Survey of ecotoxicologically-relevant reproductive endpoint coverage within the ECOTOX database across ToxCast ER agonists (SETAC)

Adipose tissue represents an important and understudied component of the endocrine system. Recent evidence suggests that endocrine-disrupting chemicals (EDCs) may be able to alter lipid development (e.g., adipogenesis) and/or the balance of lipid metabolism. The environmentally and ecologically relevant adverse outcomes associated with disrupted lipid homeostasis are not well understood, however lipid accumulation may play an important role in organism growth or the survival of juvenile/young-of-the-year fish. As part of the US EPA’s ToxCast program, a ~1100 chemical library was profiled in a high-content, multiplexed screening platform that utilized quantitative cell-based digital image analysis to quantify the effects of chemical exposure on adipogenesis. Chemicals were run in triplicate in six-point concentration response from 0.013 to 100 µM. ToxCast Data Analysis Pipeline identified 95 compounds that significantly increased adipocyte differentiation, including 27 chemicals that demonstrated activity less than 10 µM. Several compounds known to be PPARγ agonists (e.g. troglitazone, farglitazar) were active in this assay below cytotoxic concentrations. EDCs are able to interact with a wide variety of nuclear receptors including PPARs, a class of nuclear receptors known to play an important role in lipid homeostasis. Following ligand activation, PPARs heterodimerize with RXR and modulate the expression of target genes, including many that are known to stimulate adipogenesis. Systematic comparisons were conducted with all available Tox21 and ToxCast assay endpoints to determine which biological targets might be predictive of adipocyte differentiation. Several biological targets associated with fatty acid oxidation and lipid metabolism had specificities >0.85 and balanced accuracies of >0.60 including PPARγ, progesterone, and PDK4.These biological targets represent a possible mechanism for the induction of the adipocyte phenotype. Recent research suggests that phthalates can activate PPAR:RXR-dependent gene expression and promote changes in lipid homeostasis in sea bream, an economically important teleost fish. Thus, this mechanism may also be present in fish. Collectively, these results add important data to ToxCast that will help identify and prioritize possible endocrine disruptors to inform targeted testing strategies. This abstract does not necessarily represent US EPA policy.