The most frequent genetic alterations in melanoma are gain-of-function mutations in BRAF, which result in addiction to RAF-MEK-ERK signaling. Given the centrality of this pathway in many cancers, inhibitors have been developed against various nodes in this signaling axis. Despite the success of these inhibitors in treating BRAFV600E mutant tumors, a major problem is the inevitable emergence of drug resistance. The mechanisms of acquired resistance often include reactivation of the MAPK pathway, thus efforts to combat resistance have used vertical combination strategies to inhibit multiple nodes of the pathway. Previous work from our group showed that vemurafenib-resistant melanoma tumors displayed increased levels of MAPK signaling as indicated by elevated phospho-ERK. Strikingly, cell lines derived from these resistant tumors were addicted to vemurafenib, and removal of drug caused loss in cell viability. Consistently, in vivo, the tumors regressed upon cessation of drug treatment, which was accompanied by elevated MAPK signaling. Recent studies have reported similar results for in vitro models of melanoma resistant to MAPK inhibitors, which are addicted to drug in an ERK2-dependent manner. Together these data suggest that extreme levels of MAPK signaling are not tolerated by tumor cells. To further characterize this phenomenon, we generated models of inducible MAPK hyperactivation in BRAFV600E melanoma cells. Using these model systems, we confirmed that increasing the levels of MAPK signaling can hinder tumor cell proliferation, and demonstrated that MAPK inhibitors can rescue this phenotype. Intriguingly, the effect of MAPK hyperactivation varied depending on the RAS/RAF mutation status of the tumor cells. Upon induction of MAPK hyperactivation in sensitive cells, there was no cell cycle arrest, but instead cell death occurred, which was accompanied by activation of caspases. On a global level, MAPK hyperactivation resulted in transcriptome changes that were opposite to those produced by treatment with an ERK inhibitor. This phenomenon of MAPK hyperactivation-induced cell death was also observed in vivo, where complete tumor regression was observed in a xenograft model. Ongoing studies seek to further define the mechanism of cell death, which could suggest potential avenues to leverage hyperactivation of MAPK signaling as a therapeutic approach.
Citation Format: Grace P. Leung, Tianshu Feng, Matthew D. Shirley, Frederic D. Sigoillot, David A. Ruddy, Alyson K. Freeman, Mariela Jaskelioff, Darrin D. Stuart. Deleterious effects of MAPK pathway hyperactivation in BRAF-mutant melanoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 965.