Detection of dichromate (VI)-induced DNA strand breaks and formation of paramagnetic chromium in multiple mouse organs

Evidence for chromium crosses blood brain barrier from the hypothalamus in chromium mice model

It has been shown that exposure to hexavalent Chromium, Cr (Ⅵ), via nasal cavity can have neurotoxicological effects and induces behavioral impairment due to the fact that blood brain barrier (BBB) does not cover olfactory bulb. But whether Cr (Ⅵ) can cross the BBB and have a toxicological effects in central nervous system (CNS) remains unclear. Therefore, we investigated the effects of Cr (Ⅵ) on mice treated with different concentrations and exposure time (14 days and 28 days) of Cr (Ⅵ) via intraperitoneal injection. Results revealed that Cr accumulated in hypothalamus (HY) in a timely dependent manner. Much more severer neuropathologies was observed in the group of mice exposed to Cr (Ⅵ) for 28 days than that for 14 days. Gliosis, neuronal morphological abnormalities, synaptic degeneration, BBB disruption and neuronal number loss were observed in HY. In terms of mechanism, the Nrf2 related antioxidant stress signaling dysfunction and activated NF-κB related inflammatory pathway were observed in HY of Cr (Ⅵ) intoxication mice. And these neuropathologies and signaling defects appeared in a timely dependent manner. Taking together, we proved that Cr (Ⅵ) can enter HY due to weaker BBB in HY and HY is the most vulnerable CNS region to Cr (Ⅵ) exposure. The concentration of Cr in HY increased along with time. The accumulated Cr in HY can cause BBB disruption, neuronal morphological abnormalities, synaptic degeneration and gliosis through Nrf2 and NF-κB signaling pathway. This finding improves our understanding of the neurological dysfunctions observed in individuals who have occupational exposure to Cr (Ⅵ), and provided potential therapeutic targets to treat neurotoxicological pathologies induced by Cr (Ⅵ).

Cr (III) genotoxicity and oxidative stress: An occupational health risk for leather tannery workers of South Asian developing countries

In the leather industry, Cr (III) is used as a basic tanning agent. The wastewater discharged from the tannery industry contains a high concentration of chromium. Recent studies indicate the genotoxic effects especially DNA damage and oxidative stress of Cr (III) in tannery workers. Cr (III) interacts with DNA to form DNA cross-links and DNA strand breaks. It also modifies the oxidative DNA base through the Haber–Weiss reaction. The present study is based on an overview of scientific literature and previous observations regarding the effects of tannery chromium effluents on exposed workers and the population in the vicinity. This study strongly suggests for use of a non-toxic substitute of chromium to be used for the tanning process and placement of tannery industries on the outskirts of the city. In South Asian developing countries like India, Pakistan and Bangladesh where the economy is strongly dependent on leather manufacturing industries, there is a need to spread proper information regarding the harmful effects of chromium toxicity to the workforce employed in the tannery and also to the people living in the surrounding area. Workers should be provided with the required safety protections like gloves, aprons, foot/shoe covers, masks, etc. Last but most important on an immediate basis is the installation of the proper efficient waste treatment plant, so that, waste should be treated before moving out of the industry.

Current understanding of hexavalent chromium [Cr(VI)] neurotoxicity and new perspectives

Hexavalent chromium [Cr(VI)] is a global environmental pollutant that increases risk for several types of cancers and is increasingly being recognized as a neurotoxicant. Traditionally, the brain has been viewed as a largely post-mitotic organ due to its specialized composition of neurons, and consequently, clastogenic effects were not considered in neurotoxicology. Today, we understand the brain is composed of at least eight distinct cell types - most of which continue mitotic activity throughout lifespan. We have learned these dividing cells play essential roles in brain and body health. This review focuses on Cr(VI), a potent clastogen and known human carcinogen, as a potentially neurotoxic agent targeting mitotic cells of the brain. Despite its well-established role as a human carcinogen, Cr(VI) neurotoxicity studies have failed to find a significant link to brain cancers. In the few studies that did find a link, Cr(VI) was identified as a risk for gliomas. Instead, in the human brain, Cr(VI) appears to have more subtle deleterious effects that can impair childhood learning and attention development, olfactory function, social memory, and may contribute to motor neuron diseases. Studies of Cr(VI) neurotoxicity with animal and cell culture models have demonstrated elevated markers of oxidative damage and redox stress, with widespread neurodegeneration. One study showed mice exposed to Cr(VI)-laden tannery effluent exhibited longer periods of aggressive behavior toward an "intruder" mouse and took longer to recognize mice previously encountered, recapitulating the social memory deficits observed in humans. Here we conducted a critical review of the available literature on Cr(VI) neurotoxicity and synthesize the collective observations to thoroughly evaluate Cr(VI) neurotoxicity - much remains to be understood and recognized.

Histological, immunohistochemical, and molecular investigation on the hepatotoxic effect of potassium dichromate and the ameliorating role of Persea americana mill pulp extract

The current study has been designed to assess the role of Persea americana (P. americana) pulp extract on potassium dichromate-induced hepatotoxicity in rats. P. americana pulp extract administration improved the hepatic vascular congestion, blood extravasation, inflammatory cellular infiltration, Kupffer cell hyperplasia, and nuclear changes. It also significantly ameliorated hepatic interstitial and peri-portal fibrosis and caused retrieval of the PAS-positive reaction in the liver parenchyma and around the central vein with restoration of the glycogen granules. P. americana also significantly attenuated the immunohistochemical expression of NF-kβ p65 and its downstream inflammatory cytokines IL6 and TNFα in the liver parenchyma. The antioxidant effect of P. americana was evidenced by significant modulation of the three major components of the thioredoxin (Trx) antioxidant system, the Trx, the thioredoxin reductase (TrxR), and the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase along with significant increase in the level of superoxide dismutase and glutathione, and decrease in the lipid peroxidation product malondialdehyde. P. americana pulp extract also caused significant elevation of hepatic protein phosphatase 5 with subsequent down-regulation of Apoptosis signal-regulating kinase1 (ASK1) and its downstream signaling targets MAPK kinase 4 (MKK4), p38 mitogen-activated protein kinases (p38-MAPKs), the c-JUN N-terminal kinase (JNK), and the extracellular signal-regulated kinase 1/2 (ERK 1/2). Also, In conclusion, P. americana pulp extract has anti-oxidative and anti-inflammatory effects against potassium dichromate-induced hepatotoxicity.

Taurine attenuates Cr(VI)-induced cellular and DNA damage: an in vitro study using human erythrocytes and lymphocytes

Hexavalent chromium [(Cr(VI)] is widely used in several industries, but human exposure results in multiple organ toxicity. Enhanced generation of free radicals and reactive species is thought to play a key role in Cr(VI)-induced toxicity. We have examined the effect of taurine, a simple sulphur-containing amino acid and an antioxidant, on potassium dichromate [K₂Cr₂O₇, a Cr(VI) compound]-induced cytotoxicity and genotoxicity in human blood cells. Erythrocytes were treated with K₂Cr₂O₇, either alone or after incubation with different concentrations of taurine. Treatment of erythrocytes with K₂Cr₂O₇ alone led to marked increase in generation of reactive oxygen and nitrogen species, lipid and protein oxidation. This was accompanied by decrease in total sulfhydryl and glutathione content and lowered antioxidant power of the cells. This suggests that Cr(VI) induces oxidative stress in the cells. Incubation of erythrocytes with taurine prior to addition of K₂Cr₂O₇, resulted in a concentration-dependent decrease in the generation of reactive oxygen and nitrogen species, mitigation of oxidative stress and amelioration of antioxidant power of these cells. It also restored the activities of several metabolic, antioxidant and membrane-bound enzymes. Cr(VI)-induced damage to erythrocyte membrane and lymphocyte DNA was also significantly attenuated by prior administration of taurine. These results suggest that taurine can function as a chemoprotectant against Cr(VI)-induced oxidative injury and can be potentially used to mitigate the toxic effects of this transition metal ion.

Modulatory effect of Turnera diffusa against testicular toxicity induced by fenitrothion and/or hexavalent chromium in rats

Oxidative stress and increased production of reactive oxygen species have been implicated in pesticides and heavy metals toxicity. The objective of this study was to investigate the efficacy of Turnera diffusa Willd (damiana) on counteracting fenitrothion (FNT) and/or potassium dichromate (CrVI)-induced testicular toxicity and oxidative injury in rats. FNT and/or CrVI intoxicated animals revealed a significant increase in thiobarbituric acid reactive substances and hydrogen peroxide levels. While, reduced glutathione and protein content, as well as antioxidant enzymes, phosphatases, and aminotransferases activities, were significantly decreased. In addition, significant changes in testosterone and follicle-stimulating hormone levels were detected. Furthermore, histological and immunohistochemical alterations were observed in rat testes and this supported the observed biochemical changes. On the other hand, rats treated with damiana alone decreased lipid peroxidation and increased most of the examined parameters. Moreover, damiana pretreatment to FNT and/or CrVI-intoxicated rats showed significant improvement in lipid peroxidation, enzyme activities, and hormones as compared with their respective treated groups. Conclusively, rats treated with both FNT and/or CrVI showed pronounced hazardous effect especially in their combination group in addition, Turnera diffusa had a potential protective role against FNT and/or CrVI induced testicular toxicity.

N-Acetyl-L-Cysteine Protects Liver and Kidney Against Chromium(VI)-Induced Oxidative Stress in Mice

Acute hexavalent chromium [Cr(VI)] compound exposure may lead to hepatotoxic and nephrotoxic effects. Cr(VI) reduction may generate reactive intermediates and radicals which might be associated with damage. We investigated effects of N-acetyl-l-cysteine (NAC) pre- or post-treatment on oxidative stress and accumulation of Cr in liver and kidney of Cr(VI)-exposed mice. Intraperitoneal potassium dichromate injection (20 mg Cr/kg) caused a significant elevation of lipid peroxidation in both tissues as compared to control (p < 0.05). Significant decreases in non-protein sulfhydryl (NPSH) level, as well as enzyme activities of catalase (CAT) and superoxide dismutase (SOD) along with significant accumulation of Cr in the tissues (p < 0.05) were of note. NAC pre-treatment (200 mg/kg, ip) provided a noticeable alleviation of lipid peroxidation (p < 0.05) in both tissues, whereas post-treatment exerted significant effect only in kidney. Similarly, Cr(VI)-induced NPSH decline was restored by NAC pre-treatment in both tissues (p < 0.05); however, NAC post-treatment could only replenish NPSH in liver (p < 0.05). Regarding enzyme activities, in liver tissue NAC pre-treatment provided significant restoration on Cr(VI)-induced CAT inhibition (p < 0.05), while SOD enzyme activity was regulated to some extent. In kidney, SOD activity was efficiently restored by both treatments (p < 0.05), whereas CAT enzyme alteration could not be totally relieved. Additionally, NAC pre-treatment in both tissues and post-treatment in liver exerted significant tissue Cr level decreases (p < 0.05). Overall, especially NAC pre-treatment seems to provide beneficial effects in regulating pro-oxidant/antioxidant balance and Cr accumulation caused by Cr(VI) in liver and kidney. This finding may be due to several mechanisms including extracellular reduction or chelation of Cr(VI) by readily available NAC.

Effect of various selenium doses on chromium(IV)-induced nephrotoxicity in a male chicken model

Our study aimed to explore whether Na₂SeO₃ (Se) can alleviate the nephrotoxicity induced by K₂Cr₂O₇ [Cr(VI)]. One hundred and five male chickens were randomly divided into seven groups with 15 chickens each group: The 6 experimental groups received K₂Cr₂O₇ alone or in combination with 0.31, 0.63, 1.25, 2.50, and 5.00 mg/kg for 42 days, respectively, while control group was treated with equivalent water. Exposure to Cr(VI) significantly increased MDA contents and organ coefficient, whereas decreased T-SOD activities, Ca²⁺-ATPase activities, mitochondrial membrane potential and GSH contents, and histological studies demonstrated renal damage. Above indicators were restored by Se supplement (0.31, 0.63, and 1.25 mg/kg), in which supplement with 0.63 mg/kg Se developed more effectively than the other two groups; on the contrary, in the groups of Se supplement with 2.50 and 5.00 mg/kg, the above indicators were not ameliorated and even exacerbated. This study demonstrated that Cr(VI) can result in kidney oxidative damage in male chickens, and Se of certain dose has the protective effects against Cr(VI)-induced nephrptoxicity.

Gallic acid attenuates chromium-induced thyroid dysfunction by modulating antioxidant status and inflammatory cytokines

Hexavalent chromium-mediated oxidative stress causes severe organ damage. The present study was designed to investigate the possible thyroprotective effect and underlying mechanisms of gallic acid using rat model of potassium dichromate-induced thyroid dysfunction. Forty adult male albino rats were divided into 4 groups: control, gallic acid (20mg GA/kg b. wt), potassium dichromate (2mg PD/kg b. wt) and the fourth group was co-treated with PD and GA. PD-injection resulted in decreased serum free triiodothyonine (FT3), free thyroxine (FT4) with concomitant significant increase in thyroid stimulating hormone (TSH) levels. Superoxide dismutase (SOD), glutathione-S-transferase (GST) activities and their respective mRNA expression and reduced glutathione (GSH) content were significantly decreased. Thyroid nitrosative stress marker (NO level and iNOS mRNA and protein expression) and pro-inflammatory cytokines (serum TNF-α, IL-6 and thyroid TNF-α, IL-6 and COX-2 gene and protein expression levels) were disturbed. Histopathological changes revealed distended, collapsed and degenerated follicles with vacuolated cytoplasm. GA co-treatment attenuated pro-inflammatory cytokines, the thyroid expression of iNOS, TNF-α, IL-6 and COX-2, decreased the elevated lipid peroxidation biomarkers and NO level and up- regulated SOD and GST mRNA expression levels. In conclusion, GA has shown strong modulatory potential against PD-induced inflammation and oxidative stress in albino rats.

Comparative histological studies on liver of mice exposed to Cr(VI) and Cr(V) compounds

Chromium toxicity is strongly dependent on its oxidation state. Cr(VI) is carcinogenic and mutagenic, although its in vivo and in vitro toxic effects are related to its intracellular fate. Inside the cells, Cr(VI) is rapidly reduced to stable Cr(III). As Cr(V) and Cr(IV) species have been reported to be formed in the Cr(VI) reduction pathways, Cr(VI)induced damage is thought, at least in part, to arise from these hypervalent species. The study of Cr(VI) reduction mechanisms and the characterization of the effects of each reactive intermediate constitute important steps towards a better understanding of chromium toxicity. The purpose of this work is to enlarge the scope of Cr(VI)induced alterations in mouse to other chromium species. Our studies have led to the in situ preparation of a new Cr(V) complex, [CrV–BT]2–, a stable compound at neutral pH, which mimics Cr(VI) reduction intermediates. The effect of Cr(V) on the histology of mice liver is assessed and compared with similar Cr(VI) assays. Liver toxicity was examined after single administrations of Cr(VI) or [CrV–BT]2– to mice. Both compounds produced reversible hepatic damage in a time-dependent manner. However, Cr(V) toxic effects have proved to be more rapid than with Cr(VI), permitting the role of Cr(VI) intermediates formed during intracellular chromium reduction to be highlighted.

Cytotoxic and genotoxic potential of Cr(VI), Cr(III)-nitrate and Cr(III)-EDTA complex in human hepatoma (HepG2) cells

Chromium (Cr) and ethylenediaminetetraacetate (EDTA) are common environmental pollutants and can be present in high concentrations in surface waters at the same time. Therefore, chelation of Cr with EDTA can occur and thereby stable Cr(III)-EDTA complex is formed. Since there are no literature data on Cr(III)-EDTA toxicity, the aim of our work was to evaluate and compare Cr(III)-EDTA cytotoxic and genotoxic activity with those of Cr(VI) and Cr(III)-nitrate in human hepatoma (HepG2) cell line. First the effect of Cr(VI), Cr(III)-nitrate and Cr(III)-EDTA on cell viability was studied in the concentration range from 0.04 μg mL(-1) to 25 μg mL(-1) after 24 h exposure. Further the influence of non-cytotoxic concentrations of Cr(VI), Cr(III)-nitrate and Cr(III)-EDTA on DNA damage and genomic stability was determined with the comet assay and cytokinesis block micronucleus cytome assay, respectively. Cell viability was decreased only by Cr(VI) at concentrations above 1.0 μg mL(-1). Cr(VI) at ≥0.2 μg mL(-1) and Cr(III) at ≥1.0 μg mL(-1) induced DNA damage, while after Cr(III)-EDTA exposure no formation DNA strand breaks was determined. Statistically significant formation of micronuclei was induced only by Cr(VI) at ≥0.2 μg mL(-1), while no influence on the frequency of nuclear buds nor nucleoplasmic bridges was observed at any exposure. This study provides the first evidence that Cr(III)-EDTA did not induce DNA damage and had no influence on the genomic stability of HepG2 cells.

Spatial Evaluation of Heavy Metals Concentrations in the Surface Sediment of Taihu Lake

With regard to the size of China's freshwater lakes, Taihu Lake ranks third and it plays an important role in the supply of drinking water, flood prevention, farming and navigation, as well as in the travelling industry. The problem of environmental pollution has attracted widespread attention in recent years. In order to understand the levels, distribution and sources of heavy metals in sediments of Taihu Lake, random selection was carried out to obtain 59 samples of surface sediment from the entire lake and study the concentrations of Pb, Cd, Cu, Zn, Cr and Ni. Toxic units were also calculated to normalize the toxicities caused by various heavy metals. As a result, Cd and Cu in sediment were considered lower than the effect range low (ERL) at all regions where samples were gathered, while Pb and Ni were categorized into ERL-effect range median (ERM) at over 22% of the regions where samples were obtained. Nevertheless, all average concentrations of the samples were below the level of potential effect. According to the findings of this research, significant spatial heterogeneity existed in the above heavy metals. In conclusion, the distribution areas of heavy metals with higher concentrations were mainly the north bays, namely Zhushan Bay, Meiliang Bay as well as Gonghu Bay. The distribution areas of Cu, Zn, Cr and Ni with higher concentration also included the lake's central region, whereas the uniform distribution areas of those with lower concentrations were the lake's southeast region. In addition, it was most probable that the spatial distribution of heavy metals was determined by river inputs, whereas atmospheric precipitation caused by urban and traffic contamination also exerted considerable effects on the higher concentrations of Pb and Cd. Through evaluating the total amount of toxic units (ΣTU), it was found that higher toxicity existed primarily in the north bays and central region of the lake. If the heavy metals were sorted by the reduction of mean heavy metal toxic units in Taihu Lake in descending order, it would be Pb, Cr, Ni, Cd, Zn and Cu. Generally speaking, these result of analyses are conducive to alleviating the contamination of heavy metals in Taihu Lake.

Oxidative Stress Markers and Histological Analysis in Diverse Organs from Rats Treated with a Hepatotoxic Dose of Cr(VI): Effect of Curcumin

Hexavalent chromium [Cr(VI)] compounds are extremely toxic and carcinogenic. Despite the vast quantity of reports about Cr(VI) toxicity, the information regarding its effects when it is intraperitoneally (i.p.) administered is still limited. In contrast, it has been shown that curcumin prevents hepatotoxicity induced by a single intraperitoneal injection of 15 mg/kg body weight (b.w.) of potassium dichromate (K2Cr2O7). This study aims to evaluate oxidative stress markers, the activity of antioxidant enzymes, and the potential histological injury in brain, heart, lung, kidney, spleen, pancreas, stomach, and intestine from rats treated with a hepatotoxic dose of K2Cr2O7 (15 mg/kg b.w.), and the effect of curcumin pretreatment. Rats were divided into four groups: control, curcumin, K2Cr2O7, and curcumin+K2Cr2O7. At the end of the treatment, plasma and ascites fluid were collected and target organs were dissected out for biochemical and histological analysis. K2Cr2O7 induced hepatotoxicity but failed to induce in all the other studied organs either oxidative or histological injury, since levels of malondialdehyde (MDA), glutathione (GSH), and the activity of superoxide dismutase (SOD), catalase (CAT), and related GSH enzymes were unchanged. As expected, curcumin was safe. Lack of K2Cr2O7-induced toxicity in those target organs could be due to the following: (1) route of administration, (2) absorption through the portal circulation, (3) lower dose than needed, (4) short time of exposure, or (5) repeated doses are required to produce damage. Thus, the intraperitoneal injection of 15 mg/kg of K2Cr2O7, that is able to induce hepatotoxicity, was unable to induce histological and oxidative damage in other target organs.

Temporal changes in rat liver gene expression after acute cadmium and chromium exposure

U.S. Service Members and civilians are at risk of exposure to a variety of environmental health hazards throughout their normal duty activities and in industrial occupations. Metals are widely used in large quantities in a number of industrial processes and are a common environmental toxicant, which increases the possibility of being exposed at toxic levels. While metal toxicity has been widely studied, the exact mechanisms of toxicity remain unclear. In order to further elucidate these mechanisms and identify candidate biomarkers, rats were exposed via a single intraperitoneal injection to three concentrations of CdCl₂ and Na₂Cr₂O₇, with livers harvested at 1, 3, or 7 days after exposure. Cd and Cr accumulated in the liver at 1 day post exposure. Cd levels remained elevated over the length of the experiment, while Cr levels declined. Metal exposures induced ROS, including hydroxyl radical (•OH), resulting in DNA strand breaks and lipid peroxidation. Interestingly, ROS and cellular damage appeared to increase with time post-exposure in both metals, despite declines in Cr levels. Differentially expressed genes were identified via microarray analysis. Both metals perturbed gene expression in pathways related to oxidative stress, metabolism, DNA damage, cell cycle, and inflammatory response. This work provides insight into the temporal effects and mechanistic pathways involved in acute metal intoxication, leading to the identification of candidate biomarkers.

Protective effect of curcumin against heavy metals-induced liver damage

Occupational or environmental exposures to heavy metals produce several adverse health effects. The common mechanism determining their toxicity and carcinogenicity is the generation of oxidative stress that leads to hepatic damage. In addition, oxidative stress induced by metal exposure leads to the activation of the nuclear factor (erythroid-derived 2)-like 2/Kelch-like ECH-associated protein 1/antioxidant response elements (Nrf2/Keap1/ARE) pathway. Since antioxidant and chelating agents are generally used for the treatment of heavy metals poisoning, this review is focused on the protective role of curcumin against liver injury induced by heavy metals. Curcumin has shown, in clinical and preclinical studies, numerous biological activities including therapeutic efficacy against various human diseases and anti-hepatotoxic effects against environmental or occupational toxins. Curcumin reduces the hepatotoxicity induced by arsenic, cadmium, chromium, copper, lead and mercury, prevents histological injury, lipid peroxidation and glutathione (GSH) depletion, maintains the liver antioxidant enzyme status and protects against mitochondrial dysfunction. The preventive effect of curcumin on the noxious effects induced by heavy metals has been attributed to its scavenging and chelating properties, and/or to the ability to induce the Nrf2/Keap1/ARE pathway. However, additional research is needed in order to propose curcumin as a potential protective agent against liver damage induced by heavy metals.

Curcumin pretreatment prevents potassium dichromate-induced hepatotoxicity, oxidative stress, decreased respiratory complex I activity, and membrane permeability transition pore opening

Curcumin is a polyphenol derived from turmeric with recognized antioxidant properties. Hexavalent chromium is an environmental toxic and carcinogen compound that induces oxidative stress. The objective of this study was to evaluate the potential protective effect of curcumin on the hepatic damage generated by potassium dichromate (K₂Cr₂O₇) in rats. Animals were pretreated daily by 9-10 days with curcumin (400 mg/kg b.w.) before the injection of a single intraperitoneal of K₂Cr₂O₇ (15 mg/kg b.w.). Groups of animals were sacrificed 24 and 48 h later. K₂Cr₂O₇-induced damage to the liver was evident by histological alterations and increase in the liver weight and in the activity of alanine aminotransferase, aspartate aminotransferase, lactate dehydrogenase, and alkaline phosphatase in plasma. In addition, K₂Cr₂O₇ induced oxidative damage in liver and isolated mitochondria, which was evident by the increase in the content of malondialdehyde and protein carbonyl and decrease in the glutathione content and in the activity of several antioxidant enzymes. Moreover, K₂Cr₂O₇ induced decrease in mitochondrial oxygen consumption, in the activity of respiratory complex I, and permeability transition pore opening. All the above-mentioned alterations were prevented by curcumin pretreatment. The beneficial effects of curcumin against K₂Cr₂O₇-induced liver oxidative damage were associated with prevention of mitochondrial dysfunction.

The effects of chromium(VI) on the thioredoxin system: implications for redox regulation

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.

Strategies for chromium bioremediation of tannery effluent

Bioremediation offers the possibility of using living organisms (bacteria, fungi, algae,or plants), but primarily microorganisms, to degrade or remove environmental contaminants, and transform them into nontoxic or less-toxic forms. The major advantages of bioremediation over conventional physicochemical and biological treatment methods include low cost, good efficiency, minimization of chemicals, reduced quantity of secondary sludge, regeneration of cell biomass, and the possibility of recover-ing pollutant metals. Leather industries, which extensively employ chromium compounds in the tanning process, discharge spent-chromium-laden effluent into nearby water bodies. Worldwide, chromium is known to be one of the most common inorganic contaminants of groundwater at pollutant hazardous sites. Hexavalent chromium poses a health risk to all forms of life. Bioremediation of chromium extant in tannery waste involves different strategies that include biosorption, bioaccumulation,bioreduction, and immobilization of biomaterial(s). Biosorption is a nondirected physiochemical interaction that occurs between metal species and the cellular components of biological species. It is metabolism-dependent when living biomass is employed, and metabolism-independent in dead cell biomass. Dead cell biomass is much more effective than living cell biomass at biosorping heavy metals, including chromium. Bioaccumulation is a metabolically active process in living organisms that works through adsorption, intracellular accumulation, and bioprecipitation mechanisms. In bioreduction processes, microorganisms alter the oxidation/reduction state of toxic metals through direct or indirect biological and chemical process(es).Bioreduction of Cr6+ to Cr3+ not only decreases the chromium toxicity to living organisms, but also helps precipitate chromium at a neutral pH for further physical removal,thus offering promise as a bioremediation strategy. However, biosorption, bioaccumulation, and bioreduction methods that rely on free cells for bioremediation suffer from Cr6 toxicity, and cell damage. Therefore, immobilization of microbial cell biomass enhances bioremediation and renders industrial bioremediation processes more economically viable from reduced free-cells toxicity, easier separation of biosorbents from the tannery effluent, ability to achieve multiple biosorption cycles, and desorption (elution) of metal(s) from matrices for reuse. Thus, microbial bioremediation can be a cost competitive strategy and beneficial bioresource for removing many hazardous contaminants from tannery and other industrial wastes.

Distinct contributions of JNK and p38 to chromium cytotoxicity and inhibition of murine embryonic stem cell differentiation

BACKGROUND

Potassium dichromate [Cr(VI)] is a widespread environmental toxicant responsible for increased risk of several human diseases. Cr(VI) exposure leads to activation of mitogen-activated protein kinases (MAPKs), including c-Jun N-terminal kinase (JNK)1/2, p38, and extracellular-signal regulated kinase (ERK)1/2.

OBJECTIVES

We evaluated the contribution of MAPKs to Cr(VI) toxicity.

METHODS

Phosphorylation of MAPKs and their downstream effectors was evaluated by Western immunoblotting; reactive oxygen species were measured by DCFDA (5',6'-chloromethyl-2'-7'-dichlorofluorescin diacetate) labeling and flow cytometry, and glutathione and glutathione disulfide levels were determined by monochrome graphic spectroflurometer. Cytotoxicity was assessed by the MTS [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium] assay and colony formation. Embryoid body (EB) differentiation was evaluated by contracting cardiomyocyte formation, and real-time polymerase chain reaction (RT-PCR) was used for cardiomyocyte-specific and stem-cell-specific gene expression.

RESULTS

Acute treatment of mouse embryonic stem (ES) cells with 50 microM Cr(VI) induced the rapid phosphorylation of JNK, p38, and ERK and their respective downstream transcription factors, c-JUN, activating transcription factor-2, and ELK1. MAPK activation and cytotoxicity induction were partially blocked by pretreatment with the antioxidant N-acetyl cysteine. Ablation of the upstream MAP kinase kinase (MAP2K7) in ES cells prevented JNK activation, whereas ablation of MAP2K4 prevented both JNK and p38 activation. Using specific MAPK inhibitors and MAP2K4- and MAP2K7-deficient ES cells, we showed that JNK reduced acute Cr(VI) cytotoxicity, p38 potentiated it, and ERK had no effect. At low submicromolar concentrations, Cr(VI) caused MAP2K4/7-dependent JNK activation and MAP2K4-dependent p38 activation and strongly inhibited contracting cardiomyocyte development in wild-type ES cells, but much less so in Map2k7((-/-)) cells.

CONCLUSION

Each MAPK distinctly contributes to chromium toxicity. Whereas JNK prevents and p38 promotes acute cytotoxicity, JNK contributes to optimal inhibition of ES cell differentiation by chromium.

A role for Mus81 in the repair of chromium-induced DNA damage

Hexavalent chromium (Cr[VI]) is a toxic environmental contaminant that is capable of producing a broad spectrum of DNA damage. The ability of Cr[VI] to induce mutagenesis and neoplastic transformation has been attributed to its genotoxic action, however our understanding of molecular mechanisms involved in the repair of Cr[VI]-induced DNA damage remains incomplete. Here, we report that Mus81, an enzyme that participates with Eme1 in the resolution of replication fork damage caused by certain lesions, is involved in the repair of Cr[VI]-induced DNA damage. Mus81-deficient cells were found to be more susceptible to Cr[VI]-induced proliferation arrest and more sensitive to the long-term cytotoxic effects of Cr[VI] than isogenic wild-type cells. Following Cr[VI] exposure, Mus81-deficient cells displayed a lag in the disappearance of Rad51 foci, exhibited elevated replication-associated gamma-H2AX and showed an increased incidence of chromosomal instability compared to wild-type cells. Our findings support a role for Mus81 in the resolution of replication-associated DNA damage associated with this genotoxic agent, by converting Cr[VI]-DNA lesions into a form more amenable for homologous recombination.

Addition of DNA to Cr(VI) and cytochrome b5 containing proteoliposomes leads to generation of DNA strand breaks and Cr(III) complexes

Chromium (Cr) is a cytotoxic metal that can be associated with a variety of types of DNA damage, including Cr-DNA adducts and strand breaks. Prior studies with purified human cytochrome b(5) and NADPH:P450 reductase in reconstituted proteoliposomes (PLs) demonstrated rapid reduction of Cr(VI) (hexavalent chromium, as CrO(4)(2-), and the generation of Cr(V), superoxide (O(2)(*-)), and hydroxyl radical (HO(*)). Studies reported here examined the potential for the species produced by this system to interact with DNA. Strand breaks of purified plasmid DNA increased over time aerobically, but were not observed in the absence of O(2). Cr(V) is formed under both conditions, so the breaks are not mediated directly by Cr(V). The aerobic strand breaks were significantly prevented by catalase and EtOH, but not by the metal chelator diethylenetriaminepentaacetic acid (DTPA), suggesting that they are largely due to HO(*) from Cr-mediated redox cycling. EPR was used to assess the formation of Cr-DNA complexes. Following a 10-min incubation of PLs, CrO(4)(2-), and plasmid DNA, intense EPR signals at g=5.7 and g=5.0 were observed. These signals are attributed to specific Cr(III) complexes with large zero field splitting (ZFS). Without DNA, the signals in the g=5 region were weak. The large ZFS signals were not seen, when Cr(III)Cl(3) was incubated with DNA, suggesting that the Cr(III)-DNA interactions are different when generated by the PLs. After 24 h, a broad signal at g=2 is attributed to Cr(III) complexes with a small ZFS. This g=2 signal was observed without DNA, but it was different from that seen with plasmid. It is concluded that EPR can detect specific Cr(III) complexes that depend on the presence of plasmid DNA and the manner in which the Cr(III) is formed.

Genetic and epigenetic mechanisms in metal carcinogenesis and cocarcinogenesis: nickel, arsenic, and chromium

Chronic exposure to nickel(II), chromium(VI), or inorganic arsenic (iAs) has long been known to increase cancer incidence among affected individuals. Recent epidemiological studies have found that carcinogenic risks associated with chromate and iAs exposures were substantially higher than previously thought, which led to major revisions of the federal standards regulating ambient and drinking water levels. Genotoxic effects of Cr(VI) and iAs are strongly influenced by their intracellular metabolism, which creates several reactive intermediates and byproducts. Toxic metals are capable of potent and surprisingly selective activation of stress-signaling pathways, which are known to contribute to the development of human cancers. Depending on the metal, ascorbate (vitamin C) has been found to act either as a strong enhancer or suppressor of toxic responses in human cells. In addition to genetic damage via both oxidative and nonoxidative (DNA adducts) mechanisms, metals can also cause significant changes in DNA methylation and histone modifications, leading to epigenetic silencing or reactivation of gene expression. In vitro genotoxicity experiments and recent animal carcinogenicity studies provided strong support for the idea that metals can act as cocarcinogens in combination with nonmetal carcinogens. Cocarcinogenic and comutagenic effects of metals are likely to stem from their ability to interfere with DNA repair processes. Overall, metal carcinogenesis appears to require the formation of specific metal complexes, chromosomal damage, and activation of signal transduction pathways promoting survival and expansion of genetically/epigenetically altered cells.

Hexavalent chromium-induced DNA damage and repair mechanisms

Hexavalent chromium is a commonly used industrial metal that has been shown to induce lung cancer in workers having long term exposure. In the particulate form, Cr(VI) dissolves slowly in vivo, leading to an extended exposure of lung cells. Hexavalent chromium is taken into the cell and rapidly reduced to Cr(V), Cr(IV), Cr(III), and reactive oxygen species. Cells treated with Cr(VI) are subject to several types of DNA damage resulting from this reduction, including base modification, single-strand breaks, double-strand breaks, Cr-DNA adducts, DNA-Cr-DNA adducts, and protein-Cr-DNA adducts. These types of damage, if left unrepaired or are misrepaired, can lead to growth arrest, cytotoxicity, and apoptosis, as well as mutations leading to neoplastic transformation and ultimately tumorigenesis. Here we review the current literature on Cr-induced DNA damage and its repair.

Toxicity and carcinogenicity of chromium compounds in humans

Chromium is a human carcinogen primarily by inhalation exposure in occupational settings. Although lung cancer has been established as a consequence of hexavalent chromium exposure in smokers and nonsmokers, some cancers of other tissues of the gastrointestinal and central nervous systems have also been noted. Except for a few reports from China, little is known about the health risks of environmental exposures to chromium. Likewise, there has been a lack of epidemiological studies of human exposure to hexavalent Cr by drinking water or ingestion, and it has been suggested that humans can perhaps tolerate hexavalent Cr at higher levels than the current drinking water standard of 50 ppb. This review highlights the most recent data on the induction of skin tumors in mice by chronic drinking-water exposure to hexavalent chromium in combination with solar ultraviolet light. This experimental system represents an important new animal model for chromate-induced cancers by ingestion of drinking water, and it suggests by extrapolation that chromate can likely be considered a human carcinogen by ingestion as well. The potential use of this animal model for future risk assessment is discussed.

Causes of DNA single-strand breaks during reduction of chromate by glutathione in vitro and in cells

Carcinogenic chromates induce DNA single-strand breaks (SSB) that are detectable by conventional alkali-based assays. However, the extent of direct breakage has been uncertain because excision repair and hydrolysis of Cr-DNA adducts at alkaline pH also generate SSB. We examined mechanisms of SSB production during chromate reduction by glutathione (GSH) and assessed the significance of these lesions in cells using genetic approaches. Cr(VI) reduction was biphasic and the formation of SSB occurred exclusively during the slow reaction phase. Catalase or iron chelators completely blocked DNA breakage, as did the use of GSH purified by a modified Chelex procedure. Thus, the direct intermediates of GSH-chromate reactions were unable to cause SSB unless activated by H2O2. SSB repair-deficient XRCC1(-/-) and proficient XRCC1+ EM9 cells had identical survival at doses causing up to 60% clonogenic death and accumulation of 1 mM Cr(VI). However, XRCC1(-/-) cells displayed higher lethality in the more toxic range and the depletion of GSH made them hypersensitive even to moderate doses. Elevation of cellular catalase or GSH levels eliminated survival differences between XRCC1(-/-) and XRCC1+ cells. In summary, formation of toxic SSB in cells occurs at relatively high chromate doses, requires H2O2, and is suppressed by high GSH concentrations.

Interindividual variability in response to sodium dichromate-induced oxidative DNA damage: role of the Ser326Cys polymorphism in the DNA-repair protein of 8-oxo-7,8-dihydro-2'-deoxyguanosine DNA glycosylase 1

Although the genotoxic mechanism(s) of hexavalent chromium (CrVI) carcinogenicity remain to be fully elucidated, intracellular reduction of CrVI and concomitant generation of reactive intermediates including reactive oxygen species and subsequent oxidative damage to DNA is believed to contribute to the process of carcinogenesis. In the current study, substantial interindividual variation (7.19-25.84% and 8.79-34.72% tail DNA as assessed by conventional and FPG-modified comet assay, respectively) in levels of DNA strand breaks after in vitro treatment of WBC with sodium dichromate (100 micromol/L, 1 hour) was shown within a group of healthy adult volunteers (n = 72) as assessed by both comet and formamidopyrimidine glycosylase-modified comet assays. No statistically significant correlation between glutathione S-transferases M1 or T1, NADPH quinone oxidoreductase 1 (codon 187) and X-ray repair cross complementation factor 1 (codon 194) genotypes and individual levels of DNA damage were observed. However, individuals homozygous for the Cys(326) 8-oxo 7,8-dihydro-2'-deoxyguanosine glycosylase 1 (OGG1) polymorphism had a statistically significant elevation of formamidopyrimidine glycosylase-dependent oxidative DNA damage after treatment with sodium dichromate when compared with either Ser(326)/Ser(326) or Ser(326)/Cys(326) individuals (P = 0.008 and P = 0.003, respectively). In contrast, no effect of OGG1 genotype on background levels of oxidative DNA damage was observed. When individuals were divided on the basis of OGG1 genotype, Cys(326)/Cys(326) individuals had a statistically significant (P < 0.05, one-way ANOVA followed by Tukey test) higher ratio of oxidative DNA damage to plasma antioxidant capacity than either Ser(326)/Ser(326) or Ser(326)/Cys(326) individuals. The results of this study suggest that the Cys(326)/Cys(326) OGG1 genotype may represent a phenotype that is deficient in the repair of 8-oxo-7,8-dihydro-2'-deoxyguanosine, but only under conditions of cellular oxidative stress. We hypothesize that this may be due to oxidation of the Cys(326) residue. In conclusion, the homozygous Cys(326) genotype may represent a biomarker of individual susceptibility of lung cancer risk in individuals that are occupationally exposed to CrVI.

Oxidative stress damage in the liver of fish and rats receiving an intraperitoneal injection of hexavalent chromium as evaluated by chemiluminescence

The livers fractions of Oreochromis niloticus (Tilapia) and Wistar rats taken from treated animals to single intraperitoneal doses of hexavalent chromium (K(2)Cr(2)O(7)), were analyzed for tert butyl hydroperoxide-initiated chemiluminescence (CL), lipid peroxidation using thiobarbituric acid reactive substances (TBARS), enzymes superoxide dismutase (SOD) and catalase activities, and the quantification of cytochromes P450 and b5. The CL time course curve was significantly higher in O. niloticus treated with Cr(VI) at all times studied. The maximum CL was observed after 24h of exposure. The CL mean ratio treated/control was 4.6 and the initial velocity (V(0)) increased 7.4 times at 24h of intoxication. The TBARS levels however increased only 24h after intoxication. The CL time course curve was significantly higher in rats treated with Cr(VI) as early as 3h after intoxication. The maximum CL occurred 24h after exposure. The CL mean ratio treated/control was 2.1 and the V(0) increased 3.8 times at 24h of intoxication. On the contrary, was not observed any increase in TBARS in this study. Compared to the controls, in fish, SOD activity increased significantly only 24h after of exposure. In rats, there was a significant increase in SOD activity after 3 and 24h of intoxication. There was no catalase activity, nor cytochrome P450 and cytochrome b5 variation in both species studied. Through CL approach, it was possible to detect oxidative stress as early as 15min in fish and 3h in rats. Also a marked oxidative stress was revealed by the increased CL parameters that at 24h of intoxication was accompanied by arose SOD activity in liver of O. niloticus and Wistar rats and increased TBARS in O. niloticus. In addition, it was possible to show higher levels of oxidative stress in fish compared to the rat in spite of the dose to be four times smaller. Furthermore, CL provide a sensitive method for possible use to detect earlier biological impact in contaminated environments.

Potential role of vitamins in chromium induced spermatogenesis in Swiss mice

Chromium (Cr) (VI) compounds are known carcinogens and mutagens. The mechanism of carcinogenicity and mutagenicity caused by chromium(VI) compounds remained unclear for several years. However, in the recent past chromium-induced carcinogenicity and/or mutagenicity was known to happen due to the generation of reactive oxygen species (ROS). In the present context, chromic acid (CrO(3)), a potential Cr(VI) compound could be able to generate reactive oxygen radicals in the testes of Swiss mice as evidenced from significantly higher lipid peroxidation compared to untreated controls. The cytotoxic effects of the compound on the testes are depicted in terms of significantly reduced sperm count level accompanied with increased abnormal sperm population in treated mice. Supplementation of vitamins like Vitamin C and Vitamin E (Vit C and Vit E) to CrO(3) injected mice groups could partially prevent the incidence of abnormal sperm population and increased the sperm count. Of the two vitamins, taken for the study, Vit C happens to be more effective in ameliorating germ cells from degeneration and from mutation to abnormal sperm. Possible antioxidative role of both the vitamins have been studied for significant decrease in lipid peroxidation associated with marked elevation in sperm count level and significant decrease in the percentage of abnormal sperm formation in CrO(3)-treated mice.