Differential expression of angiopoietin-1 and angiopoietin-2 may enhance recruitment of bone marrow-derived endothelial precursor cells into brain tumors

Objectives: Angiogenesis is necessary for sustained neoplastic development. The angiopoietins Ang-1 and Ang-2 have been implicated in the regulation of this process; recent reports have suggested that a net gain in Ang-2 activity may be an initiating factor for tumor angiogenesis. We examined the recruitment of bone marrow-derived endothelial precursor cells into developing tumor neovasculature, and the spatial relationship between these cells and angiopoietin (Ang-1 and Ang-2) expression. Methods: For this study T-cell depleted knockout mice (RAG-2/KO-5.2) were lethally irradiated and their bone marrow was reconstituted by bone marrow cells (BMCs) from transgenic mice (C57BL/Ka-Thy1.1) expressing green fluorescent protein (GFP). Rat glioma cells (RT-2/RAG) were then injected into the transplanted animals to form solid brain tumors. The animals were killed and their brains were analysed using immunohistochemistry and fluorescence-activated cell sorting. Results: We found that BMCs migrated preferentially into the tumor when compared to adjacent healthy brain parenchyma. Furthermore, GFP+/CD34+ cells represented up to 8% of endothelial-like cells within the walls of tumor blood vessels. In the tumor, significant colocalization of Ang-2 with GFP+/CD34+ cells was noted (> 80%), but colocalization with Ang-1 never exceeded 20%. In normal tissue directly surrounding the tumor, GFP+/CD34+ cells colocalized strongly with both angiopoietins (> 75% and > 70% for Ang-1 and Ang-2, respectively). Discussion: The relative increase in angiopoietin-2 activity in brain tumors may result in the creation of a pro-angiogenic environment that enhances the recruitment of putative bone marrow-derived endothelial precursor cells into the tumor's developing vascular tree.