Central melanin-concentrating hormone influences liver and adipose metabolism via specific hypothalamic nuclei and efferent autonomic/JNK1 pathways.

BACKGROUND & AIMS: Specific neuronal circuits modulate autonomic outflow to liver and white adipose tissue. Melanin-concentrating hormone (MCH)-deficient mice are hypophagic, lean, and do not develop hepatosteatosis when fed a high-fat diet. Herein, we sought to investigate the role of MCH, an orexigenic neuropeptide specifically expressed in the lateral hypothalamic area, on hepatic and adipocyte metabolism. METHODS: Chronic central administration of MCH and adenoviral vectors increasing MCH signaling were performed in rats and mice. Vagal denervation was performed to assess its effect on liver metabolism. The peripheral effects on lipid metabolism were assessed by real-time polymerase chain reaction and Western blot. RESULTS: We showed that the activation of MCH receptors promotes nonalcoholic fatty liver disease through the parasympathetic nervous system, whereas it increases fat deposition in white adipose tissue via the suppression of sympathetic traffic. These metabolic actions are independent of parallel changes in food intake and energy expenditure. In the liver, MCH triggers lipid accumulation and lipid uptake, with c-Jun N-terminal kinase being an essential player, whereas in adipocytes MCH induces metabolic pathways that promote lipid storage and decreases lipid mobilization. Genetic activation of MCH receptors or infusion of MCH specifically in the lateral hypothalamic area modulated hepatic lipid metabolism, whereas the specific activation of this receptor in the arcuate nucleus affected adipocyte metabolism. CONCLUSIONS: Our findings show that central MCH directly controls hepatic and adipocyte metabolism through different pathways.

The endocannabinoid system: role in glucose and energy metabolism.

The endocannabinoid system (ECS) has emerged as one of the most relevant regulators of energy balance. The ECS acts through two cannabinoid receptors: types 1 and 2 (CB1 and CB2). CB1 receptors are widely expressed in the brain, but are also expressed in adipose tissue, skeletal muscle, the liver, the gut, and the pancreas. Blockade of CB1 receptors causes a reduction in food intake and a sustained weight loss. This system contributes also to the control of lipid and glucose metabolism, and it is well established that blockade of CB1 receptors enhances insulin sensitivity in both humans and rodents. In obese states, endocannabinoid levels are increased and might exert unfavorable effects on insulin-sensitive tissues. This review summarizes the effects of the endocannabinoid system on glucose metabolism in humans and rodents.

Hypothalamic κ-opioid receptor modulates the orexigenic effect of ghrelin.

The opioid system is well recognized as an important regulator of appetite and energy balance. We now hypothesized that the hypothalamic opioid system might modulate the orexigenic effect of ghrelin. Using pharmacological and gene silencing approaches, we demonstrate that ghrelin utilizes a hypothalamic κ-opioid receptor (KOR) pathway to increase food intake in rats. Pharmacological blockade of KOR decreases the acute orexigenic effect of ghrelin. Inhibition of KOR expression in the hypothalamic arcuate nucleus is sufficient to blunt ghrelin-induced food intake. By contrast, the specific inhibition of KOR expression in the ventral tegmental area does not affect central ghrelin-induced feeding. This new pathway is independent of ghrelin-induced AMP-activated protein kinase activation, but modulates the levels of the transcription factors and orexigenic neuropeptides triggered by ghrelin to finally stimulate feeding. Our novel data implicate hypothalamic KOR signaling in the orexigenic action of ghrelin.

The central Sirtuin 1/p53 pathway is essential for the orexigenic action of ghrelin.

OBJECTIVE: Ghrelin is a stomach-derived peptide that increases food intake through the activation of hypothalamic AMP-activated protein kinase (AMPK). However, the molecular mechanisms initiated by the activation of the ghrelin receptor, which in turn lead to AMPK activation, remain unclear. Sirtuin 1 (SIRT1) is a deacetylase activated in response to calorie restriction that acts through the tumor suppressor gene p53. We tested the hypothesis that the central SIRT1/p53 pathway might be mediating the orexigenic action of ghrelin. RESEARCH DESIGN AND METHODS: SIRT1 inhibitors, such as Ex527 and sirtinol, and AMPK activators, such as AICAR, were administered alongside ghrelin in the brain of rats and mice (wild-type versus p53 knockout [KO]). Their hypothalamic effects on lipid metabolism and changes in transcription factors and neuropeptides were assessed by Western blot and in situ hybridization. RESULTS: The central pretreatment with Ex527, a potent SIRT1 inhibitor, blunted the ghrelin-induced food intake in rats. Mice lacking p53, a target of SIRT1 action, failed to respond to ghrelin in feeding behavior. Ghrelin failed to phosphorylate hypothalamic AMPK when rats were pretreated with Ex527, as it did in p53 KO mice. It is noteworthy that the hypothalamic SIRT1/p53 pathway seems to be specific for mediating the orexigenic action of ghrelin, because central administration of AICAR, a potent AMPK activator, increased food intake in p53 KO mice. Finally, blockade of the central SIRT1 pathway did not modify ghrelin-induced growth hormone secretion. CONCLUSIONS: Ghrelin specifically triggers a central SIRT1/p53 pathway that is essential for its orexigenic action, but not for the release of growth hormone.

Central GLP-1 actions on energy metabolism.

Glucagon-like peptide 1 (GLP-1) is secreted mainly by the intestine in a nutrient-dependent manner and stimulates glucose-induced insulin secretion, inhibits gastric emptying, food intake, and glucagon secretion. All these beneficial effects make GLP-1 as a promising, and currently in the market, drug candidate for the treatment of type 2 diabetes. More recently, it has been also demonstrated that within the central nervous system, GLP-1 also exerts important metabolic actions inhibiting food intake, increasing insulin secretion, and modulating behavioral responses. In this review, we will focus on the metabolic actions and mechanisms of the central GLP-1 system: modulation of energy intake, glucose metabolism, and fatty acid metabolism.