Arsenite Binds to the RING Finger Domain of FANCL E3 Ubiquitin Ligase and Inhibits DNA Interstrand Crosslink Repair

Chemoproteomic Approach for Identifying Nuclear Arsenite-Binding Proteins

Trivalent arsenic, i.e., As(III), is the main form of arsenic species in the environment. Prolonged exposure to arsenicals through ingesting contaminated food and water has been implicated in the development of cancer and diabetes as well as cardiovascular and neurodegenerative diseases. A number of studies have been conducted to examine the mechanisms underlying the toxic effects of arsenite exposure, where As(III) was shown to displace Zn(II) and impair the functions of zinc-binding proteins. Considering that many zinc-binding proteins can bind to nucleic acids, we reason that systematic identification of arsenite-binding proteins in the nucleus may provide additional insights into the molecular targets of arsenite, thereby improving our understanding of the mechanisms of arsenic toxicity. Here, we conducted a quantitative proteomics experiment relying on affinity pull-down from nuclear protein lysate with a biotin-As(III) probe to identify nuclear arsenite-binding proteins. We uncovered a number of candidate As(III)-binding proteins that are involved in mRNA splicing, DNA repair, and replication. We also found that As(III) could bind to splicing factor 1 (SF1) and that this binding perturbs mRNA splicing in human cells. Together, our work provided insights into the mechanisms of As(III) toxicity by revealing new nuclear protein targets of As(III).

Comparison of Radio- and Phototoxicity in Association with an Enhancing Effect of the Photosensitizers Psoralen, Trioxsalen and Ortho-Iodo-Hoechst33258 on FaDu, PC-3, 4T1 and B16-F10 Cells

Background: Energy delivered at different wavelengths causes different types of damage to DNA. Methods: PC-3, FaDu, 4T1 and B16-F10 cells were irradiated with different wavelengths of ultraviolet light (UVA/UVC) and ionizing radiation (X-ray). Furthermore, different photosensitizers (ortho-iodo-Hoechst33258/psoralen/trioxsalen) were tested for their amplifying effect. Survival fraction and damage analysis using the γH2A.X assay (double-strand breaks) and the ELISA assay (cyclobutane pyrimidine dimers) were compared. Results: The PC-3 cells were found to be the most sensitive cells to the treatment strategies used. FaDu and PC-3 showed a strong sensitivity to UVA. Analysis of the damage showed that the cell lines exhibited different sensitivities. Conclusions: Thus, an enhancing effect of photosensitizers (PS) in combination with UVA could be demonstrated in some cases. However, this is cell- and dose-dependent.

Update of the risk assessment of inorganic arsenic in food

The European Commission asked EFSA to update its 2009 risk assessment on arsenic in food carrying out a hazard assessment of inorganic arsenic (iAs) and using the revised exposure assessment issued by EFSA in 2021. Epidemiological studies show that the chronic intake of iAs via diet and/or drinking water is associated with increased risk of several adverse outcomes including cancers of the skin, bladder and lung. The CONTAM Panel used the benchmark dose lower confidence limit based on a benchmark response (BMR) of 5% (relative increase of the background incidence after adjustment for confounders, BMDL05) of 0.06 μg iAs/kg bw per day obtained from a study on skin cancer as a Reference Point (RP). Inorganic As is a genotoxic carcinogen with additional epigenetic effects and the CONTAM Panel applied a margin of exposure (MOE) approach for the risk characterisation. In adults, the MOEs are low (range between 2 and 0.4 for mean consumers and between 0.9 and 0.2 at the 95th percentile exposure, respectively) and as such raise a health concern despite the uncertainties.

Recent progress in analytical strategies of arsenic-binding proteomes in living systems

Arsenic (As) is one of the most concerning elements due to its high exposure risks to organisms and ecosystems. The interaction between arsenicals and proteins plays a pivotal role in inducing their biological effects on living systems, e.g., arsenicosis. In this review article, the recent advances in analytical techniques and methods of As-binding proteomes were well summarized and discussed, including chromatographic separation and purification, biotin-streptavidin pull-down probes, in situ imaging using novel fluorescent probes, and protein identification. These analytical technologies could provide a growing body of knowledge regarding the composition, level, and distribution of As-binding proteomes in both cells and biological samples, even at the organellar level. The perspectives on analysis of As-binding proteomes are also proposed, e.g., isolation and identification of minor proteins, in vivo targeted protein degradation (TPD) technologies, and spatial As-binding proteomics. The application and development of sensitive, accurate, and high-throughput methodologies of As-binding proteomics would enable us to address the key molecular mechanisms underlying the adverse health effects of arsenicals.

Mechanisms of genotoxicity and proteotoxicity induced by the metalloids arsenic and antimony

Arsenic and antimony are metalloids with profound effects on biological systems and human health. Both elements are toxic to cells and organisms, and exposure is associated with several pathological conditions including cancer and neurodegenerative disorders. At the same time, arsenic- and antimony-containing compounds are used in the treatment of multiple diseases. Although these metalloids can both cause and cure disease, their modes of molecular action are incompletely understood. The past decades have seen major advances in our understanding of arsenic and antimony toxicity, emphasizing genotoxicity and proteotoxicity as key contributors to pathogenesis. In this review, we highlight mechanisms by which arsenic and antimony cause toxicity, focusing on their genotoxic and proteotoxic effects. The mechanisms used by cells to maintain proteostasis during metalloid exposure are also described. Furthermore, we address how metalloid-induced proteotoxicity may promote neurodegenerative disease and how genotoxicity and proteotoxicity may be interrelated and together contribute to proteinopathies. A deeper understanding of cellular toxicity and response mechanisms and their links to pathogenesis may promote the development of strategies for both disease prevention and treatment.

Chemoproteomic Approach for the Quantitative Identification of Arsenic-Binding Proteins

Arsenic is a widespread environmental contaminant, and long-term exposure to arsenic in drinking water is known to be associated with the development of many human diseases. Identification of arsenic-binding proteins is important for understanding the mechanisms underlying the toxic effects of arsenic species. Here, we developed a chemoproteomic strategy, relying on the use of a biotin-As(III) probe, stable isotope labeling by amino acids in cell culture, and liquid chromatography-tandem mass spectrometry analysis, to identify quantitatively As(III)-binding proteins. Over 400 proteins were enriched from the lysate of HEK293T cells with streptavidin beads immobilized with the biotin-As(III) probe. Competitive labeling experiments in the presence or absence of p-aminophenylarsenoxide (PAPAO) revealed 51 candidate As(III)-binding proteins, including several molecular chaperones and cochaperones, that is, HSPA4, HSPA4L, HSPH1, HOP1, FKBP51, and FKBP52. We also validated, by employing western blot analysis, the ability of HSPA4, a member of heat shock protein 70 (HSP70) family, in binding with PAPAO and sodium arsenite in vitro. Together, our work led to the identification of a number of new As(III)-interaction proteins, and our results suggest that As(III) may perturb proteostasis partly through binding directly with molecular chaperones.

Arsenic and cancer: Evidence and mechanisms

Arsenic is a potent carcinogen and poses a significant health concern worldwide. Exposure occurs through ingestion of drinking water and contaminated foods and through inhalation due to pollution. Epidemiological evidence shows arsenic induces cancers of the skin, lung, liver, and bladder among other tissues. While studies in animal and cell culture models support arsenic as a carcinogen, the mechanisms of arsenic carcinogenesis are not fully understood. Arsenic carcinogenesis is a complex process due its ability to be metabolized and because of the many cellular pathways it targets in the cell. Arsenic metabolism and the multiple forms of arsenic play distinct roles in its toxicity and contribute differently to carcinogenic endpoints, and thus must be considered. Arsenic generates reactive oxygen species increasing oxidative stress and damaging DNA and other macromolecules. Concurrently, arsenic inhibits DNA repair, modifies epigenetic regulation of gene expression, and targets protein function due its ability to replace zinc in select proteins. While these mechanisms contribute to arsenic carcinogenesis, there remain significant gaps in understanding the complex nature of arsenic cancers. In the future improving models available for arsenic cancer research and the use of arsenic induced human tumors will bridge some of these gaps in understanding arsenic driven cancers.

Quantitative Assessment of Arsenite-Induced Perturbation of Ubiquitinated Proteome

Arsenic contamination in food and groundwater constitutes a public health concern for more than 200 million people worldwide. Individuals chronically exposed to arsenic through drinking and ingestion exhibit a higher risk of developing cancers and cardiovascular diseases. Nevertheless, the underlying mechanisms of arsenic toxicity are not fully understood. Arsenite is known to bind to and deactivate RING finger E3 ubiquitin ligases; thus, we reason that a systematic interrogation about how arsenite exposure modulates global protein ubiquitination may reveal novel molecular targets for arsenic toxicity. By employing liquid chromatography-tandem mass spectrometry, in combination with stable isotope labeling by amino acids in cell culture (SILAC) and immunoprecipitation of di-glycine-conjugated lysine-containing tryptic peptides, we assessed the alterations in protein ubiquitination in GM00637 human skin fibroblast cells upon arsenite exposure at the entire proteome level. We observed that arsenite exposure led to altered ubiquitination of many proteins, where the alterations in a large majority of ubiquitination events are negatively correlated with changes in expression of the corresponding proteins, suggesting their modulation by the ubiquitin-proteasomal pathway. Moreover, we observed that arsenite exposure confers diminished ubiquitination of a rate-limiting enzyme in cholesterol biosynthesis, HMGCR, at Lys248. We also revealed that TRC8 is the major E3 ubiquitin ligase for HMGCR ubiquitination in HEK293T cells, and the arsenite-induced diminution of HMGCR ubiquitination is abrogated upon genetic depletion of TRC8. In summary, we systematically characterized arsenite-induced perturbations in a ubiquitinated proteome in human cells and found that the arsenite-elicited attenuation of HMGCR ubiquitination in HEK293T cells involves TRC8.

Natural Dietary Compounds in the Treatment of Arsenic Toxicity

Chronic exposure to arsenic (As) compounds leads to its accumulation in the body, with skin lesions and cancer being the most typical outcomes. Treating As-induced diseases continues to be challenging as there is no specific, safe, and efficacious therapeutic management. Therapeutic and preventive measures available to combat As toxicity refer to chelation therapy, antioxidant therapy, and the intake of natural dietary compounds. Although chelation therapy is the most commonly used method for detoxifying As, it has several side effects resulting in various toxicities such as hepatotoxicity, neurotoxicity, and other adverse consequences. Drugs of plant origin and natural dietary compounds show efficient and progressive relief from As-mediated toxicity without any particular side effects. These natural compounds have also been found to aid the elimination of As from the body and, therefore, can be more effective than conventional therapeutic agents in ameliorating As toxicity. This review provides an overview of the recently updated knowledge on treating As poisoning through natural dietary compounds. This updated information may serve as a basis for defining novel prophylactic and therapeutic formulations.

Arsenic-protein interactions as a mechanism of arsenic toxicity

Millions of people worldwide are exposed to arsenic, a metalloid listed as one of the top chemical pollutants of concern to human health. Epidemiological and experimental studies link arsenic exposure to the development of cancer and other diseases. Several mechanisms have been proposed to explain the effects induced by arsenic. Notably, arsenic and its metabolites interact with proteins by direct binding to individual cysteine residues, cysteine clusters, zinc finger motifs, and RING finger domains. Consequently, arsenic interactions with proteins disrupt the functions of proteins and may lead to the development and progression of diseases. In this review, we focus on current evidence in the literature that implicates the interaction of arsenic with proteins as a mechanism of arsenic toxicity. Data show that arsenic-protein interactions affect multiple cellular processes and alter epigenetic regulation, cause endocrine disruption, inhibit DNA damage repair mechanisms, and deregulate gene expression, among other adverse effects.

Arsenic co-carcinogenesis: Inhibition of DNA repair and interaction with zinc finger proteins

Arsenic is widely present in the environment and is associated with various population health risks including cancers. Arsenic exposure at environmentally relevant levels enhances the mutagenic effect of other carcinogens such as ultraviolet radiation. Investigation on the molecular mechanisms could inform the prevention and intervention strategies of arsenic carcinogenesis and co-carcinogenesis. Arsenic inhibition of DNA repair has been demonstrated to be an important mechanism, and certain DNA repair proteins have been identified to be extremely sensitive to arsenic exposure. This review will summarize the recent advances in understanding the mechanisms of arsenic carcinogenesis and co-carcinogenesis, including DNA damage induction and ROS generation, particularly how arsenic inhibits DNA repair through an integrated molecular mechanism which includes its interactions with sensitive zinc finger DNA repair proteins.

Ectopic expression of wheat aquaglyceroporin TaNIP2;1 alters arsenic accumulation and tolerance in Arabidopsis thaliana

Arsenic (As) is one of the most toxic contaminants to food crops, and as such, decreasing crops uptake and accumulation of As cannot be overemphasized. Here, we characterized a functional wheat NIP2;1 homolog of the As transporter, TaNIP2;1. TaNIP2;1 expression was suppressed by arsenite (As(III)) in wheat. Ectopic expression of TaNIP2;1 in the Δfps1 yeast mutant enhanced yeast sensitivity towards As(III). Conversely, the elevated expression of TaNIP2;1 in Δacr3 mutants decreased yeast sensitivity to arsenate (As(V)), demonstrating that TaNIP2;1 showed both influx and efflux transport activities for As(III) in yeasts. This is further supported by increased As concentration in the yeast cells that overproduce TaNIP2;1 in Δfps1, while As concentration decreased in Δacr3. Furthermore, ectopic expression of TaNIP2;1 in Arabidopsis confirmed that TaNIP2;1 can transport As into plants, as supported by increased sensitivity to and uptake of As(III). No change in plant sensitivity was found to Cu(II), Cd(II), Zn(II) or Ni(II), indicating that transport activity of TaNIP2;1 is specific for As(III). Taken together, our data show that TaNIP2;1 may be involved in As(III) transportation in plants. This finding reveals a functional gene that can be manipulated to reduce As content in wheat.

Arsenic Exposure and Compromised Protein Quality Control

Exposure to arsenic in contaminated drinking water is a worldwide public health problem that affects more than 200 million people. Protein quality control constitutes an evolutionarily conserved mechanism for promoting proper folding of proteins, refolding of misfolded proteins, and removal of aggregated proteins, thereby maintaining homeostasis of the proteome (i.e., proteostasis). Accumulating lines of evidence from epidemiological and laboratory studies revealed that chronic exposure to inorganic arsenic species can elicit proteinopathies that contribute to neurodegenerative disorders, cancer, and type II diabetes. Here, we review the effects of arsenic exposure on perturbing various elements of the proteostasis network, including mitochondrial homeostasis, molecular chaperones, inflammatory response, ubiquitin-proteasome system, autophagy, as well as asymmetric segregation and axonal transport of misfolded proteins. We also discuss arsenic-induced disruptions of post-translational modifications of proteins, for example, ubiquitination, and their implications in proteostasis. Together, studies in the past few decades support that disruption of protein quality control may constitute an important mechanism underlying the arsenic-induced toxicity.

Arsenite Binds to ZNF598 to Perturb Ribosome-Associated Protein Quality Control

Arsenic pollution in drinking water is a widespread public health problem, and it affects approximately 200 million people in over 70 countries. Many human diseases, including neurodegenerative disorders, are engendered by the malfunction of proteins involved in important biological processes and are elicited by protein misfolding and/or loss of protein quality control during translation. Arsenic exposure results in proteotoxic stress, though the detailed molecular mechanisms remain poorly understood. Here, we showed that arsenite interacts with ZNF598 protein in cells and exposure of human skin fibroblasts to arsenite results in significant decreases in the ubiquitination levels of lysine residues 138 and 139 in RPS10 and lysine 8 in RPS20, which are regulatory post-translational modifications important in ribosome-associated protein quality control. Furthermore, the arsenite-elicited diminutions in ubiquitinations of RPS10 and RPS20 gave rise to augmented read-through of poly(adenosine)-containing stalling sequences, which was abolished in ZNF598 knockout cells. Together, our study revealed a novel mechanism underlying the arsenic-induced proteostatic stress in human cells.

Arsenite Exposure Displaces Zinc from ZRANB2 Leading to Altered Splicing

Exposure to arsenic, a class I carcinogen, affects 200 million people globally. Skin is the major target organ, but the molecular etiology of arsenic-induced skin carcinogenesis remains unclear. Arsenite (As³⁺)-induced disruption of alternative splicing could be involved, but the mechanism is unknown. Zinc finger proteins play key roles in alternative splicing. As³⁺ can displace zinc (Zn²⁺) from C3H1 and C4 zinc finger motifs (zfm's), affecting protein function. ZRANB2, an alternative splicing regulator with two C4 zfm's integral to its structure and splicing function, was chosen as a candidate for this study. We hypothesized that As³⁺ could displace Zn²⁺ from ZRANB2, altering its structure, expression, and splicing function. As³⁺/Zn²⁺ binding and mutual displacement experiments were performed with synthetic apo-peptides corresponding to each ZRANB2 zfm, employing a combination of intrinsic fluorescence, ultraviolet spectrophotometry, zinc colorimetric assay, and liquid chromatography-tandem mass spectrometry. ZRANB2 expression in HaCaT cells acutely exposed to As³⁺ (0 or 5 μM, 0-72 h; or 0-5 μM, 6 h) was examined by RT-qPCR and immunoblotting. ZRANB2-dependent splicing of TRA2B mRNA, a known ZRANB2 target, was monitored by reverse transcription-polymerase chain reaction. As³⁺ bound to, as well as displaced Zn²⁺ from, each zfm. Also, Zn²⁺ displaced As³⁺ from As³⁺-bound zfm's acutely, albeit transiently. As³⁺ exposure induced ZRANB2 protein expression between 3 and 24 h and at all exposures tested but not ZRANB2 mRNA expression. ZRANB2-directed TRA2B splicing was impaired between 3 and 24 h post-exposure. Furthermore, ZRANB2 splicing function was also compromised at all As³⁺ exposures, starting at 100 nm. We conclude that As³⁺ exposure displaces Zn²⁺ from ZRANB2 zfm's, changing its structure and compromising splicing of its targets, and increases ZRANB2 protein expression as a homeostatic response both at environmental/toxicological exposures and therapeutically relevant doses.

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.

Identification of Histone H3 and H4 Amino Acid Residues Important for the Regulation of Arsenite Stress Signaling in Saccharomyces cerevisiae

Arsenic is an environmental carcinogen that causes many diseases in humans, including cancers and organ failures, affecting millions of people in the world. Arsenic trioxide is a drug used for the treatment of acute promyelocytic leukemia (APL). In the present study, we screened the synthetic histone H3 and H4 library in the presence of arsenite to understand the role of histone residues in arsenic toxicity. We identified residues of histone H3 and H4 crucial for arsenite stress response. The residues H3T3, H3G90, H4K5, H4G13, and H4R95 are required for the activation of Hog1 kinase in response to arsenite exposure. We showed that a reduced level of Hog1 activation increases the intracellular arsenic content in these histone mutants through the Fps1 channel. We have also noticed the reduced expression of ACR3 exporter in the mutants. The growth defect of mutants caused by arsenite exposure was suppressed in hyperosmotic conditions, in a higher concentration of glucose, and upon deletion of the FPS1 gene. The arsenite sensitive histone mutants also showed a lack of H3K4 methylation and reduced H4K16 acetylation. Altogether, we have identified the key residues in histone H3 and H4 proteins important for the regulation of Hog1 signaling, Fps1 activity, and ACR3 expression during arsenite stress.

A review of arsenic exposure and lung cancer

As a well-established human carcinogen, arsenic has increased the risk of lung cancer over the past decades. Wide exposure to arsenic in the environment has attracted the attention of scientists. Its carcinogenicity at early life stages has been observed in certain animal studies already, yet current evidence is insufficient to extrapolate this to humans. Although the mechanisms of lung cancer induced by arsenic remain unclear, most of them are related to the biotransformation of arsenic, which would further provide target sites for precaution and therapy. This review comprehensively summarizes current studies associated to arsenic exposure and lung cancer and the mechanism of its carcinogenesis in lung cancer in three sections, namely, epidemiological studies, experimental studies, and mechanistic studies. In addition, prevention and treatment strategies as well as directions for future studies are discussed.

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.

Fanconi Anemia Signaling and Cancer

The extremely high cancer incidence associated with patients suffering from a rare human genetic disease, Fanconi anemia (FA), demonstrates the importance of FA genes. Over the course of human tumor development, FA genes perform critical tumor-suppression roles. In doing so, FA provides researchers with a unique genetic model system to study cancer etiology. Here, we review how aberrant function of the 22 FA genes and their signaling network contributes to malignancy. From this perspective, we will also discuss how the knowledge discovered from FA research serves basic and translational cancer research.

Arsenite Binds to the Zinc Finger Motif of TIP60 Histone Acetyltransferase and Induces Its Degradation via the 26S Proteasome

Arsenic is a ubiquitous environmental contaminant with widespread public health concern. Epidemiological studies have revealed that chronic human exposure to arsenic in drinking water is associated with the prevalence of skin, lung, and bladder cancers. Aberrant histone modifications (e.g., methylation, acetylation, and ubiquitination) were previously found to be accompanied by arsenic exposure; thus, perturbation of epigenetic pathways is thought to contribute to arsenic carcinogenesis. Arsenite is known to interact with zinc finger motifs of proteins, and zinc finger motif is present in and indispensable for the enzymatic activities of crucial histone-modifying enzymes especially the MYST family of histone acetyltransferases (e.g., TIP60). Hence, we reasoned that trivalent arsenic may target the zinc finger motif of these enzymes, disturb their enzymatic activities, and alter histone acetylation. Herein, we found that As³⁺ could bind directly to the zinc-finger motif of TIP60 in vitro and in cells. In addition, exposure to As³⁺ could lead to a dose-dependent decrease in TIP60 protein level via the ubiquitin-proteasome pathway. Thus, the results from the present study revealed, for the first time, that arsenite may target cysteine residues in the zinc-finger motif of the TIP60 histone acetyltransferase, thereby altering the H4K16Ac histone epigenetic mark. Our results also shed some new light on the mechanisms underlying the arsenic-induced epigenotoxicity and carcinogenesis in humans.