RAF-Mutant Melanomas Differentially Depend on ERK2 Over ERK1 to Support Aberrant MAPK Pathway Activation and Cell Proliferation

Abstract

Half of advanced human melanomas are driven by mutant BRAF and dependent on MAPK signaling. Interestingly, the results of three independent genetic screens highlight a dependency of BRAF-mutant melanoma cell lines on BRAF and ERK2, but not ERK1. ERK2 is expressed higher in melanoma compared with other cancer types and higher than ERK1 within melanoma. However, ERK1 and ERK2 are similarly required in primary human melanocytes transformed with mutant BRAF and are expressed at a similar, lower amount compared with established cancer cell lines. ERK1 can compensate for ERK2 loss as seen by expression of ERK1 rescuing the proliferation arrest mediated by ERK2 loss (both by shRNA or inhibition by an ERK inhibitor). ERK2 knockdown, as opposed to ERK1 knockdown, led to more robust suppression of MAPK signaling as seen by RNA-sequencing, qRT-PCR, and Western blot analysis. In addition, treatment with MAPK pathway inhibitors led to gene expression changes that closely resembled those seen upon knockdown of ERK2 but not ERK1. Together, these data demonstrate that ERK2 drives BRAF-mutant melanoma gene expression and proliferation as a function of its higher expression compared with ERK1. Selective inhibition of ERK2 for the treatment of melanomas may spare the toxicity associated with pan-ERK inhibition in normal tissues. Implications: BRAF-mutant melanomas overexpress and depend on ERK2 but not ERK1, suggesting that ERK2-selective inhibition may be toxicity sparing.

Identification of cell type–specific correlations between ERK activity and cell viability upon treatment with ERK1/2 inhibitors

“…For example, our results indicate that, for SH-SY5Y and HCT-116 cells, ulixertinib exhibits less excessive toxicity than SCH772984, which is supported by KinomeScan data that show less off-targets for ulixertinib ( 49 ) than for SCH772984 ( 38 ). Also, it was recently suggested that BRAF mutant cancer cells are dependent on ERK2, but not ERK1 ( 50 ), so selective targeting of ERK2 rather than both kinases may reduce the toxicity of ERK1/2 inhibitors. Ulixertinib and ravoxertinib, which showed lower toxicity than other drugs, have a higher affinity toward ERK2 than ERK1 ( 28 , 48 ).…”

Alternative Splicing of MAPKs in the Regulation of Signaling Specificity

“…The main proteins translated by the ERK1 and ERK2 genes are the 44 and 42 kDa isoforms, respectively. These isoforms usually share similar characteristics, although some differences between them do exist under restricted conditions or in specific cell types [ 108 , 109 , 110 , 111 , 112 , 113 , 114 , 115 , 116 , 117 ]. Aside from these main isoforms, initial studies reported the existence of additional, slightly different transcripts called ERK1psi [ 118 ] and ERK2a [ 119 ]; however, these transcripts exist only on the RNA level and are not translated into a stable protein.…”