title: Cyclin A/Cdk2 complexes regulate activation of Cdk1 and Cdc25 phosphatases in human cells.
abstract: 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.
authors.affiliation: Departments of Medicine and Genetics and Cancer Center, University of Pennsylvania, Philadelphia, PA 19104-6140, USA. jmitra@mail.med.upenn.edu
authors.forename: Jayashree
authors.initials: J
authors.lastname: Mitra
authors.forename: Greg H
authors.initials: GH
authors.lastname: Enders
mesh_terms: D016203:CDC2 Protein Kinase / Q000378:metabolism*; D042846:CDC2-CDC28 Kinases / Q000502:physiology*; D002478:Cells, Cultured; D019925:Cyclin A / Q000502:physiology*; D051357:Cyclin-Dependent Kinase 2; D004789:Enzyme Activation; D016195:G2 Phase; D006801:Humans; D010766:Phosphorylation; D016196:S Phase; D020687:cdc25 Phosphatases / Q000378:metabolism*
chemical_list: D019925:Cyclin A; D016203:CDC2 Protein Kinase; D042846:CDC2-CDC28 Kinases; C495894:CDK2 protein, human; D051357:Cyclin-Dependent Kinase 2; D020687:cdc25 Phosphatases