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Mek1 phosphorylation site mutants activate Raf-1 in NIH 3T3 cells
A Alessandrini1, H Greulich, W Huang
1Department of Molecular and Cellular Biology, Harvard University, Cambridge, Massachusetts 02138, USA.
Abstract:
MAP (mitogen-activated protein) kinases are activated by a family of dual specificity kinases called Meks (MAP kinase/Erk kinase). Mek1 can be activated by Raf by phosphorylation on serine 218 and serine 222. Mutation of these sites to acidic residues leads to constitutively active Mek1 in some cases. When fibroblast lines were infected with high titer retroviral stocks carrying these Mek1 genes, the resultant transformation and morphological changes correlated with the kinase activity of the respective Mek1 enzymes. Although [Asp218]- and [Asp218,Asp222]Mek immunoprecipitated from clonal cell lines could phosphorylate kinase-inactive Erk1 equally well in vitro, the endogenous MAP kinase activity was 5-7-fold greater in [Asp218]Mek1-infected clonal lines, and did not correlate with the degree of transformation. Analysis of the Erk1 pathway revealed Raf-1 activation, which correlated qualitatively with the MAP kinase activity seen in the [Asp218]- and [Asp218,Asp222]Mek1-infected clonal cell lines. Expression of dominant negative Ras did not affect the elevated Raf-1 activity observed in these cells, however. These data suggest that Mek1 phosphorylation site mutants activate Raf-1 and MAP kinase by a Ras-independent pathway and that the mechanism by which transformation occurs may utilize pathways that are MAP kinase-independent.
Insights
Mutant Mek1 (MAP kinase/Erk kinase) activates Raf-1 and MAP kinase independently of Ras, suggesting transformation pathways may not rely on MAP kinase signaling.
Area of Science:
- Cellular signaling pathways
- Molecular biology
- Cancer research
Background:
- Mitogen-activated protein (MAP) kinases are crucial signaling molecules.
- MAP kinase/Erk kinase (MEK) enzymes, particularly MEK1, are activated by Raf kinases.
- Specific phosphorylation sites on MEK1 (serine 218 and 222) are key for its activation.
Purpose of the Study:
- To investigate the activation mechanism of MEK1 and its downstream effects.
- To determine if mutated MEK1 activates MAP kinase and Raf-1 in a Ras-dependent or independent manner.
- To explore the relationship between MEK1 activity, MAP kinase signaling, and cellular transformation.
Main Methods:
- Retroviral infection of fibroblast lines with modified MEK1 genes (e.g., [Asp218]MEK1, [Asp218,Asp222]MEK1).
- In vitro kinase assays to measure MEK1 and Erk1 activity.
- Analysis of endogenous MAP kinase activity and Raf-1 activation.
- Experiments using dominant-negative Ras to assess pathway dependency.
Main Results:
- Mutant MEK1 ([Asp218]- and [Asp218,Asp222]MEK1) showed significantly increased endogenous MAP kinase activity (5-7 fold).
- Raf-1 activation was observed and correlated with MAP kinase activity in MEK1 mutant-infected cells.
- Elevated Raf-1 activity persisted even when Ras signaling was inhibited by dominant-negative Ras.
- Cellular transformation did not directly correlate with the observed MAP kinase activity levels.
Conclusions:
- MEK1 phosphorylation site mutants activate Raf-1 and MAP kinase through a Ras-independent pathway.
- Cellular transformation induced by these MEK1 mutants may involve signaling pathways distinct from canonical MAP kinase activation.
- These findings highlight a potential divergence between MEK1-driven signaling and transformation mechanisms.