WITHDRAWAL OF DIFFERENTIATION INHIBITORY ACTIVITY LEUKEMIA INHIBITORYFACTOR UP-REGULATES D-TYPE CYCLINS AND CYCLIN-DEPENDENT KINASE INHIBITORS IN MOUSE EMBRYONIC STEM-CELLS

The expression of E and D-type cyclins, Cyclin-Dependent Kinase (CDK) 2 and 4, as web as CDK inhibitors p21(Cip1) and p27(Kip1) were examine d during in vitro differentiation of mouse embryonic stem (ES) cells. ES cells cultured in presence of Differentiation Inhibitory Activity/L eukemia Inhibitory Factor (DIA/LIF) express very low levels of cylin E /CDK2 complexes, p21(Cip1) and p27(Kip1) CDK inhibitors, while cylin D /CDK4-associated kinase activity is undetectable. Withdrawal of DIA/LI F, which induces differentiation, results in the progressive up-regula tion of all. Up-regulation of D cyclins occurs through an increase in the steady-state levels of mRNA, concomitantly with the activation of Brachyury and Goosecoid, two early markers of mesoderm differentiation . Similarly, cells from the epiblast of the early postimplantation mou se embryo do not express any cyclin D/CDK4 complexes, These are progre ssively up-regulated at gastrulation and early organogenesis, DIA/LIF- stimulated ES cells are not growth-arrested by overexpression of p16(I nk4a), a specific inhibitor of CDK4 and CDK6. We propose that the G1/S transition may be regulated by a minimal mechanism in mouse embryonic stem cells. Induction of differentiation triggers the establishment o f a more sophisticated mechanism involving both cyclin D/CDK4- and CDK inhibitor-associated control of G1-phase progression.