Objective: Corepressor proteins have an inhibitory effect on gene expression by chromatin compaction. Altered gene expression can be used to switch the endothelial phenotype from a quiescent to a pro-angiogenic state. We hypothesised that by specifically targeting endothelial corepressors angiogenesis can be improved. To study this aspect, we characterized the function of nuclear repressor complexes in human umbilical vein endothelial cells (HUVEC) as well as in organ culture of mouse aortic tissue.

Results: RNAseq revealed that numerous copressors are expressed in HUVEC with NCoR1, SMRT and CoREST showing the highest expression. siRNA knockdown of these repressors demonstrated the selective loss of NCoR1 increased endothelial cell angiogenic capacity as determined by spheroid outgrowth assay. Confrontation assays comparing cells with and without NCoR1 deficiency revealed that loss of NCoR1 promoted a tip-cell position. Interestingly, these effects were independent of VEGF signalling as the VEGFR inhibitor KRN633 did not affect NCoR1 mediated sprouting. However, DLL4 activated Notch signalling abolished NCoR1 evoked sprouting back to basal level. To determine the underlying mechanism, RNAseq was performed. Loss of NCoR1 significantly upregulated the expression of 1200 genes among them many associated with tip cell formation or pro-angiogenic processes such as ESM1, FLT4 and NOTCH4.

Conclusion: NCoR1 is an interesting target allowing the positive modulation of pathophysiological angiogenesis related processes such as diabetes or atherosclerosis.