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

Dehydrogenase reductase 9 (SDR9C4) and related homologs recognize a broad spectrum of lipid mediator oxylipins as substrates

Bioactive oxylipins play multiple roles during inflammation and in the immune response, with termination of their actions partly dependent on the activity of yet-to-be characterized dehydrogenases. Here, we report that human microsomal dehydrogenase reductase 9 (DHRS9, also known as SDR9C4 of the short-chain dehydrogenase/reductase (SDR) superfamily) exhibits a robust oxidative activity toward oxylipins with hydroxyl groups located at carbons C9 and C13 of octadecanoids, C12 and C15 carbons of eicosanoids, and C14 carbon of docosanoids. DHRS9/SDR9C4 is also active toward lipid inflammatory mediator dihydroxylated Leukotriene B₄ and proresolving mediators such as tri-hydroxylated Resolvin D1 and Lipoxin A₄, although notably, with lack of activity on the 15-hydroxyl of prostaglandins. We also found that the SDR enzymes phylogenetically related to DHRS9, i.e., human SDR9C8 (or retinol dehydrogenase 16), the rat SDR9C family member known as retinol dehydrogenase 7, and the mouse ortholog of human DHRS9 display similar activity toward oxylipin substrates. Mice deficient in DHRS9 protein are viable, fertile, and display no apparent phenotype under normal conditions. However, the oxidative activity of microsomal membranes from the skin, lung, and trachea of Dhrs9^−/− mice toward 1 μM Leukotriene B₄ is 1.7- to 6-fold lower than that of microsomes from wild-type littermates. In addition, the oxidative activity toward 1 μM Resolvin D1 is reduced by about 2.5-fold with DHRS9-null microsomes from the skin and trachea. These results strongly suggest that DHRS9 might play an important role in the metabolism of a wide range of bioactive oxylipins in vivo.

New insights into the stereospecific reduction by an (S) specific carbonyl reductase from Candida parapsilosis ATCC 7330: experimental and QM/MM studies

The quantum mechanics/molecular mechanics study of an (S) specific carbonyl reductase from C. parapsilosis ATCC 7330 showing a dual kinetic response for the reduction of ketones and α-ketoesters suggests different reaction mechanisms for the same.

Conditional Ablation of Retinol Dehydrogenase 10 in the Retinal Pigmented Epithelium Causes Delayed Dark Adaption in Mice

Regeneration of the visual chromophore, 11-cis-retinal, is a crucial step in the visual cycle required to sustain vision. This cycle consists of sequential biochemical reactions that occur in photoreceptor cells and the retinal pigmented epithelium (RPE). Oxidation of 11-cis-retinol to 11-cis-retinal is accomplished by a family of enzymes termed 11-cis-retinol dehydrogenases, including RDH5 and RDH11. Double deletion of Rdh5 and Rdh11 does not limit the production of 11-cis-retinal in mice. Here we describe a third retinol dehydrogenase in the RPE, RDH10, which can produce 11-cis-retinal. Mice with a conditional knock-out of Rdh10 in RPE cells (Rdh10 cKO) displayed delayed 11-cis-retinal regeneration and dark adaption after bright light illumination. Retinal function measured by electroretinogram after light exposure was also delayed in Rdh10 cKO mice as compared with controls. Double deletion of Rdh5 and Rdh10 (cDKO) in mice caused elevated 11/13-cis-retinyl ester content also seen in Rdh5(-/-)Rdh11(-/-) mice as compared with Rdh5(-/-) mice. Normal retinal morphology was observed in 6-month-old Rdh10 cKO and cDKO mice, suggesting that loss of Rdh10 in the RPE does not negatively affect the health of the retina. Compensatory expression of other retinol dehydrogenases was observed in both Rdh5(-/-) and Rdh10 cKO mice. These results indicate that RDH10 acts in cooperation with other RDH isoforms to produce the 11-cis-retinal chromophore needed for vision.

Signaling through retinoic acid receptors in cardiac development: Doing the right things at the right times

Retinoic acid (RA) is a terpenoid that is synthesized from vitamin A/retinol (ROL) and binds to the nuclear receptors retinoic acid receptor (RAR)/retinoid X receptor (RXR) to control multiple developmental processes in vertebrates. The available clinical and experimental data provide uncontested evidence for the pleiotropic roles of RA signaling in development of multiple embryonic structures and organs such eyes, central nervous system, gonads, lungs and heart. The development of any of these above-mentioned embryonic organ systems can be effectively utilized to showcase the many strategies utilized by RA signaling. However, it is very likely that the strategies employed to transfer RA signals during cardiac development comprise the majority of the relevant and sophisticated ways through which retinoid signals can be conveyed in a complex biological system. Here, we provide the reader with arguments indicating that RA signaling is exquisitely regulated according to specific phases of cardiac development and that RA signaling itself is one of the major regulators of the timing of cardiac morphogenesis and differentiation. We will focus on the role of signaling by RA receptors (RARs) in early phases of heart development. This article is part of a Special Issue entitled: Nuclear receptors in animal development.

Regulation of retinoid-mediated signaling involved in skin homeostasis by RAR and RXR agonists/antagonists in mouse skin

Endogenous retinoids like all-trans retinoic acid (ATRA) play important roles in skin homeostasis and skin-based immune responses. Moreover, retinoid signaling was found to be dysregulated in various skin diseases. The present study used topical application of selective agonists and antagonists for retinoic acid receptors (RARs) α and γ and retinoid-X receptors (RXRs) for two weeks on mouse skin in order to determine the role of retinoid receptor subtypes in the gene regulation in skin. We observed pronounced epidermal hyperproliferation upon application of ATRA and synthetic agonists for RARγ and RXR. ATRA and the RARγ agonist further increased retinoid target gene expression (Rbp1, Crabp2, Krt4, Cyp26a1, Cyp26b1) and the chemokines Ccl17 and Ccl22. In contrast, a RARα agonist strongly decreased the expression of ATRA-synthesis enzymes, of retinoid target genes, markers of skin homeostasis, and various cytokines in the skin, thereby markedly resembling the expression profile induced by RXR and RAR antagonists. Our results indicate that RARα and RARγ subtypes possess different roles in the skin and may be of relevance for the auto-regulation of endogenous retinoid signaling in skin. We suggest that dysregulated retinoid signaling in the skin mediated by RXR, RARα and/or RARγ may promote skin-based inflammation and dysregulation of skin barrier properties.

Cholestenoic Acid is an important elimination product of cholesterol in the retina: comparison of retinal cholesterol metabolism with that in the brain

PURPOSE

Accumulating evidence indicates a link between cholesterol and age-related macular degeneration. Yet, little is known about cholesterol elimination from the retina and retinal pigment epithelium (RPE), the two layers that are damaged in this blinding disease. Several different pathways of enzymatic cholesterol removal exist in extraocular tissues. The authors tested whether metabolites from these pathways could also be quantified in the bovine and human retina and RPE. For comparison, they measured cholesterol oxidation products in two regions of the bovine and human brain and in the bovine liver and adrenal glands.

METHODS

Sterol quantification was carried out by isotope dilution gas chromatography-mass spectrometry. Bovine tissues were used first to optimize analytical procedures and to investigate postmortem changes in oxysterol concentrations. Then human specimens were analyzed for oxysterol concentrations.

RESULTS

Qualitatively, oxysterol profiles were similar in the bovine and human tissues. In the human retina and RPE, the authors could not detect 27-hydroxycholesterol but unexpectedly found that its oxidation product, 5-cholestenoic acid, is the most abundant oxysterol, varying up to threefold in different persons. 24S-Hydroxysterol and pregnenolone were also present in the retina, but at much lower quantities and without significant interindividual variability. In the brain, the predominant oxysterol was 24S-hydroxycholesterol.

CONCLUSIONS

The oxysterol profile of the retina suggests that all known pathways of cholesterol elimination in extraocular organs are operative in the retina and that they likely vary depending on specific cell type. However, overall oxidation to 5-cholestenoic acid appears to be the predominant mechanism for cholesterol elimination from this organ.

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.

The search for non-chordate retinoic acid signaling: lessons from chordates

Signaling by retinoic acid (RA) is an important pathway in the development and homeostasis of vertebrate and invertebrate chordates, with a critical role in mesoderm patterning. Classical studies on the distribution of nuclear receptors of animals suggested that the family of RA receptors (RARs/NR1B) was restricted to chordates, while the family of RA X receptors (RXR/NR2B) was distributed from cnidarians to chordates. However, the accumulation of data from genome projects and studies in non-model species is questioning this traditional view. Here we discuss the evidence for non-chordate RA signaling systems in the light of recent advances in our understanding of carotene (pro-Vitamin A) metabolism and of the identification of potential RARs and members of the NR1 family in echinoderms and lophotrochozoan trematodes, respectively. We conclude, as have others before (Bertrand et al., 2004. Mol Biol Evol 21(10):1923-1937), that signaling by RA is more likely an ancestral feature of bilaterians than a chordate innovation.

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.

Crystal structure and enzyme kinetics of the (S)-specific 1-phenylethanol dehydrogenase of the denitrifying bacterium strain EbN1

(S)-1-Phenylethanol dehydrogenase (PED) from the denitrifying bacterium strain EbN1 catalyzes the NAD+-dependent, stereospecific oxidation of (S)-1-phenylethanol to acetophenone and the biotechnologically interesting reverse reaction. This novel enzyme belongs to the short-chain alcohol dehydrogenase/aldehyde reductase family. The coding gene (ped) was heterologously expressed in Escherichia coli and the purified protein was crystallized. The X-ray structures of the apo-form and the NAD+-bound form were solved at a resolution of 2.1 and 2.4 A, respectively, revealing that the enzyme is a tetramer with two types of hydrophobic dimerization interfaces, similar to beta-oxoacyl-[acyl carrier protein] reductase (FabG) from E. coli. NAD+-binding is associated with a conformational shift of the substrate binding loop of PED from a crystallographically unordered "open" to a more ordered "closed" form. Modeling the substrate acetophenone into the active site revealed the structural prerequisites for the strong enantioselectivity of the enzyme and for the catalytic mechanism. Studies on the steady-state kinetics of PED indicated a highly positive cooperativity of both catalytic directions with respect to the substrates. This is contrasted by the behavior of FabG. Moreover, PED exhibits extensive regulation on the enzyme level, being inhibited by elevated concentrations of substrates and products, as well as the wrong enantiomer of 1-phenylethanol. These regulatory properties of PED are consistent with the presence of a putative "transmission module" between the subunits. This module consists of the C-terminal loops of all four subunits, which form a special interconnected structural domain and mediate close contact of the subunits, and of a phenylalanine residue in each subunit that reaches out between substrate-binding loop and C-terminal domain of an adjacent subunit. These elements may transmit the substrate-induced conformational change of the substrate binding loop from one subunit to the others in the tetrameric complex and thus mediate the cooperative behavior of PED.

The C-terminal region of cis-retinol/androgen dehydrogenase 1 (CRAD1) confers ER localization and in vivo enzymatic function

Retinoic acid is generated from retinol (vitamin A) by the sequential actions of two different classes of enzymes, retinol dehydrogenases and retinal dehydrogenases. Several enzymes implicated in this process have been identified and characterized in vitro. However, our understanding of the cell biological function and regulation of this process is limited. To get further knowledge regarding the regulation of RA biosynthesis, we have determined possible regulatory mechanisms at the transcriptional and post-transcriptional levels for the prototypic microsomal retinol dehydrogenase cis-retinol/androgen dehydrogenase 1 (CRAD1). We note that the expression and stability of the enzyme are only moderately controlled by the retinoid status. Instead, we find that the cytosolic tail dramatically affects the activity of the enzyme, and we have mapped the structural elements required for ER retention and in vivo functional activity, respectively. Although inactive tail-deletion mutants display an abnormal subcellular localization, restoration of ER localization per se is not sufficient for enzymatic activity suggesting that additional trans-acting components interacting with, or modifying, the cytosolic tail are required for controlling the activity of the enzyme in vivo.

Effect of the tumor promoter phenobarbital on the pattern of global gene expression in liver of connexin32-wild-type and connexin32-deficient mice

The antiepileptic drug phenobarbital (PB) is used frequently as a model tumor promoter in rodent liver. It is believed to increase the probability of cancer by accelerating the clonal expansion of cells transformed during tumor initiation. The molecular mechanism underlying this process is only partly understood but seems to require the function of connexin32 (Cx32), one of the 2 gap junction proteins expressed in hepatocytes. PB mediates transcriptional activation of various genes in liver but which of these are relevant for tumor promotion is unknown. We have used oligonucleotide microarrays to identify genes differentially modulated in expression by PB in liver of Cx32-wild-type and Cx32-null mice. Mice of both strains were kept on PB containing (0.05%) or control diet for 2 weeks. Total liver RNA was isolated from 3 mice per experimental group and reverse transcribed; cDNAs were hybridized to oligonucleotide microarrays and a gene-by-gene linear model was used for statistical analysis of data. Five genes were identified as induced or repressed in untreated Cx32-null as compared to untreated Cx32-wild-type mice. PB affected the expression of 53 genes, of which 13 code for members of Phase-I/II of drug metabolism, and 12 genes were differentially affected in expression by PB in Cx32-null as compared to Cx32-wild-type mice. Among the differentially affected genes that could be verified by quantitative RT-PCR or Western analysis were the insulin like growth factor binding protein-1, retinol dehydrogenase-6 and the Y-chromosomally located gene Dby, among which may be a candidate of relevance for PB-mediated tumor promotion.

Delayed dark adaptation in 11-cis-retinol dehydrogenase-deficient mice: a role of RDH11 in visual processes in vivo

The oxidation of 11-cis-retinol to 11-cis-retinal in the retinal pigment epithelium (RPE) represents the final step in a metabolic cycle that culminates in visual pigment regeneration. Retinol dehydrogenase 5 (RDH5) is responsible for a majority of the 11-cis-RDH activity in the RPE, but the formation of 11-cis-retinal in rdh5-/- mice suggests another enzyme(s) is present. We have previously shown that RDH11 is also highly expressed in RPE cells and has dual specificity for both cis- and trans-retinoid substrates. To investigate the role of RDH11 in the retinoid cycle, we generated rdh11-/- and rdh5-/-rdh11-/- mice and examined their electrophysiological responses to various intensities of illumination and during dark adaptation. Retinoid profiles of darkadapted rdh11-/- mice did not show significant differences compared with wild-type mice, whereas an accumulation of cis-esters was detected in rdh5-/- and rdh5-/-rdh11-/- mice. Following light stimulation, 73% more cis-retinyl esters were stored in rdh5-/-rdh11-/- mice compared with rdh5-/- mice. Single-flash ERGs of rdh11-/- showed normal responses under dark- and light-adapted conditions, but exhibited delayed dark adaptation following high bleaching levels. Double knockout mice also had normal ERG responses in dark- and light-adapted conditions, but had a further delay in dark adaptation relative to either rdh11-/- or rdh5-/- mice. Taken together, these results suggest that RDH11 has a measurable role in regenerating the visual pigment by complementing RDH5 as an 11-cis-RDH in RPE cells, and indicate that an additional unidentified enzyme(s) oxidizes 11-cis-retinol or that an alternative pathway contributes to the retinoid cycle.

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.

Dark adaptation and the retinoid cycle of vision

Following exposure of our eye to very intense illumination, we experience a greatly elevated visual threshold, that takes tens of minutes to return completely to normal. The slowness of this phenomenon of "dark adaptation" has been studied for many decades, yet is still not fully understood. Here we review the biochemical and physical processes involved in eliminating the products of light absorption from the photoreceptor outer segment, in recycling the released retinoid to its original isomeric form as 11-cis retinal, and in regenerating the visual pigment rhodopsin. Then we analyse the time-course of three aspects of human dark adaptation: the recovery of psychophysical threshold, the recovery of rod photoreceptor circulating current, and the regeneration of rhodopsin. We begin with normal human subjects, and then analyse the recovery in several retinal disorders, including Oguchi disease, vitamin A deficiency, fundus albipunctatus, Bothnia dystrophy and Stargardt disease. We review a large body of evidence showing that the time-course of human dark adaptation and pigment regeneration is determined by the local concentration of 11-cis retinal, and that after a large bleach the recovery is limited by the rate at which 11-cis retinal is delivered to opsin in the bleached rod outer segments. We present a mathematical model that successfully describes a wide range of results in human and other mammals. The theoretical analysis provides a simple means of estimating the relative concentration of free 11-cis retinal in the retina/RPE, in disorders exhibiting slowed dark adaptation, from analysis of psychophysical measurements of threshold recovery or from analysis of pigment regeneration kinetics.

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.

Isorhodopsin rather than rhodopsin mediates rod function in RPE65 knock-out mice

The chromophore of visual pigments is 11-cis-retinal and, thus, in its absence, opsin is not photosensitive and no visual function exists. However, in the RPE65 knockout (Rpe65-/-) mouse, where synthesis of 11-cis-retinal does not occur, a minimal visual response from rod photoreceptors is obtained. We have examined if an alternative pathway exists for cis-retinoid generation in the absence of RPE65. Cyclic-light-reared, 2-month-old Rpe65-/- mice were placed in complete darkness. No exogenous retinoids were administered. After 4 weeks, enhanced a- and b-wave amplitudes were obtained, increasing >10-fold for the a-wave and >3-fold for the b-wave as compared with cyclic-light-reared Rpe65-/- mice. Visual-pigment levels increased to approximately 10 pmol per retina, compared with no measurable pigment for cyclic-light-reared Rpe65-/- mice. The lambdamax of the isolated pigment was 487 nm, characteristic for isorhodopsin. Retinoid extractions confirmed the presence of 9-cis-retinal and the absence of 11-cis-retinal. Once the Rpe65-/- mice were returned to cyclic light, within 48 h the electroretinogram function returned to levels found in Rpe65-/- mice maintained in cyclic light. This dark-mediated pathway is also operational in older animals, because 13-month-old Rpe65-/- mice kept in prolonged darkness (12 weeks) had increased isorhodopsin levels and electroretinogram a- and b-wave amplitudes. These studies demonstrate that a pathway exists in the eye for the generation of 9-cis-retinal that is independent of RPE65 and light.

Oxidative 3alpha-hydroxysteroid dehydrogenase activity of human type 10 17beta-hydroxysteroid dehydrogenase

In vitro enzyme assays have demonstrated that human type 10 17beta-hydroxysteroid dehydrogenase (17beta-HSD10) catalyzes the oxidation of 5alpha-androstane-3alpha,17beta-diol (adiol), an almost inactive androgen, to dihydrotestosterone (DHT) rather than androsterone or androstanedione. To further investigate the role of this steroid-metabolizing enzyme in intact cells, we produced stable transfectants expressing 17beta-HSD10 or its catalytically inactive Y168F mutant in human embryonic kidney (HEK) 293 cells. It was found that DHT levels in HEK 293 cells expressing 17beta-HSD10, but not its catalytically inactive mutant, will dramatically increase if adiol is added to culture media. Moreover, certain malignant prostatic epithelial cells have more 17beta-HSD10 than normal controls, and can generate DHT, the most potent androgen, from adiol. This event might promote prostate cancer growth. Analysis of the 17beta-HSD10 sequence shows that this enzyme does not have any ER retention signal or transmembrane segments and has not originated by divergence from a retinol dehydrogenase. The data suggest that the unique mitochondrial location of this HSD [Eur. J. Biochem. 268 (2001) 4899] does not prevent it from oxidizing the 3alpha-hydroxyl group of a C19 sterol in living cells. The experimental results lead to the conclusion that mitochondrial 17beta-HSD10 plays a significant part in a non-classical androgen synthesis pathway along with microsomal retinol dehydrogenases.

Identification of a mouse short-chain dehydrogenase/reductase gene, retinol dehydrogenase-similar. Function of non-catalytic amino acid residues in enzyme activity

We report a mouse short-chain dehydrogenase/reductase (SDR), retinol dehydrogenase-similar (RDH-S), with intense mRNA expression in liver and kidney. The RDH-S gene localizes to chromosome 10D3 with the SDR subfamily that catalyzes metabolism of retinoids and 3 alpha-hydroxysteroids. RDH-S has no activity with prototypical retinoid/steroid substrates, despite 92% amino acid similarity to mouse RDH1. This afforded the opportunity to analyze for functions of non-catalytic SDR residues. We produced RDH-S Delta 3 by mutating RDH-S to remove an "additional" Asn residue relative to RDH1 in its center, to convert three residues into RDH1 residues (L121P, S122N, and Q123E), and to substitute RDH1 sequence G208FKTCVTSSD for RDH-S sequence F208-FLTGMASSA. RDH-S Delta 3 catalyzed all-trans-retinol and 5 alpha-androstane-3 alpha,17 alpha-diol (3 alpha-adiol) metabolism 60-70% as efficiently (Vm/Km) as RDH1. Conversely, substituting RDH-S sequence F208FLTGMASSA into RDH1 produced a chimera (viz. C3) that was inactive with all-trans-retinol, but was 4-fold more efficient with 3 alpha-adiol. A single RDH1 mutation in the C3 region (K210L) reduced efficiency for all-trans-retinol by >1250-fold. In contrast, the C3 area mutation C212G enhanced efficiency with all-trans-retinol by approximately 2.4-fold. This represents a >6000-fold difference in catalytic efficiency for two enzymes that differ by a single non-catalytic amino acid residue. Another chimera (viz. C5) retained efficiency with all-trans-retinol, but was not saturated and was weakly active with 3 alpha-adiol, stemming from three residue differences (K224Q, K229Q, and A230T). The residues studied contribute to the substrate-binding pocket: molecular modeling indicated that they would affect orientation of substrates with the catalytic residues. These data report a new member of the SDR gene family, provide insight into the function of non-catalytic SDR residues, and illustrate that limited changes in the multifunctional SDR yield major alterations in substrate specificity and/or catalytic efficiency.

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

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

Reduction of all-trans-retinal in the mouse liver peroxisome fraction by the short-chain dehydrogenase/reductase RRD: induction by the PPAR alpha ligand clofibrate

The mouse liver 16,000 g fraction, which contains peroxisomes, reduces all-trans-retinal, but has limited ability to dehydrogenate retinol enzymatically. Feeding mice for 2 weeks with a diet containing clofibrate (0.5%, w/w), a PPAR alpha ligand and peroxisome proliferator, increased the 16,000 g fraction approximately 2-fold in protein, approximately 2-fold in specific activity of retinal reduction, and approximately 4-fold in retinal reductase units compared to controls, and caused a 50% decrease in liver retinol. An increase in both reductase specific activity and units indicates that clofibrate/PPAR alpha induced expression of retinal-reducing enzymes(s), in addition to increasing reductase(s) content. We expressed a cDNA from the NCBI data bank that encodes a peroxisome short-chain dehydrogenase/reductase. The enzyme, mouse retinal reductase (RRD, also known as human 2,4-dienoyl-CoA reductase), reduces all-trans-retinal [V(m) = 40 nmol min(-1) (mg of protein)(-1); K(0.5) = 2.3 microM] and has 4- and 60-fold less activity with 13-cis-retinal and 9-cis-retinal, respectively. Recombinant RRD functions with both unbound and CRBP(I) (cellular retinol-binding protein)-bound retinal, but apo-CRBP(I) inhibits the reductase. RRD mRNA expression was initiated on embryo day 7. Most adult tissues assayed expressed the mRNA. Liver, kidney, and heart had the most intense expression, with much less intense expression in brain, spleen, and lung. Clofibrate feeding increased the amount of RRD protein in the 16,000 g fraction of liver, consistent with the clofibrate-induced increase in reductase activity. These data relate retinoid metabolism, PPAR alpha, peroxisomes, and RRD, and are consistent with a further function of CRBP(I) in retinoid metabolism.

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.

Mouse retinal dehydrogenase 4 (RALDH4), molecular cloning, cellular expression, and activity in 9-cis-retinoic acid biosynthesis in intact cells

This study describes cDNA cloning and characterization of mouse RALDH4. The 2.3-kb cDNA encodes an aldehyde dehydrogenase of 487 amino acid residues, about two-orders of magnitude more active in vitro with 9-cis-retinal than with all-trans-retinal. RALDH4 recognizes as substrate 9-cis-retinal generated in transfected cells by the short-chain dehydrogenases CRAD1, CRAD3, or RDH1, to reconstitute a path of 9-cis-retinoic acid biosynthesis in situ. Northern blot analysis showed expression of RALDH4 mRNA in adult mouse liver and kidney. In situ hybridization revealed expression of RALDH4 in liver on embryo day 14.5, in adult hepatocytes, and kidney cortex. Immunohistochemistry confirmed RALDH4 expression in hepatocytes and showed that hepatocytes also express RALDH1, RALDH2, and RALDH3. Kidney expresses the RALDH4 protein primarily in the proximal and distal convoluted tubules of the cortex but not in the glomeruli or the medulla. Kidney expresses RALDH2 in the proximal convoluted tubules of the cortex but not in the distal convoluted tubules or glomeruli. Kidney expresses RALDH1 and RALDH2 in the medulla. The enzymatic characteristics of RALDH4, its expression in fetal liver, and its unique expression pattern in adult kidney compared with RALDH1, -2, and -3 suggest that it could meet specific needs for 9-cis-retinoic acid biosynthesis.

Gene structure and minimal promoter of mouse rdh1

Mouse rdh1 encodes retinol dehydrogenase type 1 (RDH1), a short-chain dehydrogenase, which recognizes as substrates all-trans-retinol, 9-cis-retinol, 5alpha-androstan-3,17-diol and 5alpha-androstan-3-ol-17-one. RDH1 is the most efficient known mouse short-chain dehydrogenase that catalyzes dehydrogenation of all-trans-retinol, and contributes to a reconstituted path of all-trans-retinoic acid biosynthesis, when coexpressed in reporter cells with any one of three retinal dehydrogenases. Rdh1 shows widespread, if not ubiquitous, mRNA expression in the mouse beginning no later than embryo day 7. Here we report genomic organization, chromosomal localization and analysis of a minimum promoter of mouse rdh1. Rdh1 consists of four exons and three introns and spans approximately 14412 bp. Rdh1 is a single copy gene that maps to chromosome 10D3 with rdh5-9, but no known disorder maps precisely to rdh1. Rdh1 has three transcription start sites in kidney and one start site in liver. The rdh1 5'-region between -424 and +43 induces transcription maximally in COS7, mouse kidney RAG, and mouse liver NMu3Li cells. This section has no TATA box, but has a CCAAT box beginning 65 bp upstream of the major transcription start site, which is required for transcription of transfected reporter constructs. An AP1 binding site at -119 also activates transfected reporter constructs, and mediates 2-O-tetradecanoylphorbol-13-acetate (TPA) induced transcription. All-trans-retinoic acid antagonizes the TPA affect; however, no RARE or RXRE was found in the proximal promoter region, consistent with indirect regulation by all-trans-retinoic acid.

Characterization of truncated mutants of human microsomal short-chain dehydrogenase/reductase RoDH-4

Human NAD(+)-dependent microsomal short-chain dehydrogenase/reductase RoDH-4 oxidizes all-trans-retinol, 13-cis-retinol and 3alpha-hydroxysteroids to corresponding retinaldehydes and 3-ketones. RoDH-4 behaves as an integral membrane protein, but its topology in the membrane is not known. Analysis of RoDH-4 polypeptide using algorithms for secondary structure predictions suggests that RoDH-4 contains four potential membrane-spanning domains: the N-terminal, the C-terminal, and the two central hydrophobic segments. To determine the role of each segment in association of RoDH-4 with the membrane, we prepared several expression constructs coding for truncated RoDH-4 polypeptides that lacked the putative membrane-spanning domains and expressed them in insect Sf9 cells using the Baculovirus system. Association of truncated RoDH-4 constructs with the microsomal membranes was analyzed by alkaline extraction and floatation in sucrose gradient. Catalytic activity of truncated RoDH-4 constructs was assayed using the 3alpha-hydroxysteroid androsterone as substrate. Truncated RoDH-4 that lacked the first thirteen amino acids of the N-terminal segment was partially active and exhibited the apparent K(m) value for androsterone similar to that of the wild-type enzyme. Removal of 23 N-terminal hydrophobic amino acids resulted in significant loss of activity and a 14-fold increase in the apparent K(m) value. Removal of the C-terminal 27 amino acid segment resulted in a approximately 600-fold increase in the apparent K(m) value. Each truncated mutant behaved as an integral membrane protein. Furthermore, protein that lacked all four hydrophobic segments remained associated with the membrane. Thus, the N-terminal and the C-terminal ends are both important for RoDH-4 activity and the removal of the putative transmembrane segments does not convert RoDH-4 into a soluble protein, suggesting additional sites of membrane interaction.

Cytosolic retinoid dehydrogenases govern ubiquitous metabolism of retinol to retinaldehyde followed by tissue-specific metabolism to retinoic acid

The ability of vitamin A (retinol) to control growth and development depends upon tissue-specific metabolism of retinol to retinoic acid (RA). RA then functions as a ligand for retinoid receptor signaling. Mouse genetic studies support a role for cytosolic alcohol dehydrogenases (ADH) in the first step (oxidation of retinol to retinaldehyde) and a role for cytosolic retinaldehyde dehydrogenases (RALDH) in the second step (oxidation of retinaldehyde to RA). Mice lacking ADH3 have reduced survival and a growth defect that can be rescued by dietary retinol supplementation, whereas the effect of a loss of ADH1 or ADH4 is noticed only in mice subjected to vitamin A excess or deficiency, respectively. Also, genetic deficiency of both ADH1 and ADH4 does not have additive effects, verifying separate roles for these enzymes in retinoid metabolism. As for the second step of RA synthesis, a null mutation of RALDH2 is embryonic lethal, eliminating most mesodermal RA synthesis, whereas loss of RALDH1 eliminates RA synthesis only in the embryonic dorsal retina with no obvious effect on development. Analysis of RA-rescued RALDH2 mutants has also revealed that RALDH3 and at least one additional enzyme produce RA tissue-specifically in embryos. Collectively, these genetic findings indicate that metabolism of retinol to retinaldehyde is not tissue-restricted as it is catalyzed by ubiquitously-expressed ADH3 (a low activity form) as well as by tissue-specifically expressed ADH1 and ADH4 (high activity forms). In contrast, further metabolism of retinaldehyde to RA is tissue-restricted as all enzymes identified are tissue-specific. An important concept to emerge is that selective expression of enzymes catalyzing the second step is what limits the tissues that can completely metabolize retinol to RA to initiate retinoid signaling.

Novel roles of liver X receptors exposed by gene expression profiling in liver and adipose tissue

Liver X receptor (LXR) alpha and LXRbeta are nuclear oxysterol receptors whose biological function has so far been elucidated only with respect to cholesterol and lipid metabolism. To expose novel biological roles for LXRs, we performed genome-wide gene expression profiling studies in liver and white and brown adipose tissue from wild-type (LXRalpha(+/+)beta(+/+)) and knockout mice (LXRalpha(-/-)beta(-/-)) treated with a synthetic LXR agonist. By an adapted statistical analysis, we detected 319 genes significantly regulated by LXR agonist treatment in wild-type but not in knockout mice, fulfilling most stringent criteria with an overall confidence of 94%. Down-regulation of essential enzymes of gluconeogenesis in liver could point to possible beneficial effects of LXR agonists in diabetes mellitus. LXR agonist treatment also altered expression of genes involved in steroid hormone synthesis and growth hormone receptor signaling, emphasizing a potential impact on endocrine function. Notably, LXR agonist treatment up-regulated CYP4A10 and CYP4A14 together with cytochrome P450 reductase, indicating a possible enhancement of microsomal lipid peroxidation. In conclusion, these gene expression profiling data identify novel areas of regulation by LXRs and provide a highly valuable basis for further research on the biological functions of these nuclear receptors and the pharmacological characteristics of their ligands.

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.

SDR-O: an orphan short-chain dehydrogenase/reductase localized at mouse chromosome 10/human chromosome 12

We report cloning a cDNA that encodes a novel short-chain dehydrogenase/reductase, SDR-O, conserved in mouse, human and rat. Human and mouse liver express SDR-O (short-chain dehydrogenase/reductase-orphan) mRNA intensely. The mouse embryo expresses SDR-O mRNA as early as day seven. Human SDR-O localizes on chromosome 12; mouse SDR-O localizes on chromosome 10 with CRAD1, CRAD2 and RDH4. SDR-O shares highest amino acid similarity with rat RoDH1 and mouse RDH1 (69-70%), but does not have the retinol and 3alpha-hydroxysteroid dehydrogenase activity of either, nor is it active as a 17beta- or 11beta-hydroxysteroid dehydrogenase. Short-chain dehydrogenase/reductases catalyse the metabolism of ligands that bind with nuclear receptors: the occurrence of 'orphan' nuclear receptors may imply existence of 'orphan' SDR, suggesting that SDR-O may catalyse the metabolism of another class of nuclear receptor ligand. Alternatively, SDR-O may not have a catalytic function, but may regulate metabolism by binding substrates/products and/or by serving as a regulatory factor.

Distinct retinoid metabolic functions for alcohol dehydrogenase genes Adh1 and Adh4 in protection against vitamin A toxicity or deficiency revealed in double null mutant mice

The ability of class I alcohol dehydrogenase (ADH1) and class IV alcohol dehydrogenase (ADH4) to metabolize retinol to retinoic acid is supported by genetic studies in mice carrying Adh1 or Adh4 gene disruptions. To differentiate the physiological roles of ADH1 and ADH4 in retinoid metabolism we report here the generation of an Adh1/4 double null mutant mouse and its comparison to single null mutants. We demonstrate that loss of both ADH1 and ADH4 does not have additive effects, either for production of retinoic acid needed for development or for retinol turnover to minimize toxicity. During gestational vitamin A deficiency Adh4 and Adh1/4 mutants exhibit completely penetrant postnatal lethality by day 15 and day 24, respectively, while 60% of Adh1 mutants survive to adulthood similar to wild-type. Following administration of a 50-mg/kg dose of retinol to examine retinol turnover, Adh1 and Adh1/4 mutants exhibit similar 10-fold decreases in retinoic acid production, whereas Adh4 mutants have only a slight decrease. LD(50) studies indicate a large increase in acute retinol toxicity for Adh1 mutants, a small increase for Adh4 mutants, and an intermediate increase for Adh1/4 mutants. Chronic retinol supplementation during gestation resulted in 65% postnatal lethality in Adh1 mutants, whereas only approximately 5% for Adh1/4 and Adh4 mutants. These studies indicate that ADH1 provides considerable protection against vitamin A toxicity, whereas ADH4 promotes survival during vitamin A deficiency, thus demonstrating largely non-overlapping functions for these enzymes in retinoid metabolism.

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.

cis-Retinol/androgen dehydrogenase, isozyme 3 (CRAD3): a short-chain dehydrogenase active in a reconstituted path of 9-cis-retinoic acid biosynthesis in intact cells

9-cis-Retinoic acid activates retinoid X receptors, which serve as heterodimeric partners with other nuclear hormone receptors, yet the enzymology of its physiological generation remains unclear. Here, we report the identification and molecular/enzymatic characterization of a previously unknown member of the short-chain dehydrogenase/reductase family, CRAD3 (cis-retinoid/androgen dehydrogenase, type 3), which catalyzes the first step in 9-cis-retinoic acid biosynthesis, the conversion of 9-cis-retinol into 9-cis-retinal. CRAD3 shares amino acid similarity with other retinoid/steroid short-chain dehydrogenases/reductases: CRAD1, CRAD2, and RDH4. Relative to CRAD1, CRAD3 has greater 9-cis-retinol/all-trans-retinol discrimination and lower efficiency as an androgen dehydrogenase. CRAD3 has apparent efficiency (V/K(m)) for 9-cis-retinol about equivalent to that for CRAD1 and 3 orders of magnitude greater than that for RDH4. (CRAD2 does not recognize 9-cis-retinol as a substrate). CRAD3 contributes to 9-cis-retinoic acid production in intact cells, in conjunction with each of three retinal dehydrogenases that recognize 9-cis-retinal (RALDH1/AHD2, RALDH2, and ALDH12). Liver and kidney, two tissues reportedly with the highest concentrations of 9-cis-retinoids, show the most intense mRNA expression of CRAD3, but expression also occurs in testis, lung, small intestine, heart, and brain. These data are consistent with the participation of CRAD3 in the biogeneration of 9-cis-retinoic acid.

11 Beta-hydroxysteroid dehydrogenase type 1 from human liver: dimerization and enzyme cooperativity support its postulated role as glucocorticoid reductase

11Beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD 1) is a microsomal enzyme that catalyzes the reversible interconversion of receptor-active 11-hydroxy glucocorticoids (cortisol) to their receptor-inactive 11-oxo metabolites (cortisone). However, the physiological role of 11beta-HSD 1 as prereceptor control device in regulating access of glucocorticoid hormones to the glucocorticoid receptor remains obscure in light of its low substrate affinities, which is in contrast to low glucocorticoid plasma levels and low Kd values of the receptors to cortisol. To solve this enigma, we performed detailed kinetic analyses with a homogeneously purified 11beta-HSD 1 from human liver. The membrane-bound enzyme was successfully obtained in an active state by a purification procedure that took advantage of a gentle solubilization method as well as providing a favorable detergent surrounding during the various chromatographic steps. The identity of purified 11beta-HSD 1 was proven by determination of enzymatic activity, N-terminal amino acid sequencing, and immunoblot analysis. By gel-permeation chromatography we could demonstrate that 11beta-HSD 1 is active as a dimeric enzyme. The cDNA for the enzyme was cloned from a human liver cDNA library and shown to be homologous to that previously characterized in human testis. Interestingly, 11beta-HSD 1 exhibits Michaelis-Menten kinetics with cortisol and corticosterone (11beta-dehydrogenation activity) but cooperative kinetics with cortisone and dehydrocorticosterone (11-oxoreducing activity). Accordingly, this enzyme dynamically adapts to low (nanomolar) as well as to high (micromolar) substrate concentrations, thereby providing the fine-tuning required as a consequence of great variations in circadian plasma glucocorticoid levels.

Synthesis of the all-trans-retinal chromophore of retinal G protein-coupled receptor opsin in cultured pigment epithelial cells

Light-dependent production of 11-cis-retinal by the retinal pigment epithelium (RPE) and normal regeneration of rhodopsin under photic conditions involve the RPE retinal G protein-coupled receptor (RGR) opsin. This microsomal opsin is bound to all-trans-retinal which, upon illumination, isomerizes stereospecifically to the 11-cis isomer. In this paper, we investigate the synthesis of the all-trans-retinal chromophore of RGR in cultured ARPE-hRGR and freshly isolated bovine RPE cells. Exogenous all-trans-[(3)H]retinol is incorporated into intact RPE cells and converted mainly into retinyl esters and all-trans-retinal. The intracellular processing of all-trans-[(3)H]retinol results in physiological binding to RGR of a radiolabeled retinoid, identified as all-trans-[(3)H]retinal. The ARPE-hRGR cells contain a membrane-bound NADPH-dependent retinol dehydrogenase that reacts efficiently with all-trans-retinol but not the 11-cis isomer. The NADPH-dependent all-trans-retinol dehydrogenase activity in isolated RPE microsomal membranes can be linked in vitro to specific binding of the chromophore to RGR. These findings provide confirmation that RGR opsin binds the chromophore, all-trans-retinal, in the dark. A novel all-trans-retinol dehydrogenase exists in the RPE and performs a critical function in chromophore biosynthesis.

Microarray analysis of gene expression changes in mouse liver induced by peroxisome proliferator- activated receptor alpha agonists

We used a microarray technique to investigate changes of gene expression in liver induced by two peroxisome proliferator-activated receptor alpha (PPARalpha) agonists, a strong PPARalpha agonist, Wy-14,643, and a marketed fibrate drug, fenofibrate. The purposes of this work are: 1) to examine whether or not gene expression is altered in different ways by these two PPARalpha agonists and 2) to find genes whose expression has not been previously reported to be affected by PPARalpha agonists. Mice were treated orally with 100 mg/kg fenofibrate, or 30 mg/kg or 100 mg/kg Wy-14,643, and the liver was collected on Day 2 or 3. mRNA was extraction from liver, and subjected to microarray analysis. Previously reported induction or reduction of gene expression, e.g. genes involved in beta-oxidation and lipid metabolism, was confirmed in our study. Scatter plot analysis indicated that the changes of gene expression pattern induced by fenofibrate and Wy-14,643 were almost identical. However, expression levels of metallothionein 1 and 2 mRNAs were different: no change of hepatic metallothionein 1 and 2 mRNA expression was induced by 100 mg/kg fenofibrate on Day 2 or 3, while 30 mg/kg Wy-14,643 administration increased expression of both genes by 1.8-fold on Day 3. In addition to previously reported gene expression changes by PPARalpha agonists, we found expression changes of other genes, including cis-retinol/3alpha-hydroxysterol short chain dehydrogenase, vanin-1, RecA-like protein, and serum amyloid A (SAA) 2. Among them, the change of SAA2 mRNA level was noteworthy; it showed a decrease to as little as one-seventh. Seven-day fenofibrate pre-treatment of mice completely inhibited the acute-phase elevation of plasma SAA concentration triggered by acetaminophen challenge. This finding suggests that fenofibrate treatment may reduce plasma SAA concentration in patients with secondary amyloidosis.

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.

The N-terminus of retinol dehydrogenase type 1 signals cytosolic orientation in the microsomal membrane

We determined the orientation of the SDR (short-chain dehydrogenase/reductase) rat RoDH1 (retinol dehydrogenase type 1) in the endoplasmic reticulum to provide insight into its function in retinol metabolism, and to resolve whether retinoid-metabolizing SDRs differ from several other SDRs by requiring a C-terminal segment for the membrane orientation. In contrast to several soluble SDRs, the membrane-associated RoDH1 has hydrophobic extensions N- and C-terminal to the SDR core. Confocal microscopy and/or proteinase K protection assays of RoDH1, RoDH1 mutants, and RoDH1-green fluorescent protein fusion proteins showed that the N-terminal segment anchors RoDH1 to the endoplasmic reticulum membrane facing the cytosol. The C-terminal hydrophobic segment increases the relative proportion of RoDH1 associated with the endoplasmic reticulum, but has no affect on orientation. Deletion of either or both extensions causes nearly total loss of enzyme activity, possibly through altering the nature of RoDH1 association with membranes, or destabilizing the enzyme, but does not alter the expression of RoDH1 or convert it into a soluble protein. The latter suggests that the SDR core of RoDH1 has marked external hydrophobicity that causes nonspecific membrane association.

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.

Confronting complexity: the interlink of phototransduction and retinoid metabolism in the vertebrate retina

Absorption of light by rhodopsin or cone pigments in photoreceptors triggers photoisomerization of their universal chromophore, 11-cis-retinal, to all-trans-retinal. This photoreaction is the initial step in phototransduction that ultimately leads to the sensation of vision. Currently, a great deal of effort is directed toward elucidating mechanisms that return photoreceptors to the dark-adapted state, and processes that restore rhodopsin and counterbalance the bleaching of rhodopsin. Most notably, enzymatic isomerization of all-trans-retinal to 11-cis-retinal, called the visual cycle (or more properly the retinoid cycle), is required for regeneration of these visual pigments. Regeneration begins in rods and cones when all-trans-retinal is reduced to all-trans-retinol. The process continues in adjacent retinal pigment epithelial cells (RPE), where a complex set of reactions converts all-trans-retinol to 11-cis-retinal. Although remarkable progress has been made over the past decade in understanding the phototransduction cascade, our understanding of the retinoid cycle remains rudimentary. The aim of this review is to summarize recent developments in our current understanding of the retinoid cycle at the molecular level, and to examine the relevance of these reactions to phototransduction.

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.

Characterization of a novel type of human microsomal 3alpha -hydroxysteroid dehydrogenase: unique tissue distribution and catalytic properties

We report characterization of a novel member of the short chain dehydrogenase/reductase superfamily. The 1513-base pair cDNA encodes a 319-amino acid protein. The corresponding gene spans over 26 kilobase pairs on chromosome 2 and contains five exons. The recombinant protein produced using the baculovirus system is localized in the microsomal fraction of Sf9 cells and is an integral membrane protein with cytosolic orientation of its catalytic domain. The enzyme exhibits an oxidoreductase activity toward hydroxysteroids with NAD(+) and NADH as the preferred cofactors. The enzyme is most efficient as a 3alpha-hydroxysteroid dehydrogenase, converting 3alpha-tetrahydroprogesterone (allopregnanolone) to dihydroprogesterone and 3alpha-androstanediol to dihydrotestosterone with similar catalytic efficiency (V(max) values of 13-14 nmol/min/mg microsomal protein and K(m) values of 5-7 microm). Despite approximately 44-47% sequence identity with retinol/3alpha-hydroxysterol dehydrogenases, the enzyme is not active toward retinols. The corresponding message is abundant in human trachea and is present at lower levels in the spinal cord, bone marrow, brain, heart, colon, testis, placenta, lung, and lymph node. Thus, the new short chain dehydrogenase represents a novel type of microsomal NAD(+)-dependent 3alpha-hydroxysteroid dehydrogenase with unique catalytic properties and tissue distribution.

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.

Modulation of the androgenic response by recombinant human 11-cis retinol dehydrogenase

The 11-cis retinol dehydrogenase (11-cis-RoDH) enzyme catalyzes the oxidation of cis-retinols to their respective retinals, a rate limiting step in the formation of retinoic acids. Earlier, we have shown that the enzyme also exhibits an oxidative 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD) activity that can convert 5alpha-androstane-3alpha,17beta-diol (3alpha-diol) into dihydrotestosterone (DHT), the most potent natural androgen. 11-cis-RoDH could thus control the formation of two active hormones, namely 9-cis retinoic acid and DHT. Therefore, depending upon the substrate availability in the various tissues, this enzyme could provide different metabolites for specific cell functions. To further investigate the role of 11-cis-RoDH in the formation of DHT from 3alpha-diol, we stably expressed the enzyme in the human embryonic kidney cell line 293 (HEK-293). The transformation of 3alpha-diol by these cells was evaluated by assays using both microsomal fractions and intact cultured cells stably expressing 11-cis-RoDH. The results show that in the intact cells 11-cis-RoDH only catalyzes the oxidation of 3alpha-diol into DHT whereas the microsomal fraction catalyzes both the oxidation and the reduction reactions depending upon whether NAD(+) or NADH is added. Furthermore, we examined the ability of 11-cis-RoDH, through the production from 3alpha-diol of the active androgen DHT, to activate the androgen-responsive promoter of the prostate-specific antigen (PSA) gene. The co-transfection of the pCMV expression vector containing 11-cis-RoDH (pCMV-11-cisRoDH), a luciferase reporter gene driven by a PSA promoter (pCMV-PSA-Luc) and an androgen receptor (pCMV-hAR) showed that, in the presence of 3alpha-diol, the expression of the PSA promoter is increased by five to six-fold. Moreover, this stimulatory effect is inhibited by hydroxyflutamide, a well-known antiandrogen. These results suggest that 11-cis-RoDH could be involved in a non-classical pathway of androgen formation and might play a role in the modulation of the androgenic response in some peripheral tissues.

Characterization of multiple Chinese hamster carbonyl reductases

Carbonyl reductase (CR) is an enzyme which can catalyze the oxidoreduction of various carbonyl compounds in the presence of NAD(P)H. With the PCR method, using primers carrying the conserved nucleotide sequence among mammalian CRs, we isolated three different cDNAs (CHCR1, CHCR2 and CHCR3) which encode a unique carbonyl reductase from the Chinese hamster. The PCR products of CHCR1 and CHCR2 were clearly isolated with Bpu1102I, BspEI and XmaI restriction enzymes. The nucleotide-sequence of CHCR3 was completely different from those of CHCR1 and CHCR2. The predicted double-wound betaalphabetaalpha-structures of the CHCRs suggests the presence of a typical NADP(+)-binding motif and is similar to the corresponding region of 3alpha,20beta-hydroxysteroid dehydrogenase and mouse lung tetrameric carbonyl reductase. The deduced amino acid sequence of CHCR1 showed a high homology to CHCR2 (>96%) and the other mammalian CRs (>81%). However, CHCR3 showed a high homology to human CBR3 (>86%) and a relatively lower homology to the other CHCRs (<76%). Bacterial recombinant CHCRs showed typical carbonyl reductase activities towards 4-benzoylpyridine, 4-nitrobenzaldehyde and pyridine 4-carboxyaldehyde. These three CRs showed not only 3-keto reductase of steroids, but also 20-keto reductase. However, these CRs did not show any activity of 17-keto reductase activity. Both CHCR1 and CHCR2 have prostaglandin 9-keto reductase and 15-hydroxyprostaglandin dehydrogenase activities towards PGE(2) and PGF(2alpha) from the analyses of enzymatic reaction products. The results of Western blotting and RT-PCR suggest these CHCRs have a tissue-dependent-distribution in the Chinese hamster.