Design of a synthetic leptin agonist: effects on energy balance, glucose homeostasis, and thermoregulation

Assessing the Safety of MA-[D-Leu-4]-OB3, a Synthetic Peptide Leptin Mimetic: Two Pre-Clinical Toxicity Studies in Male and Female C57BL/6 Mice

Although the regulatory influence of leptin on energy balance, glycemic control, immunity, reproduction, and cognition is well established, its clinical application to common obesity and its co-morbidities has been limited by impaired transport across the blood-brain barrier, and tendencies to induce adverse side effects. To circumvent these drawbacks, MA-[D-Leu-4]-OB3, a leptin-related synthetic peptide that mimics the metabolic and neurotrophic effects of leptin in mouse models of genetic and non-genetic obesity, diabetes, and cognitive dysfunction, has been developed. This report presents the results of our initial efforts to assess the safety of orally delivered MA-[D-Leu-4]-OB3. Two pre-clinical studies were done in male and female C57BL/6 mice: a short-term study with a high dose of MA-[D-Leu-4]-OB3 (50 mg/kg/100 μL/day) and a dose-response study with 3 increasing concentrations of MA-[D-Leu-4]-OB3 (16.6, 50, and 150 mg/kg/100 μL/day). Body weight, food and water intake, glucose tolerance, and episodic memory were evaluated. Once-daily cage-side clinical observations were conducted to detect any physical or behavioral indicators of toxicity. Our results indicate that all metabolic and neurologic endpoints tested were either unaffected or improved by MA-[D-Leu-4]-OB3, and no clinical indicators of toxicity were evident. Together with our previously reported efficacy data, these results provide additional evidence supporting further development of this novel synthetic peptide leptin mimetic as a first-in-class peptide drug candidate for the treatment of a number of metabolic and/or cognitive dysfunctions in humans.

Harnessing the Power of Leptin: The Biochemical Link Connecting Obesity, Diabetes, and Cognitive Decline

In this review, the current understanding of leptin’s role in energy balance, glycemic regulation, and cognitive function is examined, and its involvement in maintaining the homeostatic “harmony” of these physiologies is explored. The effects of exercise on circulating leptin levels are summarized, and the results of clinical application of leptin to metabolic disease and neurologic dysfunction are reviewed. Finally, pre-clinical evidence is presented which suggests that synthetic peptide leptin mimetics may be useful in resolving not only the leptin resistance associated with common obesity and other elements of metabolic syndrome, but also the peripheral insulin resistance characterizing type 2 diabetes mellitus, and the central insulin resistance associated with certain neurologic deficits in humans.

Polycystic οvary syndrome revisited: An interactions network approach

BACKGROUND

The polycystic ovary syndrome (PCOS) has genetic, epigenetic, metabolic and reproductive aspects, while its complex pathophysiology has not been conclusively deciphered.

AIM

The goal of this research was to screen the gene/gene products associated with PCOS and to predict any possible interactions with the highest possible fidelity.

MATERIALS AND METHODS

STRING v10.5 database and a confidence level of 0.7 were used.

RESULTS

A highly interconnected network of 48 nodes was created, where insulin (INS) appears to be the major hub. INS upstream and downstream defects were analysed and revealed that only the kisspeptin- and glucagon-coding genes were upstream of INS.

CONCLUSION

A metabolic dominance was inferred and discussed herein with its implications in puberty, obesity, infertility and cardiovascular function. This study, thus, may contribute to the resolution of a scientific conflict between the USA and EU definitions of the syndrome and/or provide a new P4 medicine approach.

Leptin regulation of hippocampal synaptic function in health and disease

It is widely accepted that the metabolic hormone leptin regulates food intake and body weight via activation of hypothalamic leptin receptors. However, as leptin receptors are also highly expressed in other brain regions, such as the hippocampus, alterations in leptin responsiveness also impacts on key functions of the hippocampus, like learning and memory. Within the hippocampus, high levels of leptin receptors are expressed at excitatory synapses, and in accordance with a synaptic localization, leptin potently regulates synaptic transmission at both Schaffer collateral (SC) and temporoammonic (TA) inputs to CA1 pyramidal neurons. Increasing evidence from cellular and behavioral studies examining leptin action at CA1 synapses support the notion that leptin is a potential cognitive enhancer. However, the capacity of leptin to regulate synaptic efficacy at SC-CA1 and TA-CA1 synapses declines in an age-dependent manner. Moreover, clinical evidence that supports a link between circulating leptin levels and the risk of the age-related neurodegenerative disorder, Alzheimer's disease (AD) is accumulating. Consequently, it has been proposed that the leptin system is a potential therapeutic target in AD, and that boosting the hippocampal actions of leptin may be beneficial in the treatment of AD. Here we review recent progress in our understanding of the neuronal and hippocampal synaptic functions that are regulated by leptin and how alterations in the leptin system influence age-related CNS-related disorders like AD.

The Neuronal Actions of Leptin and the Implications for Treating Alzheimer's Disease

It is widely accepted that the endocrine hormone leptin controls food intake and energy homeostasis via activation of leptin receptors expressed on hypothalamic arcuate neurons. The hippocampal formation also displays raised levels of leptin receptor expression and accumulating evidence indicates that leptin has a significant impact on hippocampal synaptic function. Thus, cellular and behavioural studies support a cognitive enhancing role for leptin as excitatory synaptic transmission, synaptic plasticity and glutamate receptor trafficking at hippocampal Schaffer collateral (SC)-CA1 synapses are regulated by leptin, and treatment with leptin enhances performance in hippocampus-dependent memory tasks. Recent studies indicate that hippocampal temporoammonic (TA)-CA1 synapses are also a key target for leptin. The ability of leptin to regulate TA-CA1 synapses has important functional consequences as TA-CA1 synapses are implicated in spatial and episodic memory processes. Moreover, degeneration is initiated in the TA pathway at very early stages of Alzheimer's disease, and recent clinical evidence has revealed links between plasma leptin levels and the incidence of Alzheimer's disease (AD). Additionally, accumulating evidence indicates that leptin has neuroprotective actions in various AD models, whereas dysfunctions in the leptin system accelerate AD pathogenesis. Here, we review the data implicating the leptin system as a potential novel target for AD, and the evidence that boosting the hippocampal actions of leptin may be beneficial.

Potential Leptin Receptor Response Modifier Peptides

Drug targets for the treatment of obesity and comorbidities represent an ever-renewable source of research opportunities worldwide. One of the earliest is the leptin–leptin receptor system that was discovered in the mid-1990s. Leptin, a satiety hormone, is overproduced in overweight patients but the protein is unable to cross the blood–brain barrier and remains inactive. Circulating high levels of leptin induces a series of conditions that would not be manifested without leptin overproduction, including various forms of cancer and inflammatory and cardiovascular diseases. Current pharmaceutical research focuses on improving the blood–brain barrier penetration of leptin receptor agonists and the development of monofunctional antagonists with broad spectrum therapeutic efficacies but without unwanted side effects. Designer peptides with their expanded chemical space as well as well controllable receptor binding and elimination properties slowly replace full-sized leptin products in the drug development pipeline.

Regulation of Hippocampal Synaptic Function by the Metabolic Hormone, Leptin: Implications for Health and Neurodegenerative Disease

The role of the endocrine hormone leptin in controlling energy homeostasis in the hypothalamus are well documented. However the CNS targets for leptin are not restricted to the hypothalamus as a high density of leptin receptors are also expressed in several parts of the brain involved in higher cognitive functions including the hippocampus. Numerous studies have identified that in the hippocampus, leptin has cognitive enhancing actions as exogenous application of this hormone facilitates hippocampal-dependent learning and memory, whereas lack or insensitivity to leptin results in significant memory deficits. Leptin also markedly influences some of the main cellular changes that are involved in learning and memory including NMDA-receptor dependent synaptic plasticity and glutamate receptor trafficking. Like other metabolic hormones, there is a significant decline in neuronal sensitivity to leptin during the ageing process. Indeed, the capacity of leptin to modulate the functioning of hippocampal synapses is substantially reduced in aged compared to adult tissue. Clinical studies have also identified an association between circulating leptin levels and the risk of certain neurodegenerative disorders such as Alzheimer’s disease (AD). In view of this, targeting leptin and/or its receptor/signaling mechanisms may be an innovative approach for developing therapies to treat AD. In support of this, accumulating evidence indicates that leptin has cognitive enhancing and neuroprotective actions in various models of AD. Here we assess recent evidence that supports an important regulatory role for leptin at hippocampal CA1 synapses, and we discuss how age-related alterations in this hormonal system influences neurodegenerative disease.

Evidence for a Coupled Oscillator Model of Endocrine Ultradian Rhythms

Whereas long-period temporal structures in endocrine dynamics have been well studied, endocrine rhythms on the scale of hours are relatively unexplored. The study of these ultradian rhythms (URs) has remained nascent, in part, because a theoretical framework unifying ultradian patterns across systems has not been established. The present overview proposes a conceptual coupled oscillator network model of URs in which oscillating hormonal outputs, or nodes, are connected by edges representing the strength of node-node coupling. We propose that variable-strength coupling exists both within and across classic hormonal axes. Because coupled oscillators synchronize, such a model implies that changes across hormonal systems could be inferred by surveying accessible nodes in the network. This implication would at once simplify the study of URs and open new avenues of exploration into conditions affecting coupling. In support of this proposed framework, we review mammalian evidence for (1) URs of the gut-brain axis and the hypothalamo-pituitary-thyroid, -adrenal, and -gonadal axes, (2) UR coupling within and across these axes; and (3) the relation of these URs to body temperature. URs across these systems exhibit behavior broadly consistent with a coupled oscillator network, maintaining both consistent URs and coupling within and across axes. This model may aid the exploration of mammalian physiology at high temporal resolution and improve the understanding of endocrine system dynamics within individuals.

A Leptin Fragment Mirrors the Cognitive Enhancing and Neuroprotective Actions of Leptin

A key pathology of Alzheimer's disease (AD) is amyloid β (Aβ) accumulation that triggers synaptic impairments and neuronal death. Metabolic disruption is common in AD and recent evidence implicates impaired leptin function in AD. Thus the leptin system may be a novel therapeutic target in AD. Indeed, leptin has cognitive enhancing properties and it prevents the aberrant effects of Aβ on hippocampal synaptic function and neuronal viability. However, as leptin is a large peptide, development of smaller leptin-mimetics may be the best therapeutic approach. Thus, we have examined the cognitive enhancing and neuroprotective properties of known bioactive leptin fragments. Here we show that the leptin (116-130) fragment, but not leptin (22-56), mirrored the ability of leptin to promote AMPA receptor trafficking to synapses and facilitate activity-dependent hippocampal synaptic plasticity. Administration of leptin (116-130) also mirrored the cognitive enhancing effects of leptin as it enhanced performance in episodic-like memory tests. Moreover, leptin (116-130) prevented hippocampal synaptic disruption and neuronal cell death in models of amyloid toxicity. These findings establish further the importance of the leptin system as a therapeutic target in AD.

[D-Leu-4]-OB3 and MA-[D-Leu-4]-OB3, small molecule synthetic peptide leptin mimetics, improve glycemic control in diet-induced obese (DIO) mice

We have previously shown that following oral delivery in dodecyl maltoside (DDM), [D-Leu-4]-OB3 and its myristic acid conjugate, MA-[D-Leu-4]-OB3, improved energy balance and glucose homeostasis in genetically obese/diabetic mouse models. More recently, we have provided immunohistochemical evidence indicating that these synthetic peptide leptin mimetics cross the blood-brain barrier and concentrate in the area of the arcuate nucleus of the hypothalamus in normal C57BL/6J and Swiss Webster mice, in genetically obese ob/ob mice, and in diet-induced obese (DIO) mice. In the present study, we describe the effects of oral delivery of [D-Leu-4]-OB3 and MA-[D-Leu-4]-OB3 on glycemic control in diet-induced (DIO) mice, a non-genetic rodent model of obesity and its associated insulin resistance, which more closely recapitulates common obesity and diabetes in humans. Male C57BL/6J and DIO mice, 17, 20, and 28 weeks of age, were maintained on a low-fat or high-fat diet and given vehicle (DDM) alone or [D-Leu-4]-OB3 or MA-[D-Leu-4]-OB3 in DDM by oral gavage for 12 or 14 days. Body weight gain, food and water intake, fasting blood glucose, oral glucose tolerance, and serum insulin levels were measured. Our data indicate that (1) [D-Leu-4]-OB3 and MA-[D-Leu-4]-OB3 restore glucose tolerance in male DIO mice maintained on a high-fat diet to levels comparable to those of non-obese C57BL/6J wild-type mice of the same age and sex maintained on a low-fat diet; and (2) the influence of [D-Leu-4]-OB3 and MA-[D-Leu-4]-OB3 on glycemic control appears to be independent of their effects on energy balance. These results suggest that [D-Leu-4]-OB3 and/or MA-[D-Leu-4]-OB3 may have application to the management of the majority of cases of common obesity in humans, a state characterized at least in part, by leptin resistance resulting from a defect in leptin transport across the blood-brain barrier. They further suggest that these small molecule synthetic peptide leptin mimetics, through their influence on glycemic control, may prevent the pre-diabetic state associated with most cases of common obesity from escalating into overt type 2 diabetes mellitus.

Novel leptin OB3 peptide-induced signaling and progression in thyroid cancers: Comparison with leptin

Obesity results in increased secretion of cytokines from adipose tissue and is a risk factor for various cancers. Leptin is largely produced by adipose tissue and cancer cells. It induces cell proliferation and may serve to induce various cancers. OB3-leptin peptide (OB3) is a new class of functional leptin peptide. However, its mitogenic effect has not been determined. In the present study, because of a close link between leptin and the hypothalamic-pituitary-thyroid axis, OB3 was compared with leptin in different thyroid cancer cells for gene expression, proliferation and invasion. Neither agent stimulated cell proliferation. Leptin stimulated cell invasion, but reduced adhesion in anaplastic thyroid cancer cells. Activated ERK1/2 and STAT3 contributed to leptin-induced invasion. In contrast, OB3 did not affect expression of genes involved in proliferation and invasion. In vivo studies in the mouse showed that leptin, but not OB3, significantly increased circulating levels of thyrotropin (TSH), a growth factor for thyroid cancer. In summary, OB3 is a derivative of leptin that importantly lacks the mitogenic effects of leptin on thyroid cancer cells.

Leptin OB3 peptide suppresses leptin-induced signaling and progression in ovarian cancer cells

BACKGROUND

Obesity and its comorbidities constitute a serious health burden worldwide. Leptin plays an important role in diet control; however, it has a stimulatory potential on cancer cell proliferation. The OB3 peptide, a synthetic peptide, was shown to be more active than leptin in regulating metabolism but with no mitogenic effects in cancer cells.

METHODS

In this study, we investigated the proliferative effects, gene expressions and signaling pathways modulated by leptin and OB3 in human ovarian cancer cells. In addition, an animal study was performed.

RESULTS

Leptin, but not OB3, induced the proliferation of ovarian cancer cells. Interestingly, OB3 blocked the leptin-induced proliferative effect when it was co-applied with leptin. Both leptin and OB3 activated the phosphatidylinositol-3-kinase (PI3K) signal transduction pathway. In addition, leptin stimulated the phosphorylation of signal transducer and activator of transcription-3 (STAT3) Tyr-705 as well as estrogen receptor (ER)α, and the expression of ERα-responsive genes. Interestingly, all leptin-induced signal activation and gene expressions were blocked by the co-incubation with OB3 and the inhibition of extracellular signal-regulated kinase (ERK)1/2. Coincidently, leptin, but not OB3, increased circulating levels of follicle-stimulating hormone (FSH) which is known to play important roles in the initiation and proliferation of ovarian cancer cells.

CONCLUSIONS

In summary, our findings suggest that the OB3 peptide may prevent leptin-induced ovarian cancer initiation and progression by disrupting leptin-induced proliferative signals via STAT3 phosphorylation and ERα activation. Therefore, the OB3 peptide is a potential anticancer agent that might be employed to prevent leptin-induced cancers in obese people.

Myristic acid conjugation of [D-Leu-4]-OB3, a biologically active leptin-related synthetic peptide amide, significantly improves its pharmacokinetic profile and efficacy

We have previously described the pharmacokinetics of mouse [D-Leu-4]-OB3, a synthetic peptide amide with leptin-like activity, following delivery by subcutaneous (SC), intraperitoneal (IP), and intramuscular (IM) injection, and by oral gavage and intranasal instillation. These profiles suggested that the observed efficacy of [D-Leu-4]-OB3 on energy balance, glycemic control, and bone turnover in ob/ob and db/db mice might be improved by efforts directed toward improving its bioavailability, i.e., increasing maximum uptake (Cmax), extending serum half-life (t½), and reducing plasma clearance (CL). To address these issues, myristic (tetradecanoic) acid was conjugated to the N-terminal of [D-Leu-4]-OB3 (designated MA-[D-Leu-4]-OB3), and the pharmacokinetics of MA-[D-Leu-4]-OB3 in male Swiss Webster mice following SC, IP, and IM injection in PBS, and by oral and intranasal delivery in dodecyl maltoside (DDM, trade name Intravail®), a transmucosal absorption enhancing agent, were compared to those of [D-Leu-4]-OB. At a dose of MA-[D-Leu-4]-OB3 10-fold lower than that used previously for [D-Leu-4]-OB3 (0.1 mg vs.1.0 mg, respectively), Cmax of MA-[D-Leu-4]-OB3 was 11.1-, 7.5-, 1.9-, and 1.7-fold higher, t1/2 was 3.5-, 5.0-, 9.1-, and 86.7-fold longer, and CL was 17.0-, 11.6-, 5.7-, and 5.0-fold slower than [D-Leu-4]-OB3 following SC, IP, IM, and oral delivery, respectively. Furthermore, in leptin-resistant obese male db/db mice, oral delivery of MA-[D-Leu-4]-OB3 in DDM at concentrations up to 10-fold lower than those used with [D-Leu-4]-OB3 reduced fasting blood glucose levels in a dose-related manner.

The central question of type 2 diabetes

Type 2 diabetes (T2D) represents a significant global epidemic with more than 285 million people affected worldwide. Regulating glycemia in T2D patients can be partially achieved with currently available treatment, but intensive research during the last decades have led to the discovery of modified compounds or new targets that could represent great hope for safe and effective treatment in the future. Among them, targets in the CNS that are known to control feeding and body weight have been also shown to exert glucoregulatory actions, and could be a key in the development of a new generation of drugs in the field of T2D. Such drugs would be of great interest since they can be used both in the treatment of diabetes and obesity. This patent review aims to establish an overview of recent patents disclosing new therapeutic opportunities targeting peripheral, as well as central targets for the treatment of T2D.

Biologically active leptin-related synthetic peptides activate STAT3 via phosphorylation of ERK1/2 and PI-3K

The effects of leptin-related synthetic peptides [d-Leu-4]-OB3 and OB3 on energy balance and glucose homeostasis in ob/ob and db/db mice have been confirmed. The molecular basis of these effects, however, remains unclear. In the present study, we examined the ability of these peptides to activate signal transduction pathways known to be involved in transduction of the leptin signal. In a specific and concentration-dependent manner, [d-Leu-4]-OB3 induced phosphorylation of ERK1/2, PI-3K, Ser-727 STAT3, and Tyr-705 of STAT3. OB3 also induced activation of STAT3 via phosphorylation of ERK1/2, STAT3 Ser-727, STAT3 Tyr-705 and PI-3K p85, but to a lesser degree. Using PD98059 and LY294002, specific inhibitors of MEK and PI-3K, respectively, we were able to identify the signal transduction pathways involved in peptide-induced STAT3 activation. [d-Leu-4]-OB3 induced serine phosphorylation of STAT3 primarily through activation of ERK1/2. Tyrosine phosphorylation of STAT3, however, was induced primarily through activation of PI-3K. Our data suggest that in db/db mice, [d-Leu-4]-OB3 binding to short isoforms of the leptin receptor induces intracellular signaling cascades which do not require OB-Rb activation. These signals may ultimately result in peptide effects on transcriptional and translational events associated with energy balance and glycemic regulation. In summary, we have shown for the first time that, similar to leptin, bioactive leptin-related synthetic peptide analogs activate STAT3 via phosphorylation of serine and tyrosine residues by multiple signal transduction pathways.

[D-Leu-4]-OB3, an orally bioavailable leptin-related synthetic peptide insulin sensitizer: a study comparing the efficacies of [D-Leu-4]-OB3 and metformin on energy balance and glycemic regulation in insulin-deficient male Swiss Webster mice

The effects of oral delivery of exenatide or pramlintide acetate in dodecyl maltoside (DDM) on energy balance and glycemic control in insulin-resistant obese db/db mice are enhanced when given in combination with [D-Leu-4]-OB3. To examine the anti-hyperglycemic influence of [D-Leu-4]-OB3 in a non-obese insulin-deficient animal model, we compared the effects of metformin (200mg/kg) and [D-Leu-4]-OB3 (40 mg/kg) on energy balance and glycemic control in streptozotocin (STZ)-induced diabetic male Swiss Webster (SW) mice. Diabetic mice were given insulin (Levemir(®), sc) alone, or in combination with metformin or [D-Leu-4]-OB3 orally in DDM, for 14 days. Body weight and food and water intake were measured daily. Fasting blood glucose levels were determined every other day. Serum C-peptide was measured by ELISA. Diabetic mice receiving insulin alone for 14 days gained significantly more weight than DDM-treated control mice, or mice given insulin in combination with metformin or [D-Leu-4]-OB3. The weight gain seen in mice given insulin alone was accompanied by significant increases in both food and water intake. Mice treated with insulin in combination with metformin or [D-Leu-4]-OB3, consumed significantly less food and water. Blood glucose levels in STZ-treated mice receiving insulin alone were reduced to 65.3% of initial levels, while mice receiving insulin with metformin or [D-Leu-4]-OB3 were reduced to 44.5% and 38.9%, respectively. Our results indicate that [D-Leu-4]-OB3 is as effective as metformin in preventing the body weight gain associated with insulin therapy, and on a molar basis, that the efficacy of [D-Leu-4]-OB3 as an insulin sensitizer may equal or surpass that of metformin.

Leptin-based therapeutics

Leptin, a pluripotent adipokine, has been discovered as a hormone controlling energy balance in hypothalamic neuroendocrine centers. In addition, recent studies provided ample evidence that leptin can be produced by cells other than adipocytes, and that the hormone can regulate many physiological processes other than energy balance and appetite. In this context, it is not surprising that both leptin excess as well as leptin insufficiency have been implicated in various pathologies. Consequently, despite initially disappointing results with recombinant leptin as the drug for obesity management, new leptin receptor modifiers have been developed and emerged as potential treatment modalities for numerous metabolic, immunological and neoplastic diseases. The major focus of this paper is a systematic review of current experimental leptin-based therapies, including pharmacological advantages and limitations of each prodrug category.

Intranasal delivery of mouse [D-Leu-4]-OB3, a synthetic peptide amide with leptin-like activity, improves energy balance, glycaemic control, insulin sensitivity and bone formation in leptin-resistant C57BLK/6-m db/db mice

BACKGROUND

We have recently shown that intranasal administration of mouse [D-Leu-4]-OB3 reconstituted in Intravail(®) to male Swiss Webster mice resulted in significantly higher uptake and bioavailability when compared with commonly used injection methods of delivery.

AIM AND METHODS

In this study, we examined the effects of intranasal delivery of mouse [D-Leu-4]-OB3 in Intravail(®) on energy balance, glucose regulation, insulin secretion and serum levels of osteocalcin, a specific and sensitive marker of bone formation. Genetically obese C57BLK/6-m db/db mice were allowed food and water ad libitum and given either Intravail(®) alone or mouse [D-Leu-4]-OB3 in Intravail(®) for 14 days by intranasal instillation.

RESULTS

Mouse [D-Leu-4]-OB3 reduced body weight gain, daily food intake, daily water intake and serum glucose by 11.5, 2.2, 4.0 and 61.9%, respectively. Serum insulin levels in db/db mice given mouse [D-Leu-4]-OB3 were approximately threefold lower than those in mice receiving Intravail(®) alone. Mouse [D-Leu-4]-OB3 elevated serum osteocalcin in db/db mice by 28.7% over Intravail(®) treated control mice.

CONCLUSIONS

The results of our study indicate that intranasal delivery of biologically active mouse [D-Leu-4]-OB3 in Intravail(®) is feasible and has significant effects on regulating body weight gain, food and water intake, serum glucose, insulin sensitivity and bone formation in leptin-resistant C57BLK/6-m db/db mice.

Leptin as a potential therapeutic target for breast cancer prevention and treatment

IMPORTANCE OF THE FIELD

Obesity is considered to be an important risk factor for postmenopausal breast cancer. Elevated estrogen levels are thought to be a growth factor associated with this relationship. However, there is increasing evidence that factors produced directly in adipose tissue, adipokines, can also affect breast cancer development. Leptin is one of the adipokines that is measured in serum/plasma in increasing amounts as body weight/body fat increases.

AREAS COVERED IN THIS REVIEW

We highlight important aspects of leptin in relationship to mammary/breast tumor development. This includes findings from human, in vitro and animal studies. Information on leptin-related compounds which may have therapeutic use is presented. Additionally strategies to alter serum leptin levels by dietary and pharmacological interventions are discussed.

WHAT THE READER WILL GAIN

The reader will gain insights into the relationship of an adipose tissue protein and its potential role in breast cancer development as well as ways to intervene in leptin's actions.

TAKE HOME MESSAGE

Continued research will determine if interfering with the action of leptin has preventive or therapeutic applications in breast cancer.

Oral delivery of mouse [d-Leu-4]-OB3, a synthetic peptide amide with leptin-like activity, in male C57BL/6J wild-type and ob/ob mice: effects on energy balance, glycaemic control and serum osteocalcin levels

BACKGROUND

We have recently shown that intranasal administration of mouse [d-Leu-4]-OB3 reconstituted in Intravail to male Swiss Webster mice resulted in significantly higher bioavailability than commonly used injections methods of delivery. The absorption profile associated with intranasal delivery of mouse [d-Leu-4]-OB3 showed an early peak representing absorption across the nasal mucosa, and a later peak suggesting a gastrointestinal site of uptake.

AIM AND METHODS

In the present study, we examined the effects of orally administered (by gavage) mouse [d-Leu-4]-OB3 on energy balance, glycaemic control and serum osteocalcin levels in male C57BL/6J wild-type and ob/ob mice allowed food and water ad libitum or calorie restricted by 40% of normal intake.

RESULTS

In wild-type mice fed ad libitum, oral delivery of mouse [d-Leu-4]-OB3 reduced body weight gain, food intake and serum glucose, by 4.4, 6.8 and 28.2% respectively. Serum osteocalcin levels and water intake were essentially the same in control and treated wild-type mice. In ob/ob mice fed ad libitum, mouse [d-Leu-4]-OB3 reduced body weight gain, food intake, water intake and serum glucose by 11.6, 16.5, 22.4 and 24.4% respectively. Serum osteocalcin in ob/ob mice treated with mouse [d-Leu-4]-OB3 was elevated by 62% over controls. Calorie restriction alone caused significant weight loss in both wild-type (9.0%) and ob/ob (4.8%) mice, and mouse [d-Leu-4]-OB3 did not further enhance this weight loss. As expected, serum glucose levels in wild-type and ob/ob mice were significantly reduced by calorie restriction alone. Mouse [d-Leu-4]-OB3 further reduced serum glucose in wild-type mice and normalized levels in ob/ob mice. Calorie restriction alone reduced serum osteocalcin levels by 44.2% in wild-type mice and by 19.1% in ob/ob mice. Mouse [d-Leu-4]-OB3 prevented this decrease in groups of mice.

CONCLUSIONS

The results of this study suggest that oral delivery of mouse [d-Leu-4]-OB3 in Intravail is possible and may have potential not only as an alternative therapy in the treatment of human obesity and some of its associated metabolic dysfunctions, but also may help to prevent and/or reverse at least some of the bone loss which accompanies osteoporosis, anorexia nervosa and other wasting diseases.

Development of a pharmacologically improved peptide agonist of the leptin receptor

Leptin, a hormone produced by adipose tissue, regulates energy balance in the hypothalamus and is involved in fertility, immune response and carcinogenesis. The existence of disorders related to leptin deficit and leptin overabundance calls for the development of drugs activating or inhibiting the leptin receptor (ObR). We synthesized four proposed receptor-binding leptin fragments (sites I, IIa and IIb, III), their reportedly antagonist analogs, and a peptide chimera composed of the two discontinuous site II arms. To assess the pharmacological utility of leptin fragments, we studied the peptides' ability to stimulate the growth of ObR-positive and ObR-negative cells. The combined site II construct and site III derivatives selectively reversed leptin-induced growth of ObR-positive cells at mid-nanomolar concentrations. However, these peptides appeared to be partial agonists/antagonists as they activated cell growth in the absence of exogenous leptin. A designer site III analog, featuring non-natural amino acids at terminal positions to decrease proteolysis and a blood-brain barrier (BBB) penetration-enhancing carbohydrate moiety, proved to be full agonist to ObR, i.e., stimulated proliferation of different ObR-positive but not ObR-negative cells in the presence or absence of leptin. This glycopeptide bound to isolated ObR on solid-phase assays and activated ERK-1/2 signaling in ObR-positive MCF-7 cells at 100-500 nM concentrations. The glycopeptide was stable in mouse serum, readily crossed endothelial/astrocyte cell layers in a cellular BBB model, and was distributed into the brain of Balb/c mice after intraperitoneal administration. These characteristics suggest a potential pharmaceutical utility of the designer site III glycopeptide in leptin-deficient diseases.

Leptin as a Potential Treatment for Obesity

Despite significant reductions in the consumption of dietary fat, the prevalence of obesity is steadily rising in western civilization. Of particular concern is the recent epidemic of childhood obesity, which is expected to increase the incidence of obesity-related disorders. The obese gene (ob) protein product leptin is a hormone that is secreted from adipocytes and functions to suppress appetite and increase energy expenditure. Leptin is an attractive candidate for the treatment of obesity as it is an endogenous protein and has been demonstrated to have potent effects on bodyweight and adiposity in rodents. Whereas leptin has been successfully used in the treatment of leptin-deficient obese patients, trials in hyperleptinemic obese patients have yielded variable results. Long-acting leptins have been tried but with no greater success. Other strategies including the use of leptin analogs and other factors that bypass normal leptin delivery systems are being developed. Identifying the mechanisms at the molecular level by which leptin functions will create new avenues for pharmaceutical targeting to simulate the intracellular effects of leptin.

Pharmacological approaches for the treatment of obesity

The high incidence of obesity, its multifactorial nature, the complexity and lack of knowledge of the bodyweight control system, and the scarcity of adequate therapeutics have fuelled anti-obesity drug development during a considerable number of years. Irrespective of the efforts invested by researchers and companies, few products have reached a minimum level of effectiveness, and even fewer are available in medical practice. As a consequence of anti-obesity research, our knowledge of the bodyweight control system increased but, despite this, the pharmacological approaches to the treatment of obesity have not resulted yet in effective drugs. This review provides a panoramic of the multiple different approaches developed to obtain workable drugs. These approaches, however, rely in only four main lines of action: control of energy intake, mainly through modification of appetite;control of energy expenditure, essentially through the increase of thermogenesis;control of the availability of substrates to cells and tissues through hormonal and other metabolic factors controlling the fate of the available energy substrates; andcontrol of fat reserves through modulation of lipogenesis and lipolysis in white adipose tissue. A large proportion of current research is centred on neuropeptidic control of appetite, followed by the development of drugs controlling thermogenic mechanisms and analysis of the factors controlling adipocyte growth and fat storage. The adipocyte is also a fundamental source of metabolic signals, signals that can be intercepted, modulated and used to force the brain to adjust the mass of fat with the physiological means available. The large variety of different approaches used in the search for effective anti-obesity drugs show both the deep involvement of researchers on this field and the large amount of resources devoted to this problem by pharmaceutical companies. Future trends in anti-obesity drug research follow closely the approaches outlined; however, the increasing mass of information on the molecular basis of bodyweight control and obesity will in the end prevail in our search for effective and harmless anti-obesity drugs.