The DC electrical-field-induced Ca2+ response and growth stimulation of multicellular tumor spheroids are mediated by ATP release and purinergic receptor stimulation

It has been demonstrated that adenosine 5′-triphosphate (ATP) is actively secreted by cells, thereby eliciting Ca2+-dependent signal transduction cascades in an autocrine and paracrine manner. In the present study the effects of direct current (DC) electrical fields on ATP release, the intracellular Ca2+ concentration [Ca2+]i and growth of multicellular prostate tumor spheroids were investigated. Treatment of multicellular tumor spheroids by a single DC electrical field pulse with a field strength of 750 Vm-1 for 60 seconds resulted in a transient Ca2+ response, activation of c-Fos and growth stimulation. The initial [Ca2+]i signal was elicited at the anode-facing side of the spheroid and spread with a velocity of approximately 12 μm per second across the spheroid surface. The electrical-field-evoked Ca2+ response as well as c-Fos activation and growth stimulation of tumor spheroids were inhibited by pretreatment with the anion channel blockers NPPB, niflumic acid and tamoxifen. Furthermore, the Ca2+ response elicited by electrical field treatment was abolished following purinergic receptor desensitivation by repetitive treatment of tumor spheroids with ATP and pretreatment with the purinergic receptor antagonist suramin as well as with apyrase. Electrical field treatment of tumor spheroids resulted in release of ATP into the supernatant as evaluated by luciferin/luciferase bioluminescence. ATP release was efficiently inhibited in the presence of anion channel blockers. Our data suggest that electrical field treatment of multicellular tumor spheroids results in ATP release, which concomitantly activates purinergic receptors, elicits a Ca2+ wave spreading through the tumor spheroid tissue and stimulates tumor growth.