cDNA cloning and characterization of a cis-retinol/3alpha-hydroxysterol short-chain dehydrogenase

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.

The crystal structure of 3alpha -hydroxysteroid dehydrogenase/carbonyl reductase from Comamonas testosteroni shows a novel oligomerization pattern within the short chain dehydrogenase/reductase family

The crystal structure of 3alpha-hydroxysteroid dehydrogenase/carbonyl reductase from Comamonas testosteroni (3alpha-HSDH) as well as the structure of its binary complex with NAD(+) have been solved at 1.68-A and 1.95-A resolution, respectively. The enzyme is a member of the short chain dehydrogenase/reductase (SDR) family. Accordingly, the active center and the conformation of the bound nucleotide cofactor closely resemble those of other SDRs. The crystal structure reveals one homodimer per asymmetric unit representing the physiologically active unity. Dimerization takes place via an interface essentially built-up by helix alphaG and strand betaG of each subunit. So far this type of intermolecular contact has exclusively been observed in homotetrameric SDRs but never in the structure of a homodimeric SDR. The formation of a tetramer is blocked in 3alpha-HSDH by the presence of a predominantly alpha-helical subdomain which is missing in all other SDRs of known structure.

cDNA cloning and expression of a human aldehyde dehydrogenase (ALDH) active with 9-cis-retinal and identification of a rat ortholog, ALDH12

This report describes the isolation of a heretofore uncharacterized aldehyde dehydrogenase (ALDH) with retinal dehydrogenase activity from rat kidney and the cloning and expression of a cDNA that encodes its human ortholog, the previously unknown ALDH12. The human ALDH12 cDNA predicts a 487-residue protein with the 23 invariant amino acids, four conserved regions, cofactor binding motif (G(209)XGX(3)G), and active site cysteine residue (Cys(287)) that typify members of the ALDH superfamily. ALDH12 seems at least as efficient (V(m)/K(m)) in converting 9-cis-retinal into the retinoid X receptor ligand 9-cis-retinoic acid as two previously identified ALDHs with 9-cis-retinal dehydrogenase activity, rat retinal dehydrogenase (RALDH) 1 and RALDH2. ALDH12, however, has approximately 40-fold higher activity with 9-cis- retinal than with all-trans-retinal, whereas RALDH1 and RALDH2 have equivalent and approximately 4-fold less efficiencies for 9-cis-retinal versus all-trans-retinal, respectively. Therefore, ALDH12 is the first known ALDH to show a preference for 9-cis-retinal relative to all-trans-retinal. Evidence consistent with the possibility that ALDH12 could function in a pathway of 9-cis-retinoic acid biosynthesis in vivo includes biosynthesis of 9-cis-retinoic acid from 9-cis-retinol in cells co-transfected with cDNAs encoding ALDH12 and the 9-cis-retinol/androgen dehydrogenase, cis-retinoid/androgen dehydrogenase type 1. Intense ALDH12 mRNA expression in adult and fetal liver and kidney, two organs that reportedly have relatively high concentrations of 9-cis-retinol, reinforces this notion.

Cloning and bacterial expression of monomeric short-chain dehydrogenase/reductase (carbonyl reductase) from CHO-K1 cells

Mammalian carbonyl reductase (EC 1.1.1.184) is an enzyme that can catalyze the reduction of many carbonyl compounds, using NAD(P)H. We isolated a cDNA of carbonyl reductase (CHO-CR) from CHO-K1 cells which was 1208 bp long, including a poly(A) tail, and contained an 831-bp ORF. The deduced amino-acid sequence of 277 residues contained a typical motif for NADP+-binding (TGxxxGxG) and an SDR active site motif (S-Y-K). CHO-CR closely resembles mammalian carbonyl reductases with 71-73% identity. CHO-CR cDNA had the highest similarity to human CBR3 with 86% identity. Using the pET-28a expression vector, recombinant CHO-CR (rCHO-CR) was expressed in Escherichia coli BL21 (DE3) cells and purified with a Ni2+-affinity resin to homogeneity with a 35% yield. rCHO-CR had broad substrate specificity towards xenobiotic carbonyl compounds. RT-PCR of Chinese hamster tissues suggest that CHO-CR is highly expressed in kidney, testis, brain, heart, liver, uterus and ovary. Southern blotting analysis indicated the complexity of the Chinese hamster carbonyl reductase gene.

Stereoisomeric specificity of the retinoid cycle in the vertebrate retina

Understanding of the stereospecificity of enzymatic reactions that regenerate the universal chromophore required to sustain vision in vertebrates, 11-cis-retinal, is needed for an accurate molecular model of retinoid transformations. In rod outer segments (ROS), the redox reaction involves all-trans-retinal and pro-S-NADPH that results in the production of pro-R-all-trans-retinol. A recently identified all-trans-retinol dehydrogenase (photoreceptor retinol dehydrogenase) displays identical stereospecificity to that of the ROS enzyme(s). This result is unusual, because photoreceptor retinol dehydrogenase is a member of a short chain alcohol dehydrogenase family, which is often pro-S-specific toward their hydrophobic alcohol substrates. The second redox reaction occurring in retinal pigment epithelium, oxidation of 11-cis-retinol, which is largely catalyzed by abundantly expressed 11-cis-retinol dehydrogenase, is pro-S-specific to both 11-cis-retinol and NADH. However, there is notable presence of pro-R-specific activities. Therefore, multiple retinol dehydrogenases are involved in regeneration of 11-cis-retinal. Finally, the cellular retinaldehyde-binding protein-induced isomerization of all-trans-retinol to 11-cis-retinol proceeds with inversion of configuration at the C(15) carbon of retinol. Together, these results provide important additions to our understanding of retinoid transformations in the eye and a prelude for in vivo studies that ultimately may result in efficient pharmacological intervention to restore and prevent deterioration of vision in several inherited eye diseases.

9-cis-retinoids: biosynthesis of 9-cis-retinoic acid

Retinoids function through conformational alterations of ligand-dependent nuclear transcription factors, the retinoic acid receptors, and retinoid X receptors. 9-cis-Retinoic acid is a known biological ligand for retinoid X receptors, but its synthesis pathway in vivo is largely unknown. Recently, we identified a cis-retinol dehydrogenase (cRDH) that oxidizes 9-cis-retinol to 9-cis-retinal. Since both the expression of cRDH mRNA and its substrate are found in liver, we studied 9-cis-retinol metabolism and 9-cis-retinoic acid biosynthesis in two hepatic-derived cell types, Hep G2 hepatoma cells and HSC-T6 stellate cells. Both cell lines accumulate similar amounts of 9-cis-retinol provided in the medium. However, Hep G2 cells preferentially incorporate all-trans-retinol when equimolar concentrations of all-trans- and 9-cis-retinol were provided. In contrast, HSC-T6 cells did not exhibit a preference between all-trans- and 9-cis-retinol under the same conditions. Esterification of 9-cis-retinol occurred in both cell types, likely by acyl-CoA:retinol acyltransferase and lecithin:retinol acyltransferase. In vitro enzyme assays demonstrated that both cell types can hydrolyze 9-cis-retinyl esters via retinyl ester hydrolase(s). In Hep G2 cells, 9-cis-retinoic acid synthesis was strongly inhibited by high concentrations of 9-cis-retinol, which may explain the low levels of 9-cis-retinol in liver of mice. Cell homogenates of Hep G2 can convert all-trans-retinol to 9-cis-retinal, suggesting that the free form of all-trans-retinol may be used as a source for 9-cis-retinol and, thus, 9-cis-retinoic acid synthesis. Our studies provide the basis for identification of additional pathways for the generation of 9-cis-retinoic acid in specialized tissues.

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.

Impaired retinol utilization in Adh4 alcohol dehydrogenase mutant mice

Adh4, a member of the mouse alcohol dehydrogenase (ADH) gene family, encodes an enzyme that functions in vitro as a retinol dehydrogenase in the conversion of retinol to retinoic acid, an important developmental signaling molecule. To explore the role of Adh4 in retinoid signaling in vivo, gene targeting was used to create a null mutation at the Adh4 locus. Homozygous Adh4 mutant mice were viable and fertile and demonstrated no obvious defects when maintained on a standard mouse diet. However, when subjected to vitamin A deficiency during gestation, Adh4 mutant mice demonstrated a higher number of stillbirths than did wild-type mice. The proportion of liveborn second generation vitamin A-deficient newborn mice was only 15% for Adh4 mutant mice but 49% for wild-type mice. After retinol administration to vitamin A-deficient dams in order to rescue embryonic development, Adh4 mutant mice demonstrated a higher resorption rate at stage E12.5 (69%), compared with wild-type mice (30%). The relative ability of Adh4 mutant and wild-type mice to metabolize retinol to retinoic acid was measured after administration of a 100-mg/kg dose of retinol. Whereas kidney retinoic acid levels were below the level of detection in all vehicle-treated mice (< 1 pmol/g), retinol treatment resulted in very high kidney retinoic acid levels in wild-type mice (273 pmol/g) but 8-fold lower levels in Adh4 mutant mice (32 pmol/g), indicating a defect in metabolism of retinol to retinoic acid. These findings demonstrate that another retinol dehydrogenase can compensate for a lack of Adh4 when vitamin A is sufficient, but that Adh4 helps optimize retinol utilization under conditions of both retinol deficiency and excess.

Disruption of the 11-cis-retinol dehydrogenase gene leads to accumulation of cis-retinols and cis-retinyl esters

To elucidate the possible role of 11-cis-retinol dehydrogenase in the visual cycle and/or 9-cis-retinoic acid biosynthesis, we generated mice carrying a targeted disruption of the 11-cis-retinol dehydrogenase gene. Homozygous 11-cis-retinol dehydrogenase mutants developed normally, including their retinas. There was no appreciable loss of photoreceptors. Recently, mutations in the 11-cis-retinol dehydrogenase gene in humans have been associated with fundus albipunctatus. In 11-cis-retinol dehydrogenase knockout mice, the appearance of the fundus was normal and punctata typical of this human hereditary ocular disease were not present. A second typical symptom associated with this disease is delayed dark adaptation. Homozygous 11-cis-retinol dehydrogenase mutants showed normal rod and cone responses. 11-cis-Retinol dehydrogenase knockout mice were capable of dark adaptation. At bleaching levels under which patients suffering from fundus albipunctatus could be detected unequivocally, 11-cis-retinol dehydrogenase knockout animals displayed normal dark adaptation kinetics. However, at high bleaching levels, delayed dark adaptation in 11-cis-retinol dehydrogenase knockout mice was noticed. Reduced 11-cis-retinol oxidation capacity resulted in 11-cis-retinol/13-cis-retinol and 11-cis-retinyl/13-cis-retinyl ester accumulation. Compared with wild-type mice, a large increase in the 11-cis-retinyl ester concentration was noticed in 11-cis-retinol dehydrogenase knockout mice. In the murine retinal pigment epithelium, there has to be an additional mechanism for the biosynthesis of 11-cis-retinal which partially compensates for the loss of the 11-cis-retinol dehydrogenase activity. 11-cis-Retinyl ester formation is an important part of this adaptation process. Functional consequences of the loss of 11-cis-retinol dehydrogenase activity illustrate important differences in the compensation mechanisms between mice and humans. We furthermore demonstrate that upon 11-cis-retinol accumulation, the 13-cis-retinol concentration also increases. This retinoid is inapplicable to the visual processes, and we therefore speculate that it could be an important catabolic metabolite and its biosynthesis could be part of a process involved in regulating 11-cis-retinol concentrations within the retinal pigment epithelium of 11-cis-retinol dehydrogenase knockout mice.

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.

Analysis of rat cytosolic 9-cis-retinol dehydrogenase activity and enzymatic characterization of rat ADHII

We report the characterization of two enzymes that catalyze NAD(+)-dependent 9-cis-retinol dehydrogenase activity in rat liver cystol. Alcohol dehydrogenase class I (ADHI) contributes > 80% of the NA D+-dependent 9-cis-retinol dehydrogenase activity recovered, whereas alcohol dehydrogenase class II (ADHII), not identified previously at the protein level, nor characterized enzymatically in rat, accounts for approximately 2% of the activity. Rat ADHII exhibits properties different from those described for human ADHII. Moreover, rat ADHII-catalyzed rates of ethanol dehydrogenation are markedly lower than octanol or retinoid dehydrogenation rates. Neither ethanol nor 4-methylpyrazole inhibits the 9-cis-retinol dehydrogenase activity of rat ADHII. We propose that ADHII represents the previously observed additional retinoid oxidation activity of rat liver cytosol which occurred in the presence of either ethanol or 4-methylpyrazole. We also show that human and rat ADHII differ considerably in enzymatic properties.

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.

What are natural retinoids?

Vitamin A (retinol) is the prototype of all other natural retinoids. It is composed of a nonaromatic ring structure, a polyprenoid side chain and a carbonyl end group. These features make it liable to metabolic interconversion and protein interactions but also cause detergent-like properties and a sensitivity to UV irradiation and oxidation. Natural retinoids are present in all living organisms, either as preformed vitamin A or as carotenoids, and are required for a vast number of biological processes, e.g. vision, cellular growth and differentiation and reproduction. Although retinol is the most omnipotent compound, natural retinoids like all-trans-, 9-cis- and didehydroretinoic acid (RA) are clearly more potent outside the retina and trigger gene expression via binding to nuclear retinoid receptors. Retinaldehyde takes an intermediate position in this respect, with ability to convert to both retinol and RA. Over the years, many natural retinoids have been tried therapeutically against skin disorders with the best effects achieved with retinol, retinaldehyde, 13-cis-RA and all-trans-RA. The latter compound was the prototype when new, synthetic derivatives of vitamin A were sought, hoping for a better therapeutic index and a higher functional specificity. Inevitably, treatment with such drugs will influence the effects of coexisting natural retinoids. An understanding of the basic principles of these interactions may have major impact on patient outcome.

Pretranslational up-regulation of the hepatic microsomal delta4-3-oxosteroid 5alpha-oxidoreductase in male rat liver by all-trans-retinoic acid

Administration of all-trans-retinoic acid (ATRA; 60 mg/kg daily for 3 days) to male rats increased the rate of 5alpha-dihydrotestosterone (5alpha-DHT) formation from testosterone in microsomal fractions in vitro. The formation of androstane-3alpha,17beta-diol from testosterone was also increased because of the higher concentration of 5alpha-DHT produced in microsomal incubations. Northern analysis confirmed that the increased rate of 5alpha-DHT formation was due to the pretranslational up-regulation in delta4-3-oxosteroid 5alpha-oxidoreductase (EC 1.3.99.5) mRNA expression in ATRA-treated male rat liver. Thus, ATRA elicited in male rat liver a partial feminization of the expression of this enzyme, which normally exhibits a female-selective distribution in the rat. Subsequent experiments evaluated whether the administration of human chorionic gonadotropin or thyroxine to ATRA-treated male rats decreases 5alpha-reductase activity to that observed in untreated male rat liver. Although these treatments did not decrease 5alpha-reductase to untreated male levels, it was found that administration of ATRA to gonadectomized male rats produced complete feminization of the enzyme. Again, up-regulation was confirmed at the mRNA level. The activity of the male-specific cytochrome P450 2C11 (as reflected by microsomal testosterone 16alpha-hydroxylation activity) was correspondingly decreased by treatments that increased steroid 5alpha-reductase activity. Thus, gonadectomy in combination with ATRA administration effected a more pronounced decrease in 16alpha-hydroxylation activity than either treatment alone. These findings suggest that ATRA is a novel positive regulator of the 5alpha-reductase that in combination with the removal of circulating androgen, which normally suppresses 5alpha-reductase levels, feminizes the expression of this enzyme in rat liver.

Molecular docking studies on interaction of diverse retinol structures with human alcohol dehydrogenases predict a broad role in retinoid ligand synthesis

Some members of the human alcohol dehydrogenase (ADH) family possess retinol dehydrogenase activity and may thus function in production of the active nuclear receptor ligand retinoic acid. Many diverse natural forms of retinol exist including all-trans-retinol (vitamin A(1)), 9-cis-retinol, 3,4-didehydroretinol (vitamin A(2)), 4-oxo-retinol, and 4-hydroxy-retinol as well as their respective carboxylic acid derivatives which are active ligands for retinoid receptors. This raises the question of whether ADHs can accommodate all these different retinols and thus participate in the activation of several retinoid ligands. The crystal structures of human ADH1B and ADH4 provide the opportunity to examine their active sites for potential binding to many diverse retinol structures using molecular docking algorithms. The criteria used to score successful docking included achievement of distances of 1.9-2.4 A between the catalytic zinc and the hydroxyl oxygen of retinol and 3.2-3.6 A between C-4 of the coenzyme NAD and C-15 of retinol. These distances are sufficient to enable hydride transfer during the oxidation of an alcohol to an aldehyde. By these criteria, all-trans-retinol, 4-oxo-retinol, and 4-hydroxy-retinol were successfully docked to both ADH1B and ADH4. However, 9-cis-retinol and 3,4-didehydroretinol, which have more restrictive conformations, were successfully docked to only ADH4 which possesses a wider active site than ADH1B and more easily accommodates the C-19 methyl group. Furthermore, docking of all retinols was more favorable in the active site of ADH4 rather than ADH1B as measured by force field and contact scores. These findings suggest that ADH1B has a limited capacity to metabolize retinols, but that ADH4 is well suited to function in the metabolism of many diverse retinols and is predicted to participate in the synthesis of the active ligands all-trans-retinoic acid, 9-cis-retinoic acid, 3, 4-didehydroretinoic acid, 4-oxo-retinoic acid, and 4-hydroxy-retinoic acid.

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.

Opposing changes in 3alpha-hydroxysteroid dehydrogenase oxidative and reductive activities in rat leydig cells during pubertal development

The enzyme 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD) has an important role in androgen metabolism, catalyzing the interconversion of dihydrotestosterone (DHT) and 5alpha-androstane-3alpha,17beta-diol (3alpha-DIOL). The net direction of this interconversion will affect the amount of biologically active ligand available for androgen receptor binding. We hypothesize that in Leydig cells, differential expression of 3alpha-HSD enzymes favoring one of the two directions is a mechanism by which DHT levels are controlled. In order to characterize 3alpha-HSD in rat Leydig cells, the following properties were analyzed: rates of oxidation (3alpha-DIOL to DHT) and reduction (DHT to 3alpha-DIOL) and preference for the cofactors NADP(H) and NAD(H) (i.e., the oxidized and reduced forms of both pyridine nucleotides) in Leydig cells isolated on Days 21, 35, and 90 postpartum. Levels of 3alpha-HSD protein were measured by immunoblotting using an antibody directed against the liver type of the enzyme. Levels of 3alpha-HSD protein and rates of reduction were highest on Day 21 and lowest on Day 90. The opposite was true for the rate of 3alpha-HSD oxidation, which was barely detectable on Day 21 and highest on Day 90 (59.08 +/- 6.35 pmol/min per 10(6) cells, mean +/- SE). Therefore, the level of 3alpha-HSD protein detectable by liver enzyme was consistent with reduction but not with oxidation. There was a clear partitioning of NADP(H)-dependent activity into the cytosolic fraction of Leydig cells, whereas on Days 35 and 90, Leydig cells also contained a microsomal NAD(H)-activated 3alpha-HSD. We conclude that 1) the cytosolic 3alpha-HSD in Leydig cells on Day 21 behaves as a unidirectional NADPH-dependent reductase; 2) by Day 35, a microsomal NAD(H)-dependent enzyme activity is present and may account for predominance of 3alpha-HSD oxidation over reduction and the resultant high capacity of Leydig cells on Day 90 to synthesize DHT from 3alpha-DIOL.

Intracellular localization and membrane topology of 11-cis retinol dehydrogenase in the retinal pigment epithelium suggest a compartmentalized synthesis of 11-cis retinaldehyde

11-cis retinol dehydrogenase (EC 1.1.1.105) catalyses the last step in the biosynthetic pathway generating 11-cis retinaldehyde, the common chromophore of all visual pigments in higher animals. The enzyme is abundantly expressed in retinal pigment epithelium of the eye and is a member of the short chain dehydrogenase/reductase superfamily. In this work we demonstrate that a majority of 11-cis retinol dehydrogenase is associated with the smooth ER in retinal pigment epithelial cells and that the enzyme is an integral membrane protein, anchored to membranes by two hydrophobic peptide segments. The catalytic domain of the enzyme is confined to a lumenal compartment and is not present on the cytosolic aspect of membranes. Thus, the subcellular localization and the membrane topology of 11-cis retinol dehydrogenase suggest that generation of 11-cis retinaldehyde is a compartmentalized process.

cDNA cloning and characterization of a new human microsomal NAD+-dependent dehydrogenase that oxidizes all-trans-retinol and 3alpha-hydroxysteroids

We report the cDNA sequence and catalytic properties of a new member of the short chain dehydrogenase/reductase superfamily. The 1134-base pair cDNA isolated from the human liver cDNA library encodes a 317-amino acid protein, retinol dehydrogenase 4 (RoDH-4), which exhibits the strongest similarity with rat all-trans-retinol dehydrogenases RoDH-1, RoDH-2, and RoDH-3, and mouse cis-retinol/androgen dehydrogenase (</=73% identity). The mRNA for RoDH-4 is abundant in adult liver, where it is translated into RoDH-4 protein, which is associated with microsomal membranes, as evidenced by Western blot analysis. Significant amounts of RoDH-4 message are detected in fetal liver and lung. Recombinant RoDH-4, expressed in microsomes of Sf9 insect cells using BacoluGold Baculovirus system, oxidizes all-trans-retinol and 13-cis-retinol to corresponding aldehydes and oxidizes the 3alpha-hydroxysteroids androstane-diol and androsterone to dihydrotestosterone and androstanedione, respectively. NAD+ and NADH are the preferred cofactors, with apparent Km values 250-1500 times lower than those for NADP+ and NADPH. All-trans-retinol and 13-cis-retinol inhibit RoDH-4 catalyzed oxidation of androsterone with apparent Ki values of 5.8 and 3.5 microM, respectively. All-trans-retinol bound to cellular retinol-binding protein (type I) exhibits a similar Ki value of 3.6 microM. Unliganded cellular retinol-binding protein has no effect on RoDH activity. Citral and acyclic isoprenoids also act as inhibitors of RoDH-4 activity. Ethanol is not inhibitory. Thus, we have identified and characterized a sterol/retinol-oxidizing short chain dehydrogenase/reductase that prefers NAD+ and recognizes all-trans-retinol as substrate. RoDH-4 can potentially contribute to the biosynthesis of two powerful modulators of gene expression: retinoic acid from retinol and dihydrotestosterone from 3alpha-androstane-diol.

cDNA cloning, tissue distribution, and substrate characteristics of a cis-Retinol/3alpha-hydroxysterol short-chain dehydrogenase isozyme

We report here a mouse cDNA that encodes a 316-amino acid short-chain dehydrogenase that prefers NAD+ as its cofactor and recognizes as substrates androgens and retinols, i.e. has steroid 3alpha- and 17beta-dehydrogenase and cis/trans-retinol catalytic activities. This cis-retinol/androgen dehydrogenase type 2 (CRAD2) shares close amino acid similarity with mouse retinol dehydrogenase isozyme types 1 and 2 and CRAD1 (86, 84, and 87%, respectively). CRAD2 exhibits cooperative kinetics with 3alpha-adiol (3alpha-hydroxysteroid dehydrogenase activity) and testosterone (17beta-hydroxysteroid dehydrogenase activity), but Michaelis-Menten kinetics with androsterone (3alpha-hydroxysteroid dehydrogenase activity), 11-cis-retinol, all-trans-retinol, and 9-cis-retinol, with V/K0.5 values of 1.6, 0.2, 0.1, 0.04, 0.005, and not saturated, respectively. Carbenoxolone (IC50 = 2 microM) and 4-methylpyrazole (IC50 = 5 mM) inhibited CRAD2, but neither ethanol nor phosphatidylcholine had marked effects on its activity. Liver expressed CRAD2 mRNA intensely, with expression in lung, eye, kidney, and brain (2.9, 2, 1.6, and 0.6% of liver mRNA, respectively). CRAD2 represents the fifth isozyme in a group of short-chain dehydrogenase/reductase isozymes (retinol dehydrogenases 1-3 and CRAD1), closely related in primary amino acid sequence (approximately 85%), that are expressed in different quantities in various tissues, have different substrate specificities, and may serve different physiological functions. CRAD2 may alter the amounts of active and inactive androgens and/or convert retinols into retinals. These data expand insight into the multifunctional nature of short-chain dehydrogenases/reductases and into the enzymology of steroid and retinoid metabolism.

The visual cycle retinol dehydrogenase: possible involvement in the 9-cis retinoic acid biosynthetic pathway

The 11-cis-retinol dehydrogenase (11-cis-RoDH) gene encodes the short-chain alcohol dehydrogenase responsible for 11-cis-retinol oxidation in the visual cycle. The structure of the murine 11-cis-RoDH gene was used to reinvestigate its transcription pattern. An 11-cis-RoDH gene transcript was detected in several non-ocular tissues. The question regarding the substrate specificity of the enzyme was therefore addressed. Recombinant 11-cis-RoDH was found capable of oxidizing and reducing 9-cis-, 11-cis- and 13-cis-isomers of retinol and retinaldehyde, respectively. Dodecyl-beta-1-maltoside used to solubilize the enzyme was found to affect the substrate specificity. This is the first report on a visual cycle enzyme also present in non-retinal ocular and non-ocular tissues. A possible role in addition to its role in the visual cycle is being discussed.

Retinoids, omega-hydroxyfatty acids and cytotoxic aldehydes as physiological substrates, and H2-receptor antagonists as pharmacological inhibitors, of human class IV alcohol dehydrogenase

Kinetic constants of human class IV alcohol dehydrogenase (sigmasigma-ADH) support a role of the enzyme in retinoid metabolism, fatty acid omega-oxidation, and elimination of cytotoxic aldehydes produced by lipid peroxidation. Class IV is the human ADH form most efficient in the reduction of 4-hydroxynonenal (k(cat)/Km: 39,500 mM(-1) min(-1)). Class IV shows high activity with all-trans-retinol and 9-cis-retinol, while 13-cis-retinol is not a substrate but an inhibitor. Both all-trans-retinoic and 13-cis-retinoic acids are potent competitive inhibitors of retinol oxidation (Ki: 3-10 microM) which can be a basis for the regulation of the retinoic acid generation and of the pharmacological actions of the 13-cis-isomer. The inhibition of class IV retinol oxidation by ethanol (Ki: 6-10 mM) may be the origin of toxic and teratogenic effects of ethanol. H2-receptor antagonists are poor inhibitors of human and rat classes I and IV (Ki > 0.3 mM) suggesting a small interference in ethanol metabolism at the pharmacological doses of these common drugs.