Characterization of a Novel Endothelial Network in Normal and Cancerous Tissues

The failure of angiogenesis inhibitors as anti-cancer therapeutics has been a major disappointment in the clinic. These inhibitors target the Vascular Endothelial Growth Factor (VEGF) signaling pathway, which research has conclusively shown is involved in promoting angiogenesis. Tumors cells secrete VEGF to stimulate VEGF-receptor (VEGFR) expressing endothelial cells resulting in the formation of new blood vessels, ultimately increasing the availability of oxygen and nutrients to the growing tumor. Yet VEGF inhibitors have had very limited success in the clinic, and in some instances, cancers treated with these drugs initially respond, but surviving tumor cells grow back more aggressively than before treatment. How cancer cells that rely on angiogenesis for growth can survive, and even thrive, when treated with VEGF inhibitors has never been fully explained and has significant clinical implications. We have found a novel vascular network in zebrafish that is comprised of an endothelial cell population that is not VEGFR positive. These vessels are small in diameter and cannot carry erythrocytes to oxygenate tissues, but are leaky and can deliver nutrients and macromolecules to cells. Our molecular studies show that these vessels are not lymphatics, and importantly, express similar markers as a network of microvessels discovered in healthy human tissue in 2018. In zebrafish cancer models, we see that these microvessels are heavily intwined in growing tumors, can be found in healthy tumor areas without associated VEGFR-positive vasculature, do not respond to VEGF inhibitors, and interestingly, provide a track for cancer cells to migrate away from the primary tumor site. The goal of our Mark Foundation application will be to fully characterize these vessels at the molecular level in both zebrafish and mouse models, and identify them in human cancer tissues. We will couple in vivo imaging with targeted vascular ablation in zebrafish to better define the microvessel contribution to cancer progression, compared to VEGFR-positive vasculature. Finally, we will use zebrafish to screen for repurposed FDA-approved drugs that can eliminate these vessels, and begin pre-clinical testing in mouse cancer models. Overall, this research will define a completely new component of cancer tissue and has the potential to have a rapid and important impact in the cancer clinic.