The Ras oncogene contributes to ~30% of human cancers, but alone is not sufficient for tumourigenesis. In a screen for oncogenes that cooperate with RasACT to promote tissue overgrowth and invasion, we identified the GTP Exchange Factor, RhoGEF2, an activator of Rho-family signalling. Here we show that RhoGEF2 also cooperates with an activated allele of Ras' downstream effector, Raf (RafGOF). We dissect the downstream pathways through which RhoGEF2 cooperates with RasACT (and RafGOF), and show that RhoGEF2 requires Rho1, but not Rac for tumourigenesis. Furthermore, of the Rho1 effectors, we show that RhoGEF2 + Ras (Raf)-mediated tumourigenesis requires the Rho kinase (Rok)-Myosin II pathway, but not Diaphanous, Lim kinase or protein kinase N. The Rho1-Rok-Myosin II pathway leads to the activation of Jun kinase (JNK), in cooperation with RasACT. Moreover, we show that activation of Rok or Myosin II, using constitutively active transgenes, is sufficient for cooperative tumourigenesis with RasACT, and together with RasACT leads to strong activation of JNK. Our results show that Rok-Myosin II activity is necessary and sufficient for Ras-mediated tumourigenesis. Our observation that activation of Myosin II, which regulates Filamentous actin (F-actin) contractility with out affecting F-actin levels, cooperates with RasACT to promote JNK activation and tumourigenesis, suggests that increased cell contractility is a key factor in tumourigenesis. Furthermore, we show that signalling via the Tumour Necrosis Factor (TNF/Egr) ligand-JNK pathway is most likely the predominant pathway that activates JNK upon Rok activation. Overall, our analysis highlights the need for further analysis of the Rok-Myosin II pathway in cooperation with Ras in human cancers.
- Received April 11, 2012.
- Accepted January 5, 2013.
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