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TGF-beta receptor-activated p38 MAP kinase mediates Smad-independent TGF-beta responses
Li Yu1, Mindy C Hébert, Ying E Zhang
1Laboratory of Cellular and Molecular Biology, Center for Cancer Research, National Cancer Institute, Bethesda, MD 20892, USA.
Abstract:
Through the action of its membrane-bound type I receptors, transforming growth factor-beta (TGF-beta) elicits a wide range of cellular responses that regulate cell proliferation, differentiation and apoptosis. Many of the signaling responses induced by TGF-beta are mediated by Smad proteins, but certain evidence has suggested that TGF-beta can also signal independently of Smads. We found in mouse mammary epithelial (NMuMG) cells, which respond to TGF-beta treatment in multiple ways, that TGF-beta-induced activation of p38 MAP kinase is required for TGF-beta-induced apoptosis, epithelial-to-mesenchymal transition (EMT), but not growth arrest. We further demonstrated that activation of p38 is independent of Smads using a mutant type I receptor, which is incapable of activating Smads but still retains the kinase activity. This mutant receptor is sufficient to activate p38 and cause NMuMG cells to undergo apoptosis. However, it is not sufficient to induce EMT. These results indicate that TGF-beta receptor signals through multiple intracellular pathways and provide first-hand biochemical evidence for the existence of Smad-independent TGF-beta receptor signaling.
Insights
Transforming growth factor-beta (TGF-beta) activates p38 MAP kinase, crucial for apoptosis and EMT, independent of Smad proteins. This reveals a Smad-independent TGF-beta signaling pathway.
Area of Science:
- Cell Biology
- Molecular Biology
- Signal Transduction
Background:
- Transforming growth factor-beta (TGF-beta) is a key regulator of cellular processes like proliferation, differentiation, and apoptosis.
- While Smad proteins mediate many TGF-beta responses, evidence suggests Smad-independent signaling pathways also exist.
- Understanding these diverse signaling mechanisms is crucial for comprehending TGF-beta's complex cellular roles.
Purpose of the Study:
- To investigate the role of p38 MAP kinase in TGF-beta signaling.
- To determine whether TGF-beta-induced p38 activation is Smad-dependent or independent.
- To elucidate the involvement of Smad-independent pathways in specific TGF-beta cellular responses.
Main Methods:
- Utilized mouse mammary epithelial (NMuMG) cells for TGF-beta treatment experiments.
- Employed a mutant type I TGF-beta receptor lacking Smad-activating capability but retaining kinase activity.
- Assessed the activation of p38 MAP kinase and its role in apoptosis, epithelial-to-mesenchymal transition (EMT), and growth arrest.
Main Results:
- TGF-beta-induced activation of p38 MAP kinase is essential for apoptosis and EMT, but not growth arrest, in NMuMG cells.
- p38 MAP kinase activation by TGF-beta was demonstrated to be Smad-independent.
- A mutant TGF-beta receptor incapable of Smad activation could still induce p38 activation and apoptosis, but not EMT.
Conclusions:
- TGF-beta receptor initiates signaling through multiple intracellular pathways, including Smad-dependent and Smad-independent routes.
- p38 MAP kinase represents a critical mediator in Smad-independent TGF-beta signaling.
- These findings provide direct biochemical evidence for Smad-independent TGF-beta receptor signaling, expanding our understanding of TGF-beta's biological functions.