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Regulation of Smad signaling by protein kinase C
I Yakymovych1, P Ten Dijke, C H Heldin
1Ludwig Institute for Cancer Research, Box 595, Uppsala, Sweden.
Abstract:
Cross talk between transforming growth factor b(TGF-b) serine/threonine kinase receptor signaling and tyrosine kinase receptor signaling modulates cell responsiveness to polypeptide growth factors regulating cell proliferation, differentiation, and apoptosis. Here we provide a mechanism through which Smad-dependent TGF-b signaling is modulated by protein kinase C (PKC). PKC, for example, is activated downstream of tyrosine kinase receptors. We show that PKC directly phosphorylates receptor-regulated Smad proteins. This phosphorylation abrogates the ability of Smad3 to bind directly to DNA, which leads to subsequent inability to mediate transcriptional responses dependent on the direct binding of Smad3 to DNA. Interference with PKC regulation of Smad functions increased cell sensitivity to transformation by the tumor promoter phorbol 12-myristate 13-acetate (PMA). PKC-dependent phosphorylation of Smad3 was found also to be a key event in the PMA-dependent inactivation of TGF-b-stimulated cell death. Thus, PKC-dependent phosphorylation of Smad3 leads to down-regulation of the growth inhibitory and apoptotic action of TGF-b.
Insights
Protein kinase C (PKC) directly phosphorylates Smad3, inhibiting its DNA binding and TGF-b-mediated growth suppression and apoptosis. This reveals a key mechanism in cell signaling and cancer development.
Area of Science:
- Cellular Biology
- Molecular Signaling
- Cancer Research
Background:
- Cross-talk between TGF-b and tyrosine kinase receptor pathways influences cell growth, differentiation, and apoptosis.
- Protein kinase C (PKC) is activated downstream of tyrosine kinase receptors and plays a role in cell signaling.
Purpose of the Study:
- To elucidate the mechanism by which Smad-dependent TGF-b signaling is modulated by PKC.
- To investigate the role of PKC-mediated Smad phosphorylation in cellular responses to growth factors and tumor promoters.
Main Methods:
- Investigated the direct interaction and phosphorylation of Smad proteins by PKC.
- Assessed the effect of PKC phosphorylation on Smad3 DNA-binding ability and transcriptional activity.
- Examined the impact of modulating PKC regulation of Smad function on cell transformation and apoptosis.
Main Results:
- PKC directly phosphorylates receptor-regulated Smad proteins, including Smad3.
- PKC-dependent phosphorylation of Smad3 abrogates its direct DNA-binding capacity, impairing TGF-b-mediated transcription.
- Interference with PKC regulation of Smad3 enhanced cell sensitivity to PMA-induced transformation.
- PKC-dependent Smad3 phosphorylation is crucial for PMA-induced inactivation of TGF-b-stimulated apoptosis.
Conclusions:
- PKC-dependent phosphorylation of Smad3 down-regulates the growth inhibitory and apoptotic effects of TGF-b.
- This pathway represents a critical regulatory mechanism linking tyrosine kinase receptor signaling to TGF-b pathway output.
- Targeting the PKC-Smad3 interaction may offer therapeutic strategies for cancers involving dysregulated TGF-b signaling.