Oncogenic Ras activation of Raf/mitogen-activated protein kinase-independent pathways is sufficient to cause tumorigenic transformation
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R Khosravi-Far1, M A White, J K Westwick
1Department of Pharmacology, University of North Carolina at Chapel Hill, 27599-7365,USA.
Abstract:
Substantial evidence supports a critical role for the activation of the Raf-1/MEK/mitogen-activated protein kinase pathway in oncogenic Ras-mediated transformation. For example, dominant negative mutants of Raf-1, MEK, and mitogen-activated protein kinase all inhibit Ras transformation. Furthermore, the observation that plasma membrane-localized Raf-1 exhibits the same transforming potency as oncogenic Ras suggests that Raf-1 activation alone is sufficient to mediate full Ras transforming activity. However, the recent identification of other candidate Ras effectors (e.g., RalGDS and phosphatidylinositol-3 kinase) suggests that activation of other downstream effector-mediated signaling pathways may also mediate Ras transforming activity. In support of this, two H-Ras effector domain mutants, H-Ras(12V, 37G) and H-Ras(12V, 40C), which are defective for Raf binding and activation, induced potent tumorigenic transformation of some strains of NIH 3T3 fibroblasts. These Raf-binding defective mutants of H-Ras induced a transformed morphology that was indistinguishable from that induced by activated members of Rho family proteins. Furthermore, the transforming activities of both of these mutants were synergistically enhanced by activated Raf-1 and inhibited by the dominant negative RhoA(19N) mutant, indicating that Ras may cause transformation that occurs via coordinate activation of Raf-dependent and -independent pathways that involves Rho family proteins. Finally, cotransfection of H-Ras(12V, 37G) and H-Ras(12V, 40C) resulted in synergistic cooperation of their focus-forming activities, indicating that Ras activates at least two Raf-independent, Ras effector-mediated signaling events.
Insights
Oncogenic Ras protein triggers cell transformation through the Raf-1 pathway, but also activates independent pathways involving Rho proteins, suggesting dual mechanisms in cancer development.
Area of Science:
- Molecular Biology
- Cell Biology
- Oncology
Background:
- Oncogenic Ras proteins are key drivers of cell transformation and cancer.
- The Raf-1/MEK/mitogen-activated protein kinase (MAPK) pathway is a known mediator of Ras-driven transformation.
- Emerging evidence suggests other Ras effectors may also contribute to transformation.
Purpose of the Study:
- To investigate the role of Raf-1-independent pathways in oncogenic Ras-mediated transformation.
- To determine if other Ras effectors, such as Rho family proteins, contribute to tumorigenesis.
- To elucidate the mechanisms by which Ras activates multiple downstream signaling pathways.
Main Methods:
- Utilized H-Ras effector domain mutants defective for Raf binding (H-Ras(12V, 37G) and H-Ras(12V, 40C)).
- Assessed NIH 3T3 fibroblast transformation, including morphology and focus-forming activity.
- Investigated the synergistic effects of activated Raf-1 and inhibition by dominant-negative RhoA(19N) mutant.
Main Results:
- H-Ras mutants defective for Raf binding induced potent tumorigenic transformation.
- These mutants induced a transformed morphology similar to activated Rho family proteins.
- Ras-mediated transformation involves coordinate activation of both Raf-dependent and Raf-independent pathways, including Rho proteins.
- Cotransfection of Raf-binding defective H-Ras mutants showed synergistic cooperation, indicating at least two Raf-independent effector pathways.
Conclusions:
- Oncogenic Ras activates multiple downstream signaling pathways for transformation.
- Ras utilizes both the canonical Raf-1/MEK/MAPK pathway and Raf-independent pathways involving Rho proteins.
- These findings reveal a complex signaling network underlying Ras-driven tumorigenesis, with potential therapeutic implications.