C-Raf Inhibits MAPK Activation and Transformation by B-RafV600E

SummaryActivating B-Raf mutations that deregulate the MAPK pathway commonly occur in cancer. Whether additional proteins modulate the enzymatic activity of oncogenic B-Raf is unknown. Here we show that the proto-oncogene C-Raf paradoxically inhibits B-RafV600E kinase activity through the formation of B-RafV600E–C-Raf complexes. Although all Raf family members associate with oncogenic B-Raf, this inhibitory effect is specific to C-Raf. Indeed, a B-RafV600E isoform with impaired ability to interact with C-Raf exhibits elevated oncogenic potential. Human melanoma cells expressing B-RafV600E display a reduced C-Raf:B-Raf ratio, and further suppression of C-Raf increases MAPK activation and proliferation. Conversely, ectopic C-Raf expression lowers ERK phosphorylation and proliferation. Moreover, both oncogenic Ras and Sorafenib stabilize B-RafV600E–C-Raf complexes, thereby impairing MAPK activation. This inhibitory function of C-Raf on B-RafV600E-mediated MAPK activation may explain the lack of co-occurrence of B-RafV600E and oncogenic Ras mutations, and influence the successful clinical development of small molecule inhibitors for B-RafV600E-driven cancers.