Role of VEGF-specific gene cluster for angiogenesis

Signaling pathways and genes regulated via VEGF-A/VEGR-2 and Ang1/Tie-2 are important for vasculogenesis, sprouting angiogenesis and vessel maturation, respectively. We have recently defined gene clusters upregulated by VEGF-A in endothelial cells in comparison to the general growth factor EGF and the inflammatory cytokine IL-1a . We propose here to focus on selected genes of the VEGF-specific cluster, which are not induced by EGF and IL-1, as these are likely involved in the induction of VEGF-specific functions and constitute targets for the modulation of angiogenesis. In the major part of this project we will preferentially investigate the transcription factors HLX1 and MEF2C and will establish the secondary genes and biological functions controlled by them. In regard of HLX1 we will build on evidence already obtained by us, that HLX1 is a specific regulator of repellent guidance receptors. We will therefore investigate the potential role of HLX1 as a regulator important to balance repulsive with attracting signaling in angiogenic sprouting. For MEF2C an important role in vessel formation is supported by gene knock- out studies. Based on available evidence we will investigate whether MEF2C regulates the expression of matrix metalloproteinases and thus controls the invasive properties of the endothelial sprout. In the second part of the project we will then evaluate whether the genes of the VEGF-specific cluster or dominant negative mutants thereof can be used to modulate angiogenesis in vivo, especially if this can lead to new strategies to inhibit tumor angiogenesis. The third part of the project will compare VEGF-A-mediated with VEGF-C- and angiopoietin-induced signals and genes. We will investigate on the one hand to what extent the VEGF-A-induced genes are regulated by VEGF-C and whether the HLX1 and MEF2C-controlled processes are also functional in lymphatic endothelial cells. On the other hand we will analyze how Ang-1/Tie-2 signals interfere with VEGF-A-induced signaling and the upregulation of the VEGF-specific gene cluster, which may be important for the maturation phase of newly formed vessels.

This project has investigated the control of human blood vessel formation and defined three novel factors and their functions in either stimulating or inhibiting vessel formation. Blood vessels are of high importance for health and disease as they are essential for the distribution of oxygen, nutrients and immune cells to all organs and tissues of the body. Since deficient or excess vascularization contributes to many pathologies the modulation of vessel formation is a therapeutic goal for many diseases. For example, in myocardial ischemia it would be advantageous to trigger vessel formation as a precondition for regeneration of the heart muscle. Vice versa, the growth of all malignant forms of cancer depends on the vascularization induced by cells of the tumor or the tumor environment and in this case inhibition of vessel formation would be the therapeutic goal. New vessel formation is initiated in the adult mainly by sprouting of mature endothelial cells from the vessel wall of existing capillaries, a process called angiogenesis. It has been further proposed that endothelial progenitor cells can contribute to this process, although the extent to which this occurs is still unclear. It has been known for some time that vascular endothelial growth factor-A (VEGF-A) is the main trigger of angiogenesis and differentiation of endothelial progenitors. We have investigated in this project key transcriptional regulators induced by VEGF-A in endothelial cells and involved in angiogenesis. The first two transcription factors studied are termed HLX-1 and MEF2C. Interestingly, both factors are involved in a negative feed-back loop working like a brake on angiogenesis in the presence of sufficient oxygen, but allowing full-speed angiogenesis in the case of insufficient oxygen supply in a tissue, i.e. in ischemia. We could show that HLX achieves this function by selective upregulation of a suppressive guidance cue on the cell surface, MEF2C by the production of an inhibitive secreted protein only in the presence of oxygen, but not under ischemic conditions. The third transcription factor investigated is FOXF1, which relates to endothelial progenitor cells. We could show that in a specific endothelial progenitor cell type, which can be isolated from peripheral blood, FOXF1 is a determinant of the ability of these cells to trigger vascular sprouting. In summary, we have defined three novel pathways, two inhibitory and one stimulating, which could be used as targets to modulate vessel formation.