IκB kinase β (IKKβ) does not mediate feedback inhibition of the insulin signalling cascade

In the present study, we have examined whether IKK beta [I kappa B (inhibitor of nuclear factor kappa B)kinase beta] plays a role in feedback inhibition of the insulin signalling cascade. Insulin induces the phosphorylation of IKK beta, in vitro and in vivo, and this effect is dependent on intact signalling via PI3K (phosphoinositide 3-kinase), but not PKB (protein kinase B). To test the hypothesis that insulin activates IKK beta as a means of negative feedback, we employed a variety of experimental approaches. First, pharmacological inhibition of IKK beta via BMS-345541 did not potentiate insulin-induced IRS1 (insulin receptor substrate 1) 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 IKK beta target sites. Secondly, adenovirus-mediated overexpression of wild-type IKK beta in differentiated 3T3-L1 adipocytes did not suppress insulin-stimulated 2-deoxyglucose uptake, IRS1 tyrosine phosphorylation, IRS1 association with the p85 regulatory subunit of PI3K or PKB phosphorylation. Thirdly, insulin signalling was not potentiated in mouse embryonic fibroblasts lacking IKK beta. Finally, insulin treatment of 3T3-L1 adipocytes did not promote the recruitment of IKK beta to IRS1, supporting our findings that IKK beta, although activated by insulin, does not promote direct serine phosphorylation of IRS1 and does not contribute to the feedback inhibition of the insulin signalling cascade.