Regulated expression of cofilin and the consequent regulation of p27(kip1) are essential for G(1) phase progression.
Cofilin, a ubiquitously expressed actin binding protein, is responsible for the formation of the actin cytoskeleton and is indispensable for cell cycle control. However, the association between cofilin expression and the cell cycle remains to be elucidated. In this study, we found that the expression level of cofilin upregulated in G(1) phase-arrested confluent cells, while knockdown of cofilin expression by small interference RNA (siRNA) in these cells led to a reduction in the population of G(1) cells. To investigate the role of cofilin in the control of G(1) phase progression, a tet-on gene expression system was introduced to overexpress different concentrations of cofilin in cells. The results showed that G(1) phase progression was blocked following induction of exogenous cofilin. A survey of the cell cycle proteins controlling the G(1) phase progression revealed that the cyclin-dependent kinase inhibitor (CKI) p27(kip1) was the primary molecule induced by overexpressed cofilin in a time and dose dependent manner. Upregulated p27(kip1) repressed phosphorylation of the retinoblastoma protein (Rb) mediated by cyclin D1/CDK4 activity. Conversely, siRNA against p27(kip1) expression in the cofilin overexpressing cells released the G(1) phase arrest. Furthermore, we found that overexpression of cofilin led to induction of p27(kip1) gene promoter transactivation using luciferase reporter gene assay. This effect was associated with increase of p27(kip1) mRNA transiently. In addition, inhibition of threonine-187 phosphorylation of p27(kip1) protein for ubiquitinyl-proteasomal mediated degradation was also involved in upregulation of p27(kip1). These data suggest that cofilin expression and its regulation of p27(kip1) expression is important for the control of G(1) phase progression.
Tsai
Cheng-Han
CH
Department of Biomedical Imaging and Radiological Sciences, School of Biomedical Science and Engineering, National Yang-Ming University, Taipei, Taiwan.
Chiu
Shu-Jun
SJ
Liu
Chih-Chao
CC
Sheu
Tzong-Jen
TJ
Hsieh
Chia-Hung
CH
Keng
Peter C
PC
Lee
Yi-Jang
YJ
D008841:Actin Cytoskeleton / Q000378:metabolism*; D002453:Cell Cycle / Q000502:physiology*; D018797:Cell Cycle Proteins / Q000378:metabolism; D045744:Cell Line, Tumor; D051338:Cofilin 1 / Q000235:genetics*; D051358:Cyclin-Dependent Kinase 4 / Q000378:metabolism*; D050760:Cyclin-Dependent Kinase Inhibitor p27 / Q000235:genetics / Q000378:metabolism*; D016193:G1 Phase / Q000502:physiology*; D006801:Humans; D011401:Promoter Regions, Genetic; D034741:RNA, Small Interfering / Q000378:metabolism
D018797:Cell Cycle Proteins; D051338:Cofilin 1; D034741:RNA, Small Interfering; D050760:Cyclin-Dependent Kinase Inhibitor p27; D051358:Cyclin-Dependent Kinase 4