Oncogenic activation of MEK/ERK primes melanoma cells for adaptation to endoplasmic reticulum stress

Background: Endoplasmic reticulum (ER) stress is characterized by accumulation and aggregation of unfolded and/or misfolded proteins in the ER lumen. Cells respond by activating a range of signaling pathways to alter transcriptional and translational programs. Cancer cells are commonly subject to chronic ER stress, which must be adapted for survival and proliferation. We report here that in melanoma cells intrinsic activation of the ER stress response/unfolded protein response (UPR) is, at least in part, caused by increased outputs of protein synthesis driven by oncogenic activation of MEK/ERK. Aims: In this study, we examine the potential interaction between the constitutively activated MEK/ERK pathway and the UPR in melanoma cells. Methods: Melanoma cell lines were treated with PLX4720, U0126 or vemurafenib and subjected to analysis by western blot, QPCR, or flow cytometry. Melanocytes were lentivirally transduced with BRAFV600E before being analysed by western blot, QPCR or flow cytometry. Results: Inhibition of oncogenic BRAFV600E by PLX4720 or inhibition of MEK by U0126 attenuated activation of IRE1 and ATF6 signaling of the UPR in melanoma cells. This was associated with decreased phosphorylation of eIF4E and nascent protein synthesis, and was recapitulated by knockdown of eIF4E. In line with this, introduction of BRAFV600E into melanocytes led to increases in eIF4E phosphorylation and protein production, and triggered activation of the UPR. Paradoxically, treatment of melanoma cells with the clinically available BRAFV600E inhibitor vemurafenib resulted in induction of the UPR, an effect that may be independent of BRAFV600E inhibition. Conclusions: MEK/ERK signaling is necessary and sufficient for intrinsic activation of the UPR as a consequence of ER stress triggered by enhanced protein synthesis in melanoma cells. These results indicate that potentiation of adaptation to chronic ER stress is another mechanism by which activation of the MEK/ERK pathway promotes the pathogenesis of melanoma. The contrasting effect of PLX4720 and vemurafenib on the UPR in this study raises questions pertaining to the role of the UPR and ER stress in melanoma patients treated with BRAF inhibitors. Translational research aspect: T1: Basic Science This study illuminates the effect of PLX4720 and vemurafenib on activation of the UPR, which may be useful in developing combinational therapies or reducing side effects in melanoma patients being treated with mutant BRAF inhibitors.