The MYC oncogenic transcription factor is acetylated by the p300 and GCN5 histone acetyltransferases. The significance of MYC acetylation and the functions of specific acetylated lysine (AcK) residues have remained unclear. Here, we show that the major p300-acetylated K148(149) and K157(158) sites in human (or mouse) MYC and the main GCN5-acetylated K323 residue are reversibly acetylated in various malignant and nonmalignant cells. Oncogenic overexpression of MYC enhances its acetylation and alters the regulation of site-specific acetylation by proteasome and deacetylase inhibitors. Acetylation of MYC at different K residues differentially affects its stability in a cell type-dependent manner. Lysine-to-arginine substitutions indicate that although none of the AcK residues is required for MYC stimulation of adherent cell proliferation, individual AcK sites have gene-specific functions controlling select MYC-regulated processes in cell adhesion, contact inhibition, apoptosis, and/or metabolism and are required for the malignant cell transformation activity of MYC. Each AcK site is required for anchorage-independent growth of MYC-overexpressing cells in vitro, and both the AcK148(149) and AcK157(158) residues are also important for the tumorigenic activity of MYC transformed cells in vivo. The MYC AcK site-specific signaling pathways identified may offer new avenues for selective therapeutic targeting of MYC oncogenic activities.

Triple-negative breast cancer (TNBC) is the breast cancer subtype with the poorest clinical outcome. The PIM family of kinases has emerged as a factor that is both overexpressed in TNBC and associated with poor outcomes. Preclinical data suggest that TNBC with an elevated MYC expression is sensitive to PIM inhibition. However, clinical observations indicate that the efficacy of PIM inhibitors as single agents may be limited, suggesting the need for combination therapies. Our screening effort identifies PIM and the 20S proteasome inhibition as the most synergistic combination. PIM inhibitors, when combined with proteasome inhibitors, induce significant antitumor effects, including abnormal accumulation of poly-ubiquitinated proteins, increased proteotoxic stress, and the inability of NRF1 to counter loss in proteasome activity. Thus, the identified combination could represent a rational combination therapy against MYC-overexpressing TNBC that is readily translatable to clinical investigations.

MYC is an oncogenic DNA-binding transcription activator of many genes and is often upregulated in human cancers. MYC has an N-terminal transcription activation domain (TAD) that is also required for cell transformation. Various MYC TAD-interacting coactivators have been identified, including the transcription/transformation-associated protein (TRRAP), a subunit of different histone acetyltransferase (HAT) complexes such as the human \"SPT3-TAF9-GCN5 Acetyltransferase\" (STAGA) complex involved in MYC transactivation of the TERT gene. However, it remains unclear whether TRRAP and/or other subunits are directly contacted by MYC within these macromolecular complexes. Here, we characterize the interactions of MYC TAD with the STAGA complex. By protein crosslinking we identify both TRRAP and the GCN5 acetyltransferase as MYC TAD-interacting subunits within native STAGA. We show that purified GCN5 binds to an N-terminal sub-domain of MYC TAD (residues 21-108) and that the interaction of GCN5 and STAGA with this sub-domain is dependent on two related sequence motifs: M2 within the conserved MYC homology box I (MBI), and M3 located between residues 100-106. Interestingly, specific substitutions within the M2/3 motifs that only moderately reduce the intracellular MYC-STAGA interaction and do not influence dimerization of MYC with its DNA-binding partner MAX, strongly inhibit MYC acetylation by GCN5 and reduce MYC binding and transactivation of the GCN5-dependent TERT promoter in vivo. Hence, we propose that MYC associates with STAGA through extended interactions of the TAD with both TRRAP and GCN5 and that the TAD-GCN5 interaction is important for MYC acetylation and MYC binding to certain chromatin loci.