An Acetyl-histone Vulnerability in PI3K/AKT Inhibition-resistant Cancers is Targetable by Both BET and HDAC Inhibitors

Acquisition of resistance to phosphatidylinositol 3-kinase (PI3K)/AKT-targeted monotherapy implies the existence of common resistance mechanisms independent of cancer type. Here, we demonstrate that PI3K/AKT inhibitors cause glycolytic crisis, acetyl-coenzyme A (CoA) shortage, and a global decrease in histone acetylation. In addition, PI3K/AKT inhibitors induce drug resistance by selectively augmenting histone H3 lysine 27 acetylation (H3K27ac) and binding of CBP/p300 and BRD4 proteins at a subset of growth factor and receptor (GF/R) gene loci. BRD4 occupation at these loci and drug-resistant cell growth are vulnerable to both bromodomain and histone deacetylase (HDAC) inhibitors. Little or no occupation of HDAC proteins at the GF/R gene loci underscores the paradox that cells respond equivalently to the two classes of inhibitors with opposite modes of action. Targeting this unique acetyl-histone-related vulnerability offers two clinically viable strategies to overcome PI3K/AKT inhibitor resistance in different cancers.