Ectopic expression of Cdc25A accelerates the G(1)/S transition and leads to premature activation of cyclin E- and cyclin A-dependent kinases

Human Cdc25 phosphatases play important roles in cell cycle regulation by r emoving inhibitory phosphates from tyrosine and threonine residues of cycli n-dependent kinases. Three human Cdc25 isoforms, A, B, and C, have been dis covered. Cdc25B and Cdc25C play crucial roles at the G(2)/M transition. In the present study, we have investigated the function of human Cdc25A phosph atase. Cell lines that express human Cdc25A in an inducible manner have bee n generated. Ectopic expression of Cdc25A accelerates the G(1)/S-phase tran sition, indicating that Cdc25A controls an event(s) that is rate limiting f or entry into S phase. Furthermore, we carried out a detailed analysis of t he expression and activation of human Cdc25A Activation of endogenous Cdc25 A occurs during late G(1) phase and increases in S and G(2) phases. We furt her demonstrate that Cdc25A is activated at the same time as cyclin E- and cyclin A-dependent kinases. In vitro, Cdc25A dephosphorylates and activates the cyclin-Cdk complexes that are active during G(1). Overexpression of Cd c25A in the inducible system, however, leads to a premature activation of b oth cyclin E-Cdk2 and cyclin A-Cdk2 complexes, while no effect of cyclin D dependent kinases is observed. Furthermore, Cdc25A overexpression induces a tyrosine dephosphorylation of Cdk2. These results suggest that Cdc25A is a n important regulator of the G(1)/S-phase transition and that cyclin E- and cyclin A-dependent kinases act as direct targets.