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Department of General Surgery, University Hospital Maastricht, Maastricht, The Netherlands
Address all correspondence and requests for reprints to: J. W. M. Greve, M.D., Ph.D., Department of General Surgery, University Hospital of Maastricht, P.O. Box 5800, 6202 AZ Maastricht, The Netherlands. E-mail: . jw.greve{at}surgery.azm.nl
Abstract
To investigate soluble leptin receptor (sLR) in plasma, specific anti-sLR monoclonal antibodies were developed. Western blot analysis and size exclusion fractionation demonstrated sLR in plasma with a molecular mass of approximately 180,000. Next to this, the presence of sLR-leptin complexes in plasma was confirmed.
Using the developed monoclonal antibodies, a specific sLR ELISA was developed, which measured in plasma both free and sLR bound to leptin. sLR appeared to inhibit leptin concentrations measured in four different leptin assays indicating that these assays primarily measure free leptin and underestimate the total leptin present in plasma.
Furthermore, plasma levels of sLR and leptin were measured in 21 lean individuals and in 30 morbidly obese subjects before and 3, 6, and 12 months after gastric restrictive surgery. Preoperatively, leptin concentrations significantly correlated with body mass index (r = 0.796, P < 0.001). In contrast, sLR significantly inversely correlated with body mass index (r = -0.294, P < 0.05). In lean subjects, the molar ratio of free leptin to sLR was 1:1, whereas in morbidly obese subjects a ratio of 25:1 was found. After weight loss due to surgery, leptin levels rapidly decreased and sLR levels slowly increased to reach normal values at 12 months postoperatively.
We conclude that sLR levels are significantly decreased, whereas leptin levels are significantly increased in morbidly obese subjects compared with lean individuals.
LEPTIN, A CYTOKINE that is primarily expressed by adipose tissue (1), is considered to be involved in satiety regulation in mice (2, 3). Leptin controls food intake by its interaction with the leptin receptor in the brain (4, 5). As a consequence, leptin-deficient and leptin receptor-deficient mice show an obese phenotype (5, 6, 7, 8). Furthermore, administration of leptin to leptin-deficient or diet-induced obese mice resulted in a significant decrease of food intake (9, 10).
The action of leptin is mediated by the leptin receptor, which belongs to the class I type cytokine receptor family (11, 12, 13, 14). In humans, four different mRNA splice variants of the leptin receptor have been identified, consistent with a membrane bound receptor with a long cytoplasmatic domain and three membrane bound receptors with a short cytoplasmatic domain. Furthermore, soluble leptin receptor is generated by proteolytic cleavage of membrane-anchored receptors, indicating that the leptin receptor might have other functions besides signal transduction (15). In blood, leptin is suggested to circulate both in free form as well as bound to a soluble leptin receptor and possibly also to other as yet unidentified binding proteins (16). Binding of leptin with soluble leptin receptor has been suggested to increase the bioavailability of leptin in plasma (16, 17) as well as to decrease binding of leptin to membrane bound leptin receptors (18). The role of the soluble leptin receptor in the regulation of the physiological function of leptin is hitherto unclear.
In contrast to obesity in leptin-deficient mice, in man obesity is associated with increased leptin plasma concentrations (19). Based on murine experiments, it has been postulated that adipose tissue signals via increasing leptin levels that food intake has been sufficient (20). Assuming that a similar mechanism holds for man, lack of satiety in obese persons could indicate insensitivity to elevated leptin levels. Because circulating soluble leptin receptors have been reported to be involved in leptin function, these receptors could be involved in the satiety response.
In this study, we investigated circulating leptin receptor levels. To this end, an ELISA for the quantification of soluble leptin receptor (sLR) was developed and characterized. Using this assay, we studied circulating soluble leptin receptor and leptin in lean and obese subjects as well as in obese subjects during weight loss.
Patients and Methods
Subjects
In total, 51 subjects were included in the study. The study population comprised thirty morbidly obese and 21 healthy subjects, matched for gender and age. The morbidly obese patients were admitted to the Surgical Department of the University Hospital Maastricht for gastric restrictive surgery. Operated patients were evaluated in the outpatient clinic at 3, 6, and 12 months postoperatively. All subjects were otherwise healthy according to history, clinical examination, and routine laboratory findings. In particular, none of the studied subjects had evidence of acute or chronic inflammatory disease. Characteristics of the subjects are presented in Table 1
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In the patient group, blood was taken before surgery or at the outpatient clinic 3, 6, and 12 months after operation. All preoperative blood samples were taken at 1100 h, during preoperative assessment the day before surgery, after an overnight fast. Postoperative samples were taken at approximately 1500 h, after minimally 7 h fast. Blood samples were immediately put on melting ice, and plasma was prepared by centrifugation at 1,400 x g for 10 min at 4 C. The plasma was spun again at 2,700 x g for 10 min at 4 C and recovered plasma was stored in aliquots at -80 C. The study was approved by the ethical committee of the University Hospital Maastricht (Maastricht, The Netherlands). All subjects gave informed consent.
Reagents
BSA was purchased from Sigma (St. Louis, MO). Bovine calf serum purchased from HyClone Laboratories, Inc. (Logan, UT) was heated at 56 C for 30 min before storage at 4 C. Human recombinant leptin receptor/Fc chimera (sLR-Fc) as well as recombinant human leptin were purchased from R&D systems (Minneapolis, MN). Purification of this chimeric protein was performed using protein G-Sepharose 4 Fast Flow affinity chromatography (Amersham Pharmacia Biotech, Uppsala, Sweden). Soluble human leptin receptor-myc tagged was kindly provided by Prof. Dr. J. Tavernier (University of Ghent, Ghent, Belgium).
Production and selection of monoclonal antibodies (mAb)
BALB/c mice [obtained from the central animal facilities of the University of Maastricht (Maastricht, The Netherlands)] were immunized by ip injections with human recombinant sLR-Fc, using Special Oil Phase (Specol) as adjuvant (21). These immunizations were carried out under a protocol approved by the Institutional Animal Care Committee of the University of Maastricht. Spleen cells of the immunized mice were fused with SP2O mouse myeloma cells and plated in 96-well plates. Supernatants of wells containing proliferating cells were screened for the presence of anti-sLR Ab and antihuman IgG Ab (the latter to exclude antihuman IgG reactivity), using ELISA methodology. Hybridomas producing anti-sLR antibodies were subcloned. Isotypes of the mAb were determined using a mAb isotyping mouse test kit (HyCult Biotechnology bv, Uden, The Netherlands).
Anti-sLR mAb were purified from culture supernatant by affinity chromatography on a protein G-Sepharose 4 fast flow column (Amersham Pharmacia Biotech). Antibodies were biotinylated using biotin-X-NHS (Calbiochem, La Jolla, CA).
Soluble leptin receptor sandwich-ELISA
The four generated anti-sLR mAb were tested for their usefulness to construct a sandwich ELISA assay for sLR. Based on pilot experiments, the following method to measure sLR was developed and evaluated. sLR specific mAb 2F1 was coated at a concentration of 3 µg/ml in PBS overnight at 4 C onto 96-well plates (Immuno-Maxisorp; Nunc, Roskilde, Denmark). Free sites were blocked by 1 h of incubation with 1% BSA in PBS at room temperature. Wash buffer consisted of distilled water containing 0.1% Tween 20 and samples were diluted in PBS with 0.1% BSA. Plates were washed four times after each incubation step. Test samples as well as human recombinant sLR-Fc, used as standard, were incubated for 1 h at room temperature. In this experiment, biotinylated mAb 4C3 was used as detection antibody for 1 h at room temperature. HRP-streptavidin conjugate (Zymed Laboratories, Inc., San Francisco, CA) was used to develop the color reaction in combination with 3,3',5,5'-tetramethylbenzidine (Kirkegaard \|[amp ]\| Perry Laboratories, Gaithersburg, MD) and H2SO4. Color intensity was measured by determination of the absorbance at 450 nm using a micro-ELISA reader.
Leptin ELISA
For detection of plasma leptin levels, plates were coated overnight at 4 C with the murine antihuman leptin mAb 4G1. After blocking with 1% BSA in PBS, diluted plasma samples and a dilution series of recombinant human leptin, the standard curve, were added to the plate. Bound leptin was detected by incubation with antihuman leptin mAb 4F8, followed by peroxidase-conjugated goat antimurine IgA. 3,3',5,5'-tetramethylbenzidine was used as substrate for peroxidase, and color intensity was determined by measuring absorbency at 450 nm. The detection limit of this leptin assay is 0.04 ng/ml. Both mAb, 4G1 and 4F8, were kindly provided by Dr. R. Devos (Hoffmann La-Roche, Welwyn Garden City, UK). Leptin was also measured using two leptin ELISAs (high sensitive and normal leptin kit, BioVendor, Brno, Czech Republic), with a detection limit of 0.08 and 0.2 ng/ml respectively. In addition, a leptin RIA was used (Linco Inc., St. Charles, MO) with a detection limit of 0.5 ng/ml.
Purification of native soluble leptin receptor from plasma
To purify native sLR in a single step by affinity chromatography, the murine antihuman leptin receptor mAb 2F1 was coupled to CNBr-Sepharose beads (2 mg/ml gel; Amersham, Pharmacia Biotech). Human plasma with a high sLR concentration (>400 ng/ml) was passed over the affinity column at room temperature. After extensive washing, proteins bound to the column were eluted with 0.1 M glycine-HCl buffer, pH 2.5. The quantity of the eluted natural sLR was estimated by ELISA.
Determination of leptin binding properties of sLR present in plasma
To study the binding-capacity of sLR for leptin, a study with 125I-leptin (Linco Inc.) was performed. sLR-Fc, recombinant sLR-myc tagged or plasma derived sLR, were incubated overnight with different concentrations recombinant leptin, followed by overnight incubation with 125I-leptin. The next day, samples were added to an ELISA plate, coated with mAb 2F1 anti-sLR, incubated for 3 h, followed by five washings with distilled water containing 0.1% Tween 20. After washing, bound 125I-leptin was eluted with glycine-HCl pH 2.5, for 10 min and radioactivity in the supernatant was measured using a universal gammacounter (type 1282 compugamma CS, LKB Wallac, Inc., Turku, Finland). All incubation steps were performed at 4 C.
Furthermore, 2 mg pegylated leptin (Leptin-PEG, kindly provided by Dr. W. Saris and Dr. C. Hukshorn, University of Maastricht) was coupled to CNBr-Sepharose beads (Amerdsham Pharmacia Biotech). Plasma was incubated overnight at 4 C with leptin-PEG Sepharose. After removal of the Sepharose beads by centrifugation (500 rpm) during 5 min, leptin and sLR-levels were measured by ELISA in the supernatant.
After extensive washing, the beads were transferred to Laemmli buffer, heated, and subjected to SDS-PAGE electrophoresis and Western blotting for leptin receptor and leptin (see below).
Size exclusion fractionation of plasma
To determine the distribution of sLR and leptin in plasma, a plasma sample (2 ml) was fractionated at 4 C, using Sephacryl S-300 HR gel filtration. Fractions of 3 ml were automatically collected with a fraction collector (type 2128, Bio-Rad Laboratories, Inc., Hercules, CA). sLR and leptin concentrations in the fractions were measured with the described sLR and leptin assay. Furthermore, human IgG and human albumin were quantified by in house assays and human Liver fatty acid binding protein (L-FABP), a 14-kDa protein, by a commercial ELISA (Hbt, Uden, The Netherlands) to determine the fractions with the highest concentration of these proteins with known molecular mass.
Western blotting for soluble leptin receptor and leptin
For SDS-PAGE electrophoresis, sLR-Fc and plasma derived sLR were heated for 5 min at 95 C in Laemmli buffer and electrophoresed in a polyacrylamide/SDS gel. Proteins were transferred to nitrocellulose membranes, which were subsequently blocked with 0.1 M Tris-buffered saline 0.1% Tween 20 and 5% nonfat dry milk for 1 h at room temperature. Next, membranes were incubated with biotinylated murine antibodies or murine antibodies to human sLR or leptin respectively, in Tris-buffered saline 0.5% nonfat dry milk for 1 h. After washing, the membranes were incubated with streptavidin peroxidase or peroxidase-conjugated goat antimouse antibodies. Positive bands were detected using the chemiluminescent substrate Supersignal West Pico (Pierce Chemical Co., Rockford, IL) and were transferred onto an x-ray film.
Statistical analysis
The Mann-Whitney U test was used to analyze differences between groups. The Wilcoxon signed ranks test was used to analyze differences between preoperative and postoperative values, within the morbidly obese subjects.
Pearson correlation coefficients were computed between body mass index (BMI), sLR, and leptin for the complete group of morbidly obese subjects preoperatively and lean controls. Statistical analyses were performed using the SPSS 10.0.5 statistical package (SSPS Inc., Chicago, IL). P < 0.05 was denoted as statistically significant.
Results
Generation of anti-sLR mAb and development of a specific sLR-assay
Four positive anti-sLR mAb secreting hybridomas (all IgG1), which detected solid phase sLR-Fc chimera in ELISA, were further characterized by Western blotting. Two mAb (2F1 and 4C3) recognized recombinant sLR-Fc, sLR-myc tagged and natural derived plasma sLR, purified using anti-sLR Sepharose (Fig. 1). The antibodies reacted with plasma derived sLR with two bands: one band at approximately 180 kDa and one weak band at approximately 90 kDa. Both antibodies also reacted with recombinant sLR-myc tagged at approximately 90 kDa, which is in line with data of Lewandowski et al. (20), who reported similar results for the very same recombinant sLR protein. Furthermore, both mAb reacted with unreduced sLR-Fc at bands of approximately 380 kDa (Fig. 1). After reduction of sLR-Fc, mainly a band at approximately 180 kDa was found (data not shown).
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, similar dilution curves were obtained in the sLR assay indicating detection of sLR in plasma and sLR-Fc with similar kinetics. Next, we investigated whether the presence of leptin interfered with the detection of sLR. To this end, a concentration range of leptin was added to both sLR-Fc in PBS 0.1% BSA and plasma. The quantification of sLR was not affected by leptin added in excess (Fig. 2B). The absence of effect of leptin addition on sLR quantification strongly suggests that the sLR assay measures not only free sLR, but also sLR-leptin complexes. Next, the influence of natural leptin, as present in plasma, on the measurement of native sLR was studied. To this end, plasma samples containing respectively higher or lower leptin levels were mixed and incubated for 1 h with samples, containing respectively either low or high sLR levels. In these mixed samples, sLR levels were directly proportional to the dilution of the samples (Fig. 2C). These data support the previous data and indicate that the presence of leptin in plasma does not influence quantification of sLR.
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Earlier reports suggested the presence of sLR-leptin complexes in plasma (14, 16, 21). In this context, we determined the distribution of sLR and leptin in plasma by size exclusion fractionation. Plasma (2 ml) was fractionated using Sephacryl S-300 HR gel filtration and sLR and leptin concentrations were measured by ELISA (Fig. 3). Fractions 49–60 contained sLR with a peak at fraction 55. The sLR peak preceded the peak of human IgG (approximately 150 kDa) with a peak at fraction 56, corresponding to a molecular mass of approximately 160–180 kDa. These data are in line with size exclusion fractionation experiments of others (20) and the Western blot data as reported above. Size exclusion fractionation did not allow discrimination between putatively in plasma present sLR-leptin complexes (estimated MW 180+16 kDa) and free sLR.
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1.25 nM sLR) to undetectable levels (<1.5 ng/ml) was observed, whereas plasma incubated with antibodies to human albumin bound to Sepharose showed no change in sLR concentration (Table 2). Interestingly, the depletion of sLR was accompanied by an increase of the plasma leptin concentration. The leptin concentration raised from 21.9 to 34.9 ng/ml (1.36 to 2.18 nM leptin), an increase of 0.82 nM leptin. In the control experiments, leptin concentration did not change in plasma incubated with antibodies to human albumin which were coupled to Sepharose. Furthermore, incubation of leptin-Sepharose with PBS showed only minimal leakage of the Sepharose-bound leptin. The plasma depletion of sLR by leptin-Sepharose implicates that the soluble leptin receptor, as detected by our ELISA, has leptin-binding capacity. Moreover, the observed increase of leptin in such treated plasma strongly suggests dissociation of leptin-sLR complexes in the presence of an excess leptin bound to Sepharose. Presuming that all leptin-sLR complexes present in plasma are dissociated, and based on the assumption that leptin and sLR bind in a 1:1 ratio, these data might indicate that at least 64% of all sLR present in plasma was bound to leptin [increase in leptin in nM (0.82) divided by the concentration of sLR in nM (1.25)*100%].
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, 125I-leptin binding with recombinant sLR or sLR-Fc decreased with increasing concentration of recombinant cold leptin, which implicates that leptin binds to sLR-myc tagged as well as to sLR-Fc. The data show that mAb 2F1, used as coating antibody in the newly developed sLR-assay, detects sLR capable of binding leptin. However, no binding of 125I-leptin with plasma derived sLR was found (data not shown). An explanation for the latter might be the high percentage of saturation of sLR in plasma with leptin, as our data above indicated. To ascertain the presence of sLR-leptin complexes in plasma, plasma was incubated with leptin Sepharose, Sepharose carrying antibodies to human leptin receptor or with control Sepharose. To diminish aspecific binding to Sepharose beads, plasma was three times 24 h precleared with control Sepharose (Sepharose beads with mouse antirat MAC1 antibodies). The proteins attached to the Sepharose beads were analyzed by Western blot. Figure 5 shows that anti-sLR Sepharose bound both leptin receptor and leptin, indicating a binding of mAb 2F1 with both free sLR and the sLR-leptin complex. Furthermore, incubation of plasma with leptin-Sepharose led to binding of sLR to the leptin-Sepharose beads, indicating that the mAb used for the sLR-ELISA are capable of detecting sLR with binding capacity for leptin as well as sLR-bound to leptin.
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The above finding that plasma contains sLR, which appeared to be to a large extend bound to leptin, and the observation that leptin was not detectable in the gel filtration fractions that contained sLR, prompted us to investigate the capacity of the leptin assay used above, to detect leptin bound to sLR. In addition, other widely used leptin-assays were studied.
Recombinant human leptin (3 ng/ml 0.2 nM) was incubated for 48 h at 4 C with 0, 9, 18, and 90 ng/ml ( 0, 0.1, 0.2 and 1.0 nM) leptin free sLR-Fc or with native sLR (with unknown leptin concentration), purified from plasma with an anti-sLR column.
As depicted in Table 3, the detection of leptin in all four leptin assays was reduced with increasing sLR-Fc as well as native sLR concentrations. There appears to be a difference between the assays in extend of inhibition with the Linco RIA assay being less consistently inhibited than the other assays. Furthermore, it appeared that the native sLR was more effective in inhibiting the leptin assays, than the sLR-Fc. A fact that might be due to the presence of the Fc-tail in the molecules. Taken together, these data indicate that the leptin assays investigated are strongly inhibited by the presence of sLR in the test sample and most likely by the formation of sLR-leptin complexes.
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BMI, leptin, and sLR concentrations in lean and morbidly obese subjects
Using the characterized assays for leptin and sLR, we measured these parameters in subjects with high and low BMI. Table 1
summarizes the characteristics of 30 morbidly obese subjects and 21 healthy subjects studied. The lean subjects had a mean BMI ± SD of 24.6 ± 3.1, and the morbidly obese subjects 46.1 ± 5.8 kg/m2.
Plasma leptin concentrations in lean subjects and the preoperative concentrations in morbidly obese subjects ranged from 0.8–186.7 ng/ml. The leptin concentration in this population correlated strongly with BMI (r = 0.796, P < 0.001, Fig. 6A). Morbidly obese subjects showed significantly higher plasma leptin levels (95.0 ± 53.2 ng/ml) compared with lean subjects (15.5 ± 21.3). Most interestingly, despite the large variation, sLR concentrations in obese subjects were significantly lower (21.8 ± 47.4 ng/ml) compared with lean subjects (81.2 ± 143.2). There was a significant inverse correlation of sLR levels with BMI (r = -0.294, P < 0.05, Fig. 6B). However, leptin and sLR did not correlate with each other.
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). However, in accordance with the mean BMI of 33.2 ± 5.6 kg/m2 at 12 months, which is still substantially higher than the BMI of lean healthy subjects, leptin levels remain significantly elevated.
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In this report, we describe an investigation on the circulating soluble leptin receptor. The studies started with the generation of mAb against the extracellular part of the leptin receptor. The mAb allowed detection of sLR by Western blot and quantification by ELISA. Western blot analysis of sLR isolated from plasma with Sepharose beads with anti-sLR mAb revealed a molecular mass of approximately 180 kDa. This is slightly higher than the reported molecular mass of approximately 140 kDa [although also molecular masses of 110 kDa and 290–300 kDa were reported (18, 22, 23, 24)] for human sLR and murine sLR (25). In our gel filtration experiments, sLR was found in fractions containing proteins with a molecular mass of 160–180 kDa. The differences in molecular mass for sLR, of which the core protein molecular mass is approximately 90 kDa, may be caused by differences in level of glycosylation as reported (18, 24, 25).
In plasma, sLR have been reported to circulate both in free form and bound to leptin (16, 26). Two sets of experiments supported the presence of circulating sLR-leptin complexes. First, Western blot analysis on plasma- immunoprecipitated sLR demonstrated the presence of both leptin and soluble leptin receptor. Second, incubation of plasma with Sepharose bound leptin resulted in a replacement of leptin bound to sLR by Sepharose bound leptin, leading to enhanced free leptin levels. Next, we assessed whether the developed sLR-ELISA measured apart from free sLR, also sLR complexed with leptin. To this end, plasma-derived sLR and recombinant sLR-Fc were incubated with different leptin concentrations and quantified. Formation of sLR-leptin complexes was confirmed by incubation of sLR-Fc with 125I-leptin. The results demonstrated that the presence of leptin did not affect detection of sLR, indicating that both free and leptin bound sLR are measured by the sLR-ELISA.
Based on these results, we had expected to detect leptin in sLR-containing fractions in gel filtration experiments in analogy to Lewandowsky et al. (22). The latter used a RIA and demonstrated leptin in such sLR-containing fractions. However, leptin was only detected in fractions with a molecular mass of approximately 16 kDa. This prompted us to investigate whether the presence of sLR in plasma affects the quantification of leptin. In four widely used leptin assays, leptin detection was studied in the presence of plasma derived sLR or recombinant sLR-Fc. Both types of sLR inhibited the detection of leptin in a dose depended fashion. This explains that leptin was not measured in the gel filtration fractions containing sLR. Moreover, this implicates that these assays primarily measure free leptin and underestimate the total amount of leptin present in plasma because the amounts of sLR present in plasma are in the range in which strong inhibition of leptin detection was seen.
Next, we studied the free leptin and sLR concentrations in plasma of lean and morbidly obese subjects, using the above mentioned assays. In lean individuals (BMI < 30 kg/m2), a mean sLR concentration of 81.2 ng/ml was found, whereas in morbidly obese subjects (BMI > 40 kg/m2), a significantly lower mean sLR concentration of 21.8 ng/ml was observed. A significant inverse correlation of sLR with BMI was found. As expected, and consistent with previous reports (19, 27), significantly higher leptin levels were observed in morbidly obese individuals compared with lean subjects. Considering the molar ratio of these proteins it appeared that in plasma of lean subjects the molar ratio of free leptin to sLR was 1:1, with a maximal leptin to sLR ratio of 2:1 in case all sLR were saturated with leptin. In contrast, in morbidly obese subjects a ratio of 25:1 was found (Fig. 7B). Thus, on a molar basis, we observed in plasma always more leptin than sLR. These data are supported by Sinha et al. (16), who demonstrated that in lean subjects leptin circulates mainly in the bound form, whereas in obese subjects the majority of leptin circulates in the free form.
A significant decrease of BMI, following gastric restrictive surgery, was associated with a decrease of leptin concentrations and an increase of sLR levels. However, in the first 3 months after gastric restrictive surgery, which caused a strong reduction of food intake, leptin levels dropped dramatically, whereas sLR concentrations remained unchanged. This suggests that neither food intake, nor leptin levels, were responsible for the reduced sLR-concentrations observed in morbidly obese subjects. In line with these data, C. J. Hukshorn (Department of Human Biology, University of Maastricht, personal communication) found that sc injections of recombinant leptin to moderately obese individuals, leading to very high leptin plasma levels (>5 µg/ml), did not affect sLR-levels (measured with our ELISA).
Various functions for the circulating soluble leptin receptor have been proposed. Analogous to circulating receptors of various cytokines, sLR has been reported to function as inhibitor or stabilizer of leptin (16, 24). Recombinant sLR inhibited leptin binding to COS7 cells, expressing membrane bound full-length human leptin receptor (18), which supports the inhibitory properties of sLR. In another study, an increase in plasma leptin levels was observed in rats injected with an adenovirus encoding for the sLR (28). The latter data were explained as being proof for a stabilizing function of sLR, probably by reducing leptin clearance, implicating that sLR play a role in determining the amount of leptin in circulation. The authors of this study postulated that this may be an important mechanism to regulate the bioavailability of leptin. In line with this, Lahlou et al. (29) demonstrated high levels of leptin in patients with high sLR levels due to a defect in the leptin receptor.
Regulation of sLR could be a physiological way of regulating leptin levels in lean individuals, which have a much higher sLR/leptin ratio than obese individuals. Since leptin was reported to have strong immunoprotective effects (30, 31), it could be important to enhance the bioavailability of leptin in lean individuals with low leptin levels.
High free leptin levels were observed in morbidly obese subjects, probably caused by high leptin release by adipocytes due to abundant food intake. Interestingly, during food restriction, circulating leptin levels in lean and obese subjects drop dramatically within 1 d after onset of starvation (32), indicating that leptin release is strongly reduced. In context of the proposed stabilization function of sLR, the low sLR levels in morbidly obese individuals, could be part of a feedback mechanism aimed at reducing the increase in leptin.
At this stage, it is unclear why exogenous leptin does lead to weight reduction in CD-1 and high fat diet induced obese mice (33, 34), whereas such effects were not observed in obese individuals treated for 12 wk with leptin-PEG (35). An increase in the bioavailability of leptin in ob/ob mice by overexpression of sLR led to an improved weight reducing effect of exogenous leptin. In this context, it remains to be investigated whether the inability of exogenous leptin to induce body weight reduction in man is due to a lack of sLR. Plasma sLR might function like the soluble IL-6 receptor for the cytokine IL-6 (also a member of the gp-130 receptor family) and enhance leptin signaling. Hypothetically, the observed decreased sLR-levels in morbidly obese subjects might be a possibility for therapeutic interventions with recombinant sLR, instead of leptin, for the treatment of obesity.
Next to the function, the origin of sLR in plasma is also unclear. It could originate from alternative splicing of the leptin receptor or from full-length functional leptin receptors released by enzymatic cleavage. In the latter case, sLR levels in plasma could reflect the amount of leptin receptor expressed by tissues. This could implicate that decreased plasma levels of sLR as found in morbidly obese subjects are a sign of decreased expression of functional leptin receptors. This might be in agreement with the proposed leptin resistance in morbidly obese subjects (36, 37, 38). Elucidation of such a putative role for sLR in leptin resistance may help understand the development of obesity.
In summary, we have developed methods to study human sLR and demonstrated that circulating sLR levels decrease and leptin concentrations increase with increasing body weight. Excessive weight loss following gastric restrictive surgery was shown to result in a strong decrease in circulating leptin levels and increase in soluble leptin receptor levels.
Acknowledgments
We want to thank Dr. R. Devos from Hoffman-LaRoche Inc. (Welwyn Garden City, UK), for providing reagents for the leptin ELISA. Furthermore, we want to thank Prof. Dr. J. Tavernier, Ghent, Belgium, for providing recombinant sLR-myc tagged. We thank Prof. P. B. Soeters for creating the possibilities to perform this project. We are indebted to M. van de Watering for her technical assistance.
Footnotes
This work was supported by AGIKO Stipendium of The Netherlands Organization of Scientific Research (to F.M.H.v.D.), and by BIO4-CT97-2107 from the European Commission (to W.A.B.).
Abbreviations: BMI, Body mass index; mAb, monoclonal antibodies; sLR, soluble leptin receptor; sLR-Fc, human recombinant leptin receptor/Fc chimera.
Received July 24, 2001.
Accepted December 20, 2001.
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 C. Garofalo, D. Sisci, and E. Surmacz Leptin Interferes with the Effects of the Antiestrogen ICI 182,780 in MCF-7 Breast Cancer Cells Clin. Cancer Res., October 1, 2004; 10(19): 6466 - 6475. [Abstract] [Full Text] [PDF]
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F. M. H. van Dielen, W. A. Buurman, M. Hadfoune, J. Nijhuis, and J. W. Greve Macrophage Inhibitory Factor, Plasminogen Activator Inhibitor-1, Other Acute Phase Proteins, and Inflammatory Mediators Normalize as a Result of Weight Loss in Morbidly Obese Subjects Treated with Gastric Restrictive Surgery J. Clin. Endocrinol. Metab., August 1, 2004; 89(8): 4062 - 4068. [Abstract] [Full Text] [PDF]
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M. Misra, K. K. Miller, C. Almazan, K. Ramaswamy, A. Aggarwal, D. B. Herzog, G. Neubauer, J. Breu, and A. Klibanski 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 J. Clin. Endocrinol. Metab., July 1, 2004; 89(7): 3486 - 3495. [Abstract] [Full Text] [PDF]
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 G. Yang, H. Ge, A. Boucher, X. Yu, and C. Li Modulation of Direct Leptin Signaling by Soluble Leptin Receptor Mol. Endocrinol., June 1, 2004; 18(6): 1354 - 1362. [Abstract] [Full Text] [PDF]
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M. Schroth, J. Kratzsch, M. Groschl, M. Rauh, W. Rascher, and J. Dotsch Increased Soluble Leptin Receptor in Children with Nephrotic Syndrome J. Clin. Endocrinol. Metab., November 1, 2003; 88(11): 5497 - 5501. [Abstract] [Full Text] [PDF]
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D. R. Mann, A. O. K. Johnson, T. Gimpel, and V. D. Castracane Changes in Circulating Leptin, Leptin Receptor, and Gonadal Hormones from Infancy until Advanced Age in Humans J. Clin. Endocrinol. Metab., July 1, 2003; 88(7): 3339 - 3345. [Abstract] [Full Text] [PDF]
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 V. T. K. Chow and M. C. Phoon MEASUREMENT OF SERUM LEPTIN CONCENTRATIONS IN UNIVERSITY UNDERGRADUATES BY COMPETITIVE ELISA REVEALS CORRELATIONS WITH BODY MASS INDEX AND SEX Advan Physiol Educ, June 1, 2003; 27(2): 70 - 77. [Abstract] [Full Text] [PDF]
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T. Ueland, C. Kristo, K. Godang, P. Aukrust, and J. Bollerslev Interleukin-1 Receptor Antagonist Is Associated with Fat Distribution in Endogenous Cushing's Syndrome: A Longitudinal Study J. Clin. Endocrinol. Metab., April 1, 2003; 88(4): 1492 - 1496. [Abstract] [Full Text] [PDF]
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M. Faraj, P. J. Havel, S. Phelis, D. Blank, A. D. Sniderman, and K. Cianflone Plasma Acylation-Stimulating Protein, Adiponectin, Leptin, and Ghrelin before and after Weight Loss Induced by Gastric Bypass Surgery in Morbidly Obese Subjects J. Clin. Endocrinol. Metab., April 1, 2003; 88(4): 1594 - 1602. [Abstract] [Full Text] [PDF]
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 N. Kado, J. Kitawaki, H. Koshiba, H. Ishihara, Y. Kitaoka, M. Teramoto, and H. Honjo Relationships between the serum levels of soluble leptin receptor and free and bound leptin in non-pregnant women of reproductive age and women undergoing controlled ovarian hyperstimulation Hum. Reprod., April 1, 2003; 18(4): 715 - 720. [Abstract] [Full Text] [PDF]
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H. Ge, L. Huang, T. Pourbahrami, and C. Li Generation of Soluble Leptin Receptor by Ectodomain Shedding of Membrane-spanning Receptors in Vitro and in Vivo J. Biol. Chem., November 22, 2002; 277(48): 45898 - 45903. [Abstract] [Full Text] [PDF]
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) and of sLR-Fc (•). Both curves show similar kinetics. B, Addition of leptin does not affect quantification of soluble leptin receptor concentration. A range of leptin concentrations was added to 100 ng/ml sLR-Fc in PBS + 0.1% BSA (•) and to plasma containing 35 ng/ml native sLR (
). C, Influence of leptin on the quantification of sLR was studied by making dilution curves of 7 mixtures of each two plasma samples with a high and low soluble leptin receptor level. Every point on the curves represents a mixture of the indicated ratio of dilution of both samples.
; right peak). The three capital letters correspond to the top peaks of human IgG (A; 150 kDa), human albumin (B; 60 kDa) and human L-FABP (C; 14 kDa). All fractions were assayed using human IgG, albumin, L-FABP, sLR, and leptin ELISAs.
) or recombinant sLR-myc tagged (
), after overnight preincubation at 4 C with different concentrations of cold leptin. Concentrations of cold leptin to sLR were in a molar ratio of respectively 0 to 1, 2:1, 6:1, and 20:1 for sLR-Fc and 0:1, 1:1, 3:1, and 10:1 for sLR-myc tagged. When 125I-leptin was added to plasma derived sLR, no binding could be observed (data not shown).
