Inactivation of NF-κB p50 Leads to Insulin Sensitization in Liver through Post-translational Inhibition of p70S6K

Abstract

In this study, we investigated the metabolic phenotype of the NF-B p50 knock-out (p50-KO) mice. Compared with wild type mice, the p50-KO mice had an increase in food intake, but a decrease in body fat content. On chow diet, their blood glucose dropped much more than the wild type (WT) mice in the insulin tolerance test. Their glucose infusion rate was 30% higher than that of the WT mice in the hyperinsulinemic-euglycemic clamp. Their hepatic glucose production was suppressed more actively by insulin, and their insulin-induced glucose uptake was not altered in skeletal muscle or adipose tissue. In the liver, their p70S6K (S6K1) protein was significantly lower, and tumor necrosis factor-␣ (TNF-␣) expression was much higher. Their S6K1 protein was reduced by TNF-␣ treatment in the primary culture of hepatocytes. S6K1 reduction was blocked by the proteasome inhibitor MG132. In their livers, IKK2 (IKK␤) activity was reduced together with IKK␥. Their S6K1 degradation was dependent on IKK2 deficiency. Reconstitution of the S6K1 protein in their liver blocked the increase in insulin sensitivity. S6K1 degradation was not observed in hepatocytes of the WT mice. The data suggest that inactivation of NF-B p50 leads to suppression of IKK2 activity in the liver. IKK2 deficiency leads to S6K1 inhibition through TNF-induced protein degradation. The S6K1 reduction may contribute to insulin sensitivity in p50-KO mice. This study suggests that hepatic S6K1 may be a drug target in the treatment of insulin resistance.Chronic inflammation occurs in adipose tissue and liver in obesity (1, 2). It is generally believed that inflammation contributes to pathogenesis of insulin resistance through activation of IKK/NF-B and JNK signaling pathways. Serine kinase IKK2 (IKK␤) contributes to insulin resistance by activation of transcription factor NF-B (3-6) or induction of IRS-1 serine phosphorylation (7,8). Data from Ikk2 transgenic mice suggest that NF-B may be involved in the pathogenesis of insulin resistance (4). However, the role of NF-B has not been directly tested by gene knock-out in mice.NF-B contains two subunits, p50 (NF-B1) and p65 (RelA), in most cases. The p50 knock-out (p50-KO) mice were first reported in 1995, in which the p50 gene was globally inactivated by gene deletion (9). The mice are normal in growth, but deficient in B lymphocyte function. The p50-KO mice have been used in the study of many biological systems including the immune system (10, 11), muscle dystrophy (12), liver regeneration (13, 14), aging (15), and brain function (16). However, the metabolic phenotype remains unclear in this line of KO mice. Whole body knock-out of p65 (p65-KO) was reported in 1995 for embryonic lethality (17, 18). S6K1 (p70 ribosomal protein S6 kinase) is a serine kinase in the PI3K/Akt signaling pathway. It mediates nutrient (amino acid and glucose) and insulin signals in the regulation of insulin sensitivity (19,20). Global inactivation of S6K1 in knock-out mice protects the mice against diet-induced insulin resistance (21). An increa...

Transcriptional Repression of the Transforming Growth Factor β (TGF-β) Pseudoreceptor BMP and Activin Membrane-bound Inhibitor (BAMBI) by Nuclear Factor κB (NF-κB) p50 Enhances TGF-β Signaling in Hepatic Stellate Cells

“…Different forms of NF-B are known to have distinct functions in the inflammatory response (29,30). The most abundant form of NF-B in cells is a heterodimer of the p50 and p65 subunits.…”

Uncoupling of Inflammation and Insulin Resistance by NF-κB in Transgenic Mice through Elevated Energy Expenditure

“…The systemic inflammation is indicated by elevated proteins for TNF-␣ and IL-6 in the serum (15). The inflammation is a result of increased NF-B activity from overexpression of the p65 subunit (aP2-p65) or deletion of the p50 subunit (p50-KO) (12).…”

“…Our study indicates that energy expenditure may antagonize insulin resistance by eliminating lipid accumulation through elevated lipid oxidation, which prevents lipotoxicity in the pathogenesis of insulin resistance. In both aP2-p65 mice and p50-KO mice (15), insulin sensitivity was examined by the hyperinsulinemic-euglycemic clamp. The systemic insulin sensitivity was enhanced in both models, as indicated by the glucose infusion rate.…”

“…Expression of NF-B target genes (TNF-␣, interferon-␥, inducible nitric-oxide synthase, etc.) is increased in p50-KO mice (13)(14)(15). NF-B activity is increased by either p65 overexpression or p50 knock-out.…”

Transcription Factors NRF2 and NF-κB Are Coordinated Effectors of the Rho Family, GTP-binding Protein RAC1 during Inflammation

“…These cells were grown in DMEM containing 4.5 g/liter glucose, 4 mM L-glutamine, 10% fetal bovine serum, 50 units/ml penicillin, 50 g/ml streptomycin, 10 g/ml blasticidin and 50 g/ml Hygrogold. The p65 Ϫ/Ϫ mouse embryo fibroblasts (MEFs) and their corresponding wild type p65 ϩ/ϩ MEFs were kindly provided by Dr. Jianping Ye (Pennington Biomedical Research Center, Louisiana State University, Baton Rouge, LA) (18). The NRF2 Ϫ/Ϫ and NRF2 ϩ/ϩ MEFs were isolated around embryonal day 17.…”