Cyclin A/Cdk2 complexes regulate activation of Cdk1 and Cdc25 phosphatases in human cells

Oncogene. 2004 Apr 22;23(19):3361-7. doi: 10.1038/sj.onc.1207446.


Mitotic entry, a critical decision point for maintaining genetic stability, is governed by the cyclin B/Cyclin dependent kinase 1 (Cdc2) complex. In Xenopus oocytes and early embryos, accumulation of cyclin B activates Cdk1, which then phosphorylates and activates the positive regulator Cdc25 in an autocatalytic feedback loop. However, cyclin B levels do not increase as some human cells approach mitosis, and the key factors regulating Cdk1 activation in human cells are unknown. We report here that reducing cyclin A expression by RNA interference (RNAi) in primary human fibroblasts inhibited activation of Cdc25B and Cdc25C and dephosphorylation of Cdk1 on tyrosine (tyr) 15. These results were reproduced in U2-OS cells by inducing the expression of a dominant-negative (dn) mutant of Cdk2, the principal cyclin A binding partner. Cdk2-dn induction could inhibit Cdc25B activity and foster Cdk1 tyr phosphorylation within the S phase, temporally dissociating these events from Cdk1 activation at mitosis. In contrast, reducing Cdk1 expression delayed mitotic entry without markedly impairing Cdc25B or Cdc25C activity. These results suggest that cyclin A/Cdk2 complexes are key regulators of Cdc25 and Cdk1 activation in human cells. This pathway appears to be commonly deregulated in cancer.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • CDC2 Protein Kinase / metabolism*
  • CDC2-CDC28 Kinases / physiology*
  • Cells, Cultured
  • Cyclin A / physiology*
  • Cyclin-Dependent Kinase 2
  • Enzyme Activation
  • G2 Phase
  • Humans
  • Phosphorylation
  • S Phase
  • cdc25 Phosphatases / metabolism*


  • Cyclin A
  • CDC2 Protein Kinase
  • CDC2-CDC28 Kinases
  • CDK2 protein, human
  • Cyclin-Dependent Kinase 2
  • cdc25 Phosphatases