The oncogenic bHLH-LZ
transcription factor Myc forms binary complexes with its binding partner Max. These and other bHLH-LZ-based
protein-
protein interactions (PPI) in the Myc-Max network are essential for the physiological and oncogenic activities of Myc. We have generated a genetically determined and highly specific
protein-fragment complementation assay based on
Renilla luciferase to analyze the dynamic interplay of bHLH-LZ
transcription factors Myc, Max, and Mxd1 in vivo. We also applied this PPI reporter to quantify alterations of nuclear Myc-Max complexes in response to mutational events, competitive binding by the transcriptional repressor Mxd1, or perturbations by small-molecule Myc inhibitors, including recently identified potent PPI inhibitors from a Kröhnke
pyridine library. We show that the specificity of Myc-Max PPI reduction by the
pyridine inhibitors directly correlates with their efficient and highly specific potential to interfere with the proliferation of human and avian
tumor cells displaying deregulated Myc expression. In a direct comparison with known Myc inhibitors using human and avian cell systems, the
pyridine compounds reveal a unique inhibitory potential even at sub-micromolar concentrations combined with remarkable specificity for the inhibition of Myc-driven
tumor cell proliferation. Furthermore, we show in direct comparisons using defined avian cell systems that different Max PPI profiles for the variant members of the Myc
protein family (c-Myc, v-Myc, N-Myc, L-Myc) correlate with their diverse oncogenic potential and their variable sensitivity to the novel
pyridine inhibitors.