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Aim: Evidence has shown that 17β-estradiol induces neoplastic transformation in human breast cancer cells, promotes the growth of breast cancer cells in vivo and in vitro, and significantly increases the risk of breast cancer.The carcinogenic effect of estrogen has been shown to include genomic or/and non-genomic pathways, cell membrane-related, but ER-independent.The relationship and the balance between ERα and ERβ may be important.Functional experiments have demonstrated that ERα and ERβ have completely different roles in breast cancers, in which ERα functions as a tunor promoter whereas ERβ functions as a tumor suppressor.Experiments on ERknockout mice have also suggested a stimulatory role of ERα and an inhibitory effect of ERβ in the proliferation of different estrogen-target tissues.Further, recent studies have indicated that estrogen can affect tumor cell proliferation and metastasis by interacting with G protein-coupled receptors, which suggest that effects of estrogen on G protein-coupled receptors may play a critical role in the onset and development of breast cancer.P2Y2 is a G protein-coupled receptor, which can be activated by ATP/UTP Several results indicate that P2Y2 receptors may qualify as promising targets for innovative treatment strategies of different cancers.In breast cancer, there are reports that P2Y2 receptors are expressed in MCF-7 breast cancer tumourcells and play roles in cell proliferation.However, the underlying mechanisms between P2Y2 receptors and estrogen-independent breast cancer are unclear.Methods: In the present study, the methods of methylthiazoltetrazolium (MTT) assay, real-time RT-PCR,Western blot and fluorescent calcium imaging analysis were used to investigate whether P2Y2 receptors play a role in the effects of estrogen on the breast cancer cell lines, MCF-7 and MDA-MB-231.Results: We found that P2Y2 receptors were expressed in both the estrogen receptor alpha (ERα)-positive breast cancer cell line MCF-7and the ERα-negative breast cancer cell line MDA-MB-231.17β-Estradiol (17β-E2) (1 pM-1000 μM) promoted proliferation of MCF-7 cells, which was blocked by the ER antagonist ICI 182,780 (1 μM), the ERα antagonist methyl-piperidino-pyrazole (MPP, 50 μM), or ERα small interfering RNA, but not by the ERβ antagonist 4-(2-phenyl-5,7-bis(trifluoromethyl) (PHTPP, 50 μM) or ERβ small interfering RNA.The P2Y2 and P2Y4 receptor agonist UTP (10-100 μM) suppressed the viability of breast cancer cells in both MCF-7 and MDA-MB-231 cells.The effect was blocked by suramin (10-100 μM), known to be an effective antagonist against P2Y2, but not P2Y4, receptor-mediated responses.17β-E2 played a more positive role in promoting proliferation in MCF-7 cells when suramin blocked the functional P2Y2 receptors.17β-E2 (0.1-1000 μ.M) downregulated the expression of P2Y2 receptors in terms of both mRNA and protein levels in MCF-7 cells.The effect was blocked by ICI 182,780 and MPP, but not PHTPP or ERβ small interfering RNA.UTP (10-100 μM) led to a sharp increase in intracellular Ca2+ in MCF-7 cells.Pre-incubation with 17 β-E2 (0.1 μM) attenuated UTP-induced [Ca2+]i, which was blocked by ICI182,780 and MPP, but not PHTPP.Further, Estrogen and an ERα agonist, PPT significantly increased expression of ERK1/2 and Akt expression in MCF-7 cells, which was blocked by ICI 182,780.In contrast, 17β-E2 did not affect the cell proliferation or the expression of P2Y2 receptors in MDA-MB-231 cell line.Suramin did not affect the estrogen effect in the MDA-MB-231 cell line.Conclusion: These results suggest that activation of P2Y2 receptors inhibited the proliferation of both MCF-7 and MDA-MB-231 cells.Estrogen, via ERα receptors,promotes proliferation of breast cancer cells by down-regulating P2Y2 receptor expression and attenuating P2Y2-induced increase of [Ca2+]i in MCF-7 cells but not in MDA-MB-231 cells.These results will open new perspectives for the therapy of estrogen-dependent breast cancer.