Retinoic Acid Production Research Articles

Retinoic acid coordinates somitogenesis and left–right patterning in vertebrate embryos

A striking feature of the body plan of a majority of animals is bilateral symmetry. Almost nothing is known about the mechanisms controlling the symmetrical arrangement of the left and right body sides during development. Here we report that blocking the production of retinoic acid (RA) in chicken embryos leads to a desynchronization of somite formation between the two embryonic sides, demonstrated by a shortened left segmented region. This defect is linked to a loss of coordination of the segmentation clock oscillations. The lateralization of this defect led us to investigate the relation between somitogenesis and the left-right asymmetry machinery in RA-deficient embryos. Reversal of the situs in chick or mouse embryos lacking RA results in a reversal of the somitogenesis laterality defect. Our data indicate that RA is important in buffering the lateralizing influence of the left-right machinery, thus permitting synchronization of the development of the two embryonic sides.

Adenomatous Polyposis Coli Control of Retinoic Acid Biosynthesis Is Critical for Zebrafish Intestinal Development and Differentiation

Mutations in the APC (adenomatous polyposis coli) tumor suppressor gene cause uncontrolled proliferation and impaired differentiation of intestinal epithelial cells. Recent studies indicate that human colon adenomas and carcinomas lack retinol dehydrogenases (RDHs) and that APC regulates the expression of human RDHL. These data suggest a model wherein APC controls enterocyte differentiation by controlling retinoic acid production. However, the importance of APC and retinoic acid in mediating control of normal enterocyte development and differentiation remains unclear. To examine the relationship between APC and retinoic acid biosynthesis in normal enterocytes, we have identified two novel zebrafish retinol dehydrogenases, termed zRDHA and zRDHB, that show strong expression within the gut of developing zebrafish embryos. Morpholino knockdown of either APC or zRDHB in zebrafish embryos resulted in defects in structures known to require retinoic acid. These defects included cardiac abnormalities, pericardial edema, failed jaw and pectoral fin development, and the absence of differentiated endocrine and exocrine pancreas. In addition, APC or zRDHB morphant fish developed intestines that lacked columnar epithelial cells and failed to express the differentiation marker intestinal fatty acid-binding protein. Treatment of either APC or zRDHB morphant embryos with retinoic acid rescued the defective phenotypes. Downstream of retinoic acid production, we identified hoxc8 as a retinoic acid-induced gene that, when ectopically expressed, rescued phenotypes of APC- and zRDHB-deficient zebrafish. Our data establish a genetic link supporting a critical role for retinoic acid downstream of APC and confirm the importance of retinoic acid in enterocyte differentiation.

Inhibition of Retinoic Acid Signaling by the cdx-hox Pathway Is Essential for Blood Cell Formation during Embryogenesis.

The zebrafish mutant kugelig (kgg) is caused by a defect in the caudal-related homeobox gene cdx4 and has a deficit in both ‘primitive' and ‘definitive' hematopoietic stem cells. The embryonic anemia in the mutants can be rescued by overexpressing hox genes such as hoxb7a and hoxa9a, but not hoxb8a. This suggests that specific hox genes are required to make mesoderm competent to form blood. To further explore this, we undertook a microarray analysis to identify differentially expressed genes in kgg mutants and wild-type embryos. We found that raldh2, an enzyme required for retinoic acid (RA) production, is overexpressed in kgg mutants during the early stages of blood formation. This data led us to hypothesize that RA may act to suppress blood formation and that the cdx-hox pathway functions to limit RA production, thereby permitting blood formation to occur. To test this, we treated wild-type zebrafish embryos with RA and found that they became severely anemic. Treating kgg embryos with DEAB, a chemical that blocks raldh2 activity, restored hematopoiesis in kgg mutants. Expression of hoxa9a was not rescued in these treated embryos, indicating that RA acts downstream of the hox genes. DEAB also induced an expansion of erythroid cells in wild-type embryos, thus supporting the notion that the levels of RA during development are a critical determinant for blood formation. By performing a time-course rescue experiment, we determined that DEAB is effective when scl + hematopoietic progenitors are first formed from mesoderm, suggesting that RA acts upstream of the blood-inducer scl. In support of this, SCL overexpression rescues GATA-1 expression in embryos treated with RA. We next looked at the effects of DEAB and RA on the formation of mouse hematopoietic progenitors arising from ES cell-derived embryoid bodies (EBs). Addition of DEAB to EBs between days 2 to 3 of development resulted in a 5–8 fold increase in ‘primitive' erythroid colonies (CFU-Ep), analogous to our results in zebrafish embryos. In contrast, RA treatment caused a general inhibition in the growth of all colony types. Taken together, these results suggest a new model in which suppression of RA by the cdx-hox pathway is necessary for yolk sac hematopoiesis to occur. This model provides an explanation for how hox genes control the spatiotemporal formation of hematopoietic tissue during organogenesis and may shed new light on the pathogenesis of leukemias involving translocations of the cdx , hox, and retinoic acid receptors.

Ethanol increases retinoic acid production in cerebellar astrocytes and in cerebellum

Several characteristics of fetal alcohol syndrome (FAS) are similar to the teratogenic effects of retinoic acid (RA) exposure. It has been suggested that FAS may result from ethanol-induced alteration in endogenous RA synthesis, leading to abnormal embryonic concentrations of this morphogen. We examined whether ethanol may interfere with RA synthesis in the postnatal cerebellum, as a region of the developing CNS particularly vulnerable to both ethanol and RA teratogenesis. It was found that astrocytes are the predominant source of postnatal RA synthesis in the cerebellum. They express both retinaldehyde dehydrogenase 1 and 2. In vitro cytosolic preparations of astrocytes, as well as live cell preparations, have an increased capacity to synthesize RA in the presence of ethanol. A mechanism by which ethanol could stimulate RA synthesis is via the ethanol-activated short-chain retinol dehydrogenases, which we show to be present in the postnatal cerebellum. To determine whether ethanol stimulated RA synthesis in vivo, a sensitive and highly specific HPLC/MS n technique was used to measure cerebellar RA after administration of ethanol to postnatal day 4 rat pups. Cerebellar RA levels climbed significantly after such treatment. These results suggest that the cerebellar pathology exerted by ethanol may occur, at least in part, through increased production of RA.

Lung cancers detected by screening with spiral computed tomography have a malignant phenotype when analyzed by cDNA microarray.

Spiral computed tomography (CT) can detect lung cancer at an early stage, but the malignant potential is unknown. The question is, as follows: do these small lesions have the same lethal potential as do symptomatic tumors? We used a cDNA microarray platform and compared the gene expression profile of spiral CT-detected lung carcinomas with a matched case-control population of patients presenting with symptomatic lung cancer. CT-detected and symptomatic tumors have shown a comparable gene expression profile. Correspondence analysis has demonstrated that nine genes were differentially expressed, although with a high variability across the samples that prevented distinguishing the two groups of tumors. Analysis of these nine genes has suggested that early-detected tumors have higher levels of retinoic acid production and higher expression levels of caveolin 2, matrix Gla, and cystatin A, which are already known to be lost during tumor progression. All of the tumors observed are histologically malignant according to the WHO Classification. Early lung cancers that are detected by screening have a gene expression pattern similar to, but not identical to, that of symptomatic lung carcinomas.

LEDGF regulation of alcohol and aldehyde dehydrogenases in lens epithelial cells: stimulation of retinoic acid production and protection from ethanol toxicity.

Retinoic acid (RA) is required for the normal growth and maintenance of many cell types, including lens epithelial cells (LECs). Alcohol (ADH) and aldehyde (ALDH) dehydrogenases are implicated in cellular detoxification and conversion of vitamin A to RA. Lens epithelium-derived growth factor (LEDGF) provides cellular protection against stress by transactivating stress-associated genes. Here we show evidence that LEDGF binds and transactivates heat shock (nGAAn) and stress response (A/TGGGGA/T) elements in the promoters of ADH1, ADH4, and retinaldehyde 2 (RALDH2) genes. Electrophoretic mobility and supershift assays disclosed specific binding of LEDGF to nGAAn and A/TGGGGA/T elements in these gene promoters. Transfection experiments in LECs with promoters linked to a chloramphenicol acetyltransferase (CAT) reporter gene along with LEDGF cDNA revealed higher CAT activity. RT-PCR results confirmed that LECs overexpressing LEDGF contained increased levels of ADH1, ADH4, and RALDH2 mRNA. Notably, LECs displayed higher LEDGF mRNA and protein expression during ethanol stress. Cells overexpressing LEDGF typically exhibited elevated RA levels and survived well during ethanol stress. The present findings indicate that LEDGF is one of the transcriptional activators of these genes that facilitates cellular protection against ethanol stress and plays a role in RA production.

The Tumor Suppressor Adenomatous Polyposis Coli and Caudal Related Homeodomain Protein Regulate Expression of Retinol Dehydrogenase L

Development of normal colon epithelial cells proceeds through a systematic differentiation of cells that emerge from stem cells within the base of colon crypts. Genetic mutations in the adenomatous polyposis coli (APC) gene are thought to cause colon adenoma and carcinoma formation by enhancing colonocyte proliferation and impairing differentiation. We currently have a limited understanding of the cellular mechanisms that promote colonocyte differentiation. Herein, we present evidence supporting a lack of retinoic acid biosynthesis as a mechanism contributing to the development of colon adenomas and carcinomas. Microarray and reverse transcriptase-PCR analyses revealed reduced expression of two retinoid biosynthesis genes: retinol dehydrogenase 5 (RDH5) and retinol dehydrogenase L (RDHL) in colon adenomas and carcinomas as compared with normal colon. Consistent with the adenoma and carcinomas samples, seven colon carcinoma cell lines also lacked expression of RDH5 and RDHL. Assessment of RDH enzymatic activity within these seven cell lines showed poor conversion of retinol into retinoic acid when compared with normal cells such as normal human mammary epithelial cells. Reintroduction of wild type APC into an APC-deficient colon carcinoma cell line (HT29) resulted in increased expression of RDHL without affecting RDH5. APC-mediated induction of RDHL was paralleled by increased production of retinoic acid. Investigations into the mechanism responsible for APC induction of RDHL indicated that beta-catenin fails to repress RDHL. The colon-specific transcription factor CDX2, however, activated an RDHL promoter construct and induced endogenous RDHL. Finally, the induction of RDHL by APC appears dependent on the presence of CDX2. We propose a novel role for APC and CDX2 in controlling retinoic acid biosynthesis and in promoting a retinoid-induced program of colonocyte differentiation.

Estrogen directly induces expression of retinoic acid biosynthetic enzymes, compartmentalized between the epithelium and underlying stromal cells in rat uterus.

Estrogen (E2) has been shown to induce the biosynthesis of retinoic acid (RA) in rat uterus. Here we examined whether E2 could directly induce the enzymes involved in this process by using the ovariectomized rat. A retinol dehydrogenase that we have previously described, eRolDH, and the retinal dehydrogenase, RalDH II, were found to have markedly increased uterine mRNA levels within 4 h of E2 administration, independent of the prior administration of puromycin. eRolDH and RalDH II and their mRNAs were also increased in uteri of rats during estrus. This indicated that RA biosynthesis in rat uterus is directly controlled by E2 and provides a direct link between the action of a steroid hormone and retinoid action. We also examined the cell-specific localization of RalDH II by immunohistochemistry. The enzyme was observed in the stromal compartment, particularly in cells close to the uterine lumenal epithelium. eRolDH was observed only in the lining epithelial cells. Taken together with the previous observations of cellular retinol-binding protein and cellular retinoic acid-binding protein, type two also being expressed in the lumenal epithelium, we propose that RA production is compartmentalized, with retinol oxidation occurring in the lumenal epithelium and subsequent oxidation of retinal to RA occurring in the underlying stromal cells.

Signaling hierarchy downstream of retinoic acid that independently regulates vascular remodeling and endothelial cell proliferation.

We previously demonstrated that during vascular morphogenesis, retinoic acid (RA) is required for the control of endothelial cell proliferation and capillary plexus remodeling. Herein, we investigate the mechanisms by which RA regulates these processes in the yolk sac. We found that although the enzyme required for RA production during early embryogenesis, retinaldehyde dehydrogenase-2 (Raldh2), was expressed in the visceral endoderm, RA receptors alpha1 and alpha2 were expressed in endothelial cells in the mesoderm, indicating that they are direct targets of RA. In Raldh2(-/-) embryos, there was down-regulation of TGF-beta1, fibronectin (Fn) and integrin alpha5, which was associated with decreased visceral endoderm survival and production of VEGF-A, Indian hedgehog (IHH), and bFGF. Exogenous provision of RA or Fn to Raldh2(-/-) explants in whole mouse embryo culture restored vascular remodeling, visceral endoderm survival, as well as integrin alpha5 expression and its downstream signaling that controls endothelial growth. Exogenous provision of visceral endoderm-derived factors (VEGF-A, IHH, and bFGF) failed to rescue endothelial cell proliferative control but collectively promoted vascular remodeling, suggesting that these processes are independently regulated via a signaling hierarchy downstream of RA.

Beta-carotene storage, conversion to retinoic acid, and induction of the lipocyte phenotype in hepatic stellate cells.

Hepatic stellate cells (HSCs) are the major site of retinol (ROH) metabolism and storage. GRX is a permanent murine myofibroblastic cell line, derived from HSCs, which can be induced to display the fat-storing phenotype by treatment with retinoids. Little is known about hepatic or serum homeostasis of beta-carotene and retinoic acid (RA), although the direct biogenesis of RA from beta-carotene has been described in enterocytes. The aim of this study was to identify the uptake, metabolism, storage, and release of beta-carotene in HSCs. GRX cells were plated in 25 cm(2) tissue culture flasks, treated during 10 days with 3 micromol/L beta-carotene and subsequently transferred into the standard culture medium. beta-Carotene induced a full cell conversion into the fat-storing phenotype after 10 days. The total cell extracts, cell fractions, and culture medium were analyzed by reverse phase high-performance liquid chromatography for beta-carotene and retinoids. Cells accumulated 27.48 +/- 6.5 pmol/L beta-carotene/10(6) cells, but could not convert it to ROH nor produced retinyl esters (RE). beta-Carotene was directly converted to RA, which was found in total cell extracts and in the nuclear fraction (10.15 +/- 1.23 pmol/L/10(6) cells), promoting the phenotype conversion. After 24-h chase, cells contained 20.15 +/- 1.12 pmol/L beta-carotene/10(6) cells and steadily released beta-carotene into the medium (6.69 +/- 1.75 pmol/ml). We conclude that HSC are the site of the liver beta-carotene storage and release, which can be used for RA production as well as for maintenance of the homeostasis of circulating carotenoids in periods of low dietary uptake.

Feedback mechanisms regulate retinoic acid production and degradation in the zebrafish embryo

Retinoic acid (RA) signaling in vertebrate embryos occurs in a distinct physical and temporal pattern. Regulating this spatial distribution is crucial to the development of the embryo, as RA in excess or in inappropriate tissues is teratogenic. In order to understand how RA availability is determined in zebrafish we have investigated the expression of cyp26a1, an enzyme that inactivates RA, and its relationship to raldh2, one of the enzymes that produce RA from retinal. cyp26a1 expression follows three phases: in presumptive anterior neurectoderm and in a circumblastoporal ring during gastrulation, in the tailbud throughout somitogenesis, and in multiple specific tissue types beginning at mid-somitogenesis and continuing through 48 h postfertilization (hpf). This expression was either adjacent or opposite to those tissues expressing raldh2. We then investigated how RA production might regulate these relationships. Endogenous RA produced by raldhs did not play a role in setting cyp26a1 expression in most tissues. However, exogenous RA regulates expression of both enzymes. cyp26a1 is up regulated in the embryo in a time, concentration, and tissue-dependent manner. Conversely, raldh2 expression is reduced with RA treatment. Tests of the raldh2 promoter in cell transfections proved that RA directly represses its activity. These data demonstrate that the feedback mechanisms regulating production and degradation of RA must be considered in any experiments altering levels of RA in the developing vertebrate embryo.

α-Tocopherol and Ascorbic Acid Decrease the Production of β-Apo-carotenals and Increase the Formation of Retinoids from β-Carotene in the Lung Tissues of Cigarette Smoke–Exposed Ferrets In Vitro

Previously, we found that exposing ferrets to cigarette smoke enhanced oxidative excentric cleavage of beta-carotene. In the present study, we examined whether alpha-tocopherol, ascorbic acid, or the two combined can prevent smoke-altered beta-carotene metabolism. In vitro incubation of beta-carotene (10 micro mol/L) with lung postnuclear fractions from ferrets exposed to cigarette smoke was carried out in the absence or presence of alpha-tocopherol (50 micro mol/L), ascorbic acid (10 or 50 micro mol/L), or both vitamins to evaluate their effects on the production of beta-apo-carotenals and retinoids from beta-carotene. The oxidative cleavage metabolites of beta-carotene, beta-apo-carotenals (beta-apo-14', beta-apo-12', beta-apo-10', and beta-apo-8'), retinoic acid (RA), and retinal were analyzed by HPLC. We found that the smoke-enhanced production of individual beta-apo-carotenals was significantly decreased by 36-77% when alpha-tocopherol (50 micro mol/L) and ascorbic acid (50 micro mol/L) were added together to the incubation mixture. alpha-Tocopherol alone had a modest effect. Ascorbic acid in the presence of alpha-tocopherol inhibited the production of beta-apo-carotenals in a dose-dependent manner, although ascorbic acid alone had no effect. In contrast, the production of RA and retinal among smoke-exposed ferrets was substantially increased ( approximately 3-fold, P < 0.05) when both alpha-tocopherol and ascorbic acid were added to the incubation mixtures. However, when ascorbic acid or alpha-tocopherol alone was added, the production of RA among smoke-exposed ferrets increased only modestly (80%, P < 0.05) and did not differ from the RA levels in control ferrets. In conclusion, these data indicate that alpha-tocopherol and ascorbic acid may act synergistically in preventing the enhanced oxidative excentric cleavage of beta-carotene induced by smoking exposure, thereby facilitating the conversion of beta-carotene into RA and retinal.

Reductions in the incidence of nitrofen-induced diaphragmatic hernia by vitamin A and retinoic acid.

Congenital diaphragmatic hernia (CDH) is a serious medical condition in which the developing diaphragm forms incompletely, leaving a hole through which the abdominal contents can enter the thoracic space and interfere with lung growth. A perturbation of the retinoid system has been linked to the etiology of CDH. This includes findings that nitrofen, which induces CDH in rodents, inhibits the key enzyme for retinoic acid (RA) production, retinaldehyde dehydrogenase-2 (RALDH2) in vitro. Published studies indicate that antenatal vitamin A administration on gestational day (D) 12 in the nitrofen model of CDH reduced the severity and incidence of right-sided defects and lung hypoplasia. In this study, we administered nitrofen on D8, to include the induction of clinically more prevalent left-sided defects, and examined the efficacy of several vitamin A administration paradigms to gain insights into the developmental stage of susceptibility. Furthermore, we tested the hypothesis that administration of RA, the product of RALDH2 activity, is more potent than administering the substrate, vitamin A, in reducing the incidence of CDH. The incidence of CDH was reduced from approximately 54% (nitrofen alone) to approximately 32% with vitamin A treatment. The efficacy of RA treatment was very marked, with a reduction in the incidence of CDH to approximately 15%. Administration of vitamin A or RA on approximately D10 was most effective. These data lend further support for the potential involvement of retinoid signaling pathways and the etiology of CDH and support data from in vitro studies demonstrating a nitrofen-induced suppression of RALDH2.

Cellular Retinoic Acid-binding Protein II Gene Expression Is Directly Induced by Estrogen, but Not Retinoic Acid, in Rat Uterus

It has been suggested that cellular retinoic acid-binding protein (II) (CRABP(II)) may have a role in the movement of retinoic acid (RA) to its nuclear receptors, thereby enhancing the action of RA in the cells in which it is expressed. RA has also been shown to increase expression of CRABP(II). Previous work from our laboratory has shown that 17 beta-estradiol (E2) administration to prepubertal female rats leads to acquisition of the ability of the lining epithelium to synthesize RA as well as to express CRABP(II). To determine whether this appearance of CRABP(II) was dependent on the production of RA, both E2 and RA were administered to ovariectomized rats. E2 administration induced expression of the CRABP(II) gene in the uterus within 4 h, and this induction was not inhibited by prior administration of puromycin, indicating that the induction was direct. In contrast, RA caused no change in CRABP(II) message level, even at times as late as 48 h after administration. Isolation and analysis of 4.5 kb of the 5'-flanking region of the gene revealed no apparent E2-response element. Using this portion of the gene to drive expression of the luciferase gene in transfected cells allowed identification of a region containing an imperfect estrogen-response element and estrogen-response element half-site, necessary for E2-driven induction. A possible Sp1 binding site in the 5'-flanking region of the CRABP(II) gene was also required for this induction. The ability of E2 to induce expression of CRABP(II) suggests that it can enhance the activity of RA, directly affecting expression of retinoid-responsive genes.

The history of acute promyelocytic leukaemia.

The history of acute promyelocytic leukaemia.

Retinal Dehydrogenase-2 Is Inhibited by Compounds that Induce Congenital Diaphragmatic Hernias in Rodents

Currently, the etiology of the serious developmental anomaly congenital diaphragmatic hernia (CDH) is unknown. We have used an animal model of CDH to address this issue. We characterized four separate teratogens that produced diaphragmatic defects in embryonic rats that are similar to those in infants with CDH. We then tested the hypothesis that all these agents share the common mechanism of perturbing the retinoid-signaling pathway. Specifically, inhibition of retinal dehydrogenase-2 (RALDH2), a key enzyme necessary for the production of retinoic acid and that is expressed in the developing diaphragm, was assayed by measuring retinoic acid production in cytosolic extracts from an oligodendrocyte cell line. The following compounds all induce posterolateral defects in the rat diaphragm; nitrofen, 4-biphenyl carboxylic acid, bisdiamine, and SB-210661. Importantly, we demonstrate that they all share the common mechanism of inhibiting RALDH2. These data provide an important component of mounting evidence suggesting that the retinoid system warrants consideration in future studies of the etiology of CDH.

Steroid and cytokine regulation of matrix metalloproteinase expression in endometriosis and the establishment of experimental endometriosis in nude mice.

The cyclic expression of matrix metalloproteinases (MMPs) by human endometrium has been suggested to play a role in the invasive process necessary to establish endometriosis. The ability of progesterone exposure to inhibit endometrial MMP-3 and MMP-7 expression requires the local action of TGF beta and may also be linked to the local production of retinoic acid by stromal cells. A continuous expression of several MMPs in endometriotic lesions has been reported, indicating a failure of progesterone or locally produced factors to suppress these enzymes. To address cell-specific MMP regulation associated with endometriosis, we examined expression of MMP-3 and MMP-7 mRNA in eutopic endometrium and endometriotic lesions acquired during the secretory phase of the menstrual cycle. We examined the in vitro regulation of MMP-3 and MMP-7 protein in similar tissues. We also examined the in vitro regulation of MMP secretion by progesterone, retinoic acid, and TGF beta in endometriosis tissues relative to the establishment of experimental disease. Our studies indicate that either eutopic or ectopic tissue from women with endometriosis exhibit patterns of altered MMP regulation in vivo. A lack of responsiveness to progesterone was demonstrated in vitro, associated with a failure to suppress MMP expression and an enhanced ability of the tissue to establish experimental endometriosis. However, in vitro treatments with retinoic acid and TGF beta restored the ability of progesterone to suppress MMPs in vitro and prevented the establishment of experimental disease.

Retinol/Ethanol Drug Interaction during Acute Alcohol Intoxication in Mice Involves Inhibition of Retinol Metabolism to Retinoic Acid by Alcohol Dehydrogenase

Substantial evidence indicates that one consequence of alcohol intoxication is a reduction in retinoic acid (RA) levels. Studies on the mechanism have shown that chronic ethanol consumption induces P450 enzymes that increase RA degradation, thus accounting for much but not all of the observed decrease in RA. A reduction in RA synthesis may also be involved as ethanol competitively inhibits retinol oxidation catalyzed by alcohol dehydrogenase (ADH) in vitro. This may be important during acute ethanol intoxication and may contribute to adverse retinol/ethanol drug interactions. Here we have examined mice for the effect of either acute ethanol intoxication or Adh1 gene disruption on RA synthesis and degradation. RA produced following a dose of retinol (50 mg/kg) was reduced 87% by pretreatment with an intoxicating dose of ethanol (3.5 g/kg). RA produced in Adh1-null mutant mice following a 50-mg/kg dose of retinol was reduced 82% relative to wild-type mice, thus similar to wild-type mice pretreated with ethanol. Reduced RA production was associated with increased retinol levels in both ethanol-treated wild-type mice and Adh1-null mutant mice, indicating reduced clearance of the retinol dose. RA degradation following a dose of RA (10 mg/kg) was increased only 42% by ethanol pretreatment (3.5 g/kg) and only 26% in Adh1-null mutant mice relative to wild-type mice. These findings demonstrate that the reduced RA levels observed during acute retinol/ethanol drug interaction are due primarily to a decrease in ADH-catalyzed RA synthesis and secondarily to an increase in RA degradation.

Excessive vitamin A toxicity in mice genetically deficient in either alcohol dehydrogenase Adh1 or Adh3.

Alcohol dehydrogenase (ADH) deficiency results in decreased retinol utilization, but it is unclear what physiological roles the several known ADHs play in retinoid signaling. Here, Adh1, Adh3, and Adh4 null mutant mice have been examined following acute and chronic vitamin A excess. Following an acute dose of retinol (50 mg.kg(-1)), metabolism of retinol to retinoic acid in liver was reduced 10-fold in Adh1 mutants and 3.8-fold in Adh3 mutants, but was not significantly reduced in Adh4 mutants. Acute retinol toxicity, assessed by determination of the LD(50) value, was greatly increased in Adh1 mutants and moderately increased in Adh3 mutants, but only a minor effect was observed in Adh4 mutants. When mice were propagated for one generation on a retinol-supplemented diet containing 10-fold higher vitamin A than normal, Adh3 and Adh4 mutants had essentially the same postnatal survival to adulthood as wild-type (92-95%), but only 36% of Adh1 mutants survived to adulthood with the remainder dying by postnatal day 3. Adh1 mutants surviving to adulthood on the retinol- supplemented diet had elevated serum retinol signifying a clearance defect and elevated aspartate aminotransferase indicative of increased liver damage. These findings indicate that ADH1 functions as the primary enzyme responsible for efficient oxidative clearance of excess retinol, thus providing protection and increased survival during vitamin A toxicity. ADH3 plays a secondary role. Our results also show that retinoic acid is not the toxic moiety during vitamin A excess, as Adh1 mutants have less retinoic acid production while experiencing increased toxicity.

Novel retinoic acid generating activities in the neural tube and heart identified by conditional rescue of Raldh2 null mutant mice.

Retinoid control of vertebrate development depends upon tissue-specific metabolism of retinol to retinoic acid (RA). The RA biosynthetic enzyme RALDH2 catalyzes much, but not all, RA production in mouse embryos, as revealed here with Raldh2 null mutants carrying an RA-responsive transgene. Targeted disruption of Raldh2 arrests development at midgestation and eliminates all RA synthesis except that associated with Raldh3 expression in the surface ectoderm of the eye field. Conditional rescue of Raldh2(-/-) embryos by limited maternal RA administration allows development to proceed and results in the establishment of additional sites of RA synthesis linked to Raldh1 expression in the dorsal retina and to Raldh3 expression in the ventral retina, olfactory pit and urinary tract. Unexpectedly, conditionally rescued Raldh2(-/-) embryos also possess novel sites of RA synthesis in the neural tube and heart that do not correspond to expression of Raldh1-3. RA synthesis in the mutant neural tube was localized in the spinal cord, posterior hindbrain and portions of the midbrain and forebrain, whereas activity in the mutant heart was localized in the conotruncus and sinus venosa. In the posterior hindbrain, this novel RA-generating activity was expressed during establishment of rhombomeric boundaries. In the spinal cord, the novel activity was localized in the floorplate plus in the intermediate region where retinoid-dependent interneurons develop. These novel RA-generating activities in the neural tube and heart fill gaps in our knowledge of how RA is generated spatiotemporally and may, along with Raldh1 and Raldh3, contribute to rescue of Raldh2(-/-) embryos by producing RA locally.