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Insulin and epidermal growth factor do not affect phosphoinositide metabolism in rat liver plasma membranes and
Abstract:
Recent studies with viral oncogene tyrosine kinases have suggested that these kinases may phosphorylate phosphoinositides and diacylglycerol. Since the receptors for insulin and epidermal growth factor (EGF) also possess tyrosine kinase activity, we have investigated possible effects of insulin and EGF on phosphoinositide metabolism in rat liver plasma membranes and rat hepatocytes. In plasma membranes prepared from rats injected 18 h prior with [3H]myo-inositol or incubated with [gamma-32P]ATP, phosphatidylinositol-4-P and phosphatidylinositol-4,5-P2 were formed, but there were no effects of either insulin or EGF although these agents stimulated protein tyrosine phosphorylation. In hepatocytes incubated with [3H]myo-inositol, label was incorporated into phosphatidylinositol, phosphatidylinositol-4-P, and phosphatidylinositol-4,5-P2, but there was no effect of insulin. Incubation of hepatocytes with [3H]myo-inositol plus insulin or EGF for 2 h also did not alter the formation of [3H]myo-inositol-1,4,5-P3 from [3H]phosphatidylinositol-4,5-P2 induced by vasopressin. These findings suggest that the tyrosine kinase activity of liver insulin and EGF receptors is not important in phosphoinositide formation.
Insights
Insulin and epidermal growth factor (EGF) do not affect phosphoinositide metabolism in rat liver cells, despite their tyrosine kinase activity. This suggests their kinase function is not crucial for phosphoinositide formation in liver tissue.
Area of Science:
- Biochemistry
- Cell Signaling
- Molecular Biology
Background:
- Viral oncogene tyrosine kinases can phosphorylate phosphoinositides and diacylglycerol.
- Receptors for insulin and epidermal growth factor (EGF) possess intrinsic tyrosine kinase activity.
- The role of insulin and EGF receptor tyrosine kinase activity in phosphoinositide metabolism is unclear.
Purpose of the Study:
- To investigate the effects of insulin and EGF on phosphoinositide metabolism.
- To determine if the tyrosine kinase activity of insulin and EGF receptors influences phosphoinositide formation in rat liver.
Main Methods:
- Experiments were conducted using rat liver plasma membranes and isolated rat hepatocytes.
- Radioactive labeling ([3H]myo-inositol, [gamma-32P]ATP) was used to track phosphoinositide synthesis.
- Analysis of phosphatidylinositol (PI), phosphatidylinositol-4-P (PIP), and phosphatidylinositol-4,5-P2 (PIP2) formation.
- Measurement of inositol-1,4,5-trisphosphate ([3H]IP3) formation induced by vasopressin.
Main Results:
- Insulin and EGF stimulated protein tyrosine phosphorylation but did not affect PI, PIP, or PIP2 formation in liver plasma membranes.
- In hepatocytes, insulin did not alter the incorporation of [3H]myo-inositol into PI, PIP, and PIP2.
- Neither insulin nor EGF affected vasopressin-induced [3H]IP3 formation from [3H]PIP2 in hepatocytes.
Conclusions:
- The tyrosine kinase activity of liver insulin and EGF receptors is not essential for phosphoinositide metabolism.
- These findings indicate a dissociation between receptor tyrosine kinase activation and phosphoinositide signaling in rat liver.