V599E B-Raf as the next HSP90 client protein: A novel strategy for selective inhibition of mutated B-Raf

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The discovery of frequent mutations in the B-Raf gene in human tumors has generated enormous interest in how this Raf kinase is regulated and how mutations in B-Raf lead to transformation. In this study we provide new insight into these questions by finding that mutational activation of B-Raf is associated with gain of dependence on Hsp90 and its presence in an Hsp90-cdc37 complex. Wild type B-Raf, in contrast, is unaffected by Hsp90 inhibitors and doesn’t require Hsp90 for conformational maturation and stability. The Hsp90 inhibitor 17-allylaminogeldanamycin (17-AAG) causes the targeting of mutated B-Raf to an NP-40 insoluble fraction of the proteasome, where it is degraded. The loss of mutated B-Raf expression results in inhibition of MAP kinase, loss of D-cyclin expression with subsequent G1 arrest and apoptosis. Furthermore, degradation of B-Raf can be induced with Hsp90 inhibitors, in vivo, at non-toxic doses, and this may therefore represent a novel therapeutic strategy for this disease.