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Stimulation of c-Jun/AP-1-Activity by the Cell Cycle Inhibitor p57Kip2
Michael Keith Kullmann1, Fragka Pegka1, Christian Ploner2
1Institute of Medical Biochemistry, Biocenter, Medical University of Innsbruck, Innsbruck, Austria.
Abstract:
p57 is a member of the Cip/Kip family of cell cycle inhibitors which restrict the eukaryotic cell cycle by binding to and inhibiting cyclin/CDK complexes. They are considered as tumor suppressors and inactivating genomic mutations of p57 are associated with human overgrowth disorders. Increasing evidence suggests that p57 controls additional cellular processes beyond cell cycle control such as apoptosis, cell migration or transcription. Here we report that p57 can stimulate AP-1 promotor activity. While transactivation by c-Jun is strongly activated by p57, it did not enhance c-Fos induced transcription. This indicates that c-Jun is the target of p57 in the canonical AP-1 heterodimeric transcription factor. We could detect endogenous p57/c-Jun containing complexes in cells by co-immunoprecipitation. The strong stimulation of c-Jun activity is not the consequence of activating phosphorylation in the transactivation domain (TAD) of c-Jun, but rather due to negative interference with c-Jun repressors and positive interference with c-Jun activators. In contrast to full-length p57, the amino- and carboxy-terminal domains of p57 are insufficient for a significant activation of c-Jun induced transcription. When expressed in presence of full length p57, the p57 C-terminus abrogated and the N-terminus enhanced c-Jun activation. This indicates that the C-terminus may bind and sequester a putative activator of c-Jun, whereas the N-terminus may sequester a c-Jun repressor. Interestingly, the p57 aminoterminus is sufficient for binding to the two c-Jun repressors HDAC1 and HDAC3. These data are consistent with a model of c-Jun activation where p57 is a part of large nuclear remodeling/transcription complexes. p57 might stimulate transcription by inhibiting transcription repressor proteins like HDACs via its N-terminus and/or attracting transcription activators through its C-terminus. These data suggest that in addition to its role as a CDK inhibitor and tumor suppressor, p57 may also exert tumor promoting functions by activation of the proto-oncoprotein c-Jun.
Insights
The cell cycle inhibitor p57 (also known as CDKN1C) activates the transcription factor c-Jun, potentially promoting tumor growth. This occurs by p57 interfering with c-Jun repressors and activators, independent of its cell cycle inhibition role.
Area of Science:
- Molecular Biology
- Cell Biology
- Cancer Research
Background:
- p57 (CDKN1C) is a cell cycle inhibitor and tumor suppressor, known to inhibit cyclin-dependent kinase (CDK) complexes.
- Mutations in p57 are linked to human overgrowth disorders, and it is implicated in processes beyond cell cycle control, including apoptosis and transcription.
- The transcription factor Activator Protein-1 (AP-1), a heterodimer often composed of c-Jun and c-Fos, regulates gene expression involved in cell proliferation, differentiation, and survival.
Purpose of the Study:
- To investigate the novel role of p57 in regulating AP-1 promoter activity, specifically focusing on its interaction with c-Jun.
- To elucidate the mechanism by which p57 influences c-Jun transactivation activity.
- To determine the functional domains of p57 involved in c-Jun activation and its potential implications in cancer.
Main Methods:
- Reporter gene assays to measure AP-1 promoter activity.
- Co-immunoprecipitation to detect endogenous p57/c-Jun complexes.
- Analysis of p57 deletion mutants (N- and C-terminal domains) to assess their role in c-Jun activation.
- Assays to investigate p57's effect on c-Jun phosphorylation and its interaction with c-Jun repressors (HDAC1, HDAC3).
Main Results:
- p57 significantly stimulates AP-1 promoter activity, primarily through enhanced transactivation by c-Jun, but not c-Fos.
- Endogenous p57 and c-Jun form complexes within cells, confirmed by co-immunoprecipitation.
- p57 activates c-Jun by interfering with repressors (like HDAC1/3 via the N-terminus) and potentially activators (via the C-terminus), rather than by altering c-Jun phosphorylation.
- The N-terminus of p57 binds to c-Jun repressors HDAC1 and HDAC3, while the C-terminus appears to interact with c-Jun activators.
Conclusions:
- p57 acts as a potent activator of c-Jun-mediated transcription, suggesting a novel role beyond cell cycle inhibition.
- The distinct N- and C-terminal domains of p57 mediate this activation through differential interactions with c-Jun repressors and activators.
- These findings propose that p57 may possess tumor-promoting functions by activating the proto-oncoprotein c-Jun, potentially through its integration into nuclear remodeling complexes.
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