Mutations that activate the guanosine triphosphatase (GTPase) K-RAS promote tumorigenesis and are the most common oncogenic mutations in human cancers. Various drugs that target Ras have been developed but are ineffective in vivo. When activated, K-RAS promotes the activity of the kinase RAF by stabilizing RAF signaling complexes. The kinase suppressor of RAS (KSR) proteins are pseudokinases that act as scaffolding proteins for RAF signaling complexes by simultaneously binding to both RAF and its targets, the kinases of the MEK family. Dhawan et al. analyzed mutations in KSR that suppress RAS signaling and identified residues near the adenosine triphosphate (ATP)–binding pocket of KSR2 that were likely important for the ability of KSR2 to bind RAF and MEK1 simultaneously. To identify small molecules that might bias KSR2 towards a conformation that bound MEK1 but not RAF, the authors screened a library of kinase inhibitors for the ability to compete with ATP for binding to KSR2-MEK1. They identified a compound that competed with ATP and then generated a collection of modified forms of this compound. Among these related compounds, APS-2-79 most effectively competed with ATP for binding to KSR2-MEK1 and inhibited RAS signaling in cultured cells and in vitro. Structural analysis of APS-2-79 bound to KSR2-MEK1 and in vitro binding assays indicated that APS-2-79 disrupted RAS signaling by stabilizing an inactive conformation of KSR2 that could neither bind to RAF nor accommodate the conformational changes associated with MEK activation. APS-2-79 alone did not reduce the viability of KRAS-mutant cell lines, but the compound enhanced the ability of various MEK inhibitors, including the clinical inhibitor trametinib, to reduce growth of these cells. Thus, combination therapy with APS-2-79 or related compounds and MEK inhibitors may increase therapeutic effectiveness.
N. S. Dhawan, A. P. Scopton, A. C. Dar, Small molecule stabilization of the KSR inactive state antagonizes oncogenic Ras signalling. Nature 537, 112–116 (2016). [PubMed]