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Differential regulation of glycogen synthase kinase-3 beta by protein kinase C isotypes
N Goode1, K Hughes, J R Woodgett
1Imperial Cancer Research Fund, Lincoln's Inn Fields, London, United Kingdom.
Abstract:
In cells, stimulation of protein kinase C (PKC) results in the dephosphorylation of specific residues proximal to the DNA binding domain of c-Jun, a major component of the AP-1 transcription factor. Since phosphorylation of this region of c-Jun inhibits interaction with DNA, this pathway may contribute to PKC activation of AP-1. To determine the mechanism(s) underlying this pathway, possible interactions between PKC and proteins implicated in c-Jun regulation are being investigated. Here it is shown that glycogen synthase kinase-3 beta (GSK-3 beta), a serine/threonine kinase that specifically targets the inhibitory c-Jun phosphorylation sites, is phosphorylated in vitro by particular forms of PKC (alpha, beta 1, gamma greater than beta 2; not epsilon). By contrast, the related GSK-3 alpha is not a substrate for any of these PKC isotypes. Phosphorylation of GSK-3 beta by PKC results in its specific inactivation. These results are consistent with a model in which activation of PKC stimulates c-Jun DNA binding by inhibiting its phosphorylation by GSK-3 beta.
Insights
Protein Kinase C (PKC) activation enhances AP-1 transcription factor activity by inactivating glycogen synthase kinase-3 beta (GSK-3 beta). This prevents inhibitory phosphorylation of c-Jun, promoting DNA binding.
Area of Science:
- Molecular Biology
- Cell Signaling
- Biochemistry
Background:
- Protein Kinase C (PKC) regulates transcription factors like AP-1.
- c-Jun, a component of AP-1, is dephosphorylated upon PKC stimulation.
- Phosphorylation of c-Jun near its DNA-binding domain inhibits DNA interaction.
Purpose of the Study:
- To elucidate the mechanism by which PKC activation leads to AP-1 activation.
- To investigate interactions between PKC and proteins involved in c-Jun regulation.
- To determine if PKC directly affects c-Jun phosphorylation status.
Main Methods:
- In vitro kinase assays were performed.
- Specific PKC isotypes were used to phosphorylate glycogen synthase kinase-3 beta (GSK-3 beta).
- The effect of PKC phosphorylation on GSK-3 beta activity was assessed.
Main Results:
- Specific PKC isotypes (alpha, beta 1, gamma) phosphorylated GSK-3 beta, but not GSK-3 alpha.
- PKC-mediated phosphorylation led to the inactivation of GSK-3 beta.
- GSK-3 beta targets inhibitory phosphorylation sites on c-Jun.
Conclusions:
- PKC activation inhibits GSK-3 beta activity.
- This inhibition prevents GSK-3 beta from phosphorylating c-Jun.
- The findings support a model where PKC activation enhances c-Jun DNA binding by reducing its inhibitory phosphorylation.