title: CDK2 regulation through PI3K and CDK4 is necessary for cell cycle progression of primary rat hepatocytes.
abstract: OBJECTIVE
  Cell cycle progression is driven by the coordinated regulation of cyclin-dependent kinases (CDKs). In response to mitogenic stimuli, CDK4 and CDK2 form complexes with cyclins D and E, respectively, and translocate to the nucleus in the late G(1) phase. It is an on-going discussion whether mammalian cells need both CDK4 and CDK2 kinase activities for induction of S phase.
  
  
  RESULTS
  In this study, we have explored the role of CDK4 activity during G(1) progression of primary rat hepatocytes. We found that CDK4 activity was restricted by either inhibiting growth factor induced cyclin D1-induction with the PI3K inhibitor LY294002, or by transient transfection with a dominant negative CDK4 mutant. In both cases, we observed reduced CDK2 nuclear translocation and reduced CDK2-Thr160 phosphorylation. Furthermore, reduced pRb hyperphosphorylation and reduced cellular proliferation were observed. Ectopic expression of cyclin D1 alone was not sufficient to induce CDK4 nuclear translocation, CDK2 activity or cell proliferation.
  
  
  CONCLUSIONS
  Thus, epidermal growth factor-induced CDK4 activity was necessary for CDK2 activation and for hepatocyte proliferation. These results also suggest that, in addition to regulating cyclin D1 expression, PI3K is involved in regulation of nuclear shuttling of cyclin-CDK complexes in G(1) phase.
authors.affiliation: Laboratory for Toxicopathology, Institute of Pathology, University of Oslo, Rikshospitalet Radiumhospitalet Medical Center, Oslo, Norway. lene.wierod@medisin.uio.no
authors.forename: L
authors.initials: L
authors.lastname: Wierød
authors.forename: C M
authors.initials: CM
authors.lastname: Rosseland
authors.forename: B
authors.initials: B
authors.lastname: Lindeman
authors.forename: M P
authors.initials: MP
authors.lastname: Oksvold
authors.forename: H
authors.initials: H
authors.lastname: Grøsvik
authors.forename: E
authors.initials: E
authors.lastname: Skarpen
authors.forename: H S
authors.initials: HS
authors.lastname: Huitfeldt
mesh_terms: D021581:Active Transport, Cell Nucleus; D000818:Animals; D002453:Cell Cycle; D002467:Cell Nucleus / Q000201:enzymology; D049109:Cell Proliferation; D002478:Cells, Cultured; D019938:Cyclin D1 / Q000378:metabolism; D019927:Cyclin E / Q000096:biosynthesis; D051357:Cyclin-Dependent Kinase 2 / Q000378:metabolism*; D051358:Cyclin-Dependent Kinase 4 / Q000737:chemistry / Q000378:metabolism*; D004815:Epidermal Growth Factor / Q000494:pharmacology; D022781:Hepatocytes / Q000187:drug effects / Q000201:enzymology*; D008297:Male; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D013912:Threonine / Q000378:metabolism
chemical_list: D019927:Cyclin E; D019938:Cyclin D1; D013912:Threonine; D004815:Epidermal Growth Factor; D019869:Phosphatidylinositol 3-Kinases; C495892:Cdk2 protein, rat; C495903:Cdk4 protein, rat; D051357:Cyclin-Dependent Kinase 2; D051358:Cyclin-Dependent Kinase 4