Leptin and Appetite Control in Lipodystrophy

The worldwide epidemics of obesity and associated diseases have enormous public health consequences (1). In addition to increasing morbidity from diabetes, cardiovascular disease, liver disease, cholelithiasis, sleep apnea, arthritis, cancer, reproductive dysfunction, and various complications, obesity is independently associated with excess mortality (1). Although diet, exercise, and lifestyle modification remain the cornerstone of obesity treatment, it is obvious that these measures alone cannot reverse the burden of obesity and related diseases (1). Hence, attempts to develop effective and safe antiobesity drugs have become urgent.

Mammals, including humans, have a remarkable ability to match energy intake and expenditure leading to a near-constant body weight, despite considerable variation in the amount and composition of food consumed each day. Thus, it has been suggested that a homeostatic mechanism for regulation of energy balance and body weight must exist (2). Signals that control feeding have been divided into short-term and long-term. The former is comprised of neural, nutrient, and peptide signals from the gastrointestinal tract that rapidly trigger satiety or hunger through neuronal circuits in the brain stem, hypothalamus, and higher forebrain regions (2). The role of vagal afferents in mediating the effects of gut mechanoreceptors, nutrients, and peptides on satiety is well known (3). Classic studies by Gibbs and Smith (4) demonstrated the importance of cholecystokinin in the control of satiety. Peptide YY (PYY), a member of the neuropeptide Y (NPY) family, is produced by the small intestine and released into the circulation after meals, particularly meals of carbohydrate-rich and lipid-rich foods (reviewed in Ref.5). Administration of an N-terminal truncated form PYY 3–36 decreases food intake in rodents, likely through the Y2 receptor in the arcuate hypothalamic nucleus (5). Obese patients responded normally to exogenous PYY 3–36 treatment, despite having lower fasting levels and a blunted postprandial increase in PYY 3–36 (5). Other gut peptides that increase satiety include pancreatic polypeptide, glucagon-like peptide (GLP)-1, and oxyntomodulin (5). Ghrelin, the endogenous ligand for the GH secretagogue receptor is produced mainly by the stomach, increases during fasting, and acts as a hunger signal in the hypothalamus (6). Injection of ghrelin peripherally or into the brain stimulates food intake and weight gain (6). Conversely, feeding lowers ghrelin level. Ghrelin is markedly increased in Prader Willi syndrome and is thought to mediate the ravenous feeding in this condition (5).

Leptin and insulin are the prototypic long-term signals whose levels are tightly coupled to energy stores in adipose tissue (2). Leptin is produced mainly by adipocytes, although low levels have been detected in gastric fundic epithelium, intestine, and skeletal muscle. Leptin circulates at concentrations proportional to body fat and decreases body weight by inhibiting food intake and inducing thermogenesis (7). Several lines of evidence support a central nervous system action of leptin (7). Leptin is transported into the brain via a saturable process and binds to specific regions involved in control of feeding and energy balance. The long-form leptin receptor, which has intracellular motifs necessary for signaling via the JAK-STAT pathway, is highly expressed in hypothalamic and brain stem nuclei well known to control feeding and energy balance, e.g. arcuate, dorsomedial, paraventricular, nucleus tractus solitarius, and lateral parabrachial nuclei. A rise in leptin suppresses anorexigenic neuropeptides, e.g. NPY, agouti-related peptide, and melanin-concentrating hormone, while increasing anorexigenic peptides, e.g. α-MSH (produced by proopiomelanocortin neurons in the arcuate nucleus), cocaine and amphetamine-regulated transcript, and CRH (2). Conversely, the fall in leptin during fasting robustly triggers hyperphagia, decreases thermogenesis, and induces various hormonal changes, e.g. suppression of thyroid and reproductive hormones, and activation of the hypothalamic-pituitary-adrenal axis (in rodents) (2). Low leptin also mediates immune suppression (7). These leptin-mediated responses may have evolved as a protection against the threat of starvation, by promoting limiting energy utilization and increasing energy efficiency (7). In addition to its effects on energy balance, leptin potentiates the reward response to feeding as well as taste perception (7). Taken together, these findings led to initial excitement about the therapeutic potential of leptin.

As expected, mutations of the leptin (Lep^ob) or leptin receptor (Lepr^db) genes in rodents and humans, which lead to the absence of active leptin protein or impairment of leptin signaling, result in hyperphagia, early-onset obesity, insulin resistance, diabetes, and a variety of neuroendocrine abnormalities, notably hypothalamic hypogonadism (7). Elevated glucocorticoids are a hallmark of leptin deficiency in rodents but not humans (7). Leptin treatment profoundly suppressed appetite and obesity and restored normal neuroendocrine function in leptin-deficient rodents and humans, consistent with the idea of negative feedback regulation (7, 8). However, in contrast to congenital leptin deficiency, earlier studies produced marginal effects on food intake and body weight in normal subjects (9, 10). Because leptin is increased in proportion to body fat in obesity and yet is unable to inhibit appetite or prevent weight gain, it was surmised that the lack of response to exogenous leptin was due to leptin resistance (11). In diet-induced obese rodents, leptin resistance appears to be mediated by impaired blood-brain transport, inactivation of leptin-mediated Janus kinase (JAK) signal transducers and activators of transcription (STAT) signaling, e.g. via induction of SOCS3, or dysregulation of neuropeptides downstream of the leptin receptor (11).

Despite those disappointing results, interest in the therapeutic value of leptin in humans has been rekindled by recent studies (12, 13). Administration of leptin in humans prevented the normal suppression of TSH and T₃ levels and sex hormones during fasting (12, 13). Importantly, chronic replacement of leptin within the physiological range in normal subjects, maintained at 10% weight reduction, prevented the characteristic suppression of the thyroid axis and energy expenditure (12). These findings suggested that leptin treatment could be used to sustain weight reduction by stimulating thyroid thermogenesis (12).

Lipodystrophy provides yet another model in which the correction of low leptin proved beneficial. Lipodystrophic syndromes may be inherited or acquired, e.g. as a result of autoimmune destruction of adipose tissue or antiretroviral treatment in HIV (14). Lipodystrophy is characterized by partial or generalized loss (lipoatrophy), hypertrophy, and redistribution of body fat (14). Appetite is typically increased, although metabolic rate may be variable (14). The change in amount and distribution of adipose tissue is commonly associated with dyslipidemia, severe hepatic steatosis, insulin resistance, and diabetes, as well as low levels of leptin and adiponectin (14). It was initially thought that the absence of adequate adipose storage capacity led to accumulation of triglycerides in liver, muscle, and pancreatic β-cell, resulting in impairment of insulin action and diabetes. However, contrary to this idea, studies in rodents with congenital lipodystrophy revealed that the metabolic derangement was not due to loss of fat mass per se (15). Insulin sensitivity and lipids improved dramatically after fat transplantation from normal mice, whereas fat transplanted from Lep^ob/ob mice failed to reverse the metabolic disturbance, suggesting a crucial role of leptin and other adipose secreted factors (15). Additional support for the importance of leptin came from studies demonstrating improvement in steatosis, insulin sensitivity, and hyperlipidemia in lipodystrophic mice treated with recombinant leptin (16).

Leptin has also been successfully used to ameliorate insulin resistance, dyslipidemia, and steatosis in lipodystrophic patients (17, 18). Oral et al. (17) reported dramatic metabolic improvements in lipodystrophic patients treated with daily sc leptin injections for 4 months (17). Plasma glucose, insulin, triglycerides, and hepatic steatosis improved, whereas plasmapheresis had little effect (17). Self-reported calorie intake decreased; however, as has been described in children with congenital leptin deficiency, leptin did not increase metabolic rate (8, 17). Later on, Petersen et al. (18) showed that leptin therapy markedly reduced hepatic glucose output and increased peripheral glucose disposal in lipodystrophic patients. There were concomitant reductions in hepatic steatosis and intramyocellular triglycerides, providing a potential mechanism for the effect of leptin on insulin sensitivity (18). Studies in rodents showed that leptin acted directly on skeletal muscle or through the central and sympathetic nervous systems to stimulate AMP-activated protein kinase, leading to induction of lipid oxidation (19).

As has been described in congenital leptin deficiency, the low leptin level in lipodystrophy is associated with hypothalamic hypogonadism (8, 20). Chronic administration of recombinant leptin restored LH and estradiol levels and improved menstrual cycles in lipodystrophic patients (20). Serum T₃ and free T₄ concentrations were within the normal range before and after leptin therapy, although serum TSH fell significantly after leptin treatment (20). Moreover, in contrast to leptin-deficient Lep^ob/ob mice, leptin therapy did not affect the levels of ACTH, cortisol, or CRH response in lipodystrophic patients (7, 20).

Although the ability of leptin to suppress feeding in congenital leptin deficiency and lipodystrophy is well documented, the precise mechanisms have not been explored. For example, it is not known whether leptin controls food-seeking behavior, satiety (meal termination), or satiation (intermeal interval). In this issue of JCEM, McDuffie et al. (21) have performed a detailed analysis of leptin’s effects on feeding in an open label study in lipodystrophic patients. Eight female patients, two with acquired generalized lipodystrophy, one with Dunnigan’s familial partial lipodystrophy, and five with congenital generalized lipodystrophy (Seip-Berardinelli syndrome) were studied. Baseline serum leptin concentration was less than 4 ng/ml, and the patients had diabetes mellitus, hypertriglyceridemia, and hyperinsulinemia. After an overnight fast, blood samples were collected for measurement of fasting glucose, triglycerides, and hormones. Ad libitum food intake, food preference, hunger, satiety, and satiation were assessed in response to various meals. After baseline assessment, recombinant methionyl human leptin (A-100; Amgen Inc., Thousand Oaks, CA) was injected twice daily for 4 months. Half of the daily replacement dose (0.03–0.04 mg/kg) was given initially, increased to 200% by 2 months, and then maintained at this level for the third and fourth months. The subjects were restudied after treatment and served as their own controls.

Serum leptin concentration increased to the normal range (12.3 ± 2.1 ng/ml). Body weight decreased significantly, as did serum insulin, glucose, and triglycerides. Leptin therapy decreased the satiation time (i.e. time to voluntary cessation of eating after 12 h fast), and increased satiety time (i.e. time to consume a meal after consumption of a standardized preload meal) (21). The amount of food consumed to attain satiation decreased after leptin treatment, whereas the amount of food desired after the postabsorptive period decreased. Importantly, the macronutrient intake was not affected by leptin. Moreover, leptin did not substantially alter hunger or food preference (21). Ghrelin level decreased significantly after leptin administration, suggesting a role in the satiety effect of leptin. In contrast, GLP-1 was not affected by leptin (21).

McDuffie et al. (21) have provided an objective template for future studies into the specific actions of leptin on control of appetite and feeding behavior. The findings, albeit in a small number of subjects, offer a comprehensive and rational explanation of the increase in food consumption in low leptin states, such as lipodystrophy and possibly congenital leptin deficiency (8, 17, 18). Although the contribution of energy expenditure to leptin’s effect on weight loss was not assessed here, other studies have demonstrated a reduction rather than an increase in energy expenditure in response to leptin therapy in congenital leptin deficiency and lipodystrophy (8, 17, 22). Leptin decreases fat mass and to a lesser extent lean mass, but it does not significantly alter bone mineral density (8, 17, 22). Taken together, these studies indicate that in contrast to rodents, the antiobesity action of leptin in humans is mediated primarily through inhibition of feeding, specifically satiety (8, 21, 22). Importantly, leptin did not affect macronutrient intake, consistent with apparent lack of effect on food preference (21).

Overall, these studies demonstrate that leptin can be administered safely and effectively in leptin-deficient patients. However, additional questions remain. Do the findings in low leptin states apply to normal leptin-sufficient subjects? Does leptin exert similar effects on feeding, energy balance, neuroendocrine axis, glucose, and lipids in normal subjects? Well-controlled studies are needed to clarify the role of in obese hyperleptinemic patients. Because circulating leptin concentrations are highly variable in the latter, perhaps a subset of obese patients with relative leptin deficiency may respond better to chronic leptin therapy. Alternatively, supplementation of leptin may be useful in maintaining weight reduction after caloric restriction has been initiated, as shown in patients maintained at 10% weight reduction (12). In the latter case, although a small number of subjects were studied, the results revealed a clear and significant effect of leptin in increasing energy expenditure, likely through stimulation of thyroid thermogenesis and sympathetic nervous activity (12). This finding is contrary to the lack of effect or reduction in energy expenditure when patients with congenital leptin deficiency or lipodystrophy were treated with exogenous leptin (8, 22). Thus, it will be important to establish whether leptin controls satiety as well as energy expenditure in normal subjects.

A major deficiency of the current study was the paucity of detailed long-term analyses of putative gastrointestinal peptide and hormonal targets of leptin. The fall in ghrelin at the end of the study suggested that the satiety effect of leptin was mediated, at least in part, through inhibition of ghrelin. However, it would have been more instructive to assess the temporal changes in ghrelin, GLP-1, PYY, pancreatic polypeptide, cholecystokinin, NPY, melanocortins, and other putative mediators at various times during leptin treatment.

The study by McDuffie et al. (21) is an example of how basic science can be translated into experiments in humans. Energy homeostasis involves a complex interplay of physiological and behavioral mechanisms. Given the limitations of rodent models in these areas, especially the differences in brown adipose tissue metabolism and neuroendocrine function, it is crucial to expand such studies to humans, where possible. Human experiments, albeit challenging, will allow the measurement of subjective parameters such as hunger and satiety in conjunction with relevant biological markers. The role of the hypothalamus and other brain regions that mediate the responses to leptin, insulin, nutrients, and other signals could be explored using positron emission tomography or magnetic resonance imaging (23). States of hunger or satiation could be functionally correlated with brain activity and metabolites in plasma and cerebrospinal fluid (23). We have entered an exciting era of clinical research in which the uniqueness of the human model should be explored using powerful physiological, anatomical, and biochemical tools to provide insight into the pathogenesis, diagnosis, and treatment of obesity and related diseases.

Due to the limitation in the number of references, we were unable to cite important original papers.

Abbreviations:

 
• GLP,

  Glucagon-like peptide;

 
• NPY,

  neuropeptide Y;

 
• PYY,

  peptide YY.