Angiogenesis, the formation of new blood vessels from existing ones, is dysregulated during tumor progression as a result of chronic hypoxia and inflammation. Such alterations lead to a lack of vessel hierarchy, and the formation of poorly perfused, leaky and blunt-ended vessels, contributing to disease progression. This thesis explores the impact of neuropilin-1 (NRP1) presentation of vascular endothelial growth factor-A (VEGF-A) to its cognate receptor, VEGFR2. NRP1 presentation of VEGF-A occurs in cis (when NRP1 and VEGFR2 are present on the same cell) or in trans (when molecules are present on adjacent cells). As shown in this thesis, the different modes of NRP1 presentation influence endothelial cell signaling and tumor angiogenesis. The overall aim with the studies has been to identify new biomarkers for cancer survival and potential therapeutic targets.

In paper I, we explored if signaling downstream of VEGFR2 was affected by NRP1 presentation in cis compared to trans. Complex formation in trans was readily identified, however, the kinetics were delayed and prolonged, inhibiting VEGFR2 internalization and downstream signaling. Additionally, in vivo tumor studies in mice demonstrated that trans presentation of NRP1 led to early inhibition of angiogenesis and suppressed tumor initiation.

In paper II, the presence and clinical impact of trans VEGFR2/NRP1 complexes in human cancer was investigated. We first identified gastric and pancreatic adenocarcinomas (PDAC) as candidates for further investigation. VEGFR2/NRP1 complexes were identified in both tumor types but were more prevalent in PDAC. Trans presentation of NRP1 in PDAC correlated with a reduction in several vessel parameters and tumor cell proliferation. Importantly, this study identified the presence of trans complexes as an independent marker of longer overall survival for PDAC patients.

In paper III, we explored the impact of NRP1 presentation modes on renal cell carcinoma (RCC) patient survival. We performed in situ proximity ligation assay (PLA) and immunofluorescence staining on a RCC cohort. Tumor cell NRP1, either trans-complexed with endothelial cell-expressed VEGFR2 as detected by in situ PLA, or alternatively, detected by immunofluorescent staining, was identified as an independent predictor of increased overall survival. These data reinforce the importance of the cell type-specific expression of cancer biomarkers.