RAF Complexities Spark Caution

ReseaRch highlights

SIGNALLING RAF complexities spark caution Several RAF inhibitors are in clinical to RAF inhibitors could not dimerize activation of CRAF, their model trials, but three recently published with CRAF, but expression of a CRAF of activation is different. RAF proteins papers indicate that the function gatekeeper mutant did not prevent need to bind ATP to be active, and of the RAS pathway in a patient’s binding to BRAF or MEK–ERK obviously one protein cannot bind tumour will need to be assessed to activation in the presence of a RAF both an ATP-competitive inhibitor avoid potential tumour-promoting inhibitor. Use of a kinase-dead mutant and ATP at the same time. Instead, effects of these inhibitors. of BRAF showed that the kinase activ- these authors showed that binding Although ATP-competitive RAF ity of BRAF was not needed for BRAF of the inhibitor to one RAF protein inhibitors are active against tumours to dimerize with CRAF and activate in the dimer results in the transac- with mutant, constitutively active it. These results indicate that BRAF tivation of the other RAF protein. BRAF(V600E), which generally do inhibitors can activate the MEK–ERK Their results also indicate that RAS not harbour activating RAS muta- pathway because they induce a kinase- activity is required as it promotes tions, they do not suppress the growth dead BRAF, which — by a mechanism the formation of RAF homodimers of tumours with mutant RAS or wild- that involves activated RAS — dimer- and heterodimers. This model might type BRAF. Indeed, RAF inhibitors izes with and induces activation of also explain why low concentra- can activate RAF signalling pathways CRAF. These authors showed that tions of RAF inhibitors lead to the in some cells. The RAS–RAF pathway kinase-dead BRAF can cooperate with activation of the MEK–ERK pathway, is not linear and involves interactions oncogenic KRAS to induce melanoma whereas high concentrations result in between several kinases. BRAF can in mice, further illustrating the inhibition. bind and activate CRAF in a RAS- complexities of this pathway. Despite their differences, all the function dependent manner, leading to the Whereas BRAF is a key component three papers support several con- of the RAS activation of the downstream kinases of the mechanism proposed by Marais clusions: RAF inhibitors should pathway in MEK and ERK. Most oncogenic and colleagues, Shiva Malek, Georgia continue to prove efficacious in a patient’s mutants of BRAF activate MEK Hatzivassiliou and colleagues found tumours with oncogenic BRAF that directly; however, some act through that CRAF can be activated by RAF do not have other changes in the tumour will CRAF and occasionally BRAF inhibitors in BRAF-null tumour cell RAS pathway, and so patient screen- need to be mutants that have no kinase activity lines through the formation of CRAF ing should be mandatory prior to assessed to are selected for in human tumours. homodimers. Moreover, activation allocation of the drug; resistance to avoid potential Richard Marais and colleagues of MEK was reduced by RAF inhibi- RAF inhibitors could arise through found that drugs that inhibit BRAF tors in cells expressing the CRAF the activation of RAS or mutations tumour- result in the binding of BRAF to gatekeeper mutant that cannot bind that promote RAF activity; and acti- promoting CRAF and CRAF activation in the these inhibitors. Further investiga- vation of the MEK–ERK oncogenic effects presence of activated wild-type or tion showed that RAF inhibitors that signalling pathway by RAF inhibi- oncogenic RAS. A gatekeeper mutant transiently bind (fast off-rates) CRAF tors might explain the occurrence of of BRAF that is much less sensitive led to the activation of MEK, whereas squamous cell carcinomas in some a RAF inhibitor with a slow off-rate patients treated with these drugs in prevented the activation of MEK Phase I studies. by CRAF. Having solved the crystal Nicola McCarthy structure of the CRAF kinase domain ORIGINAL RESEARCH PAPERS Heidorn, S. J. et al. bound to an inhibitor, these authors Kinase-dead BRAF and oncogenic RAS suggest that CRAF activation by cooperate to drive tumour progression through CRAF. Cell 140, 209–221 (2010) | Poulikakos, P. I., ATP-competitive RAF kinase inhibi- Zhang, C., Bollag, G., Shokat, K. M. & Rosen, N. tors probably occurs as a result of RAF inhibitors transactivate RAF dimers and ERK inhibitor-mediated modulation of the signalling in cells with wild-type BRAF. Nature 23 Feb 2010 (doi:10.1038/nature08902) | kinase domain after transient binding. Hatzivassiliou, G. et al. RAF inhibitors prime wild- Although Neal Rosen, Kevan type RAF to activate the MAPK pathway and Shokat and colleagues also showed enhance growth. Nature 3 Feb 2010 (doi:10.1038/ nature08833) that RAF inhibitors result in the

Websum: Recent results indicate that stringent profiling of tumours will be needed prior to the use of RAF inhibitors.