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Activation of Rac1, RhoA, and mitogen-activated protein kinases is required for Ras transformation
R Khosravi-Far1, P A Solski, G J Clark
1Curriculum in Genetics and Molecular Biology, University of North Carolina at Chapel Hill, Chapel Hill 27599, USA.
Abstract:
Although substantial evidence supports a critical role for the activation of Raf-1 and mitogen-activated protein kinases (MAPKs) in oncogenic Ras-mediated transformation, recent evidence suggests that Ras may activate a second signaling pathway which involves the Ras-related proteins Rac1 and RhoA. Consequently, we used three complementary approaches to determine the contribution of Rac1 and RhoA function to oncogenic Ras-mediated transformation. First, whereas constitutively activated mutants of Rac1 and RhoA showed very weak transforming activity when transfected alone, their coexpression with a weakly transforming Raf-1 mutant caused a greater than 35-fold enhancement of transforming activity. Second, we observed that coexpression of dominant negative mutants of Rac1 and RhoA reduced oncogenic Ras transforming activity. Third, activated Rac1 and RhoA further enhanced oncogenic Ras-triggered morphologic transformation, as well as growth in soft agar and cell motility. Finally, we also observed that kinase-deficient MAPKs inhibited Ras transformation. Taken together, these data support the possibility that oncogenic Ras activation of Rac1 and RhoA, coupled with activation of the Raf/MAPK pathway, is required to trigger the full morphogenic and mitogenic consequences of oncogenic Ras transformation.
Insights
Oncogenic Ras transformation relies on both the Raf/MAPK pathway and the Rac1/RhoA pathway. Activating Rac1 and RhoA enhances Ras-mediated cell changes, suggesting a dual pathway requirement for full oncogenic effects.
Area of Science:
- Cell Biology
- Molecular Oncology
- Signal Transduction
Background:
- Oncogenic Ras proteins are key drivers of cellular transformation.
- The Raf/MAPK pathway is a well-established mediator of Ras signaling.
- Emerging evidence suggests Ras may utilize additional pathways involving Rac1 and RhoA.
Purpose of the Study:
- To investigate the role of Rac1 and RhoA in oncogenic Ras-mediated transformation.
- To determine if Rac1 and RhoA cooperate with the Raf/MAPK pathway in Ras signaling.
Main Methods:
- Utilized constitutively activated and dominant-negative mutants of Rac1 and RhoA.
- Coexpressed Ras mutants with Raf-1 and MAPK pathway components.
- Assessed morphologic transformation, soft agar growth, and cell motility.
Main Results:
- Coexpression of activated Rac1/RhoA with a weak Raf-1 mutant significantly enhanced transformation.
- Dominant-negative Rac1/RhoA mutants inhibited oncogenic Ras transforming activity.
- Activated Rac1 and RhoA amplified Ras-induced morphologic changes, anchorage-independent growth, and cell migration.
Conclusions:
- Oncogenic Ras likely activates both the Raf/MAPK and Rac1/RhoA pathways.
- Coordinated activation of these pathways is essential for the full spectrum of Ras-driven transformation.
- Targeting both pathways may offer novel therapeutic strategies for Ras-driven cancers.