IκB Kinase β (IKKβ) does not mediate feedback inhibition of the insulin-signaling cascade

Abstract

Herein, we have examined whether IκB Kinase β (IKKβ) plays a role in feedback inhibition of the insulin-signaling cascade. Insulin induces the phosphorylation of IKKβ, in vitro and in vivo, and this effect is dependent on intact signaling via phosphatidylinositol 3-kinase (PI3K), but not protein kinase B (PKB). To test the hypothesis that insulin activates IKKβ as a means of negative feedback, we employed a variety of experimental approaches. Firstly, pharmacological inhibition of IKKβ via BMS-345541 did not potentiate insulin-induced IRS1 tyrosine phosphorylation, PKB phosphorylation or 2-deoxyglucose uptake in differentiated 3T3-L1 adipocytes. BMS-345541 did not prevent insulin-induced IRS1 serine phosphorylation on known IKKβ target sites. Secondly, adenoviral-mediated over-expression of wild type (WT) IKKβ in differentiated 3T3-L1 adipocytes did not suppress insulin-stimulated 2-deoxyglucose uptake, insulin receptor substrate 1 (IRS1) tyrosine phosphorylation, IRS1 association with the p85 regulatory subunit of PI3K or PKB phosphorylation. Thirdly, insulin signaling was not potentiated in mouse embryo fibroblasts lacking IKKβ (Ikkβ-/- MEF). Finally, insulin treatment of 3T3-L1 adipocytes did not promote the recruitment of IKKβ to IRS1, supporting our data that IKKβ, while activated by insulin, does not promote direct serine phosphorylation of IRS1 and does not contribute to the feedback inhibition of the insulin-signaling cascade.