Novel inhibitory effects of cardamonin on thromboxane A₂-induced scratching response: Blocking of G_h/transglutaminase-2 binding to thromboxane A₂ receptor

Alpinia katsumadai is known to suppress thromboxane A₂ (TXA₂) receptor agonist-induced scratching in mice. The specific components of A. katsumadai responsible for these biological effects, however, are not known. In the present study, we investigated whether cardamonin (CDN), one of major principles of A. katsumadai, has suppressive effects on TXA₂-induced scratching in mice. Scratching induced by U46619 (the TXA₂ receptor agonist) at a dose of 10 nmol/site was shown to be suppressed by CDN (0.1 nmol-0.5 nmol/site). Suppression of the U46619-induced scratching response by CDN was found to be unrelated to competition with the ligand at the TXA₂ receptor, since CDN did not suppress [³H] SQ29548 (the TXA₂ receptor antagonist) binding to TXA₂ receptor. TXA₂ receptor expression in A549, HaCaT, and SH-SY5Y cell lines was examined and determined to be significant in the A549 and SH-SY5Y cell lines. Further, binding of high molecular G protein G_h/transglutaminase-2 (G_h/Tgase-2) to TXA₂ receptor was confirmed in the A549 and SH-SY5Y cells by co-immunoprecipitation. CDN suppressed the binding of TXA₂ receptor with G_h/Tgase-2, which also acts as a G protein involved in TXA₂ signaling. These results suggested that CDN suppresses TXA₂ receptor agonist-induced scratching by suppressing TXA₂ signaling, specifically via blocking of the binding of G_h/Tgase-2 to TXA₂ receptor.