Insulin Receptor Signaling in POMC, but Not AgRP, Neurons Controls Adipose Tissue Insulin Action.

Insulin is a key regulator of adipose tissue lipolysis, and impaired adipose tissue insulin action results in unrestrained lipolysis and lipotoxicity, which are hallmarks of the metabolic syndrome and diabetes. Insulin regulates adipose tissue metabolism through direct effects on adipocytes and through signaling in the central nervous system by dampening sympathetic outflow to the adipose tissue. Here we examined the role of insulin signaling in agouti-related protein (AgRP) and pro-opiomelanocortin (POMC) neurons in regulating hepatic and adipose tissue insulin action. Mice lacking the insulin receptor in AgRP neurons (AgRP IR KO) exhibited impaired hepatic insulin action because the ability of insulin to suppress hepatic glucose production (hGP) was reduced, but the ability of insulin to suppress lipolysis was unaltered. To the contrary, in POMC IR KO mice, insulin lowered hGP but failed to suppress adipose tissue lipolysis. High-fat diet equally worsened glucose tolerance in AgRP and POMC IR KO mice and their respective controls but increased hepatic triglyceride levels only in POMC IR KO mice, consistent with impaired lipolytic regulation resulting in fatty liver. These data suggest that although insulin signaling in AgRP neurons is important in regulating glucose metabolism, insulin signaling in POMC neurons controls adipose tissue lipolysis and prevents high-fat diet-induced hepatic steatosis.

Increased susceptibility to metabolic dysregulation in a mouse model of Alzheimer's disease is associated with impaired hypothalamic insulin signaling and elevated BCAA levels.

INTRODUCTION: Epidemiologic studies have demonstrated an association between diabetes and dementia. Insulin signaling within the brain, in particular within the hypothalamus regulates carbohydrate, lipid, and branched chain amino acid (BCAA) metabolism in peripheral organs such as the liver and adipose tissue. We hypothesized that cerebral amyloidosis impairs central nervous system control of metabolism through disruption of insulin signaling in the hypothalamus, which dysregulates glucose and BCAA homeostasis resulting in increased susceptibility to diabetes. METHODS: We examined whether APP/PS1 mice exhibit increased susceptibility to aging or high-fat diet (HFD)-induced metabolic impairment using metabolic phenotyping and insulin-signaling studies. RESULTS: APP/PS1 mice were more susceptible to high-fat feeding and aging-induced metabolic dysregulation including disrupted BCAA homeostasis and exhibited impaired hypothalamic insulin signaling. DISCUSSION: Our data suggest that AD pathology increases susceptibility to diabetes due to impaired hypothalamic insulin signaling, and that plasma BCAA levels could serve as a biomarker of hypothalamic insulin action in patients with AD.

TallyHO obese female mice experience poor reproductive outcomes and abnormal blastocyst metabolism that is reversed by metformin.

Obese women experience worse reproductive outcomes than normal weight women, specifically infertility, pregnancy loss, fetal malformations and developmental delay of offspring. The aim of the present study was to use a genetic mouse model of obesity to recapitulate the human reproductive phenotype and further examine potential mechanisms and therapies. New inbred, polygenic Type 2 diabetic TallyHO mice and age-matched control C57BL/6 mice were superovulated to obtain morula or blastocyst stage embryos that were cultured in human tubal fluid (HTF) medium. Deoxyglucose uptake was determined for individual insulin-stimulated blastocysts. Apoptosis was detected by confocal microscopy using the terminal deoxyribonucleotidyl transferase-mediated dUTP-digoxigenin nick end-labelling (TUNEL) assay and Topro-3 nuclear dye. Embryos were scored for TUNEL-positive as a percentage of total nuclei. AMP-activated protein kinase (AMPK) activation, tumour necrosis factor (TNF)-α expression and adiponectin expression were analysed by western immunoblot and confocal immunofluorescent microscopy. Lipid accumulation was assayed by BODIPY. Comparisons were made between TallyHO morulae cultured to blastocyst embryos in either HTF medium or HTF medium with 25 µg mL(-1) metformin. TallyHO mice developed whole body abnormal insulin tolerance, had decreased litter sizes and increased non-esterified fatty acid levels. Blastocysts from TallyHO mice exhibited increased apoptosis, decreased insulin sensitivity and decreased AMPK. A possible cause for the insulin resistance and abnormal AMPK phosphorylation was the increased TNF-α expression and lipid accumulation, as detected by BODIPY, in TallyHO blastocysts and decreased adiponectin. Culturing TallyHO morulae with the AMPK activator metformin led to a reversal of all the abnormal findings, including increased AMPK phosphorylation, improved insulin-stimulated glucose uptake and normalisation of lipid accumulation. Women with obesity and insulin resistance experience poor pregnancy outcomes. Previously we have shown in mouse models of insulin resistance that AMPK activity is decreased and that activators of AMPK reverse poor embryo outcomes. Here, we show for the first time using a genetically altered obese model, not a diet-induced model, that metformin reverses many of the adverse effects of obesity at the level of the blastocyst. Expanding on this we determine that activation of AMPK via metformin reduces lipid droplet accumulation, presumably by eliminating the inhibitory effects of TNF-α, resulting in normalisation of fatty acid oxidation and HADH2 (hydroxyacyl-CoA dehydrogenase/3-ketoacyl-CoA thiolase/enoyl-CoA hydratase (trifunctional protein), alpha subunit) activity. Metformin exposure in vitro was able to partially reverse these effects, at the level of the blastocyst, and may thus be effective in preventing the adverse effects of obesity on pregnancy and reproductive outcomes.