Activating mutations in MEK1 enhance homodimerization and promote tumorigenesis

Abstract

RAS-RAF-MEK-ERK signaling has a well-defined role in cancer biology. Although aberrant pathway activation occurs mostly upstream of the kinase MEK, mutations in MEK are prevalent in some cancer subsets. Here, we found that cancer-related, activating mutations in MEK can be classified into two groups: those that relieve inhibitory interactions with the helix A region and those that are in-frame deletions of the β3-αC loop, which enhance MEK1 homodimerization. The former, helix A–associated mutants, are inhibited by traditional MEK inhibitors. However, we found that the increased homodimerization associated with the loop-deletion mutants promoted intradimer cross-phosphorylation of the activation loop and conferred differential resistance to MEK inhibitors both in vitro and in vivo. MEK1 dimerization was required both for its activation by the kinase RAF and for its catalytic activity toward the kinase ERK. Our findings not only identify a previously unknown group of MEK mutants and provide insight into some key steps in RAF-MEK-ERK activation but also have implications for the design of therapies targeting RAS-ERK signaling in cancers.

Sporadic ERK pulses drive non-genetic resistance in drug-adapted BRAFV600Emelanoma cells

“…The precise mechanisms involved in the selective disruption of BRAF V600E -driven MEK phosphorylation by MEK inhibitors are less understood, and we pragmatically described a mechanism in which the MEK inhibitor protects MEK from phosphorylation by BRAF V600E . Previous studies suggest a potential role for drug-induced changes in the stability of RAF-MEK complexes (Hatzivassiliou et al, 2013) (Lito et al, 2014), in MEK dimerization (Yuan et al, 2018) and in association with KSR scaffolding proteins (Dhawan et al, 2016). Our use of a rule-based framework makes extending the scope and mechanistic details in MARM1 straightforward, contingent on new mechanistic evidences.…”

Targeting Aberrant RAS/RAF/MEK/ERK Signaling for Cancer Therapy

“…Based on these observations, a monomer hypothesis in which RAF inhibitors bind and inhibit monomeric BRAF(V600E), but not dimeric variants, has been suggested to explain the drug resistance of BRAF mutants. However, this hypothesis has been challenged by other findings [80,84,95,195,222,223]. Firstly, it was shown that BRAF(V600E) has an extended dimer interface in contrast to its wild-type counterpart and exists as dimer/oligomer when expressed in cells [195,223].…”

“…Given the abilities of RAF proteins to form side-to-side dimers with themselves and face-to-face dimers with MEK, it is not surprising that RAF and MEK assemble a tetramer complex of MEK:RAF:RAF:MEK in the process of activation, which has been captured in crystal structures [86]. Although how the RAF dimer phosphorylates MEK in this transient RAF/MEK tetramer is not completely understood, recent studies suggested that two MEK molecules need form a homodimer that is further phosphorylated by RAF dimer or itself [84,222], since monomeric MEK cannot be phosphorylated by RAF, and MEK homodimerization drives autophosphorylation. Moreover, phosphorylated MEK exerts its activity towards ERK as a dimer [222], suggesting that MEK/MEK homodimerization plays a critical role, as do RAF/RAF and RAF/MEK dimerizations, in the pathway activation.…”

“…The first group of MEK mutations turns on the kinase activity of MEK by disrupting the inhibitory intramolecular interaction mediated by the regulatory helix A, while the second group does so through enhancing MEK homodimerization. These two types of MEK mutants also exhibit differential sensitivities to MEK inhibitors in clinic or under clinic trials [222]. Like RAF, the elevated dimer affinity may also result in the resistance to inhibitors.…”

MEK inhibitor augments antitumor activity of B7-H3-redirected bispecific antibody

“…Aberrant activation of MAPK can be induced by a variety of mutations, such as RAS, RAF, and MEK1/2 [31]. Notably, MEK1/2 mutations are common in several cancers, including lung cancer and bladder cancer [30,[32][33][34]. Trametinib is an oral, reversible and highly selective inhibitor of MEK1/2 [34].…”