Postnatal ablation of POMC neurons induces an obese phenotype characterized by decreased food intake and enhanced anxiety-like behavior

Proopiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus are central components of systems regulating appetite and energy homeostasis. Here we report on the establishment of a mouse model in which the ribonuclease III ribonuclease Dicer-1 has been specifically deleted from POMC-expressing neurons (POMC(ΔDCR)), leading to postnatal cell death. Mice are born phenotypically normal, at the expected genetic ratio and with normal hypothalamic POMC-mRNA levels. At 6 weeks of age, no POMC neurons/cells could be detected either in the arcuate nucleus or in the pituitary of POMC(ΔDCR) mice. POMC(ΔDCR) develop progressive obesity secondary to decreased energy expenditure but unrelated to food intake, which was surprisingly lower than in control mice. Reduced expression of AgRP and ghrelin receptor in the hypothalamus and reduced uncoupling protein 1 expression in brown adipose tissue can potentially explain the decreased food intake and decreased heat production, respectively, in these mice. Fasting glucose levels were dramatically elevated in POMC(ΔDCR) mice and the glucose tolerance test revealed marked glucose intolerance in these mice. Secondary to corticotrope ablation, basal and stress-induced corticosterone levels were undetectable in POMC(ΔDCR) mice. Despite this lack of activation of the neuroendocrine stress response, POMC(ΔDCR) mice exhibited an anxiogenic phenotype, which was accompanied with elevated levels of hypothalamic corticotropin-releasing factor and arginine-vasopressin transcripts. In conclusion, postnatal ablation of POMC neurons leads to enhanced anxiety and the development of obesity despite decreased food intake and glucocorticoid deficiency.

Impact of weight gain on outcomes in type 2 diabetes

BACKGROUND

The majority of patients with type 2 diabetes mellitus (T2DM) are overweight or obese. Obesity is a significant risk factor for increased morbidity and mortality in people with T2DM, and increased weight has been shown to worsen glycemic control and increase the risk of diabetes progression.

METHODS

A search was conducted of the National Library of Medicine (PubMed) for articles published from 1990 to 2009 about the treatments of T2DM, relationship between T2DM and weight gain, obesity-related comorbidities of T2DM, and T2DM therapies associated with increased weight. Reference lists of retrieved articles were reviewed for additional publications.

FINDINGS

Results from large, prospective clinical trials have shown that weight reduction significantly improves glycemic control and blood pressure in T2DM patients and lowers the risk of progression of T2DM as well as CV disease and cancer. Treatment-related weight gain is a side effect of many oral antidiabetes agents and insulin. The thiazolidinediones (TZD), sulfonylureas, and glinides are associated with weight gain. Despite the weight gain, TZDs also redistribute fat from the central to peripheral compartments, which may lead to a beneficial effect on insulin resistance. Among insulin products, the basal insulin analog detemir is typically associated with a smaller weight increase than human insulin and insulin analog preparations, including glargine, biphasic, and prandial insulin regimens. Alpha-glucosidase inhibitors and dipeptidyl peptidase-4 inhibitors are weight neutral, whereas glucagon-like peptide1-R agonists and metformin are associated with weight loss.

DISCUSSION

An effective approach to management of the obese patient with diabetes is to communicate the significant benefits of a 1 kg reduction in body weight or prevention of weight gain on glycemic control and reduced morbidity and mortality.

LIMITATION

This article is based on an extensive literature review rather than the prospective studies needed to define further the effect of weight gain on the management of T2DM.

CONCLUSION

Weight management should be an integral part of a T2DM treatment strategy that includes selecting oral antidiabetes medications and insulin products that are weight beneficial.

Melanocortin signalling and the regulation of blood pressure in human obesity

Obesity has reached epidemic proportions worldwide. Sympathetic nervous system activation has been shown to play a major role linking obesity to the development of associated metabolic complications, such as hypertension. Recent evidence has implicated central melanocortin signalling in the regulation of blood pressure in rodents and humans. The importance of sympathetic neural activity in mediating this association has been highlighted. Humans with loss-of-function mutations in the melanocortin 4 receptor (MC4R) are an ideal group of subjects in whom the importance of melanocortin signalling in linking obesity to hypertension can be studied. Consistent with rodent studies, it was recently demonstrated that humans with MC4R deficiency have lower blood pressure, less hypertension, lower 24-h urinary catecholamine excretion, lower resting heart rate and attenuated insulin-mediated sympathetic activation compared to equally-obese humans. In overweight and obese humans without MC4R mutations, the infusion of a highly-selective MC4R agonist led to dose-dependent increases in blood pressure and heart rate. All effects were independent of insulin. This evidence supports the notion that the melanocortin system regulates blood pressure and sympathetic neural function. The results obtained in rodent and human studies, in relation to blood pressure and sympathetic function, may limit the use of MC4R agonists for the treatment of obesity. Future studies will determine whether MC4R deficiency is associated with protection from development of the detrimental cardiovascular consequences that accompany obesity.

Hypothalamic Angptl4/Fiaf is a novel regulator of food intake and body weight

OBJECTIVE

The angiopoietin-like protein 4 (Angptl4)/fasting-induced adipose factor (Fiaf) is known as a regulator of peripheral lipid and glucose metabolism. In the present study, we investigated the physiological role of Angptl4 in central regulation of body weight homeostasis.

RESEARCH DESIGN AND METHODS

Hypothalamic Angptl4 expression levels were measured using immunoblot assay during feeding manipulation or after administration of leptin, insulin, and nutrients. The effects of Angptl4 on food intake, body weight, and energy expenditure were determined following intracerebroventricular (ICV) administration of Angptl4 in C57BL/6 mice. Food intake, energy metabolism, and feeding responses to leptin, insulin, and nutrients were compared between Angptl4-null mice and their wild littermates. Finally, the relationship of hypothalamic AMP-activated protein kinase (AMPK) and Angptl4 was studied.

RESULTS

Hypothalamic Angptl4 expression levels were increased upon food intake or administration of leptin, insulin, and nutrients. Furthermore, central administration of Angptl4 suppressed food intake and body weight gain but enhanced energy expenditure. These effects were mediated via suppression of hypothalamic AMPK activities. Consistently, Angptl4-null mice displayed increased body weight and hypothalamic AMPK activity but reduced energy expenditure. Food intake following a fast was significantly greater in Angptl4-null mice, which was normalized by centrally administered Angptl4. Moreover, anorectic responses to leptin, insulin, and glucose were diminished in Angptl4-null mice. In contrast, Angptl4-null mice were resistant to diet-induced obesity, indicating obesity-promoting effects of Angptl4 under the condition of fat-enriched diet.

CONCLUSIONS

We have demonstrated that hypothalamic Angptl4 is regulated by physiological appetite regulators and mediates their anorexigenic effects via inhibition of hypothalamic AMPK activity. Therefore, Angptl4 appears to have an important role in central regulation of energy metabolism.

The melanocortin-4 receptor: physiology, pharmacology, and pathophysiology

The melanocortin-4 receptor (MC4R) was cloned in 1993 by degenerate PCR; however, its function was unknown. Subsequent studies suggest that the MC4R might be involved in regulating energy homeostasis. This hypothesis was confirmed in 1997 by a series of seminal studies in mice. In 1998, human genetic studies demonstrated that mutations in the MC4R gene can cause monogenic obesity. We now know that mutations in the MC4R are the most common monogenic form of obesity, with more than 150 distinct mutations reported thus far. This review will summarize the studies on the MC4R, from its cloning and tissue distribution to its physiological roles in regulating energy homeostasis, cachexia, cardiovascular function, glucose and lipid homeostasis, reproduction and sexual function, drug abuse, pain perception, brain inflammation, and anxiety. I will then review the studies on the pharmacology of the receptor, including ligand binding and receptor activation, signaling pathways, as well as its regulation. Finally, the pathophysiology of the MC4R in obesity pathogenesis will be reviewed. Functional studies of the mutant MC4Rs and the therapeutic implications, including small molecules in correcting binding and signaling defect, and their potential as pharmacological chaperones in rescuing intracellularly retained mutants, will be highlighted.