RESUMO
We utilized a practical, transparent approach for systematically reviewing a chemical-specific evidence base. This approach was used for a case study of ozone inhalation exposure and adverse metabolic effects (overweight/obesity, Type 1 diabetes [T1D], Type 2 diabetes [T2D], and metabolic syndrome). We followed the basic principles of systematic review. Studies were defined as "Suitable" or "Supplemental." The evidence for Suitable studies was characterized as strong or weak. An overall causality judgment for each outcome was then determined as either causal, suggestive, insufficient, or not likely. Fifteen epidemiologic and 33 toxicologic studies were Suitable for evidence synthesis. The strength of the human evidence was weak for all outcomes. The toxicologic evidence was weak for all outcomes except two: body weight, and impaired glucose tolerance/homeostasis and fasting/baseline hyperglycemia. The combined epidemiologic and toxicologic evidence was categorized as weak for overweight/obesity, T1D, and metabolic syndrome,. The association between ozone exposure and T2D was determined to be insufficient or suggestive. The streamlined approach described in this paper is transparent and focuses on key elements. As systematic review guidelines are becoming increasingly complex, it is worth exploring the extent to which related health outcomes should be combined or kept distinct, and the merits of focusing on critical elements to select studies suitable for causal inference. We recommend that systematic review results be used to target discussions around specific research needs for advancing causal determinations.
Assuntos
Diabetes Mellitus Tipo 2 , Ozônio , Humanos , Obesidade/induzido quimicamente , Ozônio/toxicidadeAssuntos
Água Potável , Cardiopatias Congênitas , Tricloroetileno , Animais , Coração , Ratos , Ratos Sprague-DawleyRESUMO
BACKGROUND: Trichloroethylene (TCE) was negative for developmental toxicity after inhalation and oral gavage exposure of pregnant rats but fetal cardiac defects were reported following drinking water exposure throughout gestation. Because of the deficiencies in this latter study, we performed another drinking water study to evaluate whether TCE causes heart defects. METHODS: Groups of 25 mated Sprague Dawley rats consumed water containing 0, 0.25, 1.5, 500, or 1,000 ppm TCE from gestational day 1-21. TCE concentrations were measured at daily formulation, when placed into water bottles each day and when water bottles were removed from cages. Four additional mated rats per group were used for plasma measurements. At termination, fetal hearts were carefully dissected fresh and examined. RESULTS: All TCE concentrations were >90% of target when initially placed in water bottles and when bottles were placed on cages. All dams survived with no clinical signs. Rats in the two higher dose groups consumed less water/day than other groups but showed no changes in maternal or fetal weights. The only fetal cardiac observation was small (<1 mm) membranous ventricular septal defect occurring in all treated and water control groups; incidences were within the range of published findings for naive animals. TCE was not detected in maternal blood, but systemic exposure was confirmed by detecting its primary oxidative metabolite, trichloroacetic acid, although only at levels above the quantitation limit in the two higher dose groups. CONCLUSIONS: Ingesting TCE in drinking water ≤1,000 ppm throughout gestation does not cause cardiac defects in rat offspring.
Assuntos
Cardiopatias Congênitas/etiologia , Tricloroetileno/efeitos adversos , Tricloroetileno/farmacologia , Animais , Água Potável , Feminino , Coração Fetal/efeitos dos fármacos , Peso Fetal/efeitos dos fármacos , Gravidez , Ratos , Ratos Sprague-Dawley , Reprodução/efeitos dos fármacos , Ácido Tricloroacético/metabolismo , Ácido Tricloroacético/farmacologia , Tricloroetileno/metabolismoRESUMO
Styrene is an important high production volume chemical used to manufacture polymeric products. In 2018, International Agency for Research on Cancer classified styrene as probably carcinogenic to humans; National Toxicology Program lists styrene as reasonably anticipated to be a human carcinogen. The genotoxicity literature for styrene and its primary metabolite, styrene 7,8-oxide (SO), begins in the 1970s. Organization of Economic Cooperation and Development (OECD) recently updated most genotoxicity test guidelines, making substantial new recommendations for assay conduct and data evaluation for the standard mutagenicity/clastogenicity assays. Thus, a critical review of the in vitro and in vivo rodent mutagenicity/clastogenicity studies for styrene and SO, based on the latest OECD recommendations, is timely. This critical review considered whether a study was optimally designed, conducted, and interpreted and provides a critical assessment of the evidence for the mutagenicity/clastogenicity of styrene/SO. Information on the ability of styrene/SO to induce other types of genotoxicity endpoints is summarized but not critically reviewed. We conclude that when styrene is metabolized to SO, it can form DNA adducts, and positive in vitro mutagenicity/clastogenicity results can be obtained. SO is mutagenic in bacteria and the in vitro mouse lymphoma gene mutation assay. No rodent in vivo mutation studies were identified. SO is clastogenic in cultured mammalian cells. Although the in vitro assays gave positive responses, styrene/SO is not clastogenic/aneugenic in vivo in rodents. In addition to providing updated information for styrene, this review demonstrates the application of the new OECD guidelines for chemicals with large genetic toxicology databases where published results may or may not be reliable. Environ. Mol. Mutagen. 2019. © 2019 Wiley Periodicals, Inc.
Assuntos
Carcinógenos/toxicidade , Mutagênicos/toxicidade , Estireno/efeitos adversos , Animais , Dano ao DNA/efeitos dos fármacos , Humanos , Mutagênese/efeitos dos fármacos , Testes de Mutagenicidade/métodosRESUMO
Methyl Chloride (MeCl; Chloromethane) is a high production volume chemical (>1000â¯t/a) and is used as an industrial solvent. Based on cardiac lesions reported in developmental toxicity studies in mice, but not in rats, manufacturers decided to classify MeCl as a developmental toxicant, cat. 2. Recently, the European Chemical Agency required a developmental toxicity study in a non-rodent species. No developmental toxicity was observed in rabbits in the recently completed, GLP, OECD 414 guideline study. In view of the absence of cardiac effects in rats and rabbits, the purpose of this review is to consider whether the cardiac effects reported in mice should be considered real effects and, if so, their potential for relevance to humans. This paper provides substantive new evidence with data from a third species and shows that an evaluation of the integrated scientific evidence indicates the reported developmental cardiac effects in mice, if not an artifact, are unlikely to be relevant to humans. As such the classification of MeCl for developmental toxicity was reconsidered.
Assuntos
Coração/efeitos dos fármacos , Cloreto de Metila/toxicidade , Animais , Camundongos , Coelhos , Ratos , Testes de ToxicidadeRESUMO
To conduct risk assessments of exogenous chemicals for which there are also endogenous exposures, knowledge of the chemistry and biology of both types of exposures needs to be integrated into problem formulation and carried through to risk characterization. This issue is framed in a risk assessment context, highlighting the importance of quantifying increments of dose from all sources of the same or similar chemicals interacting with biological targets; understanding the influence of endogenous chemical concentrations on disease risk; and assessing total dose to targets in evaluating risk from incremental environmental exposures. Examples of recent assessments illustrate the importance of addressing this issue. Evaluations of data on blood or organ concentrations of ammonia, methanol, formaldehyde, acetaldehyde, and three gaseous signaling molecules (hydrogen sulfide, carbon monoxide, and nitric oxide) provide examples where current data are already informing perspectives on relative exposures at the portal of entry and systemically. To facilitate quality risk assessments of exogenous chemicals with endogenous exposures, a series of specific questions are presented that need to be addressed in systematic review to enhance problem formulation, improve the development of holistic conceptual models, and to facilitate the identification of priority data needs for improving risk assessments.
Assuntos
Monóxido de Carbono/efeitos adversos , Monitoramento Ambiental , Poluentes Ambientais/efeitos adversos , Sulfeto de Hidrogênio/efeitos adversos , Óxido Nítrico/efeitos adversos , Monóxido de Carbono/análise , Poluentes Ambientais/análise , Humanos , Sulfeto de Hidrogênio/análise , Óxido Nítrico/análise , Medição de RiscoRESUMO
The glutathione (GSH) conjugates, S-(1,2-dichlorovinyl)-glutathione (DCVG) and S-(1,2-dichlorovinyl)-L-cysteine (DCVC), have been implicated in kidney toxicity and kidney cancer from trichloroethylene (TCE) exposure. Considerable differences in blood and tissue levels of DCVG and DCVC have been reported, depending on whether HPLC/UV (High Performance Liquid Chromatography-Ultraviolet) or HPLC/MS (HPLC-Mass Spectrometry) was used. A side-by-side comparison of analytical results with HPLC/UV and HPLC/MS/MS (High Performance Liquid Chromatography-Tandem Mass Spectrometry) detection was undertaken to quantitatively compare estimates for DCVG and DCVG using rat and human tissues. For the HPLC method, DCVG and DCVC were initially derivatized with fluorodinitrobenzene (DNP). The results from the HPLC/UV method showed that derivatized-DCVC eluted at the solvent front and could not be quantified. Derivatized-DCVG, however, was quantified but significant interference was observed in all four control tissues (rat blood, liver, kidney; and human blood), resulting in average spike recoveries of 222-22,990%. In contrast, direct analysis of spiked tissues by HPLC/MS/MS resulted in recoveries of 82-127% and 89-117% for DCVG and DCVC, respectively. These differences in analytical results were further confirmed in tissues from TCE-treated rats, e.g., DCVG levels in rat liver were 18,000 times higher by HPLC/UV as compared to HPLC/MS/MS. Fraction collection of the derivatized-DCVG peak (obtained with the HPLC-UV method), followed by peak identification via an HPLC/UV/Q-TOF/MS/MS method, identified DNP-derivatized endogenous glutamate as the primary interfering substance that contributed to and exaggerated recoveries of DCVG. Thus, estimates of DCVG based on the HPLC/UV methods are not reliable; they will over-estimate the formation of the GSH conjugates of TCE and will artifactually exaggerate the potential cancer risk in humans from TCE exposure. Therefore, it is recommended that any characterization of cancer risks from TCE exposure attributable to the GSH conjugates of TCE rely on results obtained with the more specific and reliable HPLC/MS/MS method.
Assuntos
Glutationa/metabolismo , Tricloroetileno/metabolismo , Tricloroetileno/toxicidade , Animais , Cromatografia Líquida de Alta Pressão , Humanos , Rim/metabolismo , Masculino , Ratos , Ratos Endogâmicos F344 , Medição de Risco , Espectrofotometria Ultravioleta , Espectrometria de Massas em Tandem , Tricloroetileno/sangueRESUMO
Adverse outcome pathways (AOPs) are frameworks starting with a molecular initiating event (MIE), followed by key events (KEs) linked by KE relationships (KERs), ultimately resulting in a specific adverse outcome. Relevant data for the pathway and each KE/KER are evaluated to assess biological plausibility, weight-of-evidence, and confidence. We aimed to describe an AOP relevant to chemicals directly inducing mutation in cancer critical gene(s), via the formation of chemical-specific pro-mutagenic DNA adduct(s), as an early critical step in tumor etiology. Such chemicals have mutagenic modes-of-action (MOA) for tumor induction. To assist with developing this AOP, Aflatoxin B1 (AFB1) was selected as a case study because it has a rich database and is considered to have a mutagenic MOA. AFB1 information was used to define specific KEs, KERs, and to inform development of a generic AOP for mutagen-induced hepatocellular carcinoma (HCC). In assessing the AFB1 information, it became clear that existing data are, in fact, not optimal and for some KEs/KERs, the definitive data are not available. In particular, while there is substantial information that AFB1 can induce mutations (based on a number of mutation assays), the definitive evidence - the ability to induce mutation in the cancer critical gene(s) in the tumor target tissue - is not available. Thus, it is necessary to consider the patterns of results in the weight-of-evidence for KEs and KERs. It was important to determine whether there was sufficient evidence that AFB1 can induce the necessary critical mutations early in the carcinogenic process, which was the case.
Assuntos
Rotas de Resultados Adversos , Aflatoxina B1/toxicidade , Carcinógenos/toxicidade , Carcinoma Hepatocelular/induzido quimicamente , Neoplasias Hepáticas/induzido quimicamente , Mutagênicos/toxicidade , Animais , Carcinoma Hepatocelular/genética , Adutos de DNA/genética , Humanos , Neoplasias Hepáticas/genética , MutaçãoRESUMO
Dichloromethane (DCM) is a lung and liver carcinogen in mice at inhalation exposures≥2000ppm. The modes of action (MOA) of these responses have been attributed to formation of genotoxic, reactive metabolite(s). Here, we examined gene expression in lung and liver from female B6C3F1 mice exposed to 0, 100, 500, 2000, 3000 and 4000ppm DCM for 90days. We also simulated dose measures - rates of DCM oxidation to carbon monoxide (CO) in lung and liver and expected blood carboxyhemoglobin (HbCO) time courses with a PBPK model inclusive of both conjugation and oxidation pathways. Expression of large numbers of genes was altered at 100ppm with maximal changes in the numbers occurring by 500 or 2000ppm. Most changes in genes common to the two tissues were related to cellular metabolism and circadian clock. At the lower concentrations, the changes in metabolism-related genes were discordant - up in liver and down in lung. These processes included organelle biogenesis, TCA cycle, and respiratory electron transport. Changes in circadian cycle genes - primarily transcription factors - showed strong concentration-related response at higher concentrations (Arntl, Npas2, and Clock were down-regulated; Cry2, Wee1, Bhlhe40, Per3, Nr1d1, Nr1d2 and Dbp) were up-regulated with similar directionality in both tissues. Overall, persistently elevated HbCO from DCM oxidation appears to cause extended periods of hypoxia, leading to altered circadian coupling to cellular metabolism. The dose response for altered circadian processes correlates with the cancer outcome. We found no evidence of changes in genes indicative of responses to cytotoxic, DNA-reactive metabolites.
Assuntos
Ritmo Circadiano , Hipóxia/genética , Fígado/efeitos dos fármacos , Pulmão/efeitos dos fármacos , Cloreto de Metileno/toxicidade , Transcriptoma , Animais , Carboxihemoglobina/genética , Carboxihemoglobina/metabolismo , Ritmo Circadiano/efeitos dos fármacos , Ritmo Circadiano/genética , Relação Dose-Resposta a Droga , Feminino , Regulação da Expressão Gênica , Hipóxia/induzido quimicamente , Hipóxia/patologia , Exposição por Inalação/efeitos adversos , Fígado/metabolismo , Pulmão/metabolismo , Camundongos , Camundongos Endogâmicos , Farmacocinética , Fatores de Transcrição/genética , Fatores de Transcrição/metabolismoRESUMO
The evolved World Health Organization/International Programme on Chemical Safety mode of action (MOA) framework provides a structure for evaluating evidence in pathways of causally linked key events (KE) leading to adverse health effects. Although employed globally, variability in use of the MOA framework has led to different interpretations of the sufficiency of evidence in support of hypothesized MOAs. A proof of concept extension of the MOA framework is proposed for scoring confidence in the supporting data to improve scientific justification for MOA use in characterizing hazards and selecting dose-response extrapolation methods for specific chemicals. This involves selecting hypothesized MOAs, and then, for each MOA, scoring the weight of evidence (WOE) in support of causality for each KE using evolved Bradford Hill causal considerations (biological plausibility, essentiality, dose-response concordance, consistency, and analogy). This early proof of concept method is demonstrated by comparing two potential MOAs (mutagenicity and peroxisome proliferator activated receptor-alpha) for clofibrate, a rodent liver carcinogen. Quantitative confidence scoring of hypothesized MOAs is shown to be useful in characterizing the likely operative MOA. To guide method refinement and future confidence scoring for a spectrum of MOAs, areas warranting further focus and lessons learned, including the need to incorporate a narrative discussion of the weights used in the evaluation and an overall evaluation of the plausibility of the outcome, are presented.
Assuntos
Carcinógenos/toxicidade , Segurança Química , Clofibrato/toxicidade , Testes de Mutagenicidade , Estudo de Prova de Conceito , Efeitos Colaterais e Reações Adversas Relacionados a Medicamentos , Humanos , PPAR alfa/metabolismo , Medição de RiscoRESUMO
For several decades, regulatory testing schemes for genetic damage have been standardized where the tests being utilized examined mutations and structural and numerical chromosomal damage. This has served the genetic toxicity community well when most of the substances being tested were amenable to such assays. The outcome from this testing is usually a dichotomous (yes/no) evaluation of test results, and in many instances, the information is only used to determine whether a substance has carcinogenic potential or not. Over the same time period, mechanisms and modes of action (MOAs) that elucidate a wider range of genomic damage involved in many adverse health outcomes have been recognized. In addition, a paradigm shift in applied genetic toxicology is moving the field toward a more quantitative dose-response analysis and point-of-departure (PoD) determination with a focus on risks to exposed humans. This is directing emphasis on genomic damage that is likely to induce changes associated with a variety of adverse health outcomes. This paradigm shift is moving the testing emphasis for genetic damage from a hazard identification only evaluation to a more comprehensive risk assessment approach that provides more insightful information for decision makers regarding the potential risk of genetic damage to exposed humans. To enable this broader context for examining genetic damage, a next generation testing strategy needs to take into account a broader, more flexible approach to testing, and ultimately modeling, of genomic damage as it relates to human exposure. This is consistent with the larger risk assessment context being used in regulatory decision making. As presented here, this flexible approach for examining genomic damage focuses on testing for relevant genomic effects that can be, as best as possible, associated with an adverse health effect. The most desired linkage for risk to humans would be changes in loci associated with human diseases, whether in somatic or germ cells. The outline of a flexible approach and associated considerations are presented in a series of nine steps, some of which can occur in parallel, which was developed through a collaborative effort by leading genetic toxicologists from academia, government, and industry through the International Life Sciences Institute (ILSI) Health and Environmental Sciences Institute (HESI) Genetic Toxicology Technical Committee (GTTC). The ultimate goal is to provide quantitative data to model the potential risk levels of substances, which induce genomic damage contributing to human adverse health outcomes. Any good risk assessment begins with asking the appropriate risk management questions in a planning and scoping effort. This step sets up the problem to be addressed (e.g., broadly, does genomic damage need to be addressed, and if so, how to proceed). The next two steps assemble what is known about the problem by building a knowledge base about the substance of concern and developing a rational biological argument for why testing for genomic damage is needed or not. By focusing on the risk management problem and potential genomic damage of concern, the next step of assay(s) selection takes place. The work-up of the problem during the earlier steps provides the insight to which assays would most likely produce the most meaningful data. This discussion does not detail the wide range of genomic damage tests available, but points to types of testing systems that can be very useful. Once the assays are performed and analyzed, the relevant data sets are selected for modeling potential risk. From this point on, the data are evaluated and modeled as they are for any other toxicology endpoint. Any observed genomic damage/effects (or genetic event(s)) can be modeled via a dose-response analysis and determination of an estimated PoD. When a quantitative risk analysis is needed for decision making, a parallel exposure assessment effort is performed (exposure assessment is not detailed here as this is not the focus of this discussion; guidelines for this assessment exist elsewhere). Then the PoD for genomic damage is used with the exposure information to develop risk estimations (e.g., using reference dose (RfD), margin of exposure (MOE) approaches) in a risk characterization and presented to risk managers for informing decision making. This approach is applicable now for incorporating genomic damage results into the decision-making process for assessing potential adverse outcomes in chemically exposed humans and is consistent with the ILSI HESI Risk Assessment in the 21st Century (RISK21) roadmap. This applies to any substance to which humans are exposed, including pharmaceuticals, agricultural products, food additives, and other chemicals. It is time for regulatory bodies to incorporate the broader knowledge and insights provided by genomic damage results into the assessments of risk to more fully understand the potential of adverse outcomes in chemically exposed humans, thus improving the assessment of risk due to genomic damage. The historical use of genomic damage data as a yes/no gateway for possible cancer risk has been too narrowly focused in risk assessment. The recent advances in assaying for and understanding genomic damage, including eventually epigenetic alterations, obviously add a greater wealth of information for determining potential risk to humans. Regulatory bodies need to embrace this paradigm shift from hazard identification to quantitative analysis and to incorporate the wider range of genomic damage in their assessments of risk to humans. The quantitative analyses and methodologies discussed here can be readily applied to genomic damage testing results now. Indeed, with the passage of the recent update to the Toxic Substances Control Act (TSCA) in the US, the new generation testing strategy for genomic damage described here provides a regulatory agency (here the US Environmental Protection Agency (EPA), but suitable for others) a golden opportunity to reexamine the way it addresses risk-based genomic damage testing (including hazard identification and exposure). Environ. Mol. Mutagen. 58:264-283, 2017. © 2016 The Authors. Environmental and Molecular Mutagenesis Published by Wiley Periodicals, Inc.
Assuntos
Genômica/métodos , Testes de Mutagenicidade/tendências , Animais , Saúde Ambiental , Humanos , Modelos Teóricos , Testes de Mutagenicidade/normas , Mutagênicos/toxicidade , Medição de RiscoRESUMO
From a risk assessment perspective, DNA-reactive agents are conventionally assumed to have genotoxic risks at all exposure levels, thus applying a linear extrapolation for low-dose responses. New approaches discussed here, including more diverse and sensitive methods for assessing DNA damage and DNA repair, strongly support the existence of measurable regions where genotoxic responses with increasing doses are insignificant relative to control. Model monofunctional alkylating agents have in vitro and in vivo datasets amenable to determination of points of departure (PoDs) for genotoxic effects. A session at the 2013 Society of Toxicology meeting provided an opportunity to survey the progress in understanding the biological basis of empirically-observed PoDs for DNA alkylating agents. Together with the literature published since, this review discusses cellular pathways activated by endogenous and exogenous alkylation DNA damage. Cells have evolved conserved processes that monitor and counteract a spontaneous steady-state level of DNA damage. The ubiquitous network of DNA repair pathways serves as the first line of defense for clearing of the DNA damage and preventing mutation. Other biological pathways discussed here that are activated by genotoxic stress include post-translational activation of cell cycle networks and transcriptional networks for apoptosis/cell death. The interactions of various DNA repair and DNA damage response pathways provide biological bases for the observed PoD behaviors seen with genotoxic compounds. Thus, after formation of DNA adducts, the activation of cellular pathways can lead to the avoidance of a mutagenic outcome. The understanding of the cellular mechanisms acting within the low-dose region will serve to better characterize risks from exposures to DNA-reactive agents at environmentally-relevant concentrations.
Assuntos
Alquilantes/toxicidade , Adutos de DNA/genética , Dano ao DNA/genética , Reparo do DNA/genética , Alquilação/genética , Apoptose/genética , Adutos de DNA/efeitos dos fármacos , Relação Dose-Resposta a Droga , Humanos , Testes de Mutagenicidade/métodosRESUMO
Single point estimates of human health hazard/toxicity values such as a reference dose (RfD) are generally used in chemical hazard and risk assessment programs for assessing potential risks associated with site- or use-specific exposures. The resulting point estimates are often used by risk managers for regulatory decision-making, including standard setting, determination of emission controls, and mitigation of exposures to chemical substances. Risk managers, as well as stakeholders (interested and affected parties), often have limited information regarding assumptions and uncertainty factors in numerical estimates of both hazards and risks. Further, the use of different approaches for addressing uncertainty, which vary in transparency, can lead to a lack of confidence in the scientific underpinning of regulatory decision-making. The overarching goal of this paper, which was developed from an invited participant workshop, is to offer five approaches for presenting toxicity values in a transparent manner in order to improve the understanding, consideration, and informed use of uncertainty by risk assessors, risk managers, and stakeholders. The five approaches for improving the presentation and communication of uncertainty are described using U.S. Environmental Protection Agency's (EPA's) Integrated Risk Information System (IRIS) as a case study. These approaches will ensure transparency in the documentation, development, and use of toxicity values at EPA, the Agency for Toxic Substances and Disease Registry (ATSDR), and other similar assessment programs in the public and private sector. Further empirical testing will help to inform the approaches that will work best for specific audiences and situations.
Assuntos
Tomada de Decisões , Substâncias Perigosas/toxicidade , Serviços de Informação/organização & administração , United States Environmental Protection Agency , Humanos , Serviços de Informação/estatística & dados numéricos , Medição de Risco , Incerteza , Estados UnidosRESUMO
The nature of the dose-response relationship for various in vivo endpoints of exposure and effect were investigated using the alkylating agents, methyl methanesulfonate (MMS) and methylnitrosourea (MNU). Six male F344 rats/group were dosed orally with 0, 0.5, 1, 5, 25 or 50mg/kg bw/day (mkd) of MMS, or 0, 0.01, 0.1, 1, 5, 10, 25 or 50 mkd of MNU, for 4 consecutive days and sacrificed 24h after the last dose. The dose-responses for multiple biomarkers of exposure and genotoxic effect were investigated. In MMS-treated rats, the hemoglobin adduct level, a systemic exposure biomarker, increased linearly with dose (r (2) = 0.9990, P < 0.05), indicating the systemic availability of MMS; however, the N7MeG DNA adduct, a target exposure biomarker, exhibited a non-linear dose-response in blood and liver tissues. Blood reticulocyte micronuclei (MN), a genotoxic effect biomarker, exhibited a clear no-observed-genotoxic-effect-level (NOGEL) of 5 mkd as a point of departure (PoD) for MMS. Two separate dose-response models, the Lutz and Lutz model and the stepwise approach using PROC REG both supported a bilinear/threshold dose-response for MN induction. Liver gene expression, a mechanistic endpoint, also exhibited a bilinear dose-response. Similarly, in MNU-treated rats, hepatic DNA adducts, gene expression changes and MN all exhibited clear PoDs, with a NOGEL of 1 mkd for MN induction, although dose-response modeling of the MNU-induced MN data showed a better statistical fit for a linear dose-response. In summary, these results provide in vivo data that support the existence of clear non-linear dose-responses for a number of biologically significant events along the pathway for genotoxicity induced by DNA-reactive agents.
Assuntos
Adutos de DNA , Fígado/efeitos dos fármacos , Metanossulfonato de Metila/toxicidade , Metilnitrosoureia/toxicidade , Reticulócitos/efeitos dos fármacos , Alquilantes/toxicidade , Animais , Biomarcadores , DNA/efeitos dos fármacos , DNA/metabolismo , Relação Dose-Resposta a Droga , Hemoglobinas/efeitos dos fármacos , Fígado/metabolismo , Masculino , Modelos Biológicos , Mutagênicos/toxicidade , Especificidade de Órgãos , Ratos , Ratos Endogâmicos F344 , Reticulócitos/metabolismoRESUMO
This study investigated the time- and concentration-dependent effects of inhaled ethylene on eosinophilic rhinitis and nasal epithelial remodeling in Fisher 344 rats exposed to 0, 10, 50, 300, or 10,000 ppm ethylene, 6 h/day, 5 days/week for up to 4 weeks. Morphometric quantitation of eosinophilic inflammation and mucous cell metaplasia/hyperplasia (MCM) and nasal mucosal gene expression were evaluated at anatomic sites previously shown to undergo ethylene-induced epithelial remodeling. Serum levels of total IgE, IgG1 and IgG2a were measured to determine if ethylene exposure increased the expression of Th2-associated (IgE and IgG1) relative to Th1-associated (IgG2a) antibody isotypes. Rats exposed to 0 or 10,000 ppm for 1, 3, 5, 10, or 20 days were analyzed to assess the temporal pattern of ethylene-induced alterations in nasal epithelial cell proliferation, morphology and gene expression. Rats exposed to 0, 10, 50, 300, and 10,000 ppm ethylene for 20 days were analyzed to assess concentration-dependent effects on lesion development. Additional rats exposed 4 weeks to 0, 300, or 10,000 ppm ethylene were held for 13 weeks post-exposure to examine the persistence of ethylene-induced mucosal alterations. The data indicate that cell death and reparative cell proliferation were not a part of the pathogenesis of ethylene-induced nasal lesions. Enhanced gene expression of Th2 cytokines (e.g., IL-5, IL-13) and chitinase (YM1/2) in the nasal mucosa was much greater than that of Th1 cytokines (e.g., IFNγ) after ethylene exposure. A significant increase in MCM was measured after 5 days of exposure to 10,000 ppm ethylene and after 20 days of exposure 10 ppm ethylene. Ethylene-induced MCM was reversible after cessation of exposure. No increase in total serum IgE, IgG1 or IgG2a was measured in any ethylene-exposed group. These data do not support involvement of an immune-mediated allergic mechanism in the pathogenesis of ethylene-induced nasal lesions in rats. Repeated inhalation of ethylene can induce a local Th2-mediated response in the nasal mucosa of rats, however the mechanisms which induce nasal inflammatory and epithelial responses are yet to be determined.
Assuntos
Eosinófilos/efeitos dos fármacos , Etilenos/efeitos adversos , Exposição por Inalação/efeitos adversos , Mucosa Nasal/efeitos dos fármacos , Mucosa Respiratória/efeitos dos fármacos , Rinite/induzido quimicamente , Animais , Morte Celular/efeitos dos fármacos , Proliferação de Células/efeitos dos fármacos , Eosinófilos/metabolismo , Feminino , Expressão Gênica/efeitos dos fármacos , Hiperplasia/induzido quimicamente , Hiperplasia/metabolismo , Imunoglobulinas/metabolismo , Inflamação/induzido quimicamente , Inflamação/metabolismo , Metaplasia/induzido quimicamente , Metaplasia/metabolismo , Mucosa Nasal/metabolismo , Ratos , Ratos Endogâmicos F344 , Mucosa Respiratória/metabolismo , Rinite/metabolismoRESUMO
Dietary exposure to pronamide resulted in higher incidences of Leydig cell tumors (LCT) at 1000ppm in a 2-year cancer bioassay, but there were no testes effects at 40 or 200ppm, and no testes effects at 12-months at any concentration. A 90-day mode-of-action (MoA) study was conducted at concentrations of 0, 200, 1000 and 2000ppm. Standard parameters and stereological and proliferation analyses of LCs, targeted testis and liver gene expression, in vitro metabolism of testosterone by liver microsomes, and quantification of serum hormones and testosterone metabolites were evaluated. Increased testosterone metabolism due to increases in hepatic microsomal activity, alterations in serum hormone levels, and other data suggest that LCTs were mediated through a perturbation of the HPG-axis. Data suggest that this occurs after a threshold of exposure is reached, indicating a nonlinear/threshold dose-response. Pronamide-induced rat LCTs mediated by alterations to the HPG-axis have low relevance to humans due to quantitative differences in sensitivity between rats and humans to LCTs. Pronamide displayed no genotoxicity or direct endocrine effects. A margin of exposure approach for risk assessment and derivation of the chronic reference dose based on a point of departure of 200ppm is most appropriate and protective of human health.
Assuntos
Benzamidas/toxicidade , Carcinógenos/toxicidade , Herbicidas/toxicidade , Tumor de Células de Leydig/induzido quimicamente , Testosterona/metabolismo , Animais , Expressão Gênica/efeitos dos fármacos , Humanos , Tumor de Células de Leydig/metabolismo , Fígado/efeitos dos fármacos , Fígado/metabolismo , Masculino , Microssomos Hepáticos/efeitos dos fármacos , Microssomos Hepáticos/metabolismo , Ratos , Medição de Risco , Testículo/efeitos dos fármacos , Testículo/metabolismo , Testosterona/sangueRESUMO
Based on the exposure potential to humans and environment, pronamide was one of 52 chemicals on the first list evaluated under US EPA's Endocrine Disruptor Screening Program (EDSP). The purpose of EDSP is to screen chemicals for their potential to interact with estrogen-, androgen-, or thyroid-signaling pathways. A battery of 11 Tier 1 assays was completed for pronamide in accordance with EDSP test guidelines. In addition, Other Scientifically Relevant Information, which included existing data from regulatory guideline studies and published literature, was used in a weight-of-evidence (WoE) evaluation of potential endocrine activity. The WoE conclusion is that pronamide does not interact directly with estrogen, androgen, or thyroid receptors or post-receptor events. Across in vivo studies, the liver is consistently and reproducibly the target organ for pronamide's effects. Pronamide activates hepatocytic nuclear receptors (including constitutive androstane receptor), induces hepatic enzymes, produces hepatocellular hypertrophy and increases liver weights. These changes are coupled with increased metabolic activity and a subsequent increased metabolism and/or clearance of both steroid and thyroid hormones. Thus, while pronamide alters some endocrine-sensitive endpoints in EDSP Tier 1 assays, effects on liver metabolism likely explain altered hormone levels and indirect endocrine changes.
Assuntos
Benzamidas/toxicidade , Disruptores Endócrinos/toxicidade , Herbicidas/toxicidade , Fígado/efeitos dos fármacos , Animais , Hormônios Gonadais/metabolismo , Gonadotropinas Hipofisárias/metabolismo , Humanos , Fígado/metabolismo , Hormônios Tireóideos/metabolismoRESUMO
Pronamide, a selective, systemic, pre- and post-emergence herbicide, caused an increased incidence of thyroid follicular cell adenomas in a rat carcinogenicity study. Thyroid tumors, as well as liver and pituitary changes, were limited only to the high-dose group. The evidence for and against specific potential modes of action (MoAs) for rat thyroid follicular cell adenomas and their relevance to humans is discussed. Pronamide is not mutagenic and therefore, direct DNA reactivity is not relevant as a MoA. The hypothesized MoA for this effect is altered homeostasis of the hypothalamic-pituitary-thyroid (HPT) axis mediated by the induction of hepatic enzymes, including uridine diphosphate glucuronosyltransferase (UGT). Evaluation of data from a series of regulatory guideline and MoA studies aimed at identifying the causative and associated key events supported a UGT-mediated MoA in the development of thyroid follicular tumors. This MoA for pronamide-induced thyroid tumors in rats, which involves increased thyroid hormone metabolism/clearance, altered thyroid hormone homeostasis and HPT stimulation is not considered relevant to humans based on quantitative species differences, making rats markedly more sensitive than humans to thyroid perturbations.
Assuntos
Adenoma/induzido quimicamente , Benzamidas/toxicidade , Transformação Celular Neoplásica/induzido quimicamente , Herbicidas/toxicidade , Neoplasias da Glândula Tireoide/induzido quimicamente , Adenoma/metabolismo , Adenoma/patologia , Animais , Transformação Celular Neoplásica/metabolismo , Transformação Celular Neoplásica/patologia , Glucuronosiltransferase/metabolismo , Humanos , Sistema Hipotálamo-Hipofisário/efeitos dos fármacos , Sistema Hipotálamo-Hipofisário/metabolismo , Fígado/efeitos dos fármacos , Fígado/enzimologia , Ratos , Medição de Risco , Especificidade da Espécie , Glândula Tireoide/efeitos dos fármacos , Glândula Tireoide/metabolismo , Hormônios Tireóideos/metabolismo , Neoplasias da Glândula Tireoide/metabolismo , Neoplasias da Glândula Tireoide/patologia , Regulação para CimaRESUMO
The key events responsible for mouse liver tumors induced by a pesticide (viz., pronamide) were investigated in a series of studies employing molecular, biochemical, cellular, and apical endpoints. Based on these studies, it was demonstrated that the liver tumors were mediated by a mode of action (MoA) involving nuclear receptors (NRs) through the following key events: (1) CAR and PPAR-α receptor activation, (2) increased hepatocellular proliferation, eventually leading to (3) hepatocellular tumors. Specifically, gene expression analysis indicated robust, simultaneous coactivation of the CAR and PPAR-α NRs, as indicated by the induction of hepatic Cyp2b10 and Cyp4a10 transcripts, in response to dietary administration of pronamide to mice. The presence of hepatocellular hypertrophy and peroxisome proliferation was indicative of the activation of these two NRs at carcinogenic dose levels. Demonstrated induction of Cyp2b10 gene and protein, however, was not accompanied by enhancement of the corresponding enzyme activity (7-pentoxyresorufin-O-dealkylase (PROD)), suggesting that pronamide administration resulted in mechanism-based (suicide) inhibition of the enzyme in vivo. This was confirmed with an in vitro assay for suicide inhibition, where pronamide and/or its metabolites irreversibly inhibited Cyp2b10-mediated PROD activity. Analysis of hepatocellular proliferation via BrdU incorporation indicated a clear dose- and duration-related induction of S-phase DNA synthesis only in animals treated at and above the carcinogenic dose level. The available MoA data were evaluated for weight-of-evidence based upon the Bradford Hill criteria, followed by a human relevance framework. The conclusion from this evaluation is that pronamide-induced mouse liver tumors occur via an NR-mediated MoA involving CAR and PPAR-α activation and this MoA is not relevant to humans based on qualitative/quantitative differences between mice and humans.
Assuntos
Benzamidas/toxicidade , Expressão Gênica/efeitos dos fármacos , Herbicidas/toxicidade , Neoplasias Hepáticas Experimentais/induzido quimicamente , Fígado/efeitos dos fármacos , Receptores Citoplasmáticos e Nucleares/metabolismo , Animais , Hidrocarboneto de Aril Hidroxilases/genética , Proliferação de Células/efeitos dos fármacos , Receptor Constitutivo de Androstano , Sistema Enzimático do Citocromo P-450/genética , Família 2 do Citocromo P450 , Relação Dose-Resposta a Droga , Feminino , Humanos , Fígado/enzimologia , Fígado/metabolismo , Fígado/ultraestrutura , Neoplasias Hepáticas Experimentais/genética , Neoplasias Hepáticas Experimentais/metabolismo , Neoplasias Hepáticas Experimentais/patologia , Masculino , Camundongos Endogâmicos , PPAR alfa/genética , PPAR alfa/metabolismo , Receptor de Pregnano X , Receptores Citoplasmáticos e Nucleares/genética , Receptores de Esteroides/genética , Receptores de Esteroides/metabolismo , Especificidade da Espécie , Esteroide Hidroxilases/genética , Fatores de TempoRESUMO
The framework analysis previously presented for using DNA adduct information in the risk assessment of chemical carcinogens was applied in a series of case studies which place the adduct information into context with the key events in carcinogenesis to determine whether they could be used to support a mutagenic mode of action (MOA) for the examined chemicals. Three data-rich chemicals, aflatoxin B1 (AFB1), tamoxifen (Tam) and vinyl chloride (VCl) were selected for this exercise. These chemicals were selected because they are known human carcinogens and have different characteristics: AFB1 forms a unique adduct and human exposure is through contaminated foods; Tam is a pharmaceutical given to women so that the dose and duration of exposure are known, forms unique adducts in rodents, and has both estrogenic and genotoxic properties; and VCl, to which there is industrial exposure, forms a number of adducts that are identical to endogenous adducts found in unexposed people. All three chemicals produce liver tumors in rats. AFB1 and VCl also produce liver tumors in humans, but Tam induces human uterine tumors, only. To support a mutagenic MOA, the chemical-induced adducts must be characterized, shown to be pro-mutagenic, be present in the tumor target tissue, and produce mutations of the class found in the tumor. The adducts formed by AFB1 and VCl support a mutagenic MOA for their carcinogenicity. However, the data available for Tam shows a mutagenic MOA for liver tumors in rats, but its carcinogenicity in humans is most likely via a different MOA.