The proto-oncogene c-myc acts through the cyclin-dependent kinase (Cdk) inhibitor p27(Kip1) to facilitate the activation of Cdk4/6 and early G(1) phase progression.
Progression through the early G(1) phase of the cell cycle requires mitogenic stimulation, which ultimately leads to the activation of cyclin-dependent kinases 4 and 6 (Cdk4/6). Cdk4/6 activity is promoted by D-type cyclins and opposed by Cdk inhibitor proteins. Loss of c-myc proto-oncogene function results in a defect in the activation of Cdk4/6. c-myc(-/-) cells express elevated levels of the Cdk inhibitor p27(Kip1) and reduced levels of Cdk7, the catalytic subunit of Cdk-activating kinase. We show here that in normal (c-myc(+/+)) cells, the majority of cyclin D-Cdk4/6 complexes are assembled with p27 and remain inactive during cell cycle progression; their function is presumably to sequester p27 from Cdk2 complexes. A small fraction of Cdk4/6 protein was found in lower molecular mass catalytically active complexes. Conditional overexpression of p27 in c-myc(+/+) cells caused inhibition of Cdk4/6 activity and elicited defects in G(0)-to-S phase progression very similar to those seen in c-myc(-/-) cells. Overexpression of cyclin D1 in c-myc(-/-) cells rescued the defect in Cdk4/6 activity, indicating that the limiting factor is the number of cyclin D-Cdk4/6 complexes. Cdk-activating kinase did not rescue Cdk4/6 activity. We propose that the defect in Cdk4/6 activity in c-myc(-/-) cells is caused by the elevated levels of p27, which convert the low abundance activable cyclin D-Cdk4/6 complexes into unactivable complexes containing higher stoichiometries of p27. These observations establish p27 as a physiologically relevant regulator of cyclin D-Cdk4/6 activity as well as mechanistically a target of c-Myc action and provide a model by which c-Myc influences the early-to-mid G(1) phase transition.
Obaya
Alvaro J
AJ
Department of Molecular Biology, Cell Biology, and Biochemistry, Brown University, 69 Brown Street, Providence, RI 02912, USA.
Kotenko
Iulia
I
Cole
Michael D
MD
Sedivy
John M
JM
D000818:Animals; D002460:Cell Line; D019938:Cyclin D1 / Q000502:physiology; D051358:Cyclin-Dependent Kinase 4; D051361:Cyclin-Dependent Kinase 6; D018844:Cyclin-Dependent Kinases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D004789:Enzyme Activation; D016193:G1 Phase; D016259:Genes, myc / Q000502:physiology*; D008840:Microfilament Proteins / Q000502:physiology*; D009124:Muscle Proteins; D017346:Protein Serine-Threonine Kinases / Q000037:antagonists & inhibitors* / Q000378:metabolism / Q000502:physiology; D011518:Proto-Oncogene Proteins; D051381:Rats; D000095043:Cyclin-Dependent Kinase-Activating Kinase
D008840:Microfilament Proteins; D009124:Muscle Proteins; D011518:Proto-Oncogene Proteins; C058992:Tagln protein, mouse; D019938:Cyclin D1; D017346:Protein Serine-Threonine Kinases; C495903:Cdk4 protein, rat; C495913:Cdk6 protein, rat; D051358:Cyclin-Dependent Kinase 4; D051361:Cyclin-Dependent Kinase 6; D018844:Cyclin-Dependent Kinases; D000095043:Cyclin-Dependent Kinase-Activating Kinase