M-CSF Inhibition Selectively Targets Tumor Angiogenesis and Lymphangiogenesis

Anti-angiogenic therapy in cancer is desired to be selective for tumor angiogenesis. The best validated approaches for limiting tumor angiogenesis involve blockade of the VEGF pathway. However, recent studies show that VEGF blockade damages healthy vessels resulting in toxic side effects (Maharaj et al. J. Exp. Med. 2008) and that interrupting the VEGF blockade induces rapid vascular regrowth in tumors (Mancuso et al. J. Clin. Invest. 2006). Therefore, other targets are currently being explored. Macrophage-colony stimulating factor (M-CSF), a cytokine required for differentiation of monocyte-lineage cells including macrophages promotes formation of a high-density vessel network in tumors suggesting therapeutic potential of M-CSF inhibition (Lin et al. J. Exp. Med. 2001). Moreover, M-CSF is abundantly expressed in highly-metastatic cancers such as breast cancer and osteosarcoma, suggesting high dependency on macrophages. However, the physiological role of M-CSF in vascular and lymphatic development as well as the precise mechanisms underlying anti-angiogenic effects of M-CSF inhibition is not clarified. Here, utilizing osteopetrotic (op/op) mice, we show that M-CSF deficiency leads to lack of both LYVE-1⁺ and LYVE1⁻ macrophages, resulting in defects not only in vascular but also lymphatic development. To clarify the anti-cancer effects of M-CSF inhibition, we utilized a novel mouse model of osteosarcoma, in which mice were transplanted with a newly established osteosarcoma cell line, AX, which were developed from c-Myc-overexpressing Ink4a/ARF^−/− bone marrow-derived stromal cells. In this model, systemic treatment with small molecule c-fms inhibitors (Ki20227) effectively suppressed tumor angiogenesis and lymphangiogenesis disorganizing extracellular matrices. Consequently, c-fms inhibitors almost completely suppressed tumor metastasis, and protected mice from cancer death. Different from VEGF blockade, interruption of M-CSF inhibition did not promote rapid tumor regrowth. Continuous M-CSF inhibition did not affect healthy vascular and lymphatic systems outside the tumor. These results suggest M-CSF-targeted therapy is an ideal therapeutic strategy against cancer alternative to VEGF blockade.