p27kip protein levels and E2F activity are targets of Cot kinase during G1 phase progression in T cells.

Cot/Tpl-2 kinase, homologous to members of mitogen-activated protein kinase kinase kinase, was initially discovered by its capacity to promote cell transformation. Cot/Tpl-2 mRNA levels are increased during G(0) to G(1) phase progression in T lymphocytes, suggesting a role for this kinase later on in the cell cycle. The IL-2-dependent CTLL-2 cells were used to investigate the role of Cot kinase in G(1) to S phase transition. Transient expression of Cot kinase in CTLL-2 cells increases DNA synthesis triggered by IL-2 and the transient expression of a dominant negative form of Cot kinase in CTLL-2 markedly reduces the DNA synthesis triggered by this cytokine. Cell cycle analysis of synchronized CTLL-2 stabling overexpressing Cot kinase indicates that this kinase contributes to the passage to S and G(2)-M phases of the cell cycle. Cot kinase reduces the levels of the cyclin kinase inhibitor p27(kip), whereas bcl-x(L) expression is unaffected. Cot kinase also increases E2F transcriptional activity in a phosphatidylinositol 3 kinase-independent way and acts in synergy with this kinase. These data give evidence, for the first time, of the regulation of different G(1) progression events by Cot kinase.