Intravital imaging of BRAF-mutant melanoma cells containing an ERK/MAPK biosensor reveals how the tumor microenvironment affects response to BRAF inhibition by PLX4720.,Initially, melanoma cells respond to PLX4720, but rapid reactivation of ERK/MAPK is observed in areas of high stromal density.,This is linked to “paradoxical” activation of melanoma-associated fibroblasts by PLX4720 and the promotion of matrix production and remodeling leading to elevated integrin β1/FAK/Src signaling in melanoma cells.,Fibronectin-rich matrices with 3-12 kPa elastic modulus are sufficient to provide PLX4720 tolerance.,Co-inhibition of BRAF and FAK abolished ERK reactivation and led to more effective control of BRAF-mutant melanoma.,We propose that paradoxically activated MAFs provide a “safe haven” for melanoma cells to tolerate BRAF inhibition.,•BRAF mutant melanoma cells respond to PLX4720 heterogeneously in vivo•BRAF inhibition activates MAFs, leading to FAK-dependent melanoma survival signaling•ECM-derived signals can support residual disease•BRAF and FAK inhibition synergize in pre-clinical models,BRAF mutant melanoma cells respond to PLX4720 heterogeneously in vivo,BRAF inhibition activates MAFs, leading to FAK-dependent melanoma survival signaling,ECM-derived signals can support residual disease,BRAF and FAK inhibition synergize in pre-clinical models,Hirata et al. show that the BRAF inhibitor PLX4720 promotes melanoma-associated fibroblasts in BRAF-mutant melanomas to produce and remodel matrix, leading to integrin β1-FAK-Src signaling and reactivation of ERK and MAPK in melanoma cells.,Co-inhibition of BRAF and FAK blocks ERK reactivation.
YAP and its paralog protein TAZ are downstream effectors of the Hippo pathway.,Both are amplified in many human cancers and promote cell proliferation and epithelial-mesenchymal transition.,Little is known about the status of the Hippo pathway in cutaneous melanoma.,We profiled Hippo pathway component expression in a panel of human melanoma cell lines and melanocytic lesions, and characterized the capacity of YAP and TAZ to control melanoma cell behavior.,YAP and TAZ immuno-staining in human samples revealed mixed cytoplasmic and nuclear staining for both proteins in benign nevi and superficial spreading melanoma.,TAZ was expressed at higher levels than YAP1/2 in all cell lines and in those with high invasive potential.,Stable YAP or TAZ knockdown dramatically reduced the expression of the classical Hippo target CCN2/connective-tissue growth factor (CTGF), as well as anchorage-independent growth, capacity to invade Matrigel, and ability form lung metastases in mice following tail-vein injection.,YAP knockdown also reduced invasion in a model of skin reconstruct.,Inversely, YAP overexpression increased melanoma cell invasiveness, associated with increased TEA domain-dependent transcription and CCN2/CTGF expression.,Together, these results demonstrate that both YAP and TAZ contribute to the invasive and metastatic capacity of melanoma cells and may represent worthy targets for therapeutic intervention.