Recent evidence suggests a role for the P2X7 purinergic receptor (P2X7R) in cell proliferation, but the underlying mechanisms of its effects on estrogen related breast cancer are unclear. Here, we examined the role of P2X7R in estrogen-induced proliferation and the underlying mechanisms regulating estrogen modulation of P2X7R in the breast cancer cell lines MCF-7 and MDA-MB-231, using MTT assays, qRT-PCR, western blotting analysis and siRNA transfection. In MCF-7 and MDA-MB-231 cells, benzo ATP (BzATP) significantly promoted cell proliferation, which was blocked by the P2X7R antagonist A438079. In MCF-7 cells, the promoting effect of estrogen was blocked by the nonselective purinergic receptor antagonist PPADS, A438079 or P2X7R siRNA. Estrogen upregulated P2X7R expression at both the mRNA and protein levels, which was blocked by the estrogen receptor (ER) antagonist ICI 182,780, ER? antagonist MPP or ER? siRNA, but not by ER? antagonist PHTPP. The selective ER? agonist PPT, but not ER? agonist DPN, significantly increased P2X7R expression. Estrogen and PPT significantly increased expression of ERK1/2 and Akt expression, which was blocked by ICI 182,780. Antagonists against MAPK (U0126) and PI3k/Akt (LY294002) blocked the estrogen effect on P2X7R expression and phosphorylation of cAMP response element-binding protein (CREB). Administration of three different P2X7R antagonists or silencing P2X7R in MCF-7 cells injected in nude mice reduced MCF-7-derived tumor growth and decreased vessel formation. These findings suggested that estrogen promoted breast cancer cell proliferation by upregulating P2X7R via ER? through the ERK1/2 or Akt-pCREB signaling pathways.