From Classical Toxicology to Tox21: Some Critical Conceptual and Technological Advances in the Molecular Understanding of the Toxic Response Beginning From the Last Quarter of the 20th Century

Abstract

Toxicology has made steady advances over the last 60+ years in understanding the mechanisms of toxicity at an increasingly finer level of cellular organization. Traditionally, toxicological studies have used animal models. However, the general adoption of the principles of 3R (Replace, Reduce, Refine) provided the impetus for the development of in vitro models in toxicity testing. The present commentary is an attempt to briefly discuss the transformation in toxicology that began around 1980. Many genes important in cellular protection and metabolism of toxicants were cloned and characterized in the 80s, and gene expression studies became feasible, too. The development of transgenic and knockout mice provided valuable animal models to investigate the role of specific genes in producing toxic effects of chemicals or protecting the organism from the toxic effects of chemicals. Further developments in toxicology came from the incorporation of the tools of "omics" (genomics, proteomics, metabolomics, interactomics), epigenetics, systems biology, computational biology, and in vitro biology. Collectively, the advances in toxicology made during the last 30-40 years are expected to provide more innovative and efficient approaches to risk assessment. A goal of experimental toxicology going forward is to reduce animal use and yet be able to conduct appropriate risk assessments and make sound regulatory decisions using alternative methods of toxicity testing. In that respect, Tox21 has provided a big picture framework for the future. Currently, regulatory decisions involving drugs, biologics, food additives, and similar compounds still utilize data from animal testing and human clinical trials. In contrast, the prioritization of environmental chemicals for further study can be made using in vitro screening and computational tools.

Keywords: Tox21; adverse outcome pathway; in vitro; nuclear receptors; toxicology; transcription factors.

Figure 1

Structure, function and regulation of nuclear receptor superfamily of transcription factors. A, The structure of a typical nuclear receptor transcription factor. Structural divergence of the LBD determines the species-specific differences in the response to different ligands. B, nuclear receptor function. On entering the cell, the ligand (L) binds to the nuclear receptor, forming the NR-L complex. The NR-L complex translocates to the nucleus and heterodimerizes with the RXR. Formation of the heterodimer results in the dissociation of the transcriptional corepressor complex and recruitment of the coactivator complex, which enhances transcription. C, The organization of the direct repeat, inverted repeat, and everted repeat binding motifs. In this example the number of intervening bases shown is 4 (DR-4, IR-4, and ER-4); it usually varies between 1 and 6. The DR usually involves one strand whereas the IR and ER involve both strands.

Figure 2

Potential stages in the development of toxicity following chemical exposure. Toxicity involves the delivery of the toxicant to the target tissue where an interaction with its target molecule(s) may result in cellular dysfunction, which is primary mediated by aberrant cell signaling and inadequate cell maintenance. The process may involve the toxicant itself or its metabolism product, which could be more reactive than the parent compound. Damaged cells could die off or attempt to repair, recover, and restore homeostasis. Cells that attempt to repair and recover tend to undergo apoptosis if repair and recovery fails. The surviving damaged cells could trigger the development of an adverse outcome, including carcinogenesis.

Figure 3

A, The difference between TP and AOP is depicted. A TP focuses on initiating events and proximal cellular responses that can be measured in vitro. In contrast, an AOP is defined as a conceptual construct that portrays existing knowledge concerning the linkage between a direct MIE and an adverse outcome at a biological level of organization relevant to risk assessment. The steps (events) that lie between the MIE and the adverse outcome constitute the KEs, and their causal links are called the KERs. B, The exposure-to-outcome-continuum model discussed in the recent NAS report. The interaction with biological molecules step is similar to the MIE step in Ankley’s AOP pathway model. Elaboration of the exposure events as external exposure, internal exposure, and target exposure reflects the recent developments in the exposure science.