Leptin in the Pathophysiology of Human Obesity and the Clinical Potential of Leptin-Based Therapy
Leptin is a circulating hormone that is secreted in proportion to fat mass. It can reduce bodyweight by activating signalling molecules in the brain. Leptin appears to affect bodyweight primarily by decreasing food intake; there is no direct evidence that it significantly influences energy expenditure in humans. Its discovery in 1994 raised the possibility that it may be a useful, satiety-inducing, anti-obesity drug. However, treating obese patients with leptin alone does not induce substantial bodyweight loss because most obese patients are insensitive to leptin and are not leptin deficient. In combination with diet therapy, however, leptin treatment has the potential to eliminate the dramatic fall in circulating leptin levels (and the subsequent increase in hunger) caused by calorie restriction. Used in this manner, leptin may play a very useful role in the maintenance of body-weight loss. In the future, leptin analogues and the development of compounds that increase leptin sensitivity may also prove to be valuable therapeutic approaches for obesity.
1. Endogenous Leptin
The discovery of leptin resulted in the identification of a feedback signalling system between adipose tissue and the brain which allows the hypothalamus to monitor and influence the amount of energy stored as body fat. The demonstration that body fat can be biologically regulated in this manner means we can no longer regard body fat as solely a product of our environment or willpower. Obesity is a serious, chronic medical condition and drugs will be an increasing part of therapy for those with severe obesity. This review looks at the role of leptin in the pathophysiology of obesity and the potential for leptin-based therapy in the treatment of obesity.
1.1 Structure and Production
In the 1970s, parabiosis studies on lean and obese mice strongly suggested that circulatory factors can influence bodyweight but these remained unidentified until the ob gene was isolated by positional cloning in 1994. This new gene was found to encode a 167-amino acid-secreted protein which was named leptin after the Greek word leptos meaning thin.
Leptin is predominantly expressed in adipose tissue but is also expressed at lower levels in gastric epithelium, placenta and heart. Serum levels of leptin normally reflect the amount of energy stored in adipose tissue. Leptin expression varies in different adipose tissue depots with a stronger leptin signal from subcutaneous adipocytes than from visceral adipocytes, particularly in women. This suggests a possible role for leptin in controlling both the amount of adipose tissue and its distribution.
Leptin is a cytokine which circulates as a 16kD hormone in human plasma. It is not modified post-translationally. However, a large proportion of leptin circulates bound to serum proteins.[3, 4, 5] For example, leptin can circulate in a complex with a soluble form of the leptin receptor. It is possible that the proportion of free versus bound leptin may affect bodyweight by influencing leptin action.
Several cytokines [e.g. tumour necrosis factor α (TNFα)], insulin and corticosteroids can increase circulating leptin levels, while fasting dramatically lowers circulating leptin levels. Long chain fatty acids, catecholamines, thiazolidinediones and circulating leptin itself can also inhibit leptin expression. Negative correlations between dietary fat intake and circulating leptin levels independent of body fat mass have been reported in several papers.[8,9]
For any given measure of obesity, leptin levels are higher in women than in men, consistent with a state of leptin resistance. Lower levels in men may be due to a suppressive effect of androgens on leptin or differences in fat distribution between men and women, or both. Leptin levels are high in the fetus but decline dramatically after birth. This may be important in the stimulation of feeding behaviour. As humans age, the relationship between fat content and leptin levels is disrupted, and there is a blunted diurnal excursion in leptin in the elderly, possibly contributing to the increased adiposity which often occurs with age. Leptin is cleared from the plasma by the kidneys.
1.2 Physiological Function
1.2.1 Regulation of Energy Intake
Leptin initiates the communication of information about body fat mass to the brain by binding to a transmembrane leptin receptor which causes receptor dimerisation and signalling responses. These alter the expression of several hypothalamic neuropeptides [e.g. inhibition of neuropeptide Y (NPY) and stimulation of pro-opiomelanocortin (POMC) expression]. In this way, leptin alters energy intake, although the exact signalling mechanism by which this occurs is not known.
The leptin receptor exists as 4 splice variants in humans: the long isoform huOb-R and the shorter isoforms B219.1 to B219.3.[15,16] The long isoform has full intracellular signalling capacity via the janus kinases which subsequently phosphorylate transcription factors of the signal transducer and activator of transcription (STAT) family. The roles of the other isoforms remain to be elucidated. One of the shorter isoforms is likely to function as a leptin transporter across the blood-brain barrier.
Leptin decreases food intake by acting as a long term adiposity-related signal rather than a short term meal-related factor. In rats it can selectively decrease visceral adiposity.
1.2.2 Energy Expenditure and Activity Levels
It is unclear whether leptin significantly influences energy expenditure in humans. Treating leptin-deficient ob/ob mice with leptin increases resting energy expenditure and physical activity, but treating leptin-deficient humans with leptin does not appear to alter energy expenditure. A lack of correlation between leptin levels and resting energy expenditure in humans has been reported in many studies, although there are some discrepancies in the literature.[8,10,21,22] Studies examining correlations between leptin and physical activity are also divided; there are studies in adults showing no significant association between physical activity and leptin levels[8,23] but a study in children did find such an association.
Interestingly, overfeeding produces a variable amount of bodyweight gain in humans. This appears to be inversely related to the level of nonexercise spontaneous physical activity. However, activation of spontaneous physical activity in those who do not gain in bodyweight does not appear to be mediated by leptin.
The energy expenditure response to leptin may depend upon the state of energy balance. In lean mice, leptin only appears to be able to control thermoregulatory energy expenditure when food supplies are scarce. It is, therefore, likely that leptin regulates body fat predominantly by altering eating behaviour rather than by altering energy expenditure.
1.2.3 Other Effects
The ability of leptin to regulate energy intake makes it a prime candidate for drug therapies. As well as altering fat metabolism, leptin can influence a wide array of other metabolic processes — some beneficial, such as the onset of puberty and immunological responses, and some potentially deleterious, such as increasing blood pressure and platelet aggregation.
Leptin can also improve insulin action, particularly in the liver where it can redistribute glucose fluxes. Supportive evidence for this comes from a recent study showing that leptin can reverse insulin resistance and diabetes mellitus in mice with congenital lipodystrophy. Whether leptin exerts these effects via the CNS or by interacting with peripheral tissue leptin receptors has not been fully established, although the effect of leptin on insulin action is at least partly centrally mediated.
1.3 Role in Human Obesity
The vast majority of obese patients are leptin insensitive rather than leptin deficient, that is, they have an excessive amount of circulating leptin that does not appear to be functioning appropriately. However, 5 to 10% of obese study participants have relatively low levels of leptin, suggesting a reduced rate of leptin production in this subgroup.
Relative hypoleptinaemia has been reported in women with mild gestational diabetes mellitus. It has also been suggested that relatively low plasma leptin levels may play a role in the development of obesity in Pima Indians, a population prone to obesity.
A total absence of leptin is very rare and there are only two published reports of human leptin gene mutations which cause leptin deficiency A homozygous frame-shift mutation in the leptin gene has been identified in 2 severely obese children from the same highly consanguineous pedigree, while a missense mutation in the leptin gene has been found in 3 members of a Turkish kindred who are extremely obese. Mutations in the leptin receptor are equally rare in humans. A homozygous mutation in the leptin receptor gene that results in a truncated leptin receptor has been reported in 3 morbidly obese members of a French family.
The associations of these mutations with obesity confirms the importance of leptin in regulating energy balance in humans However, their rarity means that the pathogenesis of obesity in most patients is not due to mutations in the leptin or leptin receptor genes.
The capacity for leptin transport (based on CSF/serum leptin ratios) appears to be lower in obese human individuals, suggesting that an abnormality in leptin metabolism proximal to the hypothalamic leptin receptor may cause the leptin insensitivity commonly seen in obese humans Alternatively, leptin insensitivity may be due to abnormalities in the signalling cascade or transducer system downstream of the leptin receptor
An increasing number of factors (such as melanocortin receptor defects or the presence of corticosteroids) have been shown to induce leptin resistance in animals.[38,39] Whether these play a significant role in the aetiology of human obesity remains to be determined, although mutations in the melanocortin-4 receptor have recently been associated with dominantly inherited morbid obesity in humans in several families. It is possible that hyperleptinaemia itself induces leptin insensitivity since peripheral leptin administration for 48 hours causes a several-fold increase in mRNA encoding the suppressor of cytokine signalling 3 (SOCS-3) and cytokine inducible sequence (CIS) in hypothalamus and peripheral tissues. Both SOCS-3 and CIS can inhibit leptin signalling. In addition, overnight exposure to leptin down-regulates all isoforms of the human leptin receptor in cell culture.
2. Therapy with Leptin
2.1 Clinical Trials of Leptin Treatment for Obesity
2.1.1 Hypoleptinaemic Patients
Replacing leptin in a leptin-deficient child has been shown to ameliorate hyperphagia and promote bodyweight loss while preserving lean mass. Total energy expenditure was unchanged; thus, the therapeutic effects of leptin on bodyweight in this child were largely attributable to changes in energy intake. In the small subgroup of obese patients who have leptin deficiency, leptin treatment is therefore likely to prove very beneficial.
2.1.2 Hyperleptinaemic Patients
But what effect does leptin treatment have in the majority of obese patients, those who are hyperleptinaemic? Three years after the discovery of leptin, a multicentre clinical trial using recombinant methionyl human leptin (a synthesised form of the natural human hormone, using the DNA of leptin) was begun in the US. The results of this trial have recently been published. 73 moderately obese study participants (with a body mass index between 27.6 and 36.0 kg/m2) were given daily subcutaneous injections of placebo or 1 of 4 leptin dosages, ranging from 0.01 to 0.30 mg/kg/day, for 24 weeks. All the patients were also placed on a mildly hypocaloric diet (500 kcal/day deficit).
After 4 weeks of leptin treatment, 10 patients had dropped out of the study; 8 of these had been receiving the highest dosage of leptin. By 24 weeks, 36% of the patients had dropped out of the study. This high dropout rate may have been due to the inconvenience of daily injections or skin reactions at the injection site, which were a common adverse effect.
Of the obese participants who completed the study, 8 were taking the highest dosage of leptin and these patients lost an average of 7.1kg. Leptin did not appear to cause any clinically significant adverse effects on major organ systems according to the incidence of adverse events, physical examinations, laboratory values, electrocardiograms and vital signs. The drug did not alter serum glucose levels or insulin profiles.
Although leptin treatment induces only a small amount of bodyweight loss in obese patients who are not leptin-deficient, this amount of bodyweight loss may still be clinically relevant since even small amounts (5% of total bodyweight) diminish the health risk of obesity. Smaller and more soluble leptin analogues are being developed and these may have greater therapeutic potential in the treatment of obesity.
2.2 Likely Role of Leptin in Obesity Management
The major role of leptin may be to signal starvation and protect against bodyweight loss. Several studies have shown large rapid falls in leptin levels associated with even modest bodyweight loss.[45,46] This decrease in leptin levels is much greater than that expected from the loss of fat mass. Keim et al. have shown that adiposity-adjusted leptin levels decreased by 54% while fat mass only decreased by 4% after 1 week of a moderate energy deficit in women. Leptin levels remained low during the 12-week intervention period.
The low leptin levels were associated with self-reported hunger, desire to eat and prospective consumption. The greatest hunger increase coincided with the largest percentage drop in circulating leptin levels and the lowest final leptin level. Interestingly, the relationship between leptin and hunger was not influenced by the amount of bodyweight or body fat loss.
Decreased adipose tissue glucose metabolism or increased lipolysis may be responsible for this decreased leptin secretion in response to fasting.[47,48] Others have shown that this decrease in leptin correlates with changes in glycaemia whereas, in men with type 2 diabetes mellitus (NIDDM), the reduction in leptin levels was directly associated with reductions in serum triglyceride and cholesterol levels, independently of improvements in glycemic control.
Whatever the cause, the fall in leptin levels was associated with an induced drive to eat which is considered likely to be responsible for the almost universal bodyweight regain after dieting is attempted. Under these conditions it is possible that leptin treatment may have a very beneficial effect. When used in combination with calorie restriction, leptin therapy may eliminate both the fall in leptin levels and the increase in hunger which is normally observed. If this is the case, leptin may have a very useful role in the maintenance of bodyweight loss.
There is an urgent need to develop more effective treatments for obesity. Leptin treatment alone does not induce dramatic weight loss in most obese humans. However, leptin treatment may have the capacity to maintain weight loss by preventing the large fall in circulating leptin levels (and consequent increase in hunger) observed after dieting. In other words, leptin drug therapy may be able to overcome the defensive mechanisms the body puts into place to prevent weight loss. If this proves to be the case, leptin therapy may prove to be of great clinical importance.
Anne Thorburn is supported by the National Health and Medical Research Council of Australia. Deborah Ainslie is the recipient of a Melbourne University Research Scholarship. Barbara Fam is the recipient of an Australian Postgraduate Award.
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