Organophosphate Esters Bind to and Inhibit Estrogen-Related Receptor γ in Cells

Organophosphate esters (OPEs) have been reported to induce endocrine disruption effects, and several well-known nuclear receptors have been investigated as cellular targets of OPEs in their mode of action. Here, we demonstrated for the first time that an orphan nuclear receptor estrogen-related receptor gamma (ERR gamma) is another possible target of OPEs. Using the fluorescence competitive binding assays that we established, we measured the binding affinity of nine OPEs with different substitution groups, including aromatic rings, chlorinated alkyl chains, and alkyl chains. Seven of the OPEs were found to bind to ERR gamma, with tri-m-cresyl phosphate (TCrP) showing the highest binding affinity (K-d, 0.34 mu M). By using an ERR gamma-mediated luciferase reporter gene assay, we found seven OPEs showed inhibitory effects toward ERR gamma. Both the binding affinity and the inhibitory effect of the OPEs correlate positively with the hydrophobicity of their substitution groups in the following rank order: aromatic rings > chlorinated alkyl chains > alkyl chains. On the basis of molecular docking, the mechanism of the inhibitory effect of OPEs was proposed to be ligand-triggered displacement of the activation function-2 helix from the active position in the receptor. The ERR gamma pathway may provide a new mechanism for the endocrine disruption effects of OPEs.