Related Experiment Videos
Growth arrest by the cyclin-dependent kinase inhibitor p27Kip1 is abrogated by c-Myc
J Vlach1, S Hennecke, K Alevizopoulos
1Swiss Institute for Experimental Cancer Research, Epalinges, Switzerland.
Abstract:
We show here that c-Myc antagonizes the cyclin-dependent kinase (CDK) inhibitor p27Kip1. p27 expressed from recombinant retroviruses in Rat1 cells associated with and inhibited cyclin E/CDK2 complexes, induced accumulation of the pRb and p130 proteins in their hypophosphorylated forms, and arrested cells in G1. Prior expression of c-Myc prevented inactivation of cyclin E/CDK2 as well as dephosphorylation of pRb and p130, and allowed continuous cell proliferation in the presence of p27. This effect did not require ubiquitin-mediated degradation of p27. Myc altered neither the susceptibility of cyclin E/CDK2 to inhibition by p27, nor the intrinsic CDK-inhibitory activity of p27, but induced sequestration of p27 in a form unable to bind cyclin E/CDK2. Neither Myc itself nor other G1-cyclin/CDK complexes were directly responsible for p27 sequestration. Retroviral expression of G1 cyclins (D1-3, E or A) or of the Cdc25A phosphatase did not overcome p27-induced arrest. Growth rescue by Myc required dimerization with Max, DNA binding and an intact transcriptional activation domain, as previously shown for cellular transformation. We propose that this activity is mediated by the product of an as yet unknown Myc-Max target gene(s) and represents an essential aspect of Myc's mitogenic and oncogenic functions.
Insights
The oncogene c-Myc prevents the cell cycle inhibitor p27Kip1 from binding to cyclin E/CDK2 complexes, allowing cells to proliferate. This mechanism is crucial for c-Myc
Area of Science:
- Cell Biology
- Molecular Oncology
- Cancer Research
Background:
- The cell cycle is tightly regulated by cyclin-dependent kinases (CDKs) and their inhibitors.
- p27Kip1 is a CDK inhibitor that plays a critical role in cell cycle arrest, particularly in the G1 phase.
- c-Myc is a potent oncogene implicated in cell proliferation, differentiation, and apoptosis.
Purpose of the Study:
- To investigate the mechanism by which c-Myc antagonizes the function of the CDK inhibitor p27Kip1.
- To elucidate how c-Myc promotes continuous cell proliferation in the presence of p27.
- To determine the role of this interaction in c-Myc's mitogenic and oncogenic activities.
Main Methods:
- Expression of p27Kip1 and c-Myc in Rat1 cells using recombinant retroviruses.
- Analysis of cyclin E/CDK2 complex activity, pRb and p130 protein phosphorylation status.
- Assessment of cell cycle progression and requirement for c-Myc dimerization with Max, DNA binding, and transcriptional activation.
Main Results:
- p27Kip1 inhibited cyclin E/CDK2, induced hypophosphorylation of pRb and p130, and caused G1 arrest.
- c-Myc expression prevented p27Kip1-mediated inactivation of cyclin E/CDK2 and dephosphorylation of pRb/p130, enabling proliferation.
- c-Myc sequestered p27Kip1 without degrading it or altering its inhibitory activity, and this effect required Max dimerization and DNA binding.
Conclusions:
- c-Myc antagonizes p27Kip1 by sequestering it, thereby preventing cell cycle arrest and promoting proliferation.
- This sequestration mechanism is independent of p27 degradation and does not involve direct interaction with Myc or other G1 cyclin/CDK complexes.
- The observed growth rescue is mediated by a Myc-Max target gene(s) and represents a key aspect of c-Myc's oncogenic function.