Anthraquinone emodin inhibits human cancer cell invasiveness by antagonizing P2X7 receptors

The adenosine 5'-triphosphate (ATP)-gated Ca2+-permeable channel P2X7 receptor (P2X7R) is strongly upregulated in many tumors and cancer cells, and has an important role in cancer cell invasion associated with metastases. Emodin (1,3,8-trihydroxy-6-methylanthraquinone) is an anthraquinone derivative originally isolated from Rheum officinale Baill known for decades to possess anticancer properties. In this study, we examined the effects of emodin on P2X7R-dependent Ca2+signaling, extracellular matrix degradation, and in vitro and in vivo cancer cell invasiveness using highly aggressive human cancer cells. Inclusion of emodin at doses ≤10 μM in cell culture had no or very mild effect on the cell viability. ATP elicited increases in intracellular Ca2+ concentration were reduced by 35 and 60% by 1 and 10 μM emodin, respectively. Emodin specifically inhibited P2X7R-mediated currents with an IC50 of 3 μM and did not inhibit the currents mediated by the other human P2x receptors heterologously expressed in human embryonic kidney (HEK293T) cells. ATPinduced increase in gelatinolytic activity, in cancer cell invasiveness in vitro and in cell morphology changes were prevented by 1 μM emodin. Furthermore, such ATP-evoked effects and inhibition by emodin were almost completely ablated in cancer cells transfected with P2X7R-specific small interfering RNA (siRNA)but not with scrambled siRNA. Finally, the in vivo invasiveness of the P2X7R-positive MDA-MB-435s breast cancer cells, assessed using a zebrafish model of micrometastases, was suppressed by 40 and 50% by 1 and 10 μM emodin. Taken together, these results provide consistent evidence to indicate that emodin inhibits human cancer cell invasiveness by specifically antagonizing the P2X7R.