Mechanism of Nm23/NDPK in Breast Cancer; Heterologous Signaling in Support of Tumor Angiogenesis and Metastasis

Human breast carcinoma cells (MDA-MB-435) secrete an adenosine 5'-diphosphate trans-phosphorylase identified as shed or secreted (s) human (h) nucleoside diphosphate kinase (shNDPK-B) known to induce endothelial cell tubulogenesis in a P2Y-receptor dependent manner. Here, we examined whether sNDPK-A/B secretion is a common feature of breast cancer cell lines and what affects sNDPK has on endothelial cell growth and migration. We demonstrated for the first time that shNDPK-B can be detected specifically with high sensitivity in an ELISA assay of our own design. Our ELISA assay also revealed that a panel of breast cancer cell lines with origins in ductal carcinoma, adenocarcinoma and medullary carcinoma secreted/shed NDPK-A/B into their growth media, while sNDPK-A/B cannot be detected in the media of a control breast cell line (MCF-12f) by western blot. Secreted NDPK-A/B acted as a transphosphorylase converting ADP to ATP in the presence of a phosphoryl donor and promoted endothelial proliferation and cell migration through activation of P2Y purine nucleotide receptors (P2YRs). This finding is consistent with the hypothesis that human breast tumor cell-secreted NDPK-A/B increases extracellular ATP levels in the immediate environment of metastatic tumors. We also extended our studies of purinergic regulation of angiogenesis. We hypothesized that in the absence of growth factors, activated P2Y1 receptors transactivate the proto-oncogene Src kinase and vascular endothelial growth factor receptor-2 (VEGFR-2) in angiogenic signaling. The Immunoprecipitation and direct western blots results showed that P2Y1 receptors activated by the P2Y1 specific agonist 2-methylthio-ATP (2MeS-ATP) induce phosphorylation of VEGFR-2 (Tyr 1059 and Tyr 1175). The transactivation of VEGFR-2 by P2Y1 activation was blocked by pretreatment with the P2Y1 specific antagonist, Src or the VEGFR-2. Further, the results showed that the addition of extracellular NDPK to human endothelial cultures resulted in transactivation of VEGFR-2 and transactivation was blocked by the NDPK inhibitor, EA. Activation of the endothelial nucleotide P2Y1 receptor by its specific agonist 2MeS-ATP or NDPK, transactivates VEGFR-2 in the absence of VEGF. We suggest that sNDPK and activated P2Y1R support tumor angiogenesis via VEGFR-2. Next, we examined nucleotide regulation of angiogenesis linking sNDPK to MAPK activation using western blot analysis, immunofluorescence and measurement of human endothelial cell migration. Our results demonstrated that the addition of extracellular NDPK to HEC cultures activated Erk1/2 and induced cell migration, both of which were blocked by inhibitors of NDPK and P2Y receptors. Activation of pErk1/2 by 2MeS-ATP was blocked by pretreatment with the P2Y1 specific antagonist, the proto-oncogene non-receptor tyrosine kinase (Src) inhibitor, or the VEGFR-2 antagonist. The ability of ATP-P2 receptor stimulation to transactivate the endothelial VEGF receptor and downstream Erk1/2 activation in the absence of VEGF suggests the possibility that cancer cell-secreted NDPK-A/B, endothelial P2Y receptors, and VEGF receptors interact as partners in tumor angiogenesis in vitro. We extended our studies of purinergic regulation of angiogenesis in vivo in a breast cancer metastasis model in mice carrying luciferase-tagged human breast carcinoma cells (MDA-MB-23-Luc2) to permit tracking of primary tumor development and tumor metastases. We hypothesized that orthotopic-human tumors growing in SCID mice modulate extracellular sNDPK-A/B levels to attract angiogenic growth, development and metastasis; and that inhibition of NDPK-A/B's extracellular action as transphosphorylase and that inhibition of P2Y1 receptors prevents/delays the growth of metastatic breast tumor in a mouse model. Examination of serum using Western blot and an ELISA assay of our own design revealed that tumors secreted sNDPK-A/B in large amounts two weeks after subcutaneous injection of cancer cells in the mammary fat pad. The antitumor therapy showed that NDPK-A/B and activation of P2Y1R promotes tumor growth in a mouse model of human breast cancer and that the inhibition of NDPK activity by EA and inhibition of P2Y1R activation by MRS2179 inhibited primary tumor growth and reduced metastases in MRS2179 or EA alone and completely eradicated the metastatic lesions when combined. Therefore, we proposed that cancer cell-secreted sNDPK-A/B positively regulates angiogenesis through its regeneration of extracellular ATP levels at the surface of capillary blood vessels in an animal model of breast cancer, and that these data support the hypothesis that shNDPK-A/B may be responsible for the early events in angiogenesis supporting both primary and metastatic tumor growth and development