The MYC oncogene induces specific surfaceome changes associated with TGF-β hypersensitivity in mammary cells.

Aubert, Léo [UCL] Nourreddine, Sami Gouronnec, Alizé Lavoie, Genevieve Méant, Antoine Roux, Philippe P.

The c-Myc oncoprotein belongs to a family of “super-transcription factors” that regulates the transcription of several downstream effectors, thereby orchestrating a broad range of biological responses. c-Myc has long been considered the Higgs boson of anticancer drug targets because of its high expression levels and functional deregulation in many human cancers. Although c-Myc inhibition would be a powerful approach for the treatment of many cancers, direct targeting of c-Myc has been challenging owing to its “undruggable” protein structure. Hence, alternatives to c-Myc direct blockade have been widely explored to achieve desirable anti-tumor effects, including c-Myc/Max complex disruption, c-Myc destabilization, as well as the targeting of synthetic lethality proteins. Nevertheless, this enthusiasm has been mitigated by the recognition that c-Myc is almost universally involved in normal physiological processes, thereby raising concerns that c-Myc inhibition would lead to unacceptable toxicities. Intriguingly, recent transcriptomic data from our lab predict that most of the changes associated with c-Myc overexpression involved transcripts coding for cell surface proteins. Elucidating how oncogenic c-Myc modifies the cell surface proteome (surfaceome) could help understand its complex mechanism of action, and possibly identify new “druggable” targets and/or tumor-specific biomarkers. Herein, we have optimized a cutting-edge chemoproteomic approach based on the labeling of cell surface proteins with biotin reagents, their subsequent purification with avidin chromatography, and quantification using label-free quantitative proteomics with liquid chromatography-tandem mass spectrometry (LC-MS/MS). Using mammary epithelial cells overexpressing c-Myc as model system, our LC-MS/MS analyses allowed the identification of >500 cell surface molecules from which 32% and 21% were significantly upregulated and downregulated in c-Myc-overexpressing cells, respectively. Interestingly, we found that c-Myc modulates the surface expression of a large network of proteins associated with TGF-β-induced Epithelial-Mesenchymal Transition (EMT). These results were confirmed using several in vitro assays, which highlighted the existence of a TGF-β autocrine loop induced specifically by c-Myc overexpression. Taken together, these results indicate that oncogenic c-Myc leads to a massive reprogramming of the epithelial cell surface, and suggest new options for targeting c-Myc-dependent cancers.