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Serine/threonine phosphorylation of insulin receptor substrate 1 modulates insulin receptor signaling
J F Tanti1, T Grémeaux, E van Obberghen
1Institut National de la Santé et de la Recherche Médicale U 145, Faculté de Médecine, Nice, France.
Abstract:
Treatment of cells with okadaic acid, a protein phosphatase inhibitor, leads to an insulin-resistant state without modification in the tyrosine kinase activity of the receptor toward exogenous substrates. In 3T3-L1 adipocytes, okadaic acid induced a similar dose-dependent inhibition of the insulin effect on deoxyglucose uptake, phosphatidylinositol 3-kinase (PI 3-kinase) activation, and insulin receptor substrate (IRS) 1 tyrosine phosphorylation. Simultaneously, in okadaic acid-treated 3T3-L1 adipocytes, the reduced IRS 1 tyrosine phosphorylation was linked to a decrease in its electrophoretic mobility due to phosphorylation on serine/threonine residues. This phosphorylation appeared to result from the activation of cytosolic kinase(s). Furthermore, using in vitro reconstitution, we show that, compared to IRS 1 immunopurified from untreated cells, the IRS 1 obtained from okadaic acid-treated cells had a reduced capacity to be phosphorylated by insulin receptors and, concomitantly, to bind PI 3-kinase. Taken together these data suggest that serine/threonine phosphorylation of IRS 1 induced by okadaic acid reduces the ability of the insulin receptor to phosphorylate IRS 1 and to dock one of its interacting molecules, i.e. PI 3-kinase. Finally, the inhibitory effect of okadaic acid on the stimulatory action of insulin on glucose transport suggests that the serine/threonine phosphorylation of IRS 1 might represent a key regulatory mechanism of insulin action.
Insights
Okadaic acid induces insulin resistance by promoting serine/threonine phosphorylation of insulin receptor substrate 1 (IRS 1). This modification impairs IRS 1
Area of Science:
- Cellular Biology
- Metabolic Signaling
- Endocrinology
Background:
- Insulin signaling is crucial for glucose homeostasis.
- Insulin resistance is a hallmark of type 2 diabetes and metabolic syndrome.
- Protein phosphatases play a role in regulating insulin signaling pathways.
Purpose of the Study:
- To investigate the mechanism by which okadaic acid induces insulin resistance.
- To determine the effect of okadaic acid on insulin receptor substrate 1 (IRS 1) phosphorylation and function.
- To elucidate the role of serine/threonine phosphorylation of IRS 1 in insulin action.
Main Methods:
- Treatment of 3T3-L1 adipocytes with okadaic acid.
- Assays for deoxyglucose uptake, PI 3-kinase activation, and IRS 1 tyrosine phosphorylation.
- Analysis of IRS 1 electrophoretic mobility and in vitro reconstitution assays.
- Measurement of insulin receptor kinase activity and IRS 1 binding to PI 3-kinase.
Main Results:
- Okadaic acid treatment inhibited insulin-stimulated deoxyglucose uptake, PI 3-kinase activation, and IRS 1 tyrosine phosphorylation in a dose-dependent manner.
- Reduced IRS 1 tyrosine phosphorylation correlated with increased serine/threonine phosphorylation and decreased electrophoretic mobility.
- In vitro reconstitution demonstrated that okadaic acid-modified IRS 1 had reduced capacity for insulin receptor phosphorylation and PI 3-kinase binding.
- Okadaic acid inhibited the stimulatory action of insulin on glucose transport.
Conclusions:
- Serine/threonine phosphorylation of IRS 1, induced by okadaic acid, impairs insulin receptor-mediated IRS 1 phosphorylation and PI 3-kinase docking.
- This modification of IRS 1 leads to insulin resistance and reduced glucose transport.
- Serine/threonine phosphorylation of IRS 1 represents a key regulatory mechanism in insulin action.