Evidence for ligand-independent homo-oligomerization of leptin receptor (OB-R) isoforms: A proposed mechanism permitting productive long-form signaling in the presence of excess short-form expression

Recent progress in leptin signaling from a structural perspective and its implications for diseases

As a multi-potency cytokine, leptin not only plays a crucial role in controlling weight and energy homeostasis but also participates in the metabolic balance in the human body. Leptin is a small helical protein with a molecular weight of 16 kDa. It can interact with multiple subtypes of its receptors to initiate intracellular signal transduction and exerts physiological effects. Disturbances in leptin signaling may lead to obesity and a variety of metabolic diseases. Leptin was also found to be a critical factor in many diseases of the elderly. In this review, we focus on recent advances in the structural and molecular mechanisms of leptin signaling through its receptors with the aim of a deeper understanding of leptin-related diseases.

Rare Antagonistic Leptin Variants and Severe, Early-Onset Obesity

Hormone absence or inactivity is common in congenital disease, but hormone antagonism remains controversial. Here, we characterize two novel homozygous leptin variants that yielded antagonistic proteins in two unrelated children with intense hyperphagia, severe obesity, and high circulating levels of leptin. Both variants bind to the leptin receptor but trigger marginal, if any, signaling. In the presence of nonvariant leptin, the variants act as competitive antagonists. Thus, treatment with recombinant leptin was initiated at high doses, which were gradually lowered. Both patients eventually attained near-normal weight. Antidrug antibodies developed in the patients, although they had no apparent effect on efficacy. No severe adverse events were observed. (Funded by the German Research Foundation and others.).

A novel leptin receptor antagonist uncouples leptin's metabolic and immune functions

Leptin links body energy stores to high energy demanding processes like reproduction and immunity. Based on leptin's role in autoimmune diseases and cancer, several leptin and leptin receptor (LR) antagonists have been developed, but these intrinsically lead to unwanted weight gain. Here, we report on the uncoupling of leptin's metabolic and immune functions based on the cross talk with the epidermal growth factor receptor (EGFR). We show that both receptors spontaneously interact and, remarkably, that this complex can partially overrule the lack of LR activation by a leptin antagonistic mutein. Moreover, this leptin mutant induces EGFR phosphorylation comparable to wild-type leptin. Exploiting this non-canonical leptin signalling pathway, we identified a camelid single-domain antibody that selectively inhibits this LR-EGFR cross talk without interfering with homotypic LR signalling. Administration in vivo showed that this single-domain antibody did not interfere with leptin's metabolic functions, but could reverse the leptin-driven protection against starvation-induced thymic and splenic atrophy. These findings offer new opportunities for the design and clinical application of selective leptin and LR antagonists that avoid unwanted metabolic side effects.

Uterine artery leptin receptors during the ovarian cycle and pregnancy regulate angiogenesis in ovine uterine artery endothelial cells†

Leptin regulates body weight, reproductive functions, blood pressure, endothelial function, and fetoplacental angiogenesis. Compared to the luteal phase, the follicular phase and pregnancy are physiological states of elevated estrogen, angiogenesis, and uterine blood flow (UBF). Little is known concerning regulation of uterine artery (UA) angiogenesis by leptin and its receptors. We hypothesized that (1) ex vivo expression of leptin receptors (LEPR) in UA endothelium (UAendo) and UA vascular smooth muscle (UAvsm) is elevated in pregnant versus nonpregnant (Luteal and Follicular) sheep; (2) in vitro leptin treatments differentially modulate mitogenesis in uterine artery endothelial cells from pregnant (P-UAECs) more than in nonpregnant (NP-UAECs) ewes; and (3) LEPR are upregulated in P-UAECs versus NP-UAECs in association with leptin activation of phospho-STAT3 signaling. Local UA adaptations were evaluated using a unilateral pregnant sheep model where prebreeding uterine horn isolation (nongravid) restricted gravidity to one horn. Immunolocalization revealed LEPR in UAendo and UAvsm from pregnant and nonpregnant sheep. Contrary to our hypothesis, western analysis revealed that follicular UAendo and UAvsm LEPR were greater than luteal, nongravid, gravid, and control pregnant. Compared to pregnant groups, LEPR were elevated in renal artery endothelium of follicular and luteal sheep. Leptin treatment significantly increased mitogenesis in follicular phase NP-UAECs and P-UAECs, but not luteal phase NP-UAECs. Although UAEC expression of LEPR was similar between groups, leptin treatment only activated phospho-STAT3 in follicular NP-UAECs and P-UAECs. Thus, leptin may play an angiogenic role particularly in preparation for the increased UBF during the periovulatory period and subsequently to meet the demands of the growing fetus.

The Leptin Receptor Complex: Heavier Than Expected?

Under normal physiological conditions, leptin and the leptin receptor (ObR) regulate the body weight by balancing food intake and energy expenditure. However, this adipocyte-derived hormone also directs peripheral processes, including immunity, reproduction, and bone metabolism. Leptin, therefore, can act as a metabolic switch connecting the body's nutritional status to high energy consuming processes. We provide an extensive overview of current structural insights on the leptin-ObR interface and ObR activation, coupling to signaling pathways and their negative regulation, and leptin functioning under normal and pathophysiological conditions (obesity, autoimmunity, cancer, … ). We also discuss possible cross-talk with other receptor systems on the receptor (extracellular) and signaling cascade (intracellular) levels.

Regulation of intestinal SGLT1 by catestatin in hyperleptinemic type 2 diabetic mice

The small intestine is the major site for nutrient absorption that is critical in maintenance of euglycemia. Leptin, a key hormone involved in energy homeostasis, directly affects nutrient transport across the intestinal epithelium. Catestatin (CST), a 21-amino acid peptide derived from proprotein chromogranin A, has been shown to modulate leptin signaling. Therefore, we reasoned that leptin and CST could modulate intestinal Na(+)-glucose transporter 1 (SGLT1) expression in the context of obesity and diabetes. We found that hyperleptinemic db/db mice exhibit increased mucosal mass, associated with an enhanced proliferative response and decreased apoptosis in intestinal crypts, a finding absent in leptin-deficient ob/ob mice. Intestinal SGLT1 abundance was significantly decreased in hyperleptinemic but not leptin-deficient mice, indicating leptin regulation of SGLT1 expression. Phlorizin, a SGLT1/2 inhibitor, was without effect in an oral glucose tolerance test in db/db mice. The alterations in architecture and SGLT1 abundance were not accompanied by changes in the localization of intestinal alkaline phosphatase, indicating intact differentiation. Treatment of db/db mice with CST restored intestinal SGLT1 abundance and intestinal turnover, suggesting a cross-talk between leptin and CST, without affecting plasma leptin levels. Consistent with this hypothesis, we identified structural homology between CST and the AB-loop of leptin and protein-protein docking revealed binding of CST and leptin with the Ig-like binding site-III of the leptin receptor. In summary, downregulation of SGLT1 in an obese type 2 diabetic mouse model with hyperleptinemia is presumably mediated via the short form of the leptin receptor and reduces overt hyperglycemia.

Leptin: From structural insights to the design of antagonists

After its discovery in 1994, it soon became clear that leptin acts as an adipocyte-derived hormone with a central role in the control of body weight and energy homeostasis. However, a growing body of evidence has revealed that leptin is a pleiotropic cytokine with activities on many peripheral cell types. Inappropriate leptin signaling can promote autoimmunity, certain cardiovascular diseases, elevated blood pressure and cancer, which makes leptin and the leptin receptor interesting targets for antagonism. Profound insights in the leptin receptor (LR) activation mechanisms are a prerequisite for the rational design of these antagonists. In this review, we focus on the molecular mechanisms underlying leptin receptor activation and signaling. We also discuss the current strategies to interfere with leptin signaling and their therapeutic potential.

The interleukin (IL)-31/IL-31R axis contributes to tumor growth in human follicular lymphoma

Interleukin (IL)-31A binds to an heterodimer composed of IL-31 receptor A (IL-31RA) and Oncostatin M Receptor (OSMR). The IL-31/IL-31R complex is involved in the pathogenesis of various skin diseases, including cutaneous T-cell lymphoma. No information is available on the relations between the IL-31/IL-31R complex and B-cell lymphoma. Here we have addressed this issue in follicular lymphoma (FL), a prototypic germinal center(GC)-derived B-cell malignancy. IL-31 enhanced primary FL cell proliferation through IL-31R-driven signal transducer and activator of transcription factor 1/3 (STAT1/3), extracellular signal-regulated kinase 1/2 (ERK1/2) and Akt phosphorylation. In contrast, GC B cells did not signal to IL-31 in spite of IL-31R expression. GC B cells expressed predominantly the inhibitory short IL-31RA isoform, whereas FL cells expressed predominantly the long signaling isoform. Moreover, GC B cells lacked expression of other IL-31RA isoforms potentially involved in the signaling pathway. IL-31 protein expression was significantly higher in surface membrane than in cytosol of both FL and GC B cells. IL-31 was detected in plasma membrane microvesicles from both cell types but not released in soluble form in culture supernatants. IL-31 and IL-31RA expression was higher in lymph nodes from FL patients with grade IIIa compared with grade I/II, suggesting a paracrine and/or autocrine role of IL-31/IL-31RA complex in tumor progression through microvesicle shedding.

20 years of leptin: insights into signaling assemblies of the leptin receptor

Leptin plays a central role in the control of body weight and energy homeostasis, but is a pleiotropic cytokine with activities on many peripheral cell types. In this review, we discuss the interaction of leptin with its receptor, and focus on the structural and mechanistic aspects of the extracellular aspects of leptin receptor (LR) activation. We provide an extensive overview of all structural information that has been obtained for leptin and its receptor via X-ray crystallography, electron microscopy, small-angle X-ray scattering, homology modeling, and mutagenesis studies. The available knowledge is integrated into putative models toward a recapitulation of the LR activation mechanism.

An updated view of leptin on implantation and pregnancy: a review

The hormone leptin, which is thought to be primarily produced by adipose tissue, is a polypeptide that was initially characterized by its ability to regulate food intake and energy metabolism. Leptin appears to signal the status of body energy stores to the brain, resulting in the regulation of food intake and whole-body energy expenditure. Subsequently, it was recognized as a cytokine with a wide range of peripheral actions and is involved in the regulation of a number of physiological systems including reproduction. In the fed state, leptin circulates in the plasma in proportion to body adiposity in all species studied to date. However other factors such as sex, age, body mass index (BMI), sex steroids and pregnancy may also affect leptin levels in plasma. In pregnant mice and humans, the placenta is also a major site of leptin expression. Leptin circulates in biological fluids both as free protein and in a form that is bound to the soluble isoform of its receptor or other binding proteins such as one of the immunoglobulin superfamily members Siglec-6 (OB-BP1). Although the actions of leptin in the control of reproductive function are thought to be exerted mainly via the hypothalamic-pituitary-gonadal axis, there have also been reports of local direct effects of leptin at the peripheral level, however, these data appear contradictory. Therefore, there is a need to summarize the current status of research outcomes and analyze the possible reasons for differing results and thus provide researchers with new insight in designing experiments to investigate leptin effect on reproduction. Most importantly, our recent experimental data suggesting that reproductive performance is improved by decreasing concentrations of peripheral leptin was unexpected and cannot be explained by hypotheses drawn from the experiments of excessive exogenous leptin administration to normal animals or ob/ob mice.

Quantitative FRET imaging of leptin receptor oligomerization kinetics in single cells

BACKGROUND INFORMATION

Leptin, an adipocyte-secreted hormone, signals through activation of its membrane-embedded receptor (LEPR). To study the leptin-induced events occurring in short (LEPRa) and long (LEPRb) LEPRs in the cell membrane, by FRET (fluorescence resonance energy transfer) methodology, the respective receptors, tagged at their C-terminal with CFP (cyan fluorescent protein) or YFP (yellow fluorescent protein), were prepared.

RESULTS

The constructs encoding mLEPRa (mouse LEPRa)-YFP and mLEPRa-CFP, mLEPRb-YFP and mLEPRb-CFP were tested for biological activity in transiently transfected CHO cells (Chinese-hamster ovary cells) and HEK-293T cells (human embryonic kidney 293 T cells) for activation of STAT3 (signal transduction and activators of transcription 3)-mediated LUC (luciferase) activity and binding of radiolabelled leptin. All four constructs were biologically active and were as potent as their untagged counterparts. The localization pattern of the fused protein appeared to be confined almost entirely to the cell membrane. The leptin-dependent interaction between various types of receptors in fixed cells were studied by measuring FRET, using fluorescence lifetime imaging microscopy and acceptor photobleaching methods.

CONCLUSIONS

Both methods yielded similar results, indicating that (1) leptin receptors expressed in the cell membrane exist mostly as preformed LEPRa/LEPRa or LEPRb/LEPRb homo-oligomers but not as LEPRb/LEPRa hetero-oligomers; (2) the appearance of transient leptin-induced FRET in cells transfected with LEPRb/LEPRb reflects both a conformational change that leads to closer interaction in the cytosolic part and a higher FRET signal, as well as de novo homo-oligomerization; (3) in LEPRa/LEPRa, exposure to leptin does not lead to any increase in FRET signalling as the proximity of CFP and YFP fluorophores in space already gives maximal FRET efficiency of the preoligomerized receptors.

Leptin and breast cancer: an overview

Leptin is a well-known mediator of obesity. Leptin and its receptor are overexpressed in breast cancer, especially in high-grade tumors. It has an association with progression and poor survival of breast cancer. Leptin can regulate endothelial cell proliferation and promote angiogenesis. There are several other factors such as insulin and HER2 may be involved in the relationship between leptin and breast cancer. Leptin system has emerged as a new and promising therapeutic target for breast cancer. This review article summarizes the current knowledge about the relation of leptin and breast cancer.

Leptin signaling in breast cancer: an overview

The adipocyte-derived peptide leptin acts through binding to specific membrane receptors, of which six isoforms (obRa-f) have been identified up to now. Binding of leptin to its receptor induces activation of different signaling pathways, including the JAK/STAT, MAPK, IRS1, and SOCS3 signaling pathways. Since the circulating levels of leptin are elevated in obese individuals, and excess body weight has been shown to increase breast cancer risk in postmenopausal women, several studies addressed the role of leptin in breast cancer. Expression of leptin and its receptors has been demonstrated to occur in breast cancer cell lines and in human primary breast carcinoma. Leptin is able to induce the growth of breast cancer cells through activation of the Jak/STAT3, ERK1/2, and/or PI3K pathways, and can mediate angiogenesis by inducing the expression of vascular endothelial growth factor (VEGF). In addition, leptin induces transactivation of ErbB-2, and interacts in triple negative breast cancer cells with insulin like growth factor-1 (IGF-1) to transactivate the epidermal growth factor receptor (EGFR), thus promoting invasion and migration. Leptin can also affect the growth of estrogen receptor (ER)-positive breast cancer cells, by stimulating aromatase expression and thereby increasing estrogen levels through the aromatization of androgens, and by inducing MAPK-dependent activation of ER. Taken together, these findings suggest that the leptin system might play an important role in breast cancer pathogenesis and progression, and that it might represent a novel target for therapeutic intervention in breast cancer.

Convergent trafficking pattern of leptin after endocytosis mediated by ObRa-ObRd

The cellular effects of leptin are dependent on the receptor subtypes that mediate the signaling and fate of endocytosed leptin inside the cells. In this study, we examined the differences in receptor expression, endocytosis, intracellular degradation, and exocytosis of a trace amount of leptin in cells overexpressing ObRb and short forms of the leptin receptor. The relative contribution of proteasomes and lysosomes in the intracellular fate of leptin was also determined. There were three unusual findings: (1) all receptor subtypes could mediate the binding and endocytosis of leptin, although ObRb was expressed at a lower level than ObRa, ObRc, and ObRd after transient transfection. This indicates that ObRb can be a transporting receptor. (2) Once internalized, the intracellular degradation pattern and exocytosis of leptin were independent of the receptor subtype. (3) Endocytosed leptin could remain intact for at least 1 h. This stability was further enhanced by inhibition of lysosomal activity. Thus, the intracellular pool of intact leptin may allow prolonged biological functions for this adipokine.

Role of leptin and ghrelin in the regulation of gonadal function

Gonadal development and function is sustained by the complex interaction of an array of regulatory signals that operate directly on the gonads and/or indirectly via modulation of gonadotropin secretion. During the last decade, different factors primarily involved in the control of food intake and energy balance have been demonstrated as putative modulators of different elements of the reproductive axis, including the gonads, thus helping to define the neuroendocrine basis for the link between body energy stores and fertility. These factors include not only the adipocyte-derived hormone leptin, which is indispensable for proper energy balance and reproduction, but also a number of neuropeptides and hormones of central and peripheral origin. In the latter, growing evidence strongly suggests the involvement of the stomach-secreted peptide ghrelin in the control of several aspects of gonadal function. Interestingly, leptin and ghrelin have been proposed as reciprocally related regulators of energy homeostasis; however, their potential interplay in the control of reproduction remains ill defined. This work will summarize the most salient findings concerning the potential roles of leptin and ghrelin in the functional control of the gonads. In addition, open issues regarding the reproductive facets of these metabolic signals will be highlighted. Overall, the authors propose that through complementary or antagonistic actions, leptin and ghrelin may jointly cooperate to modulate a wide set of reproductive functions, thereby contributing to the physiologic integration of energy balance and reproduction.

Mapping of binding site III in the leptin receptor and modeling of a hexameric leptin.leptin receptor complex

The leptin.leptin receptor (LR) system shows strong similarities to the long chain cytokine interleukin-6 (IL-6) and granulocyte colony-stimulating factor (G-CSF) cytokine.cytokine receptor systems. The IL-6 family cytokines interact with their receptors through three different binding sites (I-III). We demonstrated previously that leptin has similar binding sites I-III and mapped the interactions between binding site II and cytokine receptor homology domain II (CRH2) (Peelman, F., Van Beneden, K., Zabeau, L., Iserentant, H., Ulrichts, P., Defeau, D., Verhee, A., Catteeuw, D., Elewaut, D., and Tavernier, J. (2004) J. Biol. Chem. 279, 41038-41046). In this study, we built homology models for the CRH1 and Ig-like domains of the LR. The Ig-like domain shows a large conserved surface patch in the beta-sheet formed by beta-strands 3, 6, and 7. Mutations in this patch almost completely abolished the leptin-induced STAT3-dependent reporter activity. We propose that a conserved cluster of residues Leu370, Ala407, Tyr409, His417, and His418 forms the center of binding site III of the LR. We built a hexameric leptin.LR complex model based on the hexameric IL-6 complex. In this model, a conserved hydrophobic protuberance of Val36, Thr37, Phe41, and Phe43 in the A-B loop of leptin fits perfectly in the CRH2 domain, corresponding to the IL-6 alpha-receptor, and forms the center of binding site I. The 2:4 hexameric leptin.LR complex offers a rational explanation for mutagenesis studies and residue conservation.

Intracellular signalling pathways activated by leptin

Leptin is a versatile 16 kDa peptide hormone, with a tertiary structure resembling that of members of the long-chain helical cytokine family. It is mainly produced by adipocytes in proportion to fat size stores, and was originally thought to act only as a satiety factor. However, the ubiquitous distribution of OB-R leptin receptors in almost all tissues underlies the pleiotropism of leptin. OB-Rs belong to the class I cytokine receptor family, which is known to act through JAKs (Janus kinases) and STATs (signal transducers and activators of transcription). The OB-R gene is alternatively spliced to produce at least five isoforms. The full-length isoform, OB-Rb, contains intracellular motifs required for activation of the JAK/STAT signal transduction pathway, and is considered to be the functional receptor. Considerable evidence for systemic effects of leptin on body mass control, reproduction, angiogenesis, immunity, wound healing, bone remodelling and cardiovascular function, as well as on specific metabolic pathways, indicates that leptin operates both directly and indirectly to orchestrate complex pathophysiological processes. Consistent with leptin's pleiotropic role, its participation in and crosstalk with some of the main signalling pathways, including those involving insulin receptor substrates, phosphoinositide 3-kinase, protein kinase B, protein kinase C, extracellular-signal-regulated kinase, mitogen-activated protein kinases, phosphodiesterase, phospholipase C and nitric oxide, has been observed. The impact of leptin on several equally relevant signalling pathways extends also to Rho family GTPases in relation to the actin cytoskeleton, production of reactive oxygen species, stimulation of prostaglandins, binding to diacylglycerol kinase and catecholamine secretion, among others.

Leptin receptor activation depends on critical cysteine residues in its fibronectin type III subdomains

The leptin receptor (LR) complex is composed of a single subunit belonging to the class I cytokine receptor family and exists as a preformed complex. The extracellular portion contains two cytokine receptor homology (CRH) domains, separated by an Ig-like domain and followed by two membrane-proximal fibronectin type III (FNIII) domains. The mechanisms underlying ligand-induced receptor activation are still poorly understood. LRs can exist as disulfide-linked dimers at the cell surface, even in the absence of leptin. We evaluated the role of the two unpaired cysteine residues (Cys-672 and Cys-751) in the FNIII domains in receptor clustering, leptin binding, and biological activity. Although mutation of cysteine on position 751 to serine has hardly any effect on ligand binding and receptor activation, the C672S mutant exhibits a marked reduction in STAT3-dependent signaling. The double mutant was completely devoid of biological activity, although leptin binding remained unaffected. Mutation of both residues resulted in complete loss of disulfide bridge formation of FNIII domains in solution. In contrast, no difference was observed in ligand-independent oligomerization of the membrane-bound receptor, suggesting a role for cysteines in the CRH2 domain in formation of the preformed LR complex. We propose a model wherein leptin-induced clustering of two preformed dimers forms the activated LR complex. Disulfide bridge formation involving Cys-672 and Cys-751 may be necessary for JAK activation and hence signaling.

BiaCore analysis of leptin-leptin receptor interaction: evidence for 1:1 stoichiometry

Leptin is a hormonal protein involved in energy homeostatis that acts to inhibit food intake, to stimulate energy expenditure, and to influence insulin secretion, lipolysis, and sugar transport. Its action is mediated by a specific receptor whose activation is highly controversial. As a member of the cytokine receptor superfamily, it has been predicted to be activated by ligand-induced dimerization. However, recent evidence has indicated that this receptor exists as a dimer in both ligand-free and ligand-bound states. Here, the BiaCore has been used to measure the kinetics and stoichiometry of the interaction between the leptin and its receptor. Human or mouse receptor chimeras comprising two receptor extracellular domains fused to the Fc region of IgG(1) were captured on to the sensor via protein G. Kinetic data fitted to the simplest 1/1 model. The observed stoichiometry at ligand saturation was 1:1. Analyzing the binding mode and the reaction stoichiometry allowed us to conclude that the leptin receptor dimerization is not induced by ligand binding. This contradicts the common paradigm of cytokine receptor activation. Furthermore, data demonstrated a high-affinity interaction. The KD was 0.23+/-0.08 nM, with ka = (1.9 +/- 0.4) x 10(6) M(-1)s(-1) and kd = (4.4 +/- 0.6) x 10(-4) s(-1) for human leptin with its cognate receptor. Similar results were observed for the affinity of different species of leptin binding to mouse leptin receptor.

Mapping of the Leptin Binding Sites and Design of a Leptin Antagonist

The leptin/leptin receptor system shows strong similarities to the long-chain cytokine interleukin-6 (IL-6) and granulocyte colony-stimulating factor cytokine/receptor systems. The IL-6 family cytokines interact with their receptors through three different binding sites I-III. The leptin structure was superposed on the crystal structures of several long-chain cytokines, and a series of leptin mutants was generated focusing on binding sites I-III. The effect of the mutations on leptin receptor (LR) signaling and on binding to the membrane proximal cytokine receptor homology domain (CRH2) of the LR was determined. Mutations in binding site I at the C terminus of helix D show a modest effect on signaling and do not affect binding to CRH2. Binding site II is composed of residues at the surface of helices A and C. Mutations in this site impair binding to CRH2 but have only limited effect on signaling. Site III mutations around the N terminus of helix D impair receptor activation without affecting binding to CRH2. We identified an S120A/T121A mutant in binding site III, which lacks any signaling capacity, but which still binds to CRH2 with wild type affinity. This leptin mutant behaves as a potent leptin antagonist both in vitro and in vivo.

Leptin: linking adipocyte metabolism with cardiovascular and autoimmune diseases

Leptin was originally discovered as an adipocyte-derived hormone involved in the central control of body weight and energy homeostasis. It is now clear that leptin is a pleiotropic cytokine, with activities on many peripheral cell types. These findings may help explain the surprising role of leptin in pathophysiological processes. Recent evidence suggests that leptin contributes to atherosclerosis and to the increased risk of cardiovascular disease in obese people. Leptin also appears to be involved in T-cell-dependent immunity and possibly in the development and maintenance of certain autoimmune diseases. Here, we review the role of leptin in cardiovascular and autoimmune diseases, and also briefly address the potential therapeutic use of leptin antagonists.

Hormonal and body composition predictors of soluble leptin receptor, leptin, and free leptin index in adolescent girls with anorexia nervosa and controls and relation to insulin sensitivity

Anorexia nervosa (AN) is associated with very low levels of leptin, a cytokine secreted by adipose tissue and known to suppress appetite. Leptin may play a permissive role in onset of puberty and in resumption of gonadal function in conditions of undernutrition. The soluble leptin receptor (sOB-R) is the main leptin binding protein, and the ratio of serum leptin to sOB-R provides a measure of the free leptin index (FLI), which may be a more accurate determinant of leptin function. Determinants of sOB-R and FLI have not been examined in an adolescent population. We examined levels of sOB-R, leptin, and FLI, and body composition and hormonal determinants of these variables in 23 adolescent girls with AN and 21 healthy adolescent girls of comparable maturity prospectively over 1 yr. Measures of insulin resistance and adiponectin were also examined. We determined changes in levels of sOB-R, leptin, and FLI with weight recovery (defined as an increase in body mass index of >/=10%, n = 11), and with resumption of menstrual cycles (n = 13). Girls with AN had significantly higher levels of sOB-R (P = 0.0008) and significantly lower levels of leptin and FLI (P < 0.0001 for both) than healthy controls, and levels of FLI were reduced more than levels of leptin in girls with AN compared with controls. An inverse correlation was noted between levels of leptin and sOB-R for the group as a whole (r = -0.64, P < 0.0001) but not in girls with AN considered alone. The most important predictor of levels of sOB-R was cortisol in the group as a whole (r = 0.61, P < 0.0001) and in girls with AN considered alone (r = 0.66, P = 0.0008). Other independent predictors of sOB-R levels for the entire group were percent body fat (r = -0.44, P = 0.003) and levels of IGF-I (r = -0.37, P = 0.01). The most important predictors of leptin and FLI were body mass index and percent body fat. An inverse relationship was noted between measures of insulin resistance and sOB-R levels, whereas a positive association was noted between these measures and leptin and FLI. Adiponectin values did not differ in girls with AN compared with healthy controls and did not correlate with sOB-R, leptin, or FLI. Weight recovery resulted in significant decreases in levels of the sOB-R (24.7 +/- 1.7 to 17.6 +/- 1.2 U/ml, P = 0.004), and increases in levels of leptin (4.4 +/- 1.0 to 13.7 +/- 2.9 microg/liter, P = 0.02). Resumption of menstrual function, but not weight recovery alone, was associated with significant increases in FLI (0.19 +/- 0.04 to 0.50 +/- 0.09 microg/U x 10(-3), P = 0.02).We demonstrate an increase in levels of sOB-R and a decrease in the FLI in adolescent girls with AN, and also demonstrate that cortisol is the most important predictor of levels of sOB-R in this condition. Levels of leptin and FLI, conversely, are primarily predicted by body composition. Weight recovery is associated with a decrease in sOB-R and an increase in leptin. Resumption of menses is associated with significant increases in the FLI, suggesting that free leptin may be an important determinant of menstrual recovery.

Expression of leptin receptor (Ob-R) isoforms and signal transducers and activators of transcription (STATs) mRNAs in the mouse taste buds

Leptin is a hormone that regulates food intake, energy expenditure and body weight. Our previous studies have demonstrated that the taste organ is a new peripheral target for leptin in mice. Leptin selectively inhibits the responses of taste nerves and receptor cells to sweet substances without affecting responses to sour, salty, and bitter substances. Still, there is no convincing evidence for the existence of leptin receptors (Ob-Rs) in taste receptor cells, especially the functional isoform Ob-Rb. We investigated the expression of 5 different Ob-R isoforms (a-e) and 6 STAT (signal transducers and activators of transcription) members in mouse taste cells. STATs are considered to be involved in the leptin signaling via Ob-Rb. Semiquantitative RT-PCR analysis showed that Ob-Rb was expressed in the taste buds of the fungiform and circumvallate papillae, but not so clearly in the surrounding epithelial tissue. The expression pattern among the three different tissues was similar to that of the taste cell specific G-protein, alpha-gustducin. The other Ob-R isoforms were widely detected in either the taste papillae or the epithelial tissue. Among 6 STAT members, STAT3 showed the highest relative abundance of mRNA in the taste buds. Consistently, in situ hybridization analysis showed that while Ob-Rb and STAT3 signals were detected in some taste bud cells, the signals were not clearly observed in the epithelial tissue cells. In conclusion, the present study provides evidence of the existence of the leptin receptor, Ob-Rb, and STAT3 in the mouse taste bud cells. This finding further confirms the involvement of leptin in the control of taste sensitivities to sweet substances in mice.

Inhibition of steroidogenic response to adrenocorticotropin by leptin: implications for the adrenal response to maternal separation in neonatal rats

Previous studies have shown that leptin can regulate the adrenocortical axis. Neonatal rodents exhibit a period of adrenal hyporesponsiveness to stress in the first 2 wk of life, and we determined the role of leptin as a mediator of this process. We examined the direct effects of leptin on neonatal adrenal steroidogenic responses to ACTH under basal conditions and after 24-h maternal separation. In isolated adrenocortical cells from as early as postnatal d 5 (PND5) and throughout the neonatal period, acute (2.5 h) incubation with leptin significantly inhibited ACTH-stimulated corticosterone and aldosterone secretion without affecting cAMP production. In PND10 pups, 24-h maternal separation and the resulting rapid decline in plasma leptin levels increased basal corticosterone and aldosterone secretion in vivo and in isolated cells, but did not modify the ability of leptin to inhibit stimulated steroid production in vitro. Maternal separation in PND10 pups increased adrenal expression of steroidogenic acute regulatory protein (StAR) and peripheral-type benzodiazepine receptor (PBR) proteins as well as all steroidogenic enzymes measured (3beta-hydroxysteroid dehydrogenase, P450C11B1, and P450C11B2). Leptin (1 mg/kg body weight, i.p.) replacement during maternal separation did not affect basal corticosterone output, but reduced corticosterone secretion and StAR and PBR protein expression induced by exogenous ACTH challenge (20 or 80 microg/kg body weight, i.p.). These results indicate that leptin inhibits ACTH-stimulated secretion of corticosterone and aldosterone, at least through a rapid reduction in the expression of StAR and PBR protein in the neonatal adrenal gland. As leptin concentrations in pups are controlled to a large extent by the maternal diet, these results emphasize the key role of leptin to mediate the maternal influence on the adrenocortical axis of the infant.

T cell receptor-independent CD4 signalling: CD4-MHC class II interactions regulate intracellular calcium and cyclic AMP

CD4 is a coreceptor on T helper (Th) cells that interacts with MHC class II molecules (MHCII). The mechanisms mediating the effects of CD4 on responses by T helper cells to stimulation of the antigen-specific T cell receptor (TCR) are still poorly understood. Here, we demonstrate T cell costimulation via CD4 signalling independent of T cell receptor-mediated signals. Incubation of T helper cells with peptide mimetics of the CD4-binding region on the MHC class II beta2 domain caused intracellular calcium mobilization in the absence of antigen or other T cell receptor stimuli. Engagement of CD4 by peptide mimetics or wild-type MHC class II, but not by mutant MHC class II molecules incapable of engaging CD4, inhibited the T cell receptor-mediated increase in cyclic AMP (cAMP) concentrations in T helper cells. CD4-mediated signals activated cyclic AMP phosphodiesterases (PDEs) and inhibited adenylyl cyclase. Full activation and clonal expansion of antigen-stimulated T helper cells required the CD4-mediated regulation of cyclic AMP. Our results suggest a costimulatory mechanism of CD4 function that acts on the second messengers, calcium and cyclic AMP.

Activation of the leptin receptor by a ligand-induced conformational change of constitutive receptor dimers

Binding of leptin to the leptin receptor is crucial for body weight and bone mass regulation in mammals. Leptin receptors were shown to exist as dimers, but the role of dimerization in receptor activation remains unknown. Using a quantitative Bioluminescence Resonance Energy Transfer approach, we show here in living cells that approximately 60% of the leptin receptor exists as constitutive dimers at physiological expression levels in the absence of leptin. No further increase in leptin receptor dimerization was detected in the presence of leptin. Importantly, in cells expressing the short leptin receptor isoform, leptin promoted a robust enhancement of energy transfer signals that reflect specific conformational changes of pre-existing leptin receptor dimers and that may be used as read-out in screening assays for leptin receptor ligands. Both leptin receptor dimerization and the leptin-induced energy transfer were Janus kinase 2-independent. Taken together, our data support a receptor activation model based on ligand-induced conformational changes rather than ligand-induced dimerization.

The ins and outs of leptin receptor activation

The adipocyte-derived hormone leptin signals the status of body energy stores by activating its receptor in hypothalamic nuclei. In contrast to the initial expectations, leptin treatment of human obesity was largely unsuccessful. One explanation for this is the marked leptin resistance, which likely operates in part at the receptor level. The leptin receptor is a member of the class I cytokine receptor family, which uses the Janus kinase/signal transducer and activator of transcription pathway as a major signaling route. In this review, we focus on the molecular mechanisms underlying leptin receptor activation. Different modes of leptin-induced clustering of the ectodomains and the subsequent signaling events will be discussed.

Leptin distribution and metabolism in the pregnant rat: transplacental leptin passage increases in late gestation but is reduced by excess glucocorticoids

Leptin is essential for the establishment of pregnancy and appears to promote fetal growth, but the mechanisms regulating fetal leptin exposure remain unclear. In rodents, indirect evidence suggests that fetal leptin is partly derived from the maternal circulation via transplacental passage. Indeed, the placenta expresses mRNA for Ob-Ra, one of the short forms of the leptin receptor (Ob-R(S)) important in leptin transport, and this expression increases markedly in late pregnancy. Therefore, we determined the transplacental passage of maternal leptin to the fetus in the rat and whether this transport increases near term in association with a rise in placental expression of Ob-R(S) protein. Because of the proposed role of leptin in promoting fetal growth, we also assessed the effect of glucocorticoid-induced fetal growth retardation on placental leptin transport. Anesthetized rats received a constant infusion of (125)I-leptin via a jugular cannula before and at d 16 and 22 of pregnancy (term = d 23); plasma samples were obtained at 10, 20, 40, 60, 80, and 100 min, and fetuses and placentas were collected at the time of the final sample. The metabolic clearance rate of leptin fell (P < 0.01) from 3.08 +/- 0.23 ml/min per kg in nonpregnant rats to 2.36 +/- 0.13 ml/min per kg by d 22. Transplacental passage of (125)I-leptin, estimated from its concentration in the whole fetus relative to maternal plasma, increased 10-fold (P < 0.005) between d 16 and d 22 of pregnancy. Over this same period, Ob-R(S) protein expression in the placental labyrinth zone increased by almost 2-fold. Transplacental leptin passage was reduced (P < 0.05) by 77% after maternal dexamethasone treatment, whereas suppression of endogenous glucocorticoid synthesis (by metyrapone) increased (P < 0.05) the transfer of maternal leptin to the fetus by 55%. These data show that transplacental passage of maternal leptin is a significant source of fetal leptin and increases markedly during late pregnancy. Consistent with the proposed role of leptin as a fetal growth factor, transplacental leptin passage is reduced in association with glucocorticoid-induced fetal growth retardation.

Leptin and melanocortin signaling in the hypothalamus

The regulation of body weight in humans is coordinated by the interplay between food intake and energy expenditure. The identification of the adipocyte-secreted hormone leptin as a key regulator on both of these processes has shed new light on the pathways involved in their regulation. Indeed, mutations in the gene's encoding leptin and its cognate receptor cause severe obesity in humans. Leptin's actions are mediated principally by target neurons in the hypothalamus where it acts to alter food intake, energy expenditure, and neuroendocrine-function. Recently, it has become clear that a number of critical neuropeptides are regulated by leptin in the hypothalamus. Among these is the proopiomelanocortin (POMC)-derived peptide, alpha-melanocyte-stimulating hormone (alpha-MSH), which is produced in the arcuate nucleus and is a potent negative regulator of food intake. Like leptin, mutations in POMC or in central melanocortin receptors lead to obesity in humans. Thus, an understanding of the mechanisms by which the leptin and melanocortin pathways signal in the hypothalamus is critical in order to begin to clarify the pathways involved in regulating body weight in humans.

Leptin: a review of its peripheral actions and interactions

Following the discovery of leptin in 1994, the scientific and clinical communities have held great hope that manipulation of the leptin axis may lead to the successful treatment of obesity. This hope is not yet dashed; however the role of the leptin axis is now being shown to be ever more complex than was first envisaged. It is now well established that leptin interacts with pathways in the central nervous system and through direct peripheral mechanisms. In this review, we consider the tissues in which leptin is synthesized and the mechanisms which mediate leptin synthesis, the structure of leptin and the knowledge gained from cloning leptin genes in aiding our understanding of the role of leptin in the periphery. The discoveries of expression of leptin receptor isotypes in a wide range of tissues in the body have encouraged investigation of leptin interactions in the periphery. Many of these interactions appear to be direct, however many are also centrally mediated. Discovery of the relative importance of the centrally mediated and peripheral interactions of leptin under different physiological states and the variations between species is beginning to show the complexity of the leptin axis. Leptin appears to have a range of roles as a growth factor in a range of cell types: as be a mediator of energy expenditure; as a permissive factor for puberty; as a signal of metabolic status and modulation between the foetus and the maternal metabolism; and perhaps importantly in all of these interactions, to also interact with other hormonal mediators and regulators of energy status and metabolism such as insulin, glucagon, the insulin-like growth factors, growth hormone and glucocorticoids. Surely, more interactions are yet to be discovered. Leptin appears to act as an endocrine and a paracrine factor and perhaps also as an autocrine factor. Although the complexity of the leptin axis indicates that it is unlikely that effective treatments for obesity will be simply derived, our improving knowledge and understanding of these complex interactions may point the way to the underlying physiology which predisposes some individuals to apparently unregulated weight gain.

Leptin signalling

The identification of leptin as the product of the obesity (ob) gene has been followed by extensive research identifying a wide spectrum of physiological effects elicited by this adipose-derived hormone. These effects are mediated via a family of cytokine-like receptor isoforms distributed in both the central nervous system and periphery. The signal transduction pathways regulated by leptin are diverse and include those characteristic of both cytokine and growth factor receptor signalling. This review describes the structure and function of leptin receptors and summarizes recent progress that has been made in characterizing the increasing number of signal transduction pathways regulated by leptin.

Identification of leptin receptors in lung and isolated fetal type II cells

Leptin is a cytokine involved in regulation of the satiety response. Receptors for this protein have been identified in brain as well as many other peripheral tissues. Some of the highest levels of receptor concentration occur in the lung. Considering the cellular diversity of lung, neither the localization nor the function of leptin in pulmonary tissues has been delineated. The purpose of the present study was to determine if fetal and adult rabbit lung displayed specific binding for leptin, to identify the binding sites, and to explore a potential functional role for leptin in lung surfactant production. Frozen sections of adult and fetal rabbit (24th gestational day) lung were prepared and incubated with increasing concentrations of [125I]leptin in the presence or absence of 1-microM-unlabeled leptin. Sections were removed and radioactivity measured. Concurrently, sections were coated with nuclear Trac emulsion and incubated in the dark at -30 degrees C. Lung showed specific binding for leptin. Microscopically, [125I]leptin was localized to acinar-lining epithelium of developing fetal lung. Larger cells within the epithelial layer appeared to bind leptin more avidly than adjacent cells. Antibodies to the leptin receptor were used to identify binding sites in adult lung and isolated fetal lung type II cells. In adult lung, both the K20 (against the extracellular amino-terminal) and the M18 antibody (against the intracellular carboxy-terminal) displayed several binding sites. In contrast, the isolated fetal type II cells showed only a single binding site for both antibodies. The apparent molecular mass of the receptor using the K20 antibody appeared to be approximately 125 kD. A protein of similar mass bound the M18 antibody suggesting that functional receptor is present in lung and expressed by fetal type II cells. Incubation of isolated fetal type II cells with leptin (0.01-10 microg/ml) stimulated [3H]choline incorporation in disaturated phosphatidylcholine. These results show that fetal and adult lung bind leptin specifically, and fetal type II cells in particular, may be responsive to leptin stimulation of phospholipid production. Leptin may therefore be important in regulating maturation of cells of the fetal lung.

Polar expression and phosphorylation of human leptin receptor isoforms in paired, syncytial, microvillous and basal membranes from human term placenta

The hormone leptin (OB) and its receptor (OB-R) are key homeostatic regulators of mammalian body weight. Two predominant isoforms of OB-R are expressed by alternative splicing: the long form, OB-RL, with full signalling capacity is highly expressed in the hypothalamus and the short, signalling-defective form, OB-Rs, is ubiquitously expressed. In a previous study we detected expression of OB-RL and OB-Rs in human syncytiotrophoblast cells using in situ hybridization and immunohistochemistry (Bodner et al., 1999). The aim of this study was to investigate leptin receptor isoform expression and phosphorylation in paired, syncytial, microvillous and basal membranes from human term placenta by Western blot analysis. Both the OB-RL and the OB-Rs isoforms were detected in the syncytial membrane preparations. The OB-RL isoform was observed exclusively in microvillous membranes, whereas the OB-Rs isoform was found in both microvillous and basal membrane preparations. No significant differences were observed between syncytial membranes from normal and type 1 diabetic pregnancies. To test the phosphorylation capacity of the OB-R isoforms, microvillous and basal membrane vesicles loaded with ATP were stimulated with leptin and the phosphorylation status of the OB-R at the tyrosine 985 (Y985) was determined. A single band at the molecular weight corresponding to the molecular weight of the OB-RL isoform was detected exclusively in the ATP-loaded microvillous vesicles. We conclude that the long form OB-RL is expressed exclusively in the microvillous membrane of the syncytiotrophoblast and is capable of being phosphorylated, suggesting that it has signal transduction capacity.

Leptin in male reproduction: the testis paradigm

Leptin, the adipocyte-derived hormone that plays a key role in body weight homeostasis, has recently emerged as a relevant neuroendocrine mediator in different systems, including the reproductive axis. Thus, compelling evidence points out a major role of leptin in the regulation of female pubertal development and fertility, both in humans and experimental animals. The contribution of leptin to the proper functioning of the male reproductive system has been less clear. However, data gathered in recent years, from independent groups and through a variety of experimental approaches, strongly suggest that leptin is able to act at different levels of the hypothalamic-pituitary-testicular axis. Herein, we review the biological effects and potential mechanisms of action of leptin upon rodent testis. Leptin appears to act as a direct inhibitory signal for testicular steroidogenesis, which may be relevant to explain the link between decreased testosterone secretion and hyperleptinaemia in obese men. Analysis of the molecular basis for leptin-induced inhibition of testosterone secretion revealed the potential involvement of decreased gene expression of several up-stream factors (e.g. SF-1, StAR and P450scc) in the steroidogenic pathway. In this context, testicular expression of leptin receptor (Ob-R) gene shows a complex pattern of alternative splicing with generation of multiple variants, including the functional leptin receptor type-b (Ob-Rb) and several short isoforms. Moreover, Ob-R mRNA expression in rat testis was regulated by homologous (leptin) as well as heterologous (gonadotropins) signals. Overall, the current data indicate that the testis is a direct target for leptin actions. Furthermore, the available evidence is suggestive of a tightly regulated, complex mode of action of leptin at different levels of the male gonadal axis that involves not only stimulatory but also inhibitory effects.

Soluble leptin receptor represents the main leptin binding activity in human blood

In human blood leptin circulates both free and bound to high molecular weight proteins. Hypothesising that these proteins may modulate ligand bioavailability and bioactivity of leptin, we investigated their molecular nature. Therefore, leptin binding activity was partially purified from human plasma using a leptin affinity column. Subjecting this preparation to size exclusion chromatography (SEC) we observed a coelution of leptin binding activity with levels of the soluble leptin receptor (sOB-R) determined by a newly developed ligand immunofunctional assay. In Western blot analysis the partially purified leptin binding activity exhibited sOB-R immunoreactivity in two bands of 110 and 140 kD. Following N-deglycosylation these bands were replaced by two bands with the molecular weight of 90 and 60 kD, suggesting two isoforms which are capable of leptin binding, as determined by cross-linking. Furthermore, different ratios of these isoforms were detectable in fractions of the leptin binding activity after separation by SEC. These findings indicate the formation of heterodimers and homodimers complexed with and without leptin. As the two sOB-R bands from Western blot analysis correspond to only two specific bands in cross-linking experiments with 125l-leptin, the role of both isoforms as leptin binding proteins appears to be exclusive. Therefore, our results indicate that sOB-R is the major leptin binding protein in the circulating human blood.

Developmental and hormonal regulation of leptin receptor (Ob-R) messenger ribonucleic acid expression in rat testis

In target tissues, leptin receptor (Ob-R) gene expression results in an array of alternatively spliced isoforms (Ob-Ra to Ob-Rf) with different functional features. Recent evidence has pointed to a direct role of leptin in the control of testicular function. However, complete elucidation of the pattern of Ob-R gene expression in the male gonad is still pending. The focus of this study was to characterize in detail the developmental pattern of expression and hormonal regulation of Ob-R gene in rat testis. To this end, the overall expression of Ob-R mRNA was compared to that of the fully functional, long Ob-Rb isoform in different experimental settings, using semiquantitative reverse transcription-polymerase chain reaction. Expression of Ob-R mRNA was detected in testes from 15-, 30-, 45-, and 75-day-old rats at rather constant relative levels. In contrast, testicular expression of Ob-Rb mRNA was higher in pubertal testes (15- to 30-day-old rats) and declined in adulthood. In testes from 30-day-old animals, analysis of isoform distribution revealed that, in addition to abundant Ob-Rb mRNA levels, expression of Ob-Ra, Ob-Rf, and, to a lesser extent, Ob-Rc and Ob-Re messages is detected. Testicular Ob-R mRNA expression appeared sensitive to neonatal imprinting as neonatal treatment with estradiol benzoate (500 microg/rat; Day 1 postpartum) resulted in a persistent increase (P: < 0.01) in the relative expression level of Ob-R mRNA, a phenomenon only partially mimicked by neonatal suppression of serum gonadotropins by means of LHRH-antagonist administration. In addition, neonatal estrogenization differentially altered the pattern of expression of Ob-R isoforms in adult rat testis, as expression of Ob-Rb mRNA was decreased to undetectable levels, whereas that of Ob-Rc remained unaltered, and Ob-Ra, Ob-Rf, and, to a lesser extent, Ob-Re mRNA levels were significantly increased (P: < 0.01) by neonatal exposure to estrogen. Finally, down-regulation of testicular Ob-R gene expression by homologous and heterologous signals was demonstrated as relative levels of Ob-R and Ob-Rb mRNAs were significantly decreased (P: < 0.01), in a coordinate manner, in rat testis after exposure to human recombinant leptin in vitro, and after stimulation with hCG and FSH in vivo. In conclusion, our results indicate that testicular Ob-R gene expression is developmentally regulated, imprinted by the neonatal endocrine milieu, and sensitive to regulation by leptin and gonadotropins. The ability of pivotal signals in testicular function to regulate Ob-R gene expression further supports the contention of a direct role of leptin in functional control of the rat testis.

Homology modeling of human leptin/leptin receptor complex

Leptin receptor mediates the weight regulatory signal carried by the adipocyte-secreted peptide hormone, leptin. It is important to understand the atomic interactions between leptin and the receptor for the therapeutic applications. However, the structure of leptin receptor has not yet been determined. Leptin shows structural similarity to G-CSF, while leptin receptor is similar in amino acid sequence to G-CSF receptor. Because of the similarity between leptin/leptin receptor complex and G-CSF/G-CSF receptor complex, we tried to build a model structure of leptin/leptin receptor complex with the crystal structure of the G-CSF/G-CSF receptor complex as the template. The obtained model for the complex was consistent with the results of the amino acid replacement and deletion experiments. The observation suggests that the model is useful to lead the experimental study on the interaction between leptin and the receptor.

The adipoinsular axis: effects of leptin on pancreatic beta-cells

The prevalence of obesity and related diabetes mellitus is increasing worldwide. Here we review evidence for the existence of an adipoinsular axis, a dual hormonal feedback loop involving the hormones insulin and leptin produced by pancreatic beta-cells and adipose tissue, respectively. Insulin is adipogenic, increases body fat mass, and stimulates the production and secretion of leptin, the satiety hormone that acts centrally to reduce food intake and increase energy expenditure. Leptin in turn suppresses insulin secretion by both central actions and direct actions on beta-cells. Because plasma levels of leptin are directly proportional to body fat mass, an increase of adiposity increases plasma leptin, thereby curtailing insulin production and further increasing fat mass. We propose that the adipoinsular axis is designed to maintain nutrient balance and that dysregulation of this axis may contribute to obesity and the development of hyperinsulinemia associated with diabetes.