A paradigm shift is required for the risk assessment of potential human health after exposure to low level chemical exposures: a response to the toxicity testing in the 21st century report

Benchmark Dose of Melamine Exposure for a Renal Injury Marker Mediated by Oxidative Stress: Examples in Patients with Urolithiasis and Occupational Workers

Establishing a safe exposure level from epidemiological studies while providing direct hazard characterization in humans often faces uncertainty in causality, especially cross-sectional data. With advances in molecular epidemiology, it is reasonable to integrate identified intermediate biomarkers into health risk assessment. In this study, by considering the mediation of the oxidative stress marker malondialdehyde (MDA), we explored the exposure threshold of melamine on the early renal injury marker N-acetyl-β-D glucosaminidase (NAG). The benchmark dose (BMD) was derived from model averaging of the composite direct effect of melamine exposure and the indirect effect through the mediation of MDA on NAG levels. As illustrative examples, we analyzed 309 adult patients with calcium urolithiasis and 80 occupational workers for the corresponding exposure thresholds. The derived threshold was subpopulation-dependent, with the one-sided lower bound BMDL10 for the patients with urolithiasis with (without) the mediator MDA for the patients with kidney stones and the occupational workers being 0.88 (0.96) μg/kg_bw/day and 22.82 (18.09) μg/kg_bw/day, respectively. The derived threshold levels, considering the oxidative stress marker MDA, were consistent with those without adjusting for the mediation effect. However, the study outcomes were further supported by the suggested mechanism pathway. The threshold for the patients with urolithiasis was up to two orders lower than the current tolerable daily intake level of 200 μg/kg_bw/day recommended by the WHO (EFSA).

Categorizing the characteristics of human carcinogens: a need for specificity

The International Agency for Research on Cancer (IARC) has recently proposed employing "ten key characteristics of human carcinogens" (TKCs) to determine the potential of agents for harmful effects. The TKCs seem likely to confuse the unsatisfactory correlation from testing regimes that have ignored the differences evident when cellular changes are compared in short and long-lived species, with their very different stem cell and somatic cell phylogenies. The proposed characteristics are so broad that their use will lead to an increase in the current unacceptably high rate of false positives. It could be an informative experiment to take well-established approved therapeutics with well-known human safety profiles and test them against this new TKC paradigm. Cancers are initiated and driven by heritable and transient changes in gene expression, expand clonally, and progress via additional associated acquired mutations and epigenetic alterations that provide cells with an evolutionary advantage. The genotoxicity testing protocols currently employed and required by regulation, emphasize testing for the mutational potential of the test agent. Two-year, chronic rodent cancer bioassays are intended to test for the entire spectrum of carcinogenic transformation. The use of cytotoxic doses causing increased, sustained cell proliferation that facilitates accumulated genetic damage leads to a high false-positive rate of tumor induction. Current cancer hazard assessment protocols and weight-of-the-evidence analysis of agent-specific cancer risk align poorly with the pathogenesis of human carcinoma and so need modernization and improvement in ways suggested here.

Drivers of and Obstacles to the Adoption of Toxicogenomics for Chemical Risk Assessment: Insights from Social Science Perspectives

BACKGROUND

Some 20 y ago, scientific and regulatory communities identified the potential of omics sciences (genomics, transcriptomics, proteomics, metabolomics) to improve chemical risk assessment through development of toxicogenomics. Recognizing that regulators adopt new scientific methods cautiously given accountability to diverse stakeholders, the scope and pace of adoption of toxicogenomics tools and data have nonetheless not met the ambitious, early expectations of omics proponents.

OBJECTIVE

Our objective was, therefore, to inventory, investigate, and derive insights into drivers of and obstacles to adoption of toxicogenomics in chemical risk assessment. By invoking established social science frameworks conceptualizing innovation adoption, we also aimed to develop recommendations for proponents of toxicogenomics and other new approach methodologies (NAMs).

METHODS

We report findings from an analysis of 56 scientific and regulatory publications from 1998 through 2017 that address the adoption of toxicogenomics for chemical risk assessment. From this purposeful sample of toxicogenomics discourse, we identified major categories of drivers of and obstacles to adoption of toxicogenomics tools and data sets. We then mapped these categories onto social science frameworks for conceptualizing innovation adoption to generate actionable insights for proponents of toxicogenomics.

DISCUSSION

We identify the most salient drivers and obstacles. From 1998 through 2017, adoption of toxicogenomics was understood to be helped by drivers such as those we labeled Superior scientific understanding, New applications, and Reduced cost & increased efficiency but hindered by obstacles such as those we labeled Insufficient validation, Complexity of interpretation, and Lack of standardization. Leveraging social science frameworks, we find that arguments for adoption that draw on the most salient drivers, which emphasize superior and novel functionality of omics as rationales, overlook potential adopters' key concerns: simplicity of use and compatibility with existing practices. We also identify two perspectives-innovation-centric and adopter-centric-on omics adoption and explain how overreliance on the former may be undermining efforts to promote toxicogenomics. https://doi.org/10.1289/EHP6500.

Gap Junction Intercellular Communication in the Carcinogenesis Hallmarks: Is This a Phenomenon or Epiphenomenon?

If occupational tumors are excluded, cancer causes are largely unknown. Therefore, it appeared useful to work out a theory explaining the complexity of this disease. More than fifty years ago the first demonstration that cells communicate with each other by exchanging ions or small molecules through the participation of connexins (Cxs) forming Gap Junctions (GJs) occurred. Then the involvement of GJ Intercellular Communication (GJIC) in numerous physiological cellular functions, especially in proliferation control, was proven and accounts for the growing attention elicited in the field of carcinogenesis. The aim of the present paper is to verify and discuss the role of Cxs, GJs, and GJIC in cancer hallmarks, pointing on the different involved mechanisms in the context of the multi-step theory of carcinogenesis. Functional GJIC acts both as a tumor suppressor and as a tumor enhancer in the metastatic stage. On the contrary, lost or non-functional GJs allow the uncontrolled proliferation of stem/progenitor initiated cells. Thus, GJIC plays a key role in many biological phenomena or epiphenomena related to cancer. Depending on this complexity, GJIC can be considered a tumor suppressor in controlling cell proliferation or a cancer ally, with possible preventive or therapeutic implications in both cases.

Environmentally prevalent polycyclic aromatic hydrocarbons can elicit co-carcinogenic properties in an in vitro murine lung epithelial cell model

Low molecular weight (LMW) polycyclic aromatic hydrocarbons (PAH) are the most abundant PAHs environmentally, occupationally, and are in cigarette smoke; however, little is known about their carcinogenic potential. We hypothesized that LMW PAHs act as co-carcinogens in the presence of a known carcinogen (benzo[a]pyrene (B[a]P)) in a mouse non-tumorigenic type II cell line (C10 cells). Gap junctions are commonly suppressed and inflammation induced during tumor promotion, while DNA-adduct formation is observed during the initiation stage of cancer. We used these endpoints together as markers of carcinogenicity in these lung adenocarcinoma progenitor cells. LMW PAHs (1-methylanthracene and fluoranthene, 1-10 µM total in a 1:1 ratio) were used based on previous studies as well as B[a]P (0-3 µM) as the classic carcinogen; non-cytotoxic doses were used. B[a]P-induced inhibition of gap junctional intercellular communication (GJIC) was observed at low doses and further reduced in the presence of the LMW PAH mixture (P < 0.05), supporting a role for GJIC suppression in cancer development. Benzo[a]pyrene diol-epoxide (BPDE)-DNA adduct levels were significantly induced in B[a]P-treated C10 cells and additionally increased with the LMW PAH mixture (P < 0.05). Significant increases in cyclooxygenase (Cox-2) were observed in response to the B[a]P/LMW PAH mixture combinations. DNA adduct formation coincided with the inhibition of GJIC and increase in Cox-2 mRNA expression. Significant cytochrome p4501b1 increases and connexin 43 decreases in gene expression were also observed. These studies suggest that LMW PAHs in combination with B[a]P can elicit increased carcinogenic potential. Future studies will further address the mechanisms of co-carcinogenesis driving these responses.

Disruptive environmental chemicals and cellular mechanisms that confer resistance to cell death

Cell death is a process of dying within biological cells that are ceasing to function. This process is essential in regulating organism development, tissue homeostasis, and to eliminate cells in the body that are irreparably damaged. In general, dysfunction in normal cellular death is tightly linked to cancer progression. Specifically, the up-regulation of pro-survival factors, including oncogenic factors and antiapoptotic signaling pathways, and the down-regulation of pro-apoptotic factors, including tumor suppressive factors, confers resistance to cell death in tumor cells, which supports the emergence of a fully immortalized cellular phenotype. This review considers the potential relevance of ubiquitous environmental chemical exposures that have been shown to disrupt key pathways and mechanisms associated with this sort of dysfunction. Specifically, bisphenol A, chlorothalonil, dibutyl phthalate, dichlorvos, lindane, linuron, methoxychlor and oxyfluorfen are discussed as prototypical chemical disruptors; as their effects relate to resistance to cell death, as constituents within environmental mixtures and as potential contributors to environmental carcinogenesis.

Automation of the in vitro micronucleus and chromosome aberration assay for the assessment of the genotoxicity of the particulate and gas-vapor phase of cigarette smoke

Total particulate matter (TPM) and the gas-vapor phase (GVP) of mainstream smoke from the Reference Cigarette 3R4F were assayed in the cytokinesis-block in vitro micronucleus (MN) assay and the in vitro chromosome aberration (CA) assay, both using V79-4 Chinese hamster lung fibroblasts exposed for up to 24 h. The Metafer image analysis platform was adapted resulting in a fully automated evaluation system of the MN assay for the detection, identification and reporting of cells with micronuclei together with the determination of the cytokinesis-block proliferation index (CBPI) to quantify the treatment-related cytotoxicity. In the CA assay, the same platform was used to identify, map and retrieve metaphases for a subsequent CA evaluation by a trained evaluator. In both the assays, TPM and GVP provoked a significant genotoxic effect: up to 6-fold more micronucleated target cells than in the negative control and up to 10-fold increases in aberrant metaphases. Data variability was lower in the automated version of the MN assay than in the non-automated. It can be estimated that two test substances that differ in their genotoxicity by approximately 30% can statistically be distinguished in the automated MN and CA assays. Time savings, based on man hours, due to the automation were approximately 70% in the MN and 25% in the CA assays. The turn-around time of the evaluation phase could be shortened by 35 and 50%, respectively. Although only cigarette smoke-derived test material has been applied, the technical improvements should be of value for other test substances.

Assessing vanadium and arsenic exposure of people living near a petrochemical complex with two-stage dispersion models

The goal of this study is to demonstrate that it is possible to construct a two-stage dispersion model empirically for the purpose of estimating air pollution levels in the vicinity of petrochemical plants. We studied oil refineries and coal-fired power plants in the No. 6 Naphtha Cracking Complex, an area of 2,603-ha situated on the central west coast of Taiwan. The pollutants targeted were vanadium (V) from oil refineries and arsenic (As) from coal-fired power plants. We applied a backward fitting method to determine emission rates of V and As, with 192 PM10 filters originally collected between 2009 and 2012. Our first-stage model estimated emission rates of V and As (median and 95% confidence intervals at 0.0202 (0.0040-0.1063) and 0.1368 (0.0398-0.4782) g/s, respectively. In our second stage model, the predicted zone-average concentrations showed a strong correlation with V, but a poor correlation with As. Our findings show that two-stage dispersion models are relatively precise for estimating V levels at residents' addresses near the petrochemical complex, but they did not work as well for As levels. In conclusion, our model-based approach can be widely used for modeling exposure to air pollution from industrial areas in countries with limited resources.

Regenerative toxicology: the role of stem cells in the development of chronic toxicities

INTRODUCTION

Human stem cell lines and their derivatives, as alternatives to the use of animal cells or cancer cell lines, have been widely discussed as cellular models in predictive toxicology. However, the role of stem cells in the development of long-term toxicities and carcinogenesis has not received great attention so far, despite growing evidence indicating the relationship of stem cell damage to adverse effects later in life. However, testing this in vitro is a scientific/technical challenge in particular due to the complex interplay of factors existing under physiological conditions. Current major research programs in stem cell toxicity are not aiming to demonstrate that stem cells can be targeted by toxicants. Therefore, this knowledge gap needs to be addressed in additional research activities developing technical solutions and defining appropriate experimental designs.

AREAS COVERED

The current review describes selected examples of the role of stem cells in the development of long-term toxicities in the brain, heart or liver and in the development of cancer.

EXPERT OPINION

The presented examples illustrate the need to analyze the contribution of stem cells to chronic toxicity in order to make a final conclusion whether stem cell toxicities are an underestimated risk in mechanism-based safety assessments. This requires the development of predictive in vitro models allowing the assessment of adverse effects to stem cells on chronic toxicity and carcinogenicity.

Oxidative stress-induced biomarkers for stem cell-based chemical screening

Stem cells have been considered for their potential in pharmaceutical research, as well as for stem cell-based therapy for many diseases. Despite the potential for their use, the challenge remains to examine the safety and efficacy of stem cells for their use in therapies. Recently, oxidative stress has been strongly implicated in the functional regulation of cell behavior of stem cells. Therefore, development of rapid and sensitive biomarkers, related to oxidative stress is of growing importance in stem cell-based therapies for treating various diseases. Since stem cells have been implicated as targets for carcinogenesis and might be the origin of "cancer stem cells", understanding of how oxidative stress-induced signaling, known to be involved in the carcinogenic process could lead to potential screening of cancer chemopreventive and chemotherapeutic agents. An evaluation of antioxidant states reducing equivalents like GSH and superoxide dismutase (SOD), as well as reactive oxygen species (ROS) and nitric oxide (NO) generation, can be effective markers in stem cell-based therapies. In addition, oxidative adducts, such as 4-hydroxynonenal, can be reliable markers to detect cellular changes during self-renewal and differentiation of stem cells. This review highlights the biomarker development to monitor oxidative stress response for stem cell-based chemical screening.

Connexin-dependent signaling in neuro-hormonal systems

The advent of multicellular organisms was accompanied by the development of short- and long-range chemical signalling systems, including those provided by the nervous and endocrine systems. In turn, the cells of these two systems have developed mechanisms for interacting with both adjacent and distant cells. With evolution, such mechanisms have diversified to become integrated in a complex regulatory network, whereby individual endocrine and neuro-endocrine cells sense the state of activity of their neighbors and, accordingly, regulate their own level of functioning. A consistent feature of this network is the expression of connexin-made channels between the (neuro)hormone-producing cells of all endocrine glands and secretory regions of the central nervous system so far investigated in vertebrates. This review summarizes the distribution of connexins in the mammalian (neuro)endocrine systems, and what we know about the participation of these proteins on hormone secretion, the life of the producing cells, and the action of (neuro)hormones on specific targets. The data gathered since the last reviews on the topic are summarized, with particular emphasis on the roles of Cx36 in the function of the insulin-producing beta cells of the endocrine pancreas, and of Cx40 in that of the renin-producing juxta-glomerular epithelioid cells of the kidney cortex. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

The biological basis and clinical significance of hormonal imprinting, an epigenetic process

The biological phenomenon, hormonal imprinting, was named and defined by us (Biol Rev, 1980, 55, 47-63) 30 years ago, after many experimental works and observations. Later, similar phenomena were also named to epigenetic imprinting or metabolic imprinting. In the case of hormonal imprinting, the first encounter between a hormone and its developing target cell receptor-usually at the perinatal period-determines the normal receptor-hormone connection for life. However, in this period, molecules similar to the target hormone (members of the same hormone family, synthetic drugs, environmental pollutants, etc), which are also able to bind to the receptor, provoke faulty imprinting also with lifelong-receptorial, behavioral, etc.,-consequences. Faulty hormonal imprinting could also be provoked later in life in continuously dividing cells and in the brain. Faulty hormonal imprinting is a disturbance of gene methylation pattern, which is epigenenetically inherited to the further generations (transgenerational imprinting). The absence of the normal or the presence of false hormonal imprinting predispose to or manifested in different diseases (e.g., malignant tumors, metabolic syndrome) long after the time of imprinting or in the progenies.

The gap junction as a "Biological Rosetta Stone": implications of evolution, stem cells to homeostatic regulation of health and disease in the Barker hypothesis

The discovery of the gap junction structure, its functions and the family of the "connexin" genes, has been basically ignored by the major biological disciplines. These connexin genes code for proteins that organize to form membrane-associated hemi-channels, "connexons", co-join with the connexons of neighboring cells to form gap junctions. Gap junctions appeared in the early evolution of the metazoan. Their fundamental functions, (e.g., to synchronize electrotonic and metabolic functions of societies of cells, and to regulate cell proliferation, cell differentiation, and apoptosis), were accomplished via integrating the extra-cellular triggering of intra-cellular signaling, and therefore, regulating gene expression. These functions have been documented by genetic mutations of the connexin genes and by chemical modulation of gap junctions. Via genetic alteration of connexins in knock-out and transgenic mice, as well as inherited connexin mutations in various human syndromes, the gap junction has been shown to be directly linked to many normal cell functions and multiple diseases, such as birth defects, reproductive, neurological disorders, immune dysfunction and cancer. Specifically, the modulation of gap junctional intercellular communication (GJIC), either by increasing or decreasing its functions by non-mutagenic chemicals or by oncogenes or tumor suppressor genes in normal or "initiated" stem cells and their progenitor cells, can have a major impact on tumor promotion or cancer chemoprevention and chemotherapy. The overview of the roles of the gap junction in the evolution of the metazoan and its potential in understanding a "systems" view of human health and aging and the diseases of aging will be attempted.