Regulation of human thioredoxin reductase expression and activity by 3'-untranslated region selenocysteine insertion sequence and mRNA instability elements.

Thioredoxin reductases function in regulating cellular redox and function through their substrate, thioredoxin, in the proper folding of enzymes and redox regulation of transcription factor activity. These enzymes are overexpressed in certain tumors and cancer cells and down-regulated in apoptosis and may play a role in regulating cell growth. Mammalian thioredoxin reductases contain a selenocysteine residue, encoded by a UGA codon, as the penultimate carboxyl-terminal amino acid. This amino acid has been proposed to carry reducing equivalents from the active site to substrates. We report expression of a wild-type thioredoxin reductase selenoenzyme, a cysteine mutant enzyme, and the UGA-terminated protein in mammalian cells and overexpression of the cysteine mutant and UGA-terminated proteins in the baculovirus insect cell system. We show that substitution of cysteine for selenocysteine decreases enzyme activity for thioredoxin by 2 orders magnitude, and that termination at the UGA codon abolishes activity. We further demonstrate the presence of a functional selenocysteine insertion sequence element that is highly active but only moderately responsive to selenium supplementation. Finally, we show that thioredoxin reductase mRNA levels are down-regulated by other sequences in the 3'-untranslated region, which contains multiple AU-rich instability elements. These sequences are found in a number of cytokine and proto-oncogene mRNAs and have been shown to confer rapid mRNA turnover.

Cloning, sequencing and functional expression of a novel human thioredoxin reductase.

The DNA sequence encoding a novel human thioredoxin reductase has been determined. The protein is predicted to have 524 amino acids including a conserved -Cys-Val-Asn-Val-Gly-Cys catalytic site and a selenocysteine containing C-terminal -Gly-Cys-SeCys-Gly. The predicted molecular mass is 56.5. The newly identified TR sequence exhibits 54% identity to a previously reported human thioredoxin reductase and 37% identity to human glutathione reductase. Transient transfection of human embryonal kidney cells results in a 5-fold increase in thioredoxin reductase activity but no increase in glutathione reductase activity.

Selenium and the thioredoxin redox system: effects on cell growth and death.

Thioredoxin is a redox protein found overexpressed in some human tumors. Thioredoxin is secreted by tumor cells and enhances the sensitivity of the cancer cells to other growth factors. Redox activity is essential for stimulation of cell growth by thioredoxin. Cells transfected with thioredoxin cDNA show increased tumor growth and decreased apoptosis in vivo and decreased sensitivity to apoptosis induced by a variety of agents both in vitro and in vivo. Cells transfected with a redox-inactive mutant thioredoxin show inhibited tumor growth in vivo. Dietary selenium has been shown to prevent some forms of human cancer. Selenocysteine is an essential component of thioredoxin reductase, the flavoenzyme that is responsible for the reduction of thioredoxin. Selenium added to the culture medium increases thioredoxin reductase activity due to an increase in thioredoxin reductase protein but mostly due to an increase in the specific activity of the enzyme. Some diaryl chalcogenide (selenium and tellurium) compounds have been studied as inhibitors of thioredoxin reductase. The most active were diaryl tellurium compounds, which were noncompetitive inhibitors of thioredoxin reductase with Ki values of 2-10 microM. Several of the compounds inhibited cancer cell colony formation in vitro with IC50s as low as 2 microM.

Transfection with human thioredoxin increases cell proliferation and a dominant-negative mutant thioredoxin reverses the transformed phenotype of human breast cancer cells.

Thioredoxin, a redox protein with growth factor activity that modulates the activity of several proteins important for cell growth, has been reported to be overexpressed in a number of human primary cancers. In the present study, the effects of stably transfecting mouse NIH 3T3 cells and MCF-7 human breast cancer cells with cDNA for wild-type human thioredoxin or a redox-inactive mutant thioredoxin, Cys32-->Ser32/Cys35-->Ser35 (C32S/C35S), on cell proliferation and transformed phenotype have been investigated. NIH 3T3 cells transfected with thioredoxin achieved increased saturation densities compared with vector alone-transfected cells, but were not transformed as assessed by tumor formation in immunodeficient mice. Thioredoxin-transfected MCF-7 cells showed unaltered monolayer growth on plastic surfaces compared with vector alone-transfected cells, but exhibited severalfold increased colony formation in soft agarose. Stable transfection of NIH 3T3 and MCF-7 cells with C32S/C35S resulted in inhibition of monolayer growth on plastic surfaces, and up to 73% inhibition of colony formation by MCF-7 cells in soft agarose. When inoculated into immunodeficient mice, thioredoxin-transfected MCF-7 cells formed tumors, although with a 38-57% growth rate compared with vector alone-transfected cells, whereas tumor formation by C32S/C35S-transfected MCF-7 cells was almost completely inhibited. The results of the study suggest that thioredoxin plays an important role in the growth and transformed phenotype of some human cancers. The inhibition of tumor cell growth by the dominant-negative redox-inactive mutant thioredoxin suggests that thioredoxin could be a novel target for the development of drugs to treat human cancer.

Thioredoxin and thioredoxin reductase gene expression in human tumors and cell lines, and the effects of serum stimulation and hypoxia.

Thioredoxin and thioredoxin reductase are redox proteins that have been implicated in the control of cell proliferation and transformation. We report the levels and activity of these proteins and their mRNAs in human primary tumors and tumor cell lines. Half of human primary colorectal carcinomas (5/10) examined had increased thioredoxin mRNA, of 3- to over 100-fold, compared to adjacent normal colonic mucosa from the same subject. Thioredoxin reductase protein and activity were increased an average of 2-fold in human colorectal tumors compared to normal mucosa. A number of human hematologic and solid tumor cell lines were studied and showed a 10-fold range of thioredoxin mRNA and a 23-fold range of thioredoxin reductase mRNA. Increased proliferation and hypoxia are factors that might contribute to the increased expression in solid tumors. We found that serum stimulation of growth arrested MCF-7 breast cancer cells caused a 59% increase in thioredoxin mRNA and a 62% increase in thioredoxin reductase mRNA by 24 hours. Exposure of HT-20 colon cancer cells to hypoxia resulted in a 14-fold increase in thioredoxin mRNA by 16 hours, and a transient 4-fold increase in thioredoxin reductase mRNA at 1 hour that had returned to control levels by 8 hours. Cancer cells were found to release thioredoxin into the medium at rates between 1 to 2 pmole/10(6) cells/3 hours. The rate of secretion was not, however, related to cellular-levels of thioredoxin. The results of the study show that the expression of thioredoxin and thioredoxin reductase are increased several fold in some human solid tumors compared to normal tissue. Secretion of thioredoxin, which is known to have a direct growth stimulating activity, by human tumor cells might lead to the stimulation of cancer cell growth.

Human thioredoxin reductase gene localization to chromosomal position 12q23-q24.1 and mRNA distribution in human tissue.

Thioredoxin reductase is a member of the pyridine nucleotide-disulfide oxidoreductase family of enzymes. By delivering reducing equivalents to thioredoxin, thioredoxin reductase exerts control over a number of redox-sensitive factors in the cell, including ribonucleotide reductase and several transcription factors. We have localized the human thioredoxin gene to chromosomal position 12q23-q24.1 by in situ hybridization. We have also determined the relative tissue distribution of thioredoxin reductase mRNA as well as thioredoxin mRNA by probing a Northern blot of several human normal tissues.

Cloning and sequencing of a human thioredoxin reductase.

The DNA sequence encoding human placental thioredoxin reductase has been determined. Of the 3826 base pairs sequenced, 1650 base pairs were in an open reading frame encoding a mature protein with 495 amino acids and a calculated molecular mass of 54,171. Sequence analysis showed strong similarity to glutathione reductases and other NADPH-dependent reductases. Human thioredoxin reductase contains the redox-active cysteines in the putative FAD binding domain and has a dimer interface domain not previously seen with prokaryote and lower eukaryote thioredoxin reductases.

Over-expression of DT-diaphorase in transfected NIH 3T3 cells does not lead to increased anticancer quinone drug sensitivity: a questionable role for the enzyme as a target for bioreductively activated anticancer drugs.

Murine NIH 3T3 cells were stably transfected with human NQO1 (DT-diaphorase) cDNA and clonal cell lines with up to 15-fold elevated DT-diaphorase activity were obtained. These cell lines showed no significant increase in cell growth inhibition by the quinone anticancer drugs mitomycin C, diaziquone and menadione, when compared to vector alone transfected control cells. There was a small increase in sensitivity to doxorubicin. The relative increase in DT-diaphorase activity in the transfected cells compared to the control cell lines is similar to the increase of DT-diaphorase activity found in some human tumors compared to their paired normal tissue. The results of this study, and other evidence, suggests that DT-diaphorase may not, as suggested by others, be a clinically useful target for the bioreductive activation of anticancer drugs.

An alternatively spliced form of NQO1 (DT-diaphorase) messenger RNA lacking the putative quinone substrate binding site is present in human normal and tumor tissues.

DT-diaphorase is a ubiquitously expressed flavoenzyme responsible for the two-electron reduction of a number of quinone and other anticancer drugs. The majority of DT-diaphorase enzyme activity in human tissues is the product of the NQO1 gene. We have now identified a novel alternatively spliced form of human NQO1 mRNA lacking exon 4 at levels equal to or exceeding those of wild-type NQO1 mRNA. Exon 4 codes for the putative quinone substrate binding site of DT-diaphorase derived from NQO1 and the recombinant protein from alternatively spliced NQO1 mRNA lacking exon 4 has minimal enzyme activity with quinoid and other known substrates of DT-diaphorase. The physiological substrate of DT-diaphorase is unknown, and it is possible that the protein derived from the alternatively spliced NQO1 mRNA could have enzyme activity with an appropriate substrate. We found full-length DT-diaphorase protein but could not detect expression of an appropriately smaller form of DT-diaphorase in human tissues using polyclonal antibody to DT-diaphorase, suggesting that alternatively spliced NQO1 mRNA lacking exon 4 may not be translated or that the protein product is rapidly degraded. Alternative splicing of NQO1 RNA could provide an important mechanism for regulating NQO1 gene expression.

Site-directed mutagenesis of Lys36 in human thioredoxin: the highly conserved residue affects reduction rates and growth stimulation but is not essential for the redox protein's biochemical or biological properties.

Previous studies have demonstrated that a recombinant form of the human redox protein thioredoxin can stimulate the growth rate of Swiss 3T3 murine fibroblasts and that this ability to promote cellular proliferation was dependent upon a redox-active form. A site-directed mutagenesis study of the highly conserved Lys36 adjacent to the two active site cysteines of thioredoxin was performed to determine whether the basic residue was essential for the biochemical and mitogenic properties of human thioredoxin. Two mutants were generated in which the lysine residue was replaced with either glutamic acid (K36E) or leucine (K36L). While K36E and K36L were both redox-active in a thioredoxin-specific assay, the mutants exhibited decreased affinities for thioredoxin reductase relative to wild-type thioredoxin since their respective KM values increased by a factor of 5 and 7. Examination of the secondary structure of the variants by circular dichroism spectroscopy revealed that both mutants had minor variations in the overall structural content when compared to thioredoxin, with K36L being most similar to the wild-type protein. Thermal equilibrium denaturation studies of the variants showed that K36E had a TM of 69.5 degrees C. A TM value for thioredoxin and K36L could not be established because the absence of a plateau above 83 degrees C rendered it difficult to establish an upper base line and, hence, the TM. The two mutants were able to stimulate cellular proliferation, albeit with reduced efficiency when compared with wild-type thioredoxin.(ABSTRACT TRUNCATED AT 250 WORDS)

The predicted amino acid sequence of human thioredoxin is identical to that of the autocrine growth factor human adult T-cell derived factor (ADF): thioredoxin mRNA is elevated in some human tumors.

The cDNA sequences of thioredoxin obtained by PCR cloning from human colon cancer cells, human lymphoblastoid cells, and human liver have been found to be identical with the cDNA sequence reported for the autocrine growth factor, human adult T-cell leukemia derived factor (ADF). Recombinant human thioredoxin was 95% reduced by dithiothreitol and was a substrate for reduction by human thioredoxin reductase. Human non-small cell primary lung tumors from subjects who were not cigarette smokers at the time of surgery showed significantly increased levels of thioredoxin mRNA compared to thioredoxin mRNA in paired normal human lung tissue. Subjects who were smokers did not show a significant increase in lung tumor thioredoxin mRNA. The results of the study show that human thioredoxin and ADF are identical species and suggest that there may be increased production of thioredoxin (ADF) by some human cancers.

Site-directed mutagenesis of active site cysteines in human thioredoxin produces competitive inhibitors of human thioredoxin reductase and elimination of mitogenic properties of thioredoxin.

Thioredoxin and thioredoxin reductase comprise a redox system ubiquitous in all organisms. To better understand the thiol chemistry of the mammalian thioredoxin-thioredoxin reductase redox system, mutants of human thioredoxin were produced by site-directed mutagenesis in which the two active site cysteines were replaced by serine residues, individually (C32S and C35S) and in combination (C32S/C35S). C35S and C32S/C35S were found to be competitive inhibitors of the reduction of human thioredoxin by human thioredoxin reductase with Ki values of 1.8 and 6.7 microM, respectively. C32S did not inhibit thioredoxin reductase, apparently due to aggregation of the oxidized C32S species. Examination of the three mutant forms of thioredoxin by circular dichroism spectroscopy indicated that there were significant differences in the secondary structures when compared with thioredoxin. There were detectable changes in the circular dichroism spectra when thioredoxin, C35S, and C32S/C35S were bound to thioredoxin reductase, whereas C32S with thioredoxin reductase underwent only a small spectral change. Recombinant human thioredoxin stimulated DNA synthesis and the proliferation of murine fibroblasts. The ability of thioredoxin to stimulate cell proliferation could not be duplicated by either dithiothreitol or glutathione. C32S, C35S, and C32S/C35S failed to stimulate cell proliferation, showing that the redox active form of thioredoxin is necessary for eliciting growth stimulation.

The thioredoxin/thioredoxin reductase redox system and control of cell growth.

Thioredoxin is a redox protein that is important for a variety of intracellular functions, possibly including regulation of transcription factor activity. We have shown that human thioredoxin has the same predicted amino acid sequence as adult T-cell-derived leukemic cell growth factor. Recombinant human thioredoxin stimulates the proliferation of Swiss murine 3T3 fibroblasts with an EC50 of 100 nM and the proliferation of a number of human cancer cells. Site-directed mutagenesis of the active-site cysteines of thioredoxin has shown that redox activity is necessary for the stimulation of cell proliferation. Added 125I-thioredoxin is taken up by cells in culture and could have intracellular action. A series of alkyl 2-imidazolyl disulfides have been shown to be competitive inhibitors of human thioredoxin reductase with Ki values of 3.3 to 8.6 microM. The compounds inhibited Swiss 3T3 serum-dependent proliferation with IC50 values of 2.0 to 4.0 microM, and one compound inhibited Swiss 3T3 thioredoxin-dependent proliferation with an IC50 value of 60 nM.

Cigarette smoking is a determinant of DT-diaphorase gene expression in human non-small cell lung carcinoma.

The levels of NAD(P)H:(quinone-acceptor) oxidoreductase (EC.1.6.99.2) (DT-diaphorase) mRNA and enzyme activity have been studied in paired human normal lung and non-small cell lung tumor samples from patients with a history of cigarette smoking. There were significantly higher levels of DT-diaphorase mRNA (1.2 kilobases) in lung tumor compared to normal lung tissue of patients who had stopped smoking more than 6 months before surgery, with relative values (normalized to beta-actin mRNA) of 29.6 +/- 7.8 (SE) in the lung tumor compared to 11.7 +/- 2.2 in normal lung tissue (P < 0.05). There was no significant difference in DT-diaphorase mRNA between lung tumor and normal lung tissue of subjects who were smokers at the time of surgery, with values of 16.5 +/- 2.1 and 15.3 +/- 2.5 (P > 0.05), respectively. DT-diaphorase enzyme activity in normal and tumor lung tissue was positively correlated with DT-diaphorase mRNA (r = 0.908, P < 0.01). The results of the study suggest that DT-diaphorase does not function as an inducible protectant enzyme in human lung against oxidant species and carcinogens present in cigarette smoke. Metabolism of some anticancer drugs by DT-diaphorase can alter their activity. Differences in DT-diaphorase between lung tumors of smokers and past smokers might alter the response to these drugs.

Purification of human thioredoxin reductase: properties and characterization by absorption and circular dichroism spectroscopy.

The flavoenzyme thioredoxin reductase (TR) and its natural substrate thioredoxin comprise a redox system generally found in all organisms. In order to better understand the biochemistry of this redox system, TR was purified (> 4000-fold) from human placenta as a dimer of 60-kDa subunits. The molecular size of native TR was determined to be 160 kDa by gel filtration chromatography whereas migration on a sucrose gradient gave a molecular mass of 130 kDa. The pI of TR was determined to be 4.85. The temperature optima for DTNB and insulin reduction by TR were 52 and 40 degrees C, respectively. Preincubation of TR at 60 degrees C for up to 1 h showed no decrease in the enzymatic rates when assayed at 28 degrees C, while temperatures above 65 degrees C resulted in an irreversible loss of activity. Circular dichroism (CD) spectra of TR indicated that the secondary structural changes at 60 degrees C were only partly reversible at 28 degrees C. CD studies showed the flavoenzyme had a TM of 63 degrees C and above 45 degrees C began to exhibit changes in the secondary structure. Equilibrium denaturation of TR by temperature and guanidine hydrochloride suggested that FAD was not displaced during inactivation of TR and that the tertiary structure was primarily disrupted prior to denaturation of the secondary structure. The results of this study show that purified human TR is a relatively thermostable flavoenzyme whose tightly bound FAD group is not displaced by elevated temperatures up to 60 degrees C or by relatively low concentrations of guanidine hydrochloride.