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Cell cycle-dependent activation of Ras
1Section of Biochemistry, Molecular and Cell Biology, Cornell University, Ithaca, New York 14853, USA.
Background:
Ras proteins play an essential role in the transduction of signals from a wide range of cell-surface receptors to the nucleus. These signals may promote cellular proliferation or differentiation, depending on the cell background. It is well established that Ras plays an important role in the transduction of mitogenic signals from activated growth-factor receptors, leading to cell-cycle entry. However, important questions remain as to whether Ras controls signalling events during cell-cycle progression and, if so, at which point in the cell-cycle it is activated.
Results:
To address these questions we have developed a novel, functional assay for the detection of cellular activated Ras. Using this assay, we found that Ras was activated in HeLa cells, following release from mitosis, and in NIH 3T3 fibroblasts, following serum-stimulated cell-cycle entry. In each case, peak Ras activation occurred in mid-G1 phase. Ras activation in HeLa cells at mid-G1 phase was dependent on RNA and protein synthesis and was not associated with tyrosine phosphorylation of Shc proteins and their binding to Grb2. Significantly, activation of Ras and the extracellular-signal regulated (ERK) sub-group of mitogen-activated protein kinases were not temporally correlated during G1-phase progression.
Conclusions:
Activation of Ras during mid-G1 phase appears to differ in many respects from its rapid activation by growth factors, suggesting a novel mechanism of regulation that may be intrinsic to cell-cycle progression. Furthermore, the temporal dissociation between Ras and ERK activation suggests that Ras targets alternate effector pathways during G1-phase progression.
Insights
Ras protein activation occurs during the G1 phase of the cell cycle, independent of growth factor signaling. This suggests a novel intrinsic regulation mechanism and alternative Ras effector pathways during cell cycle progression.
Area of Science:
- Cell Biology
- Molecular Biology
- Signal Transduction
Background:
- Ras proteins are crucial for signal transduction from cell-surface receptors to the nucleus, influencing cellular proliferation and differentiation.
- Ras is well-established to mediate mitogenic signals from growth factors, facilitating cell-cycle entry.
- The precise role of Ras in controlling signaling events during cell-cycle progression and its activation timing remain unclear.
Purpose of the Study:
- To investigate whether Ras controls signaling events during cell-cycle progression.
- To determine the specific point in the cell cycle at which Ras is activated.
Main Methods:
- Development of a novel, functional assay for detecting cellular activated Ras.
- Analysis of Ras activation in HeLa cells and NIH 3T3 fibroblasts at different cell-cycle stages.
Main Results:
- Ras activation was detected in HeLa cells post-mitosis and in NIH 3T3 fibroblasts after serum stimulation.
- Peak Ras activation occurred in mid-G1 phase in both cell types.
- Ras activation in mid-G1 phase was RNA and protein synthesis-dependent and not linked to Shc protein tyrosine phosphorylation or Grb2 binding.
- Ras activation and extracellular-signal regulated kinase (ERK) activation were not temporally correlated during G1 progression.
Conclusions:
- Ras activation during mid-G1 phase appears to involve a novel regulatory mechanism intrinsic to cell-cycle progression, distinct from rapid growth factor-induced activation.
- The temporal uncoupling of Ras and ERK activation suggests that Ras may engage alternative effector pathways during G1 phase.