Lake AD, Wood CE, Bhat VS, Carswell G, Chorley B, Simmons JE, McQueen CA, Hester SD. Molecular thresholds of early key events in liver tumorigenesis: Phthalate case study. Society of Toxicology, 2015.
Short-term changes in molecular profiles are a central component of strategies to model health effects of environmental chemicals such as phthalates, for which there is widespread human exposure and limited ability to predict long-term outcomes. In this study, 4 different doses of 3 phthalates with varying degrees of carcinogenicity, di(2-ethylhexyl) phthalate (DEHP), di-n-octyl phthalate (DNOP), and n-butyl-benzyl phthalate (BBP), were given to male B6C3F1 mice (n=10/group) in feed for 7 days (d). The goal of this study was to evaluate phthalates using early dose and effect thresholds for tumorigenic responses at 2 years (yr) with 7 d transcriptomic and apical key events. All phthalates increased gene expression at 7 d for peroxisome proliferator-activated receptor alpha (PPARα) targets, including Cyp4a genes and Acot1, while Pdk4 was only elevated in DEHP groups. Ki-67 labeling index (LI) for proliferation at 7 d was significantly increased in DEHP livers only. Fold-change thresholds for 7 d key events at doses corresponding to liver tumor outcomes at 2 yr were estimated for all phthalates. mRNA thresholds ranged from 3.2x (Pdk4) to 62x (Acot1), while proliferation thresholds (Ki-67 LI) were estimated at 2.0x. Unlike apical benchmark doses (BMDAs) for traditional endpoints (liver weight, proliferation), transcriptional BMDs (BMDT) stratified phthalates according to tumorigenic potency at 7 d. BMDT values for Acot1 as a measure of potency for PPARα activation were 29, 370, and 676 mg/kg-d for DEHP, DNOP, and BBP, respectively, compared to 2 yr BMDA estimates of 35 mg/kg-d for DEHP and 431 mg/kg-d for DNOP liver tumors. This case study reveals quantitative dose and effect threshold estimates for early receptor-mediated events that predict tumor outcomes in chronic studies. Views do not reflect EPA policy.