Rho inhibition promotes vemurafenib resistance in BRAF mutant melanoma

Current treatments for patients with BRAF mutated melanoma show limited success utilizing the drug vemurafenib by only temporarily stopping cancer cell growth. Eventually BRAF mutated cells come back completely resistant to vemurafenib. The mechanisms behind emerging resistance are not well understood, yet studies suggest that actin polymerization through Rho small family GTPase signaling have an important role. In an attempt to address this working hypothesis we created an in vitro model using the A375 cell line and treated the cells with vemurafenib combined with RNAi knockdowns of Rho or using a drug named fasudil, which inhibits ROCK kinase a downstream target of Rho. Our data showed that through either direct or indirect Rho pathway inhibition there was a promotion of acquired resistance in the A375 cell line. Additionally we see an up regulation of transcription factors Yap/Taz and increased protein expression of pPaxillin Y31, pMEK S217, and pMEK S298. All of these proteins have been associated with Rac1 activity or BRAF activity within the cell. The increase in Rac1 activity may help give us a better explanation of what cellular pathways BRAF mutated cells utilize to promote resistance.