Nongenotoxic Apoptosis Inducers Do Not Produce Misleading Positive Results in the TK6 Cell-Based GADD45a-GFP Genotoxicity Assay

The in vitro mammalian genotoxicity tests identify some carcinogens not identified by the bacterial Ames test. However, historically they have produced rather more misleading predictions of carcinogenicity than the Ames test. This liability has been reduced in pharmaceutical testing by lowering the top-testing dose and rejecting data from excessively toxic doses. It also stimulated the development of new assays with inherently higher specificity. Among these, the GADD45a-GFP assay has been recognized as a maturing technology by the International Life Sciences Institute Health and Environmental Sciences Institute In Vitro Genetic Toxicity Emerging Technologies and New Strategies workgroup and has been concluded to be suitable for inclusion in a battery of high throughput screening by the U.K. Committee on Mutagenicity of Chemicals in Food, Consumer Products and the Environment. GADD45a is induced by compounds that cause damage to or missegregation of chromosomes, and is implicated in the stimulation of repair or apoptosis where damage is overwhelming. It is therefore important to understand whether this causes a liability in the assay to produce misleading positives for nongenotoxic inducers of apoptosis. Compounds hypothesized to stimulate apoptosis in the GADD45a-GFP assay or to induce GADD45a in the absence of genotoxic stress, such as p53 activators, NF-κB and Bcl-2 inhibitors were selected. Apoptosis induction was monitored using Annexin V binding and caspase 3/7 activation assays. The majority of compounds tested were negative in the GADD45a-GFP assay. The few that generated positive data were also found positive in concurrent comet assay and/or micronucleus tests. The data presented here demonstrate that the GADD45a-GFP assay is not vulnerable to the generation of misleading positive results by apoptosis inducers.