title: Dose and Effect Thresholds for Early Key Events in a PPARα-Mediated Mode of Action.
abstract: Current strategies for predicting adverse health outcomes of environmental chemicals are centered on early key events in toxicity pathways. However, quantitative relationships between early molecular changes in a given pathway and later health effects are often poorly defined. The goal of this study was to evaluate short-term key event indicators using qualitative and quantitative methods in an established pathway of mouse liver tumorigenesis mediated by peroxisome proliferator-activated receptor alpha (PPARα). Male B6C3F1 mice were exposed for 7 days to di (2-ethylhexyl) phthalate (DEHP), di-n-octyl phthalate (DNOP), and n-butyl benzyl phthalate (BBP), which vary in PPARα activity and liver tumorigenicity. Each phthalate increased expression of select PPARα target genes at 7 days, while only DEHP significantly increased liver cell proliferation labeling index (LI). Transcriptional benchmark dose (BMDT) estimates for dose-related genomic markers stratified phthalates according to hypothetical tumorigenic potencies, unlike BMDs for non-genomic endpoints (relative liver weights or proliferation). The 7-day BMDT values for Acot1 as a surrogate measure for PPARα activation were 29, 370, and 676 mg/kg/day for DEHP, DNOP, and BBP, respectively, distinguishing DEHP (liver tumor BMD of 35 mg/kg/day) from non-tumorigenic DNOP and BBP. Effect thresholds were generated using linear regression of DEHP effects at 7 days and 2-year tumor incidence values to anchor early response molecular indicators and a later phenotypic outcome. Thresholds varied widely by marker, from 2-fold (Pdk4 and proliferation LI) to 30-fold (Acot1) induction to reach hypothetical tumorigenic expression levels. These findings highlight key issues in defining thresholds for biological adversity based on molecular changes.
authors.affiliation: *Curriculum in Toxicology, University of North Carolina, Chapel Hill, North Carolina 27599; Oak Ridge Institute for Science and Education (ORISE) participant at the National Health and Environmental Effects Research Laboratory (NHEERL), Office of Research and Development (ORD), U.S. Environmental Protection Agency (U.S. EPA), Research Triangle Park, North Carolina 27711; Integrated Systems Toxicology Division, NHEERL, ORD, U.S. EPA, Research Triangle Park, North Carolina 27711;
authors.forename: April D
authors.initials: AD
authors.lastname: Lake
authors.affiliation: Integrated Systems Toxicology Division, NHEERL, ORD, U.S. EPA, Research Triangle Park, North Carolina 27711;
authors.forename: Charles E
authors.initials: CE
authors.lastname: Wood
authors.affiliation: NSF International, Ann Arbor, Michigan 48105;
authors.forename: Virunya S
authors.initials: VS
authors.lastname: Bhat
authors.affiliation: Integrated Systems Toxicology Division, NHEERL, ORD, U.S. EPA, Research Triangle Park, North Carolina 27711;
authors.forename: Brian N
authors.initials: BN
authors.lastname: Chorley
authors.affiliation: Integrated Systems Toxicology Division, NHEERL, ORD, U.S. EPA, Research Triangle Park, North Carolina 27711;
authors.forename: Gleta K
authors.initials: GK
authors.lastname: Carswell
authors.affiliation: Integrated Systems Toxicology Division, NHEERL, ORD, U.S. EPA, Research Triangle Park, North Carolina 27711;
authors.forename: Yusupha M
authors.initials: YM
authors.lastname: Sey
authors.affiliation: Integrated Systems Toxicology Division, NHEERL, ORD, U.S. EPA, Research Triangle Park, North Carolina 27711;
authors.forename: Elaina M
authors.initials: EM
authors.lastname: Kenyon
authors.affiliation: Integrated Systems Toxicology Division, NHEERL, ORD, U.S. EPA, Research Triangle Park, North Carolina 27711;
authors.forename: Beth
authors.initials: B
authors.lastname: Padnos
authors.affiliation: Integrated Systems Toxicology Division, NHEERL, ORD, U.S. EPA, Research Triangle Park, North Carolina 27711;
authors.forename: Tanya M
authors.initials: TM
authors.lastname: Moore
authors.affiliation: Integrated Systems Toxicology Division, NHEERL, ORD, U.S. EPA, Research Triangle Park, North Carolina 27711;
authors.forename: Alan H
authors.initials: AH
authors.lastname: Tennant
authors.affiliation: Toxicology Assessment Division, NHEERL, ORD; and.
authors.forename: Judith E
authors.initials: JE
authors.lastname: Schmid
authors.affiliation: Office of the Associate Director for Health, NHEERL, ORD, U.S. EPA, Research Triangle Park, North Carolina 27711.
authors.forename: Barbara Jane
authors.initials: BJ
authors.lastname: George
authors.affiliation: Integrated Systems Toxicology Division, NHEERL, ORD, U.S. EPA, Research Triangle Park, North Carolina 27711;
authors.forename: David G
authors.initials: DG
authors.lastname: Ross
authors.affiliation: Integrated Systems Toxicology Division, NHEERL, ORD, U.S. EPA, Research Triangle Park, North Carolina 27711;
authors.forename: Michael F
authors.initials: MF
authors.lastname: Hughes
authors.affiliation: Integrated Systems Toxicology Division, NHEERL, ORD, U.S. EPA, Research Triangle Park, North Carolina 27711;
authors.forename: J Christopher
authors.initials: JC
authors.lastname: Corton
authors.affiliation: Integrated Systems Toxicology Division, NHEERL, ORD, U.S. EPA, Research Triangle Park, North Carolina 27711;
authors.forename: Jane Ellen
authors.initials: JE
authors.lastname: Simmons
authors.affiliation: Integrated Systems Toxicology Division, NHEERL, ORD, U.S. EPA, Research Triangle Park, North Carolina 27711;
authors.forename: Charlene A
authors.initials: CA
authors.lastname: McQueen
authors.affiliation: Integrated Systems Toxicology Division, NHEERL, ORD, U.S. EPA, Research Triangle Park, North Carolina 27711; hester.susan@epa.gov.
authors.forename: Susan D
authors.initials: SD
authors.lastname: Hester
mesh_terms: D000818:Animals; D019985:Benchmarking; D001835:Body Weight / Q000187:drug effects; D049109:Cell Proliferation; D004051:Diethylhexyl Phthalate / Q000633:toxicity; D004305:Dose-Response Relationship, Drug; D016014:Linear Models; D008099:Liver / Q000378:metabolism / Q000473:pathology; D008114:Liver Neoplasms, Experimental / Q000139:chemically induced*; D008297:Male; D051379:Mice; D018384:Oxidative Stress; D047493:PPAR alpha / Q000502:physiology*; D010795:Phthalic Acids / Q000633:toxicity; D016133:Polymerase Chain Reaction
chemical_list: D047493:PPAR alpha; D010795:Phthalic Acids; D004051:Diethylhexyl Phthalate; C027561:butylbenzyl phthalate
keywords: adverse outcome pathway; benchmark dose; liver carcinogenesis; mode of action; peroxisome proliferator-activated receptor-alpha; phthalate