Escape mechanisms from antiangiogenic therapy: an immune cell's perspective

Neovascularization, the formation of new blood vessels, has become a well-established hallmark of cancer. Its functional importance for the manifestation and progression of tumors has been validated further by the beneficial therapeutic effects of angiogenesis inhibitors, most notably those targeting vascular endothelial growth factor signaling pathways. However, with the transient and short-lived nature of patient response, it has become evident that tumors have the ability to adapt to the pressures of vascular growth restriction. Observations made both in the clinic and at the bench suggest the existence of several escape mechanisms that either reestablish neovascularization in tumors or change tumor behavior to enable propagation and progression without obligate neovascularization. Some of these bypass mechanisms are regulated by low oxygen conditions (hypoxia) caused by therapy-induced vessel regression. Induction of hypoxia and hypoxia-inducible factors regulate a wide range of tumor-promoting pathways, including those of neovascularization, that can upregulate additional proangiogenic factors and drive the recruitment of various bone marrow-derived cells that have the capacity to express proangiogenic factors or directly contribute to neovasculature.