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Mol Cell. 2009 Nov 13;36(3):477-86. doi: 10.1016/j.molcel.2009.10.017.

C-Raf inhibits MAPK activation and transformation by B-Raf(V600E).

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Li Ka Shing Centre, Cambridge Research Institute, Cancer Research UK, Robinson Way, Cambridge CB2 0RE, UK.


Activating 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-Raf(V600E) kinase activity through the formation of B-Raf(V600E)-C-Raf complexes. Although all Raf family members associate with oncogenic B-Raf, this inhibitory effect is specific to C-Raf. Indeed, a B-Raf(V600E) isoform with impaired ability to interact with C-Raf exhibits elevated oncogenic potential. Human melanoma cells expressing B-Raf(V600E) 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-Raf(V600E)-C-Raf complexes, thereby impairing MAPK activation. This inhibitory function of C-Raf on B-Raf(V600E)-mediated MAPK activation may explain the lack of co-occurrence of B-Raf(V600E) and oncogenic Ras mutations, and influence the successful clinical development of small molecule inhibitors for B-Raf(V600E)-driven cancers.

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