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Abstract

Corepressors negatively regulate gene expression by chromatin
compaction. Targeted regulation of gene expression could provide a means
to control endothelial cell phenotype. We hypothesize that by targeting
corepressor proteins, endothelial angiogenic function can be improved.
To study this, the expression and function of nuclear corepressors in
human umbilical vein endothelial cells (HUVEC) and in murine organ
culture was studied. RNA-seq revealed that nuclear receptor corepressor
1 (NCoR1), silencing mediator of retinoid and thyroid hormone receptors
(SMRT) and repressor element-1 silencing transcription factor (REST) are
the highest expressed corepressors in HUVECs. Knockout and knockdown
strategies demonstrated that the depletion of NCoR1 increased the
angiogenic capacity of endothelial cells, whereas depletion of SMRT or
REST did not. Interestingly, the effect was VEGF signaling independent.
NCoR1 depletion significantly upregulated angiogenesis-associated genes,
especially tip cell genes, including ESM1, DLL4 and NOTCH4, as observed
by RNA- and ATAC-seq. Confrontation assays comparing cells with and
without NCoR1-deficiency revealed that loss of NCoR1 promotes a tip-cell
position during spheroid sprouting. Moreover, a proximity ligation assay
identified NCoR1 as a direct binding partner of the
Notch-signaling-related transcription factor RBPJk. Luciferase assays
showed that siRNA-mediated knockdown of NCOR1 promotes RBPJk activity.
Furthermore, NCoR1 depletion prompts upregulation of several elements in
the Notch signaling cascade. Downregulation of NOTCH4, but not NOTCH1,
prevented the positive effect of NCOR1 knockdown on spheroid outgrowth.
Collectively, these data indicate that decreasing NCOR1 expression is an
attractive approach to promote angiogenic function.