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c-Jun inhibits transforming growth factor beta-mediated transcription by repressing Smad3 transcriptional activity
S Dennler1, C Prunier, N Ferrand
1Laboratoire GlaxoWellcome, 25 Avenue du Québec, 91951 Les Ulis Cedex and INSERM U482, Hôpital Saint-Antoine, 184 Rue du Faubourg Saint-Antoine, 75571 Paris Cedex 12, France.
Abstract:
Transforming growth factor beta (TGF-beta) is a pleiotropic cytokine that exerts its effects through a heteromeric complex of transmembrane serine/threonine kinase receptors. At least two intracellular pathways are activated by TGF-beta as follows: the SAPK/JNK, involving the MEKK1, MKK4, and JNK cascade, and the Smad pathway. Here, we report that the SAPK/JNK pathway inhibits the Smad3 pathway. Expression of dominant negative or constitutively active mutants of kinases of the SAPK/JNK pathway, respectively, activates or represses a TGF-beta-induced reporter containing Smad3-binding sites. This effect is not dependent on blocking of Smad3 nuclear translocation but involves a functional interaction between Smad3 and c-Jun, a transcription factor activated by the SAPK/JNK pathway. Overexpression of constitutively active MEKK1 or MKK4 mutants stabilizes the physical interaction between Smad3 and c-Jun, whereas dominant negative mutants inhibit this interaction. Moreover, overexpression of wild-type c-Jun inhibits Smad3-dependent transcription. However, c-Jun does not inhibit Smad3 binding to DNA in vitro. The repression obtained with a c-Jun mutant unable to activate transcription through AP-1 sites indicates that the inhibitory mechanism does not rely on the induction of a Smad3 repressor by c-Jun, suggesting that c-Jun could act as a Smad3 co-repressor. The inhibition of the Smad3 pathway by the SAPK/JNK pathway, both triggered by TGF-beta, could participate in a negative feedback loop to control TGF-beta responses.
Insights
The SAPK/JNK pathway, activated by TGF-beta, inhibits the Smad3 pathway by affecting Smad3 and c-Jun interaction. This suggests a negative feedback loop controlling TGF-beta signaling responses.
Area of Science:
- Cellular Biology
- Molecular Biology
- Signal Transduction
Background:
- Transforming growth factor beta (TGF-beta) is a key cytokine regulating cellular processes.
- TGF-beta signals through serine/threonine kinase receptors, activating distinct intracellular pathways.
- Two major pathways activated by TGF-beta are the SAPK/JNK cascade and the Smad pathway.
Purpose of the Study:
- To investigate the crosstalk between the SAPK/JNK and Smad3 pathways in response to TGF-beta.
- To elucidate the molecular mechanism by which the SAPK/JNK pathway influences Smad3 activity.
- To determine if c-Jun, a downstream target of SAPK/JNK, interacts with Smad3.
Main Methods:
- Utilized dominant-negative and constitutively active mutants of SAPK/JNK pathway kinases.
- Assessed TGF-beta-induced reporter gene activity containing Smad3-binding sites.
- Examined the physical interaction between Smad3 and c-Jun using overexpression systems.
- Investigated the effect of wild-type and mutant c-Jun on Smad3-dependent transcription and DNA binding.
Main Results:
- The SAPK/JNK pathway was found to inhibit the Smad3 pathway.
- This inhibition was independent of Smad3 nuclear translocation and involved functional interaction with c-Jun.
- Overexpression of active MEKK1/MKK4 stabilized Smad3-c-Jun interaction, while dominant-negative mutants inhibited it.
- Wild-type c-Jun inhibited Smad3-dependent transcription, and c-Jun acted as a Smad3 co-repressor, not by inducing a repressor.
Conclusions:
- The SAPK/JNK pathway negatively regulates the Smad3 pathway through interaction with c-Jun.
- c-Jun functions as a co-repressor for Smad3 transcriptional activity.
- This cross-talk may represent a negative feedback loop controlling TGF-beta signaling outcomes.