Assembly of cyclin D-dependent kinase and titration of p27Kip1 regulated by mitogen-activated protein kinase kinase (MEK1)

摘要: A constitutively active form of mitogen-activated protein kinase (MEK1) was synthesized under control a zinc-inducible promoter in NIH 3T3 fibroblasts. Zinc treatment serum-starved cells activated extracellular signal-regulated kinases (ERKs) and induced expression cyclin D1. Newly D1 assembled with cyclin-dependent kinase-4 (CDK4) to holoenzyme complexes that phosphorylated the retinoblastoma inefficiently. Activation MEK1/ERK pathway neither triggered degradation CDK inhibitor inhibitory protein-1 (p27Kip1) nor led activation E- A-dependent CDK2, such did not enter DNA synthetic (S) phase cell division cycle. In contrast, zinc induction MEK1 also engineered ectopically overexpress CDK4 subunits generated levels D-dependent activity approximating those achieved stimulated by serum. this setting, p27Kip1 mobilized into containing D1; CDK2 were activated; entered S phase. Thus, although normally is canceled through serum-dependent degradative process, overexpressed D1-CDK sequestered reduced effective threshold stoichiometric mechanism. fraction these completed divided, but they unable continuously proliferate, indicating other serum-responsive factors ultimately became rate limiting for cycle progression. Therefore, MEK/ERK only acts transcriptionally induce gene functions posttranslationally regulate assembly thereby help cancel p27Kip1-mediated inhibition.