PPARγ ligands activate the ion channel TRPA1.

The endogenous peroxisome proliferator-activated receptors (PPARγ) agonist 15-deoxy-Δ12,14-PGJ2 (15d-PGJ2) stimulates sensory neurons by activating transient receptor potential A1 (TRPA1). Synthetic thiazolidinedione PPARγ agonists have been used as antidiabetic agents but have also been explored as experimental analgesics. Here, we have used intracellular Ca2+-measurements and voltage-clamp recordings to examine the effects of several PPARγ ligands on TRPA1 in sensory neurons and cell lines and examined nociception produced by local intraplantar administration of troglitazone. Troglitazone, rosiglitazone, nTZDpa, and the PPARγ antagonist GW9662 evoked concentration-dependent Ca2+-influx responses in TRPA1 expressing, but not untransfected Chinese hamster ovary (CHO)cells. Furthermore, troglitazone, nTZDpa, and GW9662 evoked [Ca2+]i-responses in mouse DRG neurons expressing TRPA1. Responses were abolished by the TRPA1 antagonist A967079 and were absent in DRG neurons from Trpa1-/- mice. The TRPA1 agonist activity of troglitazone, nTZDpa and GW9662 were unaffected by incubation with an excess of cysteine-methyl ester, indicating that these ligands do not act by covalent modification of cysteine residues, but rather through a non-covalent interaction with TRPA1. The cysteine reducing agent DTT did not reverse the effects of Troglitazone, nTZDpa and GW9662, which suggests that the observed agonist effects were independent of cysteine oxidation. Intraplantar injections of troglitazone evoked pain-responses in wild-type mice, but not in Trpa1-/- mice. Our molecular docking studies indicate that nTZDpa and troglitazone bind to overlapping sites in a hydrophobic pocket in the pre-S1 helix These observations demonstrate that multiple PPARγ ligands stimulate TRPA1 and that nTZDpa may be a useful tool for investigations of TRPA1.