Targeting endothelial tyrosine kinase pathways in tumor growth and metastasis

Angiopoietins (Angs) and vascular endothelial growth factors (VEGFs) regulate angiogenesis, the formation of new blood vessels, and lymphangiogenesis, the formation of new lymphatic vessels. Angiogenesis is important in cancer, because for continuous growth a primary tumor needs a supply of oxygen and nutrients delivered via blood vessels. In physiological conditions, the lymphatic vasculature serves to collect interstitial fluid as well as to absorb lipid particles. In the context of cancer, lymphatic vessels serve as a route for the metastatic dissemination of tumor cells. This dissertation aimed to explore the role of vascular endothelial growth factor receptor 3 (VEGFR3) and angiopoietin-2 (Ang2) in cancer progression. In particular, the roles of these proteins in the context of tumor angiogenesis and lymphangiogenesis as well as metastasis were investigated in two separate studies. In addition, the effects of the photodynamic ablation of intralymphatic cancer cells and lymphatic vessels on the development of metastases were explored. Furthermore, the effect of the endothelial bone marrow tyrosine kinase in chromosome X (Bmx) on tumor angiogenesis was investigated. We found that the inhibition of VEGFR3 reduces tumor blood vasculature and primary tumor growth. In the second study we found that in-transit tumor cells can be targeted with verteporfin-induced photodynamic therapy. The photodynamic ablation of lymphatic vessels was improved by combining this strategy with anti-lymphangiogenic therapies via the adenovirally mediated soluble expression of the VEGF-C/D trap. Bmx, a member of the Tec non-receptor tyrosine kinase family, was previously shown to promote tumor cell survival, but relatively little is known about the effect of this kinase on tumor biology. Here, we studied the effects of Bmx tumor angiogenesis by using Bmx gene-deleted mice. Some reduction of primary tumor growth in Bmx-/- mice was detected in several isogenic and oncogenic tumor models, along with some attenuation of tumor angiogenesis. Conversely, overexpression of Bmx resulted in increased tumor progression and angiogenesis. Ang2 is known to be a context-dependent agonist of the tyrosine kinase of the Tie2 receptor. Here, we analyzed the effect of Ang2 on metastatic dissemination into the lungs, finding that the overexpression of Ang2 enhanced lung metastasis. In contrast, Ang2 inhibition decreased the occurrence of lung metastases. In the ultrastructural analysis of the metastatic lungs using transmission electron microscopy, anti-Ang2 treatment attenuated tumor-associated changes in metastasis-associated lung capillaries. This dissertation demonstrates that the blockade of VEGFR3 inhibits tumor angiogenesis, and that the inhibition of Ang2 inhibits lymphatic and lung metastasis and improves endothelial integrity. Furthermore, we demonstrated the ability of photodynamic therapy to eradicate lymphatic vessels and intralymphatic cancer cells. These data provide a rationale for developing new cancer therapies targeting the lymphatics in order to reduce metastasis and tumor progression. Taken together, these results provide new insights into endothelial tyrosine kinase-mediated angiogenesis, tumor lymphangiogenesis, and vascular-based therapeutic strategies in cancer.