Introduction Our understanding of angiogenesis has evolved from a simple endothelial-centric model into a multi-faceted process in which bone marrow-derived cells (BMDCs) are found to support the vasculature. BMDCs can act in an immediate “seek and repair” manner in response to various types of therapy, presumably to support tissue regeneration. This can induce an angiogenesis rebound after therapy and thereby impair therapy efficacy. Method By transplanting the bone marrow from the transgenic tamoxifen-inducible VE-cadherin Cre ERT2/EYFP (CIVE) mice into wild type, lethally irradiated, C57 Bl/6 mice, followed by tamoxifen treatment of the recipient mice, we created a mouse with YFP+ bone marrow-derived VE-Cadherin expressing cells (0.3% of the bone marrow). Allowing us to specifically track this lineage of BMDCs. In a second set of experiments, we used the CIVE-EYFP-notch-KO mice as bone marrow-donors. After tamoxifen induction, LLC cells were implanted subcutaneously and after eight days mice were either treated with vehicle control, cisplatin or paclitaxel. Tumor growth was measured and tumors were analyzed by FACS analysis and immunohistochemistry. Results Eight days after treatment with chemotherapy a significant increase in endothelial cells was seen in the tumor compared to the untreated tumors (p<0.01). This correlated with a rapid re-growth of the treated tumors six days after treatment initiation. This treatment-enhanced angiogenesis was accompanied by a clear increase in YFP+ cells as soon as 1 day after chemotherapy for 0.1 to 0.3% (p<0.05). The influx of YFP+ cells was specific for the tumor tissue, as no YFP+ cells were detected in control organs like lung, liver and spleen. Two populations of YFP+ cells were identified in the tumor, a macrophage-like population (Gr1/CD11b+) and an endothelial-like population (PECAM+). The YFP+ cells in the tumor were found in close association with the tumor vessels as per confocal microscopy. FACS analysis showed that in the untreated tumors housed 0.45% of all PECAM+ cells were YFP+, however, this value increased to 1-3% after chemotherapy. Transcriptional analysis of the YFP+ cells revealed high levels of Notch in this population. Using the notch KO mice as bone marrow donors resulted in a defective homing of the YFP+ cells to the tumor which completely prevented the therapy-enhanced angiogenesis and significantly enhanced anti-tumor effect of the chemotherapy. Conclusion In summary, our study identifies an important role for the BMD-VE-cadherin expressing cells in supporting angiogenesis, especially in response to chemotherapy. It reveals the critical role of notch signaling in the ability of these cells to egress the bone marrow and home to the tumor. Preventing the contribution of these angio-supportive cells significantly enhanced therapy efficacy and provides a new strategy to enhance anti-cancer therapy.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1022. doi:1538-7445.AM2012-1022