Chromium (VI) inhibits heme oxygenase-1 expression in vivo and in arsenic-exposed human airway epithelial cells

Two- and Three-Dimensional Culture Systems: Respiratory In Vitro Tissue Models for Chemical Screening and Risk-Based Decision Making

Risk of lung damage from inhaled chemicals or substances has long been assessed using animal models. However, New Approach Methodologies (NAMs) that replace, reduce, and/or refine the use of animals in safety testing such as 2D and 3D cultures are increasingly being used to understand human-relevant toxicity responses and for the assessment of hazard identification. Here we review 2D and 3D lung models in terms of their application for inhalation toxicity assessment. We highlight a key case study for the Organization for Economic Cooperation and Development (OECD), in which a 3D model was used to assess human toxicity and replace the requirement for a 90-day inhalation toxicity study in rats. Finally, we consider the regulatory guidelines for the application of NAMs and potential use of different lung models for aerosol toxicity studies depending on the regulatory requirement/context of use.

Bisphenol S induces oxidative stress-mediated impairment of testosterone synthesis by inhibiting the Nrf2/HO-1 signaling pathway

Bisphenol S (BPS) is an environmental endocrine disruptor widely used in industrial production. BPS induces oxidative stress and exhibits male reproductive toxicity in mice, but the mechanisms by which BPS impairs steroid hormone synthesis are not fully understood. Nuclear factor erythroid 2-related factor 2(Nrf2)/HO-1 signaling is a key pathway in improving cellular antioxidant defense capacities. Therefore, this study explored the effects of exposure to BPS on testosterone synthesis in adult male mice and its mechanisms with regard to the Nrf2/HO-1 signaling pathway. Adult male C57BL/6 mice were orally exposed to BPS (2, 20, and 200 mg/kg BW) with sesame oil as a vehicle (0.1 ml/10 g BW) per day for 28 consecutive days. The results showed that compared with the control group, serum testosterone levels were substantially reduced in the 20 and 200 mg/kg BPS treatment groups, and testicular testosterone levels were reduced in all BPS treatment groups. These changes were accompanied by a prominent decrease in the expression levels of testosterone synthesis-related enzymes (STAR, CYP11A1, CYP17A1, HSD3B1, and HSD17B3) in the mouse testis. In addition, BPS induced oxidative stress in the testis by upregulating the messenger RNA and protein levels of Keap1 and downregulating the levels of Nrf2, HO-1, and downstream antioxidant enzymes (CAT, SOD1, and Gpx4). In summary, our results indicate that exposure of adult male mice to BPS can inhibit Nrf2/HO-1 signaling and antioxidant enzyme activity, which induces oxidative stress and thereby may impair testosterone synthesis in testicular tissues, leading to reproductive damage.

Sulforaphane prevents chromium-induced lung injury in rats via activation of the Akt/GSK-3β/Fyn pathway

Chromium (Cr) is an internationally recognized carcinogenic hazard that causes serious pulmonary toxicity. However, Cr-induced pulmonary toxicity lacks effective treatment to date. Sulforaphane (SFN), a well-known organosulfur compound, has gained increasing attention because of its unique biological function. This study investigates if SFN could decrease K₂Cr₂O₇-induced pulmonary toxicity and a potential mechanism involved using a rat 35-day Cr-induced pulmonary toxicity model and the mouse alveolar type II epithelial cell line (MLE-12). The results showed that SFN prevented Cr-induced oxidative stress, histopathological lesions, inflammation, apoptosis, and changes in protein kinase B (Akt) and glycogen synthase kinase 3 beta (GSK-3β) levels in vivo and in vitro. However, SFN can not play the protective effect against K₂Cr₂O₇-induced cell injury after treating by an Akt-specific inhibitor (MK-2206 2HCl) in MLE-12 cells. Furthermore, SFN increased the expression of nuclear factor-E2-related factor-2 (Nrf2) phase II detoxification enzymes. Collectively, this study demonstrates that SFN prevents K₂Cr₂O₇-induced lung toxicity in rats through enhancing Nrf2-mediated exogenous antioxidant defenses via activation of the Akt/GSK-3β/Fyn signaling pathway. SFN may be a novel natural substance to cure Cr-induced lung toxicity.

Mixture toxicity of the combinations of silver nanoparticles and environmental pollutants

Although toxicity of silver nanoparticles (AgNPs) has been well studied, the mixture toxicity of the combination of AgNPs and other environmental pollutants is still largely unknown. Here, we investigated the mixture toxicity of the combinations of AgNPs and common environmental pollutants such as arsenic (As), cadmium (Cd), and chromium (Cr) on human hepatoma cell line (HepG2) at noncytotoxic concentrations based on analyses of cytotoxicity, genotoxicity, reactive oxygen species (ROS) generation, and modes of cell death. In addition, DNA microarray analysis was performed to understand the cellular responses at a molecular level. AgNPs-As and AgNPs-Cd combinations exhibited synergistic effect on cytotoxicity while AgNPs-Cr showed additive effect. The AgNPs-Cd combination caused much stronger synergism than AgNPs-As combination. Based on cellular and molecular level analyses, the synergistic effect could be explained by overproduction of reactive oxygen species (ROS), which induced DNA damage and consequently apoptotic cell death. On the other hand, the additive effect caused by AgNPs-Cr could be attributed to reduction of the mixture toxicity by precipitation of Cr ions. Taken together, our results clearly demonstrated that the mixture toxicity of AgNPs with As, Cd, or Cr at noncytotoxic concentrations had different toxicity effects. Particularly, toxicogenomic approach using DNA microarray was useful to assess the mechanisms of the mixture toxicity.

Uranium-induced rat kidney cell cytotoxicity is mediated by decreased endogenous hydrogen sulfide (H2S) generation involved in reduced Nrf2 levels

The mechanism of uranium-induced kidney cell cytotoxicity is not fully understood. Nrf2 is a transcription factor which can regulate gene expression of cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE) which are responsible for endogenous H2S formation. H2S is recognized as the gaseous mediator that exerts antioxidative and cytoprotective effects. Here, we assessed the in vitro effects of uranyl acetate on Nrf2 gene expression and endogenous H2S production in a stable rat kidney cell line (NRK-52E). The results imply that uranium treatment decreased cell viability and increased LDH release, indicating uranium-induced cytotoxicity. Uranium intoxication increased intracellular ROS and MDA contents, depleted GSH levels, and impaired SOD and CAT activities, which resulted in oxidative stress injuries. Uranium intoxication reduced CBS and CSE gene expression and endogenous H2S production. Uranium contamination decreased Nrf2 protein expression and nuclear translocation. RNA silencing of Nrf2 gene expression in kidney cells which had not been treated by uranium decreased CBS and CSE gene expression and endogenous H2S generation, which mirrored the effects of uranium exposure. In contrast, treating uranium-exposed kidney cells with Nrf2 activator (sulforaphane) preserved the protein levels of Nrf2, CBS and CSE, and endogenous H2S formation. Administration of NaHS (an H2S donor) to uranium-intoxicated kidney cells reduced cell damage and alleviated oxidative stress. These data imply that uranium-induced kidney cell cytotoxicity is mediated by decreased endogenous H2S production due to the down-regulation of CBS and CSE gene expression and reduced Nrf2 levels. Supplementary H2S generation and/or Nrf2 activation can mitigate the adverse effects of uranium on kidney cells.

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.

Long-term exposure to hexavalent chromium inhibits expression of tumor suppressor genes in cultured cells and in mice

We have used mouse hepatoma cells in culture to study acute, short-term high-dose effects of hexavalent chromium on gene regulation directed by the polycyclic aromatic hydrocarbon benzo[a]pyrene (BaP). We find that the mixture engages three major signaling pathways: (i) activation of detoxification genes; (ii) induction of signal transduction effectors; and (iii) epigenetic modification of chromatin marks. Preliminary results in mice exposed to mixtures of low doses of Cr(VI) plus BaP indicate that all three pathways are likely to be engaged also in long-term effects resulting from exposure to environmentally relevant doses of the mixture that inhibit the expression of tumor suppressor genes. Given the toxicity and carcinogenicity of these mixtures, we expect that a two-way analytical approach, from cells in culture to biological effects in vivo and vice versa, will provide a better understanding of the molecular mechanisms responsible for the biological effects of mixtures. By focusing both the in vivo and the in vitro work into long-term, low-dose, environmentally relevant exposures, we expect to develop much needed information pertinent to the type of diseases found in human populations exposed to mixtures of environmental toxicants.

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.

Nrf2-ARE stress response mechanism: a control point in oxidative stress-mediated dysfunctions and chronic inflammatory diseases

Nrf2, a redox sensitive transcription factor, plays a pivotal role in redox homeostasis during oxidative stress. Nrf2 is sequestered in cytosol by an inhibitory protein Keap1 which causes its proteasomal degradation. In response to electrophilic and oxidative stress, Nrf2 is activated, translocates to nucleus, binds to antioxidant response element (ARE), thus upregulates a battery of antioxidant and detoxifying genes. This function of Nrf2 can be significant in the treatment of diseases, such as cancer, neurodegenerative, cardiovascular and pulmonary complications, where oxidative stress causes Nrf2 derangement. Nrf2 upregulating potential of phytochemicals has been explored, in facilitating cure for various ailments while, in cancer cells, Nrf2 upregulation causes chemoresistance. Therefore, Nrf2 emerges as a key regulator in oxidative stress-mediated diseases and Nrf2 silencing can open avenues in cancer treatment. This review summarizes Nrf2-ARE stress response mechanism and its role as a control point in oxidative stress-induced cellular dysfunctions including chronic inflammatory diseases.

Chromium genotoxicity: A double-edged sword

Certain forms of hexavalent chromium [Cr(VI)] are known respiratory carcinogens that induce a broad spectrum of DNA damage. Cr(VI)-carcinogenesis may be initiated or promoted through several mechanistic processes including, the intracellular metabolic reduction of Cr(VI) producing chromium species capable of interacting with DNA to yield genotoxic and mutagenic effects, Cr(VI)-induced inflammatory/immunological responses, and alteration of survival signaling pathways. Cr(VI) enters the cell through non-specific anion channels, and is metabolically reduced by agents including ascorbate, glutathione, and cysteine to Cr(V), Cr(IV), and Cr(III). Cr(III) has a weak membrane permeability capacity and is unable to cross the cell membrane, thereby trapping it within the cell where it can bind to DNA and produce genetic damage leading to genomic instability. Structural genetic lesions produced by the intracellular reduction of Cr(VI) include DNA adducts, DNA-strand breaks, DNA-protein crosslinks, oxidized bases, abasic sites, and DNA inter- and intrastrand crosslinks. The damage induced by Cr(VI) can lead to dysfunctional DNA replication and transcription, aberrant cell cycle checkpoints, dysregulated DNA repair mechanisms, microsatelite instability, inflammatory responses, and the disruption of key regulatory gene networks responsible for the balance of cell survival and cell death, which may all play an important role in Cr(VI) carcinogenesis. Several lines of evidence have indicated that neoplastic progression is a result of consecutive genetic/epigenetic changes that provide cellular survival advantages, and ultimately lead to the conversion of normal human cells to malignant cancer cells. This review is based on studies that provide a glimpse into Cr(VI) carcinogenicity via mechanisms including Cr(VI)-induced death-resistance, the involvement of DNA repair mechanisms in survival after chromium exposure, and the activation of survival signaling cascades in response to Cr(VI) genotoxicity.

Application of the U.S. EPA mode of action Framework for purposes of guiding future research: a case study involving the oral carcinogenicity of hexavalent chromium

Mode of action (MOA) analysis provides a systematic description of key events leading to adverse health effects in animal bioassays for the purpose of informing human health risk assessment. Uncertainties and data gaps identified in the MOA analysis may also be used to guide future research to improve understanding of the MOAs underlying a specific toxic response and foster development of toxicokinetic and toxicodynamic models. An MOA analysis, consistent with approaches outlined in the MOA Framework as described in the Guidelines for Carcinogen Risk Assessment, was conducted to evaluate small intestinal tumors observed in mice chronically exposed to relatively high concentrations of hexavalent chromium (Cr(VI)) in drinking water. Based on review of the literature, key events in the MOA are hypothesized to include saturation of the reductive capacity of the upper gastrointestinal tract, absorption of Cr(VI) into the intestinal epithelium, oxidative stress and inflammation, cell proliferation, direct and/or indirect DNA modification, and mutagenesis. Although available data generally support the plausibility of these key events, several unresolved questions and data gaps were identified, highlighting the need for obtaining critical toxicokinetic and toxicodynamic data in the target tissue and in the low-dose range. Experimental assays that can address these data gaps are discussed along with strategies for comparisons between responsive and nonresponsive tissues and species. This analysis provides a practical application of MOA Framework guidance and is instructive for the design of studies to improve upon the information available for quantitative risk assessment.

Effect of chromium and cobalt ions on the expression of antioxidant enzymes in human U937 macrophage-like cells

The main concern associated with metal-on-metal (MM) hip prosthesis is the presence of metal ions, mainly chromium (Cr) and cobalt (Co), which are found both systemically and locally in the organism of patients. Previous studies revealed that Cr(III) and Co(II) ions could induce damages to proteins in macrophage-like cells in vitro, probably through the formation of reactive oxygen species (ROS). We then hypothesized that these ions can modify the expression of antioxidant enzymes in these cells. Results showed that Cr(VI) induced the protein expression of Mn-superoxide dismutase, Cu/Zn-superoxide dismutase, catalase, glutathione peroxidase, and heme oxygenase-1 (HO-1) but had no effect of the expression of their mRNA. Cr(III) have no effect on the expression of all these antioxidant enzymes. Co(II) induced the expression of both protein and mRNA of HO-1 only. In conclusion, results showed that Cr(VI), Cr(III), and Co(II) had differential effects on the expression of antioxidant enzymes in macrophage-like cells in vitro.

Induction of morphological changes in BEAS-2B human bronchial epithelial cells following chronic sub-cytotoxic and mildly cytotoxic hexavalent chromium exposures

Certain hexavalent chromium (Cr(VI)) compounds are well established occupational respiratory tract carcinogens. However, despite extensive studies, the cellular and molecular mechanisms underlying Cr(VI)-induced lung cancer remain poorly understood. In fact, the models used were often suboptimal and yielded conflicting results that were heavily dependent upon the system and experimental conditions employed. Here, we investigated the effects of chronic subcytotoxic and mildly cytotoxic (0.1-2 microM) Cr(VI) exposures on cultures of human bronchial epithelial cells, the main targets of Cr(VI) carcinogenicity. Our studies with the nontumorigenic BEAS-2B cell line suggest that relatively short exposures (h) to sublethal Cr(VI) doses (0.1-1 microM) may render these cells less sensitive to contact inhibition. We have also observed a reduced sensitivity to Cr(VI)-induced apoptosis shortly after the beginning of exposure to a mildly cytotoxic Cr(VI) dose (2 microM). Further studies are needed to determine whether these two phenotypes are involved in the Cr(VI)-induced carcinogenic process. Additionally, evidence gathered in this study strongly points to a Cr(VI) interference with cell adhesion to the substratum and with cell-cell interactions. Finally, by chronically exposing BEAS-2B cells to mildly cytotoxic Cr(VI) doses (1 and 2 microM), we were able to induce changes in cell morphology and pattern of growth characteristic of an early phase of pre-malignant progression.

Mechanism of DNA-protein cross-linking by chromium

Hexavalent chromium is a known inducer of DNA-protein cross-links (DPCs) that contribute to repression of inducible genes and genotoxicity of this metal. Lymphocytic DPCs have also shown potential utility as biomarkers of human exposure to Cr(VI). Here, we examined the mechanism of DPC formation by Cr(VI) and the impact of its main cellular reducers. In vitro reactions of Cr(VI) with one-electron reducing thiols (glutathione and cysteine) or two-electron donating ascorbate were all efficient at DPC production, indicating a dispensable role of Cr(V). No Cr(VI) reducer was able to generate DPC in the presence of Cr(III)-chelating EDTA or phosphate. A critical role of Cr(III) in DNA-protein linkages was further confirmed by dissociation of Cr(VI)-induced DPC by phosphate. EDTA was very inefficient in DPC dissociation, indicating its poor suitability for testing of Cr(III)-mediated bridging and reversal of complex DPC. Reactions containing only one Cr-modified component (protein or DNA) showed that Cr(III)-DNA adduction was the initial step in DPC formation. Cross-linking proceeded slowly after the rapid formation of Cr-DNA adducts, indicating that protein conjugation was the rate-limiting step in DPC generation. Experiments with depletion of glutathione and restoration of ascorbate levels in human lung A549 cells showed that high cellular reducing capacity promotes DPC yield. Overall, our data provide evidence for a three-step cross-linking mechanism involving (i) reduction of Cr(VI) to Cr(III), (ii) Cr(III)-DNA binding, and (iii) protein capture by DNA-bound Cr(III) generating protein-Cr(III)-DNA cross-links.

Nickel mobilizes intracellular zinc to induce metallothionein in human airway epithelial cells

We recently reported that induction of metallothionein (MT) was critical in limiting nickel (Ni)-induced lung injury in intact mice. Nonetheless, the mechanism by which Ni induces MT expression is unclear. We hypothesized that the ability of Ni to mobilize zinc (Zn) may contribute to such regulation and therefore, we examined the mechanism for Ni-induced MT2A expression in human airway epithelial (BEAS-2B) cells. Ni induced MT2A transcript levels and protein expression by 4 hours. Ni also increased the activity of a metal response element (MRE) promoter luciferase reporter construct, suggesting that Ni induces MRE binding of the metal transcription factor (MTF-1). Exposure to Ni resulted in the nuclear translocation of MTF-1, and Ni failed to induce MT in mouse embryonic fibroblasts lacking MTF-1. As Zn is the only metal known to directly bind MTF-1, we then showed that Ni increased a labile pool of intracellular Zn in cells as revealed by fluorescence-activated cell sorter using the Zn-sensitive fluorophore, FluoZin-3. Ni-induced increases in MT2A mRNA and MRE-luciferase activity were sensitive to the Zn chelator, TPEN, supporting an important role for Zn in mediating the effect of Ni. Although neither the source of labile Zn nor the mechanism by which Ni liberates labile Zn was apparent, it was noteworthy that Ni increased intracellular reactive oxygen species (ROS). Although both N-acetyl cysteine (NAC) and ascorbic acid (AA) decreased Ni-induced increases in ROS, only NAC prevented Ni-induced increases in MT2A mRNA, suggesting a special role for interactions of Ni, thiols, and Zn release.

Signal transducer and activator of transcription 1 (STAT1) is essential for chromium silencing of gene induction in human airway epithelial cells

Hexavalent chromium (Cr(VI)) promotes lung injury and pulmonary diseases through poorly defined mechanisms that may involve the silencing of inducible protective genes. The current study investigated the hypothesis that Cr(VI) actively signals through a signal transducer and activator of transcription 1 (STAT1)-dependent pathway to silence nickel (Ni)-induced expression of vascular endothelial cell growth factor A (VEGFA), an important mediator of lung injury and repair. In human bronchial airway epithelial (BEAS-2B) cells, Ni-induced VEGFA transcription by stimulating an extracellular regulated kinase (ERK) signaling cascade that involved Src kinase-activated Sp1 transactivation, as well as increased hypoxia-inducible factor-1 alpha (HIF-1 alpha) stabilization and DNA binding. Ni-stimulated ERK, Src, and HIF-1 alpha activities, as well as Ni-induced VEGFA transcript levels were inhibited in Cr(VI)-exposed cells. We previously demonstrated that Cr(VI) stimulates STAT1 to suppress VEGFA expression. In BEAS-2B cells stably expressing STAT1 short hairpin RNA, Cr(VI) increased VEGFA transcript levels and Sp1 transactivation. Moreover, in the absence of STAT1, Cr(VI), and Ni coexposures positively interacted to further increase VEGFA transcripts. This study demonstrates that metal-stimulated signaling cascades interact to regulate transcription and induction of adaptive or repair responses in airway cells. In addition, the data implicate STAT1 as a rate limiting mediator of Cr(VI)-stimulated gene regulation and suggest that cells lacking STAT1, such as many tumor cell lines, have opposite responses to Cr(VI) relative to normal cells.

Traumatic brain injury induces the activation of the Nrf2-ARE pathway in the lung in rats

BACKGROUND

Acute lung injury (ALI) is a frequent but poorly understood complication of traumatic brain injury (TBI). The Nrf2-ARE pathway has been proved to be essential for protection against diffuse inflammation and oxidative damage, which are both involved in ALI following TBI. However, whether the Nrf2-ARE pathway is activated after TBI in the lung hasn't been studied.

METHODS AND PROCEDURES

In the present study, the nuclear Nrf2 protein level was detected by Western blot and the mRNA levels of heme oxygenase-1 (HO-1) and NAD(P)H:quinone oxidoreductase-1 (NQO1), two Nrf2-regulated gene products, were determined by RT-PCR at 24 hours after TBI. In addition, the expression of Nrf2 and HO-1 was localized by immunohistochemical study.

MAIN OUTCOMES AND RESULTS

After TBI, the nuclear Nrf2 protein level in the lung was significantly increased and the mRNA levels of both HO-1 and NQO1 were also up-regulated. Moreover, immunohistochemical study showed that both Nrf2 and HO-1 were mainly localized in tracheobronchial epithelium and alveolar macrophages.

CONCLUSION

These results suggest that the Nrf2-ARE pathway is activated in the lung after TBI.

Lung injury, inflammation and Akt signaling following inhalation of particulate hexavalent chromium

Certain particulate hexavalent chromium [Cr(VI)] compounds are human respiratory carcinogens that release genotoxic soluble chromate, and are associated with fibrosis, fibrosarcomas, adenocarcinomas and squamous cell carcinomas of the lung. We postulate that inflammatory processes and mediators may contribute to the etiology of Cr(VI) carcinogenesis, however the immediate (0-24 h) pathologic injury and immune responses after exposure to particulate chromates have not been adequately investigated. Our aim was to determine the nature of the lung injury, inflammatory response, and survival signaling responses following intranasal exposure of BALB/c mice to particulate basic zinc chromate. Factors associated with lung injury, inflammation and survival signaling were measured in airway lavage fluid and in lung tissue. A single chromate exposure induced an acute immune response in the lung, characterized by a rapid and significant increase in IL-6 and GRO-alpha levels, an influx of neutrophils, and a decline in macrophages in lung airways. Histological examination of lung tissue in animals challenged with a single chromate exposure revealed an increase in bronchiolar cell apoptosis and mucosal injury. Furthermore, chromate exposure induced injury and inflammation that progressed to alveolar and interstitial pneumonitis. Finally, a single Cr(VI) challenge resulted in a rapid and persistent increase in the number of airways immunoreactive for phosphorylation of the survival signaling protein Akt, on serine 473. These data illustrate that chromate induces both survival signaling and an inflammatory response in the lung, which we postulate may contribute to early oncogenesis.

Chromium cross-links histone deacetylase 1-DNA methyltransferase 1 complexes to chromatin, inhibiting histone-remodeling marks critical for transcriptional activation

Transcriptional regulation of gene expression requires posttranslational modification of histone proteins, which, in concert with chromatin-remodeling factors, modulate chromatin structure. Exposure to environmental agents may interfere with specific histone modifications and derail normal patterns of gene expression. To test this hypothesis, we coexposed cells to binary mixtures of benzo[a]pyrene (B[a]P), an environmental procarcinogen that activates Cyp1a1 transcriptional responses mediated by the aryl hydrocarbon receptor (AHR), and chromium, a carcinogenic heavy metal that represses B[a]P-inducible AHR-mediated gene expression. We show that chromium cross-links histone deacetylase 1-DNA methyltransferase 1 (HDAC1-DNMT1) complexes to Cyp1a1 promoter chromatin and inhibits histone marks induced by AHR-mediated gene transactivation, including phosphorylation of histone H3 Ser-10, trimethylation of H3 Lys-4, and various acetylation marks in histones H3 and H4. These changes inhibit RNA polymerase II recruitment without affecting the kinetics of AHR DNA binding. HDAC1 and DNMT1 inhibitors or depletion of HDAC1 or DNMT1 with siRNAs blocks chromium-induced transcriptional repression by decreasing the interaction of these proteins with the Cyp1a1 promoter and allowing histone acetylation to proceed. By inhibiting Cyp1a1 expression, chromium stimulates the formation of B[a]P DNA adducts. Epigenetic modification of gene expression patterns may be a key element of the developmental and carcinogenic outcomes of exposure to chromium and to other environmental agents.

Mechanisms of action of inhaled fibers, particles and nanoparticles in lung and cardiovascular diseases

BACKGROUND

A symposium on the mechanisms of action of inhaled airborne particulate matter (PM), pathogenic particles and fibers such as silica and asbestos, and nanomaterials, defined as synthetic particles or fibers less than 100 nm in diameter, was held on October 27 and 28, 2005, at the Environmental Protection Agency (EPA) Conference Center in Research Triangle Park, North Carolina. The meeting was the eighth in a series of transatlantic conferences first held in Penarth, Wales, at the Medical Research Council Pneumoconiosis Unit (1979), that have fostered long-standing collaborations between researchers in the fields of mineralogy, cell and molecular biology, pathology, toxicology, and environmental/occupational health.

RESULTS

The goal of this meeting, which was largely supported by a conference grant from the NHLBI, was to assemble a group of clinical and basic research scientists who presented and discussed new data on the mechanistic effects of inhaled particulates on the onset and development of morbidity and mortality in the lung and cardiovascular system. Another outcome of the meeting was the elucidation of a number of host susceptibility factors implicated in adverse health effects associated with inhaled pathogenic particulates.

CONCLUSION

New models and data presented supported the paradigm that both genetic and environmental (and occupational) factors affect disease outcomes from inhaled particulates as well as cardiopulmonary responses. These future studies are encouraged to allow the design of appropriate strategies for prevention and treatment of particulate-associated morbidity and mortality, especially in susceptible populations.

Resistance to nitric oxide-induced necrosis in heme oxygenase-1 overexpressing pulmonary epithelial cells associated with decreased lipid peroxidation

Increased expression of heme oxygenase-1 (HO-1) increases NO resistance in several cell types, although the biochemical mechanism for this protection is unknown. To address this issue, we have measured different molecular markers of nitrosative stress in three stably transfected cell lines derived from the human lung epithelial line A549: two lines that overexpress rat HO-1 (L1 and A4), and a control line with the empty vector (Neo). Compared with the control Neo cells, L1 and A4 cells had, respectively, 5.8- and 3.8-fold greater HO activity accompanied by increased resistance to NO-induced necrosis. Compared with the Neo control, the HO-1-overexpressing cells also showed significantly less lipid peroxide formation and decreased perturbation of transition metal oxidation and coordination states following a cytotoxic NO exposure. These effects were blocked by the HO-1 inhibitors Zn- and Sn-protoporphyrin IX. In contrast, HO-1 overexpression did not significantly affect total reactive oxygen or nitrogen species, the levels of the nucleobase deamination products in DNA (xanthine, inosine, and uracil) following NO exposure, or NO-induced protein nitration. While increased HO-1 activity prevented NO-induced fluctuations in transition metal homeostasis, addition of an iron chelator decreased NO toxicity only slightly. Our results indicate that lipid peroxidation is a significant cause of NO-induced necrosis in human lung epithelial cells, and that the increased NO survival of L1 cells is due at least in part to decreased lipid peroxidation mediated by HO-1-generated biliverdin or bilirubin.