He, S., Lamers, G.E., Beenakker, J.W., Cui, C., Ghotra, V.P., Danen, E.H., Meijer, A.H., Spaink, H.P., and Snaar-Jagalska, B.E. (2012) Neutrophil-mediated experimental metastasis is enhanced by VEGFR inhibition in a zebrafish xenograft model. The Journal of pathology. 227(4):431-445.
Inhibition of VEGF signaling effectively suppresses localized tumor growth but accelerates tumor invasiveness and micrometastasis by unknown mechanisms. To study the dynamic and reciprocal interactions between tumor cells and their microenvironment during these processes, we established a xenograft model by injecting tumor cells into the blood circulation of transparent zebrafish embryos. This reproducibly results in rapid simultaneous formation of a localized tumor and experimental micrometastasis, allowing time-resolved imaging of both processes at single-cell resolution within one week. The tumor vasculature was initiated de novo by remodeling of primitive endothelial cells into a functional network. Roles of myeloid cells in critical tumorigenesis steps such as vascularization and invasion were revealed by genetic and pharmaceutical approaches. We discovered that the physiological migration of neutrophils controlled tumor invasion by conditioning the collagen matrix and forming the metastatic niche, as detected by two-photon confocal microscopy and second harmonic generation. Administration of VEGFR inhibitors blocked tumor vascularization and a localized tumor growth but enhanced migration of neutrophils, which in turn promoted tumor invasion and formation of micrometastasis. This demonstrates the in vivo cooperation between VEGF signaling and myeloid cells in metastasis and provides a new mechanism underlying the recent findings that VEGFR targeting can promote tumor invasiveness.