Elevation of cellular BPDE uptake by human cells: a possible factor contributing to co-carcinogenicity by arsenite

The dual effects of Benzo(a)pyrene/Benzo(a)pyrene-7,8-dihydrodiol-9,10-epoxide on DNA Methylation

Benzo(a)pyrene (BaP) is one of the most thoroughly studied polycyclic aromatic hydrocarbons(PAHs) and a widespread organic pollutant in various areas of human life. Its teratogenic, immunotoxic and carcinogenic effects on organisms are well documented and widely recognized by researchers. In the body, BaP is enzymatically converted to form a more active benzo(a)pyrene-7,8-dihydrodiol-9,10-epoxide (BPDE). BaP/BPDE has the potential to trigger gene mutations, influence epigenetic modifications and cause damage to cellular structures, ultimately contributing to disease onset and progression. However, there are different points of view when studying epigenetics using BaP/BPDE. On the one hand, it is claimed in cancer research that BaP/BPDE contributes to gene hypermethylation and, in particular, induces the hypermethylation of tumor's suppressor gene promoters, leading to gene silencing and subsequent cancer development. Conversely, studies in human and animal populations suggest that exposure to BaP results in genome-wide DNA hypomethylation, potentially leading to adverse outcomes in inflammatory diseases. This apparent contradiction has not been summarized in research for almost four decades. This article presents a comprehensive review of the current literature on the influence of BaP/BPDE on DNA methylation regulation. It demonstrates that BaP/BPDE exerts a dual-phase regulatory effect on methylation, which is influenced by factors such as the concentration and duration of BaP/BPDE exposure, experimental models and detection methods used in various studies. Acute/high concentration exposure to BaP/BPDE often results in global demethylation of DNA, which is associated with inhibition of DNA methyltransferase 1 (DNMT1) after exposure. At certain specific gene loci (e.g., RAR-β), BPDE can form DNA adducts, recruiting DNMT3 and leading to hypermethylation at specific sites. By integrating these different mechanisms, our goal is to unravel the patterns and regulations of BaP/BPDE-induced DNA methylation changes and provide insights into future precision therapies targeting epigenetics.

Inorganic arsenic inhibits the nucleotide excision repair pathway and reduces the expression of XPC

Chronic exposure to arsenic, most often through contaminated drinking water, has been linked to several types of cancer in humans, including skin and lung cancer. However, the mechanisms underlying its role in causing cancer are not well understood. There is evidence that exposure to arsenic can enhance the carcinogenicity of UV light in inducing skin cancers and may enhance the carcinogenicity of tobacco smoke in inducing lung cancers. The nucleotide excision repair (NER) pathway removes different types of DNA damage including those produced by UV light and components of tobacco smoke. The aim of the present study was to investigate the effect of sodium arsenite on the NER pathway in human lung fibroblasts (IMR-90 cells) and primary mouse keratinocytes. To measure NER, we employed a slot-blot assay to quantify the introduction and removal of UV light-induced 6-4 photoproducts (6-4 PP) and cyclobutane pyrimidine dimers (CPDs). We find a concentration-dependent inhibition of the removal of 6-4 PPs and CPDs in both cell types treated with arsenite. Treatment of both cell types with arsenite resulted in a significant reduction in the abundance of XPC, a protein that is critical for DNA damage recognition in NER. The abundance of RNA expressed from several key NER genes was also significantly reduced by treatment of IMR-90 cells with arsenite. Finally, treatment of IMR-90 cells with MG-132 abrogated the reduction in XPC protein, suggesting an involvement of the proteasome in the reduction of XPC protein produced by treatment of cells with arsenic. The inhibition of NER by arsenic may reflect one mechanism underlying the role of arsenic exposure in enhancing cigarette smoke-induced lung carcinogenesis and UV light-induced skin cancer, and it may provide some insights into the emergence of arsenic trioxide as a chemotherapeutic agent.

Simultaneous detection of ultraviolet B-induced DNA damage using capillary electrophoresis with laser-induced fluorescence

An immunoassay based on CE-LIF was developed for the simultaneous detection of cyclobutane pyrimidine dimers (CPDs) and pyrimidine 6-4 pyrimidone photoproducts (6-4PPs) in genomic DNA irradiated with UVB or natural sunlight. Human cells were first exposed to varying amounts of UVB or natural sunlight to induce DNA damage. Genomic DNA was extracted and incubated with anti-CPD and anti-6-4PP primary antibodies attached to secondary antibodies with a fluorescent quantum dot (QD) reporter that emitted either red or yellow fluorescence. CE was used to separate the unbound antibodies from those bound to the photoproducts, and LIF with appropriate optical filters was used to separate the fluorescence signals from each QD to individual photomultiplier tubes for simultaneous photoproduct detection. Using this strategy, photoproducts were detected from ∼6 ng (200 ng μL(-1)) of DNA under a low UVB fluence of 65 J m(-2) for CPDs or 195 J m(-2) for 6-4PPs. This assay was also the first to demonstrate the detection of CPDs in human cells after only 15 min of irradiation under natural sunlight.

Arsenic trioxide co-exposure potentiates benzo(a)pyrene genotoxicity by enhancing the oxidative stress in human lung adenocarcinoma cell

Although both arsenic trioxide (As2O3) and benzo(a)pyrene (BaP) are well-established human carcinogens, the interaction between As2O3 and BaP is synergistic or antagonistic remains controversial in terms of the existing studies. In addition, the mechanisms responsible for the combined effects are still unclear. In this study, we examined the potential interactive effects between As2O3 (1, 5, and 10 μM) and BaP (5, 10, and 20 μM) in cultured A549 cells by treating with BaP and As2O3 alone or in combination at various concentrations for 24 h. The single and combined effects of As2O3 and BaP on the cytotoxicity, DNA/chromosomal damage, and oxidative stress were examined by using tetrazolium (3-(4,5-dimethyithiazol-2-yl)-2,5-diphenyl-tetrazolium bromide) dye colorimetric assay, colony formation assay, fluorescence probe, chemical colorimetry, comet assay as well as micronucleus test. Our results showed that As2O3 synergistically enhanced the cytotoxicity, genotoxicity, and level of oxidative stress induced by BaP at various tested concentrations. Also, our experimental results showed that intracellular glutathione (GSH) contents were increased by various doses of BaP, but single or cotreatment with As2O3 significantly decreased the GSH level in the cells at all tested concentrations. Taken together, our results suggest that As2O3 may exert its synergistic cyto- and genotoxic effects with BaP mainly via elevated intracellular reactive oxygen species and reduced GSH contents and superoxide dismutase activities, thus promoting high level of oxidative stress, which may be a pivotal mechanism underlying As2O3 cocarcinogenic action.

Arsenite and its mono- and dimethylated trivalent metabolites enhance the formation of benzo[a]pyrene diol epoxide-DNA adducts in Xeroderma pigmentosum complementation group A cells

Recently, inorganic arsenite (iAs(III)) and its mono- and dimethylated metabolites have been examined for their interference with the formation and repair of benzo[a]pyrene diol epoxide (BPDE)-induced DNA adducts in human cells (Schwerdtle, ., Walter, I., and Hartwig, A. (2003) DNA Repair 2, 1449 - 1463). iAs(III) and monomethylarsonous acid (MMA(III)) were found to be able to enhance the formation of BPDE-DNA adducts, whereas dimethylarsinous acid (DMA(III)) had no enhancing effect at all. The anomaly manifested by DMA(III) prompted us to further investigate the effects of the three trivalent arsenic species on the formation of BPDE-DNA adducts. Use of a nucleotide excision repair (NER)-deficient Xeroderma pigmentosum complementation group A cell line (GM04312C) allowed us to dissect DNA damage induction from DNA repair and to examine the effects of arsenic on the formation of BPDE-DNA adducts only. At concentrations comparable to those used in the study by Schwerdtle et al., we found that each of the three trivalent arsenic species was able to enhance the formation of BPDE-DNA adducts with the potency in a descending order of MMA(III) > DMA(III) > iAs(III), which correlates well with their cytotoxicities. Similar to iAs(III), DMA(III) modulation of reduced glutathione (GSH) or total glutathione S-transferase (GST) activity could not account for its enhancing effect on DNA adduct formation. Additionally, the enhancing effects elicited by the trivalent arsenic species were demonstrated to be highly time-dependent. Thus, although our study made use of short-term assays with relatively high doses, our data may have meaningful implications for carcinogenesis induced by chronic exposure to arsenic at low doses encountered environmentally.

Enzymatic digestion and chromatographic analysis of arsenic species released from proteins

A method combining gel filtration chromatography (GFC), protease digestion, and ion pair chromatography with inductively coupled plasma mass spectrometry detection was developed for the determination of arsenic species bound to proteins. The method was first established by examining the interactions of two model proteins, metallothionein (MT) and hemoglobin, with three reactive trivalent arsenic species. It was then successfully applied to the speciation of arsenic in red blood cells of rats. Inorganic arsenite (iAs(III)), monomethylarsonous acid (MMA(III)), and dimethylarsinous acid (DMA(III)) were efficiently released from the proteins by protease digestion at pH 8.0, with the recovery ranging from 93% to 106%. There was no oxidation of iAs(III) or MMA(III) during the protease digestion process. Up to 61% DMA(III) (the least stable arsenic species) was unchanged, and the rest was oxidized to the pentavalent dimethylarsinic acid (DMA(V)). The arsenic species in the red blood cells of control rats was present as DMA(III) complex with hemoglobin. The method enabling the determination of the specific arsenic species that bind to cellular proteins is potentially useful for studying arsenic distribution, metabolism, and toxicity.

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.