VEGF₁₆₅ expression in the tumor microenvironment influences the differentiation of bone marrow-derived pericytes that contribute to the Ewing's sarcoma vasculature

We previously demonstrated that bone marrow (BM) cells migrate to Ewing's tumors and differentiate into endothelial cells within the tumor vasculature. Recent evidence suggests that the roles of BM cells in tumors are more diverse. We investigated whether non-endothelial cell types critical for tumor vessel development are also derived from migrated BM cells. We utilized BM transplantation with GFP⁺ transgenic mice as BM donors and nude mice as recipients to track the fate of migrated BM cells. After engraftment, we injected recipient mice either subcutaneously or intramuscularly with Ewing's sarcoma cells. We labeled functional tumor vessels using intravascular perfusion staining with tomato lectin. We assessed BM cell recruitment/differentiation within the tumor microenvironment using immunohistochemistry. Ewing's tumors contained BM-derived cells that had differentiated into endothelial cells lining perfused tumor vessels. A substantial fraction of recruited BM cells also resided in the vessel vicinity and expressed desmin and PDGFR-β, indicating smooth muscle cell differentiation. In order to further characterize the role of stem/progenitor cells in Ewing's sarcoma, we sorted Tie2^- BM cells from Tie2-GFP transgenic mice and then injected them intravenously into Ewing's tumor-bearing mice. Tie2^- BM progenitors migrated to Ewing's tumors and differentiated into Tie2⁺ cells occupying a perivascular residence and expressing α-smooth muscle actin, desmin and PDGFR-β, as well as VEGFR-2. We did not observe differentiation of Tie2^- cells into Tie2⁺ perivascular cells in VEGF₁₆₅-inhibited TC/siVEGF_7-1 tumors. The differentiation of Tie2^- BM cells into Tie2⁺ cells in parental but not VEGF₁₆₅-inhibited tumors indicates that the tumor microenvironment may influence the differentiation pathway.