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Raf-1 kinase and ERK2 uncoupled from mitogenic signals in rat fibroblasts
1Max-Planck-Institut für Immunobiologie, Spemann Laboratories, Freiburg, Germany.
Abstract:
The MAP kinase pathway impinging on ERK2 has been shown to be integrally associated with mitogenic signalling in many cell types. Previously, we and others have demonstrated that oncogenic forms of Raf-1 kinase, when expressed in fibroblasts, lead to the constitutive activation of ERK2, the de-regulation of c-fos expression and increased cell proliferation. Here we describe an exception to this scenario. In Rat6 cells, although both ERK1 and ERK2 are activated in response to mitogens that induce c-fos expression, such as Epidermal Growth Factor (EGF), lysophosphatidic acid (LPA) or serum, expression of v-Raf fails to induce c-fos expression and increase proliferation. However, ERK2 is activated by v-Raf expression. The co-transfection of an interfering mutant of ERK2 has no effect on the level of c-fos reporter expression in Rat6 cells whereas the analogous ERK1 mutant reduces its expression. Furthermore, the spontaneous focus formation observed in Rat6 cells is susceptible to the interfering mutant of ERK1 but resistant to that of ERK2. Thus, not only do mitogenic signals appear to by-pass both Raf-1 kinase and ERK2, the Raf-1-ERK2 pathway seems to be functionally compromised in Rat6 cells as its activation leads neither to c-fos expression nor to increased proliferation.
Insights
In Rat6 cells, the Raf-1 kinase pathway is compromised. While ERK1/ERK2 activate with mitogens, v-Raf fails to induce c-fos expression or proliferation, indicating a functional defect in this signaling cascade.
Area of Science:
- Cellular Biology
- Molecular Signaling
- Oncogenesis
Background:
- The MAP kinase pathway, particularly ERK2, is crucial for mitogenic signaling and cell proliferation in many cell types.
- Oncogenic Raf-1 kinase typically activates ERK2, leading to c-fos expression and increased proliferation in fibroblasts.
Purpose of the Study:
- To investigate an exception to the known Raf-1/ERK2 signaling pathway in Rat6 cells.
- To determine the role of ERK1 and ERK2 in mitogenic signaling and proliferation in Rat6 cells under v-Raf expression.
Main Methods:
- Expression of v-Raf in Rat6 cells.
- Analysis of ERK1 and ERK2 activation in response to mitogens (EGF, LPA, serum).
- Co-transfection with interfering mutants of ERK1 and ERK2 to assess c-fos reporter expression and focus formation.
Main Results:
- In Rat6 cells, v-Raf expression activates ERK2 but fails to induce c-fos expression or increase proliferation.
- Mitogenic signals bypass Raf-1 kinase and ERK2 in Rat6 cells.
- Interfering ERK1 mutants reduce c-fos expression and focus formation, while ERK2 mutants have no effect.
Conclusions:
- The Raf-1/ERK2 pathway is functionally compromised in Rat6 cells.
- Mitogenic signaling and proliferation in Rat6 cells do not solely rely on the canonical Raf-1/ERK2 pathway.
- ERK1, rather than ERK2, appears to play a significant role in regulating c-fos expression and focus formation in this specific cell line.