Atypical E2Fs, old acquaintances with new faces. A novel role for E2F7 and E2F8 in angiogenesis and lymphangiogenesis

Abstract

Proper functioning of the vertebrate body requires a system that efficiently transports oxygen, nutrients, hormones and circulating (immune) cells to all cells, tissues and organs. This task is carried out by two tree-like branched structures: the vascular and lymphatic system, which consist out of endothelial cells. The main axial vessels of the vascular system arise during embryonic development by coalescence and differentiation of endothelial cell progenitors, the angioblasts, in a process referred to as vasculogenesis. To form a branched like structure, endothelial cells from the main axial vessels start to sprout and migrate to form smaller blood vessels in a process called angiogenesis. At a certain developmental stage, endothelial cells specifically derived from the venous system start to sprout and differentiate into lymphatic endothelial cells and in a called lymphangiogenesis, these lymphatic endothelial cells branch into a blunt end system. Understanding how the process of angiogenesis and lymphangiogenesis during development is regulated on the physiological and genetic level is of great importance for developing treatments for diseases depending on or cause by the vascular or lymphatic system. The most well-known disease that depends on both angiogenesis and lymphangiogenesis is cancer, in which an individual cell starts to divide uncontrollably and ultimately forms a tumor. Initially, the tumor can be supplied with nutrients and oxygen by simple diffusion, however when tumor size exceeds the physical properties of simple diffusion, the lack of oxygen induces a genetic program within the tumor cells that ensures secretion of growth factors. These growth factors will induce the formation of blood and lymphatic vessels towards the tumor, thereby ensuring a constant supply of nutrients and oxygen. Moreover, the blood vessels also provide a way for the tumor cells to enter the blood stream and disseminate to distal organs. For this reason, it is of crucial importance to block angiogenesis and lymphangiogenesis during tumor formation. In this thesis we show that the transcription factors E2F7 and E2F8 are novel regulators of angiogenesis and lymphangiogenesis. More specific, we describe how E2F7 and E2F8 regulate vascular endothelial growth factor A (VEGFA), the most important driver behind angiogenesis. Furthermore, we show that E2F7 and E2F8 also regulate VEGFC, a close relative of VEGFA and indispensable for lymphangiogenesis. However, E2F7 and E2F8 regulate VEGFC not directly on the genetic level, but by modulating the expression of its receptor VEGF receptor 3 (FLT4). Moreover, E2F7 and E2F8 enhance the biological effect of VEGFC by regulating the expression of Collagen and Calcium-Binding EGF domains 1 (CCBE1), which has been shown to enhance the VEGFC-FLT4 signaling pathway. In sum, the results in this thesis link the E2F family with angiogenesis and lymphangiogenesis during development and cancer formation. Because E2F7 and E2F8 have been shown to be frequently deregulated in many human cancers, follow up studies should be directed to unravel whether E2F7 and E2F8 regulate the angiogenic potential of these human cancers. Ultimately, these results could contribute to improvement of current cancer therapies.