molecules and easily penetrate the epidermis. Their biologically active forms can modulate the expression of genes involved in cellular differentiation and proliferation. Retinoic acid (tretinoin), its 13-cis isomer isotretinoin, as well as various synthetic retinoids are used for therapeutic purposes, whereas retinaldehyde, retinol, and retinyl esters, because of their controlled conversion to retinoic acid or their direct receptor-independent biologic action, can be used as cosmeceuticals. These natural retinoic acid precursors are thus expected to be helpful in (i) renewing epidermal cells, (ii) acting as UV filters, (iii) preventing oxidative stress, (iv) controlling cutaneous bacterial flora, and (v) improving skin aging and photoaging. Retinol and retinyl esters are not irritant, whereas demonstrating only a modest clinical efficiency. On the other hand, retinaldehyde, which is fairly well tolerated, seems to be the most efficient cosmeceutical retinoid; it has significant efficiency toward oxidative stress, cutaneous bacterial flora, epidermis renewing, and photoaging.

Current perspectives on the genetic causes of neural tube defects

Neural tube defects (NTDs) are a group of severe congenital abnormalities resulting from the failure of neurulation. The pattern of inheritance of these complex defects is multifactorial, making it difficult to identify the underlying causes. Scientific research has rapidly progressed in experimental embryology and molecular genetics to elucidate the basis of neurulation. Crucial mechanisms of neurulation include the planar cell polarity pathway, which is essential for the initiation of neural tube closure, and the sonic hedgehog signaling pathway, which regulates neural plate bending. Genes influencing neurulation have been investigated for their contribution to human neural tube defects, but only genes with well-established role in convergent extension provide an exciting new set of candidate genes. Biochemical factors such as folic acid appear to be the greatest modifiers of NTDs risk in the human population. Consequently, much research has focused on genes of folate-related metabolic pathways. Variants of several such genes have been found to be significantly associated with the risk of neural tube defects in more studies. In this manuscript, we reviewed the current perspectives on the causes of neural tube defects and highlighted that we are still a long way from understanding the etiology of these complex defects.

Overview of retinoid metabolism and function

Retinoids (vitamin A) are crucial for most forms of life. In chordates, they have important roles in the developing nervous system and notochord and many other embryonic structures, as well as in maintenance of epithelial surfaces, immune competence, and reproduction. The ability of all-trans retinoic acid to regulate expression of several hundred genes through binding to nuclear transcription factors is believed to mediate most of these functions. The role of all-trans retinoic may extend beyond the regulation of gene transcription because a large number of noncoding RNAs also are regulated by retinoic acid. Additionally, extra-nuclear mechanisms of action of retinoids are also being identified. In organisms ranging from prokaryotes to humans, retinal is covalently linked to G protein-coupled transmembrane receptors called opsins. These receptors function as light-driven ion pumps, mediators of phototaxis, or photosensory pigments. In vertebrates phototransduction is initiated by a photochemical reaction where opsin-bound 11-cis-retinal is isomerized to all-trans-retinal. The photosensitive receptor is restored via the retinoid visual cycle. Multiple genes encoding components of this cycle have been identified and linked to many human retinal diseases. Central aspects of vitamin A absorption, enzymatic oxidation of all-trans retinol to all-trans retinal and all-trans retinoic acid, and esterification of all-trans retinol have been clarified. Furthermore, specific binding proteins are involved in several of these enzymatic processes as well as in delivery of all-trans retinoic acid to nuclear receptors. Thus, substantial progress has been made in our understanding of retinoid metabolism and function. This insight has improved our view of retinoids as critical molecules in vision, normal embryonic development, and in control of cellular growth, differentiation, and death throughout life.

Endocrine-disrupting organotin compounds are potent inducers of adipogenesis in vertebrates

Dietary and xenobiotic compounds can disrupt endocrine signaling, particularly of steroid receptors and sexual differentiation. Evidence is also mounting that implicates environmental agents in the growing epidemic of obesity. Despite a long-standing interest in such compounds, their identity has remained elusive. Here we show that the persistent and ubiquitous environmental contaminant, tributyltin chloride (TBT), induces the differentiation of adipocytes in vitro and increases adipose mass in vivo. TBT is a dual, nanomolar affinity ligand for both the retinoid X receptor (RXR) and the peroxisome proliferator-activated receptor gamma (PPARgamma). TBT promotes adipogenesis in the murine 3T3-L1 cell model and perturbs key regulators of adipogenesis and lipogenic pathways in vivo. Moreover, in utero exposure to TBT leads to strikingly elevated lipid accumulation in adipose depots, liver, and testis of neonate mice and results in increased epididymal adipose mass in adults. In the amphibian Xenopus laevis, ectopic adipocytes form in and around gonadal tissues after organotin, RXR, or PPARgamma ligand exposure. TBT represents, to our knowledge, the first example of an environmental endocrine disrupter that promotes adipogenesis through RXR and PPARgamma activation. Developmental or chronic lifetime exposure to organotins may therefore act as a chemical stressor for obesity and related disorders.

Retinoic acid and hindbrain patterning

Retinoid signaling plays an important role in the developmental patterning of the hindbrain. Studies of the teratogenic effects of retinoids showed early on that the hindbrain suffered patterning defects in cases of retinoid excess or deficiency. Closer examination of these effects in animal models suggested that retinoids might play a physiological role in specifying the antero-posterior axis of the hindbrain. This idea was supported by the localization of retinoid synthetic and degradative enzymes, binding proteins, and receptors to the hindbrain and neighboring regions of the neuroepithelium and the mesoderm. In parallel, it became clear that the molecular patterning of the hindbrain, in terms of the regionalized expression of Hox genes and other developmental regulatory genes, is profoundly influenced by retinoid signaling.

Synthesis of enantiopure C3- and C4-hydroxyretinals and their enzymatic reduction by ADH8 from Xenopus laevis

(R)-all-trans-3-hydroxyretinal 1, (S)-all-trans-4-hydroxyretinal and (R)-all-trans-4-hydroxyretinal have been synthesized stereoselectively by Horner-Wadsworth-Emmons and Stille cross-coupling as bond-forming reactions. The CBS method of ketone reduction was used in the enantioface-differentiation step to provide the precursors for the synthesis of the 4-hydroxyretinal enantiomers. The kinetic constants of Xenopus laevis ADH8 with these retinoids have been determined.

Global analysis of RAR-responsive genes in the Xenopus neurula using cDNA microarrays

Retinoid signaling is important for patterning the vertebrate hindbrain and midaxial regions. We recently showed that signaling through retinoic acid receptors (RARs) is essential for anteroposterior patterning along the entire body axis. To further investigate the mechanisms through which RARs act, we used microarray analysis to investigate the effects of modulating RAR activity on target gene expression. We identified 334 up-regulated genes (92% of which were validated), including known RA-responsive genes, known genes that have never been proposed as RA targets and many hypothetical and unidentified genes (n = 166). Sixty-seven validated down-regulated genes were identified, including known RA-responsive genes and anterior marker genes. The expression patterns of selected up-regulated genes (n = 45) were examined at neurula stages using whole-mount in situ hybridization. We found that most of these genes were expressed in the neural tube and many were expressed in anterior tissues such as neural crest, brain, eye anlagen, and cement gland. Some were expressed in tissues such as notochord, somites, pronephros, and blood islands, where retinoic acid (RA) plays established roles in organogenesis. Members of this set of newly identified RAR target genes are likely to play important roles in neural patterning and organogenesis under the control of RAR signaling pathways, and their further characterization will expand our understanding of RA signaling during development.

Metabolism and transactivation activity of 13,14-dihydroretinoic acid

The metabolism of vitamin A is a highly regulated process that generates essential mediators involved in the development, cellular differentiation, immunity, and vision of vertebrates. Retinol saturase converts all-trans-retinol to all-trans-13,14-dihydroretinol (Moise, A. R., Kuksa, V., Imanishi, Y., and Palczewski, K. (2004) J. Biol. Chem. 279, 50230-50242). Here we demonstrate that the enzymes involved in oxidation of retinol to retinoic acid and then to oxidized retinoic acid metabolites are also involved in the synthesis and oxidation of all-trans-13,14-dihydroretinoic acid. All-trans-13,14-dihydroretinoic acid can activate retinoic acid receptor/retinoid X receptor heterodimers but not retinoid X receptor homodimers in reporter cell assays. All-trans-13,14-dihydroretinoic acid was detected in vivo in Lrat-/- mice supplemented with retinyl palmitate. Thus, all-trans-13,14-dihydroretinoic acid is a naturally occurring retinoid and a potential ligand for nuclear receptors. This new metabolite can also be an intermediate in a retinol degradation pathway or it can serve as a precursor for the synthesis of bioactive 13,14-dihydroretinoid metabolites.

Kinetics of human alcohol dehydrogenase with ring-oxidized retinoids: effect of Tween 80

Human alcohol dehydrogenases (ADH1 and ADH4) actively use retinoids oxidized at the cyclohexenyl ring (4-oxo-, 4-hydroxy-, and 3,4-didehydro-retinoids), which are functional compounds in several cells and tissues (i.e., in human skin). Remarkably, activities with 4-oxo-retinal and 4-hydroxy-retinol (kcat = 2050 min(-1) for ADH4) are the highest among retinoids, similar to those of the best aliphatic alcohols. Thus, ADH1 and ADH4 provide a metabolic pathway for the synthesis of the corresponding retinoic acids. Tween 80, a widely used detergent in the retinoid activity assay, behaves as a competitive inhibitor. The Km values for all-trans-retinol (2-3 microM), estimated in the absence of detergent, are 10-fold lower than those obtained at the usual 0.02% Tween 80. This suggests a contribution of ADH to retinoid metabolism more relevant than previously expected. However, Tween 80 stabilizes retinoids in water solution and provides a reliable and reproducible assay, suitable for comparing different ADHs and different retinoid substrates.

Spatiotemporal retinoid-X receptor activation detected in live vertebrate embryos

Most studies on the nuclear retinoid-X receptor (RXR) have focused on its role as a heterodimeric partner but less about its own activation pattern during development and the distribution of potential endogenous ligands. The aim of this study is to visualize the distribution of activated RXRalpha in live transgenic Xenopus laevis embryos across a wide range of developmental stages. We adopted a nuclear receptor-Gal4 fusion/upstream activation sequence-based reporter system for our assay. Strong activation of the RXRalpha ligand-binding domain was observed in a segment of the spinal cord just posterior to the hindbrain. This activation is first detected in neurula stage embryos and persists up to swimming tadpole stages, after which activation strongly declines. Addition of exogenous ligands, such as 9-cis retinoic acid or all-trans retinoic acid, expands the activation of RXR throughout the spinal cord but not in the brain, whereas the RXR-specific ligand LG268 expanded the Gal4-RXR activation into the brain and olfactory epithelia. Treatment with the RAR-specific ligand 4-(E-2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)-1-propenyl)benzoic acid or thyroid hormone had no effect on Gal4-RXR activation, whereas these compounds significantly increased their corresponding Gal4/receptor fusion proteins under similar conditions. Embryos expressing a Gal4-RXR fusion protein with a deletion in the ligand-dependent activation domain (AF2) show no reporter gene activation. The results shown in this paper reveal a specific activation pattern for Gal4-RXRalpha specifically in the developing spinal cord and suggest the existence of RXR ligand "hot-spots" in this region.

Multiple points of interaction between retinoic acid and FGF signaling during embryonic axis formation

Anteroposterior (AP) patterning of the developing CNS is crucial for both regional specification and the timing of neurogenesis. Several important factors are involved in AP patterning, including members of the WNT and FGF growth factor families, retinoic acid receptors, and HOX genes. We have examined the interactions between FGF and retinoic signaling pathways. Blockade of FGF signaling downregulates the expression of members of the RAR signaling pathway, RARalpha, RALDH2 and CYP26. Overexpression of a constitutively active RARalpha2 rescues the effects of FGF blockade on the expression of XCAD3 and HOXB9. This suggests that RARalpha2 is required as a downstream target of FGF signaling for the posterior expression of XCAD3 and HOXB9. Surprisingly, we found that posterior expression of FGFR1 and FGFR4 was dependent on the expression of RARalpha2. Anterior expression was also altered with FGFR1 expression being lost, whereas FGFR4 expression was expanded beyond its normal expression domain. RARalpha2 is required for the expression of XCAD3 and HOXB9, and for the ability of XCAD3 to induce HOXB9 expression. We conclude that RARalpha2 is required at multiple points in the posteriorization pathway, suggesting that correct AP neural patterning depends on a series of mutually interactive feedback loops among FGFs, RARs and HOX genes.

Retinoid-dependent restriction of human immunodeficiency virus type 1 replication in monocytes/macrophages

Vitamin A deficiency has been correlated with increased severity of human immunodeficiency virus type 1 (HIV-1)-associated disease. Moreover, vitamin A supplementation can reduce AIDS-associated morbidity and mortality. Our group and others have shown that retinoids, the bioactive metabolites of vitamin A, repress HIV-1 replication in monocytic cell lines and primary macrophages by blocking long-terminal-repeat (LTR)-directed transcription. Based on these studies, we hypothesize that retinoids are natural repressors of HIV-1 in vivo. We show here that all-trans-retinoic acid (RA)-mediated repression of HIV-1 activation requires pretreatment for at least 12 h and is blocked by the protein synthesis inhibitors cycloheximide and puromycin. Studies of the kinetics of RA-mediated repression in U1 cells and primary monocyte-derived macrophages (MDMs) reveal that the repressive effects of RA on HIV-1 expression are long-lasting but reversible. We demonstrate that HIV-1 expression is activated when U1 cells or MDMs are cultured in retinoid-free synthetic medium and show that physiological concentrations of RA repress this activation. In addition, the synthetic pan-retinoic acid receptor antagonist BMS-204 493 activates HIV-1 replication in U1 cells in a dose-dependent manner, suggesting that RA-induced transactivation of cellular gene expression is required for HIV-1 repression. Together, these data support the hypothesis that retinoids present in tissue culture media in vitro and serum in vivo maintain HIV-1 in a transcriptionally repressed state in monocytes/macrophages.

Induction of larval tissue resorption in Xenopus laevis tadpoles by the thyroid hormone receptor agonist GC-1

A major challenge in understanding nuclear hormone receptor function is to determine how the same ligand can cause very different tissue-specific responses. Tissue specificity may result from the presence of more than one receptor subtype arising from multiple receptor genes or alternative splicing. Recently, high affinity analogs of nuclear receptor ligands have been synthesized that show subtype selectivity. These analogs can greatly facilitate the study of receptor subtype-specific functions in organisms where mutational analysis is problematic or where it is desirable for receptors to be expressed in their normal physiological contexts. We describe here the effects of the synthetic thyroid hormone analog GC-1 on the metamorphosis of the frog Xenopus laevis. The most potent natural thyroid hormone, 3,5,3'-triidothyronine or T3, shows similar binding affinity and transactivation dose-response curves for both thyroid hormone receptor isotypes, designated TRalpha and TRbeta. GC-1, however, binds to and activates TRbeta at least an order of magnitude better than it does TRalpha. GC-1 efficiently induces death and resorption of premetamorphic tadpole tissues such as the gills and the tail, two tissues that strongly induce thyroid hormone receptor beta during metamorphosis. GC-1 has less effect on the growth of adult tissues such as the hindlimbs, which express high TRalpha levels. The effectiveness of GC-1 in inducing tail resorption and tail gene expression correlates with increasing TRbeta levels. These results illustrate the utility of subtype selective ligands as probes of nuclear receptor function in vivo.

All-trans retinoic acid is a ligand for the orphan nuclear receptor ROR beta

Retinoids regulate gene expression through binding to the nuclear retinoic acid receptors (RARs) and retinoid X receptors (RXRs). In contrast, no ligands for the retinoic acid receptor-related orphan receptors beta and gamma (ROR beta and gamma) have been identified, yet structural data and structure-function analyses indicate that ROR beta is a ligand-regulated nuclear receptor. Using nondenaturing mass spectrometry and scintillation proximity assays we found that all-trans retinoic acid (ATRA) and several retinoids bind to the ROR beta ligand-binding domain (LBD). The crystal structures of the complex with ATRA and with the synthetic analog ALRT 1550 reveal the binding modes of these ligands. ATRA and related retinoids inhibit ROR beta but not ROR alpha transcriptional activity suggesting that high-affinity, subtype-specific ligands could be designed for the identification of ROR beta target genes. Our results identify ROR beta as a retinoid-regulated nuclear receptor, providing a novel pathway for retinoid action.

Vitamin A and infancy. Biochemical, functional, and clinical aspects

Vitamin A is a very intriguing natural compound. The molecule not only has a complex array of physiological functions, but also represents the precursor of promising and powerful new pharmacological agents. Although several aspects of human retinol metabolism, including absorption and tissue delivery, have been clarified, the type and amounts of vitamin A derivatives that are intracellularly produced remain quite elusive. In addition, their precise function and targets still need to be identified. Retinoic acids, undoubtedly, play a major role in explaining activities of retinol, but, recently, a large number of physiological functions have been attributed to different retinoids and to vitamin A itself. One of the primary roles this vitamin plays is in embryogenesis. Almost all steps in organogenesis are controlled by retinoic acids, thus suggesting that retinol is necessary for proper development of embryonic tissues. These considerations point to the dramatic importance of a sufficient intake of vitamin A and explain the consequences if intake of retinol is deficient. However, hypervitaminosis A also has a number of remarkable negative consequences, which, in same cases, could be fatal. Thus, the use of large doses of retinol in the treatment of some human diseases and the use of megavitamin therapy for certain chronic disorders as well as the growing tendency toward vitamin faddism should alert physicians to the possibility of vitamin overdose.

Effect of co-administration of retinoids on rat embryo development in vitro

BACKGROUND

Excess retinyl palmitate (vitamin A) induces a variety of malformations in many mammalian species, yet retinyl palmitate is not thought to be the proximate teratogen. Many metabolites are generated after oral dosing several of which are, individually teratogenic. It is not known whether these metabolites when present simultaneously interact in an additive or synergistic manner.

METHODS

Whole rat embryos were cultured with retinol, all-trans retinoic acid (tRA), 4-oxo all-trans retinoic acid (4otRA) or 9-cis retinoic acid (9cRA). Retinoids were added singly or in paired combinations at the start of culture and embryos were examined for evidence of dysmorphogenesis at the end of culture. Retinoids were added at both subthreshold and supra-threshold concentrations.

RESULTS

Co-administration of subthreshold concentrations of tRA and 4otRA, retinol and tRA or 4otRA and 4otRA and 9cRA resulted in a greater frequency of abnormal embryos compared to single administration of each retinoid.

CONCLUSIONS

Knowledge of the way in which retinyl palmitate metabolites interact should refine our ability to predict risk from a potentially teratogenic event e.g., accidental overdose with RP during human pregnancy.

Biomarkers and bioassays for detecting dioxin-like compounds in the marine environment

The presence of toxic chemical contaminants in some marine organisms, including those consumed by humans, is well known. Monitoring the levels of such contaminants and their geographic and temporal variability is important for assessing and maintaining the safety of seafood and the health of the marine environment. Chemical analyses are sensitive and specific, but can be expensive and provide little information on the actual or potential biological activity of the contaminants. Biologically-based assays can be used to indicate the presence and potential effects of contaminants in marine animals, and therefore, have potential for routine monitoring of the marine environment. Halogenated aromatic hydrocarbons (HAHs) such as chlorinated dioxins, dibenzofurans, and biphenyls comprise a major group of marine contaminants. The most toxic HAHs (dioxin-like compounds) act through an intracellular receptor protein, the aryl hydrocarbon receptor, which is present in humans and many, but not all, marine animals. A toxic equivalency approach based on an understanding of this mechanism provides an integrated measure of the biological potency or activity of HAH mixtures. Biomarkers measured in marine animals indicate their exposure to these chemicals in vivo. Similarly, in vitro biomarker responses measured in cell culture bioassays can be used to assess the concentration of 'dioxin equivalents' in extracts of environmental matrices. Here, I have reviewed the types and relative sensitivities of mechanistically-based, in vitro bioassays for dioxin-like compounds, including assays of receptor-binding, DNA-binding and transcriptional activation of native (CYP1A) or reporter (luciferase) genes. Examples of their use in environmental monitoring are provided. Cell culture bioassays are rapid and inexpensive, and thus have great potential for routine monitoring of marine resources, including seafood. Several such assays exist, or are being developed, for a variety of marine contaminants in addition to the dioxin-like chemicals. A battery of cell culture bioassays might be used to rapidly and sensitively screen seafood for the presence of contaminants of concern, including dioxin-like compounds as well as other contaminants such as natural toxins, hormonally active agents, and heavy metals. Such a battery of mechanism-based, in vitro bioassays could be incorporated into monitoring efforts under recently adopted hazard analysis and critical control point (HACCP) programs.

Influence of isomerisation on the growth inhibitory effects and cellular activity of 13-cis and all-trans retinoic acid in neuroblastoma cells

Treatment with 13-cis retinoic acid (13-cis RA) has been shown to significantly improve the clinical outcome of children with high-risk neuroblastoma. Despite the large number of studies investigating the cellular effects of retinoids in neuroblastoma cells, the influence of RA isomerisation and the factors that determine the extent of RA isomerisation and uptake are unknown. The aim of this study was to establish the extent of extra- and intracellular isomerisation of 13-cis RA and all-trans retinoic acid (ATRA) in neuroblastoma cell lines, and to investigate the influence of isomerisation on their growth inhibitory effects and on the regulation of expression of cellular retinoic acid binding protein II (CRABP II) and RAR-beta. Limited extracellular isomerisation was observed up to 72 hr after incubation of four neuroblastoma cell lines with 10 microM 13-cis RA or ATRA. The retinoic acid isomer present initially in the medium accounted for >75% of extracellular retinoid exposure. By contrast, incubation with 13-cis RA resulted in intracellular levels of ATRA comparable to those of 13-cis RA. This degree of intracellular isomerisation was not observed after ATRA incubations, with 13-cis RA accounting for <10% of total intracellular retinoids. No differences were observed in the sensitivity of three N-type neuroblastoma cell lines to either 13-cis RA (IC(50): 11.2-13.9 microM) or ATRA (IC(50): 12.9-14.4 microM), despite 10-fold differences in intracellular retinoid levels. A decrease in sensitivity to 13-cis RA (IC(50)=137 microM), as compared to ATRA (IC(50)=41 microM), was observed in the S-type cell line SH S EP. RAR-beta was induced in a dose-dependent manner in SH SY 5Y cells following incubation with ATRA, whereas a weaker and delayed induction was observed with 13-cis RA. Similarly, incubation with ATRA resulted in a greater induction of CRABP II in these cells. In summary, these results indicate either an intracellular conversion of 13-cis RA to ATRA or a selective uptake of ATRA and suggest that this may mediate the differential activity of 13-cis RA in neuroblastoma cell subtypes.

Qualitative and quantitative liquid chromatographic determination of natural retinoids in biological samples

Liquid chromatography continues to be the preferred method for determining retinoids in biological samples. The highly unstable nature of retinoids and the real possibility of artifacts or erroneous results have led to the development of rapid and highly automated protocols for retinoid extraction, separation and detection. Due to strong light absorbance in the ultraviolet region, UV detectors still predominate although mass spectrometric detection is gaining increased popularity. This paper reviews recent advances and provides major guidelines for using liquid chromatography to identify and quantify retinoids in biological samples.

Active repression of RAR signaling is required for head formation

The retinoic acid receptors (RARs) recruit coactivator and corepressor proteins to activate or repress the transcription of target genes depending on the presence of retinoic acid (RA). Despite a detailed molecular understanding of how corepressor complexes function, there is no in vivo evidence to support a necessary function for RAR-mediated repression. Signaling through RARs is required for patterning along the anteroposterior (A-P) axis, particularly in the hindbrain and posterior, although the absence of RA is required for correct anterior patterning. Because RARs and corepressors are present in regions in which RA is absent, we hypothesized that repression mediated through unliganded RARs might be important for anterior patterning. To test this hypothesis, specific reagents were used that either reduce or augment RAR-mediated repression. Derepression of RAR signaling by expressing a dominant-negative corepressor resulted in embryos that exhibited phenotypes similar to those treated by RA. Anterior structures such as forebrain and cement gland were greatly reduced, as was the expression of molecular markers. Enhancement of target gene repression using an RAR inverse agonist resulted in up-regulation of anterior neural markers and expansion of anterior structures. Morpholino antisense oligonucleotide-mediated RARalpha loss-of-function phenocopied the effects of RA treatment and dominant-negative corepressor expression. Microinjection of wild-type or dominant-negative RARalpha rescued the morpholino phenotype, confirming that RAR is functioning anteriorly as a transcriptional repressor. Lastly, increasing RAR-mediated repression potentiated head-inducing activity of the growth factor inhibitor cerberus, whereas releasing RAR-mediated repression blocked cerberus from inducing ectopic heads. We conclude that RAR-mediated repression of target genes is critical for head formation. This requirement establishes an important biological role for active repression of target genes by nuclear hormone receptors and illustrates a novel function for RARs during vertebrate development.

Regulation of cytochrome P450 (CYP) genes by nuclear receptors

Members of the nuclear-receptor superfamily mediate crucial physiological functions by regulating the synthesis of their target genes. Nuclear receptors are usually activated by ligand binding. Cytochrome P450 (CYP) isoforms often catalyse both formation and degradation of these ligands. CYPs also metabolize many exogenous compounds, some of which may act as activators of nuclear receptors and disruptors of endocrine and cellular homoeostasis. This review summarizes recent findings that indicate that major classes of CYP genes are selectively regulated by certain ligand-activated nuclear receptors, thus creating tightly controlled networks.

Retinoids in embryonal development

The key role of vitamin A in embryonal development is reviewed. Special emphasis is given to the physiological action of retinoids, as evident from the retinoid ligand knockout models. Retinoid metabolism in embryonic tissues and teratogenic consequences of retinoid administration at high doses are presented. Physiological and pharmacological actions of retinoids are outlined and explained on the basis of their interactions as ligands of the nuclear retinoid receptors. Immediate target genes and the retinoid response elements of their promoters are summarized. The fundamental role of homeobox genes in embryonal development and the actions of retinoids on their expression are discussed. The similarity of the effects of retinoid ligand knockouts to effects of compound retinoid receptor knockouts on embryogenesis is presented. Although much remains to be clarified, the emerging landscape offers exciting views for future research.

Regulation of cytochrome P450 (CYP) genes by nuclear receptors

Members of the nuclear-receptor superfamily mediate crucial physiological functions by regulating the synthesis of their target genes. Nuclear receptors are usually activated by ligand binding. Cytochrome P450 (CYP) isoforms often catalyse both formation and degradation of these ligands. CYPs also metabolize many exogenous compounds, some of which may act as activators of nuclear receptors and disruptors of endocrine and cellular homoeostasis. This review summarizes recent findings that indicate that major classes of CYP genes are selectively regulated by certain ligand-activated nuclear receptors, thus creating tightly controlled networks.