The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that has been shown to be an essential regulator of a broad spectrum of biological activities required for maintaining the body’s vital functions. AhR also plays a critical role in tumorigenesis. Its role in cancer is complex, encompassing both pro- and anti-tumorigenic activities. Its level of expression and activity are specific to each tumor and patient, increasing the difficulty of understanding the activating or inhibiting roles of AhR ligands. We explored the role of AhR in tumor cell lines and patients using genomic data sets and discuss the extent to which AhR can be considered as a therapeutic target.

The nongenetic mechanisms required to control tumor phenotypic plasticity and shape drug‐resistance remain unclear. We show here that the Aryl hydrocarbon Receptor (AhR) transcription factor directly regulates the gene expression program associated with the acquisition of resistance to BRAF inhibitor (BRAFi) in melanoma. In addition, we show in melanoma cells that canonical activation of AhR mediates the activation of the SRC pathway and promotes the acquisition of an invasive and aggressive resistant phenotype to front‐line BRAFi treatment in melanoma. This nongenetic reprogramming identifies a clinically compatible approach to reverse BRAFi resistance in melanoma. Using a preclinical BRAFi‐resistant PDX melanoma model, we demonstrate that SRC inhibition with dasatinib significantly re‐sensitizes melanoma cells to BRAFi. Together we identify the AhR/SRC axis as a new therapeutic vulnerability to trigger resistance and warrant the introduction of SRC inhibitors during the course of the treatment in combination with front‐line therapeutics to delay BRAFi resistance.

BRAF and MEK inhibitors (BRAFi and MEKi) are the standard of care for the treatment of metastatic melanoma in patients with BRAF^V600E mutations, greatly improving progression-free survival. However, the acquisition of resistance to BRAFi and MEKi remains a difficult clinical challenge, with limited therapeutic options available for these patients. Here, we investigated the therapeutic potential of natural flavonoids as specific AhR (Aryl hydrocarbon Receptor) transcription factor antagonists in combination with BRAFi. Experimental Design: Experiments were performed in vitro and in vivo with various human melanoma cell lines (mutated for BRAF^V600E) sensitive or resistant to BRAFi. We evaluated the role of various flavonoids on cell sensitivity to BRAFi and their ability to counteract resistance and the invasive phenotype of melanoma. Results: Flavonoids were highly effective in potentiating BRAFi therapy in human melanoma cell lines by increasing sensitivity and delaying the pool of resistant cells that arise during treatment. As AhR antagonists, flavonoids counteracted a gene expression program associated with the acquisition of resistance and phenotype switching that leads to an invasive and EMT-like phenotype. Conclusions: The use of natural flavonoids opens new therapeutic opportunities for the treatment of patients with BRAF-resistant disease.

Most genetic alterations that drive melanoma development and resistance to targeted therapy have been uncovered. In contrast, and despite their increasingly recognized contribution, little is known about the non-genetic mechanisms that drive these processes. Here, we performed in vivo gain-of-function CRISPR screens and identified SMAD3 , BIRC3 and SLC9A5 as key actors of BRAFi-resistance and these genes promote the tumor growth capability of persister cells. We show that their expression levels increase during acquisition of BRAFi-resistance, and remain high in persister cells and during relapse. The upregulation of the SMAD3 transcriptional activity (SMAD3-signature) promotes a mesenchymal-like phenotype and BRAFi-resistance by acting as an upstream transcriptional regulator of potent BRAFi-resistance genes such as EGFR and AXL. This SMAD3-signature predicts resistance to both current melanoma therapies in different cohorts. Critically, chemical inhibition of SMAD3 may constitute amenable target for melanoma since it efficiently abrogates persister cells survival. Interestingly, decrease of SMAD3 activity can also be reached by inhibiting the aryl hydrocarbon receptor (AhR), another druggable transcription factor governing SMAD3 expression level. Our work expands our understanding of the biology of persister cells and highlight novel drug vulnerabilities that can be exploited to develop long-lasting antimelanoma therapies.

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that has been shown to be an essential regulator of a broad spectrum of biological activities required for maintaining the body's vital functions. AhR also plays a critical role in tumorigenesis. Its role in cancer is complex, encompassing both pro- and anti-tumorigenic activities. Its level of expression and activity are specific to each tumor and patient, increasing the difficulty of understanding the activating or inhibiting roles of AhR ligands. We explored the role of AhR in tumor cell lines and patients using genomic data sets and discuss the extent to which AhR can be considered as a therapeutic target.

Le mélanome est la forme la plus mortelle de cancer cutané. La moitié des patients portent la mutation V600E de la kinase BRAF (BRAFV600E), activant de façon constitutive la voie de signalisation des MAPK impliquée dans la survie et la prolifération cellulaire. Les inhibiteurs de BRAF (BRAFi) ciblant spécifiquement la kinase mutée engendrent d’excellentes réponses chez les patients. Cette phase de réponse initiale est malheureusement suivie par le développement de mécanismes de résistance systématique conduisant à la rechute des patients. Il est aujourd’hui établi que l’hétérogénéité intratumorale et la plasticité phénotypique, c’est-à-dire la capacité des cellules tumorales à s’adapter au traitement, participent à l’émergence de cellules résistantes conduisant à la rechute. Ici, nous impliquons le facteur de transcription Aryl hydrocarbon Receptor (AhR) dans la modulation de la sensibilité ou la résistance des cellules de mélanomes aux BRAFi. Ainsi, les BRAFi sont des ligands d’AhR et l’activation non canonique du facteur de transcription par ces molécules permet l’expression d’un programme transcriptomique associé à la sensibilité des cellules aux BRAFi. À l’inverse, l’activation constitutive et canonique du facteur de transcription AhR dans les cellules de mélanome induit l’expression de gènes associés à l’acquisition de la résistance aux BRAFi. AhR participe également à l’acquisition du phénotype invasif/dédifférencié des cellules de mélanomes, associé à l’agressivité et à la résistance aux BRAFi. A l’inverse son inhibition, génétique (CRISPR-Cas9) ou chimique (antagoniste), permet de réengager les cellules vers un programme de sensibilité aux BRAFi. L’utilisation thérapeutique des antagonistes d’AhR tels les flavonoïdes, en combinaison avec les BRAFi, permet de potentialiser l’efficacité du traitement. La phase de réponse au traitement est donc allongée, limitant le développement des mécanismes de résistance in vitro et in vivo. Ainsi, AhR représente une nouvelle cible thérapeutique hautement intéressante. L’inhibition de son activité par l’utilisation d’un antagoniste, en combinaison avec les BRAFi actuellement utilisés, pourrait permettre chez les patients de retarder le développement des résistances et potentiellement la rechute.