P2Y12 Receptor Antagonists Decrease the Radiation Resistance of B16 Melanoma by Suppressing DNA Repair

Radiation therapy exerts its therapeutic effect by killing cells via the induction of DNA double-strand breaks (DSBs) in malignant tumors, but cancer cells can repair damaged DNA, leading to radiation resistance (radioresistance). Therefore, a radiosensitizing effect can be expected by suppressing mechanism(s) involved in DNA repair after irradiation. Here, we show that the P2Y12 receptor is involved in the radioresistance of mouse B16 melanoma cells, and that P2Y12 antagonist treatment decreases the radioresistance both in vitro and in vivo by inhibiting DNA repair after γ-irradiation. P2Y12 receptor antagonists Clopidogrel and PSB0739 increased cellular sites of unrepaired DNA by suppressing the DNA damage response (DDR) after γ-irradiation and enhanced radiation-induced proliferative death in B16 melanoma cells. On the other hand, ADP (a P2Y12 receptor agonist) enhanced DDR after γ-irradiation and increased radioresistance. Knockdown of the P2Y12 receptor resulted in an increase of unrepaired DNA damage and enhanced proliferative death after γ-irradiation. Suppression of the P2Y12 receptor also contributed to the enhancement of the cancer-killing effect of γ-irradiation, even in fractionated irradiation samples in which the cancer-killing effect decreased due to sublethal damage recovery. Finally, PSB0739 significantly enhanced the antitumor effect of γ-irradiation in vivo. Our results suggest that P2Y12 receptor antagonists are promising candidates as radiosensitizers to improve radiation therapy.