Leptin inhibits hypothalamic Npy and Agrp gene expression via a mechanism that requires phosphatidylinositol 3-OH-kinase signaling

Increased hypothalamic protein tyrosine phosphatase 1B contributes to leptin resistance with age

Animals at advanced ages exhibit a reduction in central leptin sensitivity. However, changes in growth, metabolism, and obesity risk occur much earlier in life, particularly during the transition from youth to middle age. To determine when initial decreases in central leptin sensitivity occur, leptin-dependent suppression of food intake was tested in 8-, 12-, and 20-wk-old male, chow-fed Sprague Dawley rats. Intracerebroventricular leptin injection (3 microg) suppressed 24-h food intake in 8- and 12-wk-old rats (P < 0.05) but not 20-wk-old rats. To identify potential cellular mediators of this resistance, we focused on protein tyrosine phosphatase 1B (PTP1B), a recently described inhibitor of leptin signaling. PTP1B protein levels, as determined by Western blot, were significantly higher in mediobasal hypothalamic punches collected from 20-wk-old rats, compared with 8-wk-old rats (P < 0.05). When 20-wk-old rats were fasted for 24 h, levels of hypothalamic PTP1B decreased (P < 0.05), coincident with a restoration of leptin sensitivity. To directly test whether inhibition of PTP1B restores leptin sensitivity, 20-wk-old chow-fed rats were pretreated with a pharmacological PTP1B inhibitor 1 h before leptin, and 24-h food intake was recorded. As expected, leptin alone produced a small but nonsignificant reduction in food intake. However, pretreatment with the PTP1B inhibitor resulted in a marked improvement in leptin-dependent suppression of food intake (P < 0.05). These data are consistent with the hypothesis that increases in PTP1B contribute to hypothalamic leptin resistance as rats transition into middle age.

Leptin signaling in neurotensin neurons involves STAT, MAP kinases ERK1/2, and p38 through c‐Fos and ATF1

The adipokine leptin signals energy status to the hypothalamus, which triggers a network of neuropeptide responses. Each hypothalamic cell type expresses a unique complement of neuropeptides, receptors, and second messengers; thus each likely responds specifically to peripheral hormones. We describe here the analysis of leptin signaling in a clonal population of mouse neurotensin (NT) -expressing hypothalamic neurons, N-39. Leptin induced phosphorylation of STAT3 and MAPK ERK1/2, but not the downstream effector of PI3K, Akt, and also induced c-Fos protein. We found activation of p38 MAPK by leptin, accompanied by phosphorylation of its downstream effector ATF-1. Phosphorylation of ATF-1 is blocked by the p38 MAPK inhibitor SB 203580. We linked this signaling directly to NT transcription. Protein binding analysis indicates that both ATF-1 and c-Fos are capable of binding to the mouse NT/N gene predominantly at physiological or high concentrations of leptin. The evidence indicates activation of distinct leptin signal transduction pathways that directly result in changes in NT gene expression and links these specific neurons to the control of energy homeostasis.

Distribution of insulin receptor substrate-2 in brain areas involved in energy homeostasis

Body weight regulation depends on neuronal signaling by adiposity-related hormones such as insulin and leptin. Activation of receptors for these hormones induces cell signaling via the insulin receptor substrate/phosphatidylinositol 3-kinase (IRS-PI3K) pathway, and growing evidence from knockout models implicates IRS-2 as a key component of this signal transduction mechanism. As a first step towards the identification of brain areas that utilize IRS-PI3K signaling in the control of energy homeostasis, we used immunohistochemical techniques to investigate the neuronal distribution of IRS-2 protein in rat brain. In the hypothalamus, strong IRS-2 staining was detected chiefly in the arcuate (ARC), ventromedial (VMN) nucleus and parvocellular paraventricular nucleus (PVN). Within the ARC, IRS-2 was co-localized with alpha melanocyte stimulating hormone (alpha-MSH) as well as neuropeptide Y (NPY). In the hindbrain, IRS-2 staining was detected in the area postrema (AP), medial nucleus of the solitary tract (mNTS), dorsal motor nucleus of the vagus nerve (DMV) and the hypoglossal nucleus (HN). Co-localization studies in the mNTS demonstrated the presence of IRS-2 in catecholamine neurons. IRS-2 protein was also found in the ventral tegmental area (VTA), an important area for reward perception, and was detected in dopamine neurons in this brain area. In summary, neurons containing IRS-2 immunoreactivity were identified in forebrain, midbrain and hindbrain areas and in cell types that are crucial for the control of food intake and autonomic function. An improved understanding of mechanisms underlying normal and abnormal energy homeostasis may be gained by analysis of the role played by signaling through IRS-2 in these brain areas.

Lipid phosphatases as a possible therapeutic target in cases of type 2 diabetes and obesity

Phosphatidyl inositol 3-kinase (PI3-kinase) functions as a lipid kinase to produce PI(3,4,5)P(3) from PI(4,5)P(2) in vivo. PI(3,4,5)P(3) is crucial as a lipid second messenger in various metabolic effects of insulin. Lipid phosphatases, src homology 2 domain containing inositol 5'-phosphatase 2 (SHIP2) and skeletal muscle and kidney-enriched inositol phosphatase (SKIP) hydrolyze PI(3,4,5)P(3) to PI(3,4)P(2) and phosphatase and tensin homolog deleted on chromosome ten (PTEN) hydrolyzes PI(3,4,5)P(3) to PI(4,5)P(2). SHIP2 negatively regulates insulin signaling relatively specifically via its 5'-phosphatase activity. Targeted disruption of the SHIP2 gene in mice resulted in increased insulin sensitivity and conferred protection from obesity induced by a high-fat diet. Polymorphisms in the human SHIP2 gene are associated, at least in part, with the insulin resistance of type 2 diabetes. Importantly, inhibition of endogenous SHIP2 through the liver-specific expression of a dominant-negative SHIP2 improves glucose metabolism and insulin resistance in diabetic db/db mice. Overexpression of PTEN and SKIP also inhibited insulin-induced phosphorylation of Akt and the uptake of glucose in cultured cells. Although a homozygous disruption of the PTEN gene in mice results in embryonic lethality, either skeletal muscle or adipose tissue-specific disruption of PTEN ameliorated glucose metabolism without formation of tumors in animal models of diabetes. The role of SKIP in glucose metabolism remains to be further clarified in vivo. Taken together, inhibition of endogenous SHIP2 in the whole body appears to be effective at improving the insulin resistance associated with type 2 diabetes and/or obesity. Inhibition of PTEN in the tissues specifically targeted, including skeletal muscle and fat, may result in an amelioration of insulin resistance in type 2 diabetes, although caution against the formation of tumors is needed.

Neuropeptide Y administration into the amygdala alters high fat food intake

The orexigenic effects of neuropeptide Y (NPY) are mediated through the hypothalamus, while the anxiolytic effects of NPY appear to be mediated through the amygdala. We hypothesized that intra-amygdalar administration of NPY might alter food preference without changing total food intake. Neuropeptide Y was administered into the central nucleus of the amygdala in both satiated and overnight-fasted rats, and intake and preference for a high fat diet (56%)/low carbohydrate (20%) diet or a low fat (10%)/high carbohydrate (66%) diet were measured. Intra-amygdalar NPY administration in satiated rats did not change total caloric intake, but it did produce a dose-dependent decrease in intake of and preference for high fat diet relative to low fat diet over 24 h. In overnight-fasted rats, intra-amygdalar NPY also decreased the intake and preference for a high fat diet relative to low fat diet over 24 h, without altering total caloric intake. Intra-amygdalar NPY administration did not produce conditioned taste aversions to a novel saccharin solution. These results suggest that amygdalar NPY may have a role in macronutrient selection, without altering total caloric intake.

Leptin regulates insulin sensitivity via phosphatidylinositol-3-OH kinase signaling in mediobasal hypothalamic neurons

To investigate whether phosphatidylinositol-3 kinase (PI3K) signaling mediates the metabolic effects of hypothalamic leptin action, adenoviral gene therapy was used to direct expression of leptin receptors to the area of the hypothalamic arcuate nucleus (ARC). This intervention markedly improved insulin sensitivity in genetically obese, leptin-receptor-deficient Koletsky (fa(k)/fa(k)) rats via a mechanism that was not dependent on reduced food intake but was attenuated by approximately 44% by third-ventricular infusion of the PI3K inhibitor LY294002. Conversely, ARC-directed expression of a constitutively active mutant of protein kinase B (PKB/Akt, an enzyme activated by PI3K) mimicked the insulin-sensitizing effect of restored hypothalamic leptin signaling in these animals, despite having no effect on food intake or body weight. These findings suggest that hypothalamic leptin signaling is an important determinant of glucose metabolism and that the underlying neuronal mechanism involves PI3K.