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Insulin-induced c-Jun N-terminal kinase activation is negatively regulated by protein kinase C delta
K Morino1, H Maegawa, T Fujita
1Third Department of Medicine, Shiga University of Medical Science, Seta, Otsu, Shiga 520-2192, Japan.
Abstract:
We investigated the role of protein kinase C (PKC) in insulin-induced c-Jun N-terminal kinase (JNK) activation in rat 1 fibroblasts expressing human insulin receptors. Insulin treatment led to increased SAPK/ERK kinase 1 (SEK1) phosphorylation, and then stimulated JNK activity in a dose- and time-dependent manner, as measured either by a solid-phase kinase assay using glutathione S-transferase (GST)-c-Jun fusion protein as a substrate, or by quantitation of the levels of phosphorylated JNK by Western blotting using anti-phospho-JNK antibody. Insulin-induced JNK activation was potentiated by either preincubating cells with 2 nM GF109203X (PKC inhibitor) or down-regulation of PKC by overnight treatment with 100 nM tetradecanoyl phorbol acetate. In contrast, brief preincubation with 100 nM tetradecanoyl phorbol acetate inhibited the insulin- induced JNK activation. Furthermore, we found that 5 microM rottlerin, a PKCdelta inhibitor, enhanced insulin-induced JNK activation, but a PKCbeta inhibitor, LY333531, had no effect. Consistent with these findings, overexpression of PKCdelta led to decreased insulin-induced JNK activation, whereas overexpression of PKCbeta had no effect. Although overexpression of wild-type PKCdelta attenuated insulin-induced JNK activation, a kinase-dead PKCdelta mutant did not cause such attenuation. Finally, we found that the magnitude of insulin-induced JNK activation was inversely correlated with the expression level of PKCdelta among different cell lines. In conclusion, the expression of PKCdelta may negatively regulate insulin-induced JNK activation.
Insights
Protein Kinase C delta (PKCδ) negatively regulates insulin-induced c-Jun N-terminal kinase (JNK) activation. Inhibiting or reducing PKCδ expression enhances insulin signaling, revealing its role in metabolic pathways.
Area of Science:
- Cellular signaling pathways
- Molecular endocrinology
- Protein kinase regulation
Background:
- Insulin signaling is crucial for glucose homeostasis and metabolic regulation.
- c-Jun N-terminal kinase (JNK) is involved in cellular stress responses and metabolic processes.
- Protein Kinase C (PKC) isoforms play diverse roles in cellular signaling.
Purpose of the Study:
- To investigate the role of Protein Kinase C (PKC) in insulin-induced c-Jun N-terminal kinase (JNK) activation.
- To elucidate the specific PKC isoforms involved in modulating insulin signaling.
- To understand the regulatory mechanisms linking insulin receptor activation to JNK signaling.
Main Methods:
- Utilized rat 1 fibroblasts expressing human insulin receptors.
- Assessed JNK activation via kinase assays and Western blotting.
- Employed PKC inhibitors (GF109203X, rottlerin, LY333531) and tetradecanoyl phorbol acetate (TPA) for modulation.
- Investigated effects of PKC isoform overexpression and knockdown on insulin-induced JNK activation.
Main Results:
- Insulin treatment dose- and time-dependently increased JNK activation.
- PKC inhibition or downregulation potentiated insulin-induced JNK activation.
- PKCδ inhibition enhanced JNK activation, while PKCβ inhibition had no effect.
- Overexpression of PKCδ, but not PKCβ, attenuated insulin-induced JNK activation.
- Higher PKCδ expression inversely correlated with insulin-induced JNK activation magnitude.
Conclusions:
- Protein Kinase C delta (PKCδ) expression negatively regulates insulin-induced JNK activation.
- PKCδ acts as a key modulator in the insulin signaling cascade.
- Findings suggest PKCδ's role in fine-tuning cellular responses to insulin.