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Cell adhesion protects c-Raf-1 against ubiquitin-dependent degradation by the proteasome
Stéphane Manenti1, Christelle Delmas, Jean-Marie Darbon
1Laboratoire de Biologie Cellulaire et Moléculaire du Contrôle de la Prolifération, CNRS UMR 5088, Université Paul Sabatier, 118 Route de Narbonne, 31062 Toulouse Cedex, France. manenti@cict.fr
Abstract:
MAP kinase activation by growth factors depends on cell adhesion to the extracellular matrix. Disrupting the cell adhesion process in NIH 3T3 fibroblasts induced an almost complete inhibition of MAP kinase, which was impaired by proteasome inhibitors. In the absence of cell anchorage, c-Raf-1 expression was dramatically decreased after 24 h. This down-regulation was suppressed by proteasome inhibitors, suggesting that a proteasome-dependent degradation of Raf occurred in the absence of cell adhesion. Proteasome inhibitors did not affect Raf-1 levels in adherent cells, indicating that this degradation only occurred in the absence of cell adhesion. Finally, ectopic coexpression of Raf-1 and ubiquitin in HEK-293 and NIH 3T3 cells generated ubiquitylated forms of Raf-1, both in adherent and suspended cells, suggesting a possible ubiquitin-dependent degradation of the protein.
Insights
Cell adhesion is crucial for MAP kinase activation. Without it, Raf-1 protein degrades via proteasomes, a process preventable with proteasome inhibitors.
Area of Science:
- Cellular Biology
- Molecular Signaling
- Extracellular Matrix Interactions
Background:
- Mitogen-activated protein (MAP) kinase activation by growth factors is known to be influenced by cell adhesion.
- The extracellular matrix plays a significant role in mediating cell adhesion and subsequent signaling pathways.
- Understanding the regulation of key signaling proteins like Raf-1 in response to adhesion status is critical.
Purpose of the Study:
- To investigate the role of cell adhesion in MAP kinase activation.
- To determine the mechanism by which cell adhesion affects c-Raf-1 expression and stability.
- To explore the involvement of proteasomal degradation in regulating Raf-1 under different adhesion conditions.
Main Methods:
- Disruption of cell adhesion in NIH 3T3 fibroblasts.
- Treatment with proteasome inhibitors.
- Analysis of MAP kinase activation and c-Raf-1 expression levels.
- Ectopic coexpression of Raf-1 and ubiquitin in HEK-293 and NIH 3T3 cells.
Main Results:
- Disrupting cell adhesion led to significant inhibition of MAP kinase activation, which was partially rescued by proteasome inhibitors.
- Absence of cell anchorage caused a dramatic decrease in c-Raf-1 expression after 24 hours, suppressed by proteasome inhibitors.
- Ubiquitylated forms of Raf-1 were detected upon coexpression with ubiquitin, suggesting a role for ubiquitination in Raf-1 degradation.
Conclusions:
- Cell adhesion to the extracellular matrix is essential for robust MAP kinase activation.
- Proteasome-dependent degradation of c-Raf-1 occurs in non-adherent cells, contributing to signaling inhibition.
- Ubiquitin-mediated degradation pathways may regulate Raf-1 stability, particularly under conditions of cell detachment.