title: Active RB elicits late G1/S inhibition.
abstract: The retinoblastoma tumor suppressor protein (RB) is activated/dephosphorylated to mediate cell cycle inhibition in response to antimitogenic signals. To elucidate the mode of RB action at this critical transition, we utilized cell lines that can be induced to express a constitutively active allele of RB (PSM-RB). As expected, induction of PSM-RB, but not wild-type protein (WT), inhibited progression into S phase. It has been well documented that active RB inhibits E2F reporter activity, and this observation was confirmed upon induction of PSM-RB. Additionally, active RB inhibited E2F-2-mediated stimulation of cyclin E. By contrast, PSM-RB did not affect the mRNA or protein levels of endogenous cyclin E when mediating cell cycle inhibition. Similarly, there was no observable effect on cyclin E protein levels when p16ink4a was utilized to activate endogenous RB. CDK2/cyclin E complex formation was not disrupted and cyclin E-associated kinase activity was retained in the presence of PSM-RB. Additionally, centrosome duplication, a CDK2/cyclin E-dependent event, was not altered in the presence of active RB. Together, these data indicate that active RB does not block the G1/S transition through inhibition of cyclin E expression or activity. In contrast, PSM-RB leads to a dramatic reduction in cyclin A protein levels by coordinate transcriptional repression and degradation. This attenuation of cyclin A protein correlates with cell cycle inhibition. These studies indicate that RB inhibits cell cycle progression by targeting CDK2/cyclin A-dependent events at the G1/S transition to inhibit cell cycle progression.
authors.affiliation: Department of Cell Biology, University of Cincinnati College of Medicine, Vontz Center for Molecular Studies, Cincinnati, Ohio 45267-0521, USA.
authors.forename: Steven P
authors.initials: SP
authors.lastname: Angus
authors.forename: Anne F
authors.initials: AF
authors.lastname: Fribourg
authors.forename: Michael P
authors.initials: MP
authors.lastname: Markey
authors.forename: Sarah L
authors.initials: SL
authors.lastname: Williams
authors.forename: Henning F
authors.initials: HF
authors.lastname: Horn
authors.forename: James
authors.initials: J
authors.lastname: DeGregori
authors.forename: Timothy F
authors.initials: TF
authors.lastname: Kowalik
authors.forename: Kenji
authors.initials: K
authors.lastname: Fukasawa
authors.forename: Erik S
authors.initials: ES
authors.lastname: Knudsen
mesh_terms: D000818:Animals; D018797:Cell Cycle Proteins / Q000235:genetics*; D002455:Cell Division / Q000235:genetics; D002471:Cell Transformation, Neoplastic / Q000235:genetics*; D002478:Cells, Cultured; D018385:Centrosome / Q000378:metabolism; D019925:Cyclin A / Q000235:genetics / Q000378:metabolism; D019927:Cyclin E / Q000235:genetics / Q000378:metabolism; D018844:Cyclin-Dependent Kinases / Q000235:genetics / Q000378:metabolism; D015536:Down-Regulation / Q000235:genetics; D005057:Eukaryotic Cells / Q000166:cytology / Q000378:metabolism*; D016193:G1 Phase / Q000235:genetics*; D015972:Gene Expression Regulation, Neoplastic / Q000235:genetics*; D009154:Mutation / Q000235:genetics; D010766:Phosphorylation; D017434:Protein Structure, Tertiary / Q000235:genetics; D051381:Rats; D016160:Retinoblastoma Protein / Q000235:genetics*; D016196:S Phase / Q000235:genetics*; D015854:Up-Regulation / Q000235:genetics
chemical_list: D018797:Cell Cycle Proteins; D019925:Cyclin A; D019927:Cyclin E; D016160:Retinoblastoma Protein; D018844:Cyclin-Dependent Kinases