Selected ginsenosides of the protopanaxdiol series are novel positive allosteric modulators of P2X7 receptors

Abstract

Background and Purpose The P2X7 receptor is an ATP-gated ion channel predominantly expressed in immune cells and plays a key role in inflammatory processes. Ginseng is a well-known Chinese herb with both pro- and anti-inflammatory properties and many of its actions have been ascribed to constituent ginsenosides. We screened a number of ginsenoside compounds for pharmacological activity at P2X7 receptors, that might contribute to the reported immunomodulatory actions of ginseng. Experimental Approach We used several assays to measure responses of P2X7 receptors, ATP-mediated dye uptake, intracellular calcium measurement and whole-cell patch-clamp recordings. HEK-293 cells stably expressing human P2X7 receptors were used in addition to mouse macrophages endogenously expressing P2X7 receptors. Key Results Four ginsenosides of the protopanaxdiol series, Rb1, Rh2, Rd and the metabolite compound K (CK) potentiated the dye uptake responses of P2X7 receptors, whereas other ginsenosides tested were ineffective (1-10 μM). The potentiation was rapid in onset, required a threshold concentration of ATP (>50 μM) and had an EC50 of 1.08 μM. CK markedly enhanced ATP-activated P2X7 currents, probably via an extracellular site of action. One of the consequences of this potentiation effect is a sustained rise in intracellular Ca2+ that could account for the decrease in cell viability in mouse macrophages after a combination of 500 μM ATP and 10 μM CK that are non-toxic when applied alone. Conclusions and Implications This study identifies selected ginsenosides as novel potent allosteric modulators of P2X7 channels that may account for some of the reported immune modulatory actions of protopanaxdiol ginsenosides in vivo.