The role of reactive oxygen species in arsenite and monomethylarsonous acid-induced signal transduction in human bladder cells: acute studies

Ethanol extract of Vitellaria paradoxa (Gaertn, F) leaves protects against sodium arsenite - induced toxicity in male wistar rats

The inadvertent exposure to arsenic has been associated with diverse diseases such as cancers. Vitellaria paradoxa is a medicinal plant with antidiabetic and antiproliferative properties. Here, we assessed the ameliorative role of Ethanol Leaf extract of Vitellaria paradoxa (ELVp) in Sodium Arsenite (SA) - induced toxicity in rats after oral treatment for two weeks as follows: Group 1 (Control, distilled water), Group 2 (Vitamin E, 100 mg/kg), Groups 3 and 4 (ELVp, 100 & 200 mg/kg respectively), Group 5 (SA, 2.5 mg/kg), Group 6 (SA + Vit E) and Group 7 (SA + ELVp (100 mg/kg) and Group 8 (SA + ELVp (200 mg/kg). The results indicated that SA significantly increased liver and kidney function markers and elevated platelet, white blood cell (WBC) count and malondialdehyde levels in rats. Additionally, SA decreased Red Blood Cell (RBC), Hemoglobin (HGB) and Hematocrit (HCT) levels in rats (p < 0.05). Sodium arsenite caused mild expression of BCL-2 protein> NF-Kb = p53 in the kidney of rats. However, ELVp ameliorated SA-induced toxicity in the liver and kidney of rats with respect to these markers. Overall, ELVp has hepatoprotective, nephroprotective and apoptotic properties against sodium arsenite-induced toxicity.

Rbfox2 dissociation from stress granules suppresses cancer progression

Stress granules (SGs) are stalled translation initiation complexes comprising untranslated mRNAs and RNA-binding proteins (RBPs). RBP fox-1 homolog 2 (Rbfox2), a component of SGs, binds to retinoblastoma 1 (RB1) mRNA, which is closely related to cancer progression; however, the role of Rbfox2 in cancer progression remains largely unknown. In this study, we confirmed that Rbfox2, which is present in the nucleus as a splicing regulator, localizes to the cytoplasm of human colon cancer tissues and that induction of Rbfox2 dissociation from SGs by resveratrol treatment inhibits cancer progression. We also observed that Rbfox2 in SGs inhibited RB1 protein expression and promoted cell cycle progression. Additionally, resveratrol treatment inhibited SG-mediated Rbfox2 localization, further inhibiting RB1 protein expression, and inhibited specific Rbfox2 localization to the cytoplasm in melanoma B16-F10 cells, thereby effectively inhibiting metastasis and tumor growth ability. These results indicate that Rbfox2 dissociation from SGs attenuates cancer progression and offer insight into the mechanism associated with Rbfox2 dissociation, thereby marking Rbfox2 as a potential candidate target for cancer therapy.

Resveratrol, an antioxidant found in red grapes, slows cancer progression by interfering with the localization and function of the RNA-binding protein Rbfox2. A study led by Kee Kim at Chungnam National University and Su-Hyung Park at Korea Advanced Institute of Science and Technology, Daejeon, Republic of Korea, showed that in human colon cancer cells Rbfox2 is located in the cytoplasm where it promotes cell proliferation by blocking the assembly of the tumor suppressor protein RB1. Treatment with resveratrol prevented the migration of Rbfox2 from the nucleus to the cytoplasm and significantly reduced tumor growth in a mouse model of melanoma. This study not only sheds light on the protective effects of resveratrol but also suggests that Rbfox2 could be a potential target for the development of new anticancer drugs.

Arsenite Targets the RING Finger Domain of Rbx1 E3 Ubiquitin Ligase to Inhibit Proteasome-Mediated Degradation of Nrf2

Activation of the nuclear factor erythroid 2-related factor 2 (Nrf2) antioxidant response signaling pathway is a major mechanism for the cellular defense against oxidative stress. Arsenite, a widespread contaminant in drinking water, is known to induce oxidative stress and activate the Nrf2-dependent signaling pathway through the stabilization of the Nrf2 protein by inhibiting its ubiquitination via the Cul3-Rbx1-Keap1 (cullin 3, RING-box 1, and Kelch-like ECH-associated protein 1) E3 ubiquitin ligase, and its degradation by the 26S proteasome, though the underlying mechanism, remains elusive. In the present study, we demonstrated that arsenite could bind to the RING finger domain of Rbx1 in vitro and in cells, which led to the suppression of Cul3-Rbx1 E3 ubiquitin ligase activity, thereby impairing the Nrf2 ubiquitination and activating the Nrf2-induced antioxidant signaling pathway. Our finding provided novel insight into arsenic toxicity by uncovering a distinct mechanism accounting for arsenite-induced Nrf2 activation.

The tumor suppressor phosphatase and tensin homolog protein (PTEN) is negatively regulated by NF-κb p50 homodimers and involves histone 3 methylation/deacetylation in UROtsa cells chronically exposed to monomethylarsonous acid

UROtsa cells have been accepted as a model to study carcinogenicity mechanisms of arsenic-associated human bladder cancer. In vitro continuous exposure to monomethylarsonous acid (MMA^III), leads UROtsa cells to commit to malignant transformation. In this process, NF-κβ-associated inflammatory response seems to play an important role since this transcription factor activates some minutes after cells are exposed in vitro to MMA^III and keeps activated during the cellular malignant transformation. It is known that a slight decrease in the protein phosphatase and tensin homologue (PTEN) gene expression is enough for some cells to become malignantly transformed. Interestingly, this tumor suppressor has been proven to be negatively regulated by NF-κβ through binding to its gene promoter. Based on these observations we propose that NF-κβ may be involved in arsenic associated carcinogenesis through the negative regulation of PTEN gene expression. Changes in PTEN expression and the binding of p50 NF-κβ subunit to PTEN promoter were evaluated in UROtsa cells exposed for 4, 12, 20, or 24 wk to 50nM MMA^III. Results showed that MMA^III induced a significant decrease in PTEN expression around 20 wk exposure to MMA^III,which correlated with increased binding of p50 subunit to the PTEN promoter. Consistent with these results, ChIP assays also showed a significant decrease in H3 acetylation (H3ac) but an increase in the repression marks H3k9me3 and H327me3 in PTEN promoter when compared with not treated cells. These results suggest that the activation of NF-κβ by MMA^III may participate in UROtsa cells malignant transformation through the negative regulation of PTEN expression involving p50 homodimers-mediated chromatin remodeling around the PTEN promoter.

Solid surface photochemistry of montmorillonite: mechanisms for the arsenite oxidation under UV-A irradiation

Transformation of inorganic arsenic species has drawn great concern in recent decades because of worldwide and speciation-dependent pollution and the hazards that they pose to the environment and to human health. As(III) photooxidation in aquatic systems has received much attention, but little is known about photochemical transformation of arsenic species on top soil. As(III) photooxidation on natural montmorillonite under UV-A radiation was investigated by using a moisture- and temperature-controlled photochemical chamber with two black-light lamps. Initial As(III) concentration, pH, layer thickness, humic acid (HA) concentration, the presence of additional iron ions, and the contribution of reactive oxygen species (ROS) were examined. The results show that pH values of the clay layers greatly influenced As(III) photooxidation on montmorillonite. As(III) photooxidation followed the Langmuir-Hinshelwood model. HA and additional iron ions greatly promoted photooxidation, but excess Fe(II) competed with As(III) for oxidation by ROS. Scavenging experiments revealed that natural montmorillonite induced the conversion of As(III) to As(V) by generating ROS (mainly HO(•) and HO2(•)/O2(•-)) and that HO(•) radical was the predominant oxidant in this system. Our work demonstrates that photooxidation on the surface of natural clay minerals in top soil can be important to As(III) transformation. This allows understanding and predicting the speciation and behavior of arsenic on the soil surface.

Photooxidation of arsenic(III) to arsenic(V) on the surface of kaolinite clay

As one of the most toxic heavy metals, the oxidation of inorganic arsenic has drawn great attention among environmental scientists. However, little has been reported on the solar photochemical behavior of arsenic species on top-soil. In the present work, the influencing factors (pH, relative humidity (RH), humic acid (HA), trisodium citrate, and additional iron ions) and the contributions of reactive oxygen species (ROS, mainly HO and HO2/O2(-)) to photooxidation of As(III) to As(V) on kaolinite surfaces under UV irradiation (λ=365nm) were investigated. Results showed that lower pH facilitated photooxidation, and the photooxidation efficiency increased with the increase of RH and trisodium citrate. Promotion or inhibition of As(III) photooxidation by HA was observed at low or high dosages, respectively. Additional iron ions greatly promoted the photooxidation, but excessive amounts of Fe(2+) competed with As(III) for oxidation by ROS. Experiments on scavengers indicated that the HO radical was the predominant oxidant in this system. Experiments on actual soil surfaces proved the occurrence of As(III) photooxidation in real topsoil. This work demonstrates that the photooxidation process of As(III) on the soil surface should be taken into account when studying the fate of arsenic in natural soil newly polluted with acidic wastewater containing As(III).

NaAsO2 induces cytotoxicity to rat bone marrow mesenchymal stem cells

AIM: To investigate the toxic effect of NaAsO₂ on rat bone marrow mesenchymal stem cells (MSCs) and to explore the possible mechanism involved.

METHODS: Cultured rat bone MSCs were exposed to different concentrations of NaAsO2 (10, 20, 30, 40 and 50 μmol/L) for 24 h. MTT assay was used to evaluate cell viability. The level of reactive oxygen species (ROS) was detected by staining cells with DCFH-DA. The content of malondialdehyde (MDA) and activity of superoxide dismutase (SOD) in rat bone MSCs were also measured.

RESULTS: Treatment with NaAsO₂ significantly decreased cell viability, GSH content and SOD activity, increased ROS and MDA formation, and induced Caspase 3 activation in rat bone MSCs cells.

CONCLUSION: Our findings suggest that NaAsO₂-induced oxidative stress may cause MSC apoptosis via mitochondria-dependent signaling pathways.

Sodium arsenite induces cyclooxygenase-2 expression in human uroepithelial cells through MAPK pathway activation and reactive oxygen species induction

Arsenic can induce reactive oxygen species (ROS) leading to oxidative stress and carcinogenesis. Bladder is one of the major target organs of arsenic, and cyclooxygenase-2 (COX-2) may play an important role in arsenic-induced bladder cancer. However, the mechanism by which arsenic induces COX-2 in bladder cells remains unclear. This study aimed at investigating arsenic-mediated intracellular redox status and signaling cascades leading to COX-2 induction in human uroepithelial cells (SV-HUC-1). SV-HUC-1 cells were exposed to sodium arsenite and COX-2 expression, mitogen-activated protein kinase (MAPK) phosphorylation, glutathione (GSH) levels, ROS induction and Nrf2 expression were quantified. Our results demonstrate that arsenite (1-10 μM) elevates COX-2 expression, GSH levels, ROS and Nrf2 expression. Arsenite treatment for 24h stimulates phosphorylation of ERK and p38, but not JNK in SV-HUC-1 cells. Induction of Cox-2 mRNA levels by arsenite was attenuated by inhibitors of ERK, p38 and JNK. Arsenite-induced ROS generation and COX-2 expression were significantly attenuated by treatment with melatonin (a ROS scavenger), but enhanced by DL-buthionine-(S, R)-sulfoximine (BSO, an inhibitor of gamma-glutamylcysteine synthetase (γ-GCS) resulting in lower GSH and increased ROS levels). These data indicate that arsenite promotes an induction of ROS, which results in an induction of COX-2 expression through activation of the MAPK pathway.

Thymoquinone prevents and ameliorates dextran sulfate sodium-induced colitis in mice

BACKGROUND

Thymoquinone (TQ), an active ingredient of the seed oil extract of Nigella sativa Linn, has previously been shown to possess antitumor, antioxidant, and anti-inflammatory bioactivity. Whether TQ has any effect on colitis remains controversial.

AIM

The aim of this study was to determine whether treatment with TQ prevents and ameliorates colonic inflammation in a mouse model of inflammatory bowel disease.

METHODS

C57BL/6 murine colitis was induced by the administration of dextran sodium sulfate (DSS) (3 % W/V) in the drinking water supplied to the mice for 7 consecutive days. The mice with colitis were treated with 5, 10, or 25 mg/kg TQ orally, and changes in body weight and macroscopic and microscopic colitis scores were examined. In addition, biochemical analyses were conducted.

RESULTS

The treatment of mice with TQ prevented and significantly reduced the appearance of diarrhea and body weight loss. These results were associated with amelioration of colitis-related damage, as measured by macroscopic and microscopic colitis scores. In addition, there was a significant reduction in colonic myeloperoxidase activity and malondialdehyde levels and an increase in glutathione levels.

CONCLUSIONS

These results indicate that TQ administration can prevent and improve murine DSS-induced colitis. These findings suggest that TQ could serve as a potential therapeutic agent for the treatment of patients with inflammatory bowel disease.

Global gene expression changes in human urothelial cells exposed to low-level monomethylarsonous acid

Bladder cancer has been associated with chronic arsenic exposure. Monomethylarsonous acid [MMA(III)] is a metabolite of inorganic arsenic and has been shown to transform an immortalized urothelial cell line (UROtsa) at concentrations 20-fold less than arsenite. MMA(III) was used as a model arsenical to examine the mechanisms of arsenical-induced transformation of urothelium. A microarray analysis was performed to assess the transcriptional changes in UROtsa during the critical window of chronic 50nM MMA(III) exposure that leads to transformation at 3 months of exposure. The analysis revealed only minor changes in gene expression at 1 and 2 months of exposure, contrasting with substantial changes observed at 3 months of exposure. The gene expression changes at 3 months were analyzed showing distinct alterations in biological processes and pathways such as a response to oxidative stress, enhanced cell proliferation, anti-apoptosis, MAPK signaling, as well as inflammation. Twelve genes selected as markers of these particular biological processes were used to validate the microarray and these genes showed a time-dependent changes at 1 and 2 months of exposure, with the most substantial changes occurring at 3 months of exposure. These results indicate that there is a strong association between the acquired phenotypic changes that occur with chronic MMA(III) exposure and the observed gene expression patterns that are indicative of a malignant transformation. Although the substantial changes that occur at 3 months of exposure may be a consequence of transformation, there are common occurrences of altered biological processes between the first 2 months of exposure and the third, which may be pivotal in driving transformation.

Interdependent genotoxic mechanisms of monomethylarsonous acid: role of ROS-induced DNA damage and poly(ADP-ribose) polymerase-1 inhibition in the malignant transformation of urothelial cells

Exposure of human bladder urothelial cells (UROtsa) to 50 nM of the arsenic metabolite, monomethylarsonous acid (MMA(III)), for 12 weeks results in irreversible malignant transformation. The ability of continuous, low-level MMA(III) exposure to cause an increase in genotoxic potential by inhibiting repair processes necessary to maintain genomic stability is unknown. Following genomic insult within cellular systems poly(ADP-ribose) polymerase-1 (PARP-1), a zinc finger protein, is rapidly activated and recruited to sites of DNA strand breaks. When UROtsa cells are continuously exposed to 50 nM MMA(III), PARP-1 activity does not increase despite the increase in MMA(III)-induced DNA single-strand breaks through 12 weeks of exposure. When UROtsa cells are removed from continuous MMA(III) exposure (2 weeks), PARP-1 activity increases coinciding with a subsequent decrease in DNA damage levels. Paradoxically, PARP-1 mRNA expression and protein levels are elevated in the presence of continuous MMA(III) indicating a possible mechanism to compensate for the inhibition of PARP-1 activity in the presence of MMA(III). The zinc finger domains of PARP-1 contain vicinal sulfhydryl groups which may act as a potential site for MMA(III) to bind, displace zinc ion, and render PARP-1 inactive. Mass spectrometry analysis demonstrates the ability of MMA(III) to bind a synthetic peptide representing the zinc-finger domain of PARP-1, and displace zinc from the peptide in a dose-dependent manner. In the presence of continuous MMA(III) exposure, continuous 4-week zinc supplementation restored PARP-1 activity levels and reduced the genotoxicity associated with MMA(III). Zinc supplementation did not produce an overall increase in PARP-1 protein levels, decrease the levels of MMA(III)-induced reactive oxygen species, or alter Cu-Zn superoxide dismutase levels. Overall, these results present two potential interdependent mechanisms in which MMA(III) may increase the susceptibility of UROtsa cells to genotoxic insult and/or malignant transformation: elevated levels of MMA(III)-induced DNA damage through the production of reactive oxygen species, and the direct MMA(III)-induced inhibition of PARP-1.

Influence of arsenate and arsenite on signal transduction pathways: an update

Arsenic has been a recognized contaminant and toxicant, as well as a medicinal compound throughout human history. Populations throughout the world are exposed to arsenic and these exposures have been associated with a number of human cancers. Not much is known about the role of arsenic as a human carcinogen and more recently its role in non-cancerous diseases, such as cardiovascular disease, hypertension and diabetes mellitus have been uncovered. The health effects associated with arsenic are numerous and the association between arsenic exposure and human disease has intensified the search for molecular mechanisms that describe the biological activity of arsenic in humans and leads to the aforementioned disease states. Arsenic poses a human health risk due in part to the regulation of cellular signal transduction pathways and over the last few decades, some cellular mechanisms that account for arsenic toxicity, as well as, signal transduction pathways have been discovered. However, given the ubiquitous nature of arsenic in the environment, making sense of all the data remains a challenge. This review will focus on our knowledge of signal transduction pathways that are regulated by arsenic.

Monomethylarsonous acid produces irreversible events resulting in malignant transformation of a human bladder cell line following 12 weeks of low-level exposure

Arsenic is a known human bladder carcinogen; however, the mechanisms underlying arsenical-induced bladder carcinogenesis are not understood. Previous research has demonstrated that exposure of a nontumorigenic human urothelial cell line, UROtsa, to 50 nM monomethylarsonous acid (MMA(III)) for 52 weeks resulted in malignant transformation. To focus research on the early mechanistic events leading to MMA(III)-induced malignancy, the goal of this research was to resolve the critical period in which continuous MMA(III) exposure (50 nM) induces the irreversible malignant transformation of UROtsa cells. An increased growth rate of UROtsa cells results after 12 weeks of MMA(III) exposure. Anchorage-independent growth occurred after 12 weeks with a continued increase in colony formation when 12-week exposed cells were cultured for an additional 12 or 24 weeks without MMA(III) exposure. UROtsa cells as early as 12 weeks MMA(III) exposure were tumorigenic in severe combined immunodeficiency mice with tumorigenicity increasing when 12-week exposed cells were cultured for an additional 12 or 24 weeks in the absence of MMA(III) exposure. To assess potential underlying mechanisms associated with the early changes that occur during MMA(III)-induced malignancy, DNA methylation was assessed in known target gene promoter regions. Although DNA methylation remains relatively unchanged after 12 weeks of exposure, aberrant DNA methylation begins to emerge after an additional 12 weeks in culture and continues to increase through 24 weeks in culture without MMA(III) exposure, coincident with the progression of a tumorigenic phenotype. Overall, these data demonstrate that 50 nM MMA(III) is capable of causing irreversible malignant transformation in UROtsa cells after 12 weeks of exposure. Having resolved an earlier timeline in which MMA(III)-induced malignant transformation occurs in UROtsa cells will allow for mechanistic studies focused on the critical biological changes taking place within these cells prior to 12 weeks of exposure, providing further evidence about potential mechanisms of MMA(III)-induced carcinogenesis.

Persistence of DNA damage following exposure of human bladder cells to chronic monomethylarsonous acid

Malignant transformation was demonstrated in UROtsa cells following 52-weeks of exposure to 50 nM monomethylarsonous acid (MMA(III)); the result was the malignantly transformed cell line, URO-MSC. URO-MSC cells were used to study the induction of DNA damage and the alteration of DNA repair enzymes in both the presence of MMA(III) [URO-MSC(+)] and after subsequent removal of MMA(III) [URO-MSC(-)] following chronic, low-level exposure. In the presence of MMA(III), URO-MSC(+) cells demonstrated a sustained increase in DNA damage following 12-weeks of exposure; in particular, a significant increase in DNA single-strand breaks at 12-weeks of exposure consistently elevated through 52 weeks. The persistence of DNA damage in URO-MSC cells was assessed after a 2-week removal of MMA(III). URO-MSC(-) cells demonstrated a decrease in DNA damage compared to URO-MSC(+); however, DNA damage in URO-MSC(-) remained significantly elevated when compared to untreated UROtsa and increased in a time-dependent manner. Reactive oxygen species (ROS) were demonstrated to be a critical component in the generation of DNA damage determined through the incubation of ROS scavengers with URO-MSC cells. Poly (ADP-ribose) polymerase (PARP) is a key repair enzyme in DNA single-strand break repair. URO-MSC(+) resulted in a slight increase in PARP activity after 36-weeks of MMA(III) exposure, suggesting the presence of MMA(III) is inhibiting the increase in PARP activity. In support, PARP activity in URO-MSC(-) increased significantly, coinciding with a subsequent decrease in DNA damage demonstrated in URO-MSC(-) compared to URO-MSC(+). These data demonstrate that chronic, low-level exposure of UROtsa cells to 50 nM MMA(III) results in: the induction of DNA damage that remains elevated upon removal of MMA(III); increased levels of ROS that play a role in MMA(III) induced-DNA damage; and decreased PARP activity in the presence of MMA(III).

Reactive oxygen species regulate properties of transformation in UROtsa cells exposed to monomethylarsonous acid by modulating MAPK signaling

UROtsa cells exposed to 50 nM monomethylarsonous acid [MMA(III)] for 52 wk (MSC52) achieved hyperproliferation, anchorage independent growth, and enhanced tumorgenicity. MMA(III) has been shown to induce reactive oxygen species (ROS), which can lead to activation of signaling cascades causing stress-related proliferation of cells and even cellular transformation. Previous research established the acute activation of MAPK signaling cascade by ROS produced by MMA(III) as well as chronic up regulation of COX-2 and EGFR in MSC52 cells. To determine if ROS played a role in the chronic pathway perturbations by acting as secondary messengers, activation of Ras was determined in UROtsa cells [exposed to MMA(III) for 0-52 wk] and found to be increased through 52 wk most dramatically after 20 wk of exposure. Ras has been shown to cause an increase in O2(-) and be activated by increases in O2(-), making ROS important to study in the transformation process. COX-2 upregulation in MSC52 cells was confirmed by real time RT-PCR. By utilizing both antioxidants or specific COX inhibitors, it was shown that COX-2 upregulation was dependent on ROS, specifically, O2(-). In addition, because previous research established the importance of MAPK activation in phenotypic changes associated with transformation in MSC52 cells, it was hypothesized that ROS play a role in maintaining phenotypic characteristics of the malignant transformation of MSC52 cells. Several studies have demonstrated that cancer cells have lowered superoxide dismutase (MnSOD) activity and protein levels. Increasing levels of MnSOD have been shown to suppress the malignant phenotype of cells. SOD was added to MSC52 cells resulting in slower proliferation rates (doubling time=42h vs. 31h). ROS scavengers of OH also slowed proliferation rates of MSC52 cells. To further substantiate the importance of ROS in these properties of transformation in MSC52 cells, anchorage independent growth was assessed after the addition of antioxidants, both enzymatic and non-enzymatic. Scavengers of OH, and O2(-) blocked the colony formation of MSC52 cells. These data support the role for the involvement of ROS in properties of transformation of UROtsa cells exposed to MMA(III).