Editors' ChoiceCancer

A New Target to Halt RAS

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Science Signaling  18 Feb 2014:
Vol. 7, Issue 313, pp. ec49
DOI: 10.1126/scisignal.2005180

Mutations that increase the activity of RAS are frequently observed in cancer; however, RAS is considered “undruggable.” RAS-induced dephosphorylation of the kinase suppressor of RAS (KSR-1) by the phosphatase PP2A stimulates the activity of the mitogen-activated protein kinase (MAPK) cascade, which promotes proliferation. Using a short-hairpin RNA (shRNA) screen, Cullis et al. found that guanine nucleotide exchange factor (GEF)–H1 was increased in abundance and promoted proliferation and inhibited apoptosis in many RAS-mutant cancer cell lines. In NIH 3T3 mouse embryonic fibroblasts (MEFs) transfected with a reporter containing the promoter of Arhgef2, which encodes GEF-H1, overexpression of constitutively active HRAS (HRASV12) increased reporter activity and GEF-H1 mRNA and protein abundance, whereas inhibitors of MAPK kinase 1 and 2 (MEK1/2) decreased reporter activity and GEF-H1 abundance. Stable knockdown of GEF-H1 in HRASV12-transformed cells reduced the phosphorylation of MEK1/2 and extracellular signal–regulated kinase 1 and 2 (ERK1/2), increased cleavage of caspase 3 (a marker of apoptosis), and markedly decreased anchorage-independent proliferation in culture and subcutaneous tumor growth in mice compared with controls. However, ectopic expression of wild-type or catalytically inactive GEF-H1 but not other related GEFs restored phosphorylation of MEK1/2 and ERK1/2 in GEF-H1–deficient MEFs. Thus, the effect of GEF-H1 on the RAS-MAPK pathway was independent of its GEF activity. GEF-H1 knockdown in human Panc-1 cells or deletion in MEFs increased the phosphorylation of KSR-1 and decreased its nuclear translocation, both indicators of decreased KSR-1 activity. GEF-H1 coimmunoprecipitated with KSR-1, and GEF-H1 overexpression induced the phosphorylation of ERK1/2 in wild-type but not Ksr1–/– MEFs. GEF-H1 also coimmunoprecipitated with PP2A, and GEF-H1 knockdown in HEK293T cells prevented the interaction between KSR-1 and PP2A, suggesting that GEF-H1 acts as a scaffold for KSR-1 and PP2A. These findings suggest that inhibiting the interaction between GEF-H1 and either KSR-1 or PP2A may be a way to therapeutically target mutant RAS signaling in cancer.

J. Cullis, D. Meiri, M. J. Sandi, N. Radulovich, O. A. Kent, M. Medrano, D. Mokady, J. Normand, J. Larose, R. Marcotte, Christopher B. Marshall, M. Ikura, T. Ketela, J. Moffat, B. G. Neel, A.-C. Gingras, M.-S. Tsao, R. Rottapel, The RhoGEF GEF-H1 is required for oncogenic RAS signaling via KSR-1. Cancer Cell 25, 181–195 (2014). [PubMed]