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Critical role of Smads and AP-1 complex in transforming growth factor-beta -dependent apoptosis
Y Yamamura1, X Hua, S Bergelson
1Whitehead Institute for Biomedical Research, Cambridge, Massachusetts 02142, USA. yama.mbch@med.tmd.ac.jp
Abstract:
Transforming growth factor-beta1 (TGF-beta1) induces not only cell growth inhibition but also apoptosis in hepatocytes, myeloid cells, and epithelial cells. Although Smad proteins are identified as key signal transducers in TGF-beta1-dependent growth inhibition, their roles in the induction of apoptosis are unclear. We show here that both Smad proteins and AP-1 complex are involved in TGF-beta1 signaling for apoptosis. Overexpression of a dominant-negative Smad3 mutant or Smad7, both of which impair Smad-mediated signal transduction, inhibits TGF-beta1-dependent apoptosis. Only the JunD. FosB form of the AP-1 complex is markedly activated during TGF-beta1-dependent apoptosis. FosB substantially enhances Smad3. Smad4-dependent transcription, and dominant-negative FosB blocks TGF-beta1-dependent apoptosis but not growth inhibition. Expression of JunD.FosB enhances induction of apoptosis by TGF-beta1. Moreover, JunD.FosB binds to the 12-O-tetradecanoyl-13-acetate-responsive gene promoter element and recruits Smad3.Smad4 to form a multicomponent complex. These results suggest that Smad proteins and AP-1 complex synergize to mediate TGF-beta1-dependent apoptosis.
Insights
Transforming growth factor-beta1 (TGF-beta1) triggers apoptosis via Smad proteins and the AP-1 complex. The JunD.FosB form of AP-1 synergizes with Smad3/Smad4 to mediate TGF-beta1-induced apoptosis.
Area of Science:
- Cellular biology
- Molecular signaling pathways
- Apoptosis research
Background:
- Transforming growth factor-beta1 (TGF-beta1) is known to inhibit cell growth and induce apoptosis in various cell types.
- While Smad proteins are established mediators of TGF-beta1-induced growth inhibition, their precise role in TGF-beta1-induced apoptosis remains incompletely understood.
Purpose of the Study:
- To elucidate the molecular mechanisms by which TGF-beta1 induces apoptosis.
- To investigate the involvement of Smad proteins and the AP-1 complex in TGF-beta1-mediated apoptotic signaling.
Main Methods:
- Utilized dominant-negative Smad3 mutants and Smad7 to assess Smad protein involvement.
- Analyzed the activation of the AP-1 complex, specifically focusing on JunD and FosB.
- Investigated the interaction between Smad proteins and the AP-1 complex using reporter gene assays and chromatin immunoprecipitation.
Main Results:
- Inhibition of Smad signaling pathways blocked TGF-beta1-induced apoptosis.
- The JunD.FosB form of the AP-1 complex was significantly activated during TGF-beta1-induced apoptosis.
- FosB enhanced Smad3.Smad4-dependent transcription, and its inhibition specifically blocked apoptosis, not growth inhibition.
- JunD.FosB directly bound to a specific gene promoter element and recruited Smad3.Smad4, forming a functional complex.
Conclusions:
- Smad proteins and the AP-1 complex (specifically JunD.FosB) are critical, cooperating components in TGF-beta1-induced apoptosis.
- This synergistic interaction between Smad and AP-1 signaling pathways is essential for mediating TGF-beta1's apoptotic effects.