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Raf induces TGFbeta production while blocking its apoptotic but not invasive responses: a mechanism leading to
K Lehmann1, E Janda, C E Pierreux
1Signal Transduction, Imperial Cancer Research Fund, London WC2A 3PX, UK.
Abstract:
c-Raf-1 is a major effector of Ras proteins, responsible for activation of the ERK MAP kinase pathway and a critical regulator of both normal growth and oncogenic transformation. Using an inducible form of Raf in MDCK cells, we have shown that sustained activation of Raf alone is able to induce the transition from an epithelial to a mesenchymal phenotype. Raf promoted invasive growth in collagen gels, a characteristic of malignant cells; this was dependent on the operation of an autocrine loop involving TGFbeta, whose secretion was induced by Raf. TGFbeta induced growth inhibition and apoptosis in normal MDCK cells: Activation of Raf led to inhibition of the ability of TGFbeta to induce apoptosis but not growth retardation. ERK has been reported previously to inhibit TGFbeta signaling via phosphorylation of the linker region of Smads, which prevents their translocation to the nucleus. However, we found no evidence in this system that ERK can significantly influence the function of Smad2, Smad3, and Smad4 at the level of nuclear translocation, DNA binding, or transcriptional activation. Instead, strong activation of Raf caused a broad protection of these cells from various apoptotic stimuli, allowing them to respond to TGFbeta with increased invasiveness while avoiding cell death. The Raf-MAP kinase pathway thus synergizes with TGFbeta in promoting malignancy but does not directly impair TGFbeta-induced Smad signaling.
Insights
Sustained Raf activation promotes cancer-like cell changes and invasive growth by inducing TGF-beta. While Raf-ERK signaling protects cells from apoptosis, it does not directly block TGF-beta
Area of Science:
- Cell Biology
- Oncology
- Molecular Biology
Background:
- c-Raf-1 is a key Ras effector, activating the ERK MAP kinase pathway.
- This pathway regulates normal growth and is implicated in oncogenic transformation.
Purpose of the Study:
- To investigate the role of Raf activation in epithelial-mesenchymal transition and invasive growth.
- To elucidate the interplay between Raf-ERK signaling and TGF-beta in cancer progression.
Main Methods:
- Utilized an inducible Raf system in Madin-Darby canine kidney (MDCK) cells.
- Assessed invasive growth in collagen gels and TGF-beta-induced apoptosis and growth inhibition.
- Examined the influence of ERK on Smad protein function (nuclear translocation, DNA binding, transcriptional activation).
Main Results:
- Sustained Raf activation induced an epithelial-to-mesenchymal transition and invasive growth.
- Raf-induced invasive growth depended on an autocrine TGF-beta loop.
- Raf activation inhibited TGF-beta-induced apoptosis but not growth retardation.
- ERK did not significantly impair Smad2, Smad3, or Smad4 function.
- Raf activation broadly protected cells from apoptotic stimuli, enhancing TGF-beta-mediated invasiveness.
Conclusions:
- The Raf-MAP kinase pathway synergizes with TGF-beta to promote malignancy.
- Raf activation enhances cellular invasiveness and survival, contributing to cancer progression.
- The mechanism involves broad anti-apoptotic effects rather than direct impairment of TGF-beta-induced Smad signaling.