Loss of the Aryl Hydrocarbon Receptor (AhR) Promotes Cancer Cells Resistance to BRAFV600E Targeted Therapies. [RNA-Seq]

Predicting a patient’s response to chemotherapies and identifying additional molecular targets to improve treatment efficacy are major objectives in cancer research. BRAFV600E inhibitors targeting the MAP kinase pathway showed promising initial clinical results in thyroid cancers (TCs) for metastatic or recurrent tumors refractory to radioiodine treatments. BRAFV600E-targeted therapies have also been approved by the FDA for the treatment of some unresectable or metastatic BRAFV600E+ solid tumors. Still, for most patients, the response to BRAFV600E blocking therapy is transient due to cell proliferation reactivation through escape pathways. We performed a genome-wide CRISPR screen to reveal targets in TCs that facilitate resistance or sensitize thyroid cancer cells to inhibitors of the MAP kinase pathway. Among the genes that consistently altered MAPK inhibitor treatment response, we identified the Aryl hydrocarbon Receptor (AhR) and its molecular partner, the AhR nuclear translocator (ARNT). The AHR-ARNT heteroduplex is an environmental sensor that integrates extracellular, endogenous, and metabolic signals to equilibrate cell activity. Inactivation of AhR or ARNT increased TC cells' resistance to targeted therapies. Our study revealed that AhR-deficient cancer cells expressed elevated activity in genes linked to the TGFβ-mediated SMAD2/3 pathway after drug treatment. Our results suggest that SMAD2/3 competes to bind ARNT and that the loss of AhR gives cancer cells a potent escape pathway to targeted therapies, counteracting MAPK inhibition with SMAD activation to sustain cancer cell proliferation.