Induction of beta-retinoic acid receptor mRNA by teratogenic doses of retinoids in murine fetuses

Adverse effects of maternal retinyl palmitate, a vitamin A compound, on the fetal liver

Background: Consuming high doses of vitamin A during pregnancy may lead to malformations in the offspring. Some reports state that low doses that do not cause macroscopic abnormalities may result in mental and behavioral disorders. However, there are few studies on the microscopic effects of these doses on the organism. Objective: The aim was to investigate the effects of early prenatal exposure to different doses of oral vitamin A on the fetal liver. Materials and methods: Twenty-five pregnant rats, divided into five groups, received oral vitamin A at doses of 10,000, 50,000, 100,000, and 200,000 IU/kg between days 10 and 12 of gestation. The fetuses were collected on day 19 of gestation, their livers were dissected, and histology, apoptosis, and proliferation were examined by hematoxylin-eosin, TUNEL assay, and Ki67 immunolabeling using stereological methods. Results: Vitamin A decreased fetal liver volume, the number of Ki67-positive cells per unit volume, and the total number of hepatocytes at all doses except 10,000 IU/kg (p<0.001). Consequently, apoptosis was significantly higher in the groups receiving 100,000 and 200,000 IU/kg vitamin A (p<0.001). Conclusion: Our study shows that vitamin A administered during gestation days 10-12 has a suppressive effect on the developing rat liver when the dose exceeds 10,000 IU/kg, probably due to increased apoptosis and suppressed cell division.

Identification of Stage-Specific Gene Expression Signatures in Response to Retinoic Acid during the Neural Differentiation of Mouse Embryonic Stem Cells

We have previously established a protocol for the neural differentiation of mouse embryonic stem cells (mESCs) as an efficient tool to evaluate the neurodevelopmental toxicity of environmental chemicals. Here, we described a multivariate bioinformatic approach to identify the stage-specific gene sets associated with neural differentiation of mESCs. We exposed mESCs (B6G-2 cells) to 10⁻⁸ or 10⁻⁷ M of retinoic acid (RA) for 4 days during embryoid body formation and then performed morphological analysis on day of differentiation (DoD) 8 and 36, or genomic microarray analysis on DoD 0, 2, 8, and 36. Three gene sets, namely a literature-based gene set (set 1), an analysis-based gene set (set 2) using self-organizing map and principal component analysis, and an enrichment gene set (set 3), were selected by the combined use of knowledge from literatures and gene information selected from the microarray data. A gene network analysis for each gene set was then performed using Bayesian statistics to identify stage-specific gene expression signatures in response to RA during mESC neural differentiation. Our results showed that RA significantly increased the size of neurosphere, neuronal cells, and glial cells on DoD 36. In addition, the gene network analysis showed that glial fibrillary acidic protein, a neural marker, remarkably up-regulates the other genes in gene set 1 and 3, and Gbx2, a neural development marker, significantly up-regulates the other genes in gene set 2 on DoD 36 in the presence of RA. These findings suggest that our protocol for identification of developmental stage-specific gene expression and interaction is a useful method for the screening of environmental chemical toxicity during neurodevelopmental periods.

Study on cytochrome p-450 dependent retinoic Acid metabolism and its inhibitors as potential agents for cancer therapy

The relative lack of clinical success with conventional anticancer agents may be due in part to the traditional concept of cancer being a biological state rather than a dynamic process. Redefining cancer as a dynamic disease commencing with carcinogenesis introduces the possibility of chemoprevention. Retinoids offer the promise of a therapeutic option based on differentiation of premalignant as well as malignant cells. Research to date has concentrated on the use of exogenous retinoids in cancer. Although this research continues with new retinoid derivatives, an alternative approach to overcoming the drawbacks associated with exogenous retinoids has been to increase the levels of endogenous retinoic acid (RA) by inhibiting the cytochrome P450- mediated catabolism of RA using a novel class of agents known as retinoic acid metabolism blocking agents (RAMBAs which increase the level of endogenous retinoic acid (RA) within the tumor cells by blocking their metabolism. This approach presents several theoretic advantages.

In the present study a wide range of established P-450 inhibitors has been screened to examine their inhibitory activity on all-trans-Retinoic acid (ATRA) metabolism. Forty-one known P450 inhibitors were tested for their inhibitory activity against RA metabolism. Most of them are nitrogen-containing compounds. The results showed that among these compounds only six compounds (N-benzyl-2-phenylethanamine, itraconazole, chlorpromazine, 5-chloro-1,3-benzoxazol-2-amine, proadifen and furazolidone) showed inhibition of RA metabolism which was > 50%. Ketoconazole and liarozole were also screened as standard potent inhibitors in the same system and gave 87.5% and 89% inhibition, respectively. The results indicate that mostly azoles with substituents in positions other than the 1-position on the ring are very weak inhibitors of RA metabolism. The most effective inhibitors (ketoconazole, itraconazole, bifonazole and clotrimazole) are 1-substituted and possess relatively large aromatic groups in the molecule. 1-Substituted imidazoles bind to cytochrome P-450 with a very high affinity but substitution in the other position of the imidazole decreases the binding affinity.

Mechanisms of action of the congenital diaphragmatic hernia-inducing teratogen nitrofen

Congenital diaphragmatic hernia (CDH) is a developmental anomaly that results in significant mortality and morbidity. The underlying etiology is poorly understood. Insights will arise from an understanding of the mechanisms by which the teratogen nitrofen induces CDH in rodent models. In this study, we use in vitro cell assays in conjunction with whole animal rodent studies to test hypotheses regarding nitrofen's mechanism of action. The first component examined the interaction of nitrofen with various aspects of the retinoid signaling pathway including uptake proteins, binding proteins, receptors, conversion, and degradation enzymes. The second component examined the interactions of nitrofen and vitamins A, C, and E to test the hypothesis that nitrofen was functioning as an antioxidant to interfere with retinoid signaling. Third, we performed a series of experiments examining the interaction of nitrofen and thyroid signaling. Collectively, the data suggest that the primary aspect of retinoid signaling affected by nitrofen is via inhibition of the rate-limiting enzymes controlling retinoic acid synthesis. Retinoid signaling perturbations do not appear to involve oxidative effects of nitrofen. Any substantial roles of nitrofen-induced perturbations of thyroid hormone signaling or receptor function are not supported.

Adverse effects of retinoic acid on embryo development and the selective expression of retinoic acid receptors in mouse blastocysts

BACKGROUND

All-trans retinoic acid (RA), the oxidative metabolite of vitamin A, is essential for normal development. In addition, high levels of RA are teratogenic in many species. We have previously shown that excess RA results in immediate effects on the preimplantation embryo and on blastocyst development. This study was conducted to clarify the long-term survival of mouse blastocyst and the effect of RA on gene expression.

METHODS AND RESULTS

Using an in vitro model, we identified the immediate adverse impact of RA on mouse blastocyst development. This involved an inhibition of cell proliferation and growth retardation. Using an in vivo model, we also identified the resorption of postimplanted blastocysts that had been treated with excess RA. Analysis of RA-mediated gene induction was also included. The retinoic acid receptors RARalpha and RARgamma were constitutively expressed in the blastocyst and the inner cell mass, whereas RARbeta was induced upon RA treatment.

CONCLUSIONS

This is the first evidence to show the impacts of RA on mouse blastocysts in vitro and any carry-over effects in the uterus. There is a retardation of early postimplantation blastocyst development and then subsequent blastocyst death. Our findings also show that there is some degree of selective induction of retinoic acid receptors when excess RA is administered to the blastocysts.

Retinoic acid, hypoxia, and GATA factors cooperatively control the onset of fetal liver erythropoietin expression and erythropoietic differentiation

The cytokine erythropoietin (Epo) is an essential factor promoting the survival, proliferation, and differentiation of erythroid progenitor cells. Epo expression and the initial phase of definitive erythropoietic differentiation in the fetal liver (E9-E12) are compromised in mouse embryos lacking the retinoic acid receptor RXRalpha. Our previous work demonstrated that the Epo gene is a direct target of retinoic acid action, via a retinoic acid receptor binding site in the Epo gene enhancer. However, Epo expression and erythropoietic differentiation become normalized in RXRalpha mutants from E12. In this study, we have investigated the molecular mechanisms underlying the transition in Epo gene regulation from RXRalpha-dependence to RXRalpha-independence. We find that three independent regulatory components are required for high level Epo expression in the early fetal liver: ligand-activated retinoic acid receptors, the hypoxia-regulated factor HIF1, and GATA factors. By E11.5, the fetal liver is no longer hypoxic, and retinoic acid signaling is no longer active; Epo expression from E11.5 onward is enhancer-independent, and is driven instead by basal promoter elements that provide a sufficient level of expression to support further erythropoietic differentiation.

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.

Retinoic acid-induced developmental defects are mediated by RARbeta/RXR heterodimers in the pharyngeal endoderm

Fusion and hypoplasia of the first two branchial arches, a defect typically observed in retinoic acid (RA) embryopathy, is generated in cultured mouse embryos upon treatment with BMS453, a synthetic compound that exhibits retinoic acid receptor beta (RARbeta) agonistic properties in transfected cells. By contrast, no branchial arch defects are observed following treatment with synthetic retinoids that exhibit RARalpha or RARgamma agonistic properties. The BMS453-induced branchial arch defects are mediated through RAR activation, as they are similar to those generated by a selective pan-RAR agonist, are prevented by a selective pan-RAR antagonist and cannot be mimicked by exposure to a pan-RXR agonist alone. They are enhanced in the presence of a pan-RXR agonist, and cannot be generated in Rarb-null embryos. Furthermore, they are accompanied, in the morphologically altered region, by ectopic expression of Rarb and of several other direct RA target genes. Therefore, craniofacial abnormalities characteristic of the RA embryopathy are mediated through ectopic activation of RARbeta/RXR heterodimers, in which the ligand-dependent activity of RXR is subordinated to that of RARbeta. Endodermal cells lining the first two branchial arches respond to treatment with the RARbeta agonist, in contrast to neural crest cells and ectoderm, which suggests that a faulty endodermal regionalization is directly responsible for RA-induced branchial arch dysmorphologies. Additionally, we provide the first in vivo evidence that the synthetic RARbeta agonist BMS453 exhibits an antagonistic activity on the two other RAR isotypes.

The synthetic retinoid AGN 193109 but not retinoic acid elevates CYP1A1 levels in mouse embryos and Hepa-1c1c7 cells

The synthetic retinoid AGN 193109 is a potent pan retinoic acid receptor (RAR) antagonist. Treatment of pregnant mice with a single oral 1 mg/kg dose of this antagonist on day 8 postcoitum results in severe craniofacial (median cleft face or frontonasal deficiency) and eye malformations in virtually all exposed fetuses. Using differential display analysis, we have determined that CYP1A1 mRNA levels are elevated in mouse embryos 6 h following treatment with AGN 193109. Similarly, an elevation in CYP1A1 mRNA levels, protein levels, and aryl hydrocarbon hydoxylase activity occurs in Hepa-1c1c7 cells, with the maximal elevation observed when the cells were treated with 10(-5) M AGN 193109 for 4 to 8 h. Elevation in CYP1A1 mRNA levels in mouse embryos and Hepa-1c1c7 cells does not occur upon treatment with the natural retinoid, all-trans-retinoic acid. Finally, elevation in CYP1A1 mRNA levels was not observed when mutant Hepa-1c1c7 cells, which are defective in either the aryl hydrocarbon receptor (AhR) or aryl hydrocarbon receptor nuclear translocator (ARNT), were treated with AGN 193109. This suggests that the AhR/ARNT pathway and not the RAR/RXR pathway is mediating the elevation of CYP1A1 mRNA levels by AGN 193109, at least in the Hepa-1c1c7 cells. This is the first example of a retinoid that displays the abililty to regulate both the RAR/RXR and AhR/ARNT transcriptional regulatory pathways.

Temporal expression of retinoic acid receptors in hamster fetus during organogenesis and alteration by retinoic acid treatment

Retinoic acid receptors alpha, beta and gamma (RAR alpha, beta, gamma) mRNAs from whole 8- to 15-day-old hamster fetuses were characterized and quantitated by Northern blots and solution hybridization using riboprobes from cloned hamster RAR cDNAs, derived from 12-day fetal hamster library. Two RAR alpha transcripts of approximately 3.1 and approximately 3.5 kb, one transcript of RAR beta approximately 2.8 kb and one transcript of RAR gamma approximately 3.1 kb were observed. The relative abundance levels of these transcripts were RAR gamma > beta > alpha. RAR beta and gamma levels peaked at day 11, increasing approximately 4-fold (beta) and approximately 2.5-fold (gamma) above their initial values at day 8. RAR alpha did not change appreciably and peaked on day 14 at 1.7 x of its lowest level at day 9. Regulation patterns of the three RARs diverged between days 8 and 9 and 13 and 14 postcoitum (p.c.) and coordinately increased between days 9 and 13 and decreased between days 14 and 15 p.c. In 12-day-old conceptuses exposed to all-trans-retinoic acid, RAR alpha did not increase significantly, but RAR beta increased 12-fold at 4 hr and RAR gamma 2-fold at 1 hr after the maternal treatments.

Cellular retinol-binding protein-supported retinoic acid synthesis. Relative roles of microsomes and cytosol

This study shows that microsomal retinol dehydrogenases, versus cytosolic retinol dehydrogenases, provide the quantitatively major share of retinal for retinoic acid (RA) biogenesis in rat tissues from the predominant substrate available physiologically, holo-cellular retinol-binding protein, type I (CRBP). With holo-CRBP as substrate in the absence of apo-CRBP microsomal retinol dehydrogenases have the higher specific activity and capacity to generate retinal used for RA synthesis by cytosolic retinal dehydrogenases. In the presence of apo-CRBP, a potent inhibitor of cytosolic retinol dehydrogenases (IC50 = approximately 1 microM), liver microsomes provide 93% of the total retinal synthesized in a combination of microsomes and cytosol. Cytosolic retinol dehydrogenase(s) and the isozymes of alcohol dehydrogenase expressed in rat liver had distinct enzymatic properties; yet ethanol inhibited cytosolic retinol dehydrogenase(s) (IC50 = 20 microM) while stimulating RA synthesis in a combination of microsomes and cytosol. At least two discrete forms of cytosolic retinol dehydrogenase were observed: NAD- and NADP-dependent forms. Multiple retinal dehydrogenases also were observed and were inhibited partially by apo-CRBP. These results provide new insights into pathways of RA biogenesis and provide further evidence that they consist of multiple enzymes that recognize both liganded and nonliganded states of CRBP.

Antisense oligonucleotides to CRABP I and II alter the expression of TGF-beta 3, RAR-beta, and tenascin in primary cultures of embryonic palate cells

The cellular retinoic acid-binding proteins (CRABPs) are thought to modulate the responsiveness of cells to retinoic acid (RA). We have previously shown that primary cultures of murine embryonic palate mesenchymal (MEPM) cells express both CRABP-I and CRABP-II genes and that this expression is regulated by RA and transforming growth factor beta (TGF-beta). These cells also express high levels of TGF-beta 3, which is also regulated by RA and TGF-beta. We have used an antisense strategy to investigate the role of the CRABPs in retinoid-induced gene expression. Subconfluent cultures of MEPM cells were treated for several days with phosphorothioate modified 18-mer oligonucleotides antisense to CRABP-I or CRABP-II and then with all-trans-retinoic acid at a concentration of 3.3 microM or 0.33 microM for 5 or 22 h. Total RNA was then extracted and the expression of TGF-beta 3, retinoic acid receptor beta (RAR-beta), and tenascin was assessed by northern blot analysis. Antisense oligonucleotides to CRABP-I partially inhibited the RA-induced TGF-beta 3, RAR-beta, and tenascin mRNA expression. The corresponding mis-sense oligonucleotides were without effect. Antisense oligonucleotides to CRABP-II also partially inhibited RA-induced expression of these genes. As with the CRABP-I antisense, mis-sense oligonucleotides to CRABP-II had no effect. These data suggest that both CRABPs modulate the responsiveness of MEPM cells to retinoic acid. Inhibition of endogenous CRABP expression renders MEPM cells less responsive to RA with respect to induction of TGF-beta 3, RAR-beta, and tenascin gene expression.(ABSTRACT TRUNCATED AT 250 WORDS)

Suppression of retinoic acid receptor-beta in premalignant oral lesions and its up-regulation by isotretinoin

BACKGROUND

Retinoids are effective in the treatment and prevention of certain human cancers. Most of their actions are thought to result from changes in gene expression mediated by nuclear retinoic acid receptors and retinoid X receptors. We conducted a study to determine whether the expression of these receptors was altered in premalignant oral lesions and, if so, whether their expression could be restored by treatment with isotretinoin.

METHODS

We performed in situ hybridization of retinoic acid receptors and retinoid X receptors using antisense riboprobes in specimens of oral mucosa from 7 normal subjects and specimens of premalignant oral lesions from 52 patients before treatment with isotretinoin and from 39 of the 52 patients after three months of treatment.

RESULTS

All the normal specimens expressed retinoic acid receptor-beta messenger RNA (mRNA). In contrast, retinoic acid receptor-beta mRNA was detected in only 21 of the 52 premalignant oral lesions (P = 0.003). Thirty-five of the 39 specimens available for evaluation after treatment expressed retinoic acid receptor-beta mRNA (P < 0.001). All normal and premalignant specimens expressed similar levels of mRNA for retinoic acid receptor-alpha and retinoic acid receptor-gamma and the three types of retinoic X receptors, alpha, beta, and gamma. The levels of retinoic acid receptor-beta mRNA increased in the specimens from 18 of the 22 patients who had responses to isotretinoin and in 8 of the 17 specimens from the patients without responses (P = 0.04).

CONCLUSIONS

The expression of retinoic acid receptor-beta mRNA is selectively lost in premalignant oral lesions and can be restored by treatment with isotretinoin. Restoration of the expression of retinoic acid receptor-beta mRNA is associated with a clinical response. Retinoic acid receptor-beta may have a role in mediating the response to retinoids and may be a useful intermediate biologic marker in trials of these agents for the prevention of oral carcinogenesis.

Expression of retinoic acid receptor genes in fetal and newborn rat lung

Lung differentiation and development are affected by vitamin A and its metabolites. One mechanism through which retinoids might exert their effects is through nuclear retinoic acid receptors (RAR). The gene expression profile of the RAR family (alpha, beta, gamma) has previously been determined in both the developing mouse embryo to 14.5 days gestation, and in the adult lung. The purpose of this study was to determine the expression of the RAR genes during the period of gestation that results in the formation of the saccular lung stage. Total RNA was extracted from fetal lungs of Sprague-Dawley rats at gestational days 17, 19, 20, 21, and 22, and from 12-hour-old newborns for Northern hybridization. Two transcripts of RAR alpha mRNA (3.7 and 2.7 kb) were found at each time point. At day 17, the 2.7 kb RAR alpha mRNA was increased two-fold or more than at any other time studied. At days 19-22 the levels of the 3.7 kb RAR alpha species were also lower than day 17 and newborn levels. One RAR beta mRNA transcript (3.4 kb), present at all time points, was significantly higher in the newborn than on days 17-22. Expression of RAR gamma mRNA could only be demonstrated by reverse transcriptase-polymerase chain reaction. We speculate that the higher RAR alpha species at day 17 indicates a role for RAR alpha in the maintenance of the columnar epithelial cells of the glandular phase of lung development.(ABSTRACT TRUNCATED AT 250 WORDS)

A sustained elevation in retinoic acid receptor-beta 2 mRNA and protein occurs during retinoic acid-induced fetal dysmorphogenesis

We have previously shown that oral treatment of pregnant mice with all-trans retinoic acid (RA) at doses which cause 100% fetal dysmorphogenesis results in a rapid elevation in the mRNA of one specific isoform of the RA receptor-beta, RAR-beta 2, in susceptible embryonic regions. To further investigate the involvement of RAR-beta 2 mRNA in teratogenesis, we have examined its expression in mouse embryos exposed to marginal/nonteratogenic and teratogenic dosing regimens of both 13-cis RA and all-trans RA. We have found that the mere elevation in embryonic RAR-beta 2 mRNA levels and free retinoid levels is not sufficient to result in dysmorphogenesis. Rather, retinoid-induced dysmorphogenesis of embryos appears to occur only when RAR-beta 2 mRNA and unbound retinoid levels remain elevated for at least 6-9 h following retinoid treatment resulting in a significant and prolonged elevation in RAR-beta protein levels.

Mouse conceptuses have a limited capacity to elevate the mRNA level of cellular retinoid binding proteins in response to teratogenic doses of retinoic acid

In these studies, we wished to determine the effect of teratogenic doses of retinoic acid on the expression of cellular retinoic acid binding protein I (CRABP-I) mRNA, cellular retinoic acid binding protein II (CRABP-II) mRNA, cellular retinol binding protein I (CRBP-I) mRNA, and cellular retinol binding protein II (CRBP-II) mRNA in mouse conceptuses. Levels of CRABP-II mRNA and CRBP-I mRNA were modestly elevated (2.5-fold and 1.5-fold, respectively) in 9-day gestation conceptuses following treatment of dams with 100 mg/kg b.w. of retinoic acid. These levels were elevated by 6 hr following treatment and remained elevated until 48 and 24 hr, respectively. Two other retinoids, etretinate and retinoyl beta-glucuronide, also moderately elevated CRABP-II mRNA and CRBP-I mRNA levels in conceptuses. In contrast, the levels of CRABP-I mRNA in the conceptuses remained unaffected by treatment with any of these three retinoids. These results demonstrate that conceptuses have a limited capacity to elevate the cellular retinoid binding proteins mRNA levels and presumably the synthesis of their respective proteins in response to high, teratogenic doses of retinoic acid. As a result, an excess of free retinoic acid becomes available to the nuclear retinoic acid receptors, which may lead to inappropriate gene expression and eventual maldevelopment.

Retinoic acid receptor beta 2 mRNA is elevated by retinoic acid in vivo in susceptible regions of mid-gestation mouse embryos

Many of the biological effects of retinoic acid are mediated by its nuclear receptors (RAR-alpha, RAR-beta, and RAR-gamma), and each of these three receptors exist in multiple isoforms. As a first step to identify if any of the receptor isoforms are involved in dysmorphogenesis which is induced in mouse embryos after treatment with retinoic acid (RA), we examined the levels of mRNA of several isoforms of each RAR in the limb buds and other embryonic regions of normal and RA-treated embryos. Within 3 to 6 hr after treatment of mice on day 11 of gestation with RA, RAR-beta 2 mRNA levels in the whole embryo increased 7-fold while both RAR-alpha 2 and RAR-gamma 1 mRNA levels were elevated only 2-fold. Since RA treatment of day 11 embryos especially produces limb defects in virtually every embryo, we next examined individual embryonic regions separately. Limb buds showed the highest elevations in RAR-beta 2 mRNA levels (12-fold) compared to a moderate elevation in the head/craniofacial region (8-fold) and a small elevation in the remainder of the body (4-fold). In contrast, RAR-alpha 2 and RAR-gamma 1 mRNA levels were elevated in all these tissues to a similar extent, which amounted to only about a 2-fold increase. Retinol, the precursor of RA in the embryo, was also capable of elevating RAR-beta 2 mRNA levels in the limb bud, but the increase was delayed, apparently indicating that metabolic conversion of retinol to RA preceded the effect on mRNA levels.(ABSTRACT TRUNCATED AT 250 WORDS)