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Signaling through mitogen-activated protein kinase and Rac/Rho does not duplicate the effects of activated Ras on
M B Ramocki1, S E Johnson, M A White
1Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907-1392, USA.
Abstract:
The ability of basic helix-loop-helix muscle regulatory factors (MRFs), such as MyoD, to convert nonmuscle cells to a myogenic lineage is regulated by numerous growth factor and oncoprotein signaling pathways. Previous studies have shown that H-Ras 12V inhibits differentiation to a skeletal muscle lineage by disrupting MRF function via a mechanism that is independent of the dimerization, DNA binding, and inherent transcriptional activation properties of the proteins. To investigate the intracellular signaling pathway(s) that mediates the inhibition of MRF-induced myogenesis by oncogenic Ras, we tested two transformation-defective H-Ras 12V effector domain variants for their ability to alter terminal differentiation. H-Ras 12V,35S retains the ability to activate the Raf/MEK/mitogen-activated protein (MAP) kinase cascade, whereas H-Ras 12V,40C is unable to interact directly with Raf-1 yet still influences other signaling intermediates, including Rac and Rho. Expression of each H-Ras 12V variant in C3H10T1/2 cells abrogates MyoD-induced activation of the complete myogenic program, suggesting that MAP kinase-dependent and -independent Ras signaling pathways individually block myogenesis in this model system. However, additional studies with constitutively activated Rac1 and RhoA proteins revealed no negative effects on MyoD-induced myogenesis. Similarly, treatment of Ras-inhibited myoblasts with the MEK1 inhibitor PD98059 revealed that elevated MAP kinase activity is not a significant contributor to the H-Ras 12V effect. These data suggest that an additional Ras pathway, distinct from the well-characterized MAP kinase and Rac/Rho pathways known to be important for the transforming function of activated Ras, is primarily responsible for the inhibition of myogenesis by H-Ras 12V.
Insights
Oncogenic Ras (H-Ras 12V) inhibits muscle cell differentiation by activating signaling pathways distinct from MAP kinase and Rac/Rho. This finding reveals a novel Ras-mediated mechanism blocking myogenesis, crucial for understanding cancer development.
Area of Science:
- Cell Biology
- Molecular Biology
- Cancer Research
Background:
- Basic helix-loop-helix muscle regulatory factors (MRFs) drive myogenesis.
- Oncogenic Ras proteins, like H-Ras 12V, can inhibit MRF function and myogenesis.
- The specific signaling pathways mediating Ras-induced inhibition of myogenesis remain unclear.
Purpose of the Study:
- To investigate the intracellular signaling pathways responsible for H-Ras 12V-mediated inhibition of myogenesis.
- To differentiate between MAP kinase-dependent and -independent Ras signaling in blocking myogenesis.
- To identify novel Ras signaling pathways affecting muscle differentiation.
Main Methods:
- Utilized transformation-defective H-Ras 12V effector domain variants (H-Ras 12V,35S and H-Ras 12V,40C) in C3H10T1/2 cells.
- Assessed the impact of Ras variants on MyoD-induced myogenic program activation.
- Investigated the roles of MAP kinase, Rac1, and RhoA signaling pathways using specific inhibitors and activated proteins.
Main Results:
- Both H-Ras 12V variants abrogated MyoD-induced myogenesis, indicating involvement of both MAP kinase-dependent and -independent pathways.
- Constitutively activated Rac1 and RhoA did not inhibit MyoD-induced myogenesis.
- MEK1 inhibition (PD98059) did not rescue Ras-inhibited myoblasts, suggesting MAP kinase is not the primary mediator.
Conclusions:
- H-Ras 12V inhibits myogenesis through a Ras signaling pathway distinct from the canonical MAP kinase and Rac/Rho pathways.
- This novel Ras pathway is primarily responsible for blocking MRF-induced muscle differentiation.
- Understanding this pathway is critical for deciphering Ras's role in cellular transformation and cancer.