Cloning of a cDNA for a second retinol dehydrogenase type II. Expression of its mRNA relative to type I

Post-natal all-trans-retinoic acid biosynthesis

Generation of the autacoid all-trans-retinoic acid (ATRA) from retinol (vitamin A) relies on a complex metabolon that includes retinol binding-proteins and enzymes from the short-chain dehydrogenase/reductase and aldehyde dehydrogenase gene families. Serum retinol binding-protein delivers all-trans-retinol (vitamin A) from blood to cells through two membrane receptors, Stra6 and Rbpr2. Stra6 and Rbpr2 convey retinol to cellular retinol binding-protein type 1 (Crbp1). Holo-Crbp1 delivers retinol to lecithin: retinol acyl transferase (Lrat) for esterification and storage. Lrat channels retinol directly into its active site from holo-Crbp1 by protein-protein interaction. The ratio apo-Crbp1/holo-Crbp1 directs flux of retinol into and out of retinyl esters, through regulating esterification vs ester hydrolysis. Multiple retinol dehydrogenases (Rdh1, Rdh10, Dhrs9, Rdhe2, Rdhe2s) channel retinol from holo-Crbp1 to generate retinal for ATRA biosynthesis. β-Carotene oxidase type 1 generates retinal from carotenoids, delivered by the scavenger receptor-B1. Retinal reductases (Dhrs3, Dhrs4, Rdh11) reduce retinal into retinol, thereby restraining ATRA biosynthesis. Retinal dehydrogenases (Raldh1, 2, 3) dehydrogenate retinal irreversibly into ATRA. ATRA regulates its own concentrations by inducing Lrat and ATRA degradative enzymes. ATRA exhibits hormesis. Its effects relate to its concentration as an inverted J-shaped curve, transitioning from beneficial in the "goldilocks" zone to toxicity, as concentrations increase. Hormesis has distorted understanding physiological effects of ATRA post-nataly using chow-diet fed, ATRA-dosed animal models. Cancer, immune deficiency and metabolic abnormalities result from mutations and/or insufficiency in Crbp1 and retinoid metabolizing enzymes.

Methoxychlor and its metabolite HPTE inhibit rat neurosteroidogenic 3α-hydroxysteroid dehydrogenase and retinol dehydrogenase 2

Methoxychlor is primarily used as an insecticide and it is widely present in the environment. The objective of the present study was to investigate the direct effects of methoxychlor and its metabolite hydroxychlor (HPTE) on rat neurosteroidogenic 3α-hydroxysteroid dehydrogenase (AKR1C14) and retinol dehydrogenase 2 (RDH2) activities. Rat AKR1C14 and RDH2 were cloned and expressed in COS-1 cells, and the effects of methoxychlor and HPTE on these enzymes were measured. HPTE was more potent to inhibit AKR1C14 and RDH2 activities than methoxychlor, with IC₅₀ values of 2.602 ± 0.057 μM and 20.473 ± 0.049 μM, respectively, while those of methoxychlor were over 100 μM. HPTE competitively inhibited AKR1C14 and RDH2 when steroid substrates were used, while it showed a mode of mixed inhibition on these enzymes when NADPH/NAD⁺ were used. We elucidated the binding mode of methoxychlor and HPTE to the crystal structure of AKR1C14 by molecular docking and found that HPTE had higher affinity with the enzyme than methoxychlor. In conclusion, HPTE is more potent than methoxychlor to inhibit both AKR1C14 and RDH2.

Functions of Intracellular Retinoid Binding-Proteins

Multiple binding and transport proteins facilitate many aspects of retinoid biology through effects on retinoid transport, cellular uptake, metabolism, and nuclear delivery. These include the serum retinol binding protein sRBP (aka Rbp4), the plasma membrane sRBP receptor Stra6, and the intracellular retinoid binding-proteins such as cellular retinol-binding proteins (CRBP) and cellular retinoic acid binding-proteins (CRABP). sRBP transports the highly lipophilic retinol through an aqueous medium. The major intracellular retinol-binding protein, CRBP1, likely enhances efficient retinoid use by providing a sink to facilitate retinol uptake from sRBP through the plasma membrane or via Stra6, delivering retinol or retinal to select enzymes that generate retinyl esters or retinoic acid, and protecting retinol/retinal from excess catabolism or opportunistic metabolism. Intracellular retinoic acid binding-proteins (CRABP1 and 2, and FABP5) seem to have more diverse functions distinctive to each, such as directing retinoic acid to catabolism, delivering retinoic acid to specific nuclear receptors, and generating non-canonical actions. Gene ablation of intracellular retinoid binding-proteins does not cause embryonic lethality or gross morphological defects. Metabolic and functional defects manifested in knockouts of CRBP1, CRBP2 and CRBP3, however, illustrate their essentiality to health, and in the case of CRBP2, to survival during limited dietary vitamin A. Future studies should continue to address the specific molecular interactions that occur between retinoid binding-proteins and their targets and their precise physiologic contributions to retinoid homeostasis and function.

Cellular retinoid binding-proteins, CRBP, CRABP, FABP5: Effects on retinoid metabolism, function and related diseases

Cellular binding-proteins (BP), including CRBP1, CRBP2, CRABP1, CRABP2, and FABP5, shepherd the poorly aqueous soluble retinoids during uptake, metabolism and function. Holo-BP promote efficient use of retinol, a scarce but essential nutrient throughout evolution, by sheltering it and its major metabolite all-trans-retinoic acid from adventitious interactions with the cellular milieu, and by imposing specificity of delivery to enzymes, nuclear receptors and other partners. Apo-BP reflect cellular retinoid status and modify activities of retinoid metabolon enzymes, or exert non-canonical actions. High ligand binding affinities and the nature of ligand sequestration necessitate external factors to prompt retinoid release from holo-BP. One or more of cross-linking, kinetics, and colocalization have identified these factors as RDH, RALDH, CYP26, LRAT, RAR and PPARβ/δ. Michaelis-Menten and other kinetic approaches verify that BP channel retinoids to select enzymes and receptors by protein-protein interactions. Function of the BP and enzymes that constitute the retinoid metabolon depends in part on retinoid exchanges unique to specific pairings. The complexity of these exchanges configure retinol metabolism to meet the diverse functions of all-trans-retinoic acid and its ability to foster contrary outcomes in different cell types, such as inducing apoptosis, differentiation or proliferation. Altered BP expression affects retinoid function, for example, by impairing pancreas development resulting in abnormal glucose and energy metabolism, promoting predisposition to breast cancer, and fostering more severe outcomes in prostate cancer, ovarian adenocarcinoma, and glioblastoma. Yet, the extent of BP interactions with retinoid metabolon enzymes and their impact on retinoid physiology remains incompletely understood.

CK1δ activity is modulated by CDK2/E- and CDK5/p35-mediated phosphorylation

CK1 protein kinases form a family of serine/threonine kinases which are highly conserved through different species and ubiquitously expressed. CK1 family members can phosphorylate numerous substrates thereby regulating different biological processes including membrane trafficking, cell cycle regulation, circadian rhythm, apoptosis, and signal transduction. Deregulation of CK1 activity and/or expression contributes to the development of neurological diseases and cancer. Therefore, CK1 became an interesting target for drug development and it is relevant to further understand the mechanisms of its regulation. In the present study, Cyclin-dependent kinase 2/Cyclin E (CDK2/E) and Cyclin-dependent kinase 5/p35 (CDK5/p35) were identified as cellular kinases able to modulate CK1δ activity through site-specific phosphorylation of its C-terminal domain. Furthermore, pre-incubation of CK1δ with CDK2/E or CDK5/p35 reduces CK1δ activity in vitro, indicating a functional impact of the interaction between CK1δ and CDK/cyclin complexes. Interestingly, inhibition of Cyclin-dependent kinases by Dinaciclib increases CK1δ activity in pancreatic cancer cells. In summary, these results suggest that CK1δ activity can be modulated by the interplay between CK1δ and CDK2/E or CDK5/p35. These findings extend our knowledge about CK1δ regulation and may be of use for future development of CK1-related therapeutic strategies in the treatment of neurological diseases or cancer.

Retinoic Acid Synthesis and Degradation

Retinoic acid (RA) was identified as the biologically active form of vitamin A almost 70 years ago and work on its function and mechanism of action is still of major interest both from a scientific and a clinical perspective. The currently accepted model postulates that RA is produced in two sequential oxidative steps: first, retinol is oxidized reversibly to retinaldehyde, and then retinaldehyde is oxidized irreversibly to RA. Excess RA is inactivated by conversion to hydroxylated derivatives. Much is left to learn, especially about retinoid binding proteins and the trafficking of the hydrophobic retinoid substrates between membrane bound and cytosolic enzymes. Here, background on development of the field and an update on recent advances in our understanding of the enzymatic pathways and mechanisms that control the rate of RA production and degradation are presented with a focus on the many questions that remain unanswered.

Fluoxetine elevates allopregnanolone in female rat brain but inhibits a steroid microsomal dehydrogenase rather than activating an aldo-keto reductase

Background and Purpose

Fluoxetine, a selective serotonin reuptake inhibitor, elevates brain concentrations of the neuroactive progesterone metabolite allopregnanolone, an effect suggested to underlie its use in the treatment of premenstrual dysphoria. One report showed fluoxetine to activate the aldo-keto reductase (AKR) component of 3α-hydroxysteroid dehydrogenase (3α-HSD), which catalyses production of allopregnanolone from 5α-dihydroprogesterone. However, this action was not observed by others. The present study sought to clarify the site of action for fluoxetine in elevating brain allopregnanolone.

Experimental Approach

Adult male rats and female rats in dioestrus were treated with fluoxetine and their brains assayed for allopregnanolone and its precursors, progesterone and 5α-dihydroprogesterone. Subcellular fractions of rat brain were also used to investigate the actions of fluoxetine on 3α-HSD activity in both the reductive direction, producing allopregnanolone from 5α-dihydroprogesterone, and the reverse oxidative direction. Fluoxetine was also tested on these recombinant enzyme activities expressed in HEK cells.

Key Results

Short-term treatment with fluoxetine increased brain allopregnanolone concentrations in female, but not male, rats. Enzyme assays on native rat brain fractions and on activities expressed in HEK cells showed fluoxetine did not affect the AKR producing allopregnanolone from 5α-dihydroprogesterone but did inhibit the microsomal dehydrogenase oxidizing allopregnanolone to 5α-dihydroprogesterone.

Conclusions and Implications

Fluoxetine elevated allopregnanolone in female rat brain by inhibiting its oxidation to 5α-dihydroprogesterone by a microsomal dehydrogenase. This is a novel site of action for fluoxetine, with implications for the development of new agents and/or dosing regimens to raise brain allopregnanolone.

Fetal brain genomic reprogramming following asphyctic preconditioning

Background

Fetal asphyctic (FA) preconditioning is effective in attenuating brain damage incurred by a subsequent perinatal asphyctic insult. Unraveling mechanisms of this endogenous neuroprotection, activated by FA preconditioning, is an important step towards new clinical strategies for asphyctic neonates. Genomic reprogramming is thought to be, at least in part, responsible for the protective effect of preconditioning. Therefore we investigated whole genome differential gene expression in the preconditioned rat brain. FA preconditioning was induced on embryonic day 17 by reversibly clamping uterine circulation. Male control and FA offspring were sacrificed 96 h after FA preconditioning. Whole genome transcription was investigated with Affymetrix Gene1.0ST chip.

Results

Data were analyzed with the Bioconductor Limma package, which showed 53 down-regulated and 35 up-regulated transcripts in the FA-group. We validated these findings with RT-qPCR for adh1, edn1, leptin, rdh2, and smad6. Moreover, we investigated differences in gene expression across different brain regions. In addition, we performed Gene Set Enrichment Analysis (GSEA) which revealed 19 significantly down-regulated gene sets, mainly involved in neurotransmission and ion transport. 10 Gene sets were significantly up-regulated, these are mainly involved in nucleosomal structure and transcription, including genes such as mecp2.

Conclusions

Here we identify for the first time differential gene expression after asphyctic preconditioning in fetal brain tissue, with the majority of differentially expressed transcripts being down-regulated. The observed down-regulation of cellular processes such as neurotransmission and ion transport could represent a restriction in energy turnover which could prevent energy failure and subsequent neuronal damage in an asphyctic event. Up-regulated transcripts seem to exert their function mainly within the cell nucleus, and subsequent Gene Set Enrichment Analysis suggests that epigenetic mechanisms play an important role in preconditioning induced neuroprotection.

The diversity of sex steroid action: novel functions of hydroxysteroid (17β) dehydrogenases as revealed by genetically modified mouse models

Disturbed action of sex steroid hormones, i.e. androgens and estrogens, is involved in the pathogenesis of various severe diseases in humans. Interestingly, recent studies have provided data further supporting the hypothesis that the circulating hormone concentrations do not explain all physiological and pathological processes observed in hormone-dependent tissues, while the intratissue sex steroid concentrations are determined by the expression of steroid metabolising enzymes in the neighbouring cells (paracrine action) and/or by target cells themselves (intracrine action). This local sex steroid production is also a valuable treatment option for developing novel therapies against hormonal diseases. Hydroxysteroid (17β) dehydrogenases (HSD17Bs) compose a family of 14 enzymes that catalyse the conversion between the low-active 17-keto steroids and the highly active 17β-hydroxy steroids. The enzymes frequently expressed in sex steroid target tissues are, thus, potential drug targets in order to lower the local sex steroid concentrations. The present review summarises the recent data obtained for the role of HSD17B1, HSD17B2, HSD17B7 and HSD17B12 enzymes in various metabolic pathways and their physiological and pathophysiological roles as revealed by the recently generated genetically modified mouse models. Our data, together with that provided by others, show that, in addition to having a role in sex steroid metabolism, several of these HSD17B enzymes possess key roles in other metabolic processes: for example, HD17B7 is essential for cholesterol biosynthesis and HSD17B12 is involved in elongation of fatty acids. Additional studies in vitro and in vivo are to be carried out in order to fully define the metabolic role of the HSD17B enzymes and to evaluate their value as drug targets.

The role of molecular regulation and targeting in regulating calcium/calmodulin stimulated protein kinases

Calcium/calmodulin-stimulated protein kinases can be classified as one of two types - restricted or multifunctional. This family of kinases contains several structural similarities: all possess a calmodulin binding motif and an autoinhibitory region. In addition, all of the calcium/calmodulin-stimulated protein kinases examined in this chapter are regulated by phosphorylation, which either activates or inhibits their kinase activity. However, as the multifunctional calcium/calmodulin-stimulated protein kinases are ubiquitously expressed, yet regulate a broad range of cellular functions, additional levels of regulation that control these cell-specific functions must exist. These additional layers of control include gene expression, signaling pathways, and expression of binding proteins and molecular targeting. All of the multifunctional calcium/calmodulin-stimulated protein kinases examined in this chapter appear to be regulated by these additional layers of control, however, this does not appear to be the case for the restricted kinases.

Physiological insights into all-trans-retinoic acid biosynthesis

All-trans-retinoic acid (atRA) provides essential support to diverse biological systems and physiological processes. Epithelial differentiation and its relationship to cancer, and embryogenesis have typified intense areas of interest into atRA function. Recently, however, interest in atRA action in the nervous system, the immune system, energy balance and obesity has increased considerably, especially concerning postnatal function. atRA action depends on atRA biosynthesis: defects in retinoid-dependent processes increasingly relate to defects in atRA biogenesis. Considerable evidence indicates that physiological atRA biosynthesis occurs via a regulated process, consisting of a complex interaction of retinoid binding-proteins and retinoid recognizing enzymes. An accrual of biochemical, physiological and genetic data have identified specific functional outcomes for the retinol dehydrogenases, RDH1, RDH10, and DHRS9, as physiological catalysts of the first step in atRA biosynthesis, and for the retinal dehydrogenases RALDH1, RALDH2, and RALDH3, as catalysts of the second and irreversible step. Each of these enzymes associates with explicit biological processes mediated by atRA. Redundancy occurs, but seems limited. Cumulative data support a model of interactions among these enzymes with retinoid binding-proteins, with feedback regulation and/or control by atRA via modulating gene expression of multiple participants. The ratio apo-CRBP1/holo-CRBP1 participates by influencing retinol flux into and out of storage as retinyl esters, thereby modulating substrate to support atRA biosynthesis. atRA biosynthesis requires the presence of both an RDH and an RALDH: conversely, absence of one isozyme of either step does not indicate lack of atRA biosynthesis at the site. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

Characterization of key residues and membrane association domains in retinol dehydrogenase 10

RDH10 (retinol dehydrogenase 10) was originally identified from the retinal pigment epithelium and retinal Müller cells. It has retinoid oxidoreductase activity and is thought to play a role in the retinoid visual cycle. A recent study showed that RDH10 is essential for generating retinoic acid at early embryonic stages. The present study demonstrated that wild-type RDH10 catalysed both oxidation of all-trans-retinol and reduction of all-trans-retinal in a cofactor-dependent manner In vitro. In cultured cells, however, oxidation is the favoured reaction catalysed by RDH10. Substitution of any of the predicted key residues in the catalytic centre conserved in the RDH family abolished the enzymatic activity of RDH10 without affecting its protein level. Unlike other RDH members, however, replacement of Ser(197), a key residue for stabilizing the substrate, by glycine and alanine did not abolish the enzymatic activity of RDH10, whereas RDH10 mutants S197C, S197T and S197V completely lost their enzymatic activity. These results suggest that the size of the residue at position 197 is critical for the activity of RDH10. Mutations of the three glycine residues (Gly(43), Gly(47) and Gly(49)) in the predicted cofactor-binding motif (Gly-Xaa(3)-Gly-Xaa-Gly) of RDH10 abolished its enzymatic activity, suggesting that the cofactor-binding motif is essential for its activity. Deletion of the two hydrophobic domains dissociated RDH10 from the membrane and abolished its activity. These studies identified the key residues for the activity of RDH10 and will contribute to the further elucidation of mechanism of this important enzyme.

Medium- and short-chain dehydrogenase/reductase gene and protein families : Medium-chain and short-chain dehydrogenases/reductases in retinoid metabolism

Retinoic acid (RA), the most active retinoid, is synthesized in two steps from retinol. The first step, oxidation of retinol to retinaldehyde, is catalyzed by cytosolic alcohol dehydrogenases (ADHs) of the medium-chain dehydrogenase/reductase (MDR) superfamily and microsomal retinol dehydrogenases (RDHs) of the short-chain dehydrogenase/reductase (SDR) superfamily. The second step, oxidation of retinaldehyde to RA, is catalyzed by several aldehyde dehydrogenases. ADH1 and ADH2 are the major MDR enzymes in liver retinol detoxification, while ADH3 (less active) and ADH4 (most active) participate in RA generation in tissues. Several NAD(+)- and NADP(+)-dependent SDRs are retinoid active. Their in vivo contribution has been demonstrated in the visual cycle (RDH5, RDH12), adult retinoid homeostasis (RDH1) and embryogenesis (RDH10). K(m) values for most retinoid-active ADHs and RDHs are close to 1 microM or lower, suggesting that they participate physiologically in retinol/retinaldehyde interconversion. Probably none of these enzymes uses retinoids bound to cellular retinol-binding protein, but only free retinoids. The large number of enzymes involved in the two directions of this step, also including aldo-keto reductases, suggests that retinaldehyde levels are strictly regulated.

Placenta defects and embryonic lethality resulting from disruption of mouse hydroxysteroid (17-beta) dehydrogenase 2 gene

Hydroxysteroid (17-beta) dehydrogenase 2 (HSD17B2) is a member of aldo-keto reductase superfamily, known to catalyze the inactivation of 17beta-hydroxysteroids to less active 17-keto forms and catalyze the conversion of 20alpha-hydroxyprogesterone to progesterone in vitro. To examine the role of HSD17B2 in vivo, we generated mice deficient in Hsd17b2 [HSD17B2 knockout (KO)] by a targeted gene disruption in embryonic stem cells. From the homozygous mice carrying the disrupted Hsd17b2, 70% showed embryonic lethality appearing at the age of embryonic d 11.5 onward. The embryonic lethality was associated with reduced placental size measured at embryonic d 17.5. The HSD17B2KO mice placentas presented with structural abnormalities in all three major layers: the decidua, spongiotrophoblast, and labyrinth. Most notable was the disruption of the spongiotrophoblast and labyrinthine layers, together with liquid-filled cysts in the junctional region and the basal layer. Treatments with an antiestrogen or progesterone did not rescue the embryonic lethality or the placenta defect in the homozygous mice. In hybrid background used, 24% of HSD17B2KO mice survived through the fetal period but were born growth retarded and displayed a phenotype in the brain with enlargement of ventricles, abnormal laminar organization, and increased cellular density in the cortex. Furthermore, the HSD17B2KO mice had unilateral renal degeneration, the affected kidney frequently appearing as a fluid-filled sac. Our results provide evidence for a role for HSD17B2 enzyme in the cellular organization of the mouse placenta.

Comparative genomic and phylogenetic analysis of short-chain dehydrogenases/reductases with dual retinol/sterol substrate specificity

Human short-chain dehydrogenases/reductases with dual retinol/sterol substrate specificity (RODH-like enzymes) are thought to contribute to the oxidation of retinol for retinoic acid biosynthesis and to the metabolism of androgenic and neuroactive 3alpha-hydroxysteroids. Here, we investigated the phylogeny and orthology of these proteins to understand better their origins and physiological roles. Phylogenetic and genomic analysis showed that two proteins (11-cis-RDH and RDHL) are highly conserved, and their orthologs can be identified in the lower taxa, such as amphibians and fish. Two other proteins (RODH-4 and 3alpha-HSD) are significantly less conserved. Orthologs for 3alpha-HSD are present in all mammals analyzed, whereas orthologs for RODH-4 can be identified in some mammalian species but not in others due to species-specific gene duplications. Understanding the evolution and divergence of RODH-like enzymes in various vertebrate species should facilitate further investigation of their in vivo functions using animal models.

Immunolocalization of retinoic acid biosynthesis systems in selected sites in rat

Vitamin A deficiency leads to focal metaplasia of numerous epithelial tissues with altered differentiation from columnar (in general) to stratified squamous cells. This process can be reversed with vitamin A repletion. Previously, we described a system of retinoic acid (RA) synthesis in the cycling rat uterus consisting of cellular retinol binding protein (Crbp), epithelial retinol dehydrogenase (eRoldh), retinal dehydrogenase 2 (Aldh1a2), and cellular retinoic acid binding protein type II (Crabp2). Western blot analysis, RT-PCR, and immunohistochemistry were performed to test whether this retinoic acid synthesis system was also present in other vitamin A sensitive tissues. We found that combinations of Crbp, eRoldh, Aldh1a2 or Aldh1a3, and Crabp2 were present in all vitamin A sensitive tissues examined. In the ureter, while eRoldh was present, another short chain alcohol dehydrogenase reductase (possibly Roldh 1, 2, or 3) was in higher concentration in the transitional epithelia. In several tissues, Crbp, Aldh1a2, and/or Aldh1a3 localized to mesenchyme and/or epithelial cells, while eRoldh and Crabp2 were expressed only in epithelial cells. This suggests that mesenchymal-epithelial interactions may be as important in the adult as they are during development and that local synthesis of RA is important in maintenance of these tissues.

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/MSn 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.

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.

Elements in the N-terminal signaling sequence that determine cytosolic topology of short-chain dehydrogenases/reductases. Studies with retinol dehydrogenase type 1 and cis-retinol/androgen dehydrogenase type 1

High affinity, retinoid-specific binding proteins chaperone retinoids to manage their transport and metabolism. Proposing mechanisms of retinoid transfer between these binding proteins and membrane-associated retinoid-metabolizing enzymes requires insight into enzyme topology. We therefore determined the topology of mouse retinol dehydrogenase type 1 (Rdh1) and cis-retinoid androgen dehydrogenase type 1 (Crad1) in the endoplasmic reticulum of intact mammalian cells. The properties of Rdh1 were compared with a chimera with a luminal signaling sequence (11beta-hydroxysteroid dehydrogenase (11beta-HSD1)(1-41)/Rdh1(23-317); the green fluorescent protein (GFP) fusion proteins Rdh1(1-22)/GFP, Crad1(1-22)/GFP, and 11beta-HSD1(1-41)/GFP; and signaling sequence charge difference mutants using confocal immunofluorescence, antibody access, proteinase K sensitivity, and deglycosylation assays. An N-terminal signaling sequence of 22 residues, consisting of a hydrophobic helix ending in a net positive charge, anchors Rdh1 and Crad1 in the endoplasmic reticulum facing the cytoplasm. Mutating arginine to glutamine in the signaling sequence did not affect topology. Inserting one or two arginine residues near the N terminus of the signaling sequence caused 28-95% inversion from cytoplasmic to luminal, depending on the net positive charge remaining at the C terminus of the signaling sequence; e.g. the mutant L3R,L5R,R16Q,R19Q,R21Q faced the lumen. Experiments with N- and C-terminal epitope-tagged Rdh1 and molecular modeling indicated that a hydrophobic helix-turn-helix near the C terminus of Rdh1 (residues 289-311) projects into the cytoplasm. These data provide insight into the features necessary to orient type III (reverse signal-anchor) proteins and demonstrate that Rdh1, Crad1, and other short-chain dehydrogenases/reductases, which share similar N-terminal signaling sequences such as human Rdh5 and mouse Rdh4, orient with their catalytic domains facing the cytoplasm.

Structure and function of retinol dehydrogenases of the short chain dehydrogenase/reductase family

All-trans-retinol is the common precursor of the active retinoids 11-cis-retinal, all-trans-retinoic acid (atRA) and 9-cis-retinoic acid (9cRA). Genetic and biochemical data supports an important role of the microsomal members of the short chain dehydrogenases/reductases (SDRs) in the first oxidative conversion of retinol into retinal. Several retinol dehydrogenases of this family have been reported in recent years. However, the structural and functional data on these enzymes is limited. The prototypic enzyme RDH5 and the related enzyme CRAD1 have been shown to face the lumen of the endoplasmic reticulum (ER), suggesting a compartmentalized synthesis of retinal. This is a matter of debate as a related enzyme has been proposed to have the opposite membrane topology. Recent data indicates that RDH5, and presumably other members of the SDRs, occur as functional homodimers, and need to interact with other proteins for proper intracellular localization and catalytic activity. Further analyses on the compartmentalization, membrane topology, and functional properties of microsomal retinol dehydrogenases, will give important clues about how retinoids are processed.

13-cis-retinoic acid competitively inhibits 3 alpha-hydroxysteroid oxidation by retinol dehydrogenase RoDH-4: a mechanism for its anti-androgenic effects in sebaceous glands?

Retinol dehydrogenase-4 (RoDH-4) converts retinol and 13-cis-retinol to corresponding aldehydes in human liver and skin in the presence of NAD(+). RoDH-4 also converts 3 alpha-androstanediol and androsterone into dihydrotestosterone and androstanedione, which may stimulate sebum secretion. This oxidative 3 alpha-hydroxysteroid dehydrogenase (3 alpha-HSD) activity of RoDH-4 is competitively inhibited by retinol and 13-cis-retinol. Here, we further examine the substrate specificity of RoDH-4 and the inhibition of its 3 alpha-HSD activity by retinoids. Recombinant RoDH-4 oxidized 3,4-didehydroretinol-a major form of vitamin A in the skin-to its corresponding aldehyde. 13-cis-retinoic acid (isotretinoin), 3,4-didehydroretinoic acid, and 3,4-didehydroretinol, but not all-trans-retinoic acid or the synthetic retinoids acitretin and adapalene, were potent competitive inhibitors of the oxidative 3 alpha-HSD activity of RoDH-4, i.e., reduced the formation of dihydrotestosterone and androstandione in vitro. Extrapolated to the in vivo situation, this effect might explain the unique sebosuppressive effect of isotretinoin when treating acne.

Expression pattern and biochemical characteristics of a major epidermal retinol dehydrogenase

The biological functions of vitamin A in the epidermis are mediated by all-trans retinoic acid, which is biosynthesized from retinol in two oxidative reactions. The first step involves enzymatic conversion of retinol to retinaldehyde. The physiological significance and relative contributions of the various retinol dehydrogenases to the oxidation of retinol in epidermal cells remain unclear. We report the characterization of a retinol dehydrogenase/reductase of the SDR superfamily, hRoDH-E2, which is abundantly expressed in the epidermis, epidermal appendages and in cultured epidermal keratinocytes. Both in live keratinocytes and in isolated keratinocyte microsomes, where the enzyme normally localizes, hRoDH-E2 functions as a bona fide retinol dehydrogenase. In the prevailing oxidative reaction it recognizes both free- and CRBP-bound retinol, and shows preference toward NADP as a co-substrate. In comparison, hRoDH-E2 retinol dehydrogenase activity in the simple epithelial HEK 293 cells is much lower and in CHO cells is non-existent. hRoDH-E2 transcripts are distributed throughout the epidermal layers but are more abundant in the basal cells. In contrast, the protein is detected predominantly in the basal and the most differentiated living layers. Its synthesis is negatively regulated by retinoic acid. The biochemical properties and the differential expression of hRoDH-E2 in the strata where retinoic acid signaling is critical for epidermal homeostasis support a conclusion that hRoDH-E2 bears the characteristics of the major microsomal retinol dehydrogenase activity in the epidermal keratinocytes in physiological circumstances.

A novel short-chain alcohol dehydrogenase from rats with retinol dehydrogenase activity, cyclically expressed in uterine epithelium

Retinoic acid is necessary for the maintenance of many lining epithelia of the body, such as the epithelium of the luminal surface of the uterus. Administration of estrogen to prepubertal rats induces in these epithelial cells the ability to synthesize retinoic acid from retinol, coincident with the appearance of cellular retinoic acid-binding protein, type two, which is normally present in these cells only at estrus in the mature, cycling animal. Here, we report the isolation, from a cDNA library prepared from uterine mRNA collected at the estrous stage and from a rat mammary adenocarcinoma cell line, of a cDNA that encodes a novel retinol dehydrogenase. A member of the short-chain alcohol dehydrogenase family, the encoded enzyme was capable of metabolizing retinol to retinal when expressed in cells after transfection of its cDNA. When cotransfected with the cDNA of human aldehyde 6, a known retinaldehyde dehydrogenase, the transfected cells synthesized retinoic acid from retinol. Immunohistochemical analysis revealed that the protein was present in the uterine lining epithelium of the mature animal only at estrus, coincident with the presence of cellular retinol-binding protein and cellular retinoic acid-binding protein, type two. Consequently, this novel short-chain alcohol dehydrogenase is an excellent candidate for the retinol dehydrogenase that catalyzes the first step in retinoic acid biosynthesis that occurs in uterine epithelial cells.

Expression of genes for alcohol and aldehyde metabolizing enzymes in mouse oocytes and preimplantation embryos

Alcohols and aldehydes are metabolized primarily by alcohol (ADH) and aldehyde (ALDH) dehydrogenase isozymes. Although significant progress has been made towards understanding the involvement of these isozymes in the oxidation of alcohol and aldehydes in the body, it is not known how these compounds are handled during fertilization and preimplantation embryogenesis. In this study, reverse transcription and the polymerase chain reaction (RT-PCR) was used to determine which ADH and ALDH isozymes are expressed at the oocyte, zygote, morula, and blastocyst stages of preimplantation development in the mouse. Transcripts of beta-actin and vimentin, assayed as controls, were detected at all stages, as well as Class III ADH (Adh-2) and Class 3 ALDH (Ahd-4), involved in the detoxification of formaldehyde and aromatic aldehydes, respectively. In contrast, transcripts for the major ethanol oxidizing isozyme, Class I ADH (Adh-1) was not detected during preimplantation development. Cytosolic retinol dehydrogenase (Adh-3) transcripts were marginally detected in oocytes and zygotes. The mRNA for cytosolic retinal dehydrogenase (Ahd-2), microsomal short-chain retinol dehydrogenases (RoDH Type I), and the mitochondrial low-Km acetaldehyde dehydrogenase (Ahd-5) only appeared as maternal transcripts. Microsomal ALDH (Ahd-3), which is induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), was not expressed until the blastocyst stage. ADH and ALDH enzyme systems may guard mouse preimplantation embryos against the toxic effects of industrial pollutants, such as formaldehyde and TCDD, as well as peroxidatic aldehydes generated during lipid peroxidation. The absence of enzymes to convert ethanol to acetaldehyde, coupled with oocyte expression of the acetaldehyde-degrading enzyme, Ahd-5, may be protective for the early embryo.

Stimulation of retinoic acid production and growth by ubiquitously expressed alcohol dehydrogenase Adh3

Influence of vitamin A (retinol) on growth depends on its sequential oxidation to retinal and then to retinoic acid (RA), producing a ligand for RA receptors essential in development of specific tissues. Genetic studies have revealed that aldehyde dehydrogenases function as tissue-specific catalysts for oxidation of retinal to RA. However, enzymes catalyzing the first step of RA synthesis, oxidation of retinol to retinal, remain unclear because none of the present candidate enzymes have expression patterns that fully overlap with those of aldehyde dehydrogenases during development. Here, we provide genetic evidence that alcohol dehydrogenase (ADH) performs this function by demonstrating a role for Adh3, a ubiquitously expressed form. Adh3 null mutant mice exhibit reduced RA generation in vivo, growth deficiency that can be rescued by retinol supplementation, and completely penetrant postnatal lethality during vitamin A deficiency. ADH3 was also shown to have in vitro retinol oxidation activity. Unlike the second step, the first step of RA synthesis is not tissue-restricted because it is catalyzed by ADH3, a ubiquitous enzyme having an ancient origin.

Expression patterns of retinoid X receptors, retinaldehyde dehydrogenase, and peroxisome proliferator activated receptor gamma in bovine preattachment embryos

In cattle, administration of retinol at the time of superovulation has been indirectly associated with enhanced developmental potential of the embryo. Vitamin A and its metabolites influence several developmental processes by interacting with 2 different types of nuclear receptors, retinoic acid receptors and retinoid X receptors (RXRs). Given the limited information available concerning the RXR-mediated retinoid signaling system, particularly in species other than rodents, this study was performed to gain insight into the potential role of retinoid signaling during preattachment embryo development in the cow. Bovine embryos were produced in vitro from oocytes harvested from abattoir ovaries and frozen in liquid nitrogen at the oocyte, 2-, 4-, 8-, and 16- to 20-cell, morula, blastocyst, and hatched blastocyst stages. Reverse transcription polymerase chain reaction (PCR) and whole mount in situ hybridization were utilized to investigate mRNA expression for RXR alpha, RXR beta, RXR gamma, alcohol dehydrogenase I (ADH-I), retinaldehyde dehydrogenase 2 (RALDH2), peroxisome proliferator activated receptor gamma (PPAR gamma), and glyceraldehyde-3-phosphate dehydrogenase. Transcripts for RXR alpha, RXR beta, RALDH2, and PPAR gamma were detected in all stages beginning from the oocyte through to the hatched blastocyst. Whole mount in situ hybridization performed using digoxigenin-labeled antisense probes detected all 4 transcripts in both the inner cell mass and the trophectoderm of hatched blastocysts. PCR products obtained for ADH-I exhibited very low homology to known human and mouse sequences. Immunohistochemistry was performed using polyclonal anti-rabbit antibodies against RXR beta and PPAR gamma to investigate whether these embryonic mRNAs were translated to the mature protein. Strong immunostaining was observed for both RXR beta and PPAR gamma in the trophectoderm and inner cell mass cells of intact and hatched blastocysts. Messenger RNA was not detected at any stage for RXR gamma. Expression of mRNA for RXR alpha, RXR beta, RALDH2, and PPAR gamma suggests that the early embryo may be competent to synthesize retinoic acid and regulate gene expression during preattachment development in vitro.

Molecular characterization of a mouse short chain dehydrogenase/reductase active with all-trans-retinol in intact cells, mRDH1

Metabolic activation of retinol (vitamin A) via sequential actions of retinol and retinal dehydrogenases produces the active metabolite all-trans-retinoic acid. This work reports cDNA cloning, enzymatic characterization, function in a reconstituted path of all-trans-retinoic acid biosynthesis in cell culture, and mRNA expression patterns in adult tissues and embryos of a mouse retinol dehydrogenase, RDH1. RDH1 represents a new member of the short chain dehydrogenase/reductase superfamily that differs from other mouse RDH in relative activity with all-trans and cis-retinols. RDH1 has a multifunctional catalytic nature, as do other short chain dehydrogenase/reductases. In addition to retinol dehydrogenase activity, RDH1 has strong 3alpha-hydroxy and weak 17beta-hydroxy steroid dehydrogenase activities. RDH1 has widespread and intense mRNA expression in tissues of embryonic and adult mice. The mouse embryo expresses RDH1 as early as 7.0 days post-coitus, and expression is especially intense within the neural tube, gut, and neural crest at embryo day 10.5. Cells cotransfected with RDH1 and any one of three retinal dehydrogenase isozymes synthesize all-trans-retinoic acid from retinol, demonstrating that RDH1contributes to a path of all-trans-retinoic acid biosynthesis in intact cells. These characteristics are consistent with RDH1 functioning in a path of all-trans-retinoic acid biosynthesis starting early during embryogenesis.

Gene structure, chromosomal localization and analysis of 3-ketosteroid reductase activity of the human 3(alpha-->beta)-hydroxysteroid epimerase

Following our previous characterization of the first human 3(alpha-->beta)hydroxysteroid epimerase (hHSE), we determined the genomic structure and chromosomal localization of the hHSE gene using fluorescent in situ hybridization (FISH) in this study. The gene spans 23 kb and contains five exons and four introns. FISH mapping assigned this gene to chromosome band 12q13. Primer extension analysis allowed the identification of a single transcription start site at 179 bp upstream from the ATG start codon. The 5'-flanking sequence lacks a typical TATA box in the proximal region of the transcription start site. However, analysis of the 2 kb promoter region revealed the presence of multiple potential transcription factor binding sites. Furthermore, we studied the 3-ketosteroid reductase activity demonstrated by hHSE in intact cells stably expressing the enzyme. It has been known that, in vitro, 3beta-hydroxysteroid dehydrogenase (3beta-HSD) shows both oxidative and reductive activity. Our results showed that hHSE catalyzes the reduction of 3-ketosteroids to form 3beta-hydroxysteroids while 3beta-HSD cannot catalyze this reaction in intact cells. However, hHSE showed 3-keto reductase activity in both microsomal fractions and intact cells. Since intact cells constitute a system which closely reflects in vivo intracellular conditions, we propose that hHSE might contribute to the cellular 3-ketosteroid reductase activity in the peripheral tissues.

Structure, promoter and chromosomal localization of rdh6

Mouse rdh6 encodes cis-retinoid/androgen dehydrogenase type 1 (CRAD1), a short-chain dehydrogenase, which recognizes as substrates 9-cis-retinol, 11-cis-retinol, 5 alpha-androstan-3 alpha,17 beta-diol and 5 alpha-androstan-3 alpha-ol-17-one, and is expressed most intensely in liver and kidney. The present study reports the genomic organization, chromosomal localization and promoter region sequence of rdh6. Rdh6 spans more than 38 kb and consists of four exons ranging from 164 to 2200 bp, and three introns ranging from 550 bp to greater than 18 kb. The gene localizes to the distal end of mouse chromosome 10, 72.5 cM from the centromere, and colocalizes with mouse rdh7, which encodes CRAD2. This corresponds to the locus of human rdh5 on human chromosome 12. Primer extension assays indicate two major transcription start sites in liver and one in kidney. The approximately 2000 kb sequenced of the 5'-flanking region contains multiple potential transcription factor binding sites, including sites for AP-1, C/EBP beta, GATA, c-Rel, ER, ROR alpha, SREBP, and CREB.

Increased retinoid signaling in vascular smooth muscle cells by proinflammatory cytokines

Retinoids have been shown to modulate inflammation and the immune response in many cell types including macrophages, endothelial cells, and vascular smooth muscle cells. However, present knowledge of whether inflammatory mediators modulate vitamin A status in these cells is limited. To identify the role of inflammation on retinoid metabolism in vascular smooth muscle cells, the cells were exposed to a combination of proinflammatory cytokines: interleukin-1beta, interferon-gamma, and lipopolysaccharides. Without stimulation with proinflammatory cytokines, vascular smooth muscle cells expressed retinol dehydrogenases-2 and 5 mRNA detected by RT-PCR. Stimulation with the combination of cytokines induced a substantial increase of retinol dehydrogenase-5 mRNA. This was associated with increased production of ligands for retinoic acid receptors, when assayed in a retinoic acid receptor-dependent luciferase reporter system. Our results demonstrate that inflammatory mediators activate the retinoid metabolic pathway in vascular smooth muscle cells, which potentially may modulate the inflammatory response in the vascular wall.

Characterization of a novel airway epithelial cell-specific short chain alcohol dehydrogenase/reductase gene whose expression is up-regulated by retinoids and is involved in the metabolism of retinol

Multiple retinoic acid responsive cDNAs were isolated from a high density cDNA microarray membrane, which was developed from a cDNA library of human tracheobronchial epithelial cells. Five selected cDNA clones encoded the sequence of the same novel gene. The predicted open reading frame of the novel gene encoded a protein of 319 amino acids. The deduced amino acid sequence contains four motifs that are conserved in the short-chain alcohol dehydrogenase/reductase (SDR) family of proteins. The novel gene shows the greatest homology to a group of dehydrogenases that can oxidize retinol (retinol dehydrogenases). The mRNA of the novel gene was found in trachea, colon, tongue, and esophagus. In situ hybridization of airway tissue sections demonstrated epithelial cell-specific gene expression, especially in the ciliated cell type. Both all-trans-retinoic acid and 9-cis-retinoic acid were able to elevate the expression of the novel gene in primary human tracheobronchial epithelial cells in vitro. This elevation coincided with an enhanced retinol metabolism in these cultures. COS cells transfected with an expression construct of the novel gene were also elevated in the metabolism of retinol. The results suggested that the novel gene represents a new member of the SDR family that may play a critical role in retinol metabolism in airway epithelia as well as in other epithelia of colon, tongue, and esophagus.

Further characterization of human microsomal 3alpha-hydroxysteroid dehydrogenase

This manuscript reports further characterization of the recently discovered human short-chain alcohol dehydrogenase, proposed to oxidize 3alpha-androstanediol to dihydrotestosterone in testis and prostate (M. G. Biswas and D. W. Russell, 1997, J. Biol. Chem. 272, 15959-15966). Enzyme expressed using the Baculovirus System localized in the microsomal fraction and catalyzed oxidation and reduction of the functional groups on steroids at carbons 3 and 17. Autoradiography assays revealed that the enzyme was most efficient as a 3alpha-hydroxysteroid oxidoreductase. High affinity of the enzyme for NADH (Km of 0.18 microM), lack of stereospecificity in the reductive direction, and poor efficiency for 3beta- versus 3alpha-hydroxyl oxidation could account for the observed transient accumulation of 3beta-stereoisomers in the oxidative reaction. Consistent with the 65% sequence identity with RoDH dehydrogenases, the enzyme oxidized all-trans-retinol with the Km value of 3.2 microM and Vmax value of 1.2 nmol/min per milligram microsomes. 13-cis-Retinol and all-trans-retinol bound to the cellular retinol-binding protein were not substrates. Neurosteroid allopregnanolone was a better substrate than all-trans-retinol with the Km and Vmax values of 0.24 microM and 14.7 nmol/min per milligram microsomes. Northern blot analysis revealed that the corresponding mRNA was present in adult human brain (caudate nucleus, amygdala, hippocampus, substantia nigra, thalamus) and spinal cord in addition to other tissues. The message was also detected in fetal lung, liver, and brain. Antibodies against the enzyme recognized a protein of approximately 35 kDa in the particulate fraction of human tissues. This study presents new information about enzymatic properties, substrate specificity, and tissue distribution of this enzyme, and provides a better insight into its possible physiological function(s).

Characterization of the oxidative 3alpha-hydroxysteroid dehydrogenase activity of human recombinant 11-cis-retinol dehydrogenase

11-cis-Retinol dehydrogenase catalyzes the oxidation of cis-retinols, a rate-limiting step in the biosynthesis of 9-cis-retinoic acid. It is also active toward 3alpha-hydroxysteroids, and thus might be involved in steroid metabolism. To better understand the role of this enzyme, we produced stable transfectants expressing 11-cis-retinol dehydrogenase in human embryonic kidney 293 cells. In vitro enzymatic assays have demonstrated that, with an appropriate exogenous cofactor, the enzyme catalyzes the interconversion of 5alpha-androstane-3alpha,17beta-diol and dihydrotestosterone and that of androsterone and androstanedione. However, using intact transfected cells, we found that the enzyme catalyzes reactions only in the oxidative direction. Thus, it is possible that 5alpha-androstane-3alpha,17beta-diol (an inactive androgen) can be converted into dihydrotestosterone, the most potent androgen, by the action of 11-cis-retinol dehydrogenase. This reaction could constitute a non-classical pathway of production of active androgens in the peripheral tissues. We also showed that all-trans-, 9-cis- and 13-cis-retinol inhibit the oxidative 3alpha-hydroxysteroid steroid activity of 11-cis-retinol dehydrogenase with similar K(i) values. Since all-trans-retinol is a precursor of cis-retinols, its inhibitory effect on the activity suggests that it could play an important role in modulating the formation of 9-cis-retinoic acid. In addition, we examined the effect of several known enzyme modulators, namely carbenoxolone, phenylarsine oxide and phosphatidylcholine, on 11-cis-retinol dehydrogenase activity. Taken together, our results suggest that, in humans, this enzyme might play a role in the biosynthesis of both 9-cis-retinoic acid and dihydrotestosterone.

Biosynthesis of 9-cis-retinoic acid in vivo. The roles of different retinol dehydrogenases and a structure-activity analysis of microsomal retinol dehydrogenases

Retinoic acid is generated by a two-step mechanism. First, retinol is converted into retinal by a retinol dehydrogenase, and, subsequently, retinoic acid is formed by a retinal dehydrogenase. In vitro, several enzymes are suggested to act in this metabolic pathway. However, little is known regarding their capacity to contribute to retinoic acid biosynthesis in vivo. We have developed a versatile cell reporter system to analyze the role of several of these enzymes in 9-cis-retinoic acid biosynthesis in vivo. Using a Gal4-retinoid X receptor fusion protein-based luciferase reporter assay, the formation of 9-cis-retinoic acid from 9-cis-retinol was measured in cells transfected with expression plasmids encoding different combinations of retinol and retinal dehydrogenases. The results suggested that efficient formation of 9-cis-retinoic acid required co-expression of retinol and retinal dehydrogenases. Interestingly, the cytosolic alcohol dehydrogenase 4 failed to efficiently catalyze 9-cis-retinol oxidation. A structure-activity analysis showed that mutants of two retinol dehydrogenases, devoid of the carboxyl-terminal cytoplasmic tails, displayed greatly reduced enzymatic activities in vivo, but were active in vitro. The cytoplasmic tails mediate efficient endoplasmic reticulum localization of the enzymes, suggesting that the unique milieu in the endoplasmic reticulum compartment is necessary for in vivo activity of microsomal retinol dehydrogenases.

Follicle-stimulating hormone and leukemia inhibitory factor regulate Sertoli cell retinol metabolism

Sertoli cells, the somatic epithelial cells of the seminiferous tubules, provide both structural and biochemical support for developing male germ cells. The Sertoli cells are targets of retinoid action in the testis. We have found that FSH, (Bu)(2)cAMP, and leukemia inhibitory factor elicit substantial changes in the metabolism of [(3)H]retinol (vitamin A) in primary cultures of purified rat Sertoli cells. Addition of (Bu)(2)cAMP for 2 h or FSH for 6 h results in a 3-fold increase in the metabolism of [(3)H]retinol to [(3)H]retinoic acid ([(3)H]RA); the esterification of [(3)H]retinol to [(3)H]retinyl esters, especially [(3)H]retinyl palmitate, is also increased by approximately 5-fold. The addition of 1 microM all-trans-RA also elicits changes in [(3)H]retinol metabolism, but in this case the metabolism of [(3)H]retinol to [(3)H]RA is inhibited, whereas the metabolism of [(3)H]retinol to [(3)H]retinyl esters is increased by over 50-fold. Leukemia inhibitory factor increases the esterification of [(3)H]retinol by 2- to 3-fold. FSH leads to a reduction in the level of cellular retinol binding protein I transcripts, whereas RA increases the cellular retinol binding protein I messenger RNA level by about 2-fold at approximately 24 h. Levels of AHD-2 (aldehyde dehydrogenase-2) and RALDH-2 (retinaldehyde dehydrogenase-2) messenger RNAs, which encode enzymes that convert [(3)H]retinaldehyde to [(3)H]RA, are increased by about 2-fold by FSH, whereas no change in CYP26 (RA hydroxylase) expression is seen. Our results suggest that one function of FSH (and/or (Bu)(2)cAMP) in Sertoli cells is to increase the metabolism of retinol to the biologically active metabolite RA and to retinyl esters.

Cloning of the human RoDH-related short chain dehydrogenase gene and analysis of its structure

We have previously characterized the first human NAD(+)-dependent short chain dehydrogenase capable of oxidizing all-trans-retinol and androgens, and found only in the liver and skin. In a search for related human enzymes, we identified a partial open reading frame, which exhibited >60% sequence identity to human RoDH-4. The full-length cDNA for this enzyme was determined in our laboratory by 5'-RACE PCR and was found to be identical to the recently reported novel type of oxidative human 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD). Analysis of the genomic structure revealed that the gene for RoDH-like 3alpha-HSD has four translated exons and, possibly, a fifth exon that codes for the 5'-untranslated region. The gene for RoDH-4 appears to have only four exons. The positions of exon-intron boundaries and the sizes of the protein coding regions are identical in 3alpha-HSD and RoDH-4. Moreover, both genes are mapped to chromosome 12q13, and are located in a close proximity to each other. Both genes appear to have satellite pseudogenes. Thus, RoDH-4 and 3alpha-HSD genes share similar structural organization and cluster on human chromosome 12, near the gene for 11-cis retinol dehydrogenase.

17beta-Hydroxysteroid dehydrogenase type 9 and other short-chain dehydrogenases/reductases that catalyze retinoid, 17beta- and 3alpha-hydroxysteroid metabolism

Subgroups of related short-chain dehydrogenase/reductase (SDR) family members serve as retinoid/androgen/estrogen metabolizing enzymes. These include retinol dehydrogenases (RoDHs) 1-3, cis-retinol/androgen dehydrogenase 1 and 2 (CRAD), retSDRs1-4, 9/11-cis-retinol dehydrogenase, and 17beta-hydroxysteroid dehydrogenase (17beta-HSD) types 6 and 9. Interaction with cellular retinol-binding protein (CRBP), the major physiological form of retinol, led to the identification and cDNA cloning of RoDH1. Probes for RoDH1 contributed to cDNA cloning many of the others. Some of these SDRs show specificity with all-trans-retinol (RoDH, retSDR, 17beta-HSD6 and 9) and others with 9 and/or 11-cis-retinol (CRAD, 9/11-cis-retinol dehydrogenase). Many have 3alpha-HSD activities with 3alpha-androstandiol as the most efficiently used substrate, followed by androsterone. In addition to 3alpha-HSD activity, CRAD2 shows relatively weak 17beta-HSD activity with testosterone. Rat 17beta-HSD6 and mouse 17beta-HSD9, which are not interspecies homologs, have efficient 17beta-HSD activities. 17beta-HSD6 has approximately 50% greater 17beta-HSD activity with estradiol than with 3alpha-androstandiol. With 3alpha-androstandiol, 17beta-HSD9 operates equally efficiently as a 17beta-HSD or a 3alpha-HSD. The multi-substrate nature of these SDRs allows for retinoid/steroid interactions. The ability of some these SDRs to access retinol bound with CRBP provides specificity in retinoid metabolism and allows retinoic acid biosynthesis and retinol esterification to continue, as CRBP protects retinol from the general cellular milieu.

Molecular characterization of a first human 3(alpha-->beta)-hydroxysteroid epimerase

In this report, we describe the isolation and characterization of a cDNA encoding an enzyme that exhibits catalytic characteristics of a 3(alpha-->beta)-hydroxysteroid epimerase (3(alpha-->beta)-HSE). The enzyme overexpressed in human 293 embryonic kidney cells transforms androsterone into epi-androsterone in two steps: the oxidation of androsterone to 5 alpha-androstane-3,17-dione, followed by the reduction of the latter to epi-androsterone. The reverse reaction, 3(beta-->alpha)-hydroxysteroid epimeration, is approximately 10-fold weaker. These results are confirmed by V(max)/K(m) determination, which shows that the enzyme catalyzes the oxidation of androsterone to 5 alpha-androstane-3,17-dione and the reduction of 5 alpha-androstane-3,17-dione to epi-androsterone more efficiently than the reverse reactions. The selective catalysis of the reaction following the 3(alpha-->beta) direction is also observed in intact transfected cells in culture, which better reflect physiological conditions. In vitro assays reveal that the recombinant enzyme prefers NAD(+) and NADH as cofactors and could recognize both C-19 and C-21 3 alpha-hydroxysteroids as substrates. DNA sequence analysis predicts a protein of 317 amino acids. Tissue distribution analysis using RT-PCR reveals that the mRNA of the enzyme is expressed in various tissues, including liver, brain, prostate, adrenal, and uterus, with the most abundant expression in the liver. Because active hydroxysteroids generally exert their effect in a stereo-specific manner, 3(alpha-->beta)-HSE could thus potentially play an important role in regulating the biological activities of various steroids.

Families of retinoid dehydrogenases regulating vitamin A function: production of visual pigment and retinoic acid

Vitamin A (retinol) and provitamin A (beta-carotene) are metabolized to specific retinoid derivatives which function in either vision or growth and development. The metabolite 11-cis-retinal functions in light absorption for vision in chordate and nonchordate animals, whereas all-trans-retinoic acid and 9-cis-retinoic acid function as ligands for nuclear retinoic acid receptors that regulate gene expression only in chordate animals. Investigation of retinoid metabolic pathways has resulted in the identification of numerous retinoid dehydrogenases that potentially contribute to metabolism of various retinoid isomers to produce active forms. These enzymes fall into three major families. Dehydrogenases catalyzing the reversible oxidation/reduction of retinol and retinal are members of either the alcohol dehydrogenase (ADH) or short-chain dehydrogenase/reductase (SDR) enzyme families, whereas dehydrogenases catalyzing the oxidation of retinal to retinoic acid are members of the aldehyde dehydrogenase (ALDH) family. Compilation of the known retinoid dehydrogenases indicates the existence of 17 nonorthologous forms: five ADHs, eight SDRs, and four ALDHs, eight of which are conserved in both mouse and human. Genetic studies indicate in vivo roles for two ADHs (ADH1 and ADH4), one SDR (RDH5), and two ALDHs (ALDH1 and RALDH2) all of which are conserved between humans and rodents. For several SDRs (RoDH1, RoDH4, CRAD1, and CRAD2) androgens rather than retinoids are the predominant substrates suggesting a function in androgen metabolism as well as retinoid metabolism.

Identification of the oxidative 3alpha-hydroxysteroid dehydrogenase activity of rat Leydig cells as type II retinol dehydrogenase

Dihydrotestosterone (DHT) is the most potent naturally occurring androgen, and its production in the testis may have important consequences in developmental and reproductive processes. In the rat testis, three factors can contribute to intracellular DHT levels: 1) synthesis of DHT from T by 5alpha-reductase, 2) conversion of DHT to 5alpha-androstane-3alpha, 17beta-diol (3alpha-DIOL) by the reductive activity of 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD), and 3) conversion of 3alpha-DIOL by an oxidative 3alpha-HSD activity. While the type I 3alpha-HSD enzyme (3alpha-HSD1 or AKR1C9) is an oxidoreductase in vitro and could theoretically be responsible for factors 2 and 3, we have shown previously that rat Leydig cells have two 3alpha-HSD activities: a cytosolic NADP(H)- dependent activity, characteristic of 3alpha-HSD1, and a microsomal NAD(H)-dependent activity. The two activities were separable by both developmental and biochemical criteria, but the identity of the second enzyme was unknown. To identify the microsomal NAD(H)-dependent 3alpha-HSD in rat Leydig cells, degenerate primers were used to amplify a number of short-chain alcohol dehydrogenases. Sequence analysis of cloned PCR products identified retinol dehydrogenase type II (RoDH2) as the prevalent species in purified Leydig cells. RoDH2 cDNA was subcloned into expression vectors and transiently transfected into CHOP and COS-1 cells. Its properties were compared with transiently transfected 3alpha-HSD1. When measured in intact CHOP and COS-1 cells, RoDH2 cDNA produced a protein that catalyzed the conversions of 3alpha-DIOL to DHT and androsterone to androstanedione, but not the reverse reactions. Therefore, the 3alpha-HSD activity of RoDH2 was exclusively oxidative. In contrast, type I 3alpha-HSD cDNA produced a protein that was exclusively a 3alpha-HSD reductase. In cell homogenates and subcellular fractions, RoDH2 catalyzed both 3alpha-HSD oxidation and reduction reactions that were NAD(H) dependent, and the enzyme activities were located in the microsomes. Type I 3alpha-HSD also catalyzed both oxidation and reduction, but was located in the cytosol and was NADP(H) dependent. We conclude that type I 3alpha-HSD and RoDH2 have distinct 3alpha-HSD activities with opposing catalytic directions, thereby controlling the rates of DHT production by Leydig cells.

Identification of endogenous retinoids, enzymes, binding proteins, and receptors during early postimplantation development in mouse: important role of retinal dehydrogenase type 2 in synthesis of all-trans-retinoic acid

Specific combinations of nuclear retinoid receptors acting as ligand-inducible transcription factors mediate the essential role of retinoids in embryonic development. Whereas some data exist on the expression of these receptors during early postimplantation development in mouse, little is known about the enzymes controlling the production of active ligands for the retinoid receptors. Furthermore, at early stages of mouse development virtually no data are available on the presence of endogenous retinoids. In the present study we have used a recently developed high-performance liquid chromatographic (HPLC) technique to identify endogenous retinoids in mouse embryos down to the egg cylinder stage. All-trans-retinoic acid, a ligand for the retinoic acid receptors, was detected in embryos dissected as early as 7.5 dpc (i.e., a combination of midstreak until late allantoic bud stage embryos). At these stages, we detected mRNA coding for all the retinoid receptors, retinoid binding proteins, and two enzymes able to convert retinol to retinal (retinol dehydrogenase 5 (RDH5) and alcohol dehydrogenase 4 (ADH4)). We also detected retinal dehydrogenase type 2 (RALDH2), an enzyme capable of oxidising the final step in the all-trans-retinoic acid synthesis. In egg cylinder stage mouse embryos no all-trans-retinoic acid was detected. However, at this stage its precursor all-trans-retinal was present. In accordance with these HPLC observations, RDH5 and ADH4 were expressed, but no transcripts coding for enzymes that oxidise retinal to retinoic acid. Therefore, our results suggest that RALDH2 is a key regulator in initiating retinoic acid synthesis sometime between the mid-primitive streak stage and the late allantoic bud stage in mouse embryos.

Complementary deoxyribonucleic acid cloning and enzymatic characterization of a novel 17beta/3alpha-hydroxysteroid/retinoid short chain dehydrogenase/reductase

17Beta-hydroxysteroid dehydrogenases (17betaHSDs) convert androgens and estrogens between their active and inactive forms, whereas retinol dehydrogenases catalyze the conversion between retinol and retinal. Retinol dehydrogenases function in the visual cycle, in the generation of the hormone retinoic acid, and some also act on androgens. Here we report cloning and expression of a complementary DNA that encodes a new mouse liver microsomal member of the short chain dehydrogenase/reductase (SDR) superfamily and its enzymatic characterization, i.e. 17betaHSD9. Although 17betaHSD9 shares 88% amino acid identity with rat 17betaHSD6, its closest homolog, the two differ in substrate specificity. In contrast to other 17betaHSD, 17betaHSD9 has nearly equivalent activities as a 17betaHSD (with estradiol approximately = adiol) and as a 3alphaHSD (with adiol approximately = androsterone). It also recognizes retinol as substrate and represents in part the NAD+-dependent liver microsomal dehydrogenase that uses unbound retinol, but not retinol complexed with cellular retinol-binding protein. Thus, this enzyme has catalytic properties that overlap with two subgroups of SDR, 17betaHSD and retinol dehydrogenases. Inactivation of estrogen and a variety of androgens seems to be its most probable function. Because of its apparent inability to access retinol bound with cellular retinol-binding protein, a function in the pathway of retinoic acid biosynthesis seems less obvious. These data provide additional insight into the enzymology of estrogen, androgen, and retinoid metabolism and illustrate how closely related members of the SDR superfamily can have strikingly different substrate specificities.

Retinoic acid: its biosynthesis and metabolism

This article presents a model that integrates the functions of retinoid-binding proteins with retinoid metabolism. One of these proteins, the widely expressed (throughout retinoid target tissues and in all vertebrates) and highly conserved cellular retinol-binding protein (CRBP), sequesters retinol in an internal binding pocket that segregates it from the intracellular milieu. The CRBP-retinol complex appears to be the quantitatively major form of retinol in vivo, and may protect the promiscuous substrate from nonenzymatic degradation and/or non-specific enzymes. For example, at least seven types of dehydrogenases catalyze retinal synthesis from unbound retinol in vitro (NAD+ vs. NADP+ dependent, cytosolic vs. microsomal, short-chain dehydrogenases/reductases vs. medium-chain alcohol dehydrogenases). But only a fraction of these (some of the short-chain de-hydrogenases/reductases) have the fascinating additional ability of catalyzing retinal synthesis from CRBP-bound retinol as well. Similarly, CRBP and/or other retinoid-binding proteins function in the synthesis of retinal esters, the reduction of retinal generated from intestinal beta-carotene metabolism, and retinoic acid metabolism. The discussion details the evidence supporting an integrated model of retinoid-binding protein/metabolism. Also addressed are retinoid-androgen interactions and evidence incompatible with ethanol causing fetal alcohol syndrome by competing directly with retinol dehydrogenation to impair retinoic acid biosynthesis.

Interactions of retinoid binding proteins and enzymes in retinoid metabolism

Naturally occurring retinoids (vitamin A or retinol and its active metabolites) are vital for vision, controlling the differentiation program of epithelial cells in the digestive tract and respiratory system, skin, bone, the nervous system, the immune system, and for hematopoiesis. Retinoids are essential for growth, reproduction (conception and embryonic development), and resistance to and recovery from infection. The functions of retinoids in the embryo begin soon after conception and continue throughout the lifespan of all vertebrates. Both naturally occurring and synthetic retinoids are used in the therapy of various skin diseases, especially acne, for augmenting the treatment of diabetes, and as cancer chemopreventive agents. Retinol metabolites serve as ligands that activate specific transcription factors in the superfamily of steroid/retinoid/thyroid/vitamin D/orphan receptors and thereby control gene expression. Additionally, retinoids may also function through non-genomic actions. Various retinoid binding proteins serve as partners in retinoid function. These binding proteins show high specificity and affinity for specific retinoids and seem to control retinoid metabolism in vivo qualitatively and quantitatively by reducing 'free' retinoid concentrations, protecting retinoids from non-specific interactions, and chaperoning access of metabolic enzymes to retinoids. Implementation of the physiological effects of retinoids depends on the spatial-temporal expressions of binding proteins, receptors and metabolic enzymes. This review will discuss current understanding of the enzymes that catalyze retinol and retinoic acid metabolism and their unique and integral relationship to retinoid binding proteins.

Endogenous casein kinase I catalyzes the phosphorylation of the lens fiber cell connexin49

The lens fiber cell-specific gap junction protein connexin49 is a substrate for a membrane-associated Ser/Thr protein kinase that can be extracted from lens cell membranes by 0.6 M KCl. However, the identity of this protein kinase has not been defined. In this report, evidence is presented indicating that it is casein kinase I. Thus, connexin49 was shown to be a substrate for purified casein kinase I but not for casein kinase II; the endogenous connexin49 protein kinase activity extracted from lens membranes with KCl was inhibited by the casein kinase I-specific inhibitor, N-(2-aminoethyl)-5-chloroisoquinoline-8-sulfonamide (CKI-7); the connexin49 protein kinase activity in the lens membrane KCl extract, which could be partially purified by gel filtration and affinity purification with a casein-Sepharose 4B column, copurified with casein kinase activity; phosphopeptide analysis showed that casein kinase I and the connexin49 protein kinase activity in the lens membrane KCl extract probably share the same phosphorylation sites in connexin49. Reverse transcription-PCR using total ovine lens RNA and casein kinase I isoform-specific oligonucleotide primers resulted in the amplification of cDNAs encoding casein kinase I-alpha and -gamma, while an in-gel casein kinase assay indicated casein kinase activity in the lens membrane KCl extract was associated with a major 39.2-kDa species, which is consistent with the 36 to 40-kDa size of casein kinase I-alpha in other animal species. These results demonstrate that the protein kinase activity present in the lens membrane 0.6 M KCl extract that catalyzes the phosphorylation of connexin49 is casein kinase I, probably the alpha isoform.

Teratology of retinoids

Either an excess or a deficiency of vitamin A and related compounds (retinoids) causes abnormal morphological development (teratogenesis). Potential retinoid sources come from dietary intake, nutritional supplements, and some therapeutic drugs. Therefore, understanding the mechanisms of retinoid teratogenesis is important. This review first gives an overview of the principles of teratology as they apply to retinoid-induced malformations. It then describes relevant aspects of the biochemical pathway and signal transduction of retinoids. The teratogenic activity of various retinoid compounds, the role of the retinoid receptors, and important toxicokinetic parameters in teratogenesis are reviewed.

Cloning and characterization of retinol dehydrogenase transcripts expressed in human epidermal keratinocytes

The normal growth and differentiation of the epidermis require an adequate supply of vitamin A. The active form of vitamin A for normal epidermal homeostasis is retinoic acid (RA). Retinoic acid controls the expression of retinoid-responsive genes via interactions of the retinoic acid/nuclear receptor complexes at specific DNA sequences in their control regions. The message conveyed by RA is likely modulated by the concentration of the ligand available for binding to the receptors. Following the uptake of plasma retinol, epidermal keratinocytes synthesize retinoic acid via two sequential reactions with retinaldehyde as an intermediate. Several retinol dehydrogenase (RDH) enzymes, members of the short-chain dehydrogenase/reductase (SDR) gene superfamily, catalyze the first and rate-limiting step that generates retinaldehyde from retinol bound to cellular retinol-binding protein (holo-CRBP). However, little is known about these enzymes and their genes in the epidermal cells. Our work describes the first member of the RDH family found in epidermis. We show that this gene is expressed predominantly in the differentiating spinous layers and that it is under positive, feed-forward regulation by retinoic acid. It encodes a protein that, using NAD+ as a preferred cofactor, utilizes free and CRBP-bound all-trans-retinol and steroids as substrates.