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AhR activity directs BRAF inhibitors resistance in metastastic melanoma

ABSTRACT: BRAF oncogene is mutated in ~50% of human cutaneous melanomas. The BRAF V600E mutation leads to constitutive activation of the mitogen-activated protein kinase (MAPK) pathway fuelling cancer growth. The inhibitors of BRAF V600E (BRAFi), lead to massive and high response rate. However, BRAFi-resistant cells that operate as a cellular reservoir for relapses severely limits the duration of the clinical response. The recent depiction of these resistant cells did not identify druggable targets to ensure long-term survival under BRAFi. Here, we identify the aryl hydrocarbon receptor (AhR) as a target to eradicate resistant cells. We show that BRAFi bind to AhR on a new site, named beta-pocket, and reprogram gene expression independently of its partner ARNT. beta-pocket activation induces a pigmentation signature, which is associated to BRAFi-induced cell death of sensitive BRAF V600E melanoma cells and tumour shrinkage. Intriguingly, in resistant cells, BRAFi does not induced a pigmentation signature since these cells display another AhR program; AhR-ARNT dependant. By this way, AhR directs several key BRAFi-resistant genes. At single cell level, this constitutive activation of AhR-ARNT is identified in rare cells before BRAFi-treatment of melanoma tumours and an enrichment of these alpha-cells is observed under BRAFi. Our data strongly suggest that an endogenous AhR ligand activates AhR-ARNT via the canonical AhR pocket (alpha-pocket), thus favouring BRAFi-resistant gene expression. Importantly, we identify the clinically compatible AhR antagonist, the resveratrol (RSV), able to abrogate the deleterious constitutive activation of AhR and to reduce the cellular reservoir for the relapse. Taken together, this work reveals that constitutive AhR signalling drives BRAFi resistance and constitutes a therapeutic target to achieve long-term patient survival under BRAFi. More broadly, the constitutive activation of AhR by endogenous ligands is in line with the ability of UV radiations to generate potent AhR ligands and to favour melanoma onset.

ORGANISM(S): Homo sapiens

PROVIDER: GSE104869 | GEO | 2018/11/29

REPOSITORIES: GEO

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Project description:Background Main drawback of BRAF/MEK inhibitors (BRAF/MEKi)-based targeted therapy in the management of BRAF- mutated cutaneous metastatic melanoma (MM) is the development of therapeutic resistance. We aimed to define in this context the specific role of mTORC2, a signaling complex defined by the presence of the essential RICTOR subunit and regarded as an oncogenic driver in several tumor types, including MM. Methods After analyzing the TCGA MM patients’ database to explore both overall survival and molecular signatures as a function of intra-tumor RICTOR levels, we investigated the effects of RICTOR downregulation in BRAF V600E MM cell lines on their response to BRAF/MEKi in vitro. We performed a proteomic screening to identify proteins modulated by changes in RICTOR expression and Seahorse analysis to evaluate the effects of RICTOR depletion on mitochondrial respiration. The combination of BRAFi with drugs targeting proteins and processes emerged in the proteomic screening was carried out on RICTOR-deficient cells in vitro and in a xenograft setting in vivo. Results We found that low RICTOR levels in MM correlate with an overall worse clinical outcome. GSEA of low- RICTOR tumors revealed a gene expression signature suggestive of activation of the mitochondrial Electron Transport Chain (ETC) energy producing pathway. RICTOR-deficient BRAF V600E cells are intrinsically tolerant to BRAFi/MEKi and anticipate the onset of resistance to BRAFi upon prolonged drug exposure. In RICTOR- depleted cells, both mitochondrial respiration and expression of nicotinamide phosphoribosyltransferase (NAMPT) are enhanced, while their pharmacological inhibition restores sensitivity to BRAFi. Conclusions Our work unveils a novel tumor suppressing role for mTORC2 in the responses of BRAF V600E melanoma cells to targeted therapy and identifies the NAMPT-ETC axis as a potential therapeutic vulnerability of low- RICTOR tumors. Importantly, our findings support the concept that the evaluation of intra-tumor RICTOR levels in MM has a prognostic value, and may help predicting the response of patients to targeted therapy.

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AhR activity directs BRAF inhibitors resistance in metastastic melanoma

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