Nrf2 activation as a key event triggered by skin sensitisers: The development of the stable KeratinoSens reporter gene assay.

The 21st century paradigm for toxicology and the adverse outcome pathway concept envisage a future toxicology largely based on mechanistic in vitro assays and relying mainly on cellular models. In the skin sensitisation field, this concept was not intuitive at the beginning. Given the high structural diversity of skin sensitising molecules, classical receptor binding as the molecular initiating event in a cell-based assay could be excluded from the start, leaving the question of how cells could sense potential skin sensitising chemicals and be able to differentiate them from non-sensitisers. When we entered this field in 2006, we realised that, in another emerging field of toxicology, detailed work on the antioxidant/electrophile sensing pathway Keap1/Nrf2/ARE was being performed. We postulated that, based on their intrinsic electrophilicity, a large structural variety of skin sensitisers would activate this pathway. This was demonstrated in a preliminary pilot study with an existing, breast cancer-derived reporter cell line. Broader confirmation of this initial hypothesis then came from a multitude of genome-wide studies, in which sensitiser-induced changes to the transcriptome were investigated. The results showed that this regulatory pathway is indeed the most common regulatory pathway activated by sensitisers at the gene expression level, and the underlying event in keratinocytes has become formalised as a Key Event in the Organisation for Economic Co-operation and Development (OECD) Adverse Outcome Pathway for sensitisation. These studies led to the development of the KeratinoSens® assay, which became the first cell-based in vitro test for skin sensitisation to be endorsed by a European Union Reference Laboratory for Alternatives to Animal Testing (EURL ECVAM) statement and an OECD Test Guideline. More recently, a number of studies have further developed this approach into 3-D skin models. Here, we review the underlying mechanism and the development of the KeratinoSens assay. We also present data on the stability of the assay over time, which is a key requirement for a cell-based biological assay to be endorsed in a regulatory context.