Insulin action in the central nervous system regulates energy homeostasis and glucose metabolism. To define the insulin-responsive neurons that mediate these effects, we generated mice with selective inactivation of the insulin receptor (IR) in either pro-opiomelanocortin (POMC)-or agouti-related peptide (AgRP)-expressing neurons of the arcuate nucleus of the hypothalamus. While neither POMC-nor AgRP-restricted IR knockout mice exhibited altered energy homeostasis, insulin failed to normally suppress hepatic glucose production during euglycemichyperinsulinemic clamps in AgRP-IR knockout (IR DAgRP ) mice. These mice also exhibited reduced insulin-stimulated hepatic interleukin-6 expression and increased hepatic expression of glucose-6-phosphatase. These results directly demonstrate that insulin action in POMC and AgRP cells is not required for steady-state regulation of food intake and body weight. However, insulin action specifically in AgRPexpressing neurons does play a critical role in controlling hepatic glucose production and may provide a target for the treatment of insulin resistance in type 2 diabetes.

Multiple hormones controlling energy homeostasis regulate the expression of neuropeptide Y (NPY) and agouti-related peptide (AgRP) in the arcuate nucleus of the hypothalamus. Nevertheless, inactivation of the genes encoding NPY and/or AgRP has no impact on food intake in mice. Here we demonstrate that induced selective ablation of AgRP-expressing neurons in adult mice results in acute reduction of feeding, demonstrating direct evidence for a critical role of these neurons in the regulation of energy homeostasis.

Leptin and insulin have been identified as fuel sensors acting in part through their hypothalamic receptors to inhibit food intake and stimulate energy expenditure. As their intracellular signaling converges at the PI3K pathway, we directly addressed the role of phosphatidylinositol 3,4,5 -trisphosphate-mediated (PIP 3 -mediated) signals in hypothalamic proopiomelanocortin (POMC) neurons by inactivating the gene for the PIP 3 phosphatase Pten specifically in this cell type. Here we show that POMC-specific disruption of Pten resulted in hyperphagia and sexually dimorphic diet-sensitive obesity. Although leptin potently stimulated Stat3 phosphorylation in POMC neurons of POMC cell-restricted Pten knockout (PPKO) mice, it failed to significantly inhibit food intake in vivo. POMC neurons of PPKO mice showed a marked hyperpolarization and a reduction in basal firing rate due to increased ATP-sensitive potassium (K ATP ) channel activity. Leptin was not able to elicit electrical activity in PPKO POMC neurons, but application of the PI3K inhibitor LY294002 and the K ATP blocker tolbutamide restored electrical activity and leptin-evoked firing of POMC neurons in these mice. Moreover, icv administration of tolbutamide abolished hyperphagia in PPKO mice. These data indicate that PIP 3 -mediated signals are critical regulators of the melanocortin system via modulation of K ATP channels.

The contribution of different leptin-induced signaling pathways in control of energy homeostasis is only partly understood. Here we show that selective Pten ablation in leptin-sensitive neurons (Pten(DeltaObRb)) results in enhanced Pi3k activation in these cells and reduces adiposity by increasing energy expenditure. White adipose tissue (WAT) of Pten(DeltaObRb) mice shows characteristics of brown adipose tissue (BAT), reflected by increased mitochondrial content and Ucp1 expression resulting from enhanced leptin-stimulated sympathetic nerve activity (SNA) in WAT. In contrast, leptin-deficient ob/ob-Pten(DeltaObRb) mice exhibit unaltered body weight and WAT morphology compared to ob/ob mice, pointing to a pivotal role of endogenous leptin in control of WAT transdifferentiation. Leanness of Pten(DeltaObRb) mice is accompanied by enhanced sensitivity to insulin in skeletal muscle. These data provide direct genetic evidence that leptin-stimulated Pi3k signaling in the CNS regulates energy expenditure via activation of SNA to perigonadal WAT leading to BAT-like differentiation of WAT.

Childhood obesity is a risk factor for asthma, but the molecular mechanisms linking both remain elusive. Since obesity leads to chronic low-grade inflammation and affects metabolic signaling we hypothesized that postnatal hyperalimentation (pHA) induced by maternal high-fat-diet during lactation leads to early-onset obesity and dysregulates pulmonary adipocytokine/insulin signaling, resulting in metabolic programming of asthma-like disease in adult mice. Offspring with pHA showed at postnatal day 21 (P21): (1) early-onset obesity, greater fat-mass, increased expression of IL-1β, IL-23, and Tnf-α, greater serum leptin and reduced glucose tolerance than Control (Ctrl); (2) less STAT3/AMPKα-activation, greater SOCS3 expression and reduced AKT/GSK3β-activation in the lung, indicative of leptin resistance and insulin signaling, respectively; (3) increased lung mRNA of IL-6, IL-13, IL-17A and Tnf-α. At P70 body weight, fat-mass, and cytokine mRNA expression were similar in the pHA and Ctrl, but serum leptin and IL-6 were greater, and insulin signaling and glucose tolerance impaired. Peribronchial elastic fiber content, bronchial smooth muscle layer, and deposition of connective tissue were not different after pHA. Despite unaltered bronchial structure mice after pHA exhibited significantly increased airway reactivity. Our study does not only demonstrate that early-onset obesity transiently activates pulmonary adipocytokine/insulin signaling and induces airway hyperreactivity in mice, but also provides new insights into metabolic programming of childhood obesity-related asthma.