Tumour necrosis factor alpha induces neuroinflammation and insulin resistance in immortalized hypothalamic neurons through independent pathways.

The links between obesity, inflammation and insulin resistance, which are all key characteristics of type 2 diabetes mellitus, are yet to be delineated in the brain. One of the key neuroinflammatory proteins detected in the hypothalamus with over-nutrition is tumour necrosis factor (TNF)alpha. Using immortalised embryonic rat and mouse hypothalamic cell lines (rHypoE-7 and mHypoE-46) that express orexigenic neuropeptide Y and agouti-related peptide, we investigated changes in insulin signalling and inflammatory gene marker mRNA expression after TNF alpha exposure. A quantitative polymerase chain reaction array of 84 inflammatory markers (cytokines, chemokines and receptors) demonstrated an increase in the expression of multiple genes encoding inflammatory markers upon exposure to 100 ng mL(-1) TNF alpha for 4 hours. Furthermore, neurones pre-exposed to TNF alpha (50 ng mL(-1)) for 6 or 16 hours exhibited a significant reduction in phosphorylated Akt compared to control after insulin treatment, indicating the attenuation of insulin signalling. mRNA expression of insulin signalling-related genes was also decreased with exposure to TNF alpha. TNF alpha significantly increased mRNA expression of I kappa B alpha, Tnfrsf1a and IL6 at 4 and 24 hours, activating a pro-inflammatory state. An inhibitor study using an inhibitor of nuclear factor kappa B kinase subunit beta (IKK-beta) inhibitor, PS1145, demonstrated that TNF alpha-induced neuroinflammatory marker expression occurs through the IKK-beta/nuclear factor-kappa B pathway, whereas oleate, a monounsaturated fatty acid, had no effect on inflammatory markers. To test the efficacy of anti-inflammatory treatment to reverse insulin resistance, neurones were treated with TNF alpha and PS1145, which did not significantly restore the TNF alpha-induced changes in cellular insulin sensitivity, indicating that an alternative pathway may be involved. In conclusion, exposure to the inflammatory cytokine TNF alpha causes cellular insulin resistance and inflammation marker expression in the rHypoE-7 and mHypoE-46 neurones, consistent with effects seen with TNF alpha in peripheral tissues. It also mimics insulin- and palmitate-induced insulin resistance in hypothalamic neurones. The present study provides further evidence that altered central energy metabolism may be caused by obesity-induced cytokine expression.