Low Levels of Circulating EPCs Following Bone Marrow Carcinogenic Stimulus Predict the Onset of Hematological Tumors.

Endothelial progenitor cells (EPCs) originate in the bone marrow (BM), proliferate, migrate and differentiate into endothelial cells, contributing to the formation of new blood vessels in normalcy and in malignant tumor growth. EPCs have been detected at increased frequency in the circulation of cancer patients and lymphoma-bearing mice, and tumor VEGF production correlates with EPCs mobilization. In order to clarify the contribution and importance of endothelial progenitor cells during tumor development, we established a murine carcinogenesis model that produces hematological neoplasias, namely thymomas and acute leukemia. This model consisted of administering 3 cycles of irradiation (sub-lethally), within one month of interval between them. Every month after the last irradiation, the levels of circulating endothelial progenitor cells (Flk-1⁺ and Sca⁺ cells) and hematopoietic progenitor cells (FLT-1⁺ and Sca⁺ cells) in peripheral blood were quantified (by flow cytometry). A first assay was done using 30 mice (FVB/N strain), divided into 2 groups: the control group (not irradiated) with 10 mice and the irradiated group with 20 mice. At the end of the assay (~11 months after last irradiation) the incidence of tumors was approximately 63% in the irradiated group (of these, there was an equal incidence of thymomas and acute leukemia). About 37% of the irradiated mice presented signs of disease 2,5 moths after the last irradiation ( called early disease group) while the other 26% presented signs of disease 7,2 months after the last irradiation ( called late disease group). Surprisingly, when comparing the EPC levels between the two groups that developed tumors and the remaining irradiated mice that did not (called no disease group), the two groups of mice that developed tumors had a decreased number of circulating EPC. This decreased was more evident in the early disease group (p<0.01). An inverse relation was observed for VEGF levels in circulation, since the mice with no disease presented lower VEGF levels (5pg/ml) comparied with the two groups that developed tumors (>10pg/ml). At this point, we hypothesized the BM derived EPC levels somehow modulated the onset of hematological disease in irradiated mice. To modulate EPC levels, we transplanted EPCs into irradiated mice with low levels of circulating EPCs. In a preliminary experiment, mice (n=2) that received exogenous EPCs survived, while the mice that did not succumbed to hematological diseases. Taken together, the data obtained suggest a role for EPCs in modulating the BM microenvironment, thus regulating the onset of hematological malignancies. Ongoing studies are addressing possibility. Finally, these results suggest that detection of reduced circulating BM EPCs levels following cyclic whole body irradiation may predict a worse disease outcome.