Editor's Highlight: Mechanistic Toxicity Tests Based on an Adverse Outcome Pathway Network for Hepatic Steatosis

Risk assessors use liver endpoints in rodent toxicology studies to assess the safety of chemical exposures. Yet, rodent endpoints may not accurately reflect human responses. For this reason and others, human-based invitro models are being developed and anchored to adverse outcome pathways to better predict adverse human health outcomes. Here, a networked adverse outcome pathway-guided selection of biology-based assays for lipid uptake, lipid efflux, fatty acid oxidation, and lipid accumulation were developed. These assays were evaluated in a metabolically competent human hepatocyte cell model (HepaRG) exposed to compounds known to cause steatosis (amiodarone, cyclosporine A, and T0901317) or activate lipid metabolism pathways (troglitazone, Wyeth-14,643, and 22(R)-hydroxycholesterol). All of the chemicals activated at least one assay, however, only T0901317 and cyclosporin A dose-dependently increased lipid accumulation. T0901317 and cyclosporin A increased fatty acid uptake, decreased lipid efflux (inferred from apolipoprotein B100 levels), and increased fatty acid synthase protein levels. Using this biologically-based evaluation of key events regulating hepatic lipid levels, we demonstrated dysregulation of compensatory pathways that normally balance hepatic lipid levels. This approach may provide biological plausibility and data needed to increase confidence in linking invitro-based measurements to chemical effects on adverse human health outcomes.