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Direct induction of cyclin D2 by Myc contributes to cell cycle progression and sequestration of p27
C Bouchard1, K Thieke, A Maier
1Institute of Molecular Biology and Tumour Research, Phillips-Universität Marburg, Emil-Mannkopff-Strasse 2, 35033 Marburg.
Abstract:
Ectopic expression of Myc induces Cdk2 kinase activity in quiescent cells and antagonizes association of p27(kip1) with Cdk2. The target gene(s) by which Myc mediates this effect is largely unknown. We now show that p27 is rapidly and transiently sequestered by cyclin D2-Cdk4 complexes upon activation of Myc and that cyclin D2 is a direct target gene of Myc. The cyclin D2 promoter is repressed by Mad-Max complexes and de-repressed by Myc via a single highly conserved E-box element. Addition of trichostatin A to quiescent cells mimics activation of Myc and induces cyclin D2 expression, suggesting that cyclin D2 is repressed in a histone deacetylase-dependent manner in quiescent cells. Inhibition of cyclin D2 function in established cell lines, either by ectopic expression of p16 or by antibody injection, inhibits Myc-dependent dissociation of p27 from Cdk2 and Myc-induced cell cycle entry. Primary mouse fibroblasts that are cyclin D2-deficient undergo accelerated senescence in culture and are not immortalized by Myc; induction of apoptosis by Myc is unimpaired in such cells. Our data identify a downstream effector pathway that links Myc directly to cell cycle progression.
Insights
Myc activates cyclin D2 expression, a key step in cell cycle progression. This finding reveals a novel pathway linking Myc to cell division and antagonizing cell cycle inhibitors like p27.
Area of Science:
- Molecular Biology
- Cell Biology
- Oncology
Background:
- The oncogene Myc drives cell proliferation but its downstream targets remain incompletely understood.
- Myc's role in regulating cell cycle progression is complex, involving interactions with cyclin-dependent kinases (CDKs) and inhibitors like p27(kip1).
Purpose of the Study:
- To identify the direct target genes of Myc responsible for inducing cell cycle progression.
- To elucidate the mechanism by which Myc antagonizes cell cycle inhibitors and promotes cell division.
Main Methods:
- Analysis of cyclin D2 promoter activity in response to Myc activation.
- Chromatin immunoprecipitation to assess Myc binding to the cyclin D2 promoter.
- Functional inhibition of cyclin D2 using p16 or antibodies.
- Assessment of Myc-induced cell cycle entry, senescence, and apoptosis in cyclin D2-deficient fibroblasts.
Main Results:
- Myc directly induces cyclin D2 expression by de-repressing its promoter, which is normally repressed by Mad-Max complexes.
- Cyclin D2 is sequestered by cyclin D2-Cdk4 complexes upon Myc activation, facilitating the dissociation of p27(kip1) from Cdk2.
- Inhibition of cyclin D2 blocks Myc-induced cell cycle entry and Myc-deficient fibroblasts exhibit accelerated senescence.
Conclusions:
- Cyclin D2 is a direct downstream effector of Myc, mediating its ability to drive cell cycle progression.
- This pathway involving Myc, cyclin D2, and p27(kip1) is crucial for overcoming cell cycle arrest and promoting cellular immortalization.