Multiple roles for the receptor tyrosine kinase Axl in tumor formation

A focus of contemporary cancer therapeutic development is the targeting of both the transformed cell and the supporting cellular microenvironment. Cell migration is a fundamental cellular behavior required for the complex interplay between multiple cell types necessary for tumor development. We therefore developed a novel retroviral-based screening technology in primary human endothelial cells to discover genes that control cell migration. We identified the receptor tyrosine kinase Ax1 as a novel regulator of endothelial cell haptotactic migration towards the matrix factor vitronectin. Using small interfering RNA-mediated silencing and overexpression of wild-type or mutated receptor proteins, we show that AA is a key regulator of multiple angiogenic behaviors including endothelial cell migration, proliferation, and tube formation in vitro. Moreover, using sustained, retrovirally delivered short hairpin RNA (shRNA) Ax1 knockdown, we show that Ax1 is necessary for in vivo angiogenesis in a mouse model. Furthermore, we show that Ax1 is also required for human breast carcinoma cells to form a tumor in vivo. These findings indicate that Ax1 regulates processes vital for both neovascularization and tumorigenesis. Disruption of Ax1 signaling using a small-molecule inhibitor will hence simultaneously affect both the tumor and stromal cell compartments and thus represents a unique approach for cancer therapeutic development.