An Adverse Outcome Pathway Network for Chemically Induced Oxidative Stress Leading to (Non)genotoxic Carcinogenesis

Abstract

Nongenotoxic (NGTX) carcinogens induce cancer via other mechanisms than direct DNA damage. A recognized mode of action for NGTX carcinogens is induction of oxidative stress, a state in which the amount of oxidants in a cell exceeds its antioxidant capacity, leading to regenerative proliferation. Currently, carcinogenicity assessment of environmental chemicals primarily relies on genetic toxicity end points. Since NGTX carcinogens lack genotoxic potential, these chemicals may remain undetected in such evaluations. To enhance the predictivity of test strategies for carcinogenicity assessment, a shift toward mechanism-based approaches is required. Here, we present an adverse outcome pathway (AOP) network for chemically induced oxidative stress leading to (NGTX) carcinogenesis. To develop this AOP network, we first investigated the role of oxidative stress in the various cancer hallmarks. Next, possible mechanisms for chemical induction of oxidative stress and the biological effects of oxidative damage to macromolecules were considered. This resulted in an AOP network, of which associated uncertainties were explored. Ultimately, development of AOP networks relevant for carcinogenesis in humans will aid the transition to a mechanism-based, human relevant carcinogenicity assessment that involves a substantially lower number of laboratory animals.

Figure 1

Hallmarks of cancer. The 10 hallmarks of cancer as described by Hanahan and Weinberg, supplemented with novel insights into carcinogenesis.^,, Translucent hallmarks are not discussed in this paper. The inner circle schematically represents the signaling networks that connect the hallmarks and the tumor microenvironment.

Figure 2

Schematic representation of ROS-induced damage and biological effects in relation to carcinogenesis. ROS can interact with and damage lipids (green/left), proteins (blue/middle) and DNA (red/right). Through signaling pathways, oxidative modification of macromolecules can contribute to various cancer hallmarks and, ultimately, carcinogenesis. Inflammation also affects neighboring cells. First layer effector molecules are depicted in orange/rectangles, second layer effector molecules in purple/rounded rectangles, and third layer effector molecules in yellow/circles. ROS: reactive oxygen species; SOD: superoxide dismutase; CAT: catalase; GPX: glutathione peroxidase; PKC: protein kinase; NF-κB: factor κ-light-chain-enhancer of B cells inhibitor; IκB: Inhibitor of NF-κB; IKK: IκB kinase; PTEN: phosphatase and tensin homologue; PI3K: phosphoinositide 3 kinase; AKT: protein kinase B; PTP1B: protein tyrosine phosphatase 1B; JAK/STAT: Janus kinase/signal transducer and activator of transcription protein; MKP: MAPK phosphatase; ERK: extracellular signal-regulated kinase; TRX: thioredoxin; AKS1: apoptosis signal-regulating kinase 1; JNK: c-Jun N-terminal kinase; AP1: activator protein 1; KEAP1: Kelch like ECH associated protein 1; NRF2: nuclear factor erythroid 2-related factor 2; HDAC: histone deacetylase; DNMT: DNA methyltransferase; MBP: methyl binding protein; Bcl2: B-cell lymphoma 2; Bcl-xL: B-cell lymphoma extra-large; Bad: Bcl-2-associated agonist of cell death; Bim: Bcl-2-interacting mediator of cell death; Bax: Bcl-2-associated X protein.

Figure 3

Proposed network of AOPs for chemically induced oxidative stress leading to carcinogenesis. Chronic or prolonged activation of the molecular initiating event and subsequent key events is required for this AOP network to trigger the adverse outcome. Molecular initiating events are depicted in green, cellular effects in orange, tissue effects in bright red and the adverse outcome in dark red. Cancer hallmarks are depicted in bold. Italicized text represent associated signaling pathways. Examples of cytochrome P450 enzymes, oxidases and key proteins in protein oxidation are given in parentheses.