Carbonyl Reductases and Pluripotent Hydroxysteroid Dehydrogenases of the Short-chain Dehydrogenase/reductase Superfamily

The Janus face of alcohol dehydrogenase 3

Many carbonyl metabolizing enzymes are equally involved in xenobiotic and endogenous metabolism, but few have been investigated in terms of substrate competition and interference between different cellular pathways. Mammalian alcohol dehydrogenase 3 (ADH3) represents the key enzyme in the formaldehyde detoxification pathway by oxidation of S-hydroxymethylglutathione [HMGSH; the glutathione (GSH) adduct of formaldehyde]. In addition, several studies have established ADH3 as S-nitrosoglutathione (GSNO) reductase in endogenous NO homeostasis during the last decade. GSNO depletion associates with various diseases including asthma, and evidence for a causal relationship between ADH3 and asthma pathology has been put forward. In a recent study, we showed that ADH3-mediated alcohol oxidation, including HMGSH oxidation, is accelerated in presence of GSNO which is concurrently reduced under immediate cofactor recycling [C.A. Staab, J. Alander, M. Brandt, J. Lengqvist, R. Morgenstern, R.C. Grafström, J.-O. Höög, Reduction of S-nitrosoglutathione by alcohol dehydrogenase 3 is facilitated by substrate alcohols via direct cofactor recycling and leads to GSH-controlled formation of glutathione transferase inhibitors, Biochem. J. 413 (2008) 493-504]. Thus, considering the usually low cytosolic free NADH/NAD(+) ratio, formaldehyde may trigger and promote GSNO reduction by enzyme-bound cofactor recycling. These findings provided evidence for formaldehyde-induced, ADH3-mediated GSNO depletion with potential direct implications for asthma. Furthermore, analysis of product formation as a function of GSH concentrations suggested that, under conditions of oxidative stress, GSNO reduction can lead to the formation of glutathione sulfinamide and its hydrolysis product glutathione sulfinic acid, both potent inhibitors of glutathione transferase activity.

[Structure and function of peroxisomal tetrameric carbonyl reductase]

In this paper, the structure and function of a new tetrameric carbonyl reductase (TCR) is reviewed. TCRs were purified from rabbit and pig heart, using 4-benzoylpyridine as a substrate. Partial peptide sequencing and cDNA cloning of rabbit and pig TCRs revealed that both enzymes belonged to the short-chain dehydrogenase/reductase family and that their subunits consisted of 260 amino acid residues. Rabbit and pig TCRs catalyzed the reduction of alkyl phenyl ketones, alpha-dicarbonyl compounds, quinones and retinals. Both enzymes were potently inhibited by flavonoids and fatty acids. 9,10-Phenanthrenequinone, which is efficiently reduced by rabbit and pig TCRs, mediated the formation of superoxide radical through its redox cycling in pig heart. The C-terminal sequences of rabbit and pig TCRs comprised a type 1 peroxisomal targeting signal (PTS1) Ser-Arg-Leu, suggesting that the enzymes are localized in the peroxisome. In fact, pig TCR was targeted into the peroxisomal matrix, in the case of transfection of HeLa cells with vectors expressing the enzyme. However, when the recombinant pig TCR was directly introduced into HeLa cells, the enzyme was not targeted into the peroxisomal matrix. The crystal structure of recombinant pig TCR demonstrated that the C-terminal PTS1 of each subunit of the enzyme was buried in the interior of the tetrameric molecule. These findings indicate that pig TCR is imported into the peroxisome as a monomer and then forms an active tetramer within this organelle.

AKR1C3 as a potential target for the inhibitory effect of dietary flavonoids

AKR1C3 (also known as 17beta-hydroxysteroid dehydrogenase type 5 or 3alpha-hydroxysteroid dehydrogenase type 2) functions as a 3-keto, 17-keto and 20-ketosteroid reductase and as a 3alpha-, 17beta- and 20alpha-hydroxysteroid oxidase. Relatively high mRNA expression of AKR1C3 was found in human prostate and mammary gland where it is implicated in regulating ligand access to the androgen and estrogen receptor, respectively. AKR1C3 is an interesting target for the development of agents for treating hormone-dependent forms of cancer like prostate cancer, breast cancer, and endometrial cancer. However, only a few clinically promising and selective inhibitors have been reported so far. Very potent inhibitors of AKR1C3 are the non-steroidal anti-inflammatory drugs, e.g. indomethacin or flufenamic acid. Also dietary phytoestrogens such as coumestrol, quercetin, and biochanin were reported to inhibit the enzyme in low micromolar concentrations. In this study, some dietary flavonoids and other phenolic compounds were tested for their ability to specifically inhibit AKR1C3. Carbonyl reduction of the anticancer drug oracin, which is a very good substrate for AKR1C3 and which could be well monitored by a sensitive HPLC system with fluorescence detection, was employed to determine the inhibitory potency of the compounds. Our results reveal that AKR1C3 could be potentially un-competitively inhibited by 2'-hydroxyflavanone, whose IC(50) value of 300nM is clinically promising. Moreover, since the inhibition is selective towards AKR1C3, 2'-hydroxyflavanone could be useful for treating or preventing hormone-dependent malignancies like prostate and breast cancer.

Genotype of human carbonyl reductase CBR3 correlates with doxorubicin disposition and toxicity

OBJECTIVES

Doxorubicin is a cytotoxic drug with potential for severe myelosuppression that is highly variable and poorly predictable.

METHODS

We correlated CBR1 and CBR3 genotypes with the pharmacokinetics and pharmacodynamics of doxorubicin in 101 Southeast Asian breast cancer patients receiving first-line doxorubicin.

RESULTS

A common CBR3 11G>A variant was associated with lower doxorubicinol area under the concentration-time curve (AUC)/doxorubicin AUC metabolite ratio (P=0.009, GG vs. AA; trend test, P=0.004), lower CBR3 expression in breast tumor tissue (P=0.001, GG vs. AA), greater tumor reduction (P=0.015, GG vs. AA), and greater percentage reduction of leukocyte and platelet counts at nadir (trend test, P < or = 0.03). Chinese and Malays had higher frequency of the CBR3 11G>A variant than Indians (P < or = 0.002). Another variant CBR3 730G>A was associated with higher doxorubicinol AUC (P=0.009, GG vs. AA) and CBR3 expression in breast tumor tissue (P=0.001, GG vs AA).

CONCLUSION

Polymorphisms in CBR3 may explain interindividual and interethnic variability of doxorubicin pharmacokinetics and pharmacodynamics.

Carbonyl reductase 1 is a predominant doxorubicin reductase in the human liver

A first step in the enzymatic disposition of the antineoplastic drug doxorubicin (DOX) is the reduction to doxorubicinol (DOX-OL). Because DOX-OL is less antineoplastic but more cardiotoxic than the parent compound, the individual rate of this reaction may affect the antitumor effect and the risk of DOX-induced heart failure. Using purified enzymes and human tissues we determined enzymes generating DOX-OL and interindividual differences in their activities. Human tissues express at least two DOX-reducing enzymes. High-clearance organs (kidney, liver, and the gastrointestinal tract) express an enzyme with an apparent Km of approximately 140 microM. Of six enzymes found to reduce DOX, Km values in this range are exhibited by carbonyl reductase 1 (CBR1) and aldo-keto reductase (AKR) 1C3. CBR1 is expressed in these three organs at higher levels than AKR1C3, whereas AKR1C3 has higher catalytic efficiency. However, inhibition constants for DOX reduction with 4-amino-1-tert-butyl-3-(2-hydroxyphenyl)pyrazolo[3,4-d]pyrimidine (an inhibitor that can discriminate between CBR1 and AKR1C3) were identical for CBR1 and human liver cytosol, but not for AKR1C3. These results suggest that CBR1 is a predominant hepatic DOX reductase. In cytosols from 80 human livers, the expression level of CBR1 and the activity of DOX reduction varied >70- and 22-fold, respectively, but showed no association with CBR1 gene variants found in these samples. Instead, the interindividual differences in CBR1 expression and activity may be mediated by environmental factors acting via recently identified xenobiotic response elements in the CBR1 promoter. The variability in the CBR1 expression may affect outcomes of therapies with DOX, as well as with other CBR1 substrates.

Enzymes without borders: mobilizing substrates, delivering products

Many cellular reactions involve both hydrophobic and hydrophilic molecules that reside within the chemically distinct environments defined by the phospholipid-based membranes and the aqueous lumens of cytoplasm and organelles. Enzymes performing this type of reaction are required to access a lipophilic substrate located in the membranes and to catalyze its reaction with a polar, water-soluble compound. Here, we explore the different binding strategies and chemical tricks that enzymes have developed to overcome this problem. These reactions can be catalyzed by integral membrane proteins that channel a hydrophilic molecule into their active site, as well as by water-soluble enzymes that are able to capture a lipophilic substrate from the phospholipid bilayer. Many chemical and biological aspects of this type of enzymology remain to be investigated and will require the integration of protein chemistry with membrane biology.

Redox-based endoplasmic reticulum dysfunction in neurological diseases

The redox homeostasis of the endoplasmic reticulum lumen is characteristically different from that of the other subcellular compartments. The concerted action of membrane transport processes and oxidoreductase enzymes maintain the oxidized state of the thiol-disulfide and the reducing state of the pyridine nucleotide redox systems, which are prerequisites for the normal functions of the organelle. The powerful thiol-oxidizing machinery allows oxidative protein folding but continuously challenges the local antioxidant defense. Alterations of the cellular redox environment either in oxidizing or reducing direction affect protein processing and may induce endoplasmic reticulum stress and unfolded protein response. The activated signaling pathways attempt to restore the balance between protein loading and processing and induce apoptosis if the attempt fails. Recent findings strongly support the involvement of this mechanism in brain ischemia, neuronal degenerative diseases and traumatic injury. The redox changes in the endoplasmic reticulum are integral parts of the pathomechanism of neurological diseases, either as causative agents, or as complications.

Characterization of human DHRS4: an inducible short-chain dehydrogenase/reductase enzyme with 3beta-hydroxysteroid dehydrogenase activity

Human DHRS4 is a peroxisomal member of the short-chain dehydrogenase/reductase superfamily, but its enzymatic properties, except for displaying NADP(H)-dependent retinol dehydrogenase/reductase activity, are unknown. We show that the human enzyme, a tetramer composed of 27kDa subunits, is inactivated at low temperature without dissociation into subunits. The cold inactivation was prevented by a mutation of Thr177 with the corresponding residue, Asn, in cold-stable pig DHRS4, where this residue is hydrogen-bonded to Asn165 in a substrate-binding loop of other subunit. Human DHRS4 reduced various aromatic ketones and alpha-dicarbonyl compounds including cytotoxic 9,10-phenanthrenequinone. The overexpression of the peroxisomal enzyme in cultured cells did not increase the cytotoxicity of 9,10-phenanthrenequinone. While its activity towards all-trans-retinal was low, human DHRS4 efficiently reduced 3-keto-C(19)/C(21)-steroids into 3beta-hydroxysteroids. The stereospecific conversion to 3beta-hydroxysteroids was observed in endothelial cells transfected with vectors expressing the enzyme. The mRNA for the enzyme was ubiquitously expressed in human tissues and several cancer cells, and the enzyme in HepG2 cells was induced by peroxisome-proliferator-activated receptor alpha ligands. The results suggest a novel mechanism of cold inactivation and role of the inducible human DHRS4 in 3beta-hydroxysteroid synthesis and xenobiotic carbonyl metabolism.

Chicken 11beta-hydroxysteroid dehydrogenase type 2: partial cloning and tissue distribution

NAD(+)-dependent 11beta-hydroxysteroid dehydrogenase (11HSD2) converts glucocorticoids to 11-oxo derivatives and thus decreases their local concentration and prevents them from activating corticosteroid receptors. In this paper we report the partial cloning, characterization and tissue distribution of chicken 11HSD2. A cDNA of 991bp was cloned from kidney mRNA by reverse transcription and polymerase chain reaction. At the amino acid level, the sequence of PCR product had 56-59% homology with mammalian and 46-48% with fish 11HSD2. The consensus sequences of the short-chain dehydrogenase/reductase superfamily such as the catalytic activity motif Tyr-X-X-X-Lys and cosubstrate-binding motif Gly-X-X-X-Gly-X-Gly, were found in the cloned cDNA. Analysis of the tissue expression of chicken 11HSD2 mRNA and NAD(+)-dependent 11beta-oxidase activity showed a similar tissue distribution pattern in the majority of tissues. High levels of expression and activity were found in kidney, small intestine, colon and oviduct; low in ovary and almost zero in brain, liver and testis.

Theoretical calculations of the catalytic triad in short-chain alcohol dehydrogenases/reductases

Three highly conserved active site residues (Ser, Tyr, and Lys) of the family of short-chain alcohol dehydrogenases/reductases (SDRs) were demonstrated to be essential for catalytic activity and have been denoted the catalytic triad of SDRs. In this study computational methods were adopted to study the ionization properties of these amino acids in SDRs from Drosophila melanogaster and Drosophila lebanonensis. Three enzyme models, with different ionization scenarios of the catalytic triad that might be possible when inhibitors bind to the enzyme cofactor complex, were constructed. The binding of the two alcohol competitive inhibitors were studied using automatic docking by the Internal Coordinate Mechanics program, molecular dynamic (MD) simulations with the AMBER program package, calculation of the free energy of ligand binding by the linear interaction energy method, and the hydropathic interactions force field. The calculations indicated that deprotonated Tyr acts as a strong base in the binary enzyme-NAD(+) complex. Molecular dynamic simulations for 5 ns confirmed that deprotonated Tyr is essential for anchoring and orientating the inhibitors at the active site, which might be a general trend for the family of SDRs. The findings here have implications for the development of therapeutically important SDR inhibitors.

Characterization of an oligomeric carbonyl reductase of dog liver: its identity with peroxisomal tetrameric carbonyl reductase

Dog liver contains an oligomeric NADPH-dependent carbonyl reductase (CR) with substrate specificity for alkyl phenyl ketones, but its endogenous substrate and primary structure remain unknown. In this study, we examined the molecular weight and substrate specificity of the enzyme purified from dog liver. The enzyme is a ca. 100-kDa tetramer composing of 27-kDa subunit, and reduces all-trans-retinal and alpha-dicarbonyl compounds including isatin, which are substrates for pig peroxisomal tetrameric carbonyl reductase (PTCR). In addition, the dog enzyme resembles pig PTCR in inhibitor sensitivity to flavonoids, myristic acid, lithocholic acid, bromosulfophthalein and flufenamic acid. Furthermore, the amino acid sequence of dog CR determined by protein sequencing and cDNA cloning was 84% identical to that of pig PTCR and had a C-terminal peroxisomal targeting signal type 1, Ser-His-Leu. The immunoprecipitation using the anti-pig PTCR antibody shows that the dog enzyme is a major form of soluble NADPH-dependent all-trans-retinal reductase in dog liver. Thus, dog oligomeric CR is PTCR, and may play a role in retinoid metabolism as a retinal reductase.

Increased resistance of tumor cells to daunorubicin after transfection of cDNAs coding for anthracycline inactivating enzymes

Carbonyl reduction is a main but undesired metabolic pathway of the anti-cancer drug daunorubicin (DRC). The resulting alcohol metabolite daunorubicinol has a far less anti-tumor potency and, in addition, is responsible for the life-threatening cardiac toxicity that limits the clinical use of DRC. Elevated levels of carbonyl-reducing enzymes in cancer cells may therefore contribute to the development of DRC chemoresistance and affect the clinical outcome. In the present investigation, human pancreas carcinoma cells were transfected with three important DRC reductases, namely carbonyl reductase (CBR1), aldehyde reductase (AKR1A1) and aldose reductase (AKR1B1), and levels of resistance towards DCR determined. Overexpression of all three reductases lead to a higher DRC inactivation and to an elevation of chemoresistance (7-fold for CBR1, 4.5-fold for AKR1A1 and 3.7-fold for AKR1B1), when IC(50)-values were considered. Coadministration of DRC reductase inhibitors in DRC chemotherapy may be desirable since this would reduce the formation of the cardiotoxic alcohol metabolite and prevent drug resistance.