The p42/p44 mitogen-activated protein kinase activation triggers p27(Kip1)degradation independently of CDK2/cyclin E in NIH 3T3 cells

The p42/p44 mitogen-activated protein (MAP) kinase is stimulated by various mitogenic stimuli, and its sustained activation is necessary for cell cycl e G(1) progression and G(1)/S transition. G(1) progression and G(1)/S trans ition also depend on sequential cyclin-dependent kinase (CDK) activation. H ere, we demonstrate that MAP kinase inhibition leads to accumulation of the CDK inhibitor P27(Kip1) in NIH 3T3 cells. Blocking the proteasome-dependen t degradation of p27(Kip1) impaired this accumulation, suggesting that MAP kinase does not act on p271(Kip1) protein synthesis. In the absence of extr acellular signals (growth factors or cell adhesion), genetic activation of MAP kinase decreased the expression of P27(Kip1) as assessed by cotransfect ion experiments and by immunofluorescence detection. Importantly, MAP kinas e activation also decreased the expression of a p27(Kip1) mutant, which can not be phosphorylated by CDK2, suggesting that MAP kinase-dependent p27(Kip 1) regulation is CDK2-independent. Accordingly, expression of dominant-nega tive CDK2 did not impair the down-regulation of p27(Kip1) induced by MAP ki nase activation. These data demonstrate that the MAP kinase pathway regulat es p27(Kip1) expression in fibroblasts essentially through a degradation me chanism, independently of p27(Kip1) phosphorylation by CDK2. This strengthe ns the role of this CDK inhibitor as a key effector of G(1) growth arrest, whose expression can be controlled by extracellular stimuli-dependent signa ling pathways.