Feed-forward signaling of TNF-α and NF-κB via IKK-β pathway contributes to insulin resistance and coronary arteriolar dysfunction in type 2 diabetic mice
We hypothesized that the interaction between tumor necrosis factor-α (TNF-α)/nuclear factor-κB (NF-κB) via the activation of IKK-β may amplify one another, resulting in the evolution of vascular disease and insulin resistance associated with diabetes. To test this hypothesis, endothelium-dependent (ACh) and -independent (sodium nitroprusside) vasodilation of isolated, pressurized coronary arterioles from mLepr db (heterozygote, normal), Lepr db (homozygote, diabetic), and Lepr db mice null for TNF-α ( dbTNF−/ dbTNF−) were examined. Although the dilation of vessels to sodium nitroprusside was not different between Lepr db and mLepr db mice, the dilation to ACh was reduced in Lepr db mice. The NF-κB antagonist MG-132 or the IKK-β inhibitor sodium salicylate (NaSal) partially restored nitric oxide-mediated endothelium-dependent coronary arteriolar dilation in Lepr db mice, but the responses in mLepr db mice were unaffected. The protein expression of IKK-α and IKK-β were higher in Lepr db than in mLepr db mice; the expression of IKK-β, but not the expression of IKK-α, was attenuated by MG-132, the antioxidant apocynin, or the genetic deletion of TNF-α in diabetic mice. Lepr db mice showed an increased insulin resistance, but NaSal improved insulin sensitivity. The protein expression of TNF-α and NF-κB and the protein modification of phosphorylated (p)-IKK-β and p-JNK were greater in Lepr db mice, but NaSal attenuated TNF-α, NF-κB, p-IKK-β, and p-JNK in Lepr db mice. The ratio of p-insulin receptor substrate (IRS)-1 at Ser307 to IRS-1 was elevated in Lepr db compared with mLepr db mice; both NaSal and the JNK inhibitor SP-600125 reduced the p-IRS-1-to-IRS-1 ratio in Lepr db mice. MG-132 or the neutralization of TNF-α reduced superoxide production in Lepr db mice. In conclusion, our results indicate that the interaction between NF-κB and TNF-α signaling induces the activation of IKK-β and amplifies oxidative stress, leading to endothelial dysfunction in type 2 diabetes.