Microsomal retinal synthesis: retinol vs. holo-CRBP as substrate and evaluation of NADP, NAD and NADPH as cofactors

Functions of Intracellular Retinoid Binding-Proteins

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

CrbpI regulates mammary retinoic acid homeostasis and the mammary microenvironment

Cellular retinol-binding protein, type I (CrbpI), encoded by retinol-binding protein, type 1 (Rbp1), is a chaperone of vitamin A (retinol) that is epigenetically silenced in ~25% of human breast cancers. CrbpI delivers vitamin A to enzymes for metabolism into an active metabolite, all-trans retinoic acid (atRA), where atRA is essential to cell proliferation, apoptosis, differentiation, and migration. Here, we show the effect of CrbpI loss on mammary atRA homeostasis using the Rbp1(-/-) mouse model. Rbp1(-/-) mouse mammary tissue has disrupted retinoid homeostasis that results in 40% depleted endogenous atRA. CrbpI loss and atRA depletion precede defects in atRA biosynthesis enzyme expression. Compensation by CrbpIII as a retinoid chaperone does not functionally replace CrbpI. Mammary subcellular fractions isolated from Rbp1(-/-) mice have altered retinol dehydrogenase/reductase (Rdh) enzyme activity that results in 24-42% less atRA production. Rbp1(-/-) mammary tissue has epithelial hyperplasia, stromal hypercellularity, increased collagen, and increased oxidative stress characteristic of atRA deficiency and early tissue dysfunction that precedes tumor formation. Consistent with the findings from the Rbp1(-/-) mouse, tumorigenic epithelial cells lacking CrbpI expression produce 51% less atRA. Together, these data show that CrbpI loss disrupts atRA homeostasis in mammary tissue, resulting in microenvironmental defects similar to those observed at the early stages of tumorigenesis.

Involvement of alcohol and aldehyde dehydrogenase activities on hepatic retinoid metabolism and its possible participation in the progression of rat liver regeneration

Liver alcohol dehydrogenase (ADH) activity is decreased towards exogenous substrates after partial hepatectomy (PH), probably due to putative endogenous substrates acting as ADH inhibitors. Hence, retinoids could be suitable candidates as such endogenous substrates. Therefore, cytosolic ADH kinetic analysis using several substrates, liver cytosolic and mitochondrial aldehyde dehydrogenase (ALDH) activities, retinal and retinol content, as well as expression of proteins for ADH and CRBPI (a retinol carrier protein) were determined in liver samples, at two stages of liver regeneration (one- or two-thirds PH). The effect of inhibiting in vivo liver ADH by 4-methylpyrazole (4-MP) was also evaluated after 70%-PH. With 70%-PH, in vitro ADH activity towards exogenous alcohols and aldehydes was diminished, but retinol oxidation was increased and retinal reduction was decreased. These activities that be due to the participation of an ADH type which did not correlate with the amount of immunoreactive ADH protein. Cytosolic and mitochondrial ALDH activities oxidized actively retinal, whereas retinol and CBRP-I expression were reduced in these animals. With 30%-PH, these changes were less evident and sometimes opposite to those found with 70%-PH. In addition, retinol readily inhibited ADH-mediated ethanol oxidation. Interestingly, in vivo 4-MP administration inhibited ADH activity in a dose-dependent manner correlating with a progressive inhibition of liver regeneration. In conclusion, PH-induced inhibition of ADH (mainly type I) seems to be related to ADH-mediated retinoid metabolism during liver proliferation. Thus, results suggest a role of ADH in retinoid metabolism, which is apparently required during rat liver regeneration.

Nutrients as trophic factors in neurons and the central nervous system: role of retinoic acid

In multicellular organisms, death, survival, proliferation, and differentiation of a given cell depend on signals produced by neighboring and/or distant cells, resulting in the coordinated development and function of the various tissues. In the nervous system, control of cell survival and differentiation is achieved through the action of a distinct group of polypeptides collectively known as neurotrophic factors. Recent findings support the view that trophic factors also are involved in the response of the nervous system to acute injury. By contrast, nutrients are not traditionally viewed as potential trophic factors; however, there is increasing evidence that at least some influence neuronal differentiation. During development the brain is responsive to variations in nutrient supply, and this increased sensitivity or vulnerability of the brain to nutrient supply may reappear during neuronal repair, a period during which a rapid membrane resynthesis and reestablishment of synthetic pathways occur. To further evaluate the potential of specific nutrients to act as pharmacologic agents in the repair of injured neurons, the effects of retinoic acid, an active metabolite of vitamin A, and its role as a trophic factor are discussed. This literature review is intended to provide background information regarding the effect of retinoic acid on the cholinergic phenotype and the differentiation of these neurons and to explain how it may promote neuronal repair and survival following injury.

The specificity of alcohol dehydrogenase with cis-retinoids. Activity with 11-cis-retinol and localization in retina

Studies in knockout mice support the involvement of alcohol dehydrogenases ADH1 and ADH4 in retinoid metabolism, although kinetics with retinoids are not known for the mouse enzymes. Moreover, a role of alcohol dehydrogenase (ADH) in the eye retinoid interconversions cannot be ascertained due to the lack of information on the kinetics with 11-cis-retinoids. We report here the kinetics of human ADH1B1, ADH1B2, ADH4, and mouse ADH1 and ADH4 with all-trans-, 7-cis-, 9-cis-, 11-cis- and 13-cis-isomers of retinol and retinal. These retinoids are substrates for all enzymes tested, except the 13-cis isomers which are not used by ADH1. In general, human and mouse ADH4 exhibit similar activity, higher than that of ADH1, while mouse ADH1 is more efficient than the homologous human enzymes. All tested ADHs use 11-cis-retinoids efficiently. ADH4 shows much higher k(cat)/K(m) values for 11-cis-retinol oxidation than for 11-cis-retinal reduction, a unique property among mammalian ADHs for any alcohol/aldehyde substrate pair. Docking simulations and the kinetic properties of the human ADH4 M141L mutant demonstrated that residue 141, in the middle region of the active site, is essential for such ADH4 specificity. The distinct kinetics of ADH4 with 11-cis-retinol, its wide specificity with retinol isomers and its immunolocalization in several retinal cell layers, including pigment epithelium, support a role of this enzyme in the various retinol oxidations that occur in the retina. Cytosolic ADH4 activity may complement the isomer-specific microsomal enzymes involved in photopigment regeneration and retinoic acid synthesis.

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.

Site-specific retinoic acid production in the brain of adult songbirds

The song system of songbirds, a set of brain nuclei necessary for song learning and production, has distinctive morphological and functional properties. Utilizing differential display, we searched for molecular components involved in song system regulation. We identified a cDNA (zRalDH) that encodes a class 1 aldehyde dehydrogenase. zRalDH was highly expressed in various song nuclei and synthesized retinoic acid efficiently. Brain areas expressing zRalDH generated retinoic acid. Within song nucleus HVC, only projection neurons not undergoing adult neurogenesis expressed zRalDH. Blocking zRalDH activity in the HVC of juveniles interfered with normal song development. Our results provide conclusive evidence for localized retinoic acid synthesis in an adult vertebrate brain and indicate that the retinoic acid-generating system plays a significant role in the maturation of a learned behavior.

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.

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.

Holo-cellular retinol-binding protein: distinction of ligand-binding affinity from efficiency as substrate in retinal biosynthesis

Microsomal enzymes that catalyze the first step in the biosynthesis of retinoic acid from retinal, retinol dehydrogenases (RDHs), access retinol bound to cellular retinol-binding protein (CRBP). This study tested the hypothesis that the RDHs interact with the region in CRBP designated as the "helical cap" by evaluating single site-directed mutations, namely, L29A, I32E, L35A, L35E, L35R, L36A, F57A, R58A, and R58E. UV analysis showed mutants had similar conformations of retinol in their binding pockets. Nevertheless, the mutants bound retinol with affinities 2-5-fold lower than wild type, except for L35 mutants, which had affinities similar to wild type. All mutants' holoforms had more relaxed conformations about their helical caps, judged by sensitivity to partial protease digestion. Mutants showed no significant differences in Km values, but two (L36A, R58A) had increased Vm values and L35 mutants had decreased Vm values. Overall, the data indicate that the residues tested contribute in varying degrees to CRBP rigidity, retinol binding, and RDH recognition/access to bound retinol. The extent of contributions can be distinguished for several residues. For example, L35 mutants had lower kcat values than wild-type CRBP; thus, L35 seems important for RDH access to retinol. F57, on the other hand, a suspected key residue in controlling retinol entrance/exit, does not make a singular contribution to retinol binding. These results suggest a role for the helical cap region as a locus for RDH interaction and as a portal for ligand access to CRBP, and show that the affinity (Kd) of CRBP for retinol alone does not determine the efficiency of holo-CRBP as substrate. These are the first experimental data of enzyme recognition by a specific exterior residue of CRBP (L35).

Effect of cellular retinol-binding protein on retinol oxidation by human class IV retinol/alcohol dehydrogenase and inhibition by ethanol

All-trans retinoic acid (atRA) is a powerful morphogen synthesized in a variety of tissues. Oxidation of all-trans retinol to all-trans retinal determines the overall rate of atRA biosynthesis. This reaction is catalyzed by multiple dehydrogenases in vitro. In the cells, most all-trans retinol is bound to cellular retinol binding protein (CRBP). Whether retinoic acid is produced from the free or CRBP-bound retinol in vivo is not known. The current study investigated whether human medium-chain alcohol/retinol dehydrogenases (ADH) can oxidize the CRBP-bound retinol. The results of this study suggest that retinol bound to CRBP cannot be channeled to the active site of ADH. Thus, the contribution of ADH isozymes to retinoic acid biosynthesis will depend on the amount of free retinol in each cell. Physiological levels of ethanol will substantially inhibit the oxidation of free retinol by human ADHs: class I, alpha alpha and beta 2 beta 2; class II, pi pi; and class IV, sigma sigma.

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.

Expression cloning and characterization of oxidative 17beta- and 3alpha-hydroxysteroid dehydrogenases from rat and human prostate

Intracellular levels of active steroid hormones are determined by their relative rates of synthesis and breakdown. In the case of the potent androgen dihydrotestosterone, synthesis from the precursor testosterone is mediated by steroid 5alpha-reductase, whereas breakdown to the inactive androgens 5alpha-androstane-3alpha, 17beta-diol (3alpha-adiol), and androsterone is mediated by reductive 3alpha-hydroxysteroid dehydrogenases (3alpha-HSD) and oxidative 17beta-hydroxysteroid dehydrogenases (17beta-HSD), respectively. We report the isolation by expression cloning of a cDNA encoding a 17beta-HSD6 isozyme that oxidizes 3alpha-adiol to androsterone. 17beta-HSD6 is a member of the short chain dehydrogenase/reductase family and shares 65% sequence identity with retinol dehydrogenase 1 (RoDH1), which catalyzes the oxidation of retinol to retinal. Expression of rat and human RoDH cDNAs in mammalian cells is associated with the oxidative conversion of 3alpha-adiol to dihydrotestosterone. Thus, 17beta-HSD6 and RoDH play opposing roles in androgen action; 17beta-HSD6 inactivates 3alpha-adiol by conversion to androsterone and RoDH activates 3alpha-adiol by conversion to dihydrotestosterone. The synthesis of an active steroid hormone by back conversion of an inactive metabolite represents a potentially important mechanism by which the steady state level of a transcriptional effector can be regulated.

Purification and characterization of a novel cytosolic NADP(H)-dependent retinol oxidoreductase from rabbit liver

Rabbit liver cytosol exhibits very high retinol dehydrogenase activity. At least two retinol dehydrogenases were demonstrated to exist in rabbit liver cytosol, and the major one, a cytosolic NADP(H)-dependent retinol dehydrogenase (systematic name: retinol oxidoreductase) was purified about 1795-fold to electrophoretic and column chromatographic homogeneity by a procedure involving column chromatography on AF-Red Toyopearl twice and then hydroxyapatite. Its molecular mass was estimated to be 34 kDa by SDS-PAGE, and 144 kDa by HPLC gel filtration, suggesting that it is a homo-tetramer. The enzyme uses free retinol and retinal, and their complexes with CRBP as substrates in vitro. The optimum pH values for retinol oxidation of free retinol and CRBP-retinol were 8.8-9.2 and 8.0-9.0, respectively, and those for retinal reduction of free retinal and retinal-CRBP were the same, 7.0-7.6. Km for free retinol and Vmax for retinal formation were 2.8 microM and 2893 nmol/min per mg protein at 37 degrees C (pH 9.0) and the corresponding values with retinol-CRBP as a substrate were 2.5 microM and 2428 nmol/min per mg protein at 37 degrees C (pH 8.6); Km for free retinal and Vmax for retinol formation were 6.5 microM and 4108 nmol/min per mg protein, and the corresponding values with retinal-CRBP as a substrate were 5.1 microM and 3067 nmol/min per mg protein at 37 degrees C, pH 7.4. NAD(H) was not effective as a cofactor. 4-Methylpyrazole was a weak inhibitor (IC50 = 28 mM) of the enzyme, and ethanol was neither a substrate nor an inhibitor of the enzyme. This enzyme exhibits relatively broad aldehyde reductase activity and some ketone reductase activity, the activity for aromatic substitutive aldehydes being especially high and effective. Whereas, except in the case of retinol, oxidative activity toward the corresponding alcohols was not detected. This novel cytosolic enzyme may play an important role in vivo in maintaining the homeostasis of retinal, the substrate of retinoic acid synthesis, at least in rabbit liver, since a high concentration of retinol in liver and the lower Km of the enzyme for retinol force the oxidative reaction, while higher activity of retinal reductase at physiological pH forces the reductive reaction.

Involvement of alcohol dehydrogenase, short-chain dehydrogenase/reductase, aldehyde dehydrogenase, and cytochrome P450 in the control of retinoid signaling by activation of retinoic acid synthesis

The effects of vitamin A (retinol) on growth and development are mediated by the active metabolite retinoic acid which controls a nuclear receptor signaling pathway. While elegant work on the retinoic acid receptor family has focused attention upon how the receptor controls this pathway, there now exists a relatively large gap in our understanding of how retinol is activated to form the ligand. During vertebrate embryogenesis and in adult organs retinoic acid is detected in a distinct spatiotemporal pattern, suggesting that it is produced from retinol in a regulated fashion. Enzymes involved in retinol and retinal metabolism are likely candidates for regulators of tissue retinoic acid levels. Members of the alcohol dehydrogenase and short-chain dehydrogenase/reductase enzyme families catalyze the reversible interconversion of retinol and retinal, the rate-limiting step, whereas members of the aldehyde dehydrogenase and cytochrome P450 enzyme families catalyze the irreversible oxidation of retinal to retinoic acid. The identification of enzymes likely to catalyze retinol oxidation in vivo has been particularly controversial, and this is made even more difficult by the reversible nature of this reaction. Taking into account enzymatic properties and coenzyme preferences, a case can be made that class IV alcohol dehydrogenase catalyzes retinol oxidation to provide retinal for retinoic acid synthesis, whereas microsomal retinol dehydrogenase (a short-chain dehydrogenase/reductase) catalyzes the reduction of retinal to retinol to promote retinoid storage. Further studies on these enzyme families will allow this layer of control in the retinoid signaling pathway to be understood.

Increased concentrations of 3,4-didehydroretinol and retinoic acid-binding protein (CRABPII) in human squamous cell carcinoma and keratoacanthoma but not in basal cell carcinoma of the skin

Retinoids are biologic response modifiers that are present in normal skin and may possibly be perturbed in carcinogenesis. To examine this possibility in human skin, we analyzed vitamin A and cytosolic retinoid binding proteins (cellular retinol binding protein and cellular retinoic acid binding protein [CRABP]) in a total of 38 non-melanoma skin tumors and 25 healthy skin samples using high performance liquid chromatography, radioligand electrophoresis, and reverse transcriptase-polymerase chain reaction. The mean +/- SEM retinol concentration was normal in basal cell carcinoma (0.60 +/- 0.10 microM) and seborrheic keratosis (0.47 +/- 0.07 microM), but increased in keratoacanthoma (1.60 +/- 0.41 microM) and squamous cell carcinoma (1.17 +/- 0.28 microM) (p < 0.05 for both). Also, the concentrations of 3,4-didehydroretinol, a major vitamin A metabolite produced in human skin, were markedly elevated (6-7 times normal) in keratoacanthoma and squamous cell cancer. All types of tumors showed moderately increased levels of cellular retinol binding protein. In addition, keratoacanthoma and squamous cell cancer showed markedly increased levels (6-7 times normal) of CRABPII protein. Transcriptional activity of the CRABPII gene was demonstrated in both normal and neoplastic epidermis, but clear CRABPI mRNA expression was found only in basal cell carcinoma. The data indicate that characteristic perturbations of the vitamin A and retinoid binding protein levels occur in squamous cell-derived skin tumors, but whether these reflect intrinsic errors in retinoid metabolism or are secondary to abnormal cellular differentiation is unknown.

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

A retinol dehydrogenase, RoDH(1), which recognizes holo-cellular retinol-binding protein (CRBP) as substrate, has been cloned, expressed, and identified as a short-chain dehydrogenase/reductase (Chai, X., Boerman, M. H. E. M., Zhai, Y., and Napoli, J. L. (1995) J. Biol. Chem. 270, 3900-3904). This work reports the cloning and expression of a cDNA encoding a RoDH isozyme, RoDH(II). The predicted amino acid sequence verifies RoDH(II) as a short-chain dehydrogenase/reductase, 82% identical with RoDH(I). RoDH(II) recognized the physiological form of retinol as substrate, CRBP, with a Km of 2 mM. Similar to microsomal RoDH and RoDH(I), RoDH(II) had higher activity with NADP rather than NAD, was stimulated by ethanol and phosphatidyl choline, was not inhibited by the medium-chain alcohol dehydrogenase inhibitor 4-methylpyrazole, but was inhibited by phenylarsine oxide and the short-chain dehydrogenase/reductase inhibitor carbenoxolone. Northern blot analysis detected RoDH(I) and RoDH(II) mRNA only in rat liver, but RNase protection assays revealed RoDH(I) and RoHD(II) mRNA in kidney, lung, testis, and brain. These data indicate that short-chain dehydrogenases/reductase isozymes expressed tissue-distinctively catalyze the first step of retinoic acid biogenesis from the physiologically most abundant substrate, CRBP.

Enzymes and binding proteins affecting retinoic acid concentrations

Free retinoids suffer promiscuous metabolism in vitro. Diverse enzymes are expressed in several subcellular fractions that are capable of converting free retinol (retinol not sequestered with specific binding proteins) into retinal or retinoic acid. If this were to occur in vivo, regulating the temporal-spatial concentrations of functionally-active retinoids, such as RA (retinoic acid), would be enigmatic. In vivo, however, retinoids occur bound to high-affinity, high-specificity binding proteins, including cellular retinol-binding protein, type I (CRBP) and cellular retinoic acid-binding protein, type I (CRABP). These binding proteins, members of the superfamily of lipid binding proteins, are expressed in concentrations that exceed those of their ligands. Considerable data favor a model pathway of RA biosynthesis and metabolism consisting of enzymes that recognize CRBP (apo and holo) and holo-CRABP as substrates and/or affecters of activity. This would restrict retinoid access to enzymes that recognize the appropriate binding protein, imparting specificity to RA homeostasis; preventing, e.g. opportunistic RA synthesis by alcohol dehydrogenases with broad substrate tolerances. An NADP-dependent microsomal retinol dehydrogenase (RDH) catalyzes the first reaction in this pathway. RDH recognizes CRBP as substrate by the dual criteria of enzyme kinetics and chemical crosslinking. A cDNA of RDH has been cloned, expressed and characterized as a short-chain alcohol dehydrogenase. Retinal generated in microsomes from holo-CRBP by RDH supports cytosolic RA synthesis by an NAD-dependent retinal dehydrogenase (RalDH). RalDH has been purified, characterized with respect to substrate specificity, and its cDNA has been cloned. CRABP is also important to modulating the steady-state concentrations of RA, through sequestering RA and facilitating its metabolism, because the complex CRABP/RA acts as a low Km substrate.

Cloning of a cDNA for liver microsomal retinol dehydrogenase. A tissue-specific, short-chain alcohol dehydrogenase

Retinoic acid, a hormone biosynthesized from retinol, controls numerous biological systems by regulating eukaryotic gene expression from conception through death. This work reports the cloning and expression of a liver cDNA encoding a microsomal retinol dehydrogenase (RoDH), which catalyzes the primary and rate-limiting step in retinoic acid synthesis. The predicted amino acid sequence and biochemical data obtained from the recombinant enzyme verify it as a short-chain alcohol dehydrogenase. Like microsomal RoDH, the recombinant enzyme recognized as substrate retinol bound to cellular retinol-binding protein, had higher activity with NADP rather than NAD, was stimulated by ethanol or phosphatidylcholine, was not inhibited by 4-methylpyrazole, was inhibited by phenylarsine oxide and carbenoxolone and localized to microsomes. RoDH recognized the physiological form of retinol, holocellular retinol-binding protein, with a Km of 0.9 microM, a value lower than the approximately 5 microM concentration of holocellular retinol binding protein in liver. Northern and Western blot analyses revealed RoDH expression only in rat liver, despite enzymatic activity in liver, brain, kidney, lung, and testes. These data suggest that tissue-specific isozyme(s) of short chain alcohol dehydrogenases catalyze the first step in retinoic acid biogenesis and further strengthen the evidence that the "cassette" of retinol bound to cellular retinol-binding protein serves as a physiological substrate.

Catalytic efficiency of human alcohol dehydrogenases for retinol oxidation and retinal reduction

Mammalian alcohol dehydrogenase (ADH) is thought to be involved in the reversible oxidation of vitamin A or retinol to retinal for retinoic acid synthesis. Retinoic acid is a potent transcriptional regulator and a morphogen. It was proposed that the competition of consumed ethanol with retinol oxidation by ADH might explain developmental disorders seen with fetal alcohol syndrome. We report herein the relative efficiency (V/Km) of eight human ADH isoenzymes for oxidation of all-trans-retinol and reduction of three retinal isomers (all-trans, 9-cis, and 13-cis-retinal). Class IV sigma sigma and class II pi pi isoenzymes are the most efficient forms, with V/Km values approximately 100 and 30 times greater, respectively, than class I beta 1 beta 1 or gamma 1 gamma 1, sigma sigma exhibits the highest V/Km (1-2 microns-1min-1), followed by pi pi, with V/Km of 0.5-0.6 microns-1min-1 for all-trans-retinol, all-trans-retinal, and 9-cis-retinal. pi pi also has the lowest Km (11-14 microns) for all-trans-retinol and three retinal isomers. alpha alpha shows an intermediate efficiency, with V/Km of 0.09-0.2 microns-1min-1 and a relatively low Km of 16-24 microns for all four substrates. alpha alpha has the highest efficiency of all tested isoenzymes for 13-cis-retinal. Class III chi chi is inactive with all the tested retinoids.(ABSTRACT TRUNCATED AT 250 WORDS)

All-trans-retinol is a ligand for the retinoic acid receptors

Competition of all-trans-retinol and all-trans-retinaldehyde with 3H-labeled all-trans-retinoic acid (RA) for binding to retinoic acid receptors (RARs) was examined in human neuroblastoma cell nuclear extracts. All-trans-retinol was 35-fold less potent than all-trans-RA, whereas all-trans-retinaldehyde was 500-fold less active in binding to the nuclear receptors. To confirm that all-trans-retinol binds to RARs, experiments were carried out with RARs alpha, beta, and gamma expressed as bacterial fusion proteins. All-trans-retinol was only 4- to 7-fold less potent than all-trans-RA in binding to all three RAR subtypes. The all-trans-retinol binding observed was not the result of metabolism of retinol to RA or some other active compound during the binding experiment. Retinyl acetate was virtually inactive in competition binding experiments, while very slight activity was observed with 13-cis-RA and all-trans-retinaldehyde. Significant competition occurred with 4-hydroxy-RA and 4-keto-RA, which were 15- to 40-fold less potent than all-trans-RA. The 9-cis isomer of RA was equipotent with all-trans-retinol in these studies. These results suggest that all-trans-retinol cannot be excluded as a physiologically significant ligand for RAR-mediated gene expression.