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Conditional inhibition of the mitogen-activated protein kinase cascade by wortmannin. Dependence on signal strength
1Department of Medicine, Division of Signal Transduction, Beth Israel Hospital, Boston, Massachusetts 02115, USA. bduckwor@bidmc.harvard.edu
Abstract:
Phosphoinositide (PI) 3-kinase and the mitogen-activated protein (MAP) kinase cascades are activated by many of the same ligands. Several groups have reported involvement of PI 3-kinase in the activation of Erk1 and Erk2, whereas many other groups have shown that activation of Erk1 and Erk2 is not sensitive to inhibitors of PI 3-kinase such as wortmannin. Here we show that wortmannin inhibition of the MAP kinase pathway is cell type- and ligand-specific. Wortmannin blocks platelet-derived growth factor (PDGF)-dependent activation of Raf-1 and the MAP kinase cascade in Chinese hamster ovary cells, which have few PDGF receptors, but has no significant effect on Erk activation in Swiss 3T3 cells, which have high levels of PDGF receptors. However, wortmannin blocks activation of Erk proteins if Swiss 3T3 cells are stimulated with lower, physiological levels of PDGF. These results suggest that PI 3-kinase is in an efficient pathway for activation of MAP kinase, but that MAP kinase can be stimulated by a redundant pathway when a large number of receptors are activated. We present evidence that a protein kinase C family member downstream of phospholipase Cgamma is involved in the redundant pathway.
Insights
Phosphoinositide 3-kinase (PI3K) inhibition of the mitogen-activated protein (MAP) kinase pathway is specific to cell type and ligand. PI3K is vital for efficient MAP kinase activation, but redundant pathways exist when many receptors are activated.
Area of Science:
- Cellular signaling pathways
- Signal transduction mechanisms
- Molecular biology
Background:
- Mitogen-activated protein (MAP) kinase and phosphoinositide 3-kinase (PI3K) pathways are activated by common ligands.
- Previous studies show conflicting results regarding PI3K's role in Erk1/Erk2 activation, with some indicating involvement and others showing no sensitivity to PI3K inhibitors like wortmannin.
Purpose of the Study:
- To investigate the cell type- and ligand-specific inhibition of the MAP kinase pathway by wortmannin.
- To elucidate the role of PI3K in MAP kinase activation under varying receptor stimulation levels.
- To identify alternative pathways involved in MAP kinase activation.
Main Methods:
- Utilized wortmannin, a PI3K inhibitor, to assess its effect on MAP kinase activation (Erk1/Erk2).
- Employed Chinese hamster ovary (CHO) cells and Swiss 3T3 cells with differing platelet-derived growth factor (PDGF) receptor levels.
- Stimulated cells with varying concentrations of PDGF to analyze ligand-specific responses.
Main Results:
- Wortmannin inhibited PDGF-dependent Raf-1 and MAP kinase cascade activation in CHO cells (low PDGF receptors).
- Wortmannin did not significantly affect Erk activation in Swiss 3T3 cells (high PDGF receptors) at high PDGF levels.
- Wortmannin blocked Erk activation in Swiss 3T3 cells at lower, physiological PDGF concentrations.
Conclusions:
- PI3K acts in an efficient pathway for MAP kinase activation.
- Redundant pathways can stimulate MAP kinase when a large number of receptors are activated.
- A protein kinase C family member downstream of phospholipase Cgamma is implicated in this redundant pathway.