Insulin rapidly stimulates tyrosine kinase activity of its receptor resulting in phosphorylation of its cytosolic substrate insulin receptor substrate 1 (IRS-1), which in turn associates with and activates the enzyme phosphatidylinositol 3-kinase (PI 3-kinase). In the present study we have examined these three initial steps in insulin action during the differentiation of 3T3-F442A adipocytes and after treatment with dexamethasone or insulin. The differentiation of 3T3-F442A cells was characterized by a 13-fold increase in insulin receptor protein, a 9-fold increase in IRS-1, and a 10- and 4.5-fold increase in their insulin- stimulated phosphorylation, respectively. The mRNA expression of these two proteins showed a similar 8-fold increase during differentiation. In addition there was a 3.5-fold increase in PI 3-kinase protein [85 kilodalton (kDa) subunit] and a 16-fold increase in IRS-1-associated PI 3-kinase activity between day 0 and day 8 of differentiation. Dexamethasone (1 μM) treatment of differentiated cells induced a further 48% (P < 0.05) increase in insulin receptor level, but the autophosphorylation of the receptor was decreased by 31 ± 1% (P < 0.02). At the same time there was a decrease by 56 ± 4% (P < 0.005) in IRS-1 protein and by 31 ± 1% (P < 0.001) in IRS-1 phosphorylation. The expression of insulin receptor mRNA was unchanged, but the expression of IRS-1 mRNA was decreased by ~75% after dexamethasone. By contrast, dexamethasone induced a 69% increase in the level of PI 3-kinase as determined by immunoblotting. The combined effect of decreased IRS-1 phosphorylation and increased PI 3-kinase protein was a minimal change (15% decrease) in the association/activation between IRS-1 and PI 3-kinase. Chronic treatment with 100 nM insulin induced a time- and dose-dependent decrease in insulin receptor and IRS-1 protein levels reaching a nadir of 34 ± 5% (P < 0.005) and 39 ± 5% (P < 0.01) of control levels after 24 h, respectively. There was an even more marked decrease in the phosphorylation level of these proteins. Chronic insulin treatment also produced a 30% decrease in PI 3-kinase protein levels and a ~50% decrease in the association/activation between IRS-1/PI 3-kinase. The expression of insulin receptor and IRS-1 mRNA was unchanged during chronic insulin treatment. Thus three of the early steps in insulin action may have an important role in the process of adipocyte differentiation and represent points of regulation in hormone-induced insulin resistance in 3T3-F442A adipocytes.
ASJC Scopus subject areas
- Molecular Biology