title: Role of Cdk1 in DNA damage-induced G1 checkpoint abrogation by the human papillomavirus E7 oncogene.
abstract: The high-risk human papillomavirus (HPV) E7 oncogene abrogates DNA damage-induced G1 checkpoint but the mechanism is not fully understood. The G1 kinase Cdk2 is activated in E7-expressing cells. However, whether Cdk2 is required for E7 to abrogate the G1 checkpoint is not known. Accumulating evidence implicates a role for the mitotic Cdk1 in G1/S phase transition in the absence of Cdk2. We therefore examined the expression and requirement of Cdk1 and Cdk2 in the G1 checkpoint abrogation in E7-expressing cells. Although both Cdk1 and Cdk2 were up-regulated in E7-expressing cells upon DNA damage, down-regulation of Cdk1 but not Cdk2 impairs the ability of E7 to abrogate the G1 checkpoint. Our study thus demonstrated an important role for Cdk1 in bypassing the G1 checkpoint in E7-expressing cells. To understand the mechanism by which E7 activates Cdk1, we examined the transcription factor B-Myb. Our studies demonstrated that downregulation of B-Myb reduced the steady-state level of Cdk1 and induced G1 arrest in E7-expressing cells upon DNA damage. In addition, it remains a mystery how E7 promotes cell cycle progression in the presence of Cdk inhibitor p21. As p21 binds Cdk1 with lower affinity than Cdk2, our results suggest a mechanism by which E7 bypasses the inhibitory effect of p21. Nonetheless, our studies demonstrated that p21 still possessed partial ability to arrest cells at G1 phase in E7-expressing cells. These studies shed light on mechanisms by which HPV E7 modulates cell cycle checkpoint.
authors.affiliation: a Department of Medicine ; University of Massachusetts Medical School ; Worcester , MA USA.
authors.forename: Xueli
authors.initials: X
authors.lastname: Fan
authors.forename: Jason J
authors.initials: JJ
authors.lastname: Chen
mesh_terms: D016203:CDC2 Protein Kinase; D002460:Cell Line; D018844:Cyclin-Dependent Kinases / Q000235:genetics / Q000378:metabolism*; D004249:DNA Damage / Q000235:genetics / Q000502:physiology*; D016193:G1 Phase / Q000235:genetics / Q000502:physiology; D006801:Humans; D050725:Papillomavirus E7 Proteins / Q000235:genetics / Q000378:metabolism*
chemical_list: D050725:Papillomavirus E7 Proteins; D016203:CDC2 Protein Kinase; C528899:CDK1 protein, human; D018844:Cyclin-Dependent Kinases
keywords: B-Myb; BrdU, bromodeoxyuridine; Cdk; Cdk, cyclin-dependent kinase; DMEM, Dulbecco's modified Eagle medium; DNA damage; E7; FBS, fetal bovine serum; G1 checkpoint; HPV, human papillomavirus; PAGE, polyacrylamide gel electrophoresis; PHKs, primary human keratinocytes; checkpoint control; oncogene; papillomavirus; siRNA, small interfering RNA