Targeting tumor vasculature is the common theme for the three separate studies presented in this thesis. The studies comprise the roles of components of three signaling pathways (networks) in several steps of blood vessel formation, and related tumor growth and dissemination. With our first study, we wanted to elucidate the role of selected Wnt signaling members in in vitro and in vivo angiogenesis. The aim of the second study was to reveal the advantages of simultaneous targeting of all of the constituents of the VEGFR family over the sole inhibition of VEGF and VEGFR2. Finally, in our third study we wanted to delineate the role of ErbB2 signaling mediator Memo in tumorigenesis, vasculogenesis and metastatic abilities of 4T1 mouse mammary carcinoma cells. Wnt signaling is highly conserved signaling pathway involved in several developmental processes and regulation of adult tissue homeostasis. Accumulating data from mouse knock out and in vitro angiogenesis studies indicate its role in blood vessel formation. To decipher the role of Wnt signaling components in angiogenesis, we used human umbilical vein endothelial cells (HUVEC), and well vascularized Lewis lung carcinomas grown in mice, as our in vitro and in vivo models, respectively. In our in vitro studies, we checked for the abilities of recombinant Wnt3a, a member of the family considered to activate only canonical Wnt pathway, and Wnt5a, so called non canonical Wnt, to induce HUVEC proliferation, migration and survival. We found that Wnt3a is a novel proangiogenic factor with the ability to induce HUVEC proliferation and migration, but without an effect on their survival. The effects were mediated through the common Wnt downstream effector protein Dishevelled. Proliferation induced by Wnt3a is VEGFR signaling independent. Wnt5a did not show an effect on any of those processes, but it activated signaling, demonstrated by Dishevelled phosphorylation. To test the role of Fz6, a member of Wnt receptor family repeatedly shown to be expressed on vasculature from different tissues, in in vivo angiogenesis, we injected Fz6 knock out mice and their wild type littermates with Lewis lung carcinoma cells. The ablation of Fz6 did not have an effect on the kinetics of tumor growth, quantity nor appearance of tumor vasculature. While the knowledge about the role of Wnt signaling in angiogenesis is only emerging, the importance of vascular endothelial growth factors (VEGFs) and their receptors (VEGFRs) in vessel formation is well documented. In our work, we wanted to test the effects of simultaneous inhibition of VEGFR1, VEGFR2 and VEGFR3 in comparison to inhibition of VEGFR2 or VEGF solely, on tumor growth and spread. We found that targeting all three VEGFRs is more effective in inhibition of functionality of the lymphatics surrounding the primary tumor and subsequent metastatic spread, than targeting VEGFR2 or VEGF alone. Moreover, we show that B16/BL6 melanoma cancer cells display an in vitro autocrine VEGF/VEGFR signaling loop whose inhibition chemosensitizes them to platinum based chemotherapeutics. ErbB2 receptor tyrosine kinase belongs to the family of epidermal growth factor receptors. It is overexpressed in 20% of human breast cancers, and its expression correlates with highly metastatic disease and poor clinical outcome for patients. In a screen for ErbB2 effectors, Memo, a novel signaling protein that is mediating migration induced through ErbB2 and several other receptor tyrosine kinases, was previoulsy identified in our lab. To test the role of Memo in cancer growth, we made use of 4T1, a highly metastatic mouse mammary carcinoma cell line. Downregulation of Memo via stable transfection of shRNA in these cells decreased in vivo primary tumor growth when compared to control cells. The inspection of tumor vasculature revealed that tumors derived from Memo knock down (KD) clones were less vascularized, which could possibly explain their growth delay. To delineate the mechanisms behind decreased vascularization, we measured the concentration of secreted VEGF in the medium of control and Memo KD clones. Our results show that Memo KD clones secrete less VEGF into the medium than control clones, indicating that this might relate to the impairment in vascularization of tumors derived from these cells. To check the role of Memo downregulation in metastasis formation, we injected control and Memo KD clones into the tail vein of BALB/c mice. We found that Memo downregulation decreases the metastatic behavior of these cells. In conclusion, with this study, we describe the effects of Memo downregulation on several aspects of tumorigenesis in mouse mammary carcinoma model. Further studies are needed to decipher the signaling pathway that Memo is a part of, and that is responsible for the described effects on primary tumor, its vascularization and metastasis formation.