Differential association of p21(Cip1) and p27(Kip1) with cyclin E-CDK2 during rat liver regeneration

Background/Aims: The cell cycle inhibitors p21(Cip1) and p27(Kip1) regulate liver regeneration by modulating the activity of cyclin-dependent kinases (CDKs), However, the specific role of these inhibitors in the regulation of CDK2 activity during liver regeneration remains unknown. The ain of this s tudy was to examine the association of p21(Cip1) and p27(Kip1) with cyclin E-CDK2 and cyclin A-CDK2 complexes during rat liver regeneration and to cor relate the association of both inhibitors with CDK2 activity, Methods: The association of p21(Cip1) Or p27(Kip1) with cyclin E-CDK2 or cy clin A-CDK2 and the activities of these complexes were analyzed bs immunopr ecipitation of rat liver homogenates obtained at different times after a pa rtial hepatectomy (PH), followed by Western blotting or kinase assays. Results: High amounts of p27(Kip1) bound to cyclin E-CDK2 were observed dur ing the first 13 h after PH, when CDK2 activity was very low At 24 h, when CDK2 activity was maximal, the amount of bound-p27(Kip1) decreased strongly The amount of p21(Cip1) bound to these complexes was low during the first 13 h but subsequently increased, No cyclin A-CDK2 complexes were found duri ng the first 13 h after PH, At 24 h, complexes containing low levels of bot h inhibitors were detected and at 28 h, a significant increase in p21(Cip1) and p27(Kip1) associated with cyclin A-CDK2 was observed, Conclusions: p27(Kip1) acts as a brake on cyclin E-CDK2 activity during the first 13 h after a PH, Both p21(Cip1) and p27(Kip1) down-regulate cyclin A -CDK2 activity at 28 h after PH, after its maximal activation.