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Effects of protein kinase C activation after epidermal growth factor binding on epidermal growth factor receptor
Abstract:
The possible role of epidermal growth factor (EGF) receptor phosphorylation at threonine 654 in modulating the protein-tyrosine kinase activity of EGF-treated A431 cells has been studied. It has been suggested that EGF could indirectly activate a protein-serine/threonine kinase, protein kinase C, that can phosphorylate the EGF receptor at threonine 654. Protein kinase C is known to be activated, and threonine 654 is phosphorylated, when A431 cells are exposed to 12-O-tetradecanoylphorbol-13-acetate (TPA). The protein-tyrosine kinase activity of EGF receptors is normally evidenced in EGF-treated cells by phosphorylation of the receptor at tyrosine. This is inhibited when TPA-treated cells are exposed to EGF. We now show that receptor phosphorylation at threonine 654 can also be detected in EGF-treated A431 cells, presumably due to indirect stimulation of protein kinase C or a similar kinase. Some receptor molecules are phosphorylated both at threonine 654 and at tyrosine. Since prior phosphorylation at threonine 654 inhibits autophosphorylation, we propose that protein kinase C can phosphorylate the threonine 654 of autophosphorylated receptors. This provides evidence for models in which protein kinase C activation, consequent upon EGF binding, could reduce the protein-tyrosine kinase activity of the EGF receptor. Indeed, we find that 12-O-tetradecanoylphorbol-13-acetate, added 10 min after EGF, further increases threonine 654 phosphorylation and induces the loss of tyrosine phosphate from A431 cell EGF receptors.
Insights
Epidermal growth factor (EGF) receptor phosphorylation at threonine 654, mediated by protein kinase C, inhibits EGF receptor tyrosine kinase activity. This finding suggests a mechanism for EGF receptor regulation in A431 cells.
Area of Science:
- Cell biology
- Molecular signaling
- Biochemistry
Background:
- Epidermal Growth Factor (EGF) receptor tyrosine kinase activity is crucial for cellular signaling.
- Protein kinase C (PKC) activation and subsequent phosphorylation of the EGF receptor at threonine 654 have been observed.
- PKC activation is induced by 12-O-tetradecanoylphorbol-13-acetate (TPA) in A431 cells.
Purpose of the Study:
- To investigate the role of epidermal growth factor (EGF) receptor phosphorylation at threonine 654 in modulating protein-tyrosine kinase activity.
- To explore the indirect activation of protein kinase C (PKC) by EGF and its effect on EGF receptor.
- To examine the interplay between threonine 654 and tyrosine phosphorylation in EGF receptors.
Main Methods:
- Studied EGF-treated A431 cells.
- Investigated the effect of 12-O-tetradecanoylphorbol-13-acetate (TPA) on EGF receptor phosphorylation.
- Analyzed receptor phosphorylation at both threonine 654 and tyrosine residues.
Main Results:
- Receptor phosphorylation at threonine 654 was detected in EGF-treated A431 cells, suggesting indirect PKC activation.
- Prior threonine 654 phosphorylation was found to inhibit EGF receptor autophosphorylation.
- Addition of TPA after EGF treatment enhanced threonine 654 phosphorylation and reduced tyrosine phosphorylation on EGF receptors.
Conclusions:
- EGF can indirectly activate protein kinase C (PKC) or a similar kinase, leading to threonine 654 phosphorylation of the EGF receptor.
- PKC-mediated phosphorylation at threonine 654 inhibits the protein-tyrosine kinase activity of the EGF receptor.
- This provides evidence for a regulatory mechanism where PKC activation by EGF binding reduces EGF receptor tyrosine kinase function.