Predicting Carcinogenic Mechanisms of Non-Genotoxic Carcinogens via Combined Analysis of Global DNA Methylation and In Vitro Cell Transformation

Glucosinolate Hydrolytic Products-A Multi-Arm Warrior

BACKGROUND

Glucosinolates (GSLs) are the most controversial yet ignored class of phytochemicals. These are the middleman phytochemicals that have low bioactivity. But once there is any injury in the plant-manmade, insect caused, or natural-magic happens. The compound is broken down into smaller phytochemicals referred to as glucosinolate hydrolytic products (GHPs; nitriles, isothiocyanates [ITCs], and thiocyanates). These hydrolytic products are like a showstopper of the fashion industry. These compounds have some of the highest bioactivity in nature. They have been associated with a varied range of bioactivities (anticancer, antioxidant, insecticidal, weedicide, etc.) by researchers across the globe.

OBJECTIVE

The objective of the current article is to provide a critical review to highlight some of the important bioactivities of these ignored compounds and for promoting researchers to at least give these compounds a chance-to glow in the dark.

METHODS

This review has been written from analysis of accessible literature, mostly from the last 5 years (2018-2023), with some critically essential exceptions.

RESULTS

The review highlighted a brief background of GSLs and its hydrolysis. Efforts were made to include most of the biological properties of the compound. Special emphasis has been given to the anticancer activities of the compound with details of the involved mechanism.

CONCLUSIONS

Considering the wide array of bioactivities of GHPs, it is essential to consider it as a prospective medicinal compound. More GHPs-in a similar manner as sulforaphane-can be proceeded to phase trials.

HIGHLIGHTS

The mechanistic pathway for production of GHPs and related biological activities have been discussed in detail. The bioactivities have been further explained using the involved mechanism.

A modular strategy for the testing and assessment of non-genotoxic carcinogens

A modular strategy is described for the testing and assessment (MoSt) of non-genotoxic carcinogenicity (NGTxC) that is suitable for regulatory applications. It utilizes and builds upon work conducted by the OECD expert group on NGTxC. The approach integrates relevant test methods from the molecular- to cellular- and further to tissue level, many of which have been recently reviewed. Six progressive modules are included in the strategy. Advice is provided for the iterative selection of the next appropriate test method within each step of the strategy. Assessment is completed by a weight of evidence conclusion, which integrates the different streams of modular information. The assessment method gives higher weight to findings that are mechanistically linked with biological relevance to carcinogenesis. With a focus on EU-REACH, and pending upon successful test method validation and acceptance, this will also enable the MoSt for NGTxC to be applied for regulatory purposes across different regulatory jurisdictions.

A vision for safer food contact materials: Public health concerns as drivers for improved testing

Food contact materials (FCMs) and food contact articles are ubiquitous in today's globalized food system. Chemicals migrate from FCMs into foodstuffs, so called food contact chemicals (FCCs), but current regulatory requirements do not sufficiently protect public health from hazardous FCCs because only individual substances used to make FCMs are tested and mostly only for genotoxicity while endocrine disruption and other hazard properties are disregarded. Indeed, FCMs are a known source of a wide range of hazardous chemicals, and they likely contribute to highly prevalent non-communicable diseases. FCMs can also include non-intentionally added substances (NIAS), which often are unknown and therefore not subject to risk assessment. To address these important shortcomings, we outline how the safety of FCMs may be improved by (1) testing the overall migrate, including (unknown) NIAS, of finished food contact articles, and (2) expanding toxicological testing beyond genotoxicity to multiple endpoints associated with non-communicable diseases relevant to human health. To identify mechanistic endpoints for testing, we group chronic health outcomes associated with chemical exposure into Six Clusters of Disease (SCOD) and we propose that finished food contact articles should be tested for their impacts on these SCOD. Research should focus on developing robust, relevant, and sensitive in-vitro assays based on mechanistic information linked to the SCOD, e.g., through Adverse Outcome Pathways (AOPs) or Key Characteristics of Toxicants. Implementing this vision will improve prevention of chronic diseases that are associated with hazardous chemical exposures, including from FCMs.

The Cell Transformation Assay: A Historical Assessment of Current Knowledge of Applications in an Integrated Approach to Testing and Assessment for Non-Genotoxic Carcinogens

The history of the development of the cell transformation assays (CTAs) is described, providing an overview of in vitro cell transformation from its origin to the new transcriptomic-based CTAs. Application of this knowledge is utilized to address how the different types of CTAs, variously addressing initiation and promotion, can be included on a mechanistic basis within the integrated approach to testing and assessment (IATA) for non-genotoxic carcinogens. Building upon assay assessments targeting the key events in the IATA, we identify how the different CTA models can appropriately fit, following preceding steps in the IATA. The preceding steps are the prescreening transcriptomic approaches, and assessment within the earlier key events of inflammation, immune disruption, mitotic signaling and cell injury. The CTA models address the later key events of (sustained) proliferation and change in morphology leading to tumor formation. The complementary key biomarkers with respect to the precursor key events and respective CTAs are mapped, providing a structured mechanistic approach to represent the complexity of the (non-genotoxic) carcinogenesis process, and specifically their capacity to identify non-genotoxic carcinogenic chemicals in a human relevant IATA.

Mechanistic Interrogation of Cell Transformation In Vitro: The Transformics Assay as an Exemplar of Oncotransformation

The Transformics Assay is an in vitro test which combines the BALB/c 3T3 Cell Transformation Assay (CTA) with microarray transcriptomics. It has been shown to improve upon the mechanistic understanding of the CTA, helping to identify mechanisms of action leading to chemical-induced transformation thanks to RNA extractions in specific time points along the process of in vitro transformation. In this study, the lowest transforming concentration of the carcinogenic benzo(a)pyrene (B(a)P) has been tested in order to find molecular signatures of initial events relevant for oncotransformation. Application of Enrichment Analysis (Metacore) to the analyses of the results facilitated key biological interpretations. After 72 h of exposure, as a consequence of the molecular initiating event of aryl hydrocarbon receptor (AhR) activation, there is a cascade of cellular events and microenvironment modification, and the immune and inflammatory responses are the main processes involved in cell response. Furthermore, pathways and processes related to cell cycle regulation, cytoskeletal adhesion and remodeling processes, cell differentiation and transformation were observed.

Gene Expression over Time during Cell Transformation Due to Non-Genotoxic Carcinogen Treatment of Bhas 42 Cells

The Bhas 42 cell transformation assay (Bhas 42 CTA) is the first Organization for Economic Cooperation and Development (OECD)-certificated method used as a specific tool for the detection of the cell-transformation potential of tumor-promoting compounds, including non-genotoxic carcinogens (NGTxCs), as separate from genotoxic carcinogens. This assay offers the great advantage of enabling the phenotypic detection of oncotransformation. A key benefit of using the Bhas 42 CTA in the study of the cell-transformation mechanisms of tumor-promoting compounds, including non-genotoxic carcinogens, is that the cell-transformation potential of the chemical can be detected directly without treatment with a tumor-initiating compound since Bhas 42 cell line was established by transfecting the v-Ha-ras gene into a mouse fibroblast cloned cell line. Here, we analyzed the gene expression over time, using DNA microarrays, in Bhas 42 cells treated with the tumor-promoting compound 12-O-tetradecanoylphorbol-13-acetate (TPA), and NGTxC, with a total of three repeat experiments. This is the first paper to report on gene expression over time during the process of cell transformation with only a tumor-promoting compound. Pathways that were activated or inactivated during the process of cell transformation in the Bhas 42 cells treated with TPA were related not only directly to RAS but also to various pathways in the hallmarks of cancer.

Integration of Epigenetic Mechanisms into Non-Genotoxic Carcinogenicity Hazard Assessment: Focus on DNA Methylation and Histone Modifications

Epigenetics involves a series of mechanisms that entail histone and DNA covalent modifications and non-coding RNAs, and that collectively contribute to programing cell functions and differentiation. Epigenetic anomalies and DNA mutations are co-drivers of cellular dysfunctions, including carcinogenesis. Alterations of the epigenetic system occur in cancers whether the initial carcinogenic events are from genotoxic (GTxC) or non-genotoxic (NGTxC) carcinogens. NGTxC are not inherently DNA reactive, they do not have a unifying mode of action and as yet there are no regulatory test guidelines addressing mechanisms of NGTxC. To fil this gap, the Test Guideline Programme of the Organisation for Economic Cooperation and Development is developing a framework for an integrated approach for the testing and assessment (IATA) of NGTxC and is considering assays that address key events of cancer hallmarks. Here, with the intent of better understanding the applicability of epigenetic assays in chemical carcinogenicity assessment, we focus on DNA methylation and histone modifications and review: (1) epigenetic mechanisms contributing to carcinogenesis, (2) epigenetic mechanisms altered following exposure to arsenic, nickel, or phenobarbital in order to identify common carcinogen-specific mechanisms, (3) characteristics of a series of epigenetic assay types, and (4) epigenetic assay validation needs in the context of chemical hazard assessment. As a key component of numerous NGTxC mechanisms of action, epigenetic assays included in IATA assay combinations can contribute to improved chemical carcinogen identification for the better protection of public health.

Effects of vanadium (sodium metavanadate) and aflatoxin-B1 on cytochrome p450 activities, DNA damage and DNA methylation in human liver cell lines

Vanadium is considered as "possibly carcinogenic to humans" (V₂O₅, IARC Group 2B), yet uncertainties persist related to the toxicity mechanisms of the multiple forms of vanadium. Exposure to vanadium often co-occurs with other metals or with organic compounds that can be transformed by cytochrome p450 (CYP) enzymes into DNA-reactive carcinogens. Therefore, effects of a soluble form of vanadium (sodium metavanadate, NaVO₃) and aflatoxin-B1 (AFB1) were tested separately and together, for induction of CYP activities, DNA damage (γH2AX and DNA alkaline unwinding assays), and DNA methylation changes (global genome and DNA repeats) in HepaRG or HepG2 liver cell lines. NaVO₃ (≥ 2.3 μM) reduced CYP1A1 and CYP3A4 activities and induced DNA damage, butcaused important cell proliferation only in HepaRG cells. As a binary mixture, NaVO₃ did not modify the effects of AFB1. There was no reproducible effect of NaVO₃ (<21 μM) on DNA methylation in AluYb8, satellite-α, satellite-2, and by the luminometric methylation assay, but DNA methylation flow-cytometry signals in HepG2 cells (25-50 μM) increased at the G1 and G2 cell cycle phases. In conclusion, cell lines responded differently to NaVO₃ supporting the importance of investigating more than one cell line, and a carcinogenic role of NaVO₃ might reside at low concentrations by stimulating the proliferation of tumorigenic cells.