Cell cycle exit during terminal erythroid differentiation is associated with accumulation of p27(Kip1) and inactivation of cdk2 kinase

Progression through the mammalian cell cycle is regulated by cyclins, cycli n- dependent kinases (CDKs), and cyclin-dependent kinase inhibitors (CKIs), The function of these proteins in the irreversible growth arrest associate d with terminally differentiated cells is largely unknown. The function of Cip/Kip proteins p21(Cip1) and p27(Kip1) during erythropoietin-induced term inal differentiation of primary erythroblasts isolated from the spleens of mice infected with the anemia-inducing strain of Friend virus was investiga ted. Both p21(Cip1) and p27(Kip1) proteins were induced during erythroid di fferentiation, but only p27(Kip1) associated with the principal G(1) CDKs-c dk4, cdk6, and cdk2, The kinetics of binding of p27(Kip1) to CDK complexes was distinct in that p27(Kip1) associated primarily with cdk4 (and, to a le sser extent, cdk6) early in differentiation, followed by subsequent associa tion with cdk2, Binding of p27(Kip1) to Cdk4 had no apparent inhibitory eff ect on cdk4 kinase activity, whereas inhibition of cdk2 kinase activity was associated with p27(Kip1) binding, accumulation of hypo-phosphorylated ret inoblastoma protein, and G1 growth arrest. Inhibition of cdk4 kinase activi ty late in differentiation resulted from events other than p27(Kip1) bindin g or loss of cyclin D from the complex. The data demonstrate that p27(Kip1) differentially regulates the activity of cdk4 and cdk2 during terminal ery throid differentiation and suggests a switching mechanism whereby cdk4 func tions to sequester p27(Kip1) until a specified time in differentiation when cdk2 kinase activity is targeted by p27(Kip1) to elicit Gt growth arrest. Further, the data Imply that p21(Cip1) may have a function independent of g rowth arrest during erythroid differentiation, (Blood, 2000;96:2746-2754) ( C) 2000 by The American Society of Hematology.