Constituents and receptor-mediated calcium signaling underlying bioactivities of Echinacea

With a long history of traditional medical uses for various ailments such as pain relief, Echinacea extracts are currently widely used mainly as herbal immunomodulators. However, their efficacy and underlying molecular mechanisms of action remain unclear. Three medicinally used species of Echinacea (E. angustifolia, E. pallida and E. purpurea) were investigated for their stimulating effects on transient receptor potential vanilloid 1 (TRPV1) expressed in Xenopus laevis oocytes. Echinacea ethanol extracts had potent activity in opening TRPV1 channel, evoking currents up to 10 times of that evoked by a saturating dose of capsaicin, the ligand used to clone TRPV1. Among the 3 species tested, E. angustifolia is the most active. Ethanol extract of E. purpurea root was fractionated by HPLC; only the most lipophilic fraction evoked a current, a current was induced by a 7 mug dry weight/ml of this fraction. Echinacea-evoked responses could not be blocked by capsazepine, a competitive TRPV1 receptor antagonist not only for capsaicin but also for many known endogenous TRPV1 agonists. TRPV1 mutants lacking either PKC phosphorylation sites (TRPV1-S502A, TRPV1-S800A and TRPV1-S502A/S800A) or critical capsaicin binding sites (TRPV1-S512Y and TRPV1-Y511A) that reduce capsaicin-dependent TRPV1 activity, do not affect Echinacea-induced TRPV1 activity. In addition, Echinacea extracts induce a much more rapid desensitization of TRPV1, compared to the calcium-dependent desensitization of TRPV1 by capsaicin. Furthermore, unlike capsaicin, Echinacea extracts rapidly desensitize TRPV1 in the presence of only minimum concentration of calcium. These results indicate the activation of TRPV1 by Echinacea involves a distinct mechanism from that of capsaicin and most likely this activation is via an indirect signaling pathway instead of direct binding.