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Regulated expression of cofilin and the consequent regulation of p27kip1 are essential for G1 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 up-regulated in G1 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 G1 cells. To investigate the role of cofilin in the control of G1 phase progression, a tet-on gene expression system was introduced to over-express different concentrations of cofilin in cells. The results showed that G1 phase progression was blocked following induction of exogenous cofilin. A survey of the cell cycle proteins controlling the G1 phase progression revealed that the cyclin-dependent kinase inhibitor (CKI) p27kip1 was the primary molecule induced by over-expressed cofilin in a time and dose dependent manner. Up-regulated p27kip1 repressed phosphorylation of the retinoblastoma protein (Rb) mediated by cyclin D1/CDK4 activity. Conversely, siRNA against p27kip1 expression in the cofilin over-expressing cells released the G1 phase arrest. Furthermore, we found that over-expression of cofilin led to induction of p27kip1 gene promoter transactivation using luciferase reporter gene assay. This effect was associated with increase of p27kip1 mRNA transiently. In addition, inhibition of threonine-187 phosphorylation of p27kip1 protein for ubiquitinyl-proteasomal mediated degradation was also involved in up-regulation of p27 kip1. These data suggest that cofilin expression and its regulation of p27kip1 expression is important for the control of G1 phase progression.