Differential control of murine aldose reductase and fibroblast growth factor (FGF)-regulated-1 gene expression in NIH 3T3 cells by FGF-1 treatment and hyperosmotic stress

Identification, characterization, and crystal structure of an aldo-keto reductase (AKR2E4) from the silkworm Bombyx mori

A new member of the aldo-keto reductase (AKR) superfamily with 3-dehydroecdysone reductase activity was found in the silkworm Bombyx mori upon induction by the insecticide diazinon. The amino acid sequence showed that this enzyme belongs to the AKR2 family, and the protein was assigned the systematic name AKR2E4. In this study, recombinant AKR2E4 was expressed, purified to near homogeneity, and kinetically characterized. Additionally, its ternary structure in complex with NADP(+) and citrate was refined at 1.3Å resolution to elucidate substrate binding and catalysis. The enzyme is a 33-kDa monomer and reduces dicarbonyl compounds such as isatin and 17α-hydroxy progesterone using NADPH as a cosubstrate. No NADH-dependent activity was detected. Robust activity toward the substrate inhibitor 3-dehydroecdysone was observed, which suggests that this enzyme plays a role in regulation of the important molting hormone ecdysone. This structure constitutes the first insect AKR structure determined. Bound NADPH is located at the center of the TIM- or (β/α)8-barrel, and residues involved in catalysis are conserved.

Role of aldose reductase in the metabolism and detoxification of carnosine-acrolein conjugates

Oxidation of unsaturated lipids generates reactive aldehydes that accumulate in tissues during inflammation, ischemia, or aging. These aldehydes form covalent adducts with histidine-containing dipeptides such as carnosine and anserine, which are present in high concentration in skeletal muscle, heart, and brain. The metabolic pathways involved in the detoxification and elimination of these conjugates are, however, poorly defined, and their significance in regulating oxidative stress is unclear. Here we report that conjugates of carnosine with aldehydes such as acrolein are produced during normal metabolism and excreted in the urine of mice and adult human non-smokers as carnosine-propanols. Our studies show that the reduction of carnosine-propanals is catalyzed by the enzyme aldose reductase (AR). Carnosine-propanals were converted to carnosine-propanols in the lysates of heart, skeletal muscle, and brain tissue from wild-type (WT) but not AR-null mice. In comparison with WT mice, the urinary excretion of carnosine-propanols was decreased in AR-null mice. Carnosine-propanals formed covalent adducts with nucleophilic amino acids leading to the generation of carnosinylated proteins. Deletion of AR increased the abundance of proteins bound to carnosine in skeletal muscle, brain, and heart of aged mice and promoted the accumulation of carnosinylated proteins in hearts subjected to global ischemia ex vivo. Perfusion with carnosine promoted post-ischemic functional recovery in WT but not in AR-null mouse hearts. Collectively, these findings reveal a previously unknown metabolic pathway for the removal of carnosine-propanal conjugates and suggest a new role of AR as a critical regulator of protein carnosinylation and carnosine-mediated tissue protection.

Aldo-Keto Reductases 1B in Endocrinology and Metabolism

The aldose reductase (AR; human AKR1B1/mouse Akr1b3) has been the focus of many research because of its role in diabetic complications. The starting point of these alterations is the massive entry of glucose in polyol pathway where it is converted into sorbitol by this enzyme. However, the issue of AR function in non-diabetic condition remains unresolved. AR-like enzymes (AKR1B10, Akr1b7, and Akr1b8) are highly related isoforms often co-expressed with bona fide AR, making functional analysis of one or the other isoform a challenging task. AKR1B/Akr1b members share at least 65% protein identity and the general ability to reduce many redundant substrates such as aldehydes provided from lipid peroxidation, steroids and their by-products, and xenobiotics in vitro. Based on these properties, AKR1B/Akr1b are generally considered as detoxifying enzymes. Considering that divergences should be more informative than similarities to help understanding their physiological functions, we chose to review specific hallmarks of each human/mouse isoforms by focusing on tissue distribution and specific mechanisms of gene regulation. Indeed, although the AR shows ubiquitous expression, AR-like proteins exhibit tissue-specific patterns of expression. We focused on three organs where certain isoforms are enriched, the adrenal gland, enterohepatic, and adipose tissues and tried to connect recent enzymatic and regulation data with endocrine and metabolic functions of these organs. We presented recent mouse models showing unsuspected physiological functions in the regulation of glucido-lipidic metabolism and adipose tissue homeostasis. Beyond the widely accepted idea that AKR1B/Akr1b are detoxification enzymes, these recent reports provide growing evidences that they are able to modify or generate signal molecules. This conceptually shifts this class of enzymes from unenviable status of scavenger to upper class of messengers.

Aldose reductase inhibition suppresses oxidative stress-induced inflammatory disorders

Oxidative stress-induced inflammation is a major contributor to several disease conditions including sepsis, carcinogenesis and metastasis, diabetic complications, allergic asthma, uveitis and after cataract surgery posterior capsular opacification. Since reactive oxygen species (ROS)-mediated activation of redox-sensitive transcription factors and subsequent expression of inflammatory cytokines, chemokines and growth factors are characteristics of inflammatory disorders, we envisioned that by blocking the molecular signals of ROS that activate redox-sensitive transcription factors, various inflammatory diseases could be ameliorated. We have indeed demonstrated that ROS-induced lipid peroxidation-derived lipid aldehydes such as 4-hydroxy-trans-2-nonenal (HNE) and their glutathione-conjugates (e.g. GS-HNE) are efficiently reduced by aldose reductase to corresponding alcohols which mediate the inflammatory signals. Our results showed that inhibition of aldose reductase (AKR1B1) significantly prevented the inflammatory signals induced by cytokines, growth factors, endotoxins, high glucose, allergens and auto-immune reactions in cellular as well as animal models. We have demonstrated that AKR1B1 inhibitor, fidarestat, significantly prevents tumor necrosis factor-alpha (TNF-α)-, growth factors-, lipopolysachharide (LPS)-, and environmental allergens-induced inflammatory signals that cause various inflammatory diseases. In animal models of inflammatory diseases such as diabetes, cardiovascular, uveitis, asthma, and cancer (colon, breast, prostate and lung) and metastasis, inhibition of AKR1B1 significantly ameliorated the disease. Our results from various cellular and animal models representing a number of inflammatory conditions suggest that ROS-induced inflammatory response could be reduced by inhibition of AKR1B1, thereby decreasing the progression of the disease and if the therapy is initiated early, the disease could be eliminated. Since fidarestat has already undergone phase III clinical trial for diabetic neuropathy and found to be safe, though clinically not very effective, our results indicate that it can be developed for the therapy of a number of inflammation-related diseases. Our results thus offer a novel therapeutic approach to treat a wide array of inflammatory diseases.

Activation of antioxidant defense during dehydration stress in the African clawed frog

The glutathione S-transferase (GST) and aldo-keto reductase (AKR) families of proteins are major groups of detoxifying enzymes that are known to be regulated by the NF-E2 related factor 2 (Nrf2) transcription factor via the antioxidant response element that is present in the promoter regions of GST and AKR genes. Expression of Nrf2, GST and AKR proteins was analyzed in the African clawed frog, Xenopus laevis, focusing on their responses to dehydration stress. Dehydration/rehydration cycles can generate oxidative stress and this could be ameliorated by enhancing antioxidant defenses. Dehydration to 28% of total body water lost triggered organ-specific changes in nrf2 mRNA expression (a 2-fold increase in liver), total Nrf2 protein (2-4-fold elevation in lung, heart, skin and liver), and a 4.3-fold increase in the content of Nrf2 in the nucleus in muscle. Protein levels of six GST and three AKR family members were assessed and showed organ-specific patterns of expression during dehydration. In particular, GSTP1 was strongly induced in liver, heart and skin, levels rising by 9-, 2.6- and 1.7-fold, respectively, whereas GSTM1 and M3 rose in skeletal muscle, kidney and skin. Selective expression of GSTK1, A3 and T1 also occurred. Dehydration also stimulated organ-specific increases in the levels of AKR family members (AKR1B4, AKR1A3, AFAR1) by 1.5-2-fold. The results show that metabolic responses to dehydration include activation of the Nrf2 transcription factor and selective up-regulation of genes under Nrf2 control.

Transcription factor AP-1 regulates TGF-beta(1)-induced expression of aldose reductase in cultured human mesangial cells

AIM

The previous studies demonstrated that transforming growth factor-beta(1) (TGF-beta(1)) could upregulate the expression of aldose reductase (AR). The aim of this study is to clarify (investigate) the mechanism of TGF-beta(1)-induced AR expression.

METHODS

Real-time polymerase chain reaction and western blot were used to analyse the AR expression in mRNA and protein levels in human mesangial cells, and reporter assay was used to analyse the function of various sites within 5'-flanking region of AR gene in AR expression.

RESULTS

TGF-beta(1) (4 ng/mL) stimulation could upregulate AR expression. The cells pretreated with pharmacological inhibitors, U0126 and PD98059 for blocking extracellular signal-related kinase (ERK) signalling pathway or SP6000125 for blocking c-Jun N-terminal kinase (JNK) signalling pathway, respectively, showed reduced expression of AR after TGF-beta(1) stimulation. Similarly, the cells transiently transfected with pCMVTAM67, which is an expression plasmid for DN-c-Jun showed decreasing AR expression. Reporter assay revealed that the 5'-promoter region of AR consisting of an AP-1 site and two putative antioxidant response elements (ARE) was responsible for TGF-beta(1) stimulation. Mutation of either ARE did not affect the promoter activity in the reporter assay while mutation of AP-1 site caused a significant decrease in the responsiveness to TGF-beta(1).

CONCLUSION

TGF-beta(1) upregulate AR expression in both mRNA and protein levels. The results demonstrated that ERK and JNK are involved in the downstream signalling pathways and transcription factor AP-1 plays an important role in the regulation of TGF-beta(1)-induced AR expression in mesangial cells.

Aldose Reductase Regulates Growth Factor-Induced Cyclooxygenase-2 Expression and Prostaglandin E2 Production in Human Colon Cancer Cells

Inhibition of prostaglandin E(2) (PGE(2)) and cyclooxygenase (COX)-2 by nonsteroidal anti-inflammatory drugs reduces the progression of colon cancer. Inhibition of aldose reductase (AR; EC. 1.1.1.21.) by sorbinil or by antisense ablation prevented fibroblast growth factor-induced and platelet-derived growth factor-induced up-regulation of PGE(2) synthesis in human colon cancer cells, Caco-2. AR besides reducing aldo-sugars efficiently reduces toxic lipid aldehydes and their conjugates with glutathione. Inhibition of AR prevented growth factor-induced COX-2 activity, protein, and mRNA and significantly decreased activation of nuclear factor-kappaB and protein kinase C (PKC) and phosphorylation of PKC-beta2 as well as progression of Caco-2 cell growth but had no effect on COX-1 activity. Cell cycle analysis suggests that inhibition of AR prevents growth factor-induced proliferation of Caco-2 cells at S phase. Treatment of Caco-2 cells with the most abundant and toxic lipid aldehyde 4-hydroxy-trans-2-nonenal (HNE) or its glutathione-conjugate [glutathionyl-HNE (GS-HNE)] or AR-catalyzed product of GS-HNE, glutathionyl-1,4-dihydroxynonane (GS-DHN), resulted in increased COX-2 expression and PGE(2) production. Inhibition of AR prevented HNE- or GS-HNE-induced but not GS-DHN-induced up-regulation of COX-2 and PGE(2). More importantly, in vivo studies showed that administration of AR-small interfering RNA (siRNA), but not control siRNA, to nude mice bearing SW480 human colon adenocarcinoma cells completely arrested tumor progression. Collectively, these observations suggest that AR is an obligatory mediator of growth factor-induced up-regulation of COX-2, PGE(2), and growth of Caco-2 cells, indicating that inhibition of AR may be a novel therapeutic approach in preventing the progression of colon cancer.

Transcription factor Nrf2 regulates promoter activity of mouse aldose reductase (AKR1B3) gene

Transcription factor Nrf2 regulates gene expression of drug metabolizing enzymes such as glutathione S-transferase via the antioxidant response element, ARE. Aldose reductase (AR), a member of the aldo-keto reductase (AKR) superfamily, metabolizes various endogenous and exogenous aldehydes. The AR gene 5'-flanking region contains a multiple stress response region (MSRR) composed of two putative AREs (ARE1 and ARE2), an AP1 site, and a tonicity response element (TonE). As this region is highly conserved among species, we examined the involvement of Nrf2 in transcriptional regulation of the AR gene. beta-Naphthoflavone, an Nrf2 activator, elevated the level of AR mRNA in HepG2 cells and increased the promoter activity of the mouse AR (AKR1B3) gene. The promoter activity of the AKR1B3 gene, containing MSRR, was also augmented by overexpression of Nrf2. Deletion and mutation analyses indicated that both ARE1 and the AP1 site were essential for the responsiveness to Nrf2, while ARE2 was nonfunctional. The presence of an ARE1 binding protein complex was revealed by electrophoretic mobility shift assay. These findings indicate that Nrf2 regulates the AKR1B3 promoter activity via ARE1 and the AP1 site.

Structural and catalytic diversity in the two family 11 aldo-keto reductases

Aldo-keto reductases (AKRs) are a large superfamily of NAD(P)H-dependent enzymes that function in a wide range of biological processes. The structures of two enzymes from the previously uncharacterized family 11 (AKR11A and AKR11B), the products of the iolS and yhdN genes of Bacillus subtilis have been determined. AKR11B appears to be a relatively conventional member of the superfamily with respect to structural and biochemical properties. It is an efficient enzyme, specific for NADPH and possesses a catalytic triad typical for AKRs. AKR11A exhibits catalytic divergence from the other members of the superfamily and, surprisingly, AKR11B, the most closely related aldo-keto reductase in sequence. Although both have conserved catalytic residues consisting of an acidic tyrosine, a lysine and an aspartate, a water molecule interrupts this triad in cofactor-bound AKR11A by inserting between the lysine and tyrosine side-chains. This results in a unique architecture for an AKR active site with scant catalytic power. In addition, the absence of a bulky tryptophan side-chain in AKR11A allows an unconventional conformation of the bound NADP+ cosubstrate, raising the possibility that it donates the 4-pro-S hydride rather than the 4-pro-R hydride seen in most other AKRs. Based upon the architecture of the active site and the resulting reaction velocities, it therefore appears that functioning as an efficient oxido-reductase is probably not the primary role of AKR11A. A comparison of the apo and holo forms of AKR11A demonstrates that the cosubstrate does not play the dramatic role in active site assembly seen in other superfamily members.

Aldose reductase mediates mitogenic signaling in vascular smooth muscle cells

Abnormal vascular smooth muscle cell (VSMC) proliferation is a key feature of atherosclerosis and restenosis; however, the mechanisms regulating growth remain unclear. Herein we show that inhibition of the aldehyde-metabolizing enzyme aldose reductase (AR) inhibits NF-kappa B activation during restenosis of balloon-injured rat carotid arteries as well as VSMC proliferation due to tumor necrosis factor alpha (TNF-alpha) stimulation. Inhibition of VSMC growth by AR inhibitors was not accompanied by increase in cell death or apoptosis. Inhibition of AR led to a decrease in the activity of the transcription factor NF-kappa B in culture and in the neointima of rat carotid arteries after balloon injury. Inhibition of AR in VSMC also prevented the activation of NF-kappa B by basic fibroblast growth factor (bFGF), angiotensin-II (Ang-II), and platelet-derived growth factor (PDGF-AB). The VSMC treated with AR inhibitors showed decreased nuclear translocation of NF-kappa B and diminished phosphorylation and proteolytic degradation of I kappa B-alpha. Under identical conditions, treatment with AR inhibitors also prevented the activation of protein kinase C (PKC) by TNF-alpha, bFGF, Ang-II, and PDGF-AB but not phorbol esters, indicating that AR inhibitors prevent PKC stimulation or the availability of its activator but not PKC itself. Treatment with antisense AR, which decreased the AR activity by >80%, attenuated PKC activation in TNF-alpha, bFGF, Ang-II, and PDGF-AB-stimulated VSMC and prevented TNF-alpha-induced proliferation. Collectively, these data suggest that inhibition of NF-kappa B may be a significant cause of the antimitogenic effects of AR inhibition and that this may be related to disruption of PKC-associated signaling in the AR-inhibited cells.

Identification of biochemical pathways for the metabolism of oxidized low-density lipoprotein derived aldehyde-4-hydroxy trans-2-nonenal in vascular smooth muscle cells

Oxidation of low-density lipoproteins (LDL) generates high concentrations of unsaturated aldehydes, such as 4-hydroxy trans-2-nonenal (HNE). These aldehydes are mitogenic to vascular smooth muscle cells and sustain a vascular inflammation. Nevertheless, the processes that mediate and regulate the vascular metabolism of these aldehydes have not been examined. In this communication, we report the identification of the major metabolic pathways and products of [(3)H]-HNE in rat aortic smooth muscle cells in culture. High-performance liquid chromatography separation of the radioactivity recovered from these cells revealed that a large (60-65%) proportion of the metabolism was linked to glutathione (GSH). Electrospray mass spectrometry showed that glutathionyl-1,4 dihydroxynonene (GS-DHN) was the major metabolite of HNE in these cells. The formation of GS-DHN appears to be due aldose reductase (AR)-catalyzed reduction of glutathionyl 4-hydroxynonanal (GS-HNE), since inhibitors of AR (tolrestat or sorbinil) prevented GS-DHN formation, and increased the fraction of the glutathione conjugate remaining as GS-HNE. Gas chromatography-chemical ionization mass spectroscopy of the metabolites identified a subsidiary route of HNE metabolism leading to the formation of 4-hydroxynonanoic acid (HNA). Oxidation to HNA accounted for 25-30% of HNE metabolism. The formation of HNA was inhibited by cyanamide, indicating that the acid is derived from an aldehyde dehydrogenase (ALDH)-catalyzed pathway. The overall rate of HNE metabolism was insensitive to inhibition of AR or ALDH, although inhibition of HNA formation by cyanamide led to a corresponding increase in the fraction of HNE metabolized by the GSH-linked pathway, indicating that ALDH-catalyzed oxidation competes with glutathione conjugation. These metabolic pathways may be the key regulators of the vascular effects of HNE and oxidized LDL.

Metabolic regulation of aldose reductase activity by nitric oxide donors

Regulation of aldose reductase (AR), a member of the aldo-keto reductase superfamily, by nitric oxide (NO) donors was examined. Incubation of human recombinant AR with S-nitrosoglutathione (GSNO) led to inactivation of the enzyme and the formation of an AR-glutathione adduct. In contrast, incubation with S-nitroso-N-acetyl penicillamine (SNAP) or N-(beta-D-glucopyranosyl)-SNAP (GlycoSNAP) led to an increase in enzyme activity which was accompanied by the direct nitrosation of the enzyme and the formation of a mixed disulfide with the NO-donor. To examine in vivo modification, red blood cells (RBC) and rat aortic vascular smooth muscle cells (VSMC) were incubated with 1 mM GSNO or SNAP. Exposure of VSMC to SNAP and GSNO for 2 h at 37 degrees C led to approximately 71% decrease in the enzyme activity with DL-glyceraldehyde as the substrate. Similarly, exposure of RBC in 5 mM glucose to NO-donors for 30 min at room temperature, followed by increasing the glucose concentration to 40 mM, resulted in >75% decrease in the formation of sorbitol. These investigations indicate that NO and/or its bioactive metabolites can regulate cellular AR, leading to either activation (by nitrosation) or inactivation (by S-thiolation).

Kinetic and structural characterization of the glutathione-binding site of aldose reductase

Aldose reductase (AR), a member of the aldo-keto reductase superfamily, has been implicated in the etiology of secondary diabetic complications. However, the physiological functions of AR under euglycemic conditions remain unclear. We have recently demonstrated that, in intact heart, AR catalyzes the reduction of the glutathione conjugate of the lipid peroxidation product 4-hydroxy-trans-2-nonenal (Srivastava, S., Chandra, A., Wang, L., Seifert, W. E., Jr., DaGue, B. B., Ansari, N. H., Srivastava, S. K., and Bhatnagar, A. (1998) J. Biol. Chem. 273, 10893-10900), consistent with a possible role of AR in the metabolism of glutathione conjugates of aldehydes. Herein, we present several lines of evidence suggesting that the active site of AR forms a specific glutathione-binding domain. The catalytic efficiency of AR in the reduction of the glutathione conjugates of acrolein, trans-2-hexenal, trans-2-nonenal, and trans,trans-2,4-decadienal was 4-1000-fold higher than for the corresponding free alkanal. Alterations in the structure of glutathione diminished the catalytic efficiency in the reduction of the acrolein adduct, consistent with the presence of specific interactions between the amino acid residues of glutathione and the AR active site. In addition, non-aldehydic conjugates of glutathione or glutathione analogs displayed active-site inhibition. Molecular dynamics calculations suggest that the conjugate adopts a specific low energy configuration at the active site, indicating selective binding. These observations support an important role of AR in the metabolism of glutathione conjugates of endogenous and xenobiotic aldehydes and demonstrate, for the first time, efficient binding of glutathione conjugates to an aldo-keto reductase.

Involvement of aldose reductase in vascular smooth muscle cell growth and lesion formation after arterial injury

Abnormal proliferation of vascular smooth muscle cells (VSMCs) is an important feature of atherosclerosis, restenosis, and hypertension. Although multiple mediators of VSMC growth have been identified, few effective pharmacological tools have been developed to limit such growth. Recent evidence indicating an important role for oxidative stress in cell growth led us to investigate the potential role of aldose reductase (AR) in the proliferation of VSMCs. Because AR catalyzes the reduction of mitogenic aldehydes derived from lipid peroxidation, we hypothesized that it might be a potential regulator of redox changes that accompany VSMC growth. Herein we report several lines of evidence suggesting that AR facilitates/mediates VSMC growth. Stimulation of human aortic SMCs in culture with mitogenic concentrations of serum, thrombin, basic fibroblast growth factor, and the lipid peroxidation product 4-hydroxy-trans-2-nonenal (HNE) led to a 2- to 4-fold increase in the steady-state levels of AR mRNA, a 4- to 7-fold increase in AR protein, and a 2- to 3-fold increase in its catalytic activity. Inhibition of the enzyme by sorbinil or tolrestat diminished mitogen-induced DNA synthesis and cell proliferation. In parallel experiments, the extent of reduction of the glutathione conjugate of HNE to glutathionyl-1,4-dihydroxynonene in HNE-exposed VSMCs was decreased by serum starvation or sorbinil. Immunohistochemical staining of cross sections from balloon-injured rat carotid arteries showed increased expression of AR protein associated with the neointima. The media of injured or uninjured arteries demonstrated no significant staining. Compared with untreated animals, rats fed sorbinil (40 mg. kg(-1). d(-1)) displayed a 51% and a 58% reduction in the ratio of neointima to the media at 10 and 21 days, respectively, after balloon injury. Taken together, these findings suggest that AR is upregulated during growth and that this upregulation facilitates growth by enhancing the metabolism of secondary products of reactive oxygen species.

The mitogen-inducible Fn14 gene encodes a type I transmembrane protein that modulates fibroblast adhesion and migration

The binding of polypeptide growth factors to their appropriate cell surface transmembrane receptors triggers numerous biochemical responses, including the transcriptional activation of specific genes. We have used a differential display approach to identify fibroblast growth factor-1-inducible genes in murine NIH 3T3 cells. Here, we report that the fibroblast growth factor-inducible-14 (Fn14) gene is a growth factor-regulated, immediate-early response gene expressed in a developmental stage- and adult tissue-specific manner in vivo. This gene, located on mouse chromosome 17, is predicted to encode an 129-amino acid type Ia membrane protein with no significant sequence similarity to any known protein. We have used two experimental approaches, direct fluorescence microscopy and immunoprecipitation analysis of biotinylated cell surface proteins, to demonstrate that Fn14 is located on the plasma membrane. To examine the biological consequences of constitutive Fn14 expression, we isolated NIH 3T3 cell lines expressing variable levels of epitope-tagged Fn14 and analyzed their phenotypic properties in vitro. These experiments revealed that Fn14 expression decreased cellular adhesion to the extracellular matrix proteins fibronectin and vitronectin and also reduced serum-stimulated cell growth and migration. These results indicate that Fn14 is a novel plasma membrane-spanning molecule that may play a role in cell-matrix interactions.

Comparisons of genomic structures and chromosomal locations of the mouse aldose reductase and aldose reductase-like genes

Aldose reductase (AR), best known as the first enzyme in the polyol pathway of sugar metabolism, has been implicated in a wide variety of physiological functions and in the etiology of diabetic complications. We have determined the structures and chromosomal locations of the mouse AR gene (Aldor1) and of two genes highly homologous to Aldor1: the fibroblast growth factor regulated protein gene (Fgfrp) and the androgen regulated vas deferens protein gene (Avdp). The number of introns and their locations in the mouse Aldor1 gene are identical to those of rat and human AR genes and also to those of Fgfrp and Avdp. Mouse Aldor1 gene was found to be located near the Cald1 (Caldesmon) and Ptn (Pleiotropin) loci at the proximal end of chromosome 6. The closely related genes Fgfrp and Avdp were also mapped in this region of the chromosome, suggesting that these three genes may have arisen by a gene duplication event.

Sequence and expression levels in human tissues of a new member of the aldo-keto reductase family

We have isolated a human cDNA clone from small intestine that represents a new member of the aldo-keto reductase family. This new member showed 70% identity at the protein level to human aldose reductase and around 80% identity to other Chinese hamster and mouse reductases. The expression pattern shows that this message is located primarily in the adrenal gland, thus suggesting an involvement in steroid metabolism. It is also strongly expressed in the intestinal tract and has been called human small intestine reductase.

Identification and characterization of a novel human aldose reductase-like gene

We have identified a novel human protein that is highly homologous to aldose reductase (AR). This protein, which we called ARL-1, consists of 316 amino acids, the same size as AR, and its amino acid sequence is 71% identical to that of AR. It is more closely related to the AR-like proteins such as mouse vas deferens protein, fibroblast growth factor-regulated protein, and Chinese hamster ovary reductase, with 81, 82, and 83%, respectively, of its amino acid sequence identical to the amino acid sequence of these proteins. The cDNA of ARL-1 was expressed in Escherichia coli to obtain recombinant protein for characterization of its enzymatic activities. For comparison, the cDNA of human AR was also expressed in E. coli and analyzed in parallel. These two enzymes differ in their pH optima and salt requirement, but they act on a similar spectrum of substrates. Similar to AR, ARL-1 can efficiently reduce aliphatic and aromatic aldehydes, and it is less active on hexoses. While AR mRNA is found in most tissues studied, ARL-1 is primarily expressed in the small intestines and in the colon, with a low level of its mRNA in the liver. The ability of ARL-1 to reduce various aldehydes and the locations of expression of this gene suggest that it may be responsible for detoxification of reactive aldehydes in the digested food before the nutrients are passed on to other organs. Interestingly, ARL-1 and AR are overexpressed in some liver cancers, but it is not clear if they contribute to the pathogenesis of this disease.