Evidence that the endogenous heat-stable glucocorticoid receptor-activating factor is thioredoxin

Genistein induces apoptosis by down-regulating thioredoxin-1 in human hepatocellular carcinoma SNU-449 cells

Genistein (GEN), a natural isoflavonoid phytoestrogen, has anti-cancer activity against various types of cancers. However, GEN has not been thoroughly investigated in human hepatocellular carcinoma cells. In this study, we evaluated the anti-cancer effects of GEN on SNU-449 cells. GEN inhibited the proliferation of SNU-449 cells in a concentration-dependent manner. We observed the typical characteristics of apoptosis, such as DNA fragmentation and caspase-3 activation. To identify proteins related to GEN-induced apoptosis, we performed two-dimensional electrophoresis and identified differentially expressed proteins. Proteomic analysis showed that the antioxidant protein thioredoxin-1 was associated with GEN-induced apoptosis. GEN treatment decreased thioredoxin-1 levels and increased intracellular accumulation of reactive oxygen species. In addition, GEN activated apoptosis signal-regulating kinase 1, c-Jun N-terminal kinases (JNK) and p38. We also observed that pretreatment with the JNK and p38 inhibitors (SP600125 and SB203580) decreased GEN-induced cell death. These results indicate that GEN has potential antitumor effects against SNU-449 cells through the down-regulation of thioredoxin-1.

Modulation of nuclear receptor function by cellular redox poise

Nuclear receptors (NRs) are ligand-responsive transcription factors involved in diverse cellular processes ranging from metabolism to circadian rhythms. This review focuses on NRs that contain redox-active thiol groups, a common feature within the superfamily. We will begin by describing NRs, how they regulate various cellular processes and how binding ligands, corepressors and/or coactivators modulate their activity. We will then describe the general area of redox regulation, especially as it pertains to thiol-disulfide interconversion and the cellular systems that respond to and govern this redox equilibrium. Lastly, we will discuss specific examples of NRs whose activities are regulated by redox-active thiols. Glucocorticoid, estrogen, and the heme-responsive receptor, Rev-erb, will be described in the most detail as they exhibit archetypal redox regulatory mechanisms.

Molecular characterization, expression, and functional analysis of two thioredoxins in the black rockfish (Sebastes schlegelii)

Thioredoxins (TRxs) are a family of small evolutionarily conserved proteins that are essential for the maintenance of cellular homeostasis. Two TRx homologue cDNAs were isolated from a black rockfish concanavalin A (Con A)/phorbol myristate acetate (PMA)-stimulated leucocyte cDNA library and named BrTPx1-1 and BrTPx1-2. As compared with other known TRx peptide sequences, the most conserved regions of both BrTRx1-1 and BrTRx1-2 peptides were found to be the redox-active site Trp-Cys-X-X-Cys (WCXXC). The TRx present in most species is a TRx1-2 protein with a Cys-Pro-Gly-Cys (CPGC) active site. However, in the larger 13 kDa BrTRx1-1 protein, a Cys-Pro-Pro-Cys (CPPC) active site was identified. Here, we report the identification of a new member of the TRx protein family from the teleost black rockfish, which defines a new subclass of 13-kDa TRx1-1 proteins. Phylogenetic analysis indicated that both BrTRx1-1 and BrTRx1-2 were grouped with other vertebrate TRx1 peptides. BrTRx1-1 expression was strongly induced in peripheral blood leucocytes (PBLs) 12-24 h following Con A/PMA stimulation, with peak expression at 24 h post-stimulation. BrTRx1-2 was induced in PBLs after stimulation with lipopolysaccharide (LPS), Con A/PMA, or poly I:C at 24 h. The BrTRx1-1 gene was predominantly expressed in the liver and gills, while BrTRx1-2 was expressed in PBLs and gills. After treatment with a high concentration (10 μg/mL) of rBrTRx1-1 or rBrTRx1-2, kidney leucocytes exhibited increased cell proliferation and viability under oxidative stress.

Antitumor indolequinones induced apoptosis in human pancreatic cancer cells via inhibition of thioredoxin reductase and activation of redox signaling

Indolequinones (IQs) were developed as potential antitumor agents against human pancreatic cancer. IQs exhibited potent antitumor activity against the human pancreatic cancer cell line MIA PaCa-2 with growth inhibitory IC(50) values in the low nanomolar range. IQs were found to induce time- and concentration-dependent apoptosis and to be potent inhibitors of thioredoxin reductase 1 (TR1) in MIA PaCa-2 cells at concentrations equivalent to those inducing growth-inhibitory effects. The mechanism of inhibition of TR1 by the IQs was studied in detail in cell-free systems using purified enzyme. The C-terminal selenocysteine of TR1 was characterized as the primary adduction site of the IQ-derived reactive iminium using liquid chromatography-tandem mass spectrometry analysis. Inhibition of TR1 by IQs in MIA PaCa-2 cells resulted in a shift of thioredoxin-1 redox state to the oxidized form and activation of the p38/c-Jun NH(2)-terminal kinase (JNK) mitogen-activated protein kinase (MAPK) signaling pathway. Oxidized thioredoxin is known to activate apoptosis signal-regulating kinase 1, an upstream activator of p38/JNK in the MAPK signaling cascade and this was confirmed in our study providing a potential mechanism for IQ-induced apoptosis. These data describe the redox and signaling events involved in the mechanism of growth inhibition induced by novel inhibitors of TR1 in human pancreatic cancer cells.

Molecular cloning and expression analysis of a thioredoxin from rock bream, Oplegnathus fasciatus, and biological activity of the recombinant protein

Thioredoxins (TRxs) are a family of small, highly conserved proteins that are essential for the maintenance of cellular homeostasis. TRx1, which contains a conserved redox-active site, Cys-Gly-Pro-Cys, is a proinflammatory cytokine, B cell growth factor, macrophage migration inhibiting factor (MIF), and an immune regulatory cytokine. The TRx1 homologue cDNA was isolated from the rock bream LPS-stimulated liver cDNA library, RbTRx1. RbTRx1 consists of 730 bp full-length cDNA with a 324 bp open reading frame encoding 108 amino acids. When compared with other known TRx1 peptide sequences, the most conserved region of the RbTRx1 peptide was the redox-active site Cys-Gly-Pro-Cys. Phylogenetic analysis grouped the RbTRx1 with other vertebrate TRx1 peptides. Quantitative real-time PCR analysis revealed the presence of RbTRx1 transcripts in liver, gill, kidney, and muscle. The expression of RbTRx1 mRNA in kidney leukocytes was upregulated after bacterial and viral challenge. The kidney leukocytes were treated with a high concentration of rRbTRx1, which significantly enhanced cell proliferation (1 μg/ml and 10 μg/ml) and viability under oxidative stress (10 μg/ml).

Increased inflammatory signaling and lethality of influenza H1N1 by nuclear thioredoxin-1

Background

Cell culture studies show that the antioxidant thiol protein, thioredoxin-1 (Trx1), translocates to cell nuclei during stress, facilitates DNA binding of transcription factors NF-κB and glucocorticoid receptor (GR) and potentiates signaling in immune cells. Excessive proinflammatory signaling in vivo contributes to immune hyper-responsiveness and disease severity, but no studies have addressed whether nuclear Trx1 mediates such responses.

Methodology/Principal Findings

Transgenic mice (Tg) expressing human Trx1 (hTrx1) with added nuclear localization signal (NLS) showed broad tissue expression and nuclear localization. The role of nuclear Trx1 in inflammatory signaling was examined in Tg and wild-type (WT) mice following infection with influenza (H1N1) virus. Results showed that Tg mice had earlier and more extensive NF-κB activation, increased TNF-α and IL-6 expression, greater weight loss, slower recovery and increased mortality compared to WT. Decreased plasma glutathione (GSH) and oxidized plasma GSH/GSSG redox potential (E_hGSSG) following infection in Tg mice showed that the increased nuclear thiol antioxidant caused a paradoxical downstream oxidative stress. An independent test of this nuclear reductive stress showed that glucocorticoid-induced thymocyte apoptosis was increased by NLS-Trx1.

Conclusion/Significance

Increased Trx1 in cell nuclei can increase severity of disease responses by potentiation of redox-sensitive transcription factor activation.

Thioredoxin-related protein 14, a new member of the thioredoxin family with disulfide reductase activity: implication in the redox regulation of TNF-alpha signaling

Thioredoxin-related protein 14 (TRP14) is a novel 14-kDa disulfide reductase with two active site Cys residues in its WCPDC motif, which is comparable to the WCGPC motif of thioredoxin (Trx). Although the active site cysteine of TRP14 is sufficiently nucleophilic, its redox potential is similar to that of Trx1, and it receives the electrons from Trx reductase 1 (TrxR1) as does Trx1. TRP14 does not target the same substrate as Trx1, suggesting that TRP14 and Trx1 might act on distinct substrate proteins. Comparison of the crystal structures of TRP14 and Trx1 reveals distinct surface structures in the vicinity of their active sites. Both TRP14 and Trx1 inhibit the pathways of nuclear factor-kappaB (NF-kappaB), mitogen-activated protein kinases, and apoptosis in cells stimulated with tumor necrosis factor-alpha (TNF-alpha), but they appear to do so by acting on target proteins, some of which do not overlap. TRP14 inhibits the TNF-alpha-induced NF-kappaB activation to a greater extent than Trx1. The dynein light chain LC8 was identified as a new target of disulfide reductase activity of TRP14, and LC8 was shown to bind IkappaBalpha in a redox-dependent manner, thereby preventing its phosphorylation by IkappaB kinase. These findings elucidate the molecular mechanism by which NF-kappaB activation is regulated through TRP14.

Thioltransferases

A family of small molecular weight proteins with thiol-disulfide exchange activity have been discovered, widely distributed from E. coli to mammalian systems, called thioltransferases or glutaredoxins. There are no substantiated reports of thioltransferases-glutaredoxins in plants; however, partially purified dehydroascorbate reductase from peas had thiol-disulfide exchange catalytic activity using glutathione as reductant and S-sulfocysteine as thiosulfate cosubstrate (unpublished data). Thus, this class of proteins is universally distributed. Based on mutagenesis studies, a sequence of Cys-Pro-Tyr(Phe)-Cys- followed by Arg-Lys- or Lys alone is critical for both the thiol-disulfide exchange reaction and the dehydroascorbate reductase activity. The dithiol-disulfide loop represented by this structure is unique since the cystine closer to the N-terminus has a highly acidic thiol pKa (3.8 as determined for the pig liver enzyme) that contributes to the protein's high S- nucleophilicity. Compared with the microbial enzyme, the mammalian thioltransferases (glutaredoxins) are extended at both N and C termini by 10-12 amino acid residues, including a second pair of cysteines toward the C-terminus with no known special function. Yeast thioltransferase is more like mammalian enzymes in length (106 amino acids) but more like E. coli glutaredoxin in being unblocked at the N-terminus and having only one set of cysteines; that is, at the active center. The three mammalian enzymes, for which sequences are available, are blocked at the N-terminus by an acetyl group linked to alanine with no known special function other than possibly to impart greater cellular turnover stability. A report of carbohydrate (8.6%) content in rat liver thioltransferase has not been verified by more sensitive methods of carbohydrate analysis, nor has carbohydrate been identified in samples of purified glutaredoxin from any source. Thiol transferase and glutaredoxin are two names for the same protein based on similarity of amino acid sequence, immunochemical cross-reactivity, and other enzyme properties. The inability of thioltransferase from some mammalian sources to act as an electron carrier in ribonucleotide reductase systems, whether homologous or heterologous in origin, remains to be explained in future studies.

Role of thioredoxin in cell growth through interactions with signaling molecules

The thioredoxin system helps maintain a reducing environment in cells, but thioredoxin functions as more than simply an antioxidant. Thioredoxin functions depend on the protein's redox state, as determined by two conserved cysteines. Key biologic activities of thioredoxin include antioxidant, growth control, and antiapoptotic properties, resulting from interaction with target molecules including transcription factors. Mechanisms by which thioredoxin regulates cell growth include binding to signaling molecules such as apoptosis signal-regulating kinase-1 (ASK-1) and thioredoxin-interacting protein (Txnip). The molecular interplay between thioredoxin, ASK-1, and Txnip potentially influences cell growth and survival in diverse human diseases such as cancer, diabetes, and heart disease. In this review, we focus on the structure of thioredoxin and its functional regulation of cell growth through the interactions with signaling molecules.

Thioredoxin reductase and cancer cell growth inhibition by organogold(III) compounds

Thioredoxin (Trx) expression is increased in several human primary cancers associated with aggressive tumor growth and decreased patient survival, and the Trx/Trx reductase (TrxR) system therefore provides an attractive target for cancer drug development. Various gold(III) compounds with none, one, two or three carbon-gold bonds were evaluated for their capacity to inhibit TrxR and the growth of MCF-7 cancer cells in vitro. Compounds with up to two carbon-gold bonds were often potent inhibitors of TrxR with IC50 values as low as 2 nmol/l. In the presence of Trx and insulin the inhibiting capacity was much lower. However, the inhibitory concentrations of the compounds did not correlate with the ability to kill cells. Out of the organometallics tested, only compound 8 with two carbon-gold bonds was able to inhibit colony formation by MCF-7 breast cancer cells at low micromolar concentrations (IC50=1.6 micromol/l). Unfortunately, the compound did not show any anti-tumor activity against MCF-7 breast cancer and HT-29 colon cancer xenografts in scid mice.

Annexin A2-S100A10 heterotetramer, a novel substrate of thioredoxin

The binding of plasminogen activators and plasminogen to the cell surface results in the rapid generation of the serine protease plasmin. Plasmin is further degraded by an autoproteolytic reaction, resulting in the release of an angiostatin, A61 (Lys78-Lys468). Previously, we demonstrated that the annexin A2-S100A10 heterotetramer (AIIt) stimulates the release of A61 from plasmin by promoting the autoproteolytic cleavage of the Lys468-Gly469 bond and reduction of the plasmin Cys462-Cys541 disulfide (Kwon, M., Caplan, J. F., Filipenko, N. R., Choi, K. S., Fitzpatrick, S. L., Zhang, L., and Waisman, D. M. (2002) J. Biol. Chem. 277, 10903-10911). Mechanistically, it was unclear if AIIt promoted a conformational change in plasmin, resulting in contortion of the plasmin disulfide, or directly reduced the plasmin disulfide. In the present study, we show that AIIt thiols are oxidized during the reduction of plasmin disulfides, establishing that AIIt directly participates in the reduction reaction. Incubation of HT1080 cells with plasminogen resulted in the rapid loss of thiol-specific labeling of AIIt by 3-(N-maleimidopropionyl)biocytin. The plasminogen-dependent oxidation of AIIt could be attenuated by thioredoxin. Thioredoxin reductase catalyzed the transfer of electrons from NADPH to the oxidized thioredoxin, thus completing the flow of electrons from NADPH to AIIt. Therefore, we identify AIIt as a substrate of the thioredoxin system and propose a new model for the role of AIIt in the redox-dependent processing of plasminogen and generation of an angiostatin at the cell surface.

The Thioredoxin-1 Inhibitor 1-Methylpropyl 2-Imidazolyl Disulfide (PX-12) Decreases Vascular Permeability in Tumor Xenografts Monitored by Dynamic Contrast Enhanced Magnetic Resonance Imaging

Purpose: The purpose of this study was to use dynamic contrast enhanced magnetic resonance imaging (DCE-MRI) to measure changes in tumor xenograft permeability produced by the antitumor thioredoxin-1 (Trx-1) inhibitor 1-methylpropyl 2-imidazolyl disulfide (PX-12) and to assess the relationship to Trx-1 and vascular endothelial growth factor (VEGF) levels.

Experimental Design: DCE-MRI was used to monitor the dynamics of gadolinium-diethylenetriaminepentaacetic acid coupled bovine serum albumin as a macromolecular contrast reagent to measure hemodynamic changes in HT-29 human colon xenografts in immunodeficient mice treated with PX-12. Blood vessel permeability was estimated from the slope of the enhancement curves, and tumor vascular volume fraction from the ordinate. Tumor Trx-1 and VEGF was also measured.

Results: PX-12 caused a rapid 63% decrease in the average tumor blood vessel permeability within 2 hours of administration. The decrease lasted 24 hours and had returned to pretreatment values by 48 hours. The changes in vascular permeability were not accompanied by alterations in average tumor vascular volume fraction. There was a decrease in tumor and tumor-derived VEGF in plasma at 24 hours after treatment with PX-12, but not at earlier time points. However, tumor redox active Trx-1 showed a rapid decline within 2 hours following PX-12 administration that was maintained for 24 hours.

Conclusion: The rapid decrease in tumor vascular permeability caused by PX-12 administration coincided with a decrease in tumor redox active Trx-1 and preceded a decrease in VEGF. DCE-MRI responses to PX-12 in patients of Trx-1 inhibition at early time points and decreased VEGF at later times, may be useful to follow tumor response and even therapeutic benefit.

A positive charge at position 33 of thioredoxin primarily affects its interaction with other proteins but not redox potential

Oxidoreductases of the thioredoxin superfamily possess the C-X-X-C motif. The redox potentials vary over a wide range for these proteins. A crucial determinant of the redox potential has been attributed to the variation of the X-X dipeptide. Here, we substitute Lys for Gly at the first X of Escherichia coli thioredoxin to investigate how a positive charge would affect the redox potential. The substitution does not affect the protein's redox potential. The equilibrium constant obtained from pairwise reaction between the mutant and wild-type proteins equals 1.1, indicating that the replacement does not significantly affect the thiol-disulfide redox equilibrium. However, the catalytic efficiency of thioredoxin reductase on the G33K mutant decreases approximately 2.8 times compared to that of the wild type. The mutation mainly affects K(m), with little effect on k(cat). The mutation also inhibits thioredoxin's ability to reduce insulin disulfide by approximately one-half. Whether the mutant protein supports the growth of phages T3/7 and f1 was tested. The efficiency of plating (EOP) of T3/7 on the mutant strain decreases 5 times at 37 degrees C and 3 x 10(4) times at 42 degrees C relative to that of the wild-type strain, suggesting that interaction between phage gene 5 protein and thioredoxin is hindered. The mutation also reduces the EOP of phage f1 by 8-fold at 37 degrees C and 1.5-fold at 42 degrees C. The global structure of the mutant protein does not change when studied by CD and fluorescence spectra. Therefore, G33K does not significantly affect the overall structure or redox potential of thioredoxin, but primarily interferes with its interaction with other proteins. Together with the G33D mutation, the overall results show that a charged residue at the first X has a greater influence on the molecular interaction of the protein than the redox potential.

Increased expression of thioredoxin-1 in human colorectal cancer is associated with decreased patient survival

Thioredoxin-1 is a redox protein that, when overexpressed, causes increased cancer-cell growth and inhibited apoptosis. Thioredoxin-1 expression has been reported to be increased in several human primary tumors, but its relationship to tumor progression and patient survival has not been established. We studied the expression of thioredoxin-1 as measured with immunohistochemical staining in paraffin-embedded human normal colonic mucosa, adenomatous polyps, and primary and metastatic colorectal cancer. Thioredoxin-1 expression was not increased in 12 colorectal adenomatous polyps, compared with 8 samples of normal colonic mucosa, but was significantly increased in 12 primary colorectal cancers (P <.01). Thioredoxin-1 expression was not significantly different in primary lymph-node metastases and the primary colorectal cancer. Using colorectal cancer samples from 37 subjects for whom survival data was available, we found that thioredoxin-1 expression increased with Dukes stage, although the association was not statistically significant (P =.077). We noted a significant association between thioredoxin-1 expression and patient survival (P =.004); higher score was associated with decreased survival. When adjusted for Dukes stage, thioredoxin-1 expression showed a statistically significant association with survival (P =.012). The work shows that increased thioredoxin-1 expression is a relatively late event in colorectal carcinogenesis and provides evidence in a small group of subjects with colorectal cancer of Dukes stages A through D that thioredoxin-1 expression may be an independent marker of patient prognosis.

Induction of hepatic thioredoxin reductase activity by sulforaphane, both in Hepa1c1c7 cells and in male Fisher 344 rats

Sulforaphane (SF), a glucosinolate-derived isothiocyanate found in cruciferous vegetables, is considered an anticarcinogenic component in broccoli. Sulforaphane induces a battery of detoxification enzymes, including quinone reductase (QR). Induction is thought to be mediated through a common regulatory region termed the antioxidant response element (ARE). To test the hypothesis that the antioxidant selenoprotein thioredoxin reductase (TR) may be induced as part of this coordinated host-defense response to dietary anticarcinogenic compounds, TR activity was measured in livers of rats pair-fed diets containing SF and/or broccoli (n = 6/group). At the doses used, neither SF nor broccoli alone significantly elevated TR activity, whereas treatments containing both broccoli and SF caused a significant increase in TR activity. Glutathione peroxidase (GSH-Px), a second selenium-dependant enzyme with antioxidant activity, was downregulated in rats fed both SF and broccoli, compared to the control diet.A second experiment, using mouse hepatoma Hepa1c1c7 cells, tested whether an interaction exists between selenium (Se) and SF in TR inducibility, since Se is known to induce TR activity. Selenium (2.5 &mgr;M) plus SF (2.0 &mgr;M) caused significantly greater TR activity than either treatment alone. All treatments with added Se or SF caused significantly greater TR activities than no Se or SF treatment. Glutathione peroxidase activity was elevated by Se, but not by SF. These data suggest that TR, known to be regulated by Se, is also upregulated as part of a host response to the dietary anticarcinogen SF, a trait not shared by another Se-dependent enzyme, GSH-Px.

Thioredoxin-related regulation of NO/NOS activities

The role of regulation of nitric oxide synthase (NOS) activity in mitigating oxidative stress in neonatal lungs and contributing to pulmonary vasodilation at birth is still unclear. Furthermore, it is known that, depending on interactions between the individual components of the mitogen-activated protein kinase (MAPK) signaling cascades, many biological consequences, including apoptosis, are initiated. Although the importance of nitric oxide (NO) in apoptosis is controversial and likely depends on NO concentrations and cell types, this highly reactive free radical can activate the p38 MAPK signal cascade. Recent studies have suggested that thioredoxin may play an important role as an effector for some of these functions. Thioredoxin is a major redox protein for many enzymes/transcription factors and is involved in cellular functions, such as viability, activation, and proliferation. In addition to its redox regulation, thioredoxin binds directly to the apoptosis signal-regulating kinase 1 (ASK1), thus inhibiting the activation of stress-induced MAPK signaling cascades that lead to apoptosis. Furthermore, NO produced from newly induced neuronal NOS was reported to induce expression of thioredoxin and several other genes for preconditioning-induced neuroprotection. Moreover, although exposure of endothelial cells to NO decreases NOS activity, this inhibition was shown to be reversed by thioredoxin. Finally, the correlation of expression of thioredoxin with endothelial NOS activity seems to suggest an important role played by this protein in perinatal changes of pulmonary artery functions. Therefore, thioredoxin may participate in the regulation of NOS activity and be involved in NO functions via multiple mechanisms.

Properties and biological activities of thioredoxins

The mammalian thioredoxins are a family of small (approximately 12 kDa) redox proteins that undergo NADPH-dependent reduction by thioredoxin reductase and in turn reduce oxidized cysteine groups on proteins. The two main thioredoxins are thioredoxin- 1, a cytosolic and nuclear form, and thioredoxin-2, a mitochondrial form. Thioredoxin-1 has been studied more. It performs many biological actions including the supply of reducing equivalents to thioredoxin peroxidases and ribonucleotide reductase, the regulation of transcription factor activity, and the regulation of enzyme activity by heterodimer formation. Thioredoxin-1 stimulates cell growth and is an inhibitor of apoptosis. Thioredoxins may play a role in a variety of human diseases including cancer. An increased level of thioredoxin-1 is found in many human tumors, where it is associated with aggressive tumor growth. Drugs are being developed that inhibit thioredoxin and that have antitumor activity.

Properties and biological activities of thioredoxins

The mammalian thioredoxins are a family of small (approximately 12 kDa) redox proteins that undergo NADPH-dependent reduction by thioredoxin reductase and in turn reduce oxidized cysteine groups on proteins. The two main thioredoxins are thioredoxin-1, a cytosolic and nuclear form, and thioredoxin-2, a mitochondrial form. Thioredoxin-1 has been studied more. It performs many biological actions including the supply of reducing equivalents to thioredoxin peroxidases and ribonucleotide reductase, the regulation of transcription factor activity, and the regulation of enzyme activity by heterodimer formation. Thioredoxin-1 stimulates cell growth and is an inhibitor of apoptosis. Thioredoxins may play a role in a variety of human diseases including cancer. An increased level of thioredoxin-1 is found in many human tumors, where it is associated with aggressive tumor growth. Drugs are being developed that inhibit thioredoxin and that have antitumor activity.

Alternative Splicing Is Associated with Decreased Expression of the Redox Proto-oncogene Thioredoxin-1 in Human Cancers

Human thioredoxin-1 (Trx-1) is a small redox protein that is overexpressed in a number of human primary tumors, where it is associated with rapid cell proliferation and inhibited apoptosis. Mutation scanning denaturing high-performance liquid chromatography of Trx-1 mRNA in 58 human tumor cell lines found no evidence for changes in the base sequence of human Trx-1 mRNA. An alternatively spliced form of Trx-1 mRNA lacking exons 2 and 3 was found in 7 of the cell lines but it was not translated. The cell lines having the alternatively spliced Trx-1 mRNA had 73% lower total Trx-1 mRNA than the other cell lines, suggesting that alternative splicing may control the level of Trx-1 mRNA in some cancer cells.

Expression and localisation of thioredoxin in mouse reproductive tissues during the oestrous cycle

Thioredoxin expression within the reproductive tissues of the female mouse was analysed during the oestrous cycle stages of dioestrus, oestrus and metoestrus by Western blot analyses and immunocytochemistry. From Western blot analyses the expression of thioredoxin was found to be increased in oestrus compared to dioestrus and metoestrus. Localisation of thioredoxin within the reproductive organs of the mouse during the oestrous cycle has shown that the expression of thioredoxin is specific for distinct areas within the reproductive organs. These areas are the stratified squamous epithelium of the vagina, the simple columnar epithelium and the uterine glands of the uterus, the ciliated columnar epithelium of the oviduct, the corpus lutea, the interstitial cells and the secondary follicles of the ovary. The discrete cellular localisation and oestrous dependence of thioredoxin expression are suggestive of specific roles in various reproductive processes.

Trace elements in regulation of NF-kappaB activity

In recent years several studies have shown that NF-kappaB might be a very important therapeutic target in the treatment ot various chronic inflammatory, degenerative and tumour diseases. Trace elements play essential roles in the regulation ot cell signaling mechanisms via transcription tactors and a large number of genes. An important aspect of the present review is the description ot the mechanisms by which trace elements might influence transcription factor NF-kappaB. DNA-binding activity of NF-kappaB is regulated by the redox state of the cysteine residue (Gys-62) in the DNA binding domain of the p50 subunit and impaired by different metals (Go, Cr, Ni, Cd, Pb). It has been hypothesised that the broad speciticity of interrelationships between NF-kappaB. AP-1 and various metals results from interactions of metals with specific moieties of transcription factors and IkappaB-kinases, as well as trom the existence of a metal-governed redox system. The hypothetical targets in the NF-kappaB signaling pathway affected by metals are: IkappaB-kinases, IkappaBs, NF-kappaB, proteasome degradation of NF-kappaB, kappaB-sites in DNA. Possibly, this system is required by the cell for adequate regulation ot the transcription machinery in response to changes in intracellular and intranuclear fluxes of metals and radicals and is very ancient evolutionary mechanism of stress adaptation. The role of the NF-kappaB-mediated mechanism in induction or prevention of chronic intlammatory, allergic, degenerative and tumor diseases by zinc, vanadium, manganese, copper, silica, iodine and other trace elements is discussed.

The role of the redox protein thioredoxin in cell growth and cancer

The thioredoxins are ubiquitous proteins containing a conserved -Trp-Cys-Gly-Pro-Cys-Lys- redox catalytic site. Mammalian thioredoxin family members include thioredoxin-1 (Trx1), mitochondrial thioredoxin-2 (Trx2), and a larger thioredoxin-like protein, p32TrxL. Thioredoxin is reduced by NADPH and thioredoxin reductase and, in turn reduces oxidized cysteine groups on proteins. When thioredoxin levels are elevated there is increased cell growth and resistance to the normal mechanism of programmed cell death. An increase in thioredoxin levels seen in many human primary cancers compared to normal tissue appears to contribute to increased cancer cell growth and resistance to chemotherapy. Mechanisms by which thioredoxin increases cell growth include an increased supply of reducing equivalents for DNA synthesis, activation of transcription factors that regulate cell growth, and an increase in the sensitivity of cells to other cytokines and growth factors. The mechanisms for the inhibition of apoptosis by thioredoxin are just now being elucidated. Because of its role in stimulating cancer cell growth and as an inhibitor of apoptosis, thioredoxin offers a target for the development of drugs to treat and prevent cancer.

cDNA sequence of bovine thioredoxin

In this paper, we report the cDNA sequence of bovine thioredoxin. We determined the full-length cDNA sequence of bovine thioredoxin by RT-PCR, 5'-RACE and 3'-RACE methods. Currently, the thioredoxin cDNA sequences of only five mammalian species (human, macaca, mouse, ovine and rat) are registered in the GenBank database. We performed sequence comparisons on the total cDNA sequence and the coding region, and produced a multialignment between the amino acid sequences of bovine and other mammalian thioredoxins. The amino acid sequences of thioredoxins are highly conserved among mammalian species, for example, only one difference exists between the amino acid sequences of bovine and ovine thioredoxin.

cDNA sequence of bovine thioredoxin

In this paper, we report the cDNA sequence of bovine thioredoxin. We determined the full-length cDNA sequence of bovine thioredoxin by RT-PCR, 5'-RACE and 3'-RACE methods. Currently, the thioredoxin cDNA sequences of only five mammalian species (human, macaca, mouse, ovine and rat) are registered in the GenBank database. We performed sequence comparisons on the total cDNA sequence and the coding region, and produced a multialignment between the amino acid sequences of bovine and other mammalian thioredoxins. The amino acid sequences of thioredoxins are highly conserved among mammalian species, for example, only one difference exists between the amino acid sequences of bovine and ovine thioredoxin.

Thioredoxin, a putative oncogene product, is overexpressed in gastric carcinoma and associated with increased proliferation and increased cell survival

Human thioredoxin is a putative oncogene that may confer both a growth and survival advantage to tumor cells. Overexpressed thioredoxin mRNA has been found in both primary human lung and colorectal cancers. To determine the intratumor distribution and amount of thioredoxin protein in human primary carcinomas, we developed an immunohistochemical assay for thioredoxin in paraffin-embedded tissue. We then studied 10 patients with primary high-risk gastric carcinoma. To further relate thioredoxin protein overexpression to cell death and survival, we used a paraffin-based in situ end-labeling (ISEL) assay. To delineate proliferation, we used the nuclear proliferation antigen detected by Ki-67. In this survey, we found that thioredoxin was localized to tumor cells and overexpressed compared with normal gastric mucosa in 8 of 10 gastric carcinomas. The thioredoxin was found at high levels in 5 of the 8 overexpressing carcinomas. The overexpression of thioredoxin was typically found in both a nuclear and cytoplasmic location in the neoplastic cells. There was a significant positive correlation (P = .0061) with cancer cell proliferation measured by Ki-67. There was a significant negative correlation (P = .0001) with DNA damage measured by the ISEL assay, suggesting decreased apoptosis and increased carcinoma cell survival. Thus, human primary gastric tumors that are highly expressive of thioredoxin have both a higher proliferative rate and a higher survival rate than tumors that do not express thioredoxin. With these newly developed assays in hand, it is now feasible to question whether this thioredoxin-related combined growth and survival advantage translates into poor clinical outcome.

G33D mutant thioredoxin primarily affects the kinetics of reaction with thioredoxin reductase. Probing the structure of the mutant protein

Escherichia coli thioredoxin is a redox-active protein. A mutant protein with an aspartic acid substitution for the largely conserved glycine at position 33 (G33D) in the active site of thioredoxin has been generated to study the effects of a negatively charged residue in the active site of the protein. Despite the close proximity of the negative-charged Asp to the redox active cysteines, the effective concentration of the cysteines does not deviate significantly from that of the wild-type protein. The redox potential (E(o)') measured by the equilibrium between NADPH and the mutant thioredoxin is also close to that of the wild-type. Kinetic measurements of the reaction between thioredoxin and thioredoxin reductase show that G33D mutant and the wild-type proteins have identical kcat values. However, the Km for G33D mutant is approximately 10-fold higher than that for the wild-type protein. In vivo assay of the growth of E. coli strain carrying wild-type or G33D mutant thioredoxin on methionine sulfoxide indicates that the G33D mutant protein is a slower electron donor for methionine sulfoxide reductase. Structural stability of the oxidized protein is not altered by the G33D substitution, as illustrated by the same unfolding free energies studied by urea. The substitution does not show significant change of the near UV and far-UV circular dichroic (CD) and the fluorescence spectra for either the reduced or the oxidized protein. Therefore, the global structure of the G33D protein is not changed. However, the surface of the active site has been altered locally by G33D substitution, which accounts for the above kinetically poor behaviors. A model of G33D structure is constructed based on these studies.

Importance of thioredoxin in the proteolysis of an immunoglobulin G as antigen by lysosomal Cys-proteases

For disulphide-bonded antigens, reduction has been postulated to be a prerequisite for proteolytic antigen processing, with subsequent production of major histocompatibility complex (MHC) class II binding fragments. The murine monoclonal immunoglobulin G (IgG) CE25/B7 was used as a multimeric antigen in a human model. Native IgG is highly resistant to proteolysis and has been previously found to be partially reduced at early steps of cell processing to become a suitable substrate for endopeptidases. The role of the oxidoreductase thioredoxin (Trx) was assessed in the reduction of the IgG by cleavage of H-L and H-H disulphide bonds. Recombinant human Trx (rTrx) has been assayed in a proteolytic in vitro system on IgG using endosomal and lysosomal subcellular fractions from B lymphoblastoid cells. rTrx is required in a dose-dependent manner for development of efficient proteolysis, catalysed by thiol-dependent Cys-proteases, such as cathepsin B. We demonstrated that cathepsin B activity was stimulated by the addition of rTrx. Thus, we propose that Trx-dependent IgG proteolysis occurred, on the one hand by means of the unfolding of the IgG after disulphide reduction, becoming a substrate of lysosomal proteases, and on the other hand by Cys-proteases such as cathepsin B that are fully active upon the regeneration of their activity by hydrogen donors.

Direct association with thioredoxin allows redox regulation of glucocorticoid receptor function

The glucocorticoid receptor (GR) is considered to belong to a class of transcription factors, the functions of which are exposed to redox regulation. We have recently demonstrated that thioredoxin (TRX), a cellular reducing catalyst, plays an important role in restoration of GR function in vivo under oxidative conditions. Although both the ligand binding domain and other domains of the GR have been suggested to be modulated by TRX, the molecular mechanism of the interaction is largely unknown. In the present study, we hypothesized that the DNA binding domain (DBD) of the GR, which is highly conserved among the nuclear receptors, is also responsible for communication with TRX in vivo. Mammalian two-hybrid assay and glutathione S-transferase pull-down assay revealed the direct association between TRX and the GR DBD. Moreover, analysis of subcellular localization of TRX and the chimeric protein harboring herpes simplex viral protein 16 transactivation domain and the GR DBD indicated that the interaction might take place in the nucleus under oxidative conditions. Together these observations indicate that TRX, via a direct association with the conserved DBD motif, may represent a key mediator operating in interplay between cellular redox signaling and nuclear receptor-mediated signal transduction.

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.

Thioredoxin redox control of cell growth and death and the effects of inhibitors

Thioredoxin is a redox protein found over-expressed in some human tumors. Thioredoxin is secreted by tumor cells and stimulates cancer cell growth. Redox activity is essential for growth stimulation 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. Thus, thioredoxin offers an attractive target for anticancer drug development. A class of disulfide inhibitors of thioredoxin has been identified. These disulfides inhibit cancer cell growth in culture and have antitumor activity against some human tumor xenografts in animals.

Mechanisms of inhibition of the thioredoxin growth factor system by antitumor 2-imidazolyl disulfides

The interactions of a series of 2-imidazolyl disulfide antitumor compounds with the thioredoxin reductase(TR)/thioredoxin (hTrx) redox system have been studied. Disulfides III-2 (n-butyl 2-mercaptoimidazolyl disulfide) and VI-2 (ethyl 2-mercaptoimidazolyl disulfide) were substrates for reduction by TR with Km values of 43 and 48 microM. Disulfides IV-2 (1-methylpropyl 2-mercaptoimidazolyl disulfide) and DLK-36 (benzyl 2-mercaptoimidazolyl disulfide) were competitive inhibitors of the reduction of hTrx by TR with Ki values of 31 microM. None of the disulfides were substrates for reduction by human glutathione reductase. The disulfides caused reversible thioalkylation of hTrx at the redox catalytic site as shown by the fact that there was no thioalkylation of a mutant hTrx where both the catalytic site Cys32 and Cys35 residues were replaced by Ser. In addition, the disulfides caused a slower irreversible inactivation of hTrx as a substrate for reduction by TR, with half-lives for III-2 of 30 min, for IV-2 of 4 hr, and for IX-2 (t-butyl 2-mercaptoimidazolyl disulfide) of 24 hr. This irreversible inactivation of hTrx occurred at concentrations of the disulfides an order of magnitude below those that inhibited TR, and involved the Cys73 of hTrx, which is outside the conserved redox catalytic site, as shown by the resistance to inactivation of a mutant hTrx where Cys73 was replaced by Ser. Electrophoretic and mass spectral analyses of the products of the reaction between the disulfides and hTrx show that modification of 1-3 Cys residues of the protein occurred in a concentration-dependent fashion. The disulfides inhibited the hTrx-dependent proliferation of MCF-7 breast cancer cells with IC50 values for III-2 and IV-2 of 0.2 and 1.2 microM, respectively. The results show that although the catalytic sites of TR and hTrx are reversibly inhibited by the 2-imidazolyl disulfides, it is the irreversible thioalkylation of Cys73 of hTrx by the disulfides that most probably accounts for the inhibition of thioredoxin-dependent cell growth by the disulfides.

Steroid receptor interactions with heat shock protein and immunophilin chaperones

We have provided a historical perspective on a body of steroid receptor research dealing with the structure and physiological significance of the untransformed 9S receptor that has often confused both novice and expert investigators. The frequent controversies and equivocations of earlier studies were due to the fact that the native, hormone-free state of these receptors is a large multiprotein complex that resisted description for many years because of its unstable and dynamic nature. The untransformed 9S state of the steroid and dioxin receptors has provided a unique system for studying the function of the ubiquitous, abundant, and conserved heat shock protein, hsp90. The hormonal control of receptor association with hsp90 provided a method of manipulating the receptor heterocomplex in a manner that was physiologically meaningful. For several steroid receptors, binding to hsp90 was required for the receptor to be in a native hormone-binding state, and for all of the receptors, hormone binding promoted dissociation of the receptor from hsp90 and conversion of the receptor to the DNA-binding state. Although the complexes between tyrosine kinases and hsp90 were discovered earlier, the hormonal regulation or steroid receptor association with hsp90 permitted much more rapid and facile study of hsp90 function. The observations that hsp90 binds to the receptors through their HBDs and that these domains can be fused to structurally different proteins bringing their function under hormonal control provided a powerful linkage between the hormonal regulation of receptor binding to hsp90 and the initial step in steroid hormone action. Because the 9S receptor hsp90 heterocomplexes could be physically stabilized by molybdate, their protein composition could be readily studied, and it became clear that these complexes are multiprotein structures containing a number of unique proteins, such as FKBP51, FKBP52, CyP-40, and p23, that were discovered because of their presence in these structures. Further analysis showed that hsp90 itself exists in a variety of native multiprotein heterocomplexes independent of steroid receptors and other 'substrate' proteins. Cell-free systems can now be used to study the formation of receptor heterocomplexes. As we outlined in the scheme of Fig. 1, the multicomponent receptor-hsp90 heterocomplex assembly system is being reconstituted, and the importance of individual proteins, such as hsp70, p60, and p23, in the assembly process is becoming recognized. It should be noted that our understanding of the mechanism and purpose of steroid receptor heterocomplex assembly is still at an early stage. We can now speculate on the roles of receptor-associated proteins in receptor action, both as individuals and as a group, but their actual functions are still vague or unknown. We can make realistic models about the chaperoning and trafficking of steroid receptors, but we don't yet know how these processes occur, we don't know where chaperoning occurs in the cell (e.g. Is it limited to the cytoplasm? Is it a diffuse process or does chaperoning occur in association with structural elements?), and, with the exception of the requirement for hormone binding, we don't know the extent to which the hsp90-based chaperone system impacts on steroid hormone action. It is not yet clear how far the discovery of this hsp90 heterocomplex assembly system will be extended to the development of a general understanding of protein processing in the cell. Because this assembly system is apparently present in all eukaryotic cells, it probably performs an essential function for many proteins. The bacterial homolog of hsp90 is not an essential protein, but hsp90 is essential in eukaryotes, and recent studies indicate that the development of the cell nucleus from prokaryotic progenitors was accompanied by the duplication of genes for hsp90 and hsp70 (698). (ABSTRACT TRUNCATED)

The effect of ultraviolet B induced adult T cell leukemia-derived factor/thioredoxin (ADF/TRX) on survival and growth of human melanocytes

Ultraviolet B (UVB) radiation is known to induce reactive oxygen species (ROS) in the skin. The skin, however, counteracts ROS by both constitutional and newly produced antioxidants. One such antioxidant, adult T cell leukemia-derived factor (ADF), a human homologue of thioredoxin (TRX), was shown to be efficiently produced in and released from cultured normal human keratinocytes after UVB irradiation by Northern and Western blot analyses and enzyme-linked immunoabsorbent assay (ELISA). Recombinant ADF (rADF) did not rescue UVB-induced melanocyte death, either when added pre- or post-UV irradiation. However, further addition of neutralizing antibody caused cell death of both keratinocytes and melanocytes. rADF was shown to induce higher expression in melanocortin-1 receptor (MC1-R) mRNA accompanied by increased binding activity using 125I labeled [Nle4, D-Phe7]-alpha-MSH in melanocytes, leading to the enhanced increment of DNA synthesis. Taken together, it was shown that released ADF from UVB-irradiated keratinocytes acts as a survival factor for both keratinocytes and melanocytes but does not rescue UV-induced melanocyte death. Further, it may work as one of the stimulatory factors for UVB-induced melanogenesis by upregulating MSH-R binding activity in combination with the enhanced DNA synthesis by alpha-MSH.

Oxidative inactivation of thioredoxin as a cellular growth factor and protection by a Cys73-->Ser mutation

Thioredoxin (Trx) is a widely distributed redox protein that regulates several intracellular redox-dependent processes and stimulates the proliferation of both normal and tumor cells. We have found that when stored in the absence of reducing agents, human recombinant Trx undergoes spontaneous oxidation, losing its ability to stimulate cell growth, but is still a substrate for NADPH-dependent reduction by human thioredoxin reductase. There is a slower spontaneous conversion of Trx to a homodimer that is not a substrate for reduction by thioredoxin reductase and that does not stimulate cell proliferation. Both conversions can be induced by chemical oxidants and are reversible by treatment with the thiol reducing agent dithiothreitol. SDS-PAGE suggests that Trx undergoes oxidation to monomeric form(s) preceding dimer formation. We have recently shown by X-ray crystallography that Trx forms a dimer that is stabilized by an intermolecular Cys73-Cys73 disulfide bond. A Cys73-->Ser mutant Trx (C73S) was prepared to determine the role of Cys73 in oxidative stability and growth stimulation. C73S was as effective as Trx in stimulating cell growth and was a comparable substrate for thioredoxin reductase. C73S did not show spontaneous or oxidant-induced loss of activity and did not form a dimer. The results suggest that Trx can exist in monomeric forms, some of which are mediated by Cys73 that do not stimulate cell proliferation but can be reduced by thioredoxin reductase. Cys73 is also involved in formation of an enzymatically inactive homodimer, which occurs on long term storage or by chemical oxidation. Thus, although clearly involved in protein inactivation, Cys73 is not necessary for the growth stimulating activity of Trx.

Role of thioltransferases on the modulation of rat liver S-adenosylmethionine synthetase activity by glutathione

Rat liver S-adenosylmethionine synthetase, high- and low-Mr forms, are regulated in vitro by the GSH/GSSG ratio at pH 8. The inhibition and oxidation constants for both forms have been calculated in the presence of thioltransferases. The mechanism of the reaction appeared to involve the formation of intramolecular disulfides. Increases of 3- to 4-fold in the oxidation constants for both S-adenosylmethionine synthetase isoenzymes in the presence of protein disulfide isomerase suggested the possibility of a thiol-disulfide exchange regulatory mechanism for this enzyme in vivo. The significance of these results is discussed on the light of the data available relating glutathione changes and modulation of enzyme activities, either in vivo and in vitro.

Crystal structures of reduced, oxidized, and mutated human thioredoxins: evidence for a regulatory homodimer

BACKGROUND

Human thioredoxin reduces the disulfide bonds of numerous proteins in vitro, and can activate transcription factors such as NFkB in vivo. Thioredoxin can also act as a growth factor, and is overexpressed and secreted in certain tumor cells.

RESULTS

Crystal structures were determined for reduced and oxidized wild type human thioredoxin (at 1.7 and 2.1 A nominal resolution, respectively), and for reduced mutant proteins Cys73-->Ser and Cys32-->Ser/Cys35-->Ser (at 1.65 and 1.8 A, respectively). Surprisingly, thioredoxin is dimeric in all four structures; the dimer is linked through a disulfide bond between Cys73 of each monomer, except in Cys73-->Ser where a hydrogen bond occurs. The thioredoxin active site is blocked by dimer formation. Conformational changes in the active site and dimer interface accompany oxidation of the active-site cysteines, Cys32 and Cys35.

CONCLUSIONS

It has been suggested that a reduced pKa in the first cysteine (Cys32 in human thioredoxin) of the active-site sequence is important for modulation of the redox potential in thioredoxin. A hydrogen bond between the sulfhydryls of Cys32 and Cys35 may reduce the pKa of Cys32 and this pKa depression probably results in increased nucleophilicity of the Cys32 thiolate group. This nucleophilicity, in tum, is thought to be necessary for the role of thioredoxin in disulfide-bond reduction. The physiological role, if any, of thioredoxin dimer formation remains unknown. It is possible that dimerization may provide a mechanism for regulation of the protein, or a means of sensing oxidative stress.

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.

Immunocytochemical localization of thioredoxin in human trophoblast and decidua

An immunocytochemical investigation into the expression of thioredoxin in human reproductive tissues was performed using monoclonal antibodies produced against recombinant human thioredoxin. First trimester and term human placental villi, decidua and term fetal membranes were examined for thioredoxin content and cellular localization. In first trimester tissue strong thioredoxin staining was observed in the underlying cytotrophoblast cells and in the stromal cells present in the decidua, but not in the syncytiotrophoblast surrounding the chorionic villi. In term placental villi very little thioredoxin was observed. Term fetal membranes proved to be a rich source of thioredoxin, the most intense staining was seen in the cytotrophoblast cells in the chorionic membrane, with the amnion and decidua also showing positive immunoreactivity. The potential role/s that thioredoxin may play within the placental bed is considered.

Characterization of mammalian thioredoxin reductase, thioredoxin and glutaredoxin by immunochemical methods

Specific polyclonal antibodies towards the oxidized form of bovine thioredoxin reductase (TR) have been obtained in rabbits, and purified. The antigenicity was lost upon reduction of TR by NADPH indicating a large conformational change upon reduction of the redox-active disulfide in the enzyme. The antibodies did not cross-react with other bovine NADPH-dependent dehydrogenases. No reactivity was observed with TR from bacteria, yeast or rat and only a slight reaction was obtained with TR from horse. Immunoaffinity purified anti-thioredoxin and anti-glutaredoxin antibodies were used to develop competitive indirect ELISA assays that were validated giving very good linearity, reproducibility, sensitivity and parallelism. The glutaredoxin (Grx) immunoassay is the first quantitative method described to measure the protein. When applied to a battery of calf tissues the contents of Grx varied from 7 to 120 micrograms per gram of fresh tissue. Skeletal and heart muscles gave the lowest values and spleen and salivary glands the highest. However, skeletal muscle showed the highest gluthathione-hydroxyethyl disulfide oxidoreductase specific activity.

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)

DNA binding activity of the glucocorticoid receptor is sensitive to redox changes in intact cells

The effect of changes of redox conditions on glucocorticoid receptor (GR) activity in intact cells has been studied using two approaches. One was to evaluate the GR-DNA binding in extracts of COS2 cells transiently overexpressing GR and in which reactive oxygen intermediates (ROI) accumulate as a consequence of glutathione (GSH) depletion. GR-DNA binding was significantly decreased in COS2 cells treated with diethylmaleate (DEM), which causes GSH depletion by forming GSH-DEM complexes. A similar effect was observed for Sp1, another Zn-finger transcription factor, whereas no difference was observed for the C/EBP transcription factor, which is known to be unaffected by redox changes in vitro. N-Acetylcysteine (NAC), which counteracts the effects of DEM by increasing GSH biosynthesis, prevents the decrease of GR-DNA binding in cells treated with DEM. The GR-DNA binding efficiency was similarly decreased using extracts from H2O2-treated COS2 cells and from COS2 cells treated with buthionine sulphoximine, which causes GSH depletion via a mechanism different from that of DEM. The other approach was to evaluate the efficiency of a GR-regulated promoter under different redox conditions. In HeLa cells, transfected with a plasmid containing the CAT gene under the control of the glucocorticoid responsive element (GRE) within the mouse mammary tumor virus promoter, and treated with dexamethasone to activate GR, exposure to DEM significantly impaired the activation of CAT gene expression induced by dexamethasone. Also in this case NAC treatment inhibited the effects of DEM.

Redox signalling and the control of cell growth and death

Cells maintain a reduced intracellular state in the face of a highly oxidizing extracellular environment. Redox signalling pathways provide a link between external stimuli, through the flavoenzyme-mediated NADPH-dependent reduction of intracellular peptide thiols, such as glutathione, thioredoxin, glutaredoxin, and redox factor-1, to the posttranslational redox modification of certain intracellular proteins. This can affect the proteins' correct folding, assembly into multimeric complexes, enzymatic activity, and their binding as transcription factors to specific DNA sequences. Such changes have been linked to altered cell growth and death.

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.

Structure of a human Clara cell phospholipid-binding protein-ligand complex at 1.9 A resolution

The Clara cell phospholipid-binding protein, previously referred to as CC10, is a homodimeric protein of M(r) 15,800. It is secreted into the bronchioalveolar lining layer in mammalian lung. A combination of X-ray crystallography and chemical analysis was used to determine that phosphatidylcholine and phosphatidylinositol are bound to the protein as isolated from human lung lavage. We now report the crystal structure of the protein-phospholipid complex at 1.9 A resolution. The phospholipid is bound inside the protein's large hydrophobic cavity. A model is proposed for the manner in which a channel may open to provide access to the cavity, allowing the binding or potential release of phospholipid.