Hyperactivation of MAPK signaling is deleterious to RAS/RAF mutant melanoma

The most frequent genetic alterations in melanoma are gain-of-function mutations in BRAF, which result in addiction to the RAF-MEK-ERK signaling pathway. Despite success of RAF and MEK inhibitors in treating BRAFV600 mutant tumors, a major challenge is the inevitable emergence of drug resistance, which often involves reactivation of the MAPK pathway. Interestingly, resistant tumors are often sensitive to drug withdrawal, suggesting that hyperactivation of the MAPK pathway is not tolerated. To further characterize this phenomenon, we generated isogenic models of inducible MAPK hyperactivation in BRAFV600E melanoma cells by overexpression of ERK2. Using this model system, we demonstrated that supra-physiological levels of MAPK signaling led to cell death, which was reversed by MAPK inhibitors. Whereas MAPK pathway inhibition led to cell stasis in BRAFV600E melanoma cells, MAPK hyperactivation induced cytotoxicity. Furthermore, complete tumor regression was observed in an ERK2 overexpressing xenograft model. To identify mediators of MAPK hyperactivation- induced cell death, we conducted a large-scale pooled screen which showed that only shRNAs against BRAF and MAP2K1 rescued loss of cell viability. This suggested that no single downstream ERK2 effector was required, consistent with pleiotropic effects on multiple cellular stress pathways. Intriguingly, the detrimental effect of MAPK hyperactivation could be partially attributed to secreted factors, and more than 100 differentially secreted proteins were identified. The effect of ERK2 overexpression was highly context dependent, as RAS/RAF mutant but not RAS/RAF wildtype melanoma were sensitive to this perturbation. This vulnerability to MAPK hyperactivation raises the possibility of a novel therapeutic approach for RAS/RAF mutant cancers.

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