MP6 Quantitative Structure Activity Relationships (QSARs)
Monday, 14 November 2005: 8:00 AM - 5:30 PM in Exhibit Hall

(TAP-1117-829199) In vitro trout assays for interpreting potential estrogenicity of industrial chemicals.

Tapper, M¹, Denny, J¹, Kolanczyk, R¹, Sheedy, B¹, Nelson, B², Wehinger, V², Schmieder, P¹, ¹ US EPA, NHEERL, Mid-Continent Ecology Division, Duluth, MN, USA² US EPA Student Services Contractor, Duluth, MN, USA

ABSTRACT- Two in vitro rainbow trout assays, cytosol estrogen receptor competitive binding, and vitellogenin (VTG) liver slice mRNA expression were used to interpret potential estrogenicity of industrial chemicals. This work is part of a larger project to develop in vitro approaches and QSAR models applicable to untested chemicals on EPA inventories. Over 150 chemicals have been tested in the competitive binding assay, with the focus on interpreting the relevance of low relative binding affinities (RBA) found for the majority of industrial chemicals of EPA concern; the RBA range of interest is from 0.5 to 0.000026%. Additionally, >50 chemicals were tested in the male trout liver VTG mRNA expression assay, each selected to enhance interpretation of binding curves. Chemicals chosen for confirmation in slice VTG tests included many that yielded characteristic competitive binding curves (shape analogous to E₂), non-binders (no indication of displacement) and many chemicals yielding non-characteristic curves (e.g., steep, low efficacy). Several low RBA chemicals tested to assess relevance of low RBA to ability to induce vitellogenesis are presented. The liver slice model proved to be an effective tool for determining relevance of low binding affinity, chemicals with RBA's as low as 0.0004% induced VTG expression at 50 to 100% of the maximum response to estradiol, as well as clarifying the relationship between chemical concentration and toxicity. Furthermore, the slice model was valuable for interpretation of abnormal binding curves, identifying both estrogens and non-estrogens with steep curves, and determining that binding displacement > 50% was needed to elicit VTG expression. In two instances of a chemical exhibiting <50% binding efficacy but increased VTG expression, a metabolite was found responsible for the induction of gene expression. Therefore, in addition to binding affinity, factors such as chemical uptake, toxicity and metabolism ultimately determined estrogenic potency in trout liver slices.

Key words: estrogenicity, rainbow trout, vitellogenin, industrial chemicals