Acute Inhibition of Central c‐Jun N‐terminal Kinase Restores Hypothalamic Insulin Signalling and Alleviates Glucose Intolerance in Diabetic Mice

The hypothalamus has been identified as a main insulin target tissue for regulating normal body weight and glucose metabolism. Recent observations suggest that c‐Jun‐N‐terminal kinase (JNK)‐signalling plays a crucial role in the development of obesity and insulin resistance because neuronal JNK‐1 ablation in the mouse prevented high‐fat diet‐induced obesity (DIO) and increased energy expenditure, as well as insulin sensitivity. In the present study, we investigated whether central JNK inhibition is associated with sensitisation of hypothalamic insulin signalling in mice fed a high‐fat diet for 3 weeks and in leptin‐deficient mice. We determined whether i.c.v. injection of a pharmacological JNK‐inhibitor (SP600125) improved impaired glucose homeostasis. By immunohistochemistry, we first observed that JNK activity was increased in the arcuate nucleus (ARC) and the ventromedial hypothalamus (VMH) in both mouse models, relative to normoglycaemic controls. This suggests that up‐regulation of JNK in these regions is associated with glucose intolerance and obesity, independent of leptin levels. Acute i.c.v. injection of SP600125 ameliorated glucose tolerance within 30 min in both leptin‐deficient and DIO mice. Given the acute nature of i.c.v. injections, these effects cannot be attributed to changes in food intake or energy balance. In a hypothalamic cell line, and in the ARC and VMH of leptin‐deficient mice, JNK inhibition by SP600125 consistently improved impaired insulin signalling. This was determined by a reduction of phospho‐insulin receptor substrate‐1 IRS‐1(Ser612)] protein in a hypothalamic cell line and a decline in the number of pIRS‐1(Ser612) immunoreactive cells in the ARC and VMH. Serine 612 phosphorylation of IRS‐1 is assumed to negatively regulate insulin signalling. In leptin‐deficient mice, in both nuclei, central inhibition of JNK increased the number of cells immunoreactive for phospho‐Akt (Ser473) and phospho‐GSK‐3β (Ser9), which are important markers of insulin signalling. Collectively, our data suggest that the acute inhibition of central JNK improves impaired glucose homeostasis and is associated with sensitisation of hypothalamic insulin signalling.