On the Utility of ToxCast™ and ToxPi as Methods for Identifying New Obesogens

Abstract

Background:In ToxCast™ Phase I, the U.S. EPA commissioned screening of 320 pesticides, herbicides, fungicides, and other chemicals in a series of high-throughput assays. The agency also developed a toxicological prioritization tool, ToxPi, to facilitate using ToxCast™ assays to predict biological function.Objectives:We asked whether top-scoring PPARγ activators identified in ToxCast™ Phase I were genuine PPARγ activators and inducers of adipogenesis. Next, we identified ToxCast™ assays that should predict adipogenesis, developed an adipogenesis ToxPi, and asked how well the ToxPi predicted adipogenic activity.Methods:We used transient transfection to test the ability of ToxCast™ chemicals to modulate PPARγ and RXRα, and differentiation assays employing 3T3-L1 preadipocytes and mouse bone marrow-derived mesenchymal stem cells (mBMSCs) to evaluate the adipogenic capacity of ToxCast™ chemicals.Results:Only 5/21 of the top scoring ToxCast™ PPARγ activators were activators in our assays, 3 were PPARγ antagonists, the remainder were inactive. The bona fide PPARγ activators we identified induced adipogenesis in 3T3-L1 cells and mBMSCs. Only 7 of the 17 chemicals predicted to be active by the ToxPi promoted adipogenesis, 1 inhibited adipogenesis, and 2 of the 7 predicted negatives were also adipogenic. Of these 9 adipogenic chemicals, 3 activated PPARγ, and 1 activated RXRα.Conclusions:ToxCast™ PPARγ and RXRα assays do not correlate well with laboratory measurements of PPARγ and RXRα activity. The adipogenesis ToxPi performed poorly, perhaps due to the performance of ToxCast™ assays. We observed a modest predictive value of ToxCast™ for PPARγ and RXRα activation and adipogenesis and it is likely that many obesogenic chemicals remain to be identified.Citation:Janesick AS, Dimastrogiovanni G, Vanek L, Boulos C, Chamorro-García R, Tang W, Blumberg B. 2016. On the utility of ToxCast™ and ToxPi as methods for identifying new obesogens. Environ Health Perspect 124:1214–1226; http://dx.doi.org/10.1289/ehp.1510352

Editor’s Highlight: Screening ToxCast Prioritized Chemicals forPPARGFunction in a Human Adipose-Derived Stem Cell Model of Adipogenesis

“…A recent report investigating the utility of ToxCast and ToxPi for evaluating adipogenic potential of novel chemicals identified, among others, triphenytlin, quinoxyfen, and fludioxonil; moderate and strong activators in our data series (Janesick et al , 2016). Discordance among the Janesick study results include, but were not limited to, pyraclostrobin and tebufenozide (moderate actives in the hASC assays, inactive in mPPARG reporter assay), as well as zoxamide and spirodiclofen (negative in the hASC screen, positive in mPPARG reporter assays).…”

“…Perhaps the most common cell model employed for investigating chemical perturbation of adipogenesis is the mouse 3T3-L1 cell line (Janesick et al , 2016; Pereira-Fernandes et al , 2013a); a committed adipocyte progenitor that has been invaluable in defining the cellular pathways involved in adipogenesis. An additional transgene assay in Xenopus (Punzon et al , 2013) or whole-organism zebrafish assay (Tingaud-Sequeira et al , 2011) provide additional understanding of PPARG signaling.…”

Transgenerational effects of obesogens

“…A variety of other potential obesogens have been identified that are known to act on pre‐adipocytes or MSCs to promote adipogenic differentiation . It was recently shown that TBT and other chemicals that activate RXR commit MSCs to the adipose lineage by activating RXR, but not PPARγ.…”

Obesogens: an emerging threat to public health

“…Some studies are correlative; for example, chlorinated persistent organic pollutants are associated with increased body mass index and/or type II diabetes in humans in cross-sectional epidemiological studies 46 . Other chemicals induce adipogenesis in cells or activate PPARγ, but have not been tested, in vivo 47-48 . Some EDCs have only been studied in adults while others predispose a developing fetus to subsequent obesity 49 .…”