RESUMO
Leptin-stimulated Stat3 activation in proopiomelanocortin (POMC)-expressing neurons of the hypothalamus plays an important role in maintenance of energy homeostasis. While Stat3 activation in POMC neurons is required for POMC expression, the role of elevated basal Stat3 activation as present in the development of obesity has not been directly addressed. Here, we have generated and characterized mice expressing a constitutively active version of Stat3 (Stat3-C) in POMC neurons (Stat3-C(POMC) mice). On normal chow diet, these animals develop obesity as a result of hyperphagia and decreased POMC expression accompanied by central leptin and insulin resistance. This unexpected finding coincides with POMC-cell-specific, Stat3-mediated upregulation of SOCS3 expression inhibiting both leptin and insulin signaling as insulin-stimulated PIP(3) (phosphatidylinositol-3,4,5 triphosphate) formation and protein kinase B (AKT) activation in POMC neurons as well as with the fact that insulin's ability to hyperpolarize POMC neurons is largely reduced in POMC cells of Stat3-C(POMC) mice. These data indicate that constitutive Stat3 activation is not sufficient to promote POMC expression but requires simultaneous PI3K (phosphoinositide 3-kinase)-dependent release of FOXO1 repression. In contrast, upon exposure to a high-fat diet, food intake and body weight were unaltered in Stat3-C(POMC) mice compared with control mice. Taken together, these experiments directly demonstrate that enhanced basal Stat3 activation in POMC neurons as present in control mice upon high-fat feeding contributes to the development of hypothalamic leptin and insulin resistance.
Assuntos
Insulina/metabolismo , Leptina/metabolismo , Proteínas de Membrana/metabolismo , Inibição Neural/fisiologia , Neurônios/fisiologia , Obesidade/fisiopatologia , Pró-Opiomelanocortina/metabolismo , Proteína Relacionada com Agouti/genética , Proteína Relacionada com Agouti/metabolismo , Animais , Composição Corporal/genética , Peso Corporal/genética , Modelos Animais de Doenças , Ingestão de Alimentos/genética , Ensaio de Desvio de Mobilidade Eletroforética , Ensaio de Imunoadsorção Enzimática/métodos , Retroalimentação/fisiologia , Regulação da Expressão Gênica/efeitos dos fármacos , Regulação da Expressão Gênica/genética , Teste de Tolerância a Glucose , Proteínas de Fluorescência Verde/genética , Hipotálamo/patologia , Técnicas In Vitro , Resistência à Insulina/genética , Fator Inibidor de Leucemia/farmacologia , Proteínas de Membrana/genética , Camundongos , Camundongos Transgênicos , Inibição Neural/efeitos dos fármacos , Inibição Neural/genética , Neuropeptídeo Y/genética , Neuropeptídeo Y/metabolismo , Obesidade/genética , Obesidade/metabolismo , Técnicas de Patch-Clamp/métodos , Fosfatidilinositol 3-Quinases/metabolismo , Transdução de Sinais/genética , Proteína 3 Supressora da Sinalização de Citocinas , Proteínas Supressoras da Sinalização de Citocina/genética , Proteínas Supressoras da Sinalização de Citocina/metabolismo , TransfecçãoRESUMO
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 phosphatidylinositol3,4,5-trisphosphate-mediated (PIP3-mediated) signals in hypothalamic proopiomelanocortin (POMC) neurons by inactivating the gene for the PIP3 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 (KATP) channel activity. Leptin was not able to elicit electrical activity in PPKO POMC neurons, but application of the PI3K inhibitor LY294002 and the KATP 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 PIP3-mediated signals are critical regulators of the melanocortin system via modulation of KATP channels.