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

BACKGROUND: In ToxCast (TM) 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 (TM) assays to predict biological function. OBJECTIVES: We asked whether top-scoring PPAR gamma activators identified in ToxCast (TM) Phase I were genuine PPAR gamma activators and inducers of adipogenesis. Next, we identified ToxCast (TM) 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 (TM) chemicals to modulate PPAR gamma and RXR alpha, and differentiation assays employing 3T3-L1 preadipocytes and mouse bone marrow-derived mesenchymal stem cells (mBMSCs) to evaluate the adipogenic capacity of ToxCast (TM) chemicals. RESULTS: Only 5/21 of the top scoring ToxCast (TM) PPAR gamma activators were activators in our assays, 3 were PPAR gamma antagonists, the remainder were inactive. The bona fide PPAR gamma 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 PPAR gamma, and 1 activated RXR alpha. CONCLUSIONS: ToxCast (TM) PPAR gamma and RXR alpha assays do not correlate well with laboratory measurements of PPAR gamma and RXR alpha activity. The adipogenesis ToxPi performed poorly, perhaps due to the performance of ToxCast (TM) assays. We observed a modest predictive value of ToxCast (TM) for PPAR gamma and RXR alpha activation and adipogenesis and it is likely that many obesogenic chemicals remain to be identified.