Fibroblasts Promote Resistance to BRAF inhibition; The Role of Molecular, Signaling and Metabolic Changes in Melanoma

Malignant melanoma is an aggressive type of skin cancer which urgently requires new and efficient treatment strategies. A novel targeted agent for advanced melanoma, vemurafenib, has shown high response rates among patients with BRAF mutation. However, relapses occur in almost all cases after a short period of progression-free survival. This project focused on the nature of vemurafenib resistance, specifically the role of the interaction between the cancer and the stromal cells. Here we used an extended melanoma cell panel to show that the stromal fibroblasts elicit strong protection of the melanoma cells against vemurafenib and do so via activation of mTORC1. We demonstrate that the fibroblast-mediated protection relies on direct cell-cell proximity and/or contacts. However, we could not find evidence of gap-junction involvement. We found that fibroblasts alter gene expression in the melanoma cells, inducing an invasive dedifferentiated molecular phenotype associated with resistance to vemurafenib. Melanoma cells which have been in contact with the fibroblasts also had altered expression of the energy metabolism regulators which suggested a decrease in the mitochondrial function. We further show that the altered cancer metabolism can serve as a target for therapeutic intervention since the melanoma cells in co-cultures were more vulnerable to the treatment with the mitochondrial stimulant, dicholoacetate, than in mono-cultures. Finally, we explored the changes in the energy metabolism of the melanoma cells induced by vemurafenib and the mTORC1 inhibitor, everolimus. We found a decrease in the mitochondrial activity triggered by both agents and a reduction of lactate production after the treatment with vemurafenib. Our findings suggest lung fibroblasts as important regulators of melanoma response to vemurafenib. The results provide hints about potential targets for therapeutic intervention in order to overcome stroma-mediated protection