RAF inhibitors prime wild-type RAF to activate the MAPK pathway and enhance growth

Abstract

Activating mutations in KRAS and BRAF are found in more than 30% of all human tumours and 40% of melanoma, respectively, thus targeting this pathway could have broad therapeutic effects. Small molecule ATP-competitive RAF kinase inhibitors have potent antitumour effects on mutant BRAF(V600E) tumours but, in contrast to mitogen-activated protein kinase kinase (MEK) inhibitors, are not potent against RAS mutant tumour models, despite RAF functioning as a key effector downstream of RAS and upstream of MEK. Here we show that ATP-competitive RAF inhibitors have two opposing mechanisms of action depending on the cellular context. In BRAF(V600E) tumours, RAF inhibitors effectively block the mitogen-activated protein kinase (MAPK) signalling pathway and decrease tumour growth. Notably, in KRAS mutant and RAS/RAF wild-type tumours, RAF inhibitors activate the RAF-MEK-ERK pathway in a RAS-dependent manner, thus enhancing tumour growth in some xenograft models. Inhibitor binding activates wild-type RAF isoforms by inducing dimerization, membrane localization and interaction with RAS-GTP. These events occur independently of kinase inhibition and are, instead, linked to direct conformational effects of inhibitors on the RAF kinase domain. On the basis of these findings, we demonstrate that ATP-competitive kinase inhibitors can have opposing functions as inhibitors or activators of signalling pathways, depending on the cellular context. Furthermore, this work provides new insights into the therapeutic use of ATP-competitive RAF inhibitors.

Functional characterization of the novel BRAF complex mutation, BRAFV600delinsYM, identified in papillary thyroid carcinoma

“…19 This mechanism might have clinical implications because promoted dimerization of CRAF and BRAF could lead to resistance to RAF inhibitors. 20,21 In this study, we report a novel complex BRAF mutation, BRAF V600delinsYM , identified in 4 out of 492 Japanese PTC cases (0.81%). We also performed its functional characterization, and it showed constitutively active kinase function and transforming ability, suggesting that it is a new PTC oncogene that activates the MAPK signaling pathway as does BRAF V600E .…”

Targeted agents and immunotherapies: optimizing outcomes in melanoma

“…This work was essential in identifying MEK as being of particular importance in BRAF-inhibitor resistance and, thus, the potential synergy between BRAF and MEK inhibitors 55 . The mechanistic specificity of this work additionally led to the description of a phenomenon dubbed 'paradoxical activation' of the MAPK pathway via BRAF inhibition in BRAF-wild-type cells, for example, keratinocytes [56][57][58][59] . This discovery suggested that some of the hyperproliferative cutaneous manifestations associated with BRAF inhibition in the clinic might be mitigated by combination therapy incorporating a MEK inhibitor.…”

Role of the pro-survival molecule Bfl-1 in melanoma

“…Furthermore, as this inhibitor is only effective on cells transformed by this V600E mutation, another BRaf-specific inhibitor, GSK2118436, which targets the V600K mutation, is also in a small phase III trial having shown great potential in patients with brain metastases 81 . However some caution remains as the inhibition of the BRaf mutation V600E (and also V600K by GSK2118436) with these BRaf inhibitors has been shown in some patients to reactivate the MAPK pathway through CRaf 82,83 , and developing resistance to BRaf inhibitors is a problem.…”