The
histone acetyltransferase (HAT)
enzymes p300 and CBP are closely related paralogs that serve as transcriptional coactivators and have been found to be dysregulated in
cancer and other diseases. p300/CBP is a multidomain
protein and possesses a highly conserved bromodomain that has been shown to bind acetylated Lys residues in both
proteins and various small molecules, including
I-CBP112 and CBP30. Here we show that the
ligand I-CBP112 can stimulate
nucleosome acetylation up to 3-fold while CBP30 does not. Activation of p300/CBP by
I-CBP112 is not observed with the isolated
histone H3 substrate but requires a
nucleosome substrate.
I-CBP112 does not impact
nucleosome acetylation by the isolated p300 HAT domain, and the effects of
I-CBP112 on p300/CBP can be neutralized by CBP30, suggesting that
I-CBP112 likely allosterically activates p300/CBP through bromodomain interactions. Using mass spectrometry and Western blots, we have found that
I-CBP112 particularly stimulates acetylation of Lys18 of
histone H3 (H3K18) in
nucleosomes, an established in vivo site of p300/CBP. In addition, we show that
I-CBP112 enhances H3K18 acetylation in acute
leukemia and
prostate cancer cells in a concentration range commensurate with its antiproliferative effects. Our findings extend the known pharmacology of bromodomain
ligands in the regulation of p300/CBP and suggest a novel approach to modulating
histone acetylation in
cancer.