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Integrin-mediated activation of MEK and mitogen-activated protein kinase is independent of Ras [corrected]
1Department of Pharmacology, School of Medicine, University of North Carolina, Chapel Hill, North Carolina 27599, USA.
Abstract:
The integrins are a family of cell surface receptors that mediate adhesive interactions with the extracellular matrix and also generate signals that influence cell growth and differentiation. Ligation and clustering of integrins causes activation and autophosphorylation of focal adhesion kinase (FAK), a cytoplasmic tyrosine kinase, and results in the transient activation of p42 and p44 mitogen-activated protein (MAP) kinases. Initial evidence has suggested that the integrin signaling pathway may share common elements with the canonical Ras signal transduction cascade activated by peptide mitogens such as epidermal growth factor (EGF). In this report we demonstrate that Raf-1 and MAP or extracellular signal-related kinase kinase (MEK), key cytoplasmic kinases of the Ras cascade, are activated subsequent to integrin-mediated adhesion of mouse NIH 3T3 fibroblasts. We also show that MAP kinase is downstream of MEK in the integrin signaling pathway. However, in contrast to the receptor tyrosine kinase signaling cascade, integrin-mediated signal transduction seems to be largely independent of Ras. Dominant negative inhibitors of Ras-dependent signaling failed to block integrin-mediated activation of MEK. In addition, while treatment with the peptide mitogen EGF clearly increased GTP-loading of Ras, little effect was observed in response to integrin-dependent cell adhesion. Thus, integrin-mediated activation of MEK and MAP kinase in 3T3 cells occurs primarily by a mechanism that is distinct from the Ras signal transduction cascade.
Insights
Integrins activate key signaling proteins like Raf-1 and MAP kinase (MEK) through cell adhesion. This integrin signaling pathway is distinct from the Ras pathway, as Ras activation is not required.
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- Integrins are cell surface receptors mediating adhesion and signaling.
- Integrin activation leads to focal adhesion kinase (FAK) phosphorylation and mitogen-activated protein (MAP) kinase activation.
- The integrin pathway was hypothesized to share elements with the Ras signal transduction cascade.
Purpose of the Study:
- To investigate the relationship between integrin signaling and the Ras pathway.
- To determine if Raf-1 and MAP kinase kinase (MEK) are activated by integrin-mediated adhesion.
- To elucidate the specific mechanism of integrin-induced MAP kinase activation.
Main Methods:
- Utilized NIH 3T3 fibroblasts for experiments.
- Assessed activation of Raf-1, MEK, and MAP kinase following integrin engagement.
- Employed dominant-negative Ras inhibitors to probe Ras dependency.
- Measured Ras GTP-loading in response to integrin adhesion and EGF stimulation.
Main Results:
- Integrin-mediated adhesion activated Raf-1 and MEK in NIH 3T3 fibroblasts.
- MAP kinase activation was confirmed to be downstream of MEK in the integrin pathway.
- Integrin signaling leading to MEK activation was largely independent of Ras.
- Integrin adhesion showed minimal effect on Ras GTP-loading compared to EGF stimulation.
Conclusions:
- Integrin-mediated activation of MEK and MAP kinase in fibroblasts proceeds through a Ras-independent mechanism.
- This finding differentiates integrin signaling from canonical receptor tyrosine kinase pathways like EGF signaling.
- The study clarifies a distinct molecular mechanism in cell adhesion and signaling.