An adverse outcome pathway for small intestinal tumors in mice involving chronic cytotoxicity and regenerative hyperplasia: a case study with hexavalent chromium, captan, and folpet

Urolithin A attenuates hexavalent chromium-induced small intestinal injury by modulating PP2A/Hippo/YAP1 pathway

Hexavalent chromium (Cr(VI)) exposure has been linked with gastrointestinal toxicity, whereas the molecular pathways and key targets remain elusive. Computational toxicology analysis predicted the correlation between protein phosphatase 2A (PP2A) and genes regarding Cr(VI)-induced intestinal injury. Here, we generated a mouse model with intestinal epithelium-specific knock-out of Ppp2r1a (encoding PP2A Aα subunit) to investigate the mechanisms underlying Cr(VI)-induced small intestinal toxicity. Heterozygous mice (HE) and matched wild-type (WT) littermates were administrated with Cr(VI) at 0, 5, 20, 80 mg/L for 28 successive days. Cr(VI) treatment led to crypt hyperplasia, epithelial cell apoptosis, and intestinal barrier dysfunction, accompanied by the decline of goblet cell counts and Occludin expression in WT mice. Notably, these effects were aggravated in HE mice, indicating that PP2A Aα deficiency conferred mice with susceptibility to Cr(VI)-induced intestinal injury. Integrated data analysis and biological experiments revealed Cr(VI) exposure could decrease YAP1 phosphorylation at Ser127 but increase protein expression and activity, together with elevated TAZ protein driving epithelial crypt cells proliferation following damage, suggesting the involvement of Hippo/YAP1 signaling pathway in Cr(VI)-induced intestinal toxicity. Nevertheless, the enhanced phosphorylation of YAP1 in HE mice resulted in proliferation/repair defects in intestinal epithelium, thereby exacerbating Cr(VI)-induced gut barrier dysfunction. Notably, by molecular docking and further studies, we identified Urolithin A, a microbial metabolite, attenuated Cr(VI)-induced disruption of intestinal barrier function, partly by modulating YAP1 expression and activity. Our findings reveal the novel molecular pathways participated in Cr(VI)-caused small intestinal injury and urolithin A could potentially protect against environmental hazards-induced intestinal diseases.

Cell proliferation and carcinogenesis: an approach to screening for potential human carcinogens

Cancer arises from multiple genetic errors occurring in a single stem cell (clonality). Every time DNA replicates, mistakes occur. Thus, agents can increase the risk of cancer either by directly damaging DNA (DNA-reactive carcinogens) or increasing the number of DNA replications (increased cell proliferation). Increased cell proliferation can be achieved either by direct mitogenesis or cytotoxicity with regenerative proliferation. Human carcinogens have a mode of action of DNA reactivity, immunomodulation (mostly immunosuppression), increased estrogenic activity (mitogenesis), or cytotoxicity and regeneration. By focusing on screening for these four effects utilizing in silico, in vitro, and short-term in vivo assays, a biologically based screening for human chemical carcinogens can be accomplished with greater predictivity than the traditional 2-year bioassay with considerably less cost, less time, and the use of fewer animals.

Revealing the mechanistic interactions of profenofos and captan pesticides with serum protein via biophysical and computational investigations

Profenofos (PF) and captan (CT) are among the most utilized organophosphorus insecticides and phthalimide fungicides, respectively. To elucidate the physicochemical and influential toxicokinetic factors, the mechanistic interactions of serum albumin and either PF or CT were carried out in the current study using a series of spectroscopy and computational analyses. Both PF and CT could bind to bovine serum albumin (BSA), a representative serum protein, with moderate binding constants in a range of 103-104 M-1. The bindings of PF and CT did not induce noticeable BSA's structural changes. Both pesticides bound preferentially to the site I pocket of BSA, where the hydrophobic interaction was the main binding mode of PF, and the electrostatic interaction drove the binding of CT. As a result, PF and CT may not only induce direct toxicity by themselves, but also compete with therapeutic drugs and essential substances to sit in the Sudlow site I of serum albumin, which may interfere with the pharmacokinetics and equilibrium of drugs and other substances causing consequent adverse effects.

Derivation of oral cancer slope factors for hexavalent chromium informed by pharmacokinetic models and in vivo genotoxicity data

Hexavalent chromium [Cr(VI)] is present in drinking water from natural and anthropogenic sources at approximately 1 ppb. Several regulatory bodies have recently developed threshold-based safety criteria for Cr(VI) of 30-100 ppb based on evidence that small intestine tumors in mice following exposure to ≥20,000 ppb are the result of a non-mutagenic mode of action (MOA). In contrast, U.S. EPA has recently concluded that Cr(VI) acts through a mutagenic MOA based, in part, on scoring numerous in vivo genotoxicity studies as having low confidence; and therefore derived a cancer slope factor (CSF) of 0.5 (mg/kg-day)-1, equivalent to ∼0.07 ppb. Herein, we demonstrate how physiologically based pharmacokinetic (PBPK) models and intestinal segment-specific tumor incidence data can form a robust dataset supporting derivation of alternative CSF values that equate to Cr(VI) concentrations ranging from below background to concentrations similar to those derived using threshold approaches-depending on benchmark response level and risk tolerance. Additionally, we highlight weaknesses in the rationale EPA used to discount critical in vivo genotoxicity studies. While the data support a non-genotoxic MOA, these alternative toxicity criteria require only PBPK models, robust tumor data, and fair interpretation of published in vivo genotoxicity data for Cr(VI).

Acute chlorothalonil exposure had the potential to influence the intestinal barrier function and micro-environment in mice

The intestinal barrier maintains intestinal homeostasis and metabolism and protects against harmful pollutants. Some environmental pollutants seriously affect intestinal barrier function. However, it remains unclear whether or how chlorothalonil (CTL) impacts the intestinal barrier function in animals. Herein, 6-week-old male mice were acutely exposed to different CTL concentrations (100 and 300 mg/kg BW) via intragastric administration once a day for 7 days. Histopathological examination revealed obvious inflammation in the mice' colon and ileum. Most notably, CTL exposure increased the intestinal permeability, particularly in the CTL-300 group. CTL exposure reduced the secretion of colonic epithelial mucus and changed the transcription levels of genes bound up with ion transport and ileal antimicrobial peptide (AMP) secretion, indicating intestinal chemical barrier damage. The results of terminal deoxynucleotidyl transferase dUTP nick end labelling (TUNEL) assay and Ki67 staining revealed abnormal apoptosis and increased intestinal epithelial cell proliferation, suggesting that CTL exposure led to cytotoxicity and inflammation. The results of 16S rRNA sequencing revealed that CTL exposure altered the intestinal microbiota composition and reduced its diversity and richness in the colon contents. Thus, acute CTL exposure affected the different intestinal barrier- and gut microenvironment-related endpoints in mice.

Toxicity analysis of endocrine disrupting pesticides on non-target organisms: A critical analysis on toxicity mechanisms

Endocrine disrupting compounds are the chemicals which mimics the natural endocrine hormones and bind to the receptors made for the hormones. Upon binding they activate the cascade of reaction which leads to permanent activating of the signalling cycle and ultimately leads to uncontrolled growth. Pesticides are one of the endocrine disrupting chemicals which cause cancer, congenital birth defects, and reproductive defects in non-target organisms. Non-target organisms are keen on exposing to these pesticides. Although several studies have reported about the pesticide toxicity. But a critical analysis of pesticide toxicity and its role as endocrine disruptor is lacking. Therefore, the presented review literature is an endeavour to understand the role of the pesticides as endocrine disruptors. In addition, it discusses about the endocrine disruption, neurological disruption, genotoxicity, and ROS induced pesticide toxicity. Moreover, biochemical mechanisms of pesticide toxicity on non-target organisms have been presented. An insight on the chlorpyrifos toxicity on non-target organisms along with species names have been presented.

Increased Cell Proliferation as a Key Event in Chemical Carcinogenesis: Application in an Integrated Approach for the Testing and Assessment of Non-Genotoxic Carcinogenesis

In contrast to genotoxic carcinogens, there are currently no internationally agreed upon regulatory tools for identifying non-genotoxic carcinogens of human relevance. The rodent cancer bioassay is only used in certain regulatory sectors and is criticized for its limited predictive power for human cancer risk. Cancer is due to genetic errors occurring in single cells. The risk of cancer is higher when there is an increase in the number of errors per replication (genotoxic agents) or in the number of replications (cell proliferation-inducing agents). The default regulatory approach for genotoxic agents whereby no threshold is set is reasonably conservative. However, non-genotoxic carcinogens cannot be regulated in the same way since increased cell proliferation has a clear threshold. An integrated approach for the testing and assessment (IATA) of non-genotoxic carcinogens is under development at the OECD, considering learnings from the regulatory assessment of data-rich substances such as agrochemicals. The aim is to achieve an endorsed IATA that predicts human cancer better than the rodent cancer bioassay, using methodologies that equally or better protect human health and are superior from the view of animal welfare/efficiency. This paper describes the technical opportunities available to assess cell proliferation as the central gateway of an IATA for non-genotoxic carcinogenicity.

Biogeochemical behaviour and toxicology of chromium in the soil-water-human nexus: A review

Chromium (Cr) affects human health if it accumulates in organs to elevated concentrations. The toxicity risk of Cr in the ecosphere depends upon the dominant Cr species and their bioavailability in the lithosphere, hydrosphere, and biosphere. However, the soil-water-human nexus that controls the biogeochemical behaviour of Cr and its potential toxicity is not fully understood. This paper synthesizes information on different dimensions of Cr ecotoxicological hazards in the soil and water and their subsequent effects on human health. The various routes of environmental exposure of Cr to humans and other organisms are also discussed. Human exposure to Cr(VI) causes both carcinogenic and non-carcinogenic health effects via complicated reactions that include oxidative stress, chromosomal and DNA damage, and mutagenesis. Chromium (VI) inhalation can cause lung cancer; however, incidences of other types of cancer following Cr(VI) exposure are low but probable. The non-carcinogenic health consequences of Cr(VI) exposure are primarily respiratory and cutaneous. Research on the biogeochemical behaviour of Cr and its toxicological hazards on human and other biological routes is therefore urgently needed to develop a holistic approach to understanding the soil-water-human nexus that controls the toxicological hazards of Cr and its detoxification.

NAM-based Prediction of Point-of-contact Toxicity in the Lung: A Case Example With 1,3-dichloropropene

Time, cost, ethical, and regulatory considerations surrounding in vivo testing methods render them insufficient to meet existing and future chemical safety testing demands. There is a need for the development of in vitro and in silico alternatives to replace traditional in vivo methods for inhalation toxicity assessment. Exposures of differentiated airway epithelial cultures to gases or aerosols at the air-liquid interface (ALI) can assess tissue responses and in vitro to in vivo extrapolation can align in vitro exposure levels with in-life exposures expected to give similar tissue exposures. Because the airway epithelium varies along its length, with various regions composed of different cell types, we have introduced a known toxic vapor to five human-derived, differentiated, in vitro airway epithelial cell culture models-MucilAir of nasal, tracheal, or bronchial origin, SmallAir, and EpiAlveolar-representing five regions of the airway epithelium-nasal, tracheal, bronchial, bronchiolar, and alveolar. We have monitored toxicity in these cultures 24hours after acute exposure using an assay for transepithelial conductance (for epithelial barrier integrity) and the lactate dehydrogenase (LDH) release assay (for cytotoxicity). Our vapor of choice in these experiments was 1,3-dichloropropene (1,3-DCP). Finally, we have developed an airway dosimetry model for 1,3-DCP vapor to predict in vivo external exposure scenarios that would produce toxic local tissue concentrations as determined by in vitro experiments. Measured in vitro points of departure (PoDs) for all tested cell culture models were similar. Calculated rat equivalent inhaled concentrations varied by model according to position of the modeled tissue within the airway, with nasal respiratory tissue being the most proximal and most sensitive tissue, and alveolar epithelium being the most distal and least sensitive tissue. These predictions are qualitatively in accordance with empirically determined in vivo PoDs. The predicted PoD concentrations were close to, but slightly higher than, PoDs determined by in vivo subchronic studies.

Long-term exposure to nanoplastics alters molecular and functional traits related to the carcinogenic process

Micro/nanoplastics (MNPLs) are considered emergent pollutants widely spread over all environmental compartments. Although their potential biological effects are being intensively evaluated, many doubts remain about their potential health effects in humans. One of the most underdeveloped fields is the determination of the potential tumorigenic risk of MNPLs exposure. To shed light on this topic, we have designed a wide battery of different hallmarks of cancer applied to prone-to-transformed progress MEF cells exposed to polystyrene nanoplastics (PSNPLs) in the long term (6 months). Interestingly, most of the evaluated hallmarks of cancer are exacerbated after exposure, independently if they are associated with an early tumoral phenotype (changes in stress-related genes, or microRNA deregulation), advanced tumoral phenotype (growing independently of anchorage ability, and migration capacity), or an aggressive tumoral phenotype (invasion potential, changes in pluripotency markers, and ability to grow to form tumorspheres). This set of obtained data constitutes a relevant warning on the potential carcinogenic risk associated with long-term exposures to MNPLs, specifically that induced by the PSNPLs evaluated in this study.

[Important Points at Interpretation ofNongenotoxic-Carcinogenicity Induced by Pesticidesin Rodent Bioassays]

Assessment of carcinogenicity is important for human health at dietary risk assessment of pesticide residues. This article indicated important points on interpretation of carcinogenicity in toxicological evaluation of pesticide residues based on principles of risk analysis in foods by CODEX to be a guide for risk assessors. This guidance was referred from the guidance on carcinogenicity evaluation by international and/or national organizations, and the interpretations of Food Safety Commissions of Japan (FSCJ) published in their risk assessment reports. We focused on carcinogenicity obtained from routine carcinogenicity bioassays in rodents. The guidance includes the purpose and usefulness of the bioassay studies, consideration points to be carcinogenicity and influencing factors to carcinogenicity in the test to judge carcinogenic hazard at hazard identification. Considering on human relevance as carcinogenic hazard also was proposed using practical case examples. Next, a carcinogenic hazard is evaluated on dose-response relationship to judge points of departure on carcinogenicity. At the end of this article, we challenged our recommendation on future assessment of carcinogenicity to progress from hazard to risk.

Crypt and Villus Transcriptomic Responses in Mouse Small Intestine Following Oral Exposure to Hexavalent Chromium

Oral exposure to hexavalent chromium (Cr(VI)) induces tumors in the mouse duodenum. Previous microarray-based transcriptomic analyses of homogenized mouse duodenal tissue have demonstrated Cr(VI)-induced alterations in various cellular pathways and processes. However, X-ray fluorescence microscopy indicates that chromium localizes primarily to the duodenal villi following exposure to Cr(VI), suggesting that previous transcriptomic analyses of homogenized tissue provide an incomplete picture of transcriptomic responses in the duodenum. Herein, transcriptomic analyses were conducted separately on crypt and villus tissue from formalin-fixed paraffin-embedded transverse duodenal sections from the same study in which microarray-based analyses were previously conducted. A total of 28 groups (7 doses × 2 timepoints × 2 tissue compartments) were analyzed for differential gene expression, dose-response, and gene set enrichment. Tissue compartment isolation was confirmed by differences in expression of typical markers of crypt and villus compartments. Fewer than 21 genes were altered in the crypt compartment of mice exposed to 0.1-5 ppm Cr(VI) for 7 or 90 days, which increased to hundreds or thousands of genes at ≥20 ppm Cr(VI). Consistent with histological evidence for crypt proliferation, a significant, dose-dependent increase in genes that regulate mitotic cell cycle was prominent in the crypt, while subtle in the villus, when compared with samples from time-matched controls. Minimal transcriptomic evidence of DNA damage response in either the crypts or the villi is consistent with published in vivo genotoxicity data. These results are also discussed in the context of modes of action that have been proposed for Cr(VI)-induced small intestine tumors in mice.

Assessment of intestinal injury of hexavalent chromium using a modified in vitro gastrointestinal digestion model

Intestinal injury assessment of hexavalent chromium (Cr-VI) in humans is crucial for quantifying assessment of adverse health risk posed by the intake of Cr (VI)-contaminated water. To overcome the deficiency in simulating human gastric reduction and intestinal absorption, we modified the constituents of simulated gastric fluid in in vitro digestion method by adding reductants glutathione (18 μM) and ascorbic acid (180 μM), which incorporated with human intestinal epithelial model to construct an in vitro gastrointestinal digestion (IVGD) model for intestinal injury assessment. Cr-VI bioaccessibility results from IVGD model showed that weak gastric acidity significantly increased the intestinal accessible Cr-VI dose by 22.41-38.43 folds. The time-course intestinal absorption indicated prolongation of intestinal exposure destroyed the intestinal epithelium, and 24 h after Cr-VI treatment was a good time point to perform intestinal absorption and toxicity assessment. A series of cell-based bioassays provided initial warning of adverse effect, suggesting that epithelial integrity exhibited greatest sensitivity to Cr-VI exposure and might be used as a sensitive marker for the toxicity assessment of oral exposure to Cr-VI. Notably, this study provides a feasible strategy for delineation of Cr-VI biotransformation and intestinal injury following ingestion exposure, which contributes to address the toxicity data gap of low-dose exposure in humans and puts forward a reference for intestinal toxicity assessment of other chemicals.

Assessment of Mechanistic Data for Hexavalent Chromium-Induced Rodent Intestinal Cancer Using the Key Characteristics of Carcinogens

Oral exposure to hexavalent chromium (Cr[VI]) induces intestinal tumors in mice. Mutagenic and nonmutagenic modes of action (MOAs) have been accepted by different regulatory bodies globally, the latter involving cytotoxicity-induced regenerative cell proliferation. However, concerns persist that all possible MOAs have not been fully considered. To address the potential for alternative MOAs, mechanistic data not represented in the existing two MOAs were evaluated. Relevant data were identified and organized by key characteristics of carcinogens (KCCs); literature related to epigenetics, immunosuppression, receptor-mediated effects, and immortalization were reviewed to identify potential key events associated with an alternative MOA. Over 200 references were screened for these four KCCs and further prioritized based on relevance to the research objective (ie, in vivo, oral exposure, gastrointestinal tissue). Minimal data were available specific to the intestine for these KCCs, and there was no evidence of any underlying mechanisms or key events that are not already represented in the two proposed MOAs. For example, while epigenetic dysregulation of DNA repair genes has been demonstrated, epigenetic effects were not measured in intestinal tissue, and it has been shown that Cr(VI) does not cause DNA damage in intestinal tissue. High-throughput screening data related to the KCCs were also evaluated, with activity generally limited to the two recognized MOAs. Collectively, no plausible alternative MOAs (or key events) were identified in addition to those previously proposed for Cr(VI) small intestine tumors.