40 CDK4 and CDK6 were both induced upon CD3/CD28 costimulation. nIL-2 abrogated the up-regulation of CDK6, and partly inhibited CDK4 induction, while BMS-345541 and PS-1145 suppressed the induction of both kinases. Taken together, these results emphasize that an important effect of IKK activation on CDK4 and CDK6 expression relies on IL-2/IL-2R Torin 1 signalling. However, as full CDK4 up-regulation requires the activation of IKK and IL-2 signalling, these data add new information about the mechanisms that govern CDK4 expression in human T cells. CDK2–cyclin E/A complexes are implicated in the
regulation of major processes governing the G1/S transition.5 In our experiments, CDK2 induction was detected in 24-hr costimulated cells, and was preserved in the presence of nIL-2, but abolished by BMS-345541 and PS-1145. We thus PD-1 antibody inhibitor conclude that, in activated T cells, CDK2
induction is independent of IL-2 signalling, and relies instead on IKK activation, which is a novel finding. To acquire catalytic activity, CDK2 must bind to cyclin E (G1/S phase transition) or cyclin A (S phase).5 We found that T-cell stimulation caused a significant increase in cyclin E and cyclin A gene expression. nIL-2 prevented cyclin A up-regulation but did not affect cyclin E, a clear indication that in activated human naïve CD4+ T cells only cyclin A expression is dependent on the IL-2/IL-2R signalling pathway, consistent with previous reports.3 Interestingly, BMS-345541 and PS-1145 prevented the expression not only of cyclin A, but also of cyclin E, providing compelling evidence for involvement of IKK in the regulation of cyclin E expression in human naïve CD4+ T cells. In light of the essential role played by the CDK2/cyclin E complex in initiating DNA replication,5 this finding underscores a critical function of IKK in the regulation of T-cell entry into S phase. Degradation of p27KIP1 by the ubiquitin–proteasome
pathway at the BCKDHB G0/G1 transition results in activation of the cyclin E/CDK2 complex, and commitment of cells to S phase.41 In our results, stimulation of human naïve CD4+ T cells resulted in a considerable decrease in p27KIP1 that was prevented by nIL-2, or BMS-345541 or PS-1145. The degradation of p27KIP1 is a complex process that requires the formation of a ternary complex with cyclin D/CDK4, followed by p27KIP1 phosphorylation on Thr187 by cyclin E/CDK2.4 The RING finger-type ubiquitin ligase complex SCFSKP2-CKS1B recognizes phosphorylated p27KIP1 through the C-terminus of two of its subunits, SKP2 and CKS1B, resulting in targeting of p27KIP1 for ubiquitination and degradation.42 SKP2 and CKS1B levels periodically oscillate during the cell cycle: they are low or absent during G0 and early G1 phases, increase in late G1 phase, and peak in S phase, dropping as cells proceed through M and early G1 phases.
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