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Regulation of the cell cycle machinery by oncogenic ras
J T Winston1, S R Coats, Y Z Wang
1Department of Cell Biology, Vanderbilt University, Nashville, Tennessee 37232, USA.
Abstract:
The ras proto-oncogene has been implicated during the formation of tumors in vivo as well as the transformation of cell lines in culture. Conditional expression of an activated ras mutant in Balb/c-3T3 fibroblasts failed to stimulate S phase entry in the absence of plasma-derived progression factors, but did shorten the G1 interval from 12 to 6 h and abrogate the normal proliferative requirement for platelet-derived growth factor. Ras-dependent alteration of the 3T3 cell cycle was accompanied by a dramatic increase in the expression of the G1 regulatory protein, cyclin D1, while expression of cyclin E and cyclin A proteins were only weakly induced. Cyclin/cdk complexes assembled in response to ectopic ras expression in the absence of growth factor stimulation bound the cdk inhibitory factor, Kip1, and were inactive. However, plasma-stimulated regulatory pathways functioned co-operatively with the oncogenic ras molecule to decrease Kip1 levels, induce the kinase activities associated with cyclins D, E and A, and trigger the initiation of DNA replication. Our results suggest that a ras-activated signal transduction pathway may link environmental mitogenic stimuli to the cell cycle machinery via modulation of G1 cyclin expression.
Insights
The ras proto-oncogene shortens the G1 phase in fibroblasts, increasing cyclin D1. Growth factors cooperate with oncogenic ras to activate cell cycle progression and DNA replication.
Area of Science:
- Oncology
- Cell Biology
- Molecular Biology
Background:
- The ras proto-oncogene is crucial in tumor formation and cell transformation.
- Understanding ras's role in cell cycle regulation is key to cancer research.
Purpose of the Study:
- To investigate how activated ras influences the cell cycle progression in Balb/c-3T3 fibroblasts.
- To identify the specific cell cycle regulatory proteins modulated by ras signaling.
Main Methods:
- Conditional expression of an activated ras mutant in Balb/c-3T3 fibroblasts.
- Analysis of cell cycle intervals (G1 and S phase) and protein expression (cyclins D1, E, A, Kip1).
- Assessment of cyclin/cdk complex activity and response to growth factors.
Main Results:
- Activated ras shortened the G1 interval and abrogated the need for platelet-derived growth factor.
- Ras significantly increased G1 cyclin D1 expression, with weaker induction of cyclins E and A.
- Oncogenic ras, in conjunction with growth factors, decreased Kip1 levels, activated cyclin/cdk complexes, and initiated DNA replication.
Conclusions:
- Ras-activated signal transduction pathways link mitogenic stimuli to the cell cycle machinery.
- Modulation of G1 cyclin expression is a key mechanism by which ras influences cell cycle progression.
- Ras oncogene cooperates with cellular pathways to drive cell proliferation.