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A Tox21 Approach to Altered Epigenetic Landscapes: Assessing Epigenetic Toxicity Pathways Leading to Altered Gene Expression and Oncogenic Transformation In Vitro. Int J Mol Sci 2017; 18:ijms18061179. [PMID: 28587163 PMCID: PMC5486002 DOI: 10.3390/ijms18061179] [Citation(s) in RCA: 28] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/12/2017] [Revised: 05/19/2017] [Accepted: 05/22/2017] [Indexed: 02/07/2023] Open
Abstract
An emerging vision for toxicity testing in the 21st century foresees in vitro assays assuming the leading role in testing for chemical hazards, including testing for carcinogenicity. Toxicity will be determined by monitoring key steps in functionally validated molecular pathways, using tests designed to reveal chemically-induced perturbations that lead to adverse phenotypic endpoints in cultured human cells. Risk assessments would subsequently be derived from the causal in vitro endpoints and concentration vs. effect data extrapolated to human in vivo concentrations. Much direct experimental evidence now shows that disruption of epigenetic processes by chemicals is a carcinogenic mode of action that leads to altered gene functions playing causal roles in cancer initiation and progression. In assessing chemical safety, it would therefore be advantageous to consider an emerging class of carcinogens, the epigenotoxicants, with the ability to change chromatin and/or DNA marks by direct or indirect effects on the activities of enzymes (writers, erasers/editors, remodelers and readers) that convey the epigenetic information. Evidence is reviewed supporting a strategy for in vitro hazard identification of carcinogens that induce toxicity through disturbance of functional epigenetic pathways in human somatic cells, leading to inactivated tumour suppressor genes and carcinogenesis. In the context of human cell transformation models, these in vitro pathway measurements ensure high biological relevance to the apical endpoint of cancer. Four causal mechanisms participating in pathways to persistent epigenetic gene silencing were considered: covalent histone modification, nucleosome remodeling, non-coding RNA interaction and DNA methylation. Within these four interacting mechanisms, 25 epigenetic toxicity pathway components (SET1, MLL1, KDM5, G9A, SUV39H1, SETDB1, EZH2, JMJD3, CBX7, CBX8, BMI, SUZ12, HP1, MPP8, DNMT1, DNMT3A, DNMT3B, TET1, MeCP2, SETDB2, BAZ2A, UHRF1, CTCF, HOTAIR and ANRIL) were found to have experimental evidence showing that functional perturbations played “driver” roles in human cellular transformation. Measurement of epigenotoxicants presents challenges for short-term carcinogenicity testing, especially in the high-throughput modes emphasized in the Tox21 chemicals testing approach. There is need to develop and validate in vitro tests to detect both, locus-specific, and genome-wide, epigenetic alterations with causal links to oncogenic cellular phenotypes. Some recent examples of cell-based high throughput chemical screening assays are presented that have been applied or have shown potential for application to epigenetic endpoints.
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Peng H, Wang H, Xue P, Hou Y, Dong J, Zhou T, Qu W, Peng S, Li J, Carmichael PL, Nelson B, Clewell R, Zhang Q, Andersen ME, Pi J. Suppression of NRF2-ARE activity sensitizes chemotherapeutic agent-induced cytotoxicity in human acute monocytic leukemia cells. Toxicol Appl Pharmacol 2015; 292:1-7. [PMID: 26708503 DOI: 10.1016/j.taap.2015.12.008] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/28/2015] [Revised: 12/14/2015] [Accepted: 12/16/2015] [Indexed: 10/22/2022]
Abstract
Nuclear factor erythroid 2-related factor 2 (NRF2), a master regulator of the antioxidant response element (ARE)-dependent transcription, plays a pivotal role in chemical detoxification in normal and tumor cells. Consistent with previous findings that NRF2-ARE contributes to chemotherapeutic resistance of cancer cells, we found that stable knockdown of NRF2 by lentiviral shRNA in human acute monocytic leukemia (AML) THP-1 cells enhanced the cytotoxicity of several chemotherapeutic agents, including arsenic trioxide (As2O3), etoposide and doxorubicin. Using an ARE-luciferase reporter expressed in several human and mouse cells, we identified a set of compounds, including isonicotinic acid amides, isoniazid and ethionamide, that inhibited NRF2-ARE activity. Treatment of THP-1 cells with ethionamide, for instance, significantly reduced mRNA expression of multiple ARE-driven genes under either basal or As2O3-challenged conditions. As determined by cell viability and cell cycle, suppression of NRF2-ARE by ethionamide also significantly enhanced susceptibility of THP-1 and U937 cells to As2O3-induced cytotoxicity. In THP-1 cells, the sensitizing effect of ethionamide on As2O3-induced cytotoxicity was highly dependent on NRF2. To our knowledge, the present study is the first to demonstrate that ethionamide suppresses NRF2-ARE signaling and disrupts the transcriptional network of the antioxidant response in AML cells, leading to sensitization to chemotherapeutic agents.
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Affiliation(s)
- Hui Peng
- The Hamner Institutes for Health Sciences, 6 Davis Drive, Research Triangle Park, NC, USA; Institute of Disease Control and Prevention, Academy of Military Medical Sciences, Beijing, China
| | - Huihui Wang
- School of Public Health, China Medical University, 77 Puhe Road, Shenyang North New Area, Shenyang, China
| | - Peng Xue
- The Hamner Institutes for Health Sciences, 6 Davis Drive, Research Triangle Park, NC, USA; Key Laboratory of the Public Health Safety, Ministry of Education, School of Public Health, Fudan University, Shanghai, China
| | - Yongyong Hou
- School of Public Health, China Medical University, 77 Puhe Road, Shenyang North New Area, Shenyang, China
| | - Jian Dong
- The Hamner Institutes for Health Sciences, 6 Davis Drive, Research Triangle Park, NC, USA; Institute of Biology and Medicine, Wuhan University of Science and Technology, Wuhan, China
| | - Tong Zhou
- The Hamner Institutes for Health Sciences, 6 Davis Drive, Research Triangle Park, NC, USA
| | - Weidong Qu
- Key Laboratory of the Public Health Safety, Ministry of Education, School of Public Health, Fudan University, Shanghai, China
| | - Shuangqing Peng
- Institute of Disease Control and Prevention, Academy of Military Medical Sciences, Beijing, China
| | - Jin Li
- Unilever, Safety & Environmental Assurance Centre, Colworth Science Park, Sharnbrook, Bedfordshire MK44 1LQ, UK
| | - Paul L Carmichael
- Unilever, Safety & Environmental Assurance Centre, Colworth Science Park, Sharnbrook, Bedfordshire MK44 1LQ, UK
| | - Bud Nelson
- The Hamner Institutes for Health Sciences, 6 Davis Drive, Research Triangle Park, NC, USA
| | - Rebecca Clewell
- The Hamner Institutes for Health Sciences, 6 Davis Drive, Research Triangle Park, NC, USA
| | - Qiang Zhang
- The Hamner Institutes for Health Sciences, 6 Davis Drive, Research Triangle Park, NC, USA
| | - Melvin E Andersen
- The Hamner Institutes for Health Sciences, 6 Davis Drive, Research Triangle Park, NC, USA
| | - Jingbo Pi
- School of Public Health, China Medical University, 77 Puhe Road, Shenyang North New Area, Shenyang, China; The Hamner Institutes for Health Sciences, 6 Davis Drive, Research Triangle Park, NC, USA.
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