Corepressors negatively regulate gene expression by chromatin compaction. Alteration of the gene expression profile could be used as strategy to switch the phenotype of endothelial cells, key players during the process of new blood vessel formation, from a quiescent to a pro-angiogenic state. Here we hypothesised that by specifically targeting corepressor proteins, the endothelial angiogenic function could be improved. To study this, we characterized the function of nuclear repressor complexes in human umbilical vein endothelial cells (HUVECs) as well as in organ culture of mouse aortic tissue.

RNA-Seq revealed that NCoR1, SMRT and REST are the most highly expressed corepressors in HUVECs. Importantly, targeted knockdown of these corepressors demonstrated that exclusively NCoR1 depletion increased the angiogenic capacity of endothelial cells as determined by tube formation and spheroid outgrowth assay. Interestingly, these effects were independent of VEGF signalling as the VEGFR inhibitor KRN633 did not block sprouting of NCoR1 deficient cells. In contrast, DLL4-activated Notch signalling abolished these siNCoR1-evoked effects. The underlying mechanism was explored by RNA- and ATAC-Seq. NCoR1 depletion significantly upregulated the expression of angiogenesis-associated genes, especially tip cell genes, including ESM1, DLL4 and NOTCH4. Confrontation assays comparing cells with and without NCoR1-deficiency revealed that loss of NCoR1 promoted a tip-cell position during spheroid sprouting. Interestingly, a proximity ligation assay identified NCoR1 as a direct binding partner of the Notch-signalling related transcription factor RBPJk in HUVECs. Furthermore, we could show that Notch activity was increased after NCoR1 depletion as determined by luciferase assays. However, RBPJk depletion in NCoR1 deficient cells abolished the previously seen effects on angiogenic capacity.

NCoR1 is an interesting target allowing the positive modulation of pathophysiological angiogenesis via its direct interaction with the notch specific transcription factor RBPJk.