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Regulation of platelet-derived growth factor signaling by activated p21Ras
L L Stice1, C Vaziri, D V Faller
1Cancer Research Center, Boston University School of Medicine, Boston, MA, USA.
Abstract:
Elucidating the molecular mechanisms regulating transduction of growth control signals and the discovery of the subversion of these pathways by oncogenes has proven critical in unraveling the biochemical factors leading to cellular transformation. One such line of investigation has been study of the effects of transforming p21Ras on platelet-derived growth factor type-beta receptor (PDGF-betaR) signaling. Platelet-derived growth factor is an important extracellular factor regulating the G0-S phase transition of mesenchymal cells. Expression of activated, oncogenic Kirsten- or Harvey-p21Ras in cells influences PDGF-betaR signaling at multiple levels. At least two separate mechanisms account for defective PDGF-betaR signaling in activated p21Ras-expressing cells: (i) transcriptional down-regulation of PDGF-betaR expression, and (ii) inhibition of ligand-induced PDGF-betaR phosphorylation by a factor which is present in the cellular membrane fraction of fibroblasts expressing activated p21Ras. The state of growth arrest in G0 is associated with increased expression of the PDGF-betaR, and oncogene-transformed cell lines, which fail to undergo growth-arrest following prolonged serum-deprivation, express constitutively low levels of the PDGF-betaR mRNA, and possess greatly reduced numbers of PDGF-BB-binding sites. This repression of PDGF-betaR expression by p21Ras is, at least in large part, transcriptional. The membrane-associated factor induced by oncogenic p21Ras provides a connection between cell morphology and cytoskeletal elements and control of ligand-dependent PDGF-betaR autophosphorylation. Reversion of the transformed phenotype results in the recovery of PDGF-betaR kinase activity. Conversely, disruption of the actin cytoskeleton of untransformed fibroblasts leads to the loss of PDGF-betaR function. These studies define two potential mechanisms for feedback control of PDGF-betaR function by downstream elements in the PDGF signaling pathway. In addition, the connection between cell morphology and the function of the PDGF-betaR established by these studies provides a new mechanistic link between the organization of the cytoskeleton, the Ras-related small G proteins, and the activity of membrane-bound receptor tyrosine kinases.
Insights
Oncogenic Ras proteins disrupt platelet-derived growth factor-beta receptor (PDGF-betaR) signaling through transcriptional downregulation and membrane-associated inhibition. Restoring normal cell growth also restores PDGF-betaR function, linking Ras, cytoskeleton, and receptor tyrosine kinases.
Area of Science:
- Molecular Biology
- Cellular Signaling
- Oncogenesis
Background:
- Cellular transformation involves subversion of growth control signaling pathways by oncogenes.
- Platelet-derived growth factor-beta receptor (PDGF-betaR) signaling is crucial for mesenchymal cell growth regulation.
- Oncogenic p21Ras proteins significantly impact PDGF-betaR signaling pathways.
Purpose of the Study:
- To elucidate how transforming p21Ras affects PDGF-betaR signaling.
- To identify molecular mechanisms underlying defective PDGF-betaR signaling in Ras-transformed cells.
- To explore the link between cell morphology, cytoskeleton, and PDGF-betaR activity.
Main Methods:
- Analysis of PDGF-betaR expression levels (mRNA and binding sites) in cells with activated p21Ras.
- Investigation of membrane-associated factors inhibiting PDGF-betaR phosphorylation.
- Assessment of PDGF-betaR kinase activity and function following reversion of transformed phenotype or cytoskeletal disruption.
Main Results:
- Activated p21Ras leads to transcriptional downregulation of PDGF-betaR expression.
- A membrane-associated factor induced by oncogenic p21Ras inhibits ligand-induced PDGF-betaR phosphorylation.
- Disruption of the actin cytoskeleton in fibroblasts impairs PDGF-betaR function, while reversion of transformation restores it.
Conclusions:
- Two mechanisms, transcriptional repression and membrane-associated inhibition, explain defective PDGF-betaR signaling by p21Ras.
- Cell morphology and cytoskeletal organization are mechanistically linked to PDGF-betaR activity.
- These findings establish a connection between cytoskeleton, Ras proteins, and receptor tyrosine kinase function.