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Decrease in beta-subunit phosphotyrosine correlates with internalization and activation of the endosomal insulin

J W Burgess¹, I Wada, N Ling

¹Polypeptide Hormone Laboratory, Royal Victoria Hospital, Montreal, Canada.

The Journal of Biological Chemistry

|May 15, 1992

Summary

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Insulin receptor (IR) in endosomes (ENs) showed higher autophosphorylation but lower phosphotyrosine content than plasma membrane (PM) receptors. This suggests a dephosphorylation-dependent activation mechanism for IR kinase activity.

Area of Science:

Molecular Cell Biology
Endocrinology
Signal Transduction

Background:

Previous studies indicated transient activation of rat hepatic insulin receptor (IR) kinase in endosomes (ENs) exceeding plasma membrane (PM) receptors post-insulin injection.
IR kinase activation was linked to beta-subunit autophosphorylation, with phosphatase treatment abolishing this activation.

Purpose of the Study:

To quantify the phosphotyrosine (PY) content of the IR beta-subunit in PM and ENs.
To investigate the relationship between IR kinase activity, phosphotyrosine content, and insulin dose in PM and ENs.
To explore potential mechanisms of IR kinase activation in ENs.

Main Methods:

In vivo labeling with 32Pi followed by receptor immunoprecipitation.
Immunoblotting using antibodies against the IR beta-subunit and phosphotyrosine (PY) to determine PY/beta-subunit content.

Related Experiment Videos

Measurement of exogenous IR kinase activity using poly(Glu:Tyr) substrate.

Main Results:

Both methods revealed significantly higher PY content in PM IR compared to EN IR, with PM IR showing 2.0-2.5 fold higher PY/beta-subunit.
Surprisingly, EN IR incorporated 3-5 times more PY/beta-subunit than PM IR due to autophosphorylation.
EN receptors exhibited a dose-dependent increase in kinase activity with decreasing PY/beta-subunit levels, unlike PM receptors.

Conclusions:

Endosomal IR contains a subpopulation of lightly phosphorylated, highly activated receptors, suggesting a dephosphorylation-dependent activation mechanism.
The findings challenge the direct correlation between phosphotyrosine content and kinase activity in endosomal insulin receptors.
Results may be explained by a dephosphorylation-dependent activation of IR kinase, similar to src family tyrosine kinases.