Research ArticleCancer

Oncogenic RAS isoforms show a hierarchical requirement for the guanine nucleotide exchange factor SOS2 to mediate cell transformation

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Sci. Signal.  04 Sep 2018:
Vol. 11, Issue 546, eaar8371
DOI: 10.1126/scisignal.aar8371

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Finding alternatives to targeting RAS

Many tumors have mutations in the guanosine triphosphatases (GTPases) HRAS, NRAS, or KRAS. Pharmacologically targeting these proteins has so far been elusive; hence, targeting critical members of their signaling pathway(s) may be more successful. Sheffels et al. found that the guanine nucleotide exchange factor SOS2, in contrast to its family member SOS1, was dispensable for two-dimensional cell proliferation but mediated oncogenic and stem-like switches in the three-dimensional growth behavior of normal and tumor cells to varying degrees, most critically for cells expressing Gly12/13 mutants of KRAS. Further analysis of the differential downstream pathways suggested that SOS2 dependence and RAS mutant type may indicate whether individual inhibitors of the kinase PI3K, AKT, or MEK may be effective against the tumor.

Abstract

About a third of tumors have activating mutations in HRAS, NRAS, or KRAS, genes encoding guanosine triphosphatases (GTPases) of the RAS family. In these tumors, wild-type RAS cooperates with mutant RAS to promote downstream effector activation and cell proliferation and transformation, suggesting that upstream activators of wild-type RAS are important modulators of mutant RAS-driven oncogenesis. The guanine nucleotide exchange factor (GEF) SOS1 mediates KRAS-driven proliferation, but little is understood about the role of SOS2. We found that RAS family members have a hierarchical requirement for the expression and activity of SOS2 to drive cellular transformation. In mouse embryonic fibroblasts (MEFs), SOS2 critically mediated mutant KRAS-driven, but not HRAS-driven, transformation. Sos2 deletion reduced epidermal growth factor (EGF)–dependent activation of wild-type HRAS and phosphorylation of the kinase AKT in cells expressing mutant RAS isoforms. Assays using pharmacological inhibitors revealed a hierarchical requirement for signaling by phosphoinositide 3-kinase (PI3K) in promoting RAS-driven cellular transformation that mirrored the requirement for SOS2. KRAS-driven transformation required the GEF activity of SOS2 and was restored in Sos2−/− MEFs by expression of constitutively activated PI3K. Finally, CRISPR/Cas9-mediated deletion of SOS2 reduced EGF-stimulated AKT phosphorylation and synergized with MEK inhibition to revert the transformed phenotype of human KRAS mutant pancreatic and lung tumor cells. These results indicate that SOS2-dependent PI3K signaling mediates mutant KRAS-driven transformation, revealing therapeutic targets in KRAS-driven cancers. Our data also reveal the importance of three-dimensional culture systems in investigating the mediators of mutant KRAS.

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