Involvement of endothelial progenitor cells in tumor vascularization

The generation of new microvasculature progresses by the process of angiogenesis, which involves the invasion and proliferation of endothelial cells from existing blood vessels into the local environment. In recent years, de novo generation of endothelial cells from circulating or local precursor endothelial cells is thought to also contribute to the neovasculature, a process that is referred to as vasculogenesis. In the adult, endothelial progenitor cells (EPC) are believed to be recruited from the bone marrow, migrate to sites requiring neovascularization and participate in the assembly of newly-forming blood vessels. A growing number of studies report that EPC participate in tumor progression and influence the efficacy of anticancer chemotherapeutics, and thus are attractive targets for cancer treatments. However, recent evidence calls into question the ability of marrow-derived EPC to act as a bona fide precursor for adult vasculogenesis. This review focuses on studies reporting or precluding the importance of EPC in tumor vasculogenesis. The putative sources of these cells and difficulties associated with their detection are discussed.