Discovery and characterization of a small molecule that modulates c-Myc mediated transcription via max homodimer stabilization

Abstract

The transcription factor Myc is a basic helix-loop-helix leucine zipper (bHLHLZ) protein with crucial roles in regulating normal cellular processes, but its transcriptional activity is deregulated in a majority of human cancers. Myc transcriptional activity is dependent on dimerization with its obligate partner Max, another bHLHLZ transcription factor. Max also forms homodimers as well as heterodimers with other proteins including the Mxd family of proteins, creating a dynamic network of protein-protein interactions to regulate transcriptional programs. Despite the significance of this network, the arsenal of chemical probes to interrogate these proteins in cancer biology remains limited. Here, we utilized small molecule microarrays and luciferase-based reporter assays to identify compounds that bind Max and modulate Myc transcriptional activity. We discovered the small molecule KI-MS2-008, which stabilizes the Max homodimer while reducing Myc protein and Myc-regulated transcript levels. KI-MS2-008 also decreases viable cancer cell growth in a Myc-dependent manner and suppresses tumor growth in mouse models of Myc-driven cancers. In a cancer cell line model treated with KI-MS2-008, the equilibrium of protein-protein interactions shifts toward a transcriptionally repressed state over time by recruiting Mxd4 and other repressive machinery to Max. This study establishes that perturbing Max dimerization with small molecules is a tractable approach to targeting Myc activity in cancer.