Identification of indoles as potential endogenous ligands of ERRγ and their modulation on drug binding

Estrogen-related receptor γ (ERRγ) is an orphan nuclear receptor in the ERR subfamily that plays a crucial role in regulating energy metabolism. To date, no endogenous ligand has been identified for ERRγ, posing a challenge for developing targeted therapeutics. Here, we identified that indole and skatole produced by the gut microbiota are potential endogenous ligands of ERRγ using biochemical, cellular, structural, and computational approaches. Indole and skatole increased ERRγ thermostability and directly bound to the ligand-binding domain (LBD) with a K_d of approximately 1-2 μM but had no significant effect or weak inhibitory activity on the transcriptional efficiency. However, RNA sequencing revealed that ERRγ could coregulate several lipid metabolism- and immune-related genes with indole, suggesting a role for ERRγ in the indole pathway. Interestingly, indole and skatole differentially attenuated the activities of ERRγ ligands: they both neutralized the agonistic activity of GSK4716, while indole reduced the antagonistic activity of 4-hydroxytamoxifen (4OHT) and GSK5182, and skatole affected the agonistic activity of endocrine disruptor bisphenol A (BPA). We further screened additional indole metabolites and analogs, resolved the complex structures of ERRγ-LBD with these compounds, and conducted molecular dynamics simulations to determine their binding site and elucidate their binding mechanisms. This study identified potential endogenous ligands of ERRγ, suggesting a novel link between the energy metabolism regulation and the indole pathway. Our findings highlight the need to consider endogenous ligands when designing and optimizing ERRγ-targeted drugs.