Activation of c-Myc uncouples DNA replication from activation of G1-cyclin-dependent kinases.
Proto-oncogenes like c-myc are thought to control exit from the cell cycle rather than progression through the cell cycle itself. We now present a different view of Myc function. Exponentially growing Rat1-MycER fibroblasts were size-fractionated by centrifugal elutriation. In these cells, activation of cyclin E- and cyclin A-dependent kinases, degradation of p27, hyperphosphorylation of retinoblastoma protein and activation of E2F occur sequentially at specific cell sizes. Upon activation of Myc, however, these transitions all occur simultaneously in small cells immediately after exit from mitosis. In contrast, Myc has no discernible effect on the cell size at which DNA replication is initiated. These data show first that Myc controls the activity of G1 cyclin-dependent kinases independently from the transition between quiescence and proliferation and from any effect on cell growth in size. These data also provide evidence of at least one dominant mechanism besides activation of E2F and of cyclin E/cdk2 kinase, which prevents DNA replication unless a critical cell size has been reached.
Pusch
O
O
Obstetrics and Gynecology, University of Vienna, Department of Prenatal Diagnosis and Therapy, Austria.
Bernaschek
G
G
Eilers
M
M
Hengstschläger
M
M
D000818:Animals; D002352:Carrier Proteins; D002453:Cell Cycle / Q000235:genetics / Q000502:physiology; D018797:Cell Cycle Proteins; D048429:Cell Size / Q000235:genetics; D002498:Centrifugation; D050760:Cyclin-Dependent Kinase Inhibitor p27; D018844:Cyclin-Dependent Kinases / Q000378:metabolism*; D016213:Cyclins / Q000032:analysis / Q000378:metabolism; D004261:DNA Replication / Q000235:genetics*; D004268:DNA-Binding Proteins; D050684:E2F Transcription Factors; D005434:Flow Cytometry; D016193:G1 Phase / Q000235:genetics; D016259:Genes, myc / Q000502:physiology*; D008869:Microtubule-Associated Proteins / Q000378:metabolism; D008938:Mitosis / Q000235:genetics; D010766:Phosphorylation; D051381:Rats; D016160:Retinoblastoma Protein / Q000378:metabolism; D056465:Retinoblastoma-Binding Protein 1; D050680:Transcription Factor DP1; D014157:Transcription Factors / Q000378:metabolism; D014407:Tumor Cells, Cultured; D025521:Tumor Suppressor Proteins
D002352:Carrier Proteins; C493962:Cdkn1b protein, rat; D018797:Cell Cycle Proteins; D016213:Cyclins; D004268:DNA-Binding Proteins; D050684:E2F Transcription Factors; D008869:Microtubule-Associated Proteins; D016160:Retinoblastoma Protein; D056465:Retinoblastoma-Binding Protein 1; D050680:Transcription Factor DP1; D014157:Transcription Factors; D025521:Tumor Suppressor Proteins; D050760:Cyclin-Dependent Kinase Inhibitor p27; D018844:Cyclin-Dependent Kinases