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Cyclin D1 represses STAT3 activation through a Cdk4-independent mechanism
F Bienvenu1, H Gascan, O Coqueret
1INSERM EMI-U 9928, Centre Hospitalier Universitaire Angers, 4 rue Larrey, 49033 Angers Cedex, France.
Abstract:
STAT3 transcription factors are cytoplasmic proteins that induce gene activation in response to cytokine receptor stimulation. Following tyrosine phosphorylation, STAT3 proteins dimerize, translocate into the nucleus, and activate specific target genes. Activation is transient, and down-regulation of STAT3 signaling occurs within a few hours. In this study, we show that cyclin D1 inhibits STAT3 activation. In co-immunoprecipitation and pull-down assays, cyclin D1 was found to associate with the activation domain of STAT3 upon interleukin-6 stimulation. Overexpression of cyclin D1 inhibited transcriptional activation by STAT3 proteins. This effect was not shared by cyclin E, was independent of association with Cdk4, and was unaffected by inhibitors of Cdk4. Whereas cyclin D1 had no effect on the steady-state level of STAT3 proteins in the cytoplasm, it was found to reduce the STAT3 nuclear level in HepG2 cells. These results suggest a model by which cyclin D1 is part of a feedback network controlling the down-regulation of STAT3 activity and highlight a new activity for this cell cycle regulatory protein.
Insights
Cyclin D1, a cell cycle regulator, inhibits STAT3 transcription factor activation by reducing its nuclear levels. This finding reveals a new feedback mechanism controlling STAT3 signaling pathways.
Area of Science:
- Molecular Biology
- Cell Biology
- Signal Transduction
Background:
- Signal transducer and activator of transcription 3 (STAT3) are key cytoplasmic proteins mediating cytokine receptor signaling.
- STAT3 activation involves tyrosine phosphorylation, dimerization, nuclear translocation, and target gene activation, followed by transient signaling.
- Understanding STAT3 regulation is crucial for deciphering cellular responses to external stimuli.
Purpose of the Study:
- To investigate the role of cyclin D1 in regulating STAT3 activation.
- To elucidate the mechanism by which cyclin D1 affects STAT3 signaling.
- To identify potential feedback loops in STAT3 pathway regulation.
Main Methods:
- Co-immunoprecipitation and pull-down assays to detect protein-protein interactions.
- Overexpression studies to assess the functional impact of cyclin D1 on STAT3 activity.
- Analysis of STAT3 nuclear and cytoplasmic levels in HepG2 cells.
Main Results:
- Cyclin D1 directly associates with the STAT3 activation domain following interleukin-6 stimulation.
- Overexpression of cyclin D1 significantly inhibits STAT3-mediated transcriptional activation.
- Cyclin D1 reduces STAT3 nuclear levels without affecting its cytoplasmic steady-state levels.
- The inhibitory effect of cyclin D1 on STAT3 is independent of cyclin E and Cdk4 association.
Conclusions:
- Cyclin D1 acts as an inhibitor of STAT3 activation, likely by preventing its nuclear accumulation.
- This interaction suggests a novel role for cyclin D1 in a feedback mechanism controlling STAT3 signaling.
- The findings highlight a new function for the cell cycle protein cyclin D1 in regulating gene transcription.