The c-Jun/RHOB/AKT pathway confers resistance of BRAF-mutant melanoma cells to MAPK inhibitors.

Audrey Delmas,Gilles Favre,Olivier Sordet,Anne Casanova,Laurence Lamant,Anne Pradines,Julia Cherier,Nicolas Meyer,Ariel Savina,Claire Medale-Giamarchi,Magdalena Pohorecka

doi:10.18632/oncotarget.3888

Abstract

The response of BRAF-mutant melanoma patients to BRAF inhibitors is dramatically impaired by secondary resistances and rapid relapse. So far, the molecular mechanisms driving these resistances are not completely understood. Here, we show that, in BRAF-mutant melanoma cells, inhibition of BRAF or its target MEK induces RHOB expression by a mechanism that depends on the transcription factor c-Jun. In those cells, RHOB deficiency causes hypersensitivity to BRAF and MEK inhibitors-induced apoptosis. Supporting these results, loss of RHOB expression in metastatic melanoma tissues is associated with an increased progression-free survival of BRAF-mutant patients treated with vemurafenib. Following BRAF inhibition, RHOB activates AKT whose inhibition causes hypersensitivity of BRAF-mutant melanoma cells to BRAF inhibitors. In mice, AKT inhibition synergizes with vemurafenib to block tumor growth of BRAF-mutant metastatic melanoma. Our findings reveal that BRAF inhibition activates a c-Jun/RHOB/AKT pathway that promotes tumor cell survival and further support a role of this pathway in the resistance of melanoma to vemurafenib. Our data also highlight the importance of using RHOB tumor levels as a biomarker to predict vemurafenib patient's response and to select those that would benefit of the combination with AKT inhibitors.

Highlights

The hypothesis of melanoma addiction to the RAF/ MEK/ERK pathway, namely the MAPK pathway, emerged with the discovery of a high frequency of the BRAFV600Eactivating mutation in melanoma cell lines and primary tumors [1]
ERK phosphorylation was recovered after 24 h indicating that BRAF inhibition was obliterated (Figure 1G). These results suggest that BRAF inhibition is required to prime RHOB induction, an adaptive mechanism of melanoma cells may occur following PLX4032 treatment to maintain high levels of RHOB
In this study we demonstrate that inhibition of mutant BRAF in melanoma cells with pharmacological inhibitors or siRNA knockdown results in the activation of the c-Jun/ RHOB/AKT signaling axis, which leads to cell survival

Summary

Introduction

The hypothesis of melanoma addiction to the RAF/ MEK/ERK pathway, namely the MAPK pathway, emerged with the discovery of a high frequency of the BRAFV600Eactivating mutation in melanoma cell lines and primary tumors [1]. MAPK inhibition modulates transcription factors expression that could regulate adaptive responses [14]. Their roles are not clearly defined, the genes whose expression is modulated could impact on the sensitivity of melanoma cells to BRAFi and could be of clinical relevance. In line with this hypothesis, the transcription factor FOXD3 and the pro-apoptotic BH3-only protein NOXA, are induced and down-regulated, respectively following PLX4032 treatment promoting resistance to cell death [15, 16]. MAPK inhibition can regulate per se the expression of resistance factors

Methods

Results

Conclusion