Histone modifications are now well-established mediators of transcriptional 
programs that distinguish cell states. However, the kinetics of histone 
modification and their role in mediating rapid, signal-responsive gene 
expression changes has been little studied on a genome-wide scale. Vascular 
endothelial growth factor A (VEGFA), a major regulator of angiogenesis, triggers 
changes in transcriptional activity of human umbilical vein endothelial cells 
(HUVECs). Here, we used chromatin immunoprecipitation followed by 
high-throughput sequencing (ChIP-seq) to measure genome-wide changes in histone 
H3 acetylation at lysine 27 (H3K27ac), a marker of active enhancers, in 
unstimulated HUVECs and HUVECs stimulated with VEGFA for 1, 4, and 12 h. We show 
that sites with the greatest H3K27ac change upon stimulation were associated 
tightly with EP300, a histone acetyltransferase. Using the variation of H3K27ac 
as a novel epigenetic signature, we identified transcriptional regulatory 
elements that are functionally linked to angiogenesis, participate in rapid 
VEGFA-stimulated changes in chromatin conformation, and mediate VEGFA-induced 
transcriptional responses. Dynamic H3K27ac deposition and associated changes in 
chromatin conformation required EP300 activity instead of altered nucleosome 
occupancy or changes in DNase I hypersensitivity. EP300 activity was also 
required for a subset of dynamic H3K27ac sites to loop into proximity of 
promoters. Our study identified thousands of endothelial, VEGFA-responsive 
enhancers, demonstrating that an epigenetic signature based on the variation of 
a chromatin feature is a productive approach to define signal-responsive genomic 
elements. Further, our study implicates global epigenetic modifications in 
rapid, signal-responsive transcriptional regulation.