Search

SUMMARY Anorexigenic pro-opiomelanocortin (Pomc)/alpha-melanocyte stimulating hormone (αMSH) neurons of the hypothalamic melanocortin system function as key regulators of energy homeostasis, also controlling somatic growth across different species. However, the mechanisms of melanocortin-dependent growth control still remain ill-defined. Here, we reveal a thus-far-unrecognized structural and functional connection between Pomc neurons and the somatotropic hypothalamo-pituitary axis. Excessive feeding of larval zebrafish causes leptin resistance and reduced levels of the hypothalamic satiety mediator pomca. In turn, this leads to reduced activation of hypophysiotropic somatostatin (Sst)-neurons that express the melanocortin receptor Mc4r, elevated growth hormone (GH) expression in the pituitary, and enhanced somatic growth. Mc4r expression and αMSH responsiveness are conserved in Sst-expressing hypothalamic neurons of mice. Thus, acquired leptin resistance and attenuation of pomca transcription in response to excessive caloric intake may represent an ancient mechanism to promote somatic growth when food resources are plentiful., Graphical abstract In Brief: The melanocortin system controls energy homeostasis and somatic growth, but the underlying mechanisms are elusive. Löhr et al. identify a functional neural circuit in which Pomc neurons stimulate hypothalamic somatostatin neurons, thereby inhibiting hypophyseal growth hormone production. Excessive feeding and acquired leptin resistance attenuate this pathway, allowing faster somatic growth when food resources are rich.

Summary: Interleukin (IL)-6 engages similar signaling mechanisms to leptin. Here, we find that central application of IL-6 in mice suppresses feeding and improves glucose tolerance. In contrast to leptin, whose action is attenuated in obesity, the ability of IL-6 to suppress feeding is enhanced in obese mice. IL-6 suppresses feeding in the absence of neuronal IL-6-receptor (IL-6R) expression in hypothalamic or all forebrain neurons of mice. Conversely, obese mice exhibit increased soluble IL-6R levels in the cerebrospinal fluid. Blocking IL-6 trans-signaling in the CNS abrogates the ability of IL-6 to suppress feeding. Furthermore, gp130 expression is enhanced in the paraventricular nucleus of the hypothalamus (PVH) of obese mice, and deletion of gp130 in the PVH attenuates the beneficial central IL-6 effects on metabolism. Collectively, these experiments indicate that IL-6 trans-signaling is enhanced in the CNS of obese mice, allowing IL-6 to exert its beneficial metabolic effects even under conditions of leptin resistance. : Timper et al. find that central IL-6 improves energy and glucose homeostasis via IL-6 trans-signaling. IL-6 trans-signaling is enhanced in the CNS of obese mice, allowing IL-6 to exert its beneficial metabolic effects even under conditions of leptin resistance. Keywords: interleukin-6, CNS, obesity, interleukin-6 trans-signaling, energy homeostasis, glucose homeostasis

SummaryActivation of c-Jun N-terminal kinase 1 (JNK1)- and inhibitor of nuclear factor kappa-B kinase 2 (IKK2)-dependent signaling plays a crucial role in the development of obesity-associated insulin and leptin resistance not only in peripheral tissues but also in the CNS. Here, we demonstrate that constitutive JNK activation in agouti-related peptide (AgRP)-expressing neurons of the hypothalamus is sufficient to induce weight gain and adiposity in mice as a consequence of hyperphagia. JNK activation increases spontaneous action potential firing of AgRP cells and causes both neuronal and systemic leptin resistance. Similarly, activation of IKK2 signaling in AgRP neurons also increases firing of these cells but fails to cause obesity and leptin resistance. In contrast to JNK activation, IKK2 activation blunts insulin signaling in AgRP neurons and impairs systemic glucose homeostasis. Collectively, these experiments reveal both overlapping and nonredundant effects of JNK- and IKK-dependent signaling in AgRP neurons, which cooperate in the manifestation of the metabolic syndrome.

SummaryObesity increases the incidence of hepatocellular carcinoma (HCC) development in part through the activation of obesity-associated proinflammatory signaling. Here, we show that in lean mice, abrogation of IL-6Rα signaling protects against diethylnitrosamine (DEN)-induced HCC development. HCC protection occurs via Mcl-1 destabilization, thus promoting hepatocyte apoptosis. IL-6 regulates Mcl-1 stability via the inhibition of PP-1α expression, promoting GSK-3β inactivation. In addition, IL-6 suppresses expression of the Mcl-1 E3 ligase (Mule). Consequently, IL-6Rα deficiency activates PP-1α and Mule expression, resulting in increased Mcl-1 turnover and protection against HCC development. In contrast, in obesity, inhibition of PP-1α and Mule expression, leading to Mcl-1 stabilization, occurs independently of IL-6 signaling. Collectively, this study provides evidence that obesity inhibits hepatocyte apoptosis through Mcl-1 stabilization independent of IL-6 signaling, thus promoting liver carcinogenesis.