Abstract 383: Investigation of endothelial progenitor cells as mediators of radiation resistance.

Proceedings: AACR 104th Annual Meeting 2013; Apr 6-10, 2013; Washington, DC Background: Radiation therapy delivered to locally advanced cancers is likely to kill most, if not all, of the endothelial cells lining the walls of blood vessels within and immediately surrounding the tumor. For the tumor to recur following radiation therapy, new vasculature must form either through angiogenesis, the sprouting of vessels from existing vessels, or by vasculogenesis, the seeding of blood vessels from circulating endothelial progenitor cells (EPCs). Because radiation is sufficient to abrogate local angiogenesis, we hypothesize that EPCs and other proangiogenic cells are recruited to tumors after radiation therapy, resulting in the recovery of the vascular network, and eventually, in the regrowth of the tumor. Methods: Nude mouse tissues and human U251 glioma xenografts were harvested, homogenized, and evaluated by flow cytometry for the EPC markers: CD117+, Flk-1+, and CD45-. EPCs were sorted and grown as primary cultures and examined for mature endothelial function using immunofluorescence for CD31 and acLDL uptake. Additionally, cultured EPCs were tested for stem cell and endothelial characteristics using tube formation and colony-forming assays. Finally, tumors and normal tissues were collected at various time points after radiation therapy and evaluated for % EPC populations present. Results: The highest concentrations of EPCs were in adipose tissue (7%), lungs (0.5%), aorta (0.5%) and glioma xenografts (0.5%) with other tissues demonstrating minimal EPC populations. EPCs could be isolated and grown as primary cultures where they demonstrated expression of the endothelial marker CD31, had endothelial function by uptake of acLDL, were able to form capillary networks on matrigel, and formed cell clusters in a colony-forming assay. Time course evaluation of EPC populations in tumors and normal lung tissue did not show an increase in EPCs at any time point after 10 Gy; however, there was approximately a 2-fold increase in EPCs present in peripheral blood at 7 days after irradiation. Additionally, an increased population of CD11b+ cells was present within the tumor 2-3 weeks after radiation therapy. Conclusions: EPCs (CD117+/Flk-1+/CD45-) were present in a variety of normal murine tissues and U251 human Glioma xenografts. While EPC populations did not increase within tumors after radiation, there was a consistent elevation of EPCs in peripheral blood during the first week after treatment with radiation. Furthermore, we were able to demonstrate a late influx of potentially proangiogenic CD11b+ monocytes within irradiated tumors. We are currently examining whether the EPC populations in irradiated tumors were originally present prior to radiation or were replaced by the radiation-induced EPC populations found in the peripheral blood. Finally, we are examining whether EPCs directly promote tumor initiation, tumor growth, and post-irradiation tumor recovery. Citation Format: Jeffery S. Russell, J. Martin Brown. Investigation of endothelial progenitor cells as mediators of radiation resistance. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 383. doi:10.1158/1538-7445.AM2013-383