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Cell transformation by ras and regulation of its protein product
D R Lowy1, M R Johnson, J E DeClue
1Laboratory of Cellular Oncology, National Cancer Institute, Bethesda, MD.
Abstract:
We are studying the biological activity and regulation of mammalian Ras protein in tumours and in physiological signalling. We have shown that GAP (the GTPase-activating protein) is a potent negative regulator of normal Ras in cells. Reduction or loss of the NF1 gene product neurofibromin, in association with genetic abnormalities of the NF1 locus, has been identified in schwannoma cell lines from patients with neurofibromatosis and in melanoma and neuroblastoma lines from patients without neurofibromatosis. Although loss of neurofibromin in the schwannoma lines was associated with a high proportion of normal Ras protein in the active GTP-bound state, Ras-GTP appeared to be appropriately regulated in the melanoma and neuroblastoma lines, which contain normal levels of GAP. Therefore the GTPase-activating activity of neurofibromin is not essential for negative regulation of Ras in some cell types and the putative tumour suppressor function of neurofibromin in such cell types is independent of its GTPase-activating activity. Mitogen activation of Ras in fibroblasts is mediated primarily by exchange factors, which probably interact with a region on the Ras protein distinct from the region required for interaction with GAP. Multiple full-length cDNAs have identified a mouse gene whose products are related to yeast CDC25 guanine nucleotide exchange factor.
Insights
Neurofibromin loss in tumors doesn't always impair Ras regulation. Its tumor suppressor function can be independent of its GTPase-activating protein activity, impacting cancer cell signaling.
Area of Science:
- Molecular Biology
- Oncology
- Cell Signaling
Background:
- Ras proteins are key regulators of cell signaling, and their dysregulation is implicated in various cancers.
- GTPase-activating protein (GAP) is a known negative regulator of normal Ras.
- Neurofibromin (NF1 gene product) is a tumor suppressor, and its loss is associated with certain tumors.
Purpose of the Study:
- To investigate the biological activity and regulation of mammalian Ras protein in tumors.
- To determine if the GTPase-activating activity of neurofibromin is essential for Ras regulation in all cell types.
- To explore the tumor suppressor function of neurofibromin independent of its GTPase-activating activity.
Main Methods:
- Analysis of Ras protein activity (GTP-bound state) in tumor cell lines with varying neurofibromin and GAP levels.
- Genetic analysis of the NF1 locus in schwannoma, melanoma, and neuroblastoma cell lines.
- Comparison of Ras regulation in cell lines with and without neurofibromin loss.
Main Results:
- Loss of neurofibromin in schwannoma lines correlated with increased active Ras-GTP.
- In melanoma and neuroblastoma lines with normal GAP levels, Ras-GTP was appropriately regulated despite neurofibromin loss.
- Mitogen activation of Ras in fibroblasts involves exchange factors interacting with a distinct Ras region.
Conclusions:
- The GTPase-activating activity of neurofibromin is not universally required for negative Ras regulation.
- Neurofibromin's tumor suppressor function can operate independently of its GTPase-activating protein activity in certain cell types.
- Further research into Ras regulation and exchange factors is warranted for understanding cancer signaling.