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Loss of protooncogene c-Myc function impedes G1 phase progression both before and after the restriction point
Christoph Schorl1, John M Sedivy
1Department of Molecular Biology, Cell Biology, and Biochemistry, Brown University, Providence, Rhode Island 02912, USA.
Abstract:
c-myc is an important protooncogene whose misregulation is believed to causally affect the development of numerous human cancers. c-myc null rat fibroblasts are viable but display a severe (two- to threefold) retardation of proliferation. The rates of RNA and protein synthesis are reduced by approximately the same factor, whereas cell size remains unaffected. We have performed a detailed kinetic cell cycle analysis of c-myc(-/-) cells by using several labeling and synchronization methods. The majority of cells (>90%) in asynchronous, exponential phase c-myc(-/-) cultures cycle continuously with uniformly elongated cell cycles. Cell cycle elongation is due to a major lengthening of G(1) phase (four- to fivefold) and a more limited lengthening of G(2) phase (twofold), whereas S phase duration is largely unaffected. Progression from mitosis to the G1 restriction point and the subsequent progression from the restriction point into S phase are both drastically delayed. These results are best explained by a model in which c-Myc directly affects cell growth (accumulation of mass) and cell proliferation (the cell cycle machinery) by independent pathways.
Insights
The protooncogene c-Myc is crucial for cell proliferation. Its absence in rat fibroblasts significantly slows cell cycles, mainly by delaying G1 and G2 phases, impacting cancer development.
Area of Science:
- Molecular Biology
- Cell Biology
- Cancer Research
Background:
- The c-Myc proto-oncogene plays a critical role in regulating cell growth and proliferation.
- Misregulation of c-Myc is implicated in the development of various human cancers.
Purpose of the Study:
- To investigate the impact of c-Myc deficiency on cell cycle kinetics in rat fibroblasts.
- To elucidate the specific cell cycle phases affected by the absence of c-Myc.
Main Methods:
- Detailed kinetic cell cycle analysis using labeling and synchronization techniques.
- Comparison of cell cycle progression in c-myc null (c-myc(-/-)) and wild-type rat fibroblasts.
- Assessment of RNA and protein synthesis rates and cell size.
Main Results:
- c-myc null fibroblasts exhibit a two- to threefold retardation in proliferation.
- Cell cycle elongation is primarily due to a four- to fivefold lengthening of the G1 phase and a twofold lengthening of the G2 phase.
- RNA and protein synthesis rates are reduced proportionally, while cell size remains unchanged.
- Progression through the G1 restriction point into S phase is significantly delayed.
Conclusions:
- c-Myc deficiency severely impacts cell cycle progression, particularly during the G1 and G2 phases.
- The findings support a model where c-Myc influences cell growth and proliferation through independent pathways.
- Understanding c-Myc's role is vital for cancer research and therapeutic strategies.