The thiazolidinedione drug troglitazone inhibits Gq signaling through direct binding to the Gq alpha subunit through inhibition of GDP release.

The cycle of GTP binding and hydrolysis controls heterotrimeric G proteins, and mutations reducing GTPase activity result in constitutive G protein signaling. In Gαq (gene: GNAQ) such mutations cause uveal melanoma and Sturge-Weber syndrome. Finding pharmacological agents that inhibit Gαq will be beneficial for research with therapeutic potential. Previously discovered bacterial depsipeptides (FR900359 and YM-254890) bind directly to Gαq and stabilize its inactive complex with GDP, but suffer from limitations of distribution and bioavailability. We used the established Gαq-YM-254890 complex structure to dock small-molecule drugs into the depsipeptide binding site of Gαq. Our in silico screen of 5000 Food Drug and Administration-approved, experimental, and withdrawn drugs predicted that thiazolidinediones are potential ligands of Gαq. Analysis of G protein coupled receptor-stimulated G protein GTPγS binding demonstrated that troglitazone (441 Da) inhibited Gq nucleotide exchange with the IC of ∼31.7 μM. The thiazolidinedione analogs, rosiglitazone and pioglitazone, had no effect. High concentrations of troglitazone modestly inhibited Gi and Gs, but not G13. In G protein thermal stability assays, troglitazone and FR900359 stabilized purified Gαq-GDP, indicating direct binding. Consistent with its negative effect on Gq signaling, in MIN6 mouse insulinoma cells, troglitazone inhibited Ca mobilization, extracellular regulated protein kinase phosphorylation, and insulin secretion stimulated by the Gq-coupled M3 muscarinic cholinergic receptor. Troglitazone and FR900359 inhibited proliferation of MEL92.1 uveal melanoma cells driven by a GNAQ-Q209L driver mutation, but not of SK-MEL-28 cells driven by BRAF-V600E. Together, our study shows that troglitazone may be a promising new lead for the development of a <500 Da small-molecule therapeutic Gαq inhibitor. SIGNIFICANCE STATEMENT: Troglitazone, unlike other thiazolidinediones, directly binds and inhibits activity of heterotrimeric G protein Gq, with a weaker effect on Gi. Troglitazone may find usage as a repurposed drug scaffold to build novel small-molecule Gαq inhibitors with better bioavailability than depsipeptide Gαq inhibitors.