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Dexamethasone down-regulation of insulin receptor substrate-1 in 3T3-L1 adipocytes
M A Turnbow1, S R Keller, K M Rice
1Department of Biochemistry and Molecular Biology, Texas Tech University Health Sciences Center, Lubbock 79430.
Abstract:
Insulin resistance resulting from prolonged exposure of intact animals or cultured cells to glucocorticoids is often attributed to postreceptor signaling defects. To better understand the specific effects of glucocorticoids on insulin signaling, we have characterized the effect of dexamethasone on the expression of an insulin signaling intermediate, the insulin receptor substrate-1 (IRS-1) in 3T3-L1 adipocytes. Addition of dexamethasone resulted in a 40-70% decline in steady-state IRS-1 protein over 24-48 h of treatment. Dexamethasone did not significantly change the degradation rate of IRS-1 protein but decreased the net rate of amino acid incorporation into IRS-1 by 87%. Between 1 and 2.5 h of treatment with dexamethasone, actinomycin D, or both drugs given simultaneously, the concentration of IRS-1 mRNA declined with a half-life of 0.7-1.0 h. However, after 4 h of dexamethasone treatment, IRS-1 mRNA concentrations stabilized at approximately 35% of the control level. The dexamethasone-induced decline in IRS-1 protein could be prevented by simultaneous administration of the glucocorticoid antagonist mifepristone, RU38486. These results suggest that in 3T3-L1 adipocytes the loss of IRS-1 protein after dexamethasone treatment can be accounted for chiefly by inhibition of the synthesis of IRS-1 mRNA.
Insights
Glucocorticoids like dexamethasone reduce insulin receptor substrate-1 (IRS-1) protein in cells. This occurs mainly by inhibiting IRS-1 mRNA synthesis, not by increasing protein degradation.
Area of Science:
- Cell biology
- Endocrinology
- Metabolic research
Background:
- Glucocorticoids induce insulin resistance, often linked to postreceptor defects.
- Understanding glucocorticoid impact on insulin signaling intermediates is crucial.
Purpose of the Study:
- To investigate the effect of dexamethasone on insulin receptor substrate-1 (IRS-1) expression in 3T3-L1 adipocytes.
- To elucidate the mechanisms behind dexamethasone-induced changes in IRS-1 protein levels.
Main Methods:
- Treatment of 3T3-L1 adipocytes with dexamethasone.
- Measurement of IRS-1 protein levels, degradation rates, and amino acid incorporation.
- Analysis of IRS-1 mRNA concentration using actinomycin D and glucocorticoid antagonist mifepristone (RU38486).
Main Results:
- Dexamethasone treatment decreased steady-state IRS-1 protein by 40-70% over 24-48 hours.
- Protein degradation rates remained unchanged, but amino acid incorporation into IRS-1 decreased by 87%.
- IRS-1 mRNA levels declined with a half-life of 0.7-1.0 h, stabilizing at 35% of control after 4h; this effect was blocked by mifepristone.
Conclusions:
- Dexamethasone-induced reduction in IRS-1 protein in 3T3-L1 adipocytes is primarily due to the inhibition of IRS-1 mRNA synthesis.
- This highlights a key mechanism in glucocorticoid-mediated insulin resistance at the level of gene expression.