The effects of chromium(VI) on the thioredoxin system: Implications for redox regulation
ABSTRACT Hexavalent chromium [Cr(VI)] compounds are highly redox active and have long been recognized as potent cytotoxins and carcinogens. The intracellular reduction of Cr(VI) generates reactive Cr intermediates, which are themselves strong oxidants, as well as superoxide, hydrogen peroxide, and hydroxyl radical. These probably contribute to the oxidative damage and effects on redox-sensitive transcription factors that have been reported. However, the identification of events that initiate these signaling changes has been elusive. More recent studies show that Cr(VI) causes irreversible inhibition of thioredoxin reductase (TrxR) and oxidation of thioredoxin (Trx) and peroxiredoxin (Prx). Mitochondrial Trx2/Prx3 are more sensitive to Cr(VI) treatment than cytosolic Trx1/Prx1, although both compartments show thiol oxidation with higher doses or longer treatments. Thiol redox proteomics demonstrate that Trx2, Prx3, and Trx1 are among the most sensitive proteins in cells to Cr(VI) treatment. Their oxidation could therefore represent initiating events that have widespread implications for protein thiol redox control and for multiple aspects of redox signaling. This review summarizes the effects of Cr(VI) on the TrxR/Trx system and how these events could influence a number of downstream redox signaling systems that are influenced by Cr(VI) exposure. Some of the signaling events discussed include the activation of apoptosis signal regulating kinase and MAP kinases (p38 and JNK) and the modulation of a number of redox-sensitive transcription factors including AP-1, NF-κB, p53, and Nrf2.
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- "PCBs have been shown to cause oxidative stress primarily through a CYP1A1 uncoupling mediated mechanism . Production of superoxide and related ROS triggers an up-regulation of a battery of antioxidant enzymes such as superoxide dismutase (SOD), catalase, glutathione reductase (GSR), glutathione transferases (GST), thioredoxins (Trx) and Trx reductases (TrxR)  . These proteins work in concert to either catalyze the transformation of ROS to benign molecules such as water and molecular oxygen or to reactivate enzymes, usually by catalytic reductions (e.g., TrxR reduces oxidized Trx to its active form ). "
ABSTRACT: Superfund chemicals such as polychlorinated biphenyls pose a serious human health risk due to their environmental persistence and link to multiple diseases. Selective bioactive food components such as flavonoids have been shown to ameliorate PCB toxicity, but primarily in an in vitro setting. Here, we show that mice fed a green tea-enriched diet and subsequently exposed to environmentally relevant doses of coplanar PCB exhibit decreased overall oxidative stress primarily due to the upregulation of a battery of antioxidant enzymes. C57BL/6 mice were fed a low fat diet supplemented with green tea extract (GTE) for 12 weeks and exposed to 5 μmol PCB 126/kg mouse weight (1.63 mg/kg-day) on weeks 10, 11 and 12 (total body burden: 4.9 mg/kg). F2-Isoprostane and its metabolites, established markers of in vivo oxidative stress, measured in plasma via HPLC-MS/MS exhibited five-fold decreased levels in mice supplemented with GTE and subsequently exposed to PCB compared to animals on a control diet exposed to PCB. Livers were collected and harvested for both mRNA and protein analyses, and it was determined that many genes transcriptionally controlled by AhR and Nrf2 proteins were upregulated in PCB-exposed mice fed the green tea supplemented diet. An increased induction of genes such as SOD1, GSR, NQO1 and GST, key antioxidant enzymes, in these mice (green tea plus PCB) may explain the observed decrease in overall oxidative stress. A diet supplemented with green tea allows for an efficient antioxidant response in the presence of PCB 126 which supports the emerging paradigm that healthful nutrition may be able to bolster and buffer a physiological system against the toxicities of environmental pollutants.The Journal of Nutritional Biochemistry 01/2013; DOI:10.1016/j.jnutbio.2013.10.003 · 4.59 Impact Factor
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ABSTRACT: Thiosemicarbazones such as triapine (Tp) and Dp44mT are tridentate iron (Fe) chelators that have well-documented anti-neoplastic activity. While Fe-thiosemicarbazones can undergo redox-cycling to generate reactive species that may have important roles in their cytotoxicity, there is only limited insight into specific cellular agents that can rapidly reduce Fe(III)-thiosemicarbazones and thereby promote their redox activity. Here we report that thioredoxin reductase-1 (TrxR1) and glutathione reductase (GR) have this activity, and that there is considerable specificity to the interactions between specific redox centers in these enzymes and different Fe(III) complexes. Site-directed variants of TrxR1 demonstrate that the selenocysteine (Sec) of the enzyme is not required, whereas the C59 residue and the flavin have important roles. While TrxR1 and GR have analogous C59/flavin motifs, TrxR is considerably faster than GR. For both enzymes, Fe(III)(Tp)2 is reduced faster than Fe(III)(Dp44mT)2. This reduction promotes redox cycling and the generation of hydroxyl radical (HO(•)) in a peroxide-dependent manner, even with low µM levels of Fe(Tp)2. TrxR also reduces Fe(III)-bleomycin and this activity is Sec-dependent. TrxR cannot reduce Fe(III)-EDTA at significant rates. Our findings are the first to demonstrate pro-oxidant reductive activation of Fe(III)-based antitumor thiosemicarbazones by interactions with specific enzyme species. The marked elevation of TrxR in many tumors could contribute to the selective tumor toxicity of these drugs by enhancing the redox activation of Fe(III)-thiosemicarbazones and the generation of reactive oxygen species such as HO(•).Free Radical Biology and Medicine 02/2013; 60. DOI:10.1016/j.freeradbiomed.2013.02.016 · 5.71 Impact Factor
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ABSTRACT: Previous studies on the impact of hexavalent chromium [Cr(VI)] on mammalian cell energetics revealed alterations suggestive of a shift to a more fermentative metabolism. Aiming at a more defined understanding of the metabolic effects of Cr(VI) and of their molecular basis, we assessed the impact of a mild Cr(VI) exposure on critical bioenergetic parameters (lactate production, oxygen consumption and intracellular ATP levels). Cells derived from normal human bronchial epithelium (BEAS-2B cell line), the main in vivo target of Cr(VI) carcinogenicity, were subjected for 48 h to 1 μM Cr(VI). We could confirm a shift to a more fermentative metabolism, resulting from the simultaneous inhibition of respiration and stimulation of glycolysis. This shift was accompanied by a decrease in the protein levels of the catalytic subunit (subunit β) of the mitochondrial H+-ATP synthase (β-F1-ATPase) and a concomitant marked increase in those of glyceraldehyde-3-phosphate dehydrogenase (GAPDH). The corresponding alteration in the β-F1-ATPase/GAPDH protein ratio (viewed as a bioenergetic signature) upon Cr(VI) exposure was in agreement with the observed attenuation of cellular respiration and enhancement of glycolytic flux. Altogether, these results constitute a novel finding in terms of the molecular mechanisms of Cr(VI) effects.FEBS Open Bio 06/2014; 4. DOI:10.1016/j.fob.2014.06.006 · 1.52 Impact Factor