Arsenite inhibits p53 phosphorylation, DNA binding activity, and p53 target gene p21 expression in mouse epidermal JB6 cells

N6-methyladenosine mediates arsenite-induced human keratinocyte transformation by suppressing p53 activation

N⁶-methyladenosine (m⁶A), the most abundant and reversible RNA modification, plays critical a role in tumorigenesis. However, whether m⁶A can regulate p53, a leading antitumor protein remains poorly understood. In this study, we explored the regulatory role of m⁶A on p53 activation using an arsenite-transformed keratinocyte model, the HaCaT-T cell line. We created the cell line by exposing human keratinocyte HaCaT cells to 1 μM arsenite for 5 months. We found that the cells exhibited an increased m⁶A level along with an aberrant expression of the methyltransferases, demethylase, and readers of m⁶A. Moreover, the cells exhibited decreased p53 activity and reduced p53 phosphorylation, acetylation, and transactivation with a high nucleus export rate of p53. Knockdown of the m⁶A methyltransferase, METTL3 significantly decreased m⁶A level, restoring p53 activation and inhibiting cellular transformation phenotypes in the arsenite-transformed cells. Further, using both a bioinformatics analysis and experimental approaches, we demonstrated that m⁶A downregulated the expression of the positive p53 regulator, PRDM2, through the YTHDF2-promoted decay of PRDM2 mRNAs. We showed that m⁶A upregulated the expression of the negative p53 regulator, YY1 and MDM2 through YTHDF1-stimulated translation of YY1 and MDM2 mRNA. Taken together, our study revealed the novel role of m⁶A in mediating arsenite-induced human keratinocyte transformation by suppressing p53 activation. This study further sheds light on the mechanisms of arsenic carcinogenesis via RNA epigenetics.

Molecular Mechanisms of Arsenic-Induced Disruption of DNA Repair

Exposure to arsenic in contaminated drinking water is an emerging public health problem that impacts more than 200 million people worldwide. Accumulating lines of evidence from epidemiological studies revealed that chronic exposure to arsenic can result in various human diseases including cancer, type 2 diabetes, and neurodegenerative disorders. Arsenic is also classified as a Group I human carcinogen. In this review, we survey extensively different modes of action for arsenic-induced carcinogenesis, with focus being placed on arsenic-mediated impairment of DNA repair pathways. Inorganic arsenic can be bioactivated by methylation, and the ensuing products are highly genotoxic. Bioactivation of arsenicals also elicits the production of reactive oxygen and nitrogen species (ROS and RNS), which can directly damage DNA and modify cysteine residues in proteins. Results from recent studies suggest zinc finger proteins as crucial molecular targets for direct binding to As3+ or for modifications by arsenic-induced ROS/RNS, which may constitute a common mechanism underlying arsenic-induced perturbations of DNA repair.

The interaction of arsenic and N-butyl-N-(4-hydroxybutyl)nitrosamine on urothelial carcinogenesis in mice

The bladder is an important organ for the storage of excreted water and metabolites. If metabolites with carcinogenic characteristics are present in urine, the urothelial lining of the bladder could be damaged and genetically altered. In this study, we analyzed the interaction of arsenic and N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) on mouse bladder carcinogenesis. Our previous study found that arsenic affects BBN-altered urothelial enzymatic activity, protein expression, DNA oxidation and global DNA CpG methylation levels. In this study, two mouse models were used. First, after administering a co-treatment of BBN and arsenic for 20 weeks, BBN alone led to a urothelial carcinoma formation of 20%, and arsenic promoted a BBN-induced urothelial carcinoma formation of 10%. The protein expression of GSTM1, GSTO1, NQO1, and p21 did not change by arsenic along with the BBN co-treatment, but the Sp1 expression increased. In the second mouse model, BBN was a pretreatment promoter; arsenic dose-dependently deteriorated BBN-promoted dysplasia by 10% and 40% at 10 ppm and 100 ppm, respectively. Conversely, BBN pretreatment also accelerated arsenic-induced dysplasia by 30%. The urothelial carcinogenic effect reversed after ceasing BBN for a period of 20 weeks. In summary, three conclusions were drawn from this study. The first is the mutual promotion of arsenic and BBN in bladder carcinogenesis. Second, arsenic dosages without bladder carcinogenicity (10 ppm) or with slight carcinogenicity (100 ppm) promote BBN-induced mice bladder cancer progression. Finally, the dysplastic urothelium had reverted to near-normal morphology after ceasing BBN intake for 20 weeks, providing a good suggestion for people who want to quit smoking.

The New Role for an Old Kinase: Protein Kinase CK2 Regulates Metal Ion Transport

The pleiotropic serine/threonine protein kinase CK2 was the first kinase discovered. It is renowned for its role in cell proliferation and anti-apoptosis. The complexity of this kinase is well reflected by the findings of past decades in terms of its heterotetrameric structure, subcellular location, constitutive activity and the extensive catalogue of substrates. With the advent of non-biased high-throughput functional genomics such as genome-wide deletion mutant screening, novel aspects of CK2 functionality have been revealed. Our recent discoveries using the model organism Saccharomyces cerevisiae and mammalian cells demonstrate that CK2 regulates metal toxicity. Extensive literature search reveals that there are few but elegant works on the role of CK2 in regulating the sodium and zinc channels. As both CK2 and metal ions are key players in cell biology and oncogenesis, understanding the details of CK2’s regulation of metal ion homeostasis has a direct bearing on cancer research. In this review, we aim to garner the recent data and gain insights into the role of CK2 in metal ion transport.

Arsenic induces polyadenylation of canonical histone mRNA by down-regulating stem-loop-binding protein gene expression

The replication-dependent histone genes are the only metazoan genes whose messenger RNA (mRNA) does not terminate with a poly(A) tail at the 3'-end. Instead, the histone mRNAs display a stem-loop structure at their 3'-end. Stem-loop-binding protein (SLBP) binds the stem-loop and regulates canonical histone mRNA metabolism. Here we report that exposure to arsenic, a carcinogenic metal, decreased cellular levels of SLBP by inducing its proteasomal degradation and inhibiting SLBP transcription via epigenetic mechanisms. Notably, arsenic exposure dramatically increased polyadenylation of canonical histone H3.1 mRNA possibly through down-regulation of SLBP expression. The polyadenylated H3.1 mRNA induced by arsenic was not susceptible to normal degradation that occurs at the end of S phase, resulting in continued presence into mitosis, increased total H3.1 mRNA, and increased H3 protein levels. Excess expression of canonical histones have been shown to increase sensitivity to DNA damage as well as increase the frequency of missing chromosomes and induce genomic instability. Thus, polyadenylation of canonical histone mRNA following arsenic exposure may contribute to arsenic-induced carcinogenesis.

Molecular features in arsenic-induced lung tumors

Arsenic is a well-known human carcinogen, which potentially affects ~160 million people worldwide via exposure to unsafe levels in drinking water. Lungs are one of the main target organs for arsenic-related carcinogenesis. These tumors exhibit particular features, such as squamous cell-type specificity and high incidence among never smokers. Arsenic-induced malignant transformation is mainly related to the biotransformation process intended for the metabolic clearing of the carcinogen, which results in specific genetic and epigenetic alterations that ultimately affect key pathways in lung carcinogenesis. Based on this, lung tumors induced by arsenic exposure could be considered an additional subtype of lung cancer, especially in the case of never-smokers, where arsenic is a known etiological agent. In this article, we review the current knowledge on the various mechanisms of arsenic carcinogenicity and the specific roles of this metalloid in signaling pathways leading to lung cancer.

Altered expression profiles of microRNAs upon arsenic exposure of human umbilical vein endothelial cells

Recent studies in our laboratory indicated that arsenite at 20μM significantly induces the apoptosis of HUVECs. In this study we analyzed miRNAs expression profiles upon arsenic exposure of these cells to explore the molecular mechanisms of arsenic-induced vascular toxicity. The expression of miRNAs was examined by Exiqon miRCURY™ LNA microRNA chips. We found that 85 miRNAs were up-regulated and 52 were down-regulated by arsenic treatment as compared to the control group. The expression of altered miRNAs was validated by quantitative reverse-transcription PCR (qRT-PCR). A number of DNA motifs were identified in the promoters of the perturbed miRNAs by promoter analysis using MEME software. Analysis of cellular functions by using DAVID Bioinformatics Resources revealed that phosphoproteins and genes involved in alternative splicing are among the top categories targeted by both up- and down-regulated miRNAs. In conclusion, the results show that the alteration of miRNAs expression might play crucial roles in arsenic-induced vascular injury.

Sodium arsenite ± hyperthermia sensitizes p53-expressing human ovarian cancer cells to cisplatin by modulating platinum-DNA damage responses

Epithelial ovarian cancer (EOC) is the leading cause of gynecological cancer death in the United States. Cisplatin is a DNA damaging agent initially effective against EOC but limited by resistance. P53 plays a critical role in cellular response to DNA damage and has been implicated in EOC response to platinum chemotherapy. In this study, we examined the role of p53 status in EOC response to a novel combination of cisplatin, sodium arsenite, and hyperthermia. Human EOC cells were treated with cisplatin ± 20μM sodium arsenite at 37°C or 39°C for 1 h. Sodium arsenite ± hyperthermia sensitized wild-type p53-expressing (A2780, A2780/CP70, OVCA 420, OVCA 429, and OVCA 433) EOC cells to cisplatin. Hyperthermia sensitized p53-null SKOV-3 and p53-mutant (OVCA 432 and OVCAR-3) cells to cisplatin. P53 small interfering RNA (siRNA) transfection abrogated sodium arsenite sensitization effect. XPC, a critical DNA damage recognition protein in global genome repair pathway, was induced by cisplatin only in wild-type p53-expressing cells. Cotreatment with sodium arsenite ± hyperthermia attenuated cisplatin-induced XPC in wild-type p53-expressing cells. XPC siRNA transfection sensitized wild-type p53-expressing cells to cisplatin, suggesting that sodium arsenite ± hyperthermia attenuation of XPC is a mechanism by which wild-type p53-expressing cells are sensitized to cisplatin. Hyperthermia ± sodium arsenite enhanced cellular and DNA accumulation of platinum in wild-type p53-expressing cells. Only hyperthermia enhanced platinum accumulation in p53-null cells. In conclusion, sodium arsenite ± hyperthermia sensitizes wild-type p53-expressing EOC cells to cisplatin by suppressing DNA repair protein XPC and increasing cellular and DNA platinum accumulation.

Trivalent arsenic inhibits the functions of chaperonin complex

The exact molecular mechanisms by which the environmental pollutant arsenic works in biological systems are not completely understood. Using an unbiased chemogenomics approach in Saccharomyces cerevisiae, we found that mutants of the chaperonin complex TRiC and the functionally related prefoldin complex are all hypersensitive to arsenic compared to a wild-type strain. In contrast, mutants with impaired ribosome functions were highly arsenic resistant. These observations led us to hypothesize that arsenic might inhibit TRiC function, required for folding of actin, tubulin, and other proteins postsynthesis. Consistent with this hypothesis, we found that arsenic treatment distorted morphology of both actin and microtubule filaments. Moreover, arsenic impaired substrate folding by both bovine and archaeal TRiC complexes in vitro. These results together indicate that TRiC is a conserved target of arsenic inhibition in various biological systems.

Arsenite induced poly(ADP-ribosyl)ation of tumor suppressor P53 in human skin keratinocytes as a possible mechanism for carcinogenesis associated with arsenic exposure

Arsenite is an environmental pollutant. Exposure to inorganic arsenic in drinking water is associated with elevated cancer risk, especially in skin. Arsenite alone does not cause skin cancer in animals, but arsenite can enhance the carcinogenicity of solar UV. Arsenite is not a significant mutagen at non-toxic concentrations, but it enhances the mutagenicity of other carcinogens. The tumor suppressor protein P53 and nuclear enzyme PARP-1 are both key players in DNA damage response. This laboratory demonstrated earlier that in cells treated with arsenite, the P53-dependent increase in p21(WAF1/CIP1) expression, normally a block to cell cycle progression after DNA damage, is deficient. Here we show that although long-term exposure of human keratinocytes (HaCaT) to a nontoxic concentration (0.1 microM) of arsenite decreases the level of global protein poly(ADP-ribosyl)ation, it increases poly(ADP-ribosyl)ation of P53 protein and PARP-1 protein abundance. We also demonstrate that exposure to 0.1 microM arsenite depresses the constitutive expression of p21 mRNA and P21 protein in HaCaT cells. Poly(ADP-ribosyl)ation of P53 is reported to block its activation, DNA binding and its functioning as a transcription factor. Our results suggest that arsenite's interference with activation of P53 via poly(ADP-ribosyl)ation may play a role in the comutagenic and cocarcinogenic effects of arsenite.

Impact of arsenic on nucleotide excision repair: XPC function, protein level, and gene expression

The ubiquitous occurrence of the human carcinogen arsenic results in multiple exposure possibilities to humans. The human diet, especially drinking water, is the primary source of inorganic arsenic intake in the general population. The ingested arsenic is metabolized to methylated derivatives; some of these metabolites are today considered to be more toxic than the inorganic species. Various modes of action have been proposed to contribute to arsenic carcinogenicity; inhibition of nucleotide excision repair (NER), removing DNA helix distorting DNA adducts induced by environmental mutagens, is likely to be of primary importance. Here, we report that arsenite and its metabolite monomethylarsonous acid (MMA(III)) strongly decreased expression and protein level of Xeroderma pigmentosum complementation group C (XPC), which is believed to be the principle initiator of global genome NER. This led to diminished association of XPC to sites of local UVC damage, resulting in decreased recruitment of further NER proteins. Additionally Xeroderma pigmentosum complementation group E protein (XPE) expression was reduced, which encodes for another important NER protein and similarly to XPC is regulated by the activity of the transcription factor p53. In summary, our data demonstrate that in human skin fibroblasts arsenite and even more pronounced MMA(III) interact with XPC expression, resulting in decreased XPC protein level and diminished assembly of the NER machinery.

Attenuation of DNA damage-induced p53 expression by arsenic: a possible mechanism for arsenic co-carcinogenesis

Inhibition of DNA repair processes has been suggested as one predominant mechanism in arsenic co-genotoxicity. However, the underlying mode of action responsible for DNA repair inhibition by arsenic remains elusive. To further elucidate the mechanism of repair inhibition by arsenic, we examined the effect of trivalent inorganic and methylated arsenic metabolites on the repair of benzo(a)pyrene diol epoxide (BPDE)-DNA adducts in normal human primary fibroblasts and their effect on repair-related protein expression. We observed that monomethylarsonous acid (MMA(III)) was the most potent inhibitor of the DNA repair. MMA(III) did not change the expression levels of some key repair proteins involved upstream of the dual incision in the global nucleotide excision repair (NER) pathway, including p48, XPC, xeroderma pigmentosum complementation group A (XPA), and p62-TFIIH. However, it led to a marked impairment of p53 induction in response to BPDE treatment. The abrogated p53 expression translated into reduced p53 DNA-binding activity, suggesting a possibility of downregulating downstream repair genes by p53. A p53-null cell line failed to exhibit the inhibitory effect of MMA(III) on NER, implicating a role for p53 in the NER inhibition by MMA(III). Further investigation revealed that MMA(III) dramatically inhibited p53 phosphorylation at serine 15, implying that MMA(III) destabilized p53 by inhibiting its phosphorylation. Because p53 is required for proficient global NER, our data suggest that arsenic inhibits NER through suppressing p53 induction in response to DNA damage in cells with normal p53 gene expression.

Global transcriptome and deletome profiles of yeast exposed to transition metals

A variety of pathologies are associated with exposure to supraphysiological concentrations of essential metals and to non-essential metals and metalloids. The molecular mechanisms linking metal exposure to human pathologies have not been clearly defined. To address these gaps in our understanding of the molecular biology of transition metals, the genomic effects of exposure to Group IB (copper, silver), IIB (zinc, cadmium, mercury), VIA (chromium), and VB (arsenic) elements on the yeast Saccharomyces cerevisiae were examined. Two comprehensive sets of metal-responsive genomic profiles were generated following exposure to equi-toxic concentrations of metal: one that provides information on the transcriptional changes associated with metal exposure (transcriptome), and a second that provides information on the relationship between the expression of ∼4,700 non-essential genes and sensitivity to metal exposure (deletome). Approximately 22% of the genome was affected by exposure to at least one metal. Principal component and cluster analyses suggest that the chemical properties of the metal are major determinants in defining the expression profile. Furthermore, cells may have developed common or convergent regulatory mechanisms to accommodate metal exposure. The transcriptome and deletome had 22 genes in common, however, comparison between Gene Ontology biological processes for the two gene sets revealed that metal stress adaptation and detoxification categories were commonly enriched. Analysis of the transcriptome and deletome identified several evolutionarily conserved, signal transduction pathways that may be involved in regulating the responses to metal exposure. In this study, we identified genes and cognate signaling pathways that respond to exposure to essential and non-essential metals. In addition, genes that are essential for survival in the presence of these metals were identified. This information will contribute to our understanding of the molecular mechanism by which organisms respond to metal stress, and could lead to an understanding of the connection between environmental stress and signal transduction pathways.

Environmental and human health threats are posed by contamination from transition metals. A variety of pathologies are associated with exposure to supraphysiological concentrations of essential metals and to non-essential metals and metalloids. To defend against metal toxicity, sophisticated defense mechanisms have evolved. Although many of the genes and regulatory pathways have been identified, the consequence of metal exposure on a systematic level has not been examined. To better define the mechanism involved in the metal response, we examined the effects of zinc, cadmium, mercury, copper, silver, chromium, and arsenic on gene expression in the yeast Saccharomyces cerevisiae. In addition, the roles of ∼4,500 non-essential genes in protecting yeast from metal toxicity were determined. Data analyses suggest that the chemical properties of the metal are major determinants in defining its biological effect on cells. Furthermore, cells may have developed common or convergent regulatory mechanisms to accommodate metal exposure. Several evolutionarily conserved regulatory pathways were identified that link metal exposure, disruption of normal metabolism and gene expression. These results provide a global understanding of the biological responses to metal exposure and the stress response.