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

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Abstract

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.

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Background

Thioredoxin was first described in 1964 as a small redox protein from Escherichia coli [1]. Mammalian thioredoxin was reported in 1967 as a redox protein present in rat Novikoff hepatoma cells [2]. Thioredoxin was subsequently rediscovered under other names, including: (i) adult T cell leukemia-derived factor (ADF), an interleukin-2 (IL-2) receptor-inducing factor produced by human T-lymphotrophic virus type 1 (HTLV-1)-infected T cells [3]; and, (ii) early pregnancy factor, part of a complex in

Redox biochemistry of thioredoxin

Members of the thioredoxin family of proteins have as a conserved catalytic site -Trp-Cys-Gly-Pro-Cys-Lys- that undergoes reversible oxidation to the cysteine-disulfide (Trx-S2) form through the transfer of reducing equivalents to a disulfide substrate (X-S2). The oxidized thioredoxin is reduced back to the cysteine-thiol form [Trx-(SH)2] by the NADPH-dependent flavoprotein thioredoxin reductase (TR). Trx-(SH)2+X-S2 Trx-S2+X(SH)2 Trx-S2+NADPH TR Trx-(SH)2+NADP+ Mammalian thioredoxin

Cloned forms

Two forms of thioredoxin have been cloned, thioredoxin-1 (Trx-1) and thioredoxin-2 (Trx-2) (shown in Fig. 1). Human Trx-1 is a 104 amino acid protein with a molecular weight of 12 kDa that contains the two catalytic site Cys residues -Trp-Cys32-Gly-Pro-Cys35-Lys found in all thioredoxin proteins, as well as three additional Cys residues, Cys62, Cys69, and Cys73, that are not found in bacterial thioredoxins [5], [6]. Trx-ls from a number of other mammalian species, including chicken, rat,

Structure of thioredoxin

Extensive structural data exists for human thioredoxin. The solution structures of oxidized and reduced mutant (Cys62 → Ala, Cys69 → Ala, Cys73 → Ala) thioredoxin [30] and crystal structures of oxidized and reduced wild-type thioredoxin, as well solution and crystal structure of various mutant forms of thioredoxin [22], [30] have been reported. These studies and earlier structure studies with E. coli thioredoxin [31], [32], [33] show that thioredoxin is a compact globular protein with a

Dimer formation by thioredoxin

Human thioredoxin-1 forms covalently-linked dimers in solution [22], [36] especially in the presence of an oxidant or when stored at high concentrations [37]. This is due to an 1100 Å2 hydrophobic patch, five hydrogen bonds, and a Cys73-Cys73 disulfide bond. The hydrophobic patch is composed of 12 hydrophobic amino acids, 10 of which are invariant in vertebrate species but not found in lower species, which suggests that dimer formation may be a recent occurrence in evolution [22]. The apparent

Subcellular localization

Thioredoxin-1 is predominantly a cytosolic protein, and although it has no known nuclear localization sequence, it has also been detected in the nucleus of tumor cells [41], [42]. Treatment of cells with H2O2 [43], PMA [42], and ultraviolet irradiation [44] results in the translocation of thioredoxin-1 from the cytoplasm to the nucleus. The mechanism for this translocation is not known, but it could be the consequence of thioredoxin-1 being carried along bound to another protein with a nuclear

Growth factor

Thioredoxin acts as a growth factor and is produced by a variety of cells, including Epstein-Barr virus-transformed B cells [3], [47], MP6 T-cell hybridoma cells [48], and hepatoma cells [49]. Thioredoxin is secreted by lymphocytes, hepatocytes, fibroblasts, and cancer cells [50], [51], [52], [53], [54]. The mechanism by which secretion occurs is not known; however, because thioredoxin has no leader sequence, a novel pathway appears to be involved independent from the endoplasmic-Golgi

Thoredoxin and human cancer

Studies with a variety of human primary tumors have shown that thioredoxin is overexpressed in the tumor compared to levels in the corresponding normal tissue (Table 1). We have recently shown by immunohistochemical studies using paraffin-embedded tissue sections that thioredoxin expression is increased in more than half of human primary gastric cancers. The thioredoxin levels showed a highly significant positive correlation (p < .001) with cell proliferation measured by nuclear proliferation

Thioredoxin as a cancer drug target

The growth-stimulating and transforming effects of thioredoxin, together with the finding that it is overexpressed by a number of human primary tumors, raise the intriguing possibility that thioredoxin is a factor leading to aggressive tumor growth and poor patient prognosis [58]. Because thioredoxin has also been shown to inhibit apoptosis caused by a number of anticancer drugs and to be a cause of resistance to the cytotoxic effects of some anticancer drugs, it is possible that increased

Summary

The thioredoxins are a family of small redox proteins whose functions include the regulation of cell growth, programmed cell death, and the development of the organism. When thioredoxin levels are elevated in cells, there is increased cell growth and resistance to normal mechanisms of programmed cell death. An increase in thioredoxin levels seen in many human primary cancers compared to normal tissue may be a contributing factor leading to increased cancer cell growth and resistance to

Acknowledgements

This work was supported by National Institutes of Health Grants CA48725 (G.P.), CA77204 (G.P.), and F32-CA79106 (D.M.).

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