Disruption of CRAF-mediated MEK activation is required for effective MEK inhibition in KRAS mutant tumors

MEK inhibitors are clinically active in BRAF V600E melanomas, but only marginally so in KRAS-mutant tumors. While MEK inhibitor treatment resulted in sustained inhibition of ERK phopshorylation in BRAF V600E tumors, it was associated with a rebound in ERK phosphorylation after prolonged exposure in KRAS mutant tumors. To understand this phenomenon, we performed an RNAi screen in a KRAS-mutant model and found that CRAF knockdown enhanced the antiproliferative effects of MEK inhibition both in vivo and in vitro. In agreement with previous findings, MEK inhibitor treatment in KRAS mutant cells resulted in CRAF reactivation and induction of MEK phosphorylation. However, MEK activated by CRAF was less susceptible to traditional MEK inhibitors, such as PD0325901, compared to when activated by BRAF V600E. Consistent with this observation, the rebound in ERK phosphorylation following MEK inhibitor treatment in KRAS mutant tumors was dependent on intact CRAF expression. Furthermore, traditional MEK inhibitors induced RAF-MEK complexes in KRAS mutant models, and disrupting these complexes enhanced the inhibition of CRAF-dependent ERK signaling. In an effort to identify more effective compounds we found that newer MEK inhibitors, such as trametinib and CH5126766, do not only target the catalytic activity of MEK, but in addition, they also impair its reactivation by CRAF. This occurs through two distinct mechanisms: trametinib disrupts RAF-MEK complex formation, as evidenced by co-immunoprecipitation of endogenous proteins and surface plasmon resonance assays with purified enzymes. Instead, CH512676 induces the interaction of MEK with RAF, yet it prevents the phosphorylation of MEK in this complex. To elucidate the mechanism responsible for the latter property, we determined the ternary structure of CH512676-bound MEK1. CH5126766 interacts with Asn 221 and Ser 222 residues in MEK1 to displace the activation segment and disrupt RAF-mediated phosphorylation. These data provide a blueprint for developing better MEK inhibitors that effectively inhibit ERK signaling in KRAS mutant tumors