title: Quiescence versus apoptosis: Myc abundance determines pathway of exit from the cell cycle.
abstract: When exposed to diverse growth conditions in vitro, cells can respond by entering states of proliferation, quiescence, differentiation or apoptosis. While the choices among these states can be influenced by proto-oncogene expression, how these disparate outcomes are achieved remains poorly understood. To address these issues, we have generated rodent fibroblast cell lines that harbor a human c-myc gene under the control of a tetracycline-regulated promoter. When Myc-induced cells are deprived of serum growth factors, they rapidly become apoptotic with the onset of apoptosis preceded by a large, transient increase in cdk2 kinase activity that is associated with the induction of cdc25A phosphatase and the later accumulation of p27Kip1 kinase inhibitor. Surprisingly, serum starvation in the absence of myc overexpression, (which leads to quiescence instead of apoptosis) also causes a marked transient elevation in cdk2 kinase activity, an induction of cdc25A and a delayed increase in p27Kip1. Transient elevations in cdk2 kinase activity and cdc25A abundance are required for cell cycle progression, but it is evident that these changes also precede entry to either apoptosis or quiescence in serum-starved cells. These findings suggest that the pathways to both quiescence and apoptosis share regulatory machinery with cell cycle control mechanisms. In addition, the abundance of Myc protein can be critical in the choices among these cellular states.
authors.affiliation: Department of Biochemistry and Molecular Biology, The University of Calgary, Alberta, Canada.
authors.forename: C C
authors.initials: CC
authors.lastname: Helbing
authors.forename: C L
authors.initials: CL
authors.lastname: Wellington
authors.forename: M
authors.initials: M
authors.lastname: Gogela-Spehar
authors.forename: T
authors.initials: T
authors.lastname: Cheng
authors.forename: G G
authors.initials: GG
authors.lastname: Pinchbeck
authors.forename: R N
authors.initials: RN
authors.lastname: Johnston
mesh_terms: D016475:3T3 Cells; D000818:Animals; D017209:Apoptosis / Q000235:genetics / Q000502:physiology*; D042846:CDC2-CDC28 Kinases; D002453:Cell Cycle / Q000187:drug effects / Q000502:physiology*; D002471:Cell Transformation, Neoplastic; D016895:Culture Media, Serum-Free; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases / Q000235:genetics / Q000378:metabolism*; D005786:Gene Expression Regulation / Q000187:drug effects; D006801:Humans; D051379:Mice; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism*; D000090063:Proto-Oncogene Mas; D016271:Proto-Oncogene Proteins c-myc / Q000096:biosynthesis / Q000235:genetics / Q000502:physiology*; D013752:Tetracycline / Q000494:pharmacology; D014162:Transfection
chemical_list: D016895:Culture Media, Serum-Free; C000715470:MAS1 protein, human; D000090063:Proto-Oncogene Mas; D016271:Proto-Oncogene Proteins c-myc; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495894:CDK2 protein, human; C495893:Cdk2 protein, mouse; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases; D013752:Tetracycline