New enzymatic assay for the AKR1C enzymes

The imbalance in expression of the human aldo-keto reductases AKR1C1-AKR1C3 is related to different hormone dependent and independent cancers and some other diseases. The AKR1C1-3 enzymes thus represent emerging targets for the development of new drugs. Currently, various enzymatic assays are used in the search for AKR1C inhibitors, and consequently the results of different research groups are not necessarily comparable. During our recent search for AKR1C inhibitors, we found a cyclopentanol derivative (2-(4-chlorobenzylidene)cyclopentanol, CBCP-ol) and its respective cyclopentanone counterpart (2-(4-chlorobenzylidene)cyclopentanone, CBCP-one) that acted as AKR1C substrates. We determined the kinetic parameters KM, kcat and kcat/KM for oxidation of CBCP-ol and reduction of CBCP-one by AKR1C enzymes in the presence of NAD(+)/NADP(+) and NADH/NADPH, respectively. The catalytic efficiencies for the oxidation of CBCP-ol in the presence of NAD(+) or NADP(+) were in general higher when compared to the catalytic efficiencies for reduction of CBCP-one in the presence of NADH or NADPH. When NADPH was used, as compared to NADH, the reductions of CBCP-one by AKR1C1, AKR1C2 and AKR1C3 were 14-, 51- and 31-fold more efficient, respectively. When comparing to oxidations of the well-known artificial substrates, 1-acenaphthenol and S-tetralol, we observed similar catalytic efficiencies as for CBCP-ol oxidation with NAD(+) and NADP(+). The comparison of CBCP-one reduction with NADPH to reductions of physiological substrates revealed in general higher efficiencies, except for reduction of 9-cis-retinaldehyde by AKR1C3. This NADPH-dependent reduction of CBCP-one was then used to re-evaluate inhibitory potencies of the known inhibitors of the target AKR1C3 and the anti-target AKR1C2, medroxyprogesterone acetate and ursodeoxycholic acid, respectively, showing Ki constants similar to the reported values. Our data thus confirm that the new enzymatic assays with two cyclopentane substrates CBP-ol and CBP-one, and especially reduction of CBCP-one with NADPH, are appropriate for the evaluation of AKR1C inhibitors.

Single-nucleotide polymorphisms in aldo-keto and carbonyl reductase genes are not associated with acute cardiotoxicity after daunorubicin chemotherapy

BACKGROUND

Evidence suggests that interpatient variability in anthracycline metabolic rate may contribute to the cardiotoxicity associated with anthracycline-based chemotherapy. Therefore, polymorphisms in the anthracycline metabolizing enzymes have been proposed as potential biomarkers of anthracycline-induced cardiotoxicity (AIC).

METHODS

We have previously shown that 13 of the naturally occurring nonsynonymous single-nucleotide polymorphisms (nsSNP) in the aldo-keto reductases (AKR) and carbonyl reductases (CBR) reduce anthracycline metabolic rate in vitro. Here, we test these SNPs individually and jointly for association with daunorubicin-induced cardiotoxicity in patients with acute myeloid leukemia (AML).

RESULTS

Five of the 13 nsSNPs exhibiting an in vitro effect on anthracycline metabolism were detected among the 185 patients with AML. No association was found between the SNPs and daunorubicin-induced cardiotoxicity in either individual or joint effect analyses.

CONCLUSIONS

Despite the shown in vitro effect of nsSNPs in reductase genes on anthracycline metabolic rate, on their own these SNPs do not explain enough variability in cardiotoxicity to be useful markers of this adverse event.

IMPACT

The results of this study provide important information for biomarker studies on side effects of anthracycline chemotherapy.

[Purification of recombinant AKR7A5 protein and measurement of substrate specificity of AKR7A5 towards naphthoquinone and its derivatives]

AIM

To investigate the substrate specificity of mouse aldo-keto reductase AKR7A5 protein towards naphthoquinone and its derivatives.

METHODS

The recombinant His-tagged AKR7A5 fusion protein in E.coli BL21pLysS cell strain was induced by IPTG and purified using FPLC system through HiTrap affinity column. The purified recombinant AKR7A5 protein was confirmed by SDS-PAGE and Western blotting. AKR enzyme assay was applied to measure the substrate specificity of recombinant AKR7A5 protein towards naphthoquinone and its derivatives.

RESULTS

Recombinant His-AKR7A5 was successfully purified as confirmed by SDS-PAGE and Western blotting. AKR enzyme assay indicated that the recombinant AKR7A5 protein exhibited mild substrate specificity towards lawsone and low specificity towards juglone and vitamine K3, but no activity towards 1, 4-naphthoquinone.

CONCLUSION

AKR7A5 has selective substrate specificity towards naphthoquinone derivatives, suggesting that the aldo-keto reductase could play an important role in metabolism of certain naphthoquinone derivatives.

Identification, cloning and regulation of cDNA encoding aldo-keto reductase 1B7 in the adrenal gland of two Saharan rodents Meriones libycus (Libyan jird) and Gerbillus gerbillus (gerbil)

Aldo-Keto Reductase 1B7 (AKR1B7) is a mouse aldose reductase-like protein with two major sites of expression, the vas deferens and the adrenal cortex. In the adrenal cortex, Akr1b7 is an adrenocorticotropin (ACTH)-responsive-gene whose product scavenges harmful byproducts of steroidogenesis and limits stress response through the biosynthesis of prostaglandin F2α. The purpose of the present study was to explore the possible expression of AKR1B7 in the adrenal glands of two saharan rodents, Libyan jird and Lesser Egyptian gerbil. Western blot analyses demonstrated that a protein related to murine/rat AKR1B7 was highly expressed in adrenals and absent from vas deferens of both saharan species. Based on conserved sequences between mouse and rat, full length cDNA were cloned and sequenced in both species while hormonal regulation and tissue localization were explored in Libyan jird. Both cDNA encoded the expected 316 amino acids protein typical of AKR1B subfamily and contained the highly conserved catalytic tetrad consisting in Asp-44, Tyr-49, Lys-78 and His-111 residues. The deduced proteins shared higher identities with aldose reductase-like, i.e. AKR1B7 (86-94%), AKR1B8 and AKR1B10 (83-86%) than with aldose reductase group, i.e. AKR1B1 and AKR1B3 (70%). Phylogenetic analysis showed that the Libyan jird and gerbil enzymes were more closely related to murine and rat AKR1B7 than to the other AKR1B members. Northern blot analyses of total RNA from Libyan jird adrenals showed a single mRNA transcript of 1.4 kb whose expression was dependent on circulating ACTH levels. In conclusion, we demonstrate here that adrenal glands of Libyan jird and gerbil express both an ortholog of the murine/rat Akr1b7 gene and that ACTH-responsiveness is at least conserved in Libyan jird.

Cadmium-induced up-regulation of aldo-keto reductase 1C3 expression in human nasal septum carcinoma RPMI-2650 cells: Involvement of reactive oxygen species and phosphatidylinositol 3-kinase/Akt

Cadmium is a well-known toxic metal and occupational exposure to it is associated with lung cancer. In probing the possible non-genotoxic molecular targets of cadmium-induced nasal toxicity, we performed an mRNA differential display analysis for cadmium-treated human nasal septum carcinoma RPMI-2650 cells. Cadmium (≥ 0.5 μM) inhibited the cell proliferation. The intracellular ROS levels were induced by cadmium treatment. In addition, cadmium elicited the AKR1C3 expression. The cadmium-induced increase in AKR1C3 protein levels was suppressed by N-acetylcysteine (NAC) and, to a lesser extent, PI3K inhibitor (Ly294002). Cells pretreated with Ly294002 were more resistant to cadmium toxicity than control. The increase in AKR1C3 protein level was accompanied by an increase in the nuclear transcription factor Nrf2. Overall, our data suggest that cadmium-induced ROS cause up-regulation of AKR1C3 expression, at least partially via the activation of PI3K-related intracellular signaling pathways, and Nrf2 activation, thereby contributing to an adaptive intracellular response to cadmium toxicity.

The cytoprotective role of the Keap1-Nrf2 pathway

An elaborate network of highly inducible proteins protects aerobic cells against the cumulative damaging effects of reactive oxygen intermediates and toxic electrophiles, which are the major causes of neoplastic and chronic degenerative diseases. These cytoprotective proteins share common transcriptional regulation, through the Keap1-Nrf2 pathway, which can be activated by various exogenous and endogenous small molecules (inducers). Inducers chemically react with critical cysteine residues of the sensor protein Keap1, leading to stabilisation and nuclear translocation of transcription factor Nrf2, and ultimately to coordinate enhanced expression of genes coding for cytoprotective proteins. In addition, inducers inhibit pro-inflammatory responses, and there is a linear correlation spanning more than six orders of magnitude of concentrations between inducer and anti-inflammatory activity. Genetic deletion of transcription factor Nrf2 renders cells and animals much more sensitive to the damaging effects of electrophiles, oxidants and inflammatory agents in comparison with their wild-type counterparts. Conversely, activation of the Keap1-Nrf2 pathway allows survival and adaptation under various conditions of stress and has protective effects in many animal models. Cross-talks with other signalling pathways broadens the role of the Keap1-Nrf2 pathway in determining the fate of the cell, impacting fundamental biological processes such as proliferation, apoptosis, angiogenesis and metastasis.

Overproduction of a rice aldo-keto reductase increases oxidative and heat stress tolerance by malondialdehyde and methylglyoxal detoxification

The accumulation of toxic compounds generated by the interaction between reactive oxygen species and polyunsaturated fatty acids of membrane lipids can significantly damage plant cells. A plethora of enzymes act on these reactive carbonyls, reducing their toxicity. Based on the chromosomal localization and on their homology with other stress-induced aldo-keto reductases (AKRs) we have selected three rice AKR genes. The transcription level of OsAKR1 was greatly induced by abscisic acid and various stress treatments; the other two AKR genes tested were moderately stress-inducible. The OsAKR1 recombinant protein exhibited a high nicotinamide adenine dinucleotide phosphate-dependent catalytic activity to reduce toxic aldehydes including glycolysis-derived methylglyoxal (MG) and lipid peroxidation-originated malondialdehyde (MDA). The function of this enzyme in MG detoxification was demonstrated in vivo in E. coli and in transgenic plants overproducing the OsAKR1 protein. Heterologous synthesis of the OsAKR1 enzyme in transgenic tobacco plants resulted in increased tolerance against oxidative stress generated by methylviologen (MV) and improved resistance to high temperature. In these plants lower levels of MDA were detected both following MV and heat treatment due to the activity of the OsAKR1 enzyme. The transgenic tobaccos also exhibited higher AKR activity and accumulated less MG in their leaves than the wild type plants; both in the presence and absence of heat stress. These results support the positive role of OsAKR1 in abiotic stress-related reactive aldehyde detoxification pathways and its use for improvement of stress tolerance in plants.

Cloning and expression pattern of 3-dehydroecdysone 3β-reductase (3DE 3β-reductase) from the silkworm, Bombyx mori L

Molting in insects is regulated by molting hormones (ecdysteroids), which are also crucial to insect growth, development, reproduction, etc. Ecdysone was inactivated to 3-dehydroecdysone (3DE) under ecdysone oxidase (EO), and followed by NAD(P)H-dependent irreversible reduction to 3-epiecdysteroid under 3DE 3a-reductase. On the other hand, 3-dehydroecdysone undergoes reversible reduction to ecdysone by 3DE 3β-reductase in the hemolymph. In this article, we cloned and characterized 3-dehydroecdysone 3β-reductase (3DE 3β-reductase) in the different tissues and the developing stage from the silkworm, Bombyx mori L. The B. mori 3DE 3β-reductase cDNA contains an ORF 972 bp and the deduced protein sequence containing 323 amino acid residues. Analysis showed that the deduced 3DE 3β-reductase belongs to the aldo-keto reductase (AKR) superfamily, which has the NAD(P)-binding domain, indicating that the function of 3DE 3β-reductase depends on the existence of NAD(P)H. Using Escherichia coli, a high level expression of a fusion polypeptide band of approx. 40 kDa was observed. The high transcription of 3DE 3β-reductase was mainly observed in the genitalia and fatty bodies in the third day of the fifth-instar larvae, followed next in the head, epidermis, and hemocytes. The expression of 3DE 3β-reductase in the early of every instar was lower than that in the late of instar. When the titer of 3DE is low, higher expression of 3DE 3β-reductase is necessary to maintain the ecdysone titer in body through converting 3DE to ecdysone, while the 3DE titer is high, the expression of 3DE 3β-reductase showed feedback inhibition.

Thermophilic enzymes and their applications in biocatalysis: a robust aldo-keto reductase

Extremophiles are providing a good source of novel robust enzymes for use in biocatalysis for the synthesis of new drugs. This is particularly true for the enzymes from thermophilic organisms which are more robust than their mesophilic counterparts to the conditions required for industrial bio-processes. This paper describes a new aldo-keto reductase enzyme from a thermophilic eubacteria, Thermotoga maritima which can be used for the production of primary alcohols. The enzyme has been cloned and over-expressed in Escherichia coli and has been purified and subjected to full biochemical characterization. The aldo-keto reductase can be used for production of primary alcohols using substrates including benzaldehyde, 1,2,3,6-tetrahydrobenzaldehyde and para-anisaldehyde. It is stable up to 80 degrees C, retaining over 60% activity for 5 hours at this temperature. The enzyme at pH 6.5 showed a preference for the forward, carbonyl reduction. The enzyme showed moderate stability with organic solvents, and retained 70% activity in 20% (v/v) isopropanol or DMSO. These properties are favourable for its potential industrial applications.

New frontiers in androgen biosynthesis and metabolism

PURPOSE OF REVIEW

To summarize recent advances in androgen biosynthesis and metabolism in peripheral tissues (e.g., liver and prostate) and how these can be exploited therapeutically.

RECENT FINDINGS

Human liver catalyzes the reduction of circulating testosterone to yield four stereoisomeric tetrahydrosteroids. Recent advances have assigned the enzymes responsible for these reactions and elucidated their structural biology. Data also suggest that for 5alpha-dihydrotestosterone (5alpha-DHT), conjugation reactions (phase II) may precede ketosteroid reduction (phase I) reactions. Human prostate is the site of benign prostatic hyperplasia and prostate cancer, which occur in the aging male. Although the importance of local androgen biosynthesis in these diseases is accepted, recent advances have identified enzymes that regulate ligand access to the androgen-receptor; a 'backdoor pathway' to 5alpha-DHT that does not require testosterone acting as an intermediate; and the finding that castrate-resistant prostate cancer (CRPC) has undergone an adaptive response to androgen deprivation, which involves intratumoral testosterone and 5alpha-DHT biosynthesis that can be targeted using inhibitors of (CYP17-hydroxylase/17,20-lyase), aldo-keto reductase 1C3, and 5alpha-reductase type 1 and type 2.

SUMMARY

Enzyme isoforms responsible for the biosynthesis and metabolism of androgens in liver and prostate have been identified and those responsible for the biosynthesis of androgens in CRPC can be therapeutically targeted.

A new member of the short-chain dehydrogenases/reductases superfamily: purification, characterization and substrate specificity of a recombinant carbonyl reductase from Pichia stipitis

A novel short-chain dehydrogenases/reductases superfamily (SDRs) reductase (PsCR) from Pichia stipitis that produced ethyl (S)-4-chloro-3-hydroxybutanoate with greater than 99% enantiomeric excess, was purified to homogeneity using fractional ammonium sulfate precipitation followed by DEAE-Sepharose chromatography. The enzyme purified from recombinant Escherichia coli had a molecular mass of about 35 kDa on SDS-PAGE and only required NADPH as an electron donor. The K(m) value of PsCR for ethyl 4-chloro-3-oxobutanoate was 4.9 mg/mL and the corresponding V(max) was 337 micromol/mg protein/min. The catalytic efficiency value was the highest ever reported for reductases from yeasts. Moreover, PsCR exhibited a medium-range substrate spectrum toward various keto and aldehyde compounds, i.e., ethyl-3-oxobutanoate with a chlorine substitution at the 2 or 4-position, or alpha,beta-diketones. In addition, the activity of the enzyme was strongly inhibited by SDS and beta-mercaptoethanol, but not by ethylene diamine tetra acetic acid.

Structural basis for substrate specificity in human monomeric carbonyl reductases

Carbonyl reduction constitutes a phase I reaction for many xenobiotics and is carried out in mammals mainly by members of two protein families, namely aldo-keto reductases and short-chain dehydrogenases/reductases. In addition to their capacity to reduce xenobiotics, several of the enzymes act on endogenous compounds such as steroids or eicosanoids. One of the major carbonyl reducing enzymes found in humans is carbonyl reductase 1 (CBR1) with a very broad substrate spectrum. A paralog, carbonyl reductase 3 (CBR3) has about 70% sequence identity and has not been sufficiently characterized to date. Screening of a focused xenobiotic compound library revealed that CBR3 has narrower substrate specificity and acts on several orthoquinones, as well as isatin or the anticancer drug oracin. To further investigate structure-activity relationships between these enzymes we crystallized CBR3, performed substrate docking, site-directed mutagenesis and compared its kinetic features to CBR1. Despite high sequence similarities, the active sites differ in shape and surface properties. The data reveal that the differences in substrate specificity are largely due to a short segment of a substrate binding loop comprising critical residues Trp229/Pro230, Ala235/Asp236 as well as part of the active site formed by Met141/Gln142 in CBR1 and CBR3, respectively. The data suggest a minor role in xenobiotic metabolism for CBR3.

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Characterization of two novel aldo-keto reductases from Arabidopsis: expression patterns, broad substrate specificity, and an open active-site structure suggest a role in toxicant metabolism following stress

Aldo-keto reductases (AKRs) are widely distributed in nature and play numerous roles in the metabolism of steroids, sugars, and other carbonyls. They have also frequently been implicated in the metabolism of exogenous and endogenous toxicants, including those stimulated by stress. Although the Arabidopsis genome includes at least 21 genes with the AKR signature, very little is known of their functions. In this study, we have screened the Arabidopsis thaliana genomic sequence for genes with significant homology to members of the mammalian AKR1 family and identified four homologues for further study. Following alignment of the predicted protein sequences with representatives from the AKR superfamily, the proteins were ascribed not to the AKR1 family but to the AKR4C subfamily, with the individual designations of AKR4C8, AKR4C9, AKR4C10, and AKR4C11. Expression of two of the genes, AKR4C8 and AKR4C9, has been shown to be coordinately regulated and markedly induced by various forms of stress. The genes have been overexpressed in bacteria, and recombinant proteins have been purified and crystallized. Both enzymes display NADPH-dependent reduction of carbonyl compounds, typical of the superfamily, but will accept a very wide range of substrates, reducing a range of steroids, sugars, and aliphatic and aromatic aldehydes/ketones, although there are distinct differences between the two enzymes. We have obtained high-resolution crystal structures of AKR4C8 (1.4 A) and AKR4C9 (1.25 A) in ternary complexes with NADP(+) and acetate. Three extended loops, present in all AKRs and responsible for defining the cofactor- and substrate-binding sites, are shorter in the 4C subfamily compared to other AKRs. Consequently, the crystal structures reveal open and accommodative substrate-binding sites, which correlates with their broad substrate specificity. It is suggested that the primary role of these enzymes may be to detoxify a range of toxic aldehydes and ketones produced during stress, although the precise nature of the principal natural substrates remains to be determined.

[Cloning, expression and the application of human, rat alcohol dehydrogenase and aldo-keto reductase]

This study is aimed to clone and express human, rat alcohol dehydrogenase (ADH) and aldo-keto reductase. Then the enantioselective metabolism of mandelic acid (MA) was studied. Human alcohol dehydrogenase 2, rat alcohol dehydrogenase 1, human and rat aldo-keto reductase 1A1 were amplified using RT-PCR from human and rat liver samples. Then subcloned into pET-28a (+) and expressed in E. coli BL21 (DE3) stably. The protein was induced with IPTG and purified by affinity chromatography. Then the enzyme activities were measured. MA enantiomers were incubated with rat, human ADH and phenylglyoxylic acid (PGA) with AKR1A1, respectively. The metabolism was analyzed with HPLC. The proper genes were cloned and purified and proteins were obtained. All of the proteins obtained showed good activity. Stereoselective-metabolism of MA was observed in human ADH2, which favors for S-MA metabolism. The expression plasmids are constructed and the recombinant proteins are expressed successfully. The recombinant alcohol dehydrogenase and aldo-keto reductase have been employed to study MA metabolism.

Crystallization and preliminary X-ray analysis of the NADPH-dependent 3-quinuclidinone reductase from Rhodotorula rubra

(R)-3-Quinuclidinol is a useful compound that is applicable to the synthesis of various pharmaceuticals. The NADPH-dependent carbonyl reductase 3-quinuclidinone reductase from Rhodotorula rubra catalyzes the stereospecific reduction of 3-quinuclidinone to (R)-3-quinuclidinol and is expected to be utilized in industrial production of this alcohol. 3-Quinuclidinone reductase from R. rubra was expressed in Escherichia coli and purified using Ni-affinity and ion-exchange column chromatography. Crystals of the protein were obtained by the sitting-drop vapour-diffusion method using PEG 8000 as the precipitant. The crystals belonged to space group P4(1)2(1)2, with unit-cell parameters a = b = 91.3, c = 265.4 A, and diffracted X-rays to 2.2 A resolution. The asymmetric unit contained four molecules of the protein and the solvent content was 48.4%.

Overexpression of GmAKR1, a stress-induced aldo/keto reductase from soybean, retards nodule development

Development of symbiotic root nodules in legumes involves the induction and repression of numerous genes in conjunction with changes in the level of phytohormones. We have isolated several genes that exhibit differential expression patterns during the development of soybean nodules. One of such genes, which were repressed in mature nodules, was identified as a putative aldo/keto reductase and thus named Glycine max aldo/keto reductase 1 (GmAKR1). GmAKR1 appears to be a close relative of a yeast aldo/keto reductase YakC whose in vivo substrate has not been identified yet. The expression of GmAKR1 in soybean showed a root-specific expression pattern and inducibility by a synthetic auxin analogue 2,4-D, which appeared to be corroborated by presence of the root-specific element and the stress-response element in the promoter region. In addition, constitutive overexpression of GmAKR1 in transgenic soybean hairy roots inhibited nodule development, which suggests that it plays a negative role in the regulation of nodule development. One of the Arabidopsis orthologues of GmAKR1 is the ARF-GAP domain 2 protein, which is a potential negative regulator of vesicle trafficking; therefore GmAKR1 may have a similar function in the roots and nodules of legume plants.

Bioconversion of norethisterone, a progesterone receptor agonist into estrogen receptor agonists in osteoblastic cells

A number of clinical studies have demonstrated that norethisterone (NET), a potent synthetic progestin, restores postmenopausal bone loss, although its mode of action on bone cells is not fully understood, while the effect of naturally occurring progesterone in bone has remained controversial. A recent report claims that the potent effects of NET on osteoblastic cell proliferation and differentiation, mimicking the action of estrogens, are mediated by non-phenolic NET derivatives. To determine whether osteoblasts possess the enzymes required to bioconvert a progesterone receptor (PR) agonist into A-ring reduced metabolites with affinity to bind estrogen receptor (ER), we studied the in vitro metabolism of [(3)H]-labeled NET in cultured neonatal rat osteoblasts and the interaction of its metabolic conversion products with cytosolic -osteoblast ER, employing a competition analysis. Results indicated that NET was extensively bioconverted (36.4%) to 5 alpha-reduced metabolites, including 5 alpha-dihydro NET, 3 alpha,5 alpha-tetrahydro NET (3 alpha,5 alpha-NET) and 3beta,5 alpha-tetrahydro NET (3beta,5 alpha-NET), demonstrating the activities of 5 alpha-steroid reductase and two enzymes of the aldo-keto reductases family. Expression of Srd5a1 in neonatal osteoblast was well demonstrated, whereas Srd5a2 expression was not detected. The most striking finding was that 3beta,5 alpha-NET and 3 alpha,5 alpha-NET were efficient competitors of [(3)H]-estradiol for osteoblast ER binding sites, exhibiting affinities similar to that of estradiol. The results support the concept that the interplay of 5 alpha-steroid reductase and aldo-keto reductases in osteoblastic cells, acting as an intracrine modulator system is capable to bioconvert a PR agonist into ER agonists, offering an explanation of the molecular mechanisms NET uses to enhance osteoblastic cell activities.

Steroid hormone transforming aldo-keto reductases and cancer

Prostate and breast cancer are hormone-dependent malignancies of the aging male and female and require the local production of androgens and estrogens to stimulate cell proliferation. Aldo-keto reductases (AKR) play key roles in this process. In the prostate, AKR1C3 (type 5 17beta-HSD) reduces Delta(4)-androstene-3,17-dione to yield testosterone while AKR1C2 (type 3 3alpha-HSD) eliminates 5alpha-dihydrotestosterone (5alpha-DHT), and AKR1C1 forms 3beta-androstanediol (a ligand for ERbeta). In the breast, AKR1C3 forms testosterone, which is converted to 17beta-estradiol by aromatase or reduces estrone to 17beta-estradiol directly. AKR1C3 also acts as a prostaglandin (PG) F synthase and forms PGF(2alpha) and 11beta-PGF(2alpha), which stimulate the FP receptor and prevent the activation of PPARgamma by PGJ(2) ligands. This proproliferative signaling may stimulate the growth of hormone-dependent and -independent prostate and breast cancer.

Structural and kinetic characterization of a maize aldose reductase

The aldo-keto reductases (AKRs) are classified as oxidoreductases and are found in organisms from prokaryotes to eukaryotes. The AKR superfamily consists of more than 120 proteins that are distributed throughout 14 families. Very few plant AKRs have been characterized and their biological functions remain largely unknown. Previous work suggests that AKRs may participate in stress tolerance by detoxifying reactive aldehyde species. In maize endosperm, the presence of an aldose reductase (AR; EC 1.1.1.21) enzyme has also been hypothesized based on the extensive metabolism of sorbitol. This manuscript identifies and characterizes an AKR from maize (Zea mays L.) with features of an AR. The cDNA clone, classified as AKR4C7, was expressed as a recombinant His-tag fusion protein in Escherichia coli. The product was purified by immobilized metal affinity chromatography followed by anion exchange chromatography. Circular dichroism spectrometry and SAXS analysis indicated that the AKR4C7 protein was stable, remained folded throughout the purification process, and formed monomers of a globular shape, with a molecular envelope similar to human AR. Maize AKR4C7 could utilize dl-glyceraldehyde and some pentoses as substrates. Although the maize AKR4C7 was able to convert sorbitol to glucose, the low affinity for this substrate indicated that AKR4C7 was probably a minimal contributor to sorbitol metabolism in maize seeds. Polyclonal antisera raised against AKR4C7 recognized at least three AR-like polypeptides in maize kernels, consistent with the presence of a small gene family. Diverse functions may have evolved for maize AKRs in association with specific physiological requirements of kernel development.

Purification, cloning, functional expression and characterization of perakine reductase: the first example from the AKR enzyme family, extending the alkaloidal network of the plant Rauvolfia

Perakine reductase (PR) catalyzes an NADPH-dependent step in a side-branch of the 10-step biosynthetic pathway of the alkaloid ajmaline. The enzyme was cloned by a "reverse-genetic" approach from cell suspension cultures of the plant Rauvolfia serpentina (Apocynaceae) and functionally expressed in Escherichia coli as the N-terminal His(6)-tagged protein. PR displays a broad substrate acceptance, converting 16 out of 28 tested compounds with reducible carbonyl function which belong to three substrate groups: benzaldehyde, cinnamic aldehyde derivatives and monoterpenoid indole alkaloids. The enzyme has an extraordinary selectivity in the group of alkaloids. Sequence alignments define PR as a new member of the aldo-keto reductase (AKR) super family, exhibiting the conserved catalytic tetrad Asp52, Tyr57, Lys84, His126. Site-directed mutagenesis of each of these functional residues to an alanine residue results in >97.8% loss of enzyme activity, in compounds of each substrate group. PR represents the first example of the large AKR-family which is involved in the biosynthesis of plant monoterpenoid indole alkaloids. In addition to a new esterase, PR significantly extends the Rauvolfia alkaloid network to the novel group of peraksine alkaloids.

Evidence for the aldo-keto reductase pathway of polycyclic aromatic trans-dihydrodiol activation in human lung A549 cells

Polycyclic aromatic hydrocarbons (PAHs) are tobacco carcinogens implicated in the causation of human lung cancer. Metabolic activation is a key prerequisite for PAHs to cause their deleterious effects. Using human lung adenocarcinoma (A549) cells, we provide evidence for the metabolic activation of (+/-)-trans-7,8dihydroxy-7,8-dihydrobenzo[a]pyrene (B[a]P-7,8-trans-dihydrodiol) by aldo-keto reductases (AKRs) to yield benzo[a]pyrene-7,8-dione (B[a]P-7,8-dione), a redox-active o-quinone. We show that B[a]P-7,8-trans-dihydrodiol (AKR substrate) and B[a]P-7,8-dione (AKR product) lead to the production of intracellular reactive oxygen species (ROS) (measured as an increase in dichlorofluorescin diacetate fluores-cence) and that similar changes were not observed with the regioisomer (+/-)-trans-4,5-dihydroxy-4,5-dihydrobenzo[a]pyrene or the diol-epoxide, (+/-)-anti-7,8-dihydroxy-9alpha,10beta-epoxy-7,8,9,10-tetrahydro-B[a]P. B[a]P-7,8-trans-dihydrodiol and B[a]P-7,8-dione also caused a decrease in glutathione levels and an increase in NADP(+)/NADPH ratios, with a concomitant increase in single-strand breaks (as measured by the comet assay) and 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxo-dGuo). The specificity of the comet assay was validated by coupling it to human 8-oxo-guanine glycosylase (hOGG1), which excises 8-oxo-Gua to yield single-strand breaks. The levels of 8-oxo-dGuo observed were confirmed by an immunoaffinity purification stable isotope dilution ([(15)N(5)]-8-oxo-dGuo) liquid chromatography-electrospray ionization/multiple reaction monitoring/mass spectrometry (LC-ESI/MRM/MS) assay. B[a]P-7,8-trans-dihydrodiol produced DNA strand breaks in the hOGG1-coupled comet assay as well as 8-oxo-dGuo (as measured by LC-ESI/MRM/MS) and was enhanced by a catechol O-methyl transferase (COMT) inhibitor, suggesting that COMT protects against o-quinone-mediated redox cycling. We conclude that activation of PAH-trans-dihydrodiols by AKRs in lung cells leads to ROS-mediated genotoxicity and contributes to lung carcinogenesis.

Pre-receptor regulation of the androgen receptor

The human androgen receptor (AR) is a ligand-activated nuclear transcription factor and mediates the induction of genes involved in the development of the male phenotype and male secondary sex characteristics, as well as the normal and abnormal growth of the prostate. We have identified the pair of hydroxysteroid dehydrogenases (HSDs) that regulate ligand access to the AR in human prostate. We find that type 3 3alpha-HSD (aldo-keto reductase (AKR)1C2) catalyzes the NADPH dependent reduction of the potent androgen 5alpha-dihydrotestosterone (5alpha-DHT) to yield the inactive androgen 3alpha-androstanediol (3alpha-diol). We also find that RoDH like 3alpha-HSD (RL-HSD) catalyzes the NAD(+) dependent oxidation of 3alpha-diol to yield 5alpha-DHT. Together these enzymes are involved in the pre-receptor regulation of androgen action. Inhibition of AKR1C2 would be desirable in cases of androgen insufficiency and inhibition of RL-HSD might be desirable in benign prostatic hyperplasia.

The aldo-keto reductase superfamily and its role in drug metabolism and detoxification

The aldo-keto reductase (AKR) superfamily comprises enzymes that catalyze redox transformations involved in biosynthesis, intermediary metabolism, and detoxification. Substrates of AKRs include glucose, steroids, glycosylation end-products, lipid peroxidation products, and environmental pollutants. These proteins adopt a (beta/alpha)(8) barrel structural motif interrupted by a number of extraneous loops and helixes that vary between proteins and bring structural identity to individual families. The human AKR family differs from the rodent families. Due to their broad substrate specificity, AKRs play an important role in the phase II detoxification of a large number of pharmaceuticals, drugs, and xenobiotics.

Identification of candidate predictive markers of anticancer drug sensitivity using a panel of human cancer cell lines

We previously investigated the correlations between the expression of 9216 genes and various chemosensitivities in a panel of 39 human cancer cell lines(1)) and found that the expression levels of AKR1B1 and CTSH were correlated with sensitivity and resistance to multiple drugs, respectively. To validate these correlations, we investigated the expression of these two genes and the chemosensitivities in 12 additional gastric cancer cell lines. The expression of AKR1B1 in the additional cell lines exhibited significant correlations with sensitivities to 8 of the 23 drugs examined, while that of CTSH displayed a significant negative correlation with only one (MS-247) of the 27 drugs examined. Their expressions were weakly correlated with sensitivity and resistance, respectively, to the remainder of the drugs. Moreover, when the 12 cell lines were divided into high-expressing and low-expressing groups, a comparison of these groups using Mann-Whitney's U test revealed that high expression levels of AKR1B1 and CTSH were related to sensitivity to 21 of the drugs and resistance to 8 of the drugs, respectively. The present results suggest that AKR1B1 and CTSH may be good markers for prediction of sensitivity to certain drugs and that our panel of 39 cell lines has the potential to identify candidate predictive marker genes.