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Stat proteins control lymphocyte proliferation by regulating p27Kip1 expression
M H Kaplan1, C Daniel, U Schindler
1Department of Immunology and Infectious Diseases, Harvard School of Public Health, Boston, Massachusetts 02115, USA.
Insights
Signal transducer and activator of transcription (Stat) proteins regulate lymphocyte proliferation by controlling cell cycle inhibitors. Stat6 deficiency impairs IL-4-induced proliferation due to increased p27Kip1 levels, hindering cell cycle progression.
Area of Science:
- Immunology
- Cell Biology
- Molecular Biology
Background:
- Lymphocyte proliferation is crucial for immune responses.
- Signal transducer and activator of transcription 6 (Stat6) is essential for interleukin-4 (IL-4)-induced lymphocyte proliferation.
Purpose of the Study:
- To investigate the mechanism behind impaired IL-4-induced proliferation in Stat6-deficient lymphocytes.
- To explore the role of Stat proteins in regulating cell cycle progression during cytokine stimulation.
Main Methods:
- Comparative analysis of Stat6-deficient and control lymphocytes.
- Cell cycle analysis using flow cytometry.
- Western blotting to assess protein levels of cell cycle regulators like p27Kip1.
- Analysis of gene and protein expression following cytokine stimulation (IL-4, IL-12).
Main Results:
- Stat6 deficiency impairs lymphocyte transition from G1 to S phase upon IL-4 stimulation.
- Elevated protein levels of the cyclin-dependent kinase inhibitor p27Kip1 in Stat6-deficient lymphocytes.
- Higher p27Kip1 levels correlate with decreased cdk2-associated kinase activity and are due to protein accumulation, not altered mRNA.
- Similar dysregulation of p27Kip1 observed in Stat4-deficient lymphocytes stimulated with IL-12.
Conclusions:
- Stat proteins regulate cytokine-induced lymphocyte proliferation by modulating cell cycle inhibitors like p27Kip1.
- Stat proteins ensure proper G1 to S phase transition by controlling the expression of cell cycle inhibitors, thereby facilitating cyclin-cdk complex function.
Abstract:
The proliferation of lymphocytes in response to cytokine stimulation is essential for a variety of immune responses. Recent studies with signal transducer and activator of transcription 6 (Stat6)-deficient mice have demonstrated that this protein is required for the normal proliferation of lymphocytes in response to interleukin-4 (IL-4). In this report, we show that the impaired IL-4-induced proliferative response of Stat6-deficient lymphocytes is not due to an inability to activate alternate signaling pathways, such as those involving insulin receptor substrates, or to a failure to upregulate IL-4 receptor levels. Cell cycle analysis showed that the percentage of Stat6-deficient lymphocytes that transit from the G1 to the S phase of the cell cycle following IL-4 stimulation is lower than that of control lymphocytes. Although the regulation of many genes involved in the control of cytokine-induced proliferation is normal in Stat6-deficient lymphocytes, protein levels of the cdk inhibitor p27Kip1 were found to be markedly dysregulated. p27Kip1 is expressed at significantly higher levels in Stat6-deficient lymphocytes than in control cells following IL-4 stimulation. The higher level of p27Kip1 expression seen in IL-4-stimulated Stat6-deficient lymphocytes correlates with decreased cdk2-associated kinase activity and is the result of the increased accumulation of protein rather than altered mRNA expression. Similarly, higher levels of p27Kip1 protein expression are also seen following IL-12 stimulation of Stat4-deficient lymphocytes than are seen following stimulation of control cells. These data suggest that Stat proteins may control the cytokine-induced proliferative response of activated T cells by regulating the expression of cell cycle inhibitors so that cyclin-cdk complexes may function to promote transition from the G1 to the S phase of the cell cycle.