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Insulin inhibits nuclear phosphatase activity: requirement for the C-terminal domain of the insulin receptor
J E Reusch1, P Hsieh, P Bhuripanyo
1Medical Research Service, Veterans Affairs Medical Center, Denver, Colorado 80220, USA.
Abstract:
Insulin's interaction with its receptor initiates a multitude of cellular effects on metabolism, growth, and differentiation. We recently described an insulin-mediated inhibition of nuclear protein phosphatase 2A (PP-2A), which is associated with an increase in phosphorylation of the transcription factor cAMP response element-binding protein. To clarify the role of nuclear PP-2A inhibition in the insulin signaling cascade, we examined the regulation of this phosphatase activity by insulin in Rat-1 fibroblasts overexpressing normal (HIRc) or mutant human insulin receptors (delta CT cells, deletion of a 43-amino acid C-terminal domain). The delta CT cells represent an excellent model of impaired metabolic and intact mitogenic action of insulin. Insulin inhibited nuclear PP-2A activity and enhanced cAMP response element-binding protein phosphorylation in HIRc cells, but not in delta CT cells. The delta CT cells exhibited normal ras activation and blunted mitogen-activating protein kinase phosphorylation and activation in response to insulin (16-fold in HIRc cells vs. 3-fold in delta CT cells), indicating that the mitogen-activating protein kinase pathway is important for the regulation of nuclear PP-2A activity by insulin. We conclude that insulin inhibits nuclear PP-2A activity, and that the carboxy-terminal domain of the insulin receptor is important for this effect.
Insights
Insulin signaling inhibits nuclear protein phosphatase 2A (PP-2A) activity, impacting transcription factor phosphorylation. The insulin receptor's C-terminal domain is crucial for this insulin-mediated effect.
Area of Science:
- Cellular biology
- Molecular endocrinology
- Signal transduction
Background:
- Insulin receptor signaling regulates critical cellular processes like metabolism and growth.
- Nuclear protein phosphatase 2A (PP-2A) activity is modulated by insulin, affecting transcription factor phosphorylation.
Purpose of the Study:
- To investigate the role of nuclear PP-2A inhibition in the insulin signaling cascade.
- To determine the importance of the insulin receptor's C-terminal domain in insulin-mediated PP-2A regulation.
Main Methods:
- Utilized Rat-1 fibroblasts overexpressing normal (HIRc) and mutant (delta CT) human insulin receptors.
- Assessed insulin's effect on nuclear PP-2A activity and cAMP response element-binding protein phosphorylation.
- Examined ras activation and mitogen-activating protein kinase (MAPK) phosphorylation in response to insulin.
Main Results:
- Insulin inhibited nuclear PP-2A activity and increased cAMP response element-binding protein phosphorylation in HIRc cells, but not in delta CT cells.
- Delta CT cells showed normal ras activation but blunted MAPK phosphorylation and activation compared to HIRc cells.
- MAPK pathway activation is critical for insulin's regulation of nuclear PP-2A activity.
Conclusions:
- Insulin actively inhibits nuclear PP-2A activity.
- The C-terminal domain of the insulin receptor is essential for mediating insulin's effect on nuclear PP-2A activity.
- The MAPK pathway plays a significant role in this regulatory mechanism.