title: ERK activity and G1 phase progression: identifying dispensable versus essential activities and primary versus secondary targets.
abstract: The ERK subfamily of MAP kinases is a critical regulator of S phase entry. ERK activity regulates the induction of cyclin D1, and a sustained ERK signal is thought to be required for this effect, at least in fibroblasts. We now show that early G1 phase ERK activity is dispensable for the induction of cyclin D1 and that the critical ERK signaling period is restricted to 3-6 h after mitogenic stimulation of quiescent fibroblasts. Similarly, early G1 phase ERK activity is dispensable for entry into S phase. Moreover, if cyclin D1 is expressed ectopically, ERK activity becomes dispensable throughout the G1 phase. In addition to its effect on cyclin D1, ERK activity is thought to contribute to the down-regulation of p27kip1. We found that this effect is restricted to late G1/S phase. Mechanistic analysis showed that the ERK effect on p27kip1 is mediated by Skp2 and is secondary to its effect on cyclin D1. Our results emphasize the importance of mid-G1 phase ERK activity and resolve primary versus secondary ERK targets within the G1 phase cyclin-dependent kinases.
authors.affiliation: Department of Pharmacology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104-6084, USA.
authors.forename: Jessie
authors.initials: J
authors.lastname: Villanueva
authors.forename: Yuval
authors.initials: Y
authors.lastname: Yung
authors.forename: Janice L
authors.initials: JL
authors.lastname: Walker
authors.forename: Richard K
authors.initials: RK
authors.lastname: Assoian
mesh_terms: D000818:Animals; D002070:Butadienes / Q000494:pharmacology; D002478:Cells, Cultured; D019938:Cyclin D1 / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D050760:Cyclin-Dependent Kinase Inhibitor p27; D004791:Enzyme Inhibitors / Q000494:pharmacology; D048049:Extracellular Signal-Regulated MAP Kinases / Q000037:antagonists & inhibitors / Q000187:drug effects / Q000378:metabolism*; D005347:Fibroblasts / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D016193:G1 Phase / Q000502:physiology*; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism; D051379:Mice; D009570:Nitriles / Q000494:pharmacology; D044786:S-Phase Kinase-Associated Proteins / Q000187:drug effects / Q000378:metabolism
chemical_list: D002070:Butadienes; C436291:CDKN1B protein, human; D004791:Enzyme Inhibitors; D047908:Intracellular Signaling Peptides and Proteins; D009570:Nitriles; D044786:S-Phase Kinase-Associated Proteins; C113580:U 0126; D019938:Cyclin D1; D050760:Cyclin-Dependent Kinase Inhibitor p27; D048049:Extracellular Signal-Regulated MAP Kinases