title: p27Kip1 and cyclin dependent kinase 2 regulate passage through the restriction point.
abstract: When quiescent cells are stimulated to reenter the cell cycle, growth factors are required only until the restriction point in G(1) phase. After this point the cell no longer requires growth factors, proliferative signaling molecules, or even protein synthesis in order to initiate DNA synthesis, which starts several hours later. Consequently, understanding the molecular nature of the restriction point constitutes one of the major goals in studies of growth regulation. We recently demonstrated that p27Kip1 (p27) regulates passage through G(1) phase in actively proliferating cultures, and initiated these studies to determine if it is also involved in passage through the restriction point following stimulation of quiescent cells. In support of this suggestion, we found that passage through the restriction point requires mitogen-dependent suppression of the high p27 levels normally present in quiescent cells. Moreover, as the culture progresses to mid-G(1) phase, the proportion of cells that pass the restriction point is increased by artificial suppression of p27 levels, while this proportion is reduced by elevation of p27 levels. p27 performs this critical function by regulating the subsequent activating phosphorylation of cyclin dependent kinase (CDK)2, which we also show is necessary for and closely associated with the initiation of DNA synthesis. We conclude that the p27 expression level at mid-G(1) phase determines when a cell passes through the restriction point, and does so by regulating subsequent CDK2 activation.
authors.affiliation: The Department of Molecular Genetics, The Lerner Research Institute, The Cleveland Clinic Foundation, Cleveland, Ohio 44195, USA. hitomim@ccf.org
authors.forename: Masahiro
authors.initials: M
authors.lastname: Hitomi
authors.forename: Ke
authors.initials: K
authors.lastname: Yang
authors.forename: Yang
authors.initials: Y
authors.lastname: Guo
authors.forename: Jonathan
authors.initials: J
authors.lastname: Fretthold
authors.forename: Jyoti
authors.initials: J
authors.lastname: Harwalkar
authors.forename: Dennis W
authors.initials: DW
authors.lastname: Stacey
mesh_terms: D016475:3T3 Cells; D000818:Animals; D002453:Cell Cycle; D051357:Cyclin-Dependent Kinase 2 / Q000378:metabolism / Q000502:physiology*; D050760:Cyclin-Dependent Kinase Inhibitor p27 / Q000032:analysis / Q000502:physiology*; D004261:DNA Replication; D016193:G1 Phase; D051379:Mice; D008934:Mitogens / Q000494:pharmacology; D010766:Phosphorylation
chemical_list: D008934:Mitogens; D050760:Cyclin-Dependent Kinase Inhibitor p27; C495894:CDK2 protein, human; D051357:Cyclin-Dependent Kinase 2