Discovery of bilirubin as novel P2X7R antagonist with anti-tumor activity.

As a unique ligand gated ion channel in the P2-receptor family, P2X7R is highly expressed in various tumors. The activated P2X7R facilitates tumor growth and metastasis. Hypoxia, inflammation and necrosis in the tumor microenvironment (TME) cause a large amount of adenosine triphosphate (ATP) accumulated in the TME. High concentration of ATP can abnormally activate P2X7R, which induces pore formation and further facilitates the Ca2+ ion influx and non-specific substance intake. Therefore, inhibition of P2X7R activation can be applied as a potential anti-tumor therapy strategy. However, there is currently no FDA approved drugs for this target for anti-tumor treatment. In this study, we identified bilirubin as novel P2X7R antagonist by using structure based virtual screening combined with cell based assays. Molecular docking studies indicated that bilirubin probably interacted with P2X7R by forming hydrogen-π interactions with residues V173, E174 and K311. The compound bilirubin inhibited the P2X7R gated EB intake by cancer cells. Meanwhile, bilirubin was capable to inhibit the cell proliferation and migration of P2X7R expressed HT29 cells. The phosphorylation of mTOR, STAT3 and GSK3β were significantly decreased when bilirubin was present. Finally, in vivo experiment exhibited the anti-tumor effect of bilirubin in the MC38 bearing mice model, but did not show tissue damage in different organs. In conclusion, bilirubin was identified as a novel P2X7R antagonist and it may have potential for anti-cancer treatment, although various functions of the molecule should be considered.