Structural basis for substrate specificity in human monomeric carbonyl reductases

Adipocyte maturation impacts daunorubicin disposition and metabolism

INTRODUCTION

Acute lymphoblastic leukaemia (ALL) is the most common type of childhood leukaemia with effective chemotherapeutic treatment. However, obesity has been associated with higher ALL chemoresistance rates and lower event-free survival rates. The molecular mechanism of how obesity promotes chemotherapy resistance is not well delineated.

OBJECTIVES

This study evaluated the effect of adipocyte maturation on sequestration and metabolism of chemotherapeutic drug daunorubicin (DNR).

METHODS

Using targeted LC-MS/MS multi-analyte assay, DNR sequestration and metabolism were studied in human preadipocyte and adipocyte cell lines, where expressions of DNR-metabolizing enzymes aldo-keto reductases (AKR) and carbonyl reductases (CBR) were also evaluated. In addition, to identify the most DNR-metabolizing AKR/CBR isoforms, recombinant human AKR and CBR enzymes were subject to DNR metabolism. The results were further validated by AKR-, CBR-specific inhibitors.

RESULTS

This report shows that adipocyte maturation upregulates expressions of AKR and CBR enzymes (by 4- to 60- folds, p < .05), which is positively associated with enhanced sequestration and metabolism of DNR in adipocytes compared to preadipocytes (by ~30%, p < .05). In particular, adipocyte maturation upregulates AKR1C3 and CBR1, which are the predominate metabolic enzyme isoforms responsible for DNR biotransformation to its metabolites.

CONCLUSION

Fat is an expandable tissue that can sequester and detoxify DNR when stimulated by obesity, likely through the upregulation of DNR-metabolizing enzymes AKR1C3 and CBR1. Our data partially explains why obese ALL patients may be more likely to become chemoresistant towards DNR, and provides evidence for potential clinical investigation targeting obesity to reduce DNR chemoresistance.

Gestational Diabetes Mellitus-Induced Milk Fat Globule Membrane Protein Changes of Human Mature Milk Based on TMT Proteomic Analysis

Breastfeeding by mothers with gestational diabetes mellitus (GDM) has been shown to reduce maternal insulin demands and diminish the risks of diabetes in infants, leading to improved long-term health outcomes. Milk fat globule membrane (MFGM) proteins play a crucial role in influencing the immunity and cognitive development of infants. Understanding the alterations in MFGM proteins in breastmilk from mothers with GDM is essential for enhancing their self-efficacy and increase breastfeeding rates. The objective of this study is to investigate and compare MFGM proteins in milk from mothers with GDM and without based on tandem mass tag (TMT) labeling and liquid chromatography tandem mass spectrometry (LC-MS) techniques. A total of 5402 proteins were identified, including 4 upregulated proteins and 24 downregulated proteins. These significantly altered proteins were found to be associated with human diseases, cellular processes, and metabolism pathways. Additionally, the oxidative phosphorylation pathway emerged as the predominant pathway through Gene Set Enrichment Analysis (GSEA) involving all genes.

Characterization of a novel porcine carbonyl reductase activated by glutathione: Relationship to carbonyl reductase 1, 3α/β-hydroxysteroid dehydrogenase and prostaglandin 9-ketoreductase

A porcine gene, LOC100622246, encodes carbonyl reductase [NADPH] 1 (pCBR-N1), whose function remains unknown. Previously, three porcine carbonyl reductases, carbonyl reductase 1 (pCBR1), 3α/β-hydroxysteroid dehydrogenase (p3α/β-HSD) and prostaglandine-9-keto reductase (pPG-9-KR), were purified from neonatal testis, adult testis and adult kidney, respectively. However, the relationship of pCBR-N1 with the three enzymes is still unknown. Here, we compare the properties of the recombinant pCBR-N1 and pCBR1. The two enzymes reduced various carbonyl compounds including 5α-dihydrotestosterone, which was converted to its 3α- and 3β-hydroxy-metabolites. Compared to pCBR1, pCBR-N1 exhibited higher Km and kcat values for most substrates, but more efficiently reduced prostaglandin E2. pCBR-N1 was inhibited by known inhibitors of p3α/β-HSD (hexestrol and indomethacin), but not by pCBR1 inhibitors. pCBR-N1 was highly expressed than pCBR1 in the several tissues of adult domestic and microminiature pigs. The results, together with partial amino acid sequence match between pCBR-N1 and pPG-9-KR, reveal that pCBR-N1 is identical to p3α/β-HSD and pPG-9-KR. Notably, pCBR-N1, but not pCBR1, reduced S-nitrosoglutathione and glutathione-adducts of alkenals including 4-oxo-2-nonenal with Km of 8.3-32 μM, and its activity toward non-glutathionylated substrates was activated 2- to 9-fold by 1 mM glutathione. Similar activation by glutathione was also observed for human CBR1. Site-directed mutagenesis revealed that the differences in kinetic constants and glutathione-mediated activation between pCBR-N1 and pCBR1 are due to differences in residue 236 and two glutathione-binding residues (at positions 97 and 193), respectively. Thus, pCBR-N1 is a glutathione-activated carbonyl reductase that functions in the metabolism of endogenous and xenobiotic carbonyl compounds.

Carbonyl reduction pathway in hepatic in vitro metabolism of anthracyclines: Impact of structure on biotransformation rate

Carbonyl reduction biotransformation pathway of anthracyclines (doxorubicin, daunorubicin) is a significant process, associated with drug metabolism and elimination. However, it also plays a pivotal role in anthracyclines-induced cardiotoxicity and cancer resistance. Herein, carbonyl reduction of eight anthracyclines, at in vivo relevant concentrations (20 μM), was studied in human liver cytosol, to describe the relationship between their structure and metabolism. Significant differences of intrinsic clearance between anthracyclines, ranging from 0,62-74,9 μL/min/mg were found and associated with data from in silico analyses, considering their binding in active sites of the main anthracyclines-reducing enzymes: carbonyl reductase 1 (CBR1) and aldo-keto reductase 1C3 (AKR1C3). Partial atomic charges of carbonyl oxygen atom were also determined and considered as a factor associated with reaction rate. Structural features, including presence or absence of side-chain hydroxy group, a configuration of sugar chain hydroxy group, and tetracyclic rings substitution, affecting anthracyclines susceptibility for carbonyl reduction were identified.

Dehydrogenase/reductase activity of human carbonyl reductase 1 with NADP(H) acting as a prosthetic group

Carbonyl reductase 1 (CBR1) is an NADP-dependent enzyme that exerts a detoxifying role, which catalyses the transformation of carbonyl-containing compounds. The ability of CBR1 to act on adducts between glutathione and lipid peroxidation derived aldehydes has recently been reported. In the present study, exploiting mass spectrometry and fluorescence spectroscopy, evidence is shown that CBR1 is able to retain NADP(H) at the active site even after extensive dialysis, and that this retention may also occur when the enzyme is performing catalysis. This property, together with the multi-substrate specificity of CBR1 in both directions of red/ox reactions, generates inter-conversion red/ox cycles. This particular feature of CBR1, in the case of the transformation of 3-glutathionyl, 4-hydroxynonanal (GSHNE), which is a key substrate of the enzyme in detoxification, supports the disproportionation reaction of GSHNE without any apparent exchange of the cofactor with the solution. The importance of the cofactor as a prosthetic group for other dehydrogenases exerting a detoxification role is discussed.

Synergistic anticancer activity of doxorubicin and piperlongumine on DU-145 prostate cancer cells - The involvement of carbonyl reductase 1 inhibition

One of the causes of therapeutic failure of chemotherapy is cancer cell resistance. In the case of anthracyclines, many resistance mechanisms have been described. One of them assumes the role of carbonyl reductase 1 (CBR1), a cytosolic enzyme that is responsible for the biotransformation process of anthracyclines to less active, undesirable metabolites. Therefore, CBR1 inhibitors are considered for use as a chemosensitizing agents. In the present study, piperlongumine (PL), a Piper longum L. alkaloid that has previously been described as a CBR1 inhibitor, was investigated for its chemosensitizing properties in co-treatment with doxorubicin (DOX). The biotransformation process of DOX in the presence of PL was tracked using human cytosol fraction and LC-MS, then a molecular modeling study was conducted to predict the interaction of PL with the active site of the CBR1. The biological interaction between DOX and PL was investigated using DU-145 prostate cancer cells. Cytotoxic and antiproliferative properties of DOX and PL were examined, and the type and potency of interaction was quantified by Combination Index. The mechanism of the cell death induced by the agents was investigated by flow cytometry and the anti-invasive properties of the drugs were determined by monitoring the movement of individual cells. PL showed dose-dependent inhibition of DOX metabolism in cytosol, which resulted in less doxorubicinol (DOXol) metabolite being formed. The possible mechanism of CBR1 inhibition was explained through molecular modeling studies by prediction of PL's binding mode in the active site of the enzyme's crystal structure-based model. DOX and PL showed a synergistic antiproliferative and proapoptotic effect on cancer cells. Significant anti-invasive properties of the combination of DOX and PL were found, but when the drugs were used separately they did not alter the cancer cells' motility. Cell motility inhibition was accompanied by significant changes in cytoskeleton architecture. DOX and PL used in co-treatment showed significant synergistic anticancer properties. Inhibition of DOX metabolism by PL was found to be a mechanism that was likely to be responsible for the observed interaction.

Carbonyl reductases from Daphnia are regulated by redox cycling compounds

Oxidative stress is a major source of reactive carbonyl compounds that can damage cellular macromolecules, leading to so-called carbonyl stress. Aside from endogenously formed carbonyls, including highly reactive short-chain aldehydes and diketones, air pollutants derived from diesel exhaust like 9,10-phenanthrenequinone (PQ) can amplify oxidative stress by redox cycling, causing tissue damage. Carbonyl reductases (CRs), which are inducible in response to ROS, represent a fundamental enzymatic defense mechanism against oxidative stress. While commonly two carbonyl reductases (CBR1 and CBR3) are found in mammalian genomes, invertebrate model organisms like Drosophila melanogaster express no CR but a functional homolog to human CBR1, termed sniffer. The microcrustacean Daphnia is an ideal model organism to investigate the function of CRs because of its unique equipment with even four copies of the CR gene (CR1, CR2, CR3, CR4) in addition to one sniffer gene. Cloning and catalytic characterization of two carbonyl reductases CR1 and CR3 from D. magna and D. pulex arenata revealed that both proteins reductively metabolize aromatic dicarbonyls (e.g., menadione, PQ) and aliphatic α-diketones (e.g., 2,3-hexanedione), while sugar-derived aldehydes (methylglyoxal, glyoxal) and lipid peroxidation products such as acrolein and butanal were poor substrates, indicating no physiological function in the metabolism of short-chain aldehydes. Treatment of D. magna with redox cyclers like menadione and the pesticide paraquat led to an upregulation of CR1 and CR3 mRNA, suggesting a role in oxidative stress defense. Further studies are needed to investigate their potential to serve as novel biomarkers for oxidative stress in Daphnia.

Virtual screening applications in short-chain dehydrogenase/reductase research

Several members of the short-chain dehydrogenase/reductase (SDR) enzyme family play fundamental roles in adrenal and gonadal steroidogenesis as well as in the metabolism of steroids, oxysterols, bile acids, and retinoids in peripheral tissues, thereby controlling the local activation of their cognate receptors. Some of these SDRs are considered as promising therapeutic targets, for example to treat estrogen-/androgen-dependent and corticosteroid-related diseases, whereas others are considered as anti-targets as their inhibition may lead to disturbances of endocrine functions, thereby contributing to the development and progression of diseases. Nevertheless, the physiological functions of about half of all SDR members are still unknown. In this respect, in silico tools are highly valuable in drug discovery for lead molecule identification, in toxicology screenings to facilitate the identification of hazardous chemicals, and in fundamental research for substrate identification and enzyme characterization. Regarding SDRs, computational methods have been employed for a variety of applications including drug discovery, enzyme characterization and substrate identification, as well as identification of potential endocrine disrupting chemicals (EDC). This review provides an overview of the efforts undertaken in the field of virtual screening supported identification of bioactive molecules in SDR research. In addition, it presents an outlook and addresses the opportunities and limitations of computational modeling and in vitro validation methods.

Human carbonyl reductase 1 participating in intestinal first-pass drug metabolism is inhibited by fatty acids and acyl-CoAs

Human carbonyl reductase 1 (CBR1), a member of the short-chain dehydrogenase/reductase (SDR) superfamily, reduces a variety of carbonyl compounds including endogenous isatin, prostaglandin E₂ and 4-oxo-2-nonenal. It is also a major non-cytochrome P450 enzyme in the phase I metabolism of carbonyl-containing drugs, and is highly expressed in the intestine. In this study, we found that long-chain fatty acids and their CoA ester derivatives inhibit CBR1. Among saturated fatty acids, myristic, palmitic and stearic acids were inhibitory, and stearic acid was the most potent (IC₅₀ 9µM). Unsaturated fatty acids (oleic, elaidic, γ-linolenic and docosahexaenoic acids) and acyl-CoAs (palmitoyl-, stearoyl- and oleoyl-CoAs) were more potent inhibitors (IC₅₀ 1.0-2.5µM), and showed high inhibitory selectivity to CBR1 over its isozyme CBR3 and other SDR superfamily enzymes (DCXR and DHRS4) with CBR activity. The inhibition by these fatty acids and acyl-CoAs was competitive with respect to the substrate, showing the K_i values of 0.49-1.2µM. Site-directed mutagenesis of the substrate-binding residues of CBR1 suggested that the interactions between the fatty acyl chain and the enzyme's Met141 and Trp229 are important for the inhibitory selectivity. We also examined CBR1 inhibition by oleic acid in cellular levels: The fatty acid effectively inhibited CBR1-mediated 4-oxo-2-nonenal metabolism in colon cancer DLD1 cells and increased sensitivity to doxorubicin in the drug-resistant gastric cancer MKN45 cells that highly express CBR1. The results suggest a possible new food-drug interaction through inhibition of CBR1-mediated intestinal first-pass drug metabolism by dietary fatty acids.

Human carbonyl reductase 1 as efficient catalyst for the reduction of glutathionylated aldehydes derived from lipid peroxidation

Human recombinant carbonyl reductase 1 (E.C. 1.1.1.184, hCBR1) is shown to efficiently act as aldehyde reductase on glutathionylated alkanals, namely 3-glutathionyl-4-hydroxynonanal (GSHNE), 3-glutathionyl-nonanal, 3-glutathionyl-hexanal and 3-glutathionyl-propanal. The presence of the glutathionyl moiety appears as a necessary requirement for the susceptibility of these compounds to the NADPH-dependent reduction by hCBR1. In fact the corresponding alkanals and alkenals, and the cysteinyl and γ-glutamyl-cysteinyl alkanals adducts were either ineffective or very poorly active as CBR1 substrates. Mass spectrometry analysis reveals the ability of hCBR1 to reduce GSHNE to the corresponding GS-dihydroxynonane (GSDHN) and at the same time to catalyze the oxidation of the hemiacetal form of GSHNE, generating the 3-glutathionylnonanoic-δ-lactone. These data are indicative of the ability of the enzyme to catalyze a disproportion reaction of the substrate through the redox recycle of the pyridine cofactor. A rationale for the observed preferential activity of hCBR1 on different GSHNE diastereoisomers is given by molecular modelling. These results evidence the potential of hCBR1 acting on GSHNE to accomplish a dual role, both in terms of HNE detoxification and, through the production of GSDHN, in terms of involvement into the signalling cascade of the cellular inflammatory response.

Environmentally Robust Rhodamine Reporters for Probe-based Cellular Detection of the Cancer-linked Oxidoreductase hNQO1

We successfully synthesized a fluorescent probe capable of detecting the cancer-associated

NAD(P)H

quinoneoxidoreductase isozyme-1 within human cells, based on results from an investigation of the stability of various rhodamines and seminaphthorhodamines toward the biological reductant NADH, present at ∼100-200 μM within cells. While rhodamines are generally known for their chemical stability, we observe that NADH causes significant and sometimes rapid modification of numerous rhodamine analogues, including those oftentimes used in imaging applications. Results from mechanistic studies lead us to rule out a radical-based reduction pathway, suggesting rhodamine reduction by NADH proceeds by a hydride transfer process to yield the reduced leuco form of the rhodamine and oxidized NAD(+). A relationship between the structural features of the rhodamines and their reactivity with NADH is observed. Rhodamines with increased alkylation on the N3- and N6-nitrogens, as well as the xanthene core, react the least with NADH; whereas, nonalkylated variants or analogues with electron-withdrawing substituents have the fastest rates of reaction. These outcomes allowed us to judiciously construct a seminaphthorhodamine-based, turn-on fluorescent probe that is capable of selectively detecting the cancer-associated, NADH-dependent enzyme

NAD(P)H

quinoneoxidoreductase isozyme-1 in human cancer cells, without the issue of NADH-induced deactivation of the seminaphthorhodamine reporter.

Inhibition of human anthracycline reductases by emodin - A possible remedy for anthracycline resistance

The clinical application of anthracyclines, like daunorubicin and doxorubicin, is limited by two factors: dose-related cardiotoxicity and drug resistance. Both have been linked to reductive metabolism of the parent drug to their metabolites daunorubicinol and doxorubicinol, respectively. These metabolites show significantly less anti-neoplastic properties as their parent drugs and accumulate in cardiac tissue leading to chronic cardiotoxicity. Therefore, we aimed to identify novel and potent natural inhibitors for anthracycline reductases, which enhance the anticancer effect of anthracyclines by preventing the development of anthracycline resistance. Human enzymes responsible for the reductive metabolism of daunorubicin were tested for their sensitivity towards anthrachinones, in particular emodin and anthraflavic acid. Intense inhibition kinetic data for the most effective daunorubicin reductases, including IC50- and Ki-values, the mode of inhibition, as well as molecular docking, were compiled. Subsequently, a cytotoxicity profile and the ability of emodin to reverse daunorubicin resistance were determined using multiresistant A549 lung cancer and HepG2 liver cancer cells. Emodin potently inhibited the four main human daunorubicin reductases in vitro. Further, we could demonstrate that emodin is able to synergistically sensitize human cancer cells towards daunorubicin at clinically relevant concentrations. Therefore, emodin may yield the potential to enhance the therapeutic effectiveness of anthracyclines by preventing anthracycline resistance via inhibition of the anthracycline reductases. In symphony with its known pharmacological properties, emodin might be a compound of particular interest in the management of anthracycline chemotherapy efficacy and their adverse effects.

Identification of a functional antioxidant responsive element in the promoter of the Chinese hamster carbonyl reductase 3 (Chcr3) gene

CHCR3, a member of the short-chain dehydrogenase/reductase superfamily, is a carbonyl reductase 3 enzyme in Chinese hamsters. Carbonyl reductase 3 in humans has been believed to involve the metabolism and/or pharmacokinetics of anthracycline drugs, and the mechanism underlying the gene regulation has been investigated. In this study, the nucleotide sequence of the Chcr3 promoter was originally determined, and its promoter activity was characterised. The proximal promoter region is TATA-less and GC-rich, similar to the promoter region of human carbonyl reductase 3. Cobalt stimulated the transcriptional activity of the Chcr3 gene. The results of a luciferase gene reporter assay demonstrated that cobalt-induced stimulation required an antioxidant responsive element. Forced expression of Nrf2, the transcription factor that binds to antioxidant responsive elements, enhanced the transcriptional activity of the Chcr3 gene. These results suggest that cobalt induces the expression of the Chcr3 gene via the Nrf2-antioxidant responsive element pathway.

Carbonyl-reducing enzymes as targets of a drug-immobilised affinity carrier

Proteins, peptides and nucleic acids are commonly isolated and purified in almost all bioscience laboratories. Methods based on molecular recognition are currently the most powerful tool in separation processes due to their selectivity and recovery. The aim of this study was to prove the versatility and the ability of an affinity carrier containing the immobilised ligand oracin (previously developed by our workgroup) to selectively bind carbonyl-reducing enzymes. These enzymes play an important role in metabolic pathways of various endogenic compounds and xenobiotics. Many important drugs, such as doxorubicin, daunorubicin, haloperidol and the model anticancer drug oracin, are metabolised by carbonyl-reducing enzymes. The functionality of the presented carrier was demonstrated with pure recombinant enzymes (AKR1A1, AKR1B1, AKR1B10, AKR1C1, AKR1C2, AKR1C3, AKR1C4, CBR1 and CBR3) as well as with two model biological samples (cell extract from genetically modified Escherichia coli and pre-purified human liver cytosol). Enzymes that show an affinity toward oracin were efficiently captured, gently eluted using 150 mM ammonium hydroxide and subsequently identified by MS. The method is highly selective and robust and may be applied to the purification and identification of various carbonyl-reducing enzymes from any biological sample.

Synthesis of 8-hydroxy-2-iminochromene derivatives as selective and potent inhibitors of human carbonyl reductase 1

Human carbonyl reductase 1 (CBR1), a member of the short-chain dehydrogenase/reductase superfamily, reduces anthracycline anticancer drugs to their less potent anticancer C-13 hydroxy metabolites, which are linked with pathogenesis of cardiotoxicity, a side effect of the drugs.

Curcumin is a tight-binding inhibitor of the most efficient human daunorubicin reductase--Carbonyl reductase 1

Curcumin is a major component of the plant Curcuma longa L. It is traditionally used as a spice and coloring in foods and is an important ingredient in curry. Curcuminoids have anti-oxidant and anti-inflammatory properties and gained increasing attention as potential neuroprotective and cancer preventive compounds. In the present study, we report that curcumin is a potent tight-binding inhibitor of human carbonyl reductase 1 (CBR1, Ki=223 nM). Curcumin acts as a non-competitive inhibitor with respect to the substrate 2,3-hexandione as revealed by plotting IC50-values against various substrate concentrations and most likely as a competitive inhibitor with respect to NADPH. Molecular modeling supports the finding that curcumin occupies the cofactor binding site of CBR1. Interestingly, CBR1 is one of the most effective human reductases in converting the anthracycline anti-tumor drug daunorubicin to daunorubicinol. The secondary alcohol metabolite daunorubicinol has significantly reduced anti-tumor activity and shows increased cardiotoxicity, thereby limiting the clinical use of daunorubicin. Thus, inhibition of CBR1 may increase the efficacy of daunorubicin in cancer tissue and simultaneously decrease its cardiotoxicity. Western-blots demonstrated basal expression of CBR1 in several cell lines. Significantly less daunorubicin reduction was detected after incubating A549 cell lysates with increasing concentrations of curcumin (up to 60% less with 50 μM curcumin), suggesting a beneficial effect in the co-treatment of anthracycline anti-tumor drugs together with curcumin.

Hepatic gene expression profiling in Nrf2 knockout mice after long-term high-fat diet-induced obesity

Introduction. The transcription factor NFE2-related factor 2 (Nrf2) is a central regulator of antioxidant and detoxification gene expression in response to electrophilic or oxidative stress. Nrf2 has recently been shown to cross-talk with metabolic pathways, and its gene deletion protected mice from high-fat-diet-(HFD-) induced obesity and insulin resistance. This study aimed to identify potential Nrf2-regulated genes of metabolic interest by comparing gene expression profiles of livers of wild-type (WT) versus Nrf2 knockout (Nrf2-KO) mice after a long-term HFD. Methods. WT and Nrf2-KO mice were fed an HFD for 180 days; total RNA was prepared from liver and used for microarray analysis and quantitative real-time RT-PCR (qRT-PCR). Results. The microarray analysis identified 601 genes that were differentially expressed between WT and Nrf2-KO mice after long-term HFD. Selected genes, including ones known to be involved in metabolic regulation, were prioritized for verification by qRT-PCR: Cyp7a1 and Fabp5 were significantly overexpressed in Nrf2-KO mice; in contrast, Car, Cyp2b10, Lipocalin 13, Aquaporin 8, Cbr3, Me1, and Nqo1 were significantly underexpressed in Nrf2-KO mice. Conclusion. Transcriptome profiling after HFD-induced obesity confirms that Nrf2 is implicated in liver metabolic gene networks. The specific genes identified here may provide insights into Nrf2-dependent mechanisms of metabolic regulation.

Characterisation of the metabolites of 1,8-cineole transferred into human milk: concentrations and ratio of enantiomers

1,8-Cineole is a widely distributed odorant that also shows physiological effects, but whose human metabolism has hitherto not been extensively investigated. The aim of the present study was, thus, to characterise the metabolites of 1,8-cineole, identified previously in human milk, after the oral intake of 100 mg of this substance. Special emphasis was placed on the enantiomeric composition of the metabolites since these data may provide important insights into potential biotransformation pathways, as well as potential biological activities of these substances, for example on the breastfed child. The volatile fraction of the human milk samples was therefore isolated via Solvent Assisted Flavour Evaporation (SAFE) and subjected to gas chromatography-mass spectrometry (GC-MS). The absolute concentrations of each metabolite were determined by matrix calibration with an internal standard, and the ratios of enantiomers were analysed on chiral capillaries. The concentrations varied over a broad range, from traces in the upper ng/kg region up to 40 µg/kg milk, with the exception of the main metabolite α2-hydroxy-1,8-cineole that showed concentrations of 100-250 µg/kg. Also, large inter- and intra-individual variations were recorded for the enantiomers, with nearly enantiomerically pure α2-hydroxy- and 3-oxo-1,8-cineole, while all other metabolites showed ratios of ~30:70 to 80:20.

Regulation of human carbonyl reductase 1 (CBR1, SDR21C1) gene by transcription factor Nrf2

Monomeric carbonyl reductase 1 (CBR1, SDR21C1) is a member of the short-chain dehydrogenase/reductase superfamily and is involved in the metabolism of anthracycline anti-cancer drugs, prostaglandins, and isatin, which is an endogenous inhibitor of monoamine oxidases. Additionally, cancer progression may be partly regulated by CBR1. In the present study, we screened more than 10 drugs for the induction of the human CBR1 gene to investigate its regulation. Of the drugs, butylated hydroxyanisole (BHA) was found to be an inducer. BHA induced the mRNA and protein expression of CBR1 in hepatoma HepG2 cells. In a luciferase reporter gene assay, the promoter region between -2062 bp and the transcription start site of CBR1 was also activated by BHA. The transcription factor Nrf2 is known to be activated by BHA. There are 2 anti-oxidant responsive elements (ARE) that are bound by Nrf2 in this region. Mutation analyses revealed that one of the AREs participates in the gene regulation of CBR1 by Nrf2. Electrophoretic mobility shift assay revealed that Nrf2 binds the site. Moreover, to determine whether the functional ARE of CBR1 is conserved with the promoter region of homologues in other species, the nucleotide sequences of the functional AREs of the Chcr1 and Chcr2 genes, which are the Chinese hamster homologues of CBR1, were determined. The region has 2 AREs, and these genes were also induced by the forced expression of Nrf2 (cotransfection of pNrf2) in the luciferase reporter gene assay. In conclusion, Nrf2 is a novel transcriptional regulator of CBR1 genes in humans and the Chinese hamster. Because the regulation of CBR1 appears to be important for diseases, the induction of CBR1 by Nrf2 may be a therapeutic target.

Effect of graded Nrf2 activation on phase-I and -II drug metabolizing enzymes and transporters in mouse liver

Nuclear factor erythroid 2-related factor 2 (Nrf2) is a transcription factor that induces a battery of cytoprotective genes in response to oxidative/electrophilic stress. Kelch-like ECH associating protein 1 (Keap1) sequesters Nrf2 in the cytosol. The purpose of this study was to investigate the role of Nrf2 in regulating the mRNA of genes encoding drug metabolizing enzymes and xenobiotic transporters. Microarray analysis was performed in livers of Nrf2-null, wild-type, Keap1-knockdown mice with increased Nrf2 activation, and Keap1-hepatocyte knockout mice with maximum Nrf2 activation. In general, Nrf2 did not have a marked effect on uptake transporters, but the mRNAs of organic anion transporting polypeptide 1a1, sodium taurocholate cotransporting polypeptide, and organic anion transporter 2 were decreased with Nrf2 activation. The effect of Nrf2 on cytochrome P450 (Cyp) genes was minimal, with only Cyp2a5, Cyp2c50, Cyp2c54, and Cyp2g1 increased, and Cyp2u1 decreased with enhanced Nrf2 activation. However, Nrf2 increased mRNA of many other phase-I enzymes, such as aldo-keto reductases, carbonyl reductases, and aldehyde dehydrogenase 1. Many genes involved in phase-II drug metabolism were induced by Nrf2, including glutathione S-transferases, UDP- glucuronosyltransferases, and UDP-glucuronic acid synthesis enzymes. Efflux transporters, such as multidrug resistance-associated proteins, breast cancer resistant protein, as well as ATP-binding cassette g5 and g8 were induced by Nrf2. In conclusion, Nrf2 markedly alters hepatic mRNA of a large number of drug metabolizing enzymes and xenobiotic transporters, and thus Nrf2 plays a central role in xenobiotic metabolism and detoxification.

Thrombospondin-1 type 1 repeats in a model of inflammatory bowel disease: transcript profile and therapeutic effects

Thrombospondin-1 (TSP-1) is a matricellular protein with regulatory functions in inflammation and cancer. The type 1 repeats (TSR) domains of TSP-1 have been shown to interact with a wide range of proteins that result in the anti-angiogenic and anti-tumor properties of TSP-1. To ascertain possible functions and evaluate potential therapeutic effects of TSRs in inflammatory bowel disease, we conducted clinical, histological and microarray analyses on a mouse model of induced colitis. We used dextran sulfate sodium (DSS) to induce colitis in wild-type (WT) mice for 7 days. Simultaneously, mice were injected with either saline or one form of TSP-1 derived recombinant proteins, containing either (1) the three type 1 repeats of the TSP-1 (3TSR), (2) the second type 1 repeat (TSR2), or (3) TSR2 with the RFK sequence (TSR2+RFK). Total RNA isolated from the mice colons were processed and hybridized to mouse arrays. Array data were validated by real-time qPCR and immunohistochemistry. Histological and disease indices reveal that the mice treated with the TSRs show different patterns of leukocytic infiltration and that 3TSR treatment was the most effective in decreasing inflammation in DSS-induced colitis. Transcriptional profiling revealed differentially expressed (DE) genes, with the 3TSR-treated mice showing the least deviation from the WT-water controls. In conclusion, this study shows that 3TSR treatment is effective in attenuating the inflammatory response to DSS injury. In addition, the transcriptomics work unveils novel genetic data that suggest beneficial application of the TSR domains in inflammatory bowel disease.

A conserved antioxidant response element (ARE) in the promoter of human carbonyl reductase 3 (CBR3) mediates induction by the master redox switch Nrf2

Carbonyl reductase activity catalyzes the two electron reduction of several endogenous and exogenous carbonyl substrates. Recent data indicate that the expression of human carbonyl reductase 3 (CBR3) is regulated by the master redox switch Nrf2. Nrf2 binds to conserved antioxidant response elements (AREs) in the promoters of target genes. The presence of functional AREs in the CBR3 promoter has not yet been reported. In this study, experiments with reporter constructs showed that the prototypical Nrf2 activator tert-butyl hydroquinone (t-BHQ) induces CBR3 promoter activity in cultures of HepG2 (2.7-fold; p<0.05) and MCF-7 cells (22-fold; p<0.01). Computational searches identified a conserved ARE in the distal CBR3 promoter region ((-2698)ARE). Deletion of this ARE from a 4212-bp CBR3 promoter construct impacted basal promoter activity and induction of promoter activity in response to treatment with t-BHQ. Deletion of (-2698)ARE also impacted the induction of CBR3 promoter activity in cells overexpressing Nrf2. Electrophoretic mobility shift assays (EMSA) demonstrated increased binding of specific protein complexes to (-2698)ARE in nuclear extracts from t-BHQ treated cells. The presence of Nrf2 in the specific nuclear protein-(-2698)ARE complexes was evidenced in EMSA experiments with anti-Nrf2 antibodies. These data suggest that the distal (-2698)ARE mediates the induction of human CBR3 in response to prototypical activators of Nrf2.

Sensitive DsRed fluorescence-based reporter cell systems for genotoxicity and oxidative stress assessment

Various in vitro test systems have been developed for genotoxic risk assessment in early drug development. However, these genotoxicity tests often show limited specificity, and provide limited insights into the mode of toxicity of the tested compounds. To identify genes that could serve as specific biomarkers for genotoxicity or oxidative stress, we exposed mouse embryonic stem (ES) cells to various genotoxic and oxidative stress-inducing compounds and performed genome-wide expression profiling. Differentially expressed genes were classified based on the fold-change of expression and their specificity for either genotoxic or oxidative stress. Promoter regions of four selected genes (Ephx1, Btg2, Cbr3 and Perp) were fused to a DsRed fluorescent reporter gene and stably integrated in mouse ES cells. Established stable reporter cell lines displayed significant induction of DsRed expression upon exposure to different classes of genotoxic and oxidative stress-inducing compounds. In contrast, exposure to non-genotoxic carcinogenic compounds did not induce DsRed expression even at cytotoxic doses. Expression of the Cbr3-DsRed reporter was more responsive to compounds that induce oxidative stress while the other three DsRed reporters reacted more specific to direct-acting genotoxic agents. Therefore, the differential response of the Btg2- and Cbr3-DsRed reporters can serve as indicator for the main action mechanism of genotoxic and oxidative stress-inducing compounds. In addition, we provide evidence that inhibition of DNA replication results in preferential activation of the Btg2-DsRed genotoxicity reporter. In conclusion, we have generated sensitive mouse ES cell reporter systems that allow detection of genotoxic and oxidative stress-inducing properties of chemical compounds and can be used in high-throughput assays.

Atomic structure of salutaridine reductase from the opium poppy (Papaver somniferum)

The opium poppy (Papaver somniferum L.) is one of the oldest known medicinal plants. In the biosynthetic pathway for morphine and codeine, salutaridine is reduced to salutaridinol by salutaridine reductase (SalR; EC 1.1.1.248) using NADPH as coenzyme. Here, we report the atomic structure of SalR to a resolution of ∼1.9 Å in the presence of NADPH. The core structure is highly homologous to other members of the short chain dehydrogenase/reductase family. The major difference is that the nicotinamide moiety and the substrate-binding pocket are covered by a loop (residues 265-279), on top of which lies a large "flap"-like domain (residues 105-140). This configuration appears to be a combination of the two common structural themes found in other members of the short chain dehydrogenase/reductase family. Previous modeling studies suggested that substrate inhibition is due to mutually exclusive productive and nonproductive modes of substrate binding in the active site. This model was tested via site-directed mutagenesis, and a number of these mutations abrogated substrate inhibition. However, the atomic structure of SalR shows that these mutated residues are instead distributed over a wide area of the enzyme, and many are not in the active site. To explain how residues distal to the active site might affect catalysis, a model is presented whereby SalR may undergo significant conformational changes during catalytic turnover.

Studies on reduction of S-nitrosoglutathione by human carbonyl reductases 1 and 3

Human carbonyl reductases 1 and 3 (CBR1 and CBR3) are monomeric NADPH-dependent enzymes of the short-chain dehydrogenase/reductase superfamily. Despite 72% identity in primary structure they exhibit substantial differences in substrate specificity. Recently, the endogenous low molecular weight S-nitrosothiol S-nitrosoglutathione (GSNO) has been added to the broad substrate spectrum of CBR1. The current study initially addressed whether CBR3 could equally reduce GSNO which was not the case. Neither the introduction of residues which contribute to glutathione binding in CBR1, i.e. K106Q and S97V/D98A, nor the exchange C143S, which prevents a theoretical disulfide bond with C227 in CBR3, could engender activity towards GSNO. However, exchanging amino acids 236-244 in CBR3 to correspond to CBR1 was sufficient to engender catalytic activity towards GSNO. Catalytic efficiency was further improved by the exchanges Q142M, C143S, P230W and H270S. Hence, the same residues previously reported as important for reduction of carbonyl compounds appear to be key to CBR1-mediated reduction of GSNO. Furthermore, for CBR1-mediated reduction of GSNO, considerable substrate inhibition at concentrations >5 K(m) was observed. Treatment of CBR1 with GSNO followed by removal of low molecular weight compounds decreased the GSNO reducing activity, suggesting a covalent modification. Treatment with dithiothreitol, but not with ascorbic acid, could rescue the activity, indicating S-glutathionylation rather than S-nitrosation as the underlying mechanism. As C227 has previously been identified as the reactive cysteine in CBR1, the variant CBR1 C227S was generated, which, in comparison to the wild-type protein, displayed a similar k(cat), but a 30-fold higher K(m), and did not show substrate inhibition. Collectively, the results clearly argue for a physiological role of CBR1, but not for CBR3, in GSNO reduction and thus ultimately in regulation of NO signaling. Furthermore, at higher concentrations, GSNO appears to work as a suicide inhibitor for CBR1, probably through glutathionylation of C227.

Crystallization and preliminary X-ray analysis of SDR-type pyridoxal dehydrogenase from Mesorhizobium loti

Pyridoxal 4-dehydrogenase from Mesorhizobium loti MAFF303099 was overexpressed in Escherichia coli. The recombinant selenomethionine-substituted enzyme was purified and crystallized by the sitting-drop vapour-diffusion method using PEG 4000 as precipitant. Crystals grew in the presence of 0.45 mM NAD(+). The crystals diffracted to 2.9 A resolution and belonged to the monoclinic space group P2(1), with unit-cell parameters a = 86.20, b = 51.11, c = 91.73 A, beta = 89.36 degrees. The calculated V(M) values suggested that the asymmetric unit contained four molecules.