Blockade of RAS-Binding Domain Interactions Inhibits RAS Signaling

Aberrant RAS signaling is present in the majority of human tumors and promotes proliferation and survival, but development of RAS inhibitors has been largely unsuccessful due to the lack of well-defined druggable pockets on the surface of RAS. Rigosertib is a non-ATP competitive inhibitor that suppresses the growth of various types of cancer cells and is currently in clinical development, but its mechanism of action is not well understood. In a mass spectrometry––based approach to identify direct targets of rigosertib, Athuluri-Divakar and colleagues identified three proteins that contain a RAS binding domain (RBD) as the primary rigosertib-binding partners: ARAF, BRAF, and CRAF. Determination of the BRAF RBD–rigosertib complex structure with nuclear magnetic resonance (NMR) revealed that rigosertib bound to the same region of the RBD as RAS, suggesting that rigosertib would prevent RAS and RAF from interacting. These findings were supported by RAF mutagenesis studies, which indicated that mutations in the RAF RBD that disrupted RAS binding also disrupted rigosertib binding. As predicted by NMR, rigosertib blocked the association between RAS and the RAF RBD. Rigosertib inhibited RAF heterodimerization and activation, and suppressed phosphorylation of the downstream effectors MEK and ERK, indicating that blocking the RAS–RAF interaction inhibits MEK–ERK pathway signaling. Rigosertib also bound to the RBD of PI3Ks and reduced AKT phosphorylation, suggesting that it also inhibited PI3K/AKT signaling by disrupting the RAS–PI3K interaction. In mouse models of mutant KRAS–driven colorectal, lung, and pancreatic cancers, rigosertib reduced tumor growth and reduced the phosphorylation of MEK, ERK, and AKT, indicating that rigosertib can inhibit RAS-driven MAPK and PI3K signaling in vivo. Taken together, these results suggest that rigosertib suppresses tumors by acting as a RAS mimetic, and illustrate that RAS can be effectively targeted by preventing its interaction with downstream effectors.

Athuluri-Divakar SK, Vasquez-Del Carpio R, Dutta K, Baker SJ, Cosenza SC, Basu I, et al. A small molecule RAS-mimetic disrupts RAS association with effector proteins to block signaling. Cell 2016;165:643–55.

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