Stat Proteins Control Lymphocyte Proliferation by Regulating p27^Kip1 Expression

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 p27^Kip1 were found to be markedly dysregulated. p27^Kip1 is expressed at significantly higher levels in Stat6-deficient lymphocytes than in control cells following IL-4 stimulation. The higher level of p27^Kip1 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 p27^Kip1 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.

ACKNOWLEDGMENTS

We thank L. Jackson-Grusby, M. Fero, J. Roberts, J. Pierce, M. White, and M. Gately for providing probes, antisera, and cytokines. We also thank M. Carroll for critical review of this paper.

M.H.K. is a Special Fellow and M.J.G. is a Scholar of the Leukemia Society of America. This work was supported by the Mathers Foundation and NIH grant AI40171.