Arcuate nucleus-dependent regulation of metabolism - pathways to obesity and diabetes mellitus

The central nervous system (CNS) receives information from afferent neurons, circulating hormones and absorbed nutrients and integrates this information to orchestrate the actions of the neuroendocrine and autonomic nervous systems in maintaining systemic metabolic homeostasis. Particularly the arcuate nucleus of the hypothalamus (ARC) is of pivotal importance for primary sensing of adiposity signals, such as leptin and insulin, and circulating nutrients, such as glucose. Importantly, energy state-sensing neurons in the ARC not only regulate feeding but at the same time control multiple physiological functions, such as glucose homeostasis, blood pressure and innate immune responses. These findings have defined them as master regulators, which adapt integrative physiology to the energy state of the organism. The disruption of this fine-tuned control leads to an imbalance between energy intake and expenditure as well as deregulation of peripheral metabolism. Improving our understanding of the cellular, molecular and functional basis of this regulatory principle in the CNS could set the stage for developing novel therapeutic strategies for the treatment of obesity and metabolic syndrome. In this review, we summarize novel insights with a particular emphasis on ARC neurocircuitries regulating food intake and glucose homeostasis and sensing factors that inform the brain of the organismal energy status.

Improvements in Long-Term Appetite-Regulating Hormones in Response to a Combined Lifestyle Intervention for Obesity

Altered levels of hormonal appetite regulators have been observed in obesity (BMI ≥ 30.0 kg/m2), most prominently increases of insulin and leptin (indicating resistance) as well as decreases of adiponectin - all of which are long-term energy regulators and adiposity signals. Disrupted signaling of these hormones may have detrimental effects on metabolism, but may also promote weight gain. Weight loss is often accompanied by normalizations of long-term adiposity signals, but findings concerning short-term appetite regulators after weight loss vary across interventions (e.g. very low calorie diets vs. exercise). Moreover, it is debated whether such weight-loss-induced hormonal changes may reflect a disposition for weight regain. Here, we investigated changes of long- and short-term appetite signals in response to an intensive 75-week combined lifestyle intervention (CLI) comprising a normocaloric healthy diet, physical activity and psychotherapy to promote improved long-term weight management. For 39 patients, data on fasting serum levels of appetite-regulating hormones (leptin, insulin, adiponectin, GIP, PP, PYY, CCK, FGF21) were available. Hormone levels were correlated to BMI at baseline (T0) and compared across three time points: T0, T1 (after 10 weeks; initial weight loss) and T2 (after 75 weeks; weight loss maintenance). T0-T1 hormone changes were correlated to BMI changes between T1 and T2 to investigate whether hormonal alterations during initial weight loss are associated with weight regain. At T0, hormone levels were not associated with BMI. BMI decreased significantly from T0 (40.13 kg/m2 ± 5.7) to T1 (38.2 ± 5.4, p < .001) which was maintained at T2 (38.2 kg/m2 ± 5.9, p < .001). There were no significant changes in GIP, PP, PYY, CCK and FGF21. Leptin decreased from T0 (44.9 ng/nl ± 15.3) to T1 (33 ng/nl ± 14.8, p < .001) and T2 (38.6 ng/nl ± 16.0, p < .01), just like insulin which was significantly decreased at T1 (123 pmol/l ± 65, p < .05) and T2 (128 pmol/l ± 64, p < .05) compared to T0 (160 pmol/l ± 80). Adiponectin did not change between T0 (3.36 ug/ml ± 2.1) and T1 (3.2 ug/ml ± 2.1), but was increased at T2 (3.7 ug/ml ± 2.9, p < .01) compared to T1. T0-T2 BMI decrease correlated positively with T0-T2 decreases in leptin (r = .667, p < .001), insulin (rho = .535, p < .001) and increases of adiponectin (r = .412, p < .01), but no other hormone. T0-T1 hormone changes did not predict T1-T2 BMI changes. Thus, a 75-week CLI was associated with beneficial changes in the long-term energy regulators adiponectin, leptin and insulin, but no changes in short-term appetite-regulating hormones were observed despite significant weight loss. Initial changes in appetite-regulating hormones were not associated with subsequent weight regain. Overall, our data suggest that a CLI does not lead to adverse changes in appetite regulation, but rather long-term improvements such as e.g. increased leptin and insulin sensitivity.

IL CONTROLLO NEUROENDOCRINO DEL COMPORTAMENTO ALIMENTARE

Appetite is regulated by a complex system of central and peripheral signals that interact in order to modulate eating behavior according the individual needs, i.e. the fasting or fed condition and the general nutritional status. Peripheral regulation includes adiposity signals and satiety signals, while central control is accomplished by several effectors, including the neuropeptidergic, monoaminergic and endocannabinoid systems. Adiposity signals inform the brain of the general nutritional status of the subject as indicated by the extent of fat depots. Indeed, leptin produced by the adipose tissue and insulin, whose pancreatic secretion tends to increase with the increase of fat mass, convey to the brain an anorexigenic message. Satiety signals, including cholecystokinin (CCK), glucagon-like peptide-1 (GLP-1) and peptide YY (PYY), originate from the gastrointestinal tract during a meal and, through the vagus nerve, reach the nucleus tractus solitarius (NTS) in the caudal brainstem. From NTS afferents fibers project to the arcuate nucleus (ARC) of the hypothalamus, where satiety signals are integrated with adiposity signals and with several hypothalamic and supra-hypothalamic inputs, thus creating a complex network of neural circuits that finally elaborate the most appropriate response, in terms of eating behavior. In more detail, ARC neurons secrete a number of neuropeptides with orexigenic properties, such as neuropeptide Y (NPY) and agouti-related peptide (AGRP), or anorexigenic effects such as pro-opiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART). Other brain areas involved in the control of food intake are located downstream the ARC: among these, the paraventricular nucleus (PVN), which produces anorexigenic peptides such as thyrotropin releasing hormone (TRH), corticotrophin releasing hormone (CRH) and oxytocin, the lateral hypothalamus (LHA) and perifornical area (PFA), secreting the orexigenic substances orexin-A (OXA) and melanin concentrating hormone (MCH). Recently, a great interest has developed for endogenous cannabinoids, important players in the regulation of food intake and energy metabolism. In the same context, increasing evidence is accumulating for a role played by the microbiota, the trillion of microorganism populating the human gastrointestinal tract. The complex interaction between the peripheral organs and the central nervous system has generated the concept of gut-brain axis, now incorporated into the physiology. A better understanding of the mechanisms governing the eating behavior will allow the development of drugs capable of reducing or enhancing food consumption.

In vivo and in vitro bisphenol A exposure effects on adiposity

In utero exposure to the ubiquitous plasticizer, bisphenol A (BPA) is associated with offspring obesity. As adipogenesis is a critical factor contributing to obesity, we determined the effects of in vivo maternal BPA and in vitro BPA exposure on newborn adipose tissue at the stem-cell level. For in vivo studies, female rats received BPA before and during pregnancy and lactation via drinking water, and offspring were studied for measures of adiposity signals. For in vitro BPA exposure, primary pre-adipocyte cell cultures from healthy newborns were utilized. We studied pre-adipocyte proliferative and differentiation effects of BPA and explored putative signal factors which partly explain adipose responses and underlying epigenetic mechanisms mediated by BPA. Maternal BPA-induced offspring adiposity, hypertrophic adipocytes and increased adipose tissue protein expression of pro-adipogenic and lipogenic factors. Consistent with in vivo data, in vitro BPA exposure induced a dose-dependent increase in pre-adipocyte proliferation and increased adipocyte lipid content. In vivo and in vitro BPA exposure promotes the proliferation and differentiation of adipocytes, contributing to an enhanced capacity for lipid storage. These findings reinforce the marked effects of BPA on adipogenesis and highlight the susceptibility of stem-cell populations during early life with long-term consequence on metabolic homeostasis.

Adiposity signals predict vocal effort in Alston's singing mice

Advertisement displays often seem extravagant and expensive, and are thought to depend on the body condition of a signaller. Nevertheless, we know little about how signallers adjust effort based on condition, and few studies find a strong relationship between natural variation in condition and display. To examine the relationship between body condition and signal elaboration more fully, we characterized physiological condition and acoustic displays in a wild rodent with elaborate vocalizations, Alston's singing mouse, Scotinomys teguina . We found two major axes of variation in condition—one defined by short-term fluctuations in caloric nutrients, and a second by longer-term variation in adiposity. Among acoustic parameters, song effort was characterized by high rates of display and longer songs. Song effort was highly correlated with measures of adiposity. We found that leptin was a particularly strong predictor of display effort. Leptin is known to influence investment in other costly traits, such as immune function and reproduction. Plasma hormone levels convey somatic state to a variety of tissues, and may govern trait investment across vertebrates. Such measures offer new insights into how animals translate body condition into behavioural and life-history decisions.

Foraging theory and the propensity to be obese: an alternative to thrift

The evolutionary origin of obesity is classically believed to be genetic or developmentally induced thrift, as an adaptation to ancestral feast and famine conditions. However, recently the thrift family of hypotheses have attracted serious criticism necessitating alternative thinking. Optimization of foraging behaviour is an important aspect of behavioural evolution. For a species evolved for optimizing nutritional benefits against predation or other foraging risks, reduction in foraging risk below a threshold dramatically increases the steady-state body weight. In modern life where feeding is detached from foraging, the behavioural regulation mechanisms are likely to fail resulting into escalation of adiposity. At a proximate level the signalling pathways for foraging optimization involve fear induced signal molecules in the brain including Cocaine and Amphetamine Regulated Transcript (CART) interacting with adiposity signals such as leptin. While leptin promotes the expression of the fear peptides, the fear peptides promote anorectic action of leptin. This interaction promotes foraging drive and risk tolerance when the stored energy is low and suppresses hunger and foraging drive when the perceived risk is high. The ecological model of foraging optimization and the molecular model of interaction of these peptides converge in the outcome that the steady state adiposity is an inverse square root function of foraging risk. The foraging optimization model is independent of thrift or insurance hypotheses, but not mutually exclusive. We review existing evidence and suggest testable predictions of the model. Understanding obesity simultaneously at proximate and ultimate levels is likely to suggest effective means to curb the obesity epidemic.

Central and peripheral control of food intake

The maintenance of the body weight at a stable level is a major determinant in keeping the higher animals and mammals survive. Th e body weight depends on the balance between the energy intake and energy expenditure. Increased food intake over the energy expenditure of prolonged time period results in an obesity. Th e obesity has become an important worldwide health problem, even at low levels. The obesity has an evil effect on the health and is associated with a shorter life expectancy. A complex of central and peripheral physiological signals is involved in the control of the food intake. Centrally, the food intake is controlled by the hypothalamus, the brainstem, and endocannabinoids and peripherally by the satiety and adiposity signals. Comprehension of the signals that control food intake and energy balance may open a new therapeutic approaches directed against the obesity and its associated complications, as is the insulin resistance and others. In conclusion, the present review summarizes the current knowledge about the complex system of the peripheral and central regulatory mechanisms of food intake and their potential therapeutic implications in the treatment of obesity.

Obesity and Appetite Control

Obesity is one of the major challenges to human health worldwide; however, there are currently no effective pharmacological interventions for obesity. Recent studies have improved our understanding of energy homeostasis by identifying sophisticated neurohumoral networks which convey signals between the brain and gut in order to control food intake. The hypothalamus is a key region which possesses reciprocal connections between the higher cortical centres such as reward-related limbic pathways, and the brainstem. Furthermore, the hypothalamus integrates a number of peripheral signals which modulate food intake and energy expenditure. Gut hormones, such as peptide YY, pancreatic polypeptide, glucagon-like peptide-1, oxyntomodulin, and ghrelin, are modulated by acute food ingestion. In contrast, adiposity signals such as leptin and insulin are implicated in both short- and long-term energy homeostasis. In this paper, we focus on the role of gut hormones and their related neuronal networks (the gut-brain axis) in appetite control, and their potentials as novel therapies for obesity.

Biology's response to dieting: the impetus for weight regain

Dieting is the most common approach to losing weight for the majority of obese and overweight individuals. Restricting intake leads to weight loss in the short term, but, by itself, dieting has a relatively poor success rate for long-term weight reduction. Most obese people eventually regain the weight they have worked so hard to lose. Weight regain has emerged as one of the most significant obstacles for obesity therapeutics, undoubtedly perpetuating the epidemic of excess weight that now affects more than 60% of U.S. adults. In this review, we summarize the evidence of biology's role in the problem of weight regain. Biology's impact is first placed in context with other pressures known to affect body weight. Then, the biological adaptations to an energy-restricted, low-fat diet that are known to occur in the overweight and obese are reviewed, and an integrative picture of energy homeostasis after long-term weight reduction and during weight regain is presented. Finally, a novel model is proposed to explain the persistence of the “energy depletion” signal during the dynamic metabolic state of weight regain, when traditional adiposity signals no longer reflect stored energy in the periphery. The preponderance of evidence would suggest that the biological response to weight loss involves comprehensive, persistent, and redundant adaptations in energy homeostasis and that these adaptations underlie the high recidivism rate in obesity therapeutics. To be successful in the long term, our strategies for preventing weight regain may need to be just as comprehensive, persistent, and redundant, as the biological adaptations they are attempting to counter.