Characterization of UBO-QIC as a G alpha(q) inhibitor in platelets

G alpha(q) plays an important role in platelet activation by agonists such as thrombin, adenosine diphosphate (ADP) and thromboxane. The significance of G alpha(q) signaling in platelets was established using YM254890, a G alpha(q/11)-specific inhibitor and G alpha(q) knockout murine platelets. However, YM-254890 is no longer available for investigators and there is a need to characterize other G alpha(q) inhibitors. The aim of this study is to characterize the specificity of a compound, {L-threonine,(3R)-N-acetyl-3-hydroxy-L-leucyl-(aR)-a-hydroxybenzenepropanoyl-2,3-idehydro-N-methylalanyl-L-alanyl-N-methyl-L-alanyl-(3R)-3-[[(2S, 3R)-3-hydroxy-4-methyl-1-oxo-2-[(1-oxo-propyl) amino]pentyl]oxy]-L-leucyl-N,O-dimethyl-,(7 -> 1)-lactone (9CI)} (UBO-QIC), as a G alpha(q) inhibitor in platelets. Human platelets treated with UBO-QIC showed a concentration-dependent inhibition of platelet aggregation and secretion by protease-activated receptors (PAR) agonists, U46619 and ADP. UBO-QIC also abolished G alpha(q) pathway signaling events such as calcium mobilization and pleckstrin phosphorylation. UBO-QIC had no nonspecific effects on the G alpha(12/13) pathway since platelet shape change was intact in G alpha(q) knockout murine platelets stimulated with PAR agonists in the presence of the inhibitor. In addition, UBO-QIC-treated platelets did not affect collagen-related peptide-induced platelet activation suggesting that this inhibitor had no non-specific effects on the GPVI pathway. Furthermore, Akt phosphorylation downstream of the G alpha(i) and G alpha(z) pathways, and vasodilator-stimulated phosphoprotein phosphorylation downstream of the G alpha(s) pathway were not inhibited in UBO-QIC-treated platelets. UBO-QIC is a specific inhibitor for G alpha(q), which can be a useful tool for investigating G alpha(q)-coupled receptor signaling pathways in platelets.