Actinobacillus actinomycetemcomitans has been implicated in the pathogenesis of periodontal diseases. This organism produces a toxin designated cytolethal distending toxin (CDT) that induces irreversible cell cycle arrest and cellular distension in various types of mammalian cells. Objectives: The aim of this study is to clarify the molecular mechanism of the cell cycle arrest induced by CDT of A. actinomycetemcomitans. Methods: The gene cdt encoding CDT of A. actinomycetemcomitans was cloned and expressed in Escherichia coli cells. HeLa cells, which are deficient of the function of tumor suppressor p53, and normal human fibroblast cells were treated with the recombinant CDT, and several regulatory factors that are responsible for cell cycle arrest (Chk2, p53, p21) or cell cycle progression (cyclin B1, Cdc2, Cdk2) were detected by Western blotting with specific antibodies. Results: When treated with CDT, both HeLa and the fibroblasts showed Chk2 activation, but only fibroblast cells accumulated p53. Furthermore, p21, an effector that inhibits the activities of a variety of cyclin-CDK complexes at downstream of p53, was induced in the fibroblasts treated with CDT, and the binding of Cdc2 or Cdk2 to the induced p21 was observed. In addition, the protein levels of cyclin B1 and Cdc2 were downregulated in CDT-treated fibroblasts but not in HeLa cells. Conclusion: The cell cycle arrest induced by CDT in normal fibroblast cells is suggested to be caused through activation of a p53-dependent checkpoint pathway.

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