Reactivation of Mitogen-activated Protein Kinase (MAPK) Pathway by FGF Receptor 3 (FGFR3)/Ras Mediates Resistance to Vemurafenib in Human B-RAF V600E Mutant Melanoma

Oncogenic B-RAF V600E mutation is found in 50% of melanomas and drives MEK/ERK pathway and cancer progression. Recently, a selective B-RAF inhibitor, vemurafenib (PLX4032), received clinical approval for treatment of melanoma with B-RAF V600E mutation. However, patients on vemurafenib eventually develop resistance to the drug and demonstrate tumor progression within an average of 7 months. Recent reports indicated that multiple complex and context-dependent mechanisms may confer resistance to B-RAF inhibition. In the study described herein, we generated B-RAF V600E melanoma cell lines of acquired-resistance to vemurafenib, and investigated the underlying mechanism(s) of resistance. Biochemical analysis revealed that MEK/ERK reactivation through Ras is the key resistance mechanism in these cells. Further analysis of total gene expression by microarray confirmed a significant increase of Ras and RTK gene signatures in the vemurafenib-resistant cells. Mechanistically, we found that the enhanced activation of fibroblast growth factor receptor 3 (FGFR3) is linked to Ras and MAPK activation, therefore conferring vemurafenib resistance. Pharmacological or genetic inhibition of the FGFR3/Ras axis restored the sensitivity of vemurafenib-resistant cells to vemurafenib. Additionally, activation of FGFR3 sufficiently reactivated Ras/MAPK signaling and conferred resistance to vemurafenib in the parental B-RAF V600E melanoma cells. Finally, we demonstrated that vemurafenib-resistant cells maintain their addiction to the MAPK pathway, and inhibition of MEK or pan-RAF activities is an effective therapeutic strategy to overcome acquired-resistance to vemurafenib. Together, we describe a novel FGFR3/Ras mediated mechanism for acquired-resistance to B-RAF inhibition. Our results have implications for the development of new therapeutic strategies to improve the outcome of patients with B-RAF V600E melanoma.