Recombinant ob protein reduces feeding and body weight in the ob/ob mouse

Serum and follicular fluid leptin during in vitro fertilization: relationship among leptin increase, body fat mass, and reduced ovarian response

The satiety factor leptin is expressed in several reproductive tissues, but its role in the control of reproductive physiology is not well understood. We studied leptin concentrations in the sera and follicle fluids of 52 women [body fat mass percentage (BFM%) range, 19.6-38.8%] undergoing pituitary down-regulation and ovarian hyperstimulation for in vitro fertilization (IVF) treatment. Fasting serum samples were collected 1) at maximal suppression before the initiation of gonadotropin treatment, 2) at maximal ovarian hyperstimulation, 3) at the time of oocyte retrieval, and 4) 16 days later when all subjects were under exogenous luteal support using 600 mg progesterone daily. Follicular fluid (FF) was obtained at oocyte retrieval from two representative preovulatory follicles in both ovaries. During ovarian hyperstimulation there was a significant 60% increase in serum leptin concentrations from 10.9 +/- 1.1 (SEM) to 15.7 +/- 1.5 ng/mL (P < 0.01) between suppression and maximal hyperstimulation, demonstrating that the ovarian functional state can affect serum leptin concentrations. A serum leptin increase of 22-198% during ovarian hyperstimulation was evident in 43 subjects, whereas in 9, leptin concentrations remained unchanged. A positive correlation between leptin change and BFM% (r = 0.55; P < 0.0005) was observed in the 43 leptin responders. The follicular fluid leptin level was similar to that in serum. In separate linear regression analysis, BFM% contributed to 59-64%, body mass index to 46-56%, and weight to 46-55% (all P < 0.001) of the variability in leptin concentrations at the 4 time points. The 20-fold increase in serum estradiol concentrations during IVF was not significantly correlated with changes in leptin concentrations. On the contrary, the relative serum leptin increase was negatively associated with the ovarian response to hyperstimulation, as revealed by the numbers of follicles (b = -0.28; r2 = 8.1%; P < 0.05) and oocytes retrieved (b = -0.39; r2 = 15.2%; P < 0.01). This relationship was further reflected in a positive correlation between the percent increases in leptin and FSH concentrations (r = 0.39; P < 0.01). The significant relationship of high leptin and reduced ovarian response was also maintained when the cumulative dose of FSH was used as a covariable. Reduced ovarian response was not a function of body mass index, BFM%, basal leptin levels, or insulin concentrations. Fasting serum insulin concentrations remained unchanged in response to IVF, but were positively correlated to serum leptin concentrations at all four time points. Our data suggest that leptin production may be influenced by the ovarian functional state. During IVF a high relative leptin increase is associated with adiposity and a reduced ovarian response. These observations support the possibility that high leptin concentrations might reduce ovarian responsiveness to gonadotropins. Hence, leptin might explain in part why obese individuals require higher amounts of gonadotropins than lean subjects to achieve ovarian hyperstimulation.

Decreased visceral adiposity accounts for leptin effect on hepatic but not peripheral insulin action

Leptin decreases visceral fat (VF) and increases peripheral and hepatic insulin action. Here, we generated similar decreases in VF using leptin (Lep), beta(3)-adrenoreceptor agonism (beta3), or food restriction (FR) and asked whether insulin action would be equally improved. For 8 days before the in vivo study, Sprague-Dawley rats (n = 24) were either fed ad libitum [control (Con)], treated with Lep or beta3 (CL-316,243) by implanted osmotic mini-pumps, or treated with FR. Total VF was similarly decreased in the latter three groups (Lep, 3.11 +/- 0.96 g; beta3, 2.87 +/- 0.48 g; and FR, 3.54 +/- 0.77 g compared with 6.91 +/- 1.41 g in Con; P < 0.001) independent of total fat mass (by (3)H(2)O) and food intake. Insulin (3 mU. kg(-1). min(-1)) clamp studies were performed to assess hepatic and peripheral insulin sensitivity. Decreased VF resulted in similar and marked improvements in insulin action on glucose production (GP) (Lep, 1.19 +/- 0.51; beta3, 1.46 +/- 0.68; FR, 2.27 +/-0.71 compared with 6.06 +/- 0.70 mg. kg(-1). min(-1) in Con; P < 0.001). By contrast, reduction in VF by beta3 and FR failed to reproduce the stimulation of insulin-mediated glucose uptake ( approximately 60%), glycogen synthesis ( approximately 80%), and glycolysis ( approximately 25%) observed with Lep. We conclude that 1) a moderate decrease in VF uniformly leads to a marked increase in hepatic insulin action, but 2) the effects of leptin on peripheral insulin action are not due to the associated changes in VF or beta3 activation.

Testosterone and leptin in a group of Chinese with and without diabetes

AIM

Leptin levels in the overweight and differences between men and women may be confounded by sex hormones in obesity-related type 2 diabetes. We carried out a study of leptin and testosterone and the effect of diabetes on this relationship in 71 Chinese subjects (51 with type 2 diabetes and 20 healthy non-diabetics) of whom 32 were overweight (body mass index (b.m.i.) > 25.0 kg/m2).

METHODS AND RESULTS

Leptin levels were similar in the diabetic (median 7.4, interquartile range 4.2-10.6 ng ml-1) and non-diabetic (11.3 (6.0-16.3) ng ml-1) subjects. Testosterone and free testosterone in diabetics and non-diabetics were similar for both gender. Regardless of diabetes status, testosterone and its free form tend to increase in overweight women and decrease in men. Leptin correlated with testosterone in women with diabetes (Pearson correlation coefficient r = 0.495, p = 0.037) but not in diabetic men. Sixty-eight per cent of leptin variance was affected by b.m.i, insulin, sex (female) and diabetes status in a regression model that excluded testosterone (total and free). However, testosterone and insulin were predictors of leptin changes in the non-diabetic subjects. In diabetes, b.m.i., sex (female) and insulin remained as predictors of leptin changes.

CONCLUSIONS

These results suggest that apart from body adiposity, testosterone also influences leptin levels and that diabetes has a significant effect on this association.

High-level production of human leptin by fed-batch cultivation of recombinant Escherichia coli and its purification

Human leptin is a 16-kDa (146-amino-acid) protein that is secreted from adipocytes and influences body weight homeostasis. In order to obtain high-level production of leptin, the human obese gene coding for leptin was expressed in Escherichia coli BL21(DE3) under the strong inducible T7 promoter. The recombinant leptin was produced as inclusion bodies in E. coli, and the recombinant leptin content was as high as 54% of the total protein content. For production of recombinant human leptin in large amounts, pH-stat fed-batch cultures were grown. Expression of leptin was induced at three different cell optical densities at 600 nm (OD600), 30, 90, and 140. When cells were induced at an OD600 of 90, the amount of leptin produced was 9.7 g/liter (37% of the total protein). After simple purification steps consisting of inclusion body isolation, denaturation and refolding, and anion-exchange chromatography, 144.9 mg of leptin that was more than 90% pure was obtained from a 50-ml culture, and the recovery yield was 41.1%.

Truncated human leptin (delta133) associated with extreme obesity undergoes proteasomal degradation after defective intracellular transport

We recently described a homozygous frameshift mutation in the human leptin (ob) gene associated with undetectable serum leptin and extreme obesity in two individuals. This represented the first identified genetic cause of morbid obesity in humans. Preliminary data suggested a defect in the secretion of this truncated (delta133) mutant leptin. In the present investigation, we have examined the mechanisms underlying the defective secretion of the delta133 leptin in transient transfection studies in Chinese hamster ovary and monkey kidney epithelium cells. Consistent with our previous observations, only immunoreactive wild-type (wt) leptin was secreted. In pulse chase experiments, intracellular wt leptin levels decreased, concomitant with secretion into the medium. In contrast, though immunoreactive delta133 leptin disappeared from cell lysates with kinetics similar to those of wt leptin (half-life, 45 min), it was not detected in the medium. Inhibition of the proteasome, using the inhibitor clastolactacystin beta-lactone, led to a significant increase in the intracellular levels of delta133 leptin, indicating a role for the proteasome in the degradation pathway. Although intracellular immunoprecipitated wt and delta133 leptin levels were comparable, analysis of total cell lysates revealed a 7-fold increase in total intracellular delta133 leptin, compared with wt leptin. Size-exclusion membrane filtration demonstrated that intracellular delta133 leptin accumulated in an aggregated form, presumably as a result of misfolding in the endoplasmic reticulum. Consistent with this, an endoplasmic reticulum-like localization for delta133 leptin was detected by immunofluorescence microscopy. In conclusion, the delta133 mutant leptin is not secreted but accumulates intracellularly, as a consequence of misfolding/aggregation, and is subsequently degraded by the proteasome. These studies further define the genotype/phenotype correlation in this paradigmatic case of human leptin deficiency.

Model for the regulation of energy balance and adiposity by the central nervous system

In 1995, we described a new model for adiposity regulation. Since then, data regarding the biology of body weight regulation has accumulated at a remarkable rate and has both modified and strengthened our understanding of this homeostatic system. In this review we integrate new information into a revised model for further understanding this important regulatory process. Our model of energy homeostasis proposes that long-term adiposity-related signals such as insulin and leptin influence the neuronal activity of central effector pathways that serve as controllers of energy balance.

Reflex effects from leptin sensors in the white adipose tissue of the epididymis to the efferent activity of the sympathetic and vagus nerve in the rat

Efferent nerve signals were recorded from the central cut end of the small nerve filament dissected from the sympathetic nerve innervating the white adipose tissue (WAT) of epididymis, inter scapular brown adipose tissue (BAT), pancreas, liver, adrenal medulla, and vagus nerve innnervating the pancreas and liver. Injection of leptin (2 ng, 0.2 ml) into the white adipose tissue of the either side of the epididymis evoked reflex activation of the sympathetic nerve activity and suppression in vagus nerve activity. These observations suggest that leptin sensors in the white adipose tissue of the epididymis play a role in reflex regulation of metabolic functions of the body through the modulatory change in sympathetic and vagal outflow.

Leptin receptor long-form splice-variant protein expression in neuron cell bodies of the brain and co-localization with neuropeptide Y mRNA in the arcuate nucleus

Reduced leptin (Ob protein) signaling is proposed to be a stimulus for the activation of neuropeptide Y (NPY) gene activity and increased expression of mRNA for the long form of the leptin receptor (Ob-Rb) in the hypothalamic arcuate nucleus. To determine if Ob-Rb protein is expressed in arcuate nucleus NPY neurons, we developed an affinity-purified polyclonal antibody against amino acids 956-1102 of human Ob-Rb. This antibody specifically recognizes the cytoplasmic tail of Ob-Rb and does not react with shorter leptin-receptor variants. Western immunoblots of Ob-Rb-transfected COS cells showed a single 150-kD band, and immunofluorescence revealed intense perinuclear staining in the cytoplasm. A 150-kD band was also present in Western immunoblots of hypothalamus. Immunocytochemical staining of brain slices revealed immunoreactive Ob-Rb protein concentrated in many neuronal cell bodies in the same regions of the forebrain that also express Ob-Rb mRNA. In the hypothalamus, Ob-Rb-positive cell bodies were abundant in the arcuate nucleus and ventromedial nucleus, with lesser numbers in the dorsomedial nucleus and paraventricular nucleus. Immunostaining was also detected in cell bodies of pyramidal cell neurons of the pyriform cortex and cerebral cortex, in neurons of the thalamus, and on the surface of ependymal cells lining the third ventricle. The choroid plexus, which expresses the short Ob-Ra form, was negative. Combined immunocytochemistry for Ob-Rb protein and fluorescence in situ hybridization for NPY mRNA identified arcuate nucleus neurons containing both NPY mRNA and Ob-Rb protein. The present finding of Ob-Rb protein in neurons that express NPY mRNA supports the hypothesis that arcuate nucleus NPY neurons are direct targets of leptin and play an important role in regulation of food intake and body weight.

Serum leptin concentrations in women taking oral contraceptives

OBJECTIVES

To investigate serum leptin concentrations in women taking oral contraceptives containing the same gestagen and different doses of ethinyl estradiol.

STUDY DESIGN

30 women received tablets containing 20 microg of ethinyl estradiol (EE) and 150 microg of desogestrel (DSG) (Mercilon) whereas another group of 30 women received 30 microg of EE and 150 microg of DSG (Marvelon). Serum leptin concentrations were estimated using a Leptin RIA kit (Linco Research USA) after an overnight fast on the first day of the cycle prior to the onset of therapy as well as after the 3rd and 6th treated cycles.

RESULTS

In both groups a positive correlation between serum leptin and body mass index (BMI) was found (r=0.56; P<0.001 and r=0.67; P<0.001). The initial serum leptin concentration in the Mercilon group was 7.62+/-8.46 ng/ml. This value was not statistically different from values after 3 months (9.31 8.23 ng/ml) and after 6 months (10.53+/-8.03 ng/ml) of treatment. Very similar results were found in patients receiving Marvelon: 8.81+/-6.56 ng/ml initially; 11.62+/-11.16 ng/ml at 3 months, and 10.38+/-7.32 ng/ml at 6 months. The statistical analysis did not reveal any significant difference at each investigated time point in either study group.

CONCLUSIONS

Modern low dose OC containing third generation gestagen and low dose of ethinyl estradiol, does not have any influence on serum leptin or BMI, and therefore does not exert a significant influence on body energy metabolism.

Effect of the genetic background on the reproduction of leptin-deficient obese mice

Obesity is often associated with an impairment of the hypothalamic-pituitary-gonadal axis. The leptin-deficient ob/ob mouse model is characterized by a morbid obesity with a sterility in males and females that is corrected by continuous leptin treatment. Since ob/ob mice are maintained on the C57BL/6J inbred genetic background, we sought to determine whether their infertility can be corrected without leptin treatment but via the effect of modifier genes brought into the obese-sterile phenotype by a different genetic background. Thus, we generated via an F2 intercross ob/ob mice on a mixed C57BL/6J-BALB/cJ genetic background and assayed them for fertility by mating with wild-type C57BL/6J mice. Whereas genetically heterogeneous F2 obese females remained sterile like male and female C57BL/6J ob/ob mice, 41% of F2 C57BL/6J-BALB/cJ obese males were capable of reproducing despite a morbidly obese state. Therefore, the sterility of the original C57BL/6J ob/ob mouse model was genetically corrected independently of its obese state via the effects of modifier genes. Unlike testosterone levels, triglyceride levels, and testes weight-to-body weight ratios, which were all higher in fertile vs. sterile mice, glucose levels were similar in both groups, indicating that the underlying hyperglycemia of ob/ob mice was not an impediment to the onset of fertility. A genome-wide scan in F2 ob/ob males resulted in the localization of four modifier loci on chromosomes 1, 3, 5, and 14 with respective quantitative traits consisting of number of pregnancies, testes weights normalized to body weights, body weight at 8 weeks of age, and circulating testosterone. We conclude that the inheritance of modifier genes at the identified loci acts to promote fertility of otherwise sterile leptin-deficient obese male mice.

Cord blood leptin concentrations in relation to intrauterine growth

OBJECTIVE

Leptin, a hormone that signals the amount of energy stores to the brain, has recently been shown to play a role in the regulation of several hypothalamic pituitary axes, including the growth hormone axis. To investigate a potential association between cord blood leptin concentrations and intrauterine growth we measured leptin concentrations in the cord blood of small for gestational age (SGA), appropriate for gestational age (AGA) and large for gestational age (LGA) healthy newborns.

PATIENTS AND MEASUREMENTS

Cord blood leptin concentrations were evaluated in 25 SGA, 100 AGA, and 45 LGA, neonates.

RESULTS

Leptin was detectable in all newborns in concentrations comparable with those found in adults. Moreover, SGA newborns had lower leptin concentrations (3.70 +/- 1.81 micrograms/l) than AGA (5.65 +/- 4.98 micrograms/l) and LGA newborns (11.99 +/- 7.06 micrograms/ l)(P < 0.01). Cord blood leptin concentrations were significantly associated with ponderal index, cord blood insulin concentrations, placental weight and maternal serum leptin concentrations. Importantly, the association between cord blood leptin concentrations and intrauterine growth status persisted after adjusting for adiposity, placental weight, maternal serum leptin concentrations and cord blood insulin concentrations.

CONCLUSIONS

Cord blood leptin concentrations are independently associated with intrauterine growth. Future studies are needed to elucidate the underlying mechanism and clarify the role of leptin in regulating growth and controlling appetite in newborns.

Parabiosis between db/db and ob/ob or db/+ mice

Genetically obese C57B1/6J-m db/db mice were parabiosed with either lean male db/+ or obese female ob/ob mice. Male db/db mice had lower serum leptin than females, and this was reflected in the amount of protein that crossed the parabiotic union into their partners. Eighteen days post operation, ob/ob partners of db/db mice had increased body temperature, lost 50% body weight and 60% body fat, but maintained carcass protein. The db/+ partners of db/db mice had a normal gut content and (by implication) food intake, did not raise their body temperature, but lost significant amounts of both lean and fat tissue during 25 days of parabiosis. The differences between the db/+ and ob/ob partners of db/db mice may be caused by leptin inhibiting growth of male mice, but not of female mice that are on a slower growth curve, or by the excess lipid in ob/ob mice sparing body protein. The db/db partners of ob/ob mice lost a small amount of body fat, but carcass protein was increased by 30%, compared with their controls. These results imply that leptin stimulated release of a circulating growth factor, possibly through activation of the long-form leptin receptor, in ob/ob partners of db/db mice.

Human uncoupling proteins and obesity

Uncoupling protein (UCP) 2 and UCP3 are newly discovered proteins that can uncouple ATP production from mitochondrial respiration, thereby dissipating energy as heat and affecting energy metabolism efficiency. In contrast to UCP1, which is only present in brown adipose tissue, UCP2 has a wide tissue distribution, whereas UCP3 is expressed predominantly in skeletal muscle. Some evidence of a role for UCPs in modulating metabolic rate was provided by linkage and association studies. Furthermore, UCP3 gene expression was found to correlate negatively with body mass index and positively with sleeping metabolic rate in Pima Indians. Treatment with thyroid hormone increases expression of the UCP2 and UCP3 genes. Other regulators of UCP2 and UCP3 gene expression are beta3-adrenergic agonists and glucocorticoids. Surprisingly, fasting has a stimulatory effect on UCP2 and UCP3 mRNA levels, possibly explained by the effects of free fatty acid on UCP2 and UCP3 gene expression.

Serum leptin levels in males with delayed puberty during short-term pulsatile GnRH administration

Leptin may be a possible trigger for puberty. In normal males, it has been shown that leptin increases from the pre-pubertal to the early pubertal stage, and then declines in the late pubertal stage. We examined leptin levels in six male adolescents (mean age 16.3+/-0.6 yr; range 14.2-17.6 yr) with delayed puberty (constitutional delay of puberty no.=2; idiopathic hypogonadotropic hypogonadism no.=4) during 120 days of subcutaneous pulsatile GnRH administration. A group of subjects in pre-puberty (no.=11), early-puberty (n=10) and mid-puberty (no.=7) were evaluated as controls. Morning blood samples were taken for determination of leptin, testosterone, LH and FSH levels. In delayed puberty subjects blood samples were taken every 30 days after the start of GnRH administration. At each examination BMI and testicular volume were evaluated. A follow-up examination was performed in the 6 patients 1.3-7.5 yr after the end of the 120 days of GnRH therapy. At baseline evaluation in delayed puberty mean leptin levels were 11.3+/-2.0 microg/l (median 11.3 microg/l; range 4.7-17.3 microg/l) and were higher than those found in pre-puberty (p=0.04) and mid-puberty (p=0.001). During GnRH administration there was no change in BMI and leptin levels but there was an increase in gonadotrophin levels, testosterone and testicular volume. One hundred and twenty days after, mean serum leptin were 10.1+/-2.1 microg/l (median 9.1 microg/l; range 3.4-16.8 microg/l). At the end of the study, leptin levels were higher in delayed puberty than in mid-puberty (p=0.002). At the follow-up examination leptin levels were 4.3+/-1.3 microg/l (median 3.4 microg/l; range 1.4-9.1 microg/l) (p=0.03 vs end of 120 days GnRH therapy) while testosterone and BMI were not changed. In conclusion 120-day pulsatile GnRH administration induced in males with delayed puberty physiological-like pubertal changes but not the decline in leptin levels reported during the progression of puberty. Therefore, in males with delayed puberty an impairment in the phenomenon of leptin decline associated with progression of puberty could be suggested. However after retrospective diagnosis of pubertal delay and long-term therapy in subjects with idiopathic hypogonadotropic hypogonadism leptin levels declined. These data seem to indicate that time more than increase in testosterone levels and testicular volume is the determinant of leptin decline at puberty.

Total parenteral nutrition increases serum leptin concentration in hospitalized, undernourished patients

BACKGROUND

The hormone leptin has putative roles in both body weight homeostasis (chronic) and satiety (acute). To determine if this dual regulation is observed in hospitalized, undernourished patients, serum leptin concentration was measured before and during total parenteral nutrition (TPN) infusion.

METHODS

Six consecutive patients were considered undernourished, as assessed by an independent multidisciplinary nutrition team, and TPN was prescribed at an initial rate of between 5023.2 and 7333.2 kJ in the first 24 hours. Serum leptin, insulin, and glucose were measured before the infusion and at 3 and 22 hours after initiation of TPN.

RESULTS

Baseline serum leptin concentrations correlated well with the patient's body mass index (BMI; r2 = .85, p<.05). Three hours of TPN infusion produced only modest changes in circulating leptin. However, after 22 hours, leptin concentrations increased by 1.8+/-0.5-fold (p<.05), and this increase was independent of any change in body weight.

CONCLUSIONS

Basal leptin concentrations correlate well with BMI. TPN induces a rise in leptin concentration independent of body weight. Leptin secretion is dually regulated in hospitalized, undernourished patients.

Splice variants of the OB receptor gene are differentially expressed in brain and peripheral tissues of mice

A high affinity receptor for OB protein was recently cloned from the choroid plexus of mice. At least six alternatively spliced forms of the OB receptor (OB-R) gene have been described, all of which encode proteins containing the OB-R extracellular domain. One splice variant encodes a receptor with a long intracellular domain, OB-RL, that has been implicated in OB-R signaling. Here, we have used in situ hybridization to examine the localization of OB-R splice variants in brain and peripheral tissues of adult and newborn mice. Using a probe hybridizing with all known splice variants, we confirmed that OB-R mRNA was widely distributed in the adult tissues. In the CNS, choroid plexus was the major site of expression. We now demonstrate that OB-R mRNA is expressed in peripheral tissues; primarily associated with connective tissues. In addition, OB-R mRNA was detected at higher levels in peripheral tissues of newborn mice than in adult mice. With a probe specific for OB-RL, we confirmed that high mRNA expression was detected in hypothalamic nuclei, while low levels were observed in choroid plexus. We now report that in peripheral tissues of adult mice, OB-RL mRNA expression was either very low or undetectable. In newborn mice, the pattern of OB-RL message expression in the CNS was similar to that of adult mice, while bone was the site of highest OB-RL message expression in the peripheral tissue. These data suggest different biological roles for OB-R splice variants encoding the short and long forms of OB-R. The localization of OB-RL to hypothalamic nuclei supports the idea that OB-RL is the brain receptor that mediates OB protein signaling and actions. In addition, the expression of OB-R message in newborn mice also suggests a biological role of OB-R during development in mice.

Therapeutic controversy: Obesity--a modern-day epidemic

While the hyperleptinemia of obesity is likely to be associated with the metabolic complications of obesity/hyperinsulinemia/insulin resistance, it is not associated with diabetes, with the relative hypercortisolism of upper body obesity, with hypertension in women, (it is in men), or with dyslipidemia. Overall, the correlations between leptin and the metabolic diseases associated with obesity are weak. The equivocal results of an association of leptin with components of the metabolic syndrome make it unlikely that leptin affects these directly. (On the other hand, these correlations, when found, preclude any causal relationship between leptin and metabolic diseases.) There are experimental data showing a definite role for insulin and glucocorticoids in the regulation of leptin, and of leptin in the regulation of insulin. More data are required on the effects of leptin, but it is likely that leptin will not be a major link between obesity and the metabolic syndrome. Certainly, however, when leptin is available for clinical use, its effect on different aspects of the metabolic syndrome will be worth studying.

Production of leptin in Escherichia coli: a comparison of methods

A procedure is described for gram-scale refolding of Escherichia coli-derived human leptin inclusion bodies. Refolding was achieved by gradually reducing denaturant using a diafiltration method. Refolded leptin is characterized by in vivo modulation of food intake, reduction in body weight, and lowering of insulin and glucose levels in ob/ob mice. In addition, refolded leptin is characterized by radioimmunoassay (RIA) and activation of the leptin receptor in a cell-based assay. For comparison we also refolded leptin by a simple dilution method and produced periplasmic derived leptin, which did not require ex vivo folding. Leptin produced by these three methods and leptin obtained from commercial sources were compared using the RIA and the cell-based assay and appeared to be of comparable quality and potency.

The obese gene product, leptin: possible role in obesity-related hypertension in adolescents

OBJECTIVE

The mechanism of the association between obesity and hypertension is not clear. The recently discovered obese gene product, leptin, the levels of which increase in obese subjects, has been shown to reduce food intake and increase sympathetic nervous system activity in animal studies. The present study was undertaken to elucidate the relationship between blood pressure and factors related to obesity, including leptin, in different age groups.

METHODS

The subjects were 348 Japanese male adolescents (15-17 years old) and 165 men (40-59 years old) not taking medication for hypertension, diabetes mellitus or hyperlipidaemia. Height, weight, blood pressure, heart rate, plasma glucose, lipid profiles, serum insulin and leptin levels were measured in the morning after an overnight fast

RESULTS

Body mass index (BMI), serum leptin level and the homeostasis model insulin resistance index increased in the order of blood-pressure category (i.e. normotensive < high normal < hypertensive) in both the male adolescents and the middle-aged men. In addition, simple linear regressions revealed that both systolic and diastolic blood pressure correlated significantly with serum leptin and the insulin resistance index in both groups. Even after adjustment for age and BMI, the correlation of mean blood pressure with leptin remained in the obese adolescents (r2 = 0.390, P = 0.02). The heart rate also correlated with leptin in the adolescents (r = 0.18, P< 0.001), but not in the middle-aged subjects (r = 0.04). Even after adjustment for age and BMI in adolescents, serum leptin correlated significantly with heart rate.

CONCLUSION

These results suggest a role for leptin in obesity-related hypertension, especially in adolescents.

Fate of leptin after intracerebroventricular injection into the mouse brain

The fate of the metabolic regulatory protein leptin was studied after intracerebroventricular (i.c.v.) administration into the lateral ventricle of the brain. In the brain, a mean of 72% of the recovered radioiodinated leptin was intact. Efflux from the brain for leptin occurred with the reabsorption of the cerebrospinal fluid into the blood. Leptin appearing in the blood was 71% intact over the course of the study. The amount of leptin in the blood rose slowly, and 20 min after i.c.v. injection equaled or exceeded levels previously seen 20 min after i.v. administration. Autoradiography showed the slow disappearance of leptin from the ventricular system over time. The degree of periventricular penetration of radiolabeled leptin also was determined. By 30 min, leptin was detected 600 microm from the midline, but computer-assisted image analysis showed that the amount of radioactivity had fallen to half the midline value by 300 microm. The concentration of leptin within the arcuate nucleus, previously observed after i.v. administration, was not seen after i.c.v. injection. High concentrations of leptin were found at the choroid plexus, suggesting the presence of leptin receptors on the brain side of the blood-cerebrospinal fluid barrier and within the lumen of the middle cerebral arteries.

Leptin does not fully account for the satiety activity of adipose tissue-conditioned medium

To determine whether leptin alone accounts for the satiety activity secreted by native adipose tissue, we prepared culture media conditioned by microdissected adipose tissue from overfed Long-Evans rats, fa/fa rats, or db/db mice (media A, B, and C, respectively). Medium A significantly suppressed food intake following intracerebroventricular delivery to Long-Evans rats (2-h chow intake = 68 +/- 5% of baseline, P < 0.001). Media B and C significantly suppressed food intake following intraperitoneal delivery to ob/ob mice (24-h chow intake = 56 +/- 7% of baseline for medium B, P = 0. 001; 4-day chow intake = 78 +/- 3% of baseline for medium C, P = 0. 004). Using a leptin receptor-based bioassay, we determined that the leptin concentration of medium C was 392 +/- 18 ng/ml. This concentration was 20-fold lower than the concentration of recombinant murine leptin required to produce a similar degree of feeding suppression following 5 days of administration to ob/ob mice. Neither medium conditioned by adipose tissue from ob/ob mice nor medium conditioned by adipose tissue from fa/fa rats and subsequently immunodepleted of leptin had significant satiety activity. We conclude that leptin is necessary but not sufficient to account for the satiety activity of native adipose tissue, perhaps due to the production by adipocytes of a cofactor that augments the ability of leptin to suppress feeding.

Abnormal regulation of the leptin gene in the pathogenesis of obesity

A subset of obese humans has relatively low plasma levels of leptin. This finding has suggested that in some cases abnormal regulation of the leptin gene in adipose tissue is etiologic in the pathogenesis of the obese state. The possibility that a relative decrease in leptin production can lead to obesity was tested by mating animals carrying a weakly expressed adipocyte specific aP2-human leptin transgene to C57BL/6J ob/ob mice (which do not express leptin). The transgene does not contain the regulatory elements of the leptin gene and is analogous to a circumstance in which the cis elements and/or trans factors regulating leptin RNA production are abnormal. The ob/ob mice carrying the transgene had a plasma leptin level of 1. 78 ng/ml, which is approximately one-half that found in normal, nontransgenic mice (3.72 ng/ml, P < 0.01). The ob/ob animals expressing the leptin transgene were markedly obese though not as obese as ob/ob mice without the transgene. The infertility as well as several of the endocrine abnormalities generally evident in ob/ob mice were normalized in the ob/ob transgenic mice. However, the ob/ob transgenic mice had an abnormal response when placed at an ambient temperature of 4 degreesC, suggesting that different thresholds exist for the different biologic effects of leptin. Leptin treatment of the ob/ob transgenic mice resulted in marked weight loss with efficacy similar to that seen after treatment of wild-type mice. In aggregate these data suggest that dysregulation of leptin gene can result in obesity with relatively normal levels of leptin and that this form of obesity is responsive to leptin treatment.

Functional properties of leptin receptor isoforms containing the gln-->pro extracellular domain mutation of the fatty rat

Mutations of the leptin receptor have been found to cause obesity in rodents. The fa mutation that is responsible for obesity in Zucker rats is a missense mutation (269 gln-->pro) in the extracellular domain of the leptin receptor. We have characterized the effects of this mutation on the two major isoforms of the leptin receptor, Ob-Rb and Ob-Ra, by studying cell-surface expression, leptin binding affinity, signaling capacity, and receptor-mediated internalization and degradation of leptin in transfected mammalian cell lines. Both Ob-Rb(269 gln-->pro) and Ob-Ra(269 gln-->pro) have decreased cell-surface expression and decreased leptin binding affinity. Ob-Rb(269 gln-->pro) was shown to have defective signaling to the JAK-STAT pathway and markedly diminished ability to activate transcription of the egr-1 promoter. Constitutive ligand-independent activation of Ob-Rb(269 gln-->pro) was observed for activation of egr-1-luc but only under conditions when JAK2 was coexpressed with Ob-Rb(269 gln-->pro), Finally, Ob-Ra(269 gln-->pro) has an increased ability to internalize leptin but is less efficient at degrading leptin, as compared with Ob-Ra. In conclusion, both Ob-Ra(269 gln-->pro) and Ob-Rb(269 gln-->pro) have multiple functional defects.

Monkey leptin receptor mRNA: sequence, tissue distribution, and mRNA expression in the adipose tissue of normal, hyperinsulinemic, and type 2 diabetic rhesus monkeys

OBJECTIVE

We have cloned the rhesus monkey leptin receptor and examined its mRNA expression levels in the adipose tissue of monkeys to investigate the regulation of gene expression of the leptin receptor.

RESEARCH METHODS AND PROCEDURES

Monkey leptin receptor cDNA was cloned by reverse transcriptase-polymerase chain reaction (RT-PCR). Tissue distribution of monkey leptin receptor was examined by Northern blot analysis and RT-PCR. The mRNA levels of monkey leptin receptor in adipose tissue of normal (n=10), hyperinsulinemic obese (n=8), and type 2 diabetic monkeys (n=8) were measured by quantitative RT-PCR.

RESULTS

Monkey leptin receptor cDNA had at least two alternatively spliced isoforms (long and short forms). The long form of the leptin receptor mRNA was expressed relatively highly in liver, adipose tissue, hypothalamus, and choroid plexus, whereas the total leptin receptors were expressed in every tissue examined. The mRNA levels of the long form of the leptin receptor in adipose tissue were not correlated to body weight, fasting plasma insulin, plasma glucose, or plasma leptin levels. The mRNA levels of the long form of the leptin receptor were highly correlated to that of the total leptin receptor (long and short form).

DISCUSSION

The long form of leptin receptor mRNA existed in adipose tissue as well as in liver and hypothalamus, suggesting that the leptin receptor in adipose tissue may be functional in adipose tissue. The expression of the leptin receptor mRNA in adipose tissue is not affected by obesity, hyperinsulinemia, or diabetes.

Do leptin levels predict weight gain?--A 5-year follow-up study in Mauritius. Mauritius Non-communicable Disease Study Group

OBJECTIVE

To investigate whether relative baseline leptin levels predict long-term changes in adiposity and/or its distribution.

RESEARCH METHODS AND PROCEDURES

In a longitudinal study of 2888 nondiabetic Mauritians aged 25 years to 74 years who participated in population-based surveys in 1987 and 1992, changes in body mass index (BMI), waist/hip ratio (WHR), and waist circumference were compared between "hyperleptinemic," "normoleptinemic," and "hypoleptinemic" groups. "Relative leptin levels" were calculated as standardized residuals from the regression of log10 leptin on baseline BMI to provide a leptin measure independent of BMI. Analyses were performed within each sex. A linear regression model was used to assess the effect of standardized residuals on changes in BMI, WHR, and waist circumference, independent of baseline BMI, age, fasting insulin, and ethnicity.

RESULTS

After adjusting for age and baseline BMI by analysis of covariance, there was no difference in changes in BMI, WHR, or waist circumference between men with low, normal, or high relative leptin levels. Among women, there was a significant difference in deltaWHR across leptin groups, such that the largest increase occurred in the "normal" leptin group. For both men and women, the linear regression models explained approximately 10% of variation in dependent variables, and the only significant independent variables were age, BMI, and being of Chinese origin, compared with Indian origin.

DISCUSSION

These findings do not support a role for leptin concentration in predicting weight gain or changes in fat distribution in adults over a 5-year period.

Serum leptin concentration in women: effect of age, obesity, and estrogen administration

OBJECTIVE

To compare serum leptin levels in normally cycling reproductive females (20-35 years old) with those in age-matched males, in women who were receiving oral contraceptives, and in older (postmenopausal) women (50-65 years old) who were or who were not receiving hormone replacement therapy.

DESIGN

Case-control study.

SETTING

Obstetrics and Gynecology Clinic, Texas Tech University Health Sciences Center-Amarillo, or the Exercise Physiology Laboratory at Southeastern Louisiana University.

PATIENT(S)

Normally cycling women between the ages of 20-35 years and age-matched controls who were receiving oral contraceptives. Postmenopausal women between the ages of 50-65 years who were or who were not receiving hormone replacement therapy.

MAIN OUTCOME MEASURE(S)

Serum leptin concentration.

RESULT(S)

In all groups, serum leptin concentrations were correlated significantly with body mass index. Leptin levels were significantly higher in young women than young men (P <.001), but no other statistically significant differences were found for the other three comparisons.

CONCLUSION(S)

Serum leptin concentrations expressed as a measure of adiposity (body mass index) are greater in young normally cycling females (20-35 years old) than in age-matched males. There is no difference in levels of serum leptin between young and postmenopausal (50-65 years old) women. Estrogen administration, either in young women who are receiving estrogen-progestin oral contraceptives or in postmenopausal women who are receiving hormone replacement therapy, does not effect serum leptin concentrations.

Afferent signals from leptin sensors in the white adipose tissue of the epididymis, and their reflex effect in the rat

Afferent nerve signals were recorded from a peripheral cut end of the small nerve bundle innervating the white adipose tissue (WAT) of the epididymis in the anesthetized rat. An injection of leptin (2 ng, 0.2 ml) into the white adipose tissue facilitated the afferent activity. The response was dose dependent and the least effective dose was 100 pg (0.1 ml). An injection of 2 ng (0.2 ml) leptin into the one side of the WAT resulted in a reflex activation of efferent activity of the sympathetic nerve innervating the WAT of the bilateral epididymis. The observations suggest the existence of leptin sensors in WAT which send afferent signals from the WAT to the central nervous system and evoke a reflex activation of sympathetic outflow to the WAT which may accelerate lipolysis. This WAT to WAT reflex can explain a part of the effect of leptin on metabolic function of the fatty tissue such as the reduction of body weight and increase in energy expenditure as a negative feed-back reflex response.

[Leptin--new knowledge on the pathogenesis of obesity]

Cloning of the ob-gene and characterization of its gene product leptin has led to the identification of a satiety factor, which signals the amount of peripheral fat stores to the central nervous system and regulates further feeding behaviour, thus playing a central role in the regulation of body weight. Soon after cloning of the ob-gene, a leptin-binding receptor has been identified in the central nervous system as well as in various peripheral organs. A feedback loop between peripheral fat stores and leptin receptors in the central nervous system appears to play an important role in normal body weight regulation. In contrast to human obesity, which associated with leptin resistance of uncertain etiology, the obesity syndromes associated with several animal models are now known to result from the interruption of the feedback loop at different points. Moreover, leptin may play a role in manifestation of insulin resistance and type II diabetes. Since the identification of leptin, a vast number of studies have been conducted to assess the molecular mechanisms and signal transduction pathways that are involved in the development and manifestation of obesity. From the large body of data generated to date, novel concepts of the regulation of energy balance and target strategies to control human obesity should soon be forthcoming.

Leptin, the hypothalamus and the regulation of adiposity

The hypothalamus contains a wealth of peptide and non-peptide neurotransmitters, many of which have been shown experimentally to influence feeding behaviour and energy metabolism. Regulatory activities as complex as these are likely to be controlled by numerous neurotransmitters interacting at a variety of levels. The hierarchy of command of these neuronal circuits is not known, but it is possible that some converge on to a final common pathway, governed by the actions of a single neurotransmitter, through which all other influences ultimately operate. This review will discuss several of the more recently identified neurotransmitters and consider their validity as candidates in the regulation of energy homeostasis.

Effect of a high-fat diet on food intake and hypothalamic neuropeptide gene expression in streptozotocin diabetes

Insulin-deficient diabetic rats are markedly hyperphagic when fed a high-carbohydrate (HC) diet, but normophagic when fed a high-fat (HF) diet. When maintained on a HC diet, diabetic rats also exhibit increased gene expression of the orexigenic peptide neuropeptide Y (NPY) in the hypothalamic arcuate nucleus, and reduced expression of the anorectic peptide corticotropin-releasing hormone (CRH) in the paraventricular nucleus, and these changes are hypothesized to contribute to diabetic hyperphagia. In this experiment we assessed whether the normophagia displayed by HF-fed diabetic rats is associated with the opposite profile of NPY and CRH expression. Our results show that relative to diabetic rats on the HC diet, the diabetic rats on the HF diet exhibited significantly reduced caloric intake (-40%), NPY expression in the arcuate nucleus (-27%), and elevated CRH expression in the paraventricular nucleus (+37%). Insulin and corticosterone, which are known to affect hypothalamic NPY and CRH expression, were not different between these two groups, making it unlikely that they can account for the differences in either feeding behavior or hypothalamic peptide expression. There was a small but significant increase in plasma leptin levels in the diabetic animals maintained on the HF, and large differences in parameters associated with elevated fat oxidation. These observations support the hypothesis that the normalization of food intake observed in diabetic rats consuming a HF diet may in part be mediated by reductions in NPY expression and elevations in CRH expression.

Leptin produces anorexia and weight loss without inducing an acute phase response or protein wasting

The ob gene product leptin is known to produce anorexia and loss of body fat when chronically administered to both lean and genetically obese mice. The current study was undertaken to examine whether administration of recombinant leptin in quantities sufficient to produce decreases in food intake and body weight and alterations in body composition would elicit either an hepatic acute phase protein response or preferential loss of carcass lean tissue. Mice were administered increasing quantities of recombinant human leptin or human tumor necrosis factor-alpha as a positive control. Although leptin (at 10 mg/kg body wt) produced significant anorexia and weight loss (both P < 0.05), human leptin administration did not appear to induce an hepatic acute phase protein response in either lean or genetically obese mice, as determined by protein synthetic rates in the liver or changes in the plasma concentration of the murine acute phase protein reactants, amyloid A, amyloid P, or seromucoid (alpha1-acid glycoprotein). In addition, human leptin administration did not induce a loss of fat-free dry mass (protein) in lean or obese animals. The findings suggest that at doses adequate to alter food intake and body weight leptin is not a significant inducer of the hepatic acute phase response nor does leptin promote the preferential loss of somatic protein characteristic of a chronic inflammatory process.

Zinc may regulate serum leptin concentrations in humans

OBJECTIVE

Leptin, the product of the ob gene, plays a key role in a feedback loop that maintains energy balance by signaling the state of energy stores to the brain and by influencing the regulation of appetite and energy metabolism. Zinc also plays an important role in appetite regulation. Thus, we evaluated the relationship between zinc status and the leptin system in humans.

METHODS

We studied nine healthy men with marginal zinc deficiency, induced by dietary means, before and after zinc supplementation.

RESULTS

Zinc restriction decreased leptin levels while zinc supplementation of zinc-depleted subjects increased circulating leptin levels. In addition, zinc supplementation increased IL-2 and TNF-alpha production that could be responsible for the observed increase in leptin concentrations.

CONCLUSIONS

Zinc may influence serum leptin levels, possibly by increasing the production of IL-2 and TNF-alpha.

Leptin indirectly affects estrous cycles by increasing metabolic fuel oxidation

In previous experiments, lean Syrian hamsters fasted on days 1 and 2 of the estrous cycle failed to show sex behavior and ovulation normally expected to occur on the evening of day 4. The first goal of the present experiment was to determine whether systemic treatment with the ob (obese) protein leptin could reverse the effects of fasting on estrous cyclicity, social behaviors, and ovulation rate. Fasting-induced anestrus was reversed and normal sex and social behavior and ovulation rate were restored in hamsters injected intraperitoneally with 5 mg/kg leptin every 12 h during fasting on days 1 and 2 of the estrous cycle. A second goal was to test whether the effects of leptin could be prevented by treatment with pharmacological agents that block the oxidation of metabolic fuels. Glucose oxidation was blocked by treatment with 2-deoxy-d-glucose (2DG) and fatty acid oxidation was blocked by treatment with methyl palmoxirate (MP). 2DG (1000 mg/kg) or MP (20 mg/kg) was administered at doses that did not induce anestrus in hamsters fed ad libitum. As in the first experiment, fasting-induced anestrus was reversed by leptin treatment. However, when each injection of leptin was preceded by an injection of 2DG or MP, leptin treatment did not reverse fasting-induced anestrus. In summary, estrous cyclicity was not restored when oxidation of metabolic fuels was blocked, despite high endogenous levels of leptin. These results are consistent with the hypothesis that leptin acts indirectly on the reproductive system by increasing fuel oxidation.

Circulating insulin concentrations, smoking, and alcohol intake are important independent predictors of leptin in young healthy men

OBJECTIVE

Leptin, an adipocyte-secreted hormone, has been shown to signal the status of energy stores to the brain, regulate energy homeostasis, and mediate the neuroendocrine response to food deprivation. Obesity is associated with increased leptin levels, and several hormones, including insulin and glucocorticoids, have been associated with leptin levels and expression in rodents. Although obesity has been strongly associated with increased leptin in humans, a significant percentage of leptin's variability remains unexplained. The role of endogenous hormones, demographic factors, or certain life-style factors in explaining the residual variability of leptin levels has not yet been clarified. We performed this cross-sectional study to document the relative importance of obesity, lifestyle factor, and endogenous hormones in determining serum leptin levels.

RESEARCH METHODS AND PROCEDURES

We measured serum concentrations of insulin, cortisol, testosterone, growth hormone, and dehydroepiandrosterone sulfate; ascertained anthropometric, demographic, and lifestyle characteristics; and studied these variables in relationship to serum leptin concentrations in a sample of young healthy men.

RESULTS

Obesity and alcohol intake were independently and positively associated with circulating leptin concentrations. Additionally, cigarette smoking was negatively and independently associated with leptin concentrations. Finally, serum insulin concentration was an independent hormonal determinant of circulating leptin concentrations, whereas serum testosterone was negatively associated with leptin only by bivariate analysis.

DISCUSSION

We conclude that, in addition to obesity, cigarette smoking, alcohol intake, and serum insulin levels are associated with leptin levels in a population of healthy young men.

Acute effects of recombinant murine leptin on rat pituitary-adrenocortical function

Leptin, the product of the ob gene, is an adipose-tissue secreted hormone, which regulates satiety, metabolic rate and thermogenesis. Many lines of evidence suggest the existence of mutual relationships between leptin and adrenal-cortex secretion, but in vivo studies gave rather conflicting results. Therefore, we have investigated the acute effect of the systemic bolus administration of leptin (5 or 10 nmol/kg) on the function of rat pituitary-adrenocortical function. The blood concentrations of ACTH, aldosterone and corticosterone were measured by specific RIA 60 or 120 min after the leptin injection. Leptin did not affect the blood concentrations of ACTH and aldosterone at 60 min, but at 120 min the lower dose of the peptide increased them. In contrast, the blood level of corticosterone was markedly enhanced by both doses of leptin at 60 and 120 min. Collectively, these findings indicate that leptin exerts a moderate acute activation of the pituitary-adrenocortical function in rats. They also suggest that the adrenocortical secretagogue action of leptin is not only mediated by the enhanced pituitary ACTH release, but is also consequent to a direct stimulatory effect of the peptide on adrenocortical cells.

Central nervous system effects of leptin

Evidence exists demonstrating the importance of leptin in the control of energy homeostasis, feeding behavior and reproductive function. Leptin receptors are localized in several regions of the brain implicated in regulation of energy balance and reproductive function, including the arcuate nucleus/median eminence, paraventricular nucleus, and ventromedial nucleus. Administration of exogenous leptin has been shown to alter function of the hypothalamic-pituitary-adrenal axis and stimulate gonadotropin release through hypothalamic and pituitary actions. Results from in situ hybridization studies demonstrate the ability of leptin to modulate the expression of key neuropeptides (neuropeptide Y, corticotropin-releasing hormone) implicated in the regulation of energy homeostasis. This suggests that leptin is an important component in the neuroendocrine transmission line that regulates appetite, energy balance and reproduction.

Longitudinal changes in maternal serum leptin concentrations, body composition, and resting metabolic rate in pregnancy

OBJECTIVE

We sought to evaluate the longitudinal changes in maternal serum leptin concentrations, body composition, and resting metabolic rate during pregnancy.

STUDY DESIGN

Ten women were evaluated before pregnancy, in early pregnancy (12 to 14 weeks), and in late pregnancy (34 to 36 weeks). Leptin concentrations were measured by radioimmunoassay, body composition with hydrodensitometry with adjustment for total body water, and resting metabolic rate by use of indirect calorimetry.

RESULTS

Using analysis of variance with repeated measures from pregravid to late pregnancy, a 66% increase (mean +/- SD) was found in leptin concentrations (in nanograms per milliliter) (before pregnancy, 25.4 +/- 19.9; in early pregnancy, 37.5 +/- 26.2; and in late pregnancy, 38.4 +/- 27.3, p = 0.003); a 9% increase in body fat (in kilograms) (before pregnancy, 29.4 +/- 15.7; in early pregnancy, 28.7 +/- 14.0; in late pregnancy, 31.4 +/- 14.6; p = 0.04); a 28% increase in oxygen consumption (in milliliters of oxygen per minute) (before pregnancy, 221.2 +/- 29.5; in early pregnancy, 230.4 +/- 42.9; in late pregnancy, 285.3 +/- 51.9; p < 0.0001); and a 9% increase in oxygen consumption (milliliters of oxygen per kilogram per minute) (before pregnancy, 3.02 +/- 0.43; in early pregnancy, 3.05 +/- 0.30; in late pregnancy, 3.31 +/- 0.37, p = 0.002) with advancing gestation. A significant positive correlation was present between leptin and body fat before pregnancy (r = 0.90, p < 0.0001), in early pregnancy (r = 0.91, p < 0.0001), and in late pregnancy (r = 0.87, p = 0.0005) and between leptin and oxygen consumption before pregnancy (r = 0.80, p = 0.004), in early pregnancy (r = 0.92, p < 0.0001), and in late pregnancy (r = 0.62, p = 0.06). When oxygen consumption was adjusted for maternal and fetal tissue mass, a significant negative correlation was found between leptin and oxygen consumption before pregnancy (r = -0.96, p < 0.0001), in early pregnancy (r = -0.80, p = 0.0034), and in late pregnancy (r = -0.70, p = 0.02).

CONCLUSION

We conclude that leptin increases significantly during early pregnancy before any major changes in body fat and resting metabolic rate. These data suggest that pregnancy represents a leptin-resistant state.

Acute and chronic effects of leptin on glucose utilization in lean mice

Experiments described here show that in vivo glucose uptake is impaired in mice given 30 micrograms leptin by intraperitoneal injection 2 hours before an oral glucose tolerance test (GTT). When mice were infused for 7 days with 10 micrograms/day leptin, the 4-fold increase in circulating leptin caused a transient hypophagia, a sustained weight loss and significantly inhibited insulin release in response to an oral GTT. Adipocytes from these mice were not insulin responsive whereas insulin-stimulated muscle and liver glycogen synthesis were increased. In contrast, leptin added to 2 hour in vitro incubations had an insulin-like effect on muscle glucose utilization and augmented insulin stimulation of adipocyte lipid synthesis. Thus, normal mice treated chronically with leptin develop tissue specific changes in insulin sensitivity and compensate for inhibition of glucose-stimulated insulin release. The contrasting response to acute leptin exposure suggests these changes are not a direct effect of the protein.

Clinical aspects of leptin

Hyperleptinemia is an essential feature of human obesity. Total body fat mass > % body fat > BMI are the best predictors of circulating leptin levels. Although ob gene is differentially expressed in different fat compartments, apart from total body fat, upper or lower body adiposity or visceral fat does not influence basal leptin levels. Similarly, age, basal glucose levels, and ethnicity do not influence circulating leptin levels. Only in insulin-sensitive individuals do basal levels of insulin and leptin correlate positively even after factoring in body fat. Diabetes does not influence leptin secretion in both lean and obese subjects per se. Independent of adiposity, leptin levels are higher in women than in men. This sexual dimorphism is also present in adolescent children. In eating disorders anorexia nervosa and bulimea nervosa, leptin levels are not upregulated but simply reflect BMI and probably body fat. In spite of strong correlation between body fat and leptin levels, there is great heterogeneity in leptin levels at any given index of body fat. About 5% of obese populations can be regarded as "relatively" leptin deficient which could benefit from leptin therapy. Leptin has dual regulation in human physiology. During the periods of weight maintenance, when energy intake and energy output are equal, leptin levels reflect total bodyfat mass. However, in conditions of negative (weight-loss programs) and positive (weight-gain programs) energy balances, the changes in leptin levels function as a sensor of energy imbalance. This latter phenomenon is best illustrated by short-term fasting and overfeeding experiments. Within 24 h of fasting leptin levels decline to approximately 30% of initial basal values. Massive overfeeding over a 12-h period increases leptin levels by approximately 50% of initial basal values. Meal ingestion does not acutely regulate serum leptin levels. A few studies have shown a modest increase in leptin secretion at supraphysiological insulin concentrations 4-6 h following insulin infusion. Under in vitro conditions, insulin stimulates leptin production only after four days in primary cultures of human adipocytes, which is apparently due to its trophic effects and an increased fat-cell size. Similar to other hormones, leptin secretion shows circadian rhythm and oscillatory pattern. The nocturnal rise of leptin secretion is entrained to mealtime probably due to cumulative hyperinsulinemia of the entire day. Like other growth factors and cytokines, leptin binding proteins including soluble leptin receptor are present in human serum. In lean subjects, the majority of leptin circulates in the bound form whereas in obese subjects, the majority of leptin is present in the free form. When free-leptin levels are compared between lean and obese subjects, even more pronounced hyperleptinemia in obesity is observed than that reported by measuring total leptin levels. During short-term fasting, free-leptin levels in lean subjects decrease in much greater proportion than those in obese subjects. In lean subjects with a relatively small energy store and particularly during food deprivation, leptin circulating predominantly in the bound form could be the mechanism to restrict its availability to hypothalamic leptin receptors for inhibiting leptin's effect on food intake and/or energy metabolism. Unlike marked changes in serum leptin, CSF leptin is only modestly increased in obese subjects and the CSF leptin/serum leptin ratio decreases logarithmically with increasing BMI. If CSF leptin levels are any indication of brain interstitial fluid levels, then hypothalami of obese subjects are not exposed to abnormally elevated leptin concentrations. In the presence of normal leptin receptor (functional long form, i.e., OB-Rb) mRNA expression and in the absence of leptin receptor gene mutations, it is logical to assume defective leptin signaling and/or impaired affector system(s) are the likely causes of leptin resistance in

Regional localization of specific [125I]leptin binding sites in rat forebrain

Specific [125I]leptin receptor binding sites have been identified in choroid plexus (CP), but have eluded regional localization within the brain parenchyma. To optimize specific [125I]leptin binding in brain loci, we ran experiments varying the pH of incubation buffers. We found that specific [125I]leptin binding in CP was strikingly pH dependent with the most acidic buffer, pH 5.5, resulting in a greater than 100% increase over the amount of specific binding measured at pH 7.5. While low pH permitted detection of specific binding in parenchymal loci, clear pH dependency was only observed in the CP. In the caudate putamen (CauP), a locus with low specific binding, values for specific binding did not differ significantly across the range of pH conditions tested. Using incubation buffers at pH 6.0 in subsequent binding experiments, we localized specific [125I]leptin binding in several brain loci including thalamus and hypothalamus. In CP and thalamus, where the range of OD permitted analysis of binding parameters, [125I]leptin binding was saturable with increasing concentrations of unlabelled leptin. In all loci, specific [125I]leptin binding was insensitive to competition by high concentrations of other unlabelled compounds. Our results varying pH conditions of the incubation buffer suggest leptin receptors may be divided into subclassifications based on pH sensitivity of the specific binding. Furthermore, our results suggest that although densities are low, high affinity leptin receptors are present in neural loci implicated in food intake and energy balance, and are more widespread in the forebrain than previously determined.

Leptin: a molecule integrating somatic energy stores, energy expenditure and fertility

The signaling of fat mass to central nervous system (CNS) regulators of food intake, energy expenditure and fertility has been inferred by experimental physiologists for over 75 years. The ability to modify such phenotypes based upon the status of body energy stores (fat) has critical survival value and, therefore, has been the object of potent selection pressure in evolution. The recent molecular cloning of the mouse ob mutation and the subsequent elucidation of the fundamentals of its regulatory physiology has identified a protein secreted by adipocytes, leptin, as a plausible candidate for a humoral signal with the requisite endocrinology and neurobiology.

Plasma leptin and hunger ratings in healthy humans

Leptin, the ob gene product, is considered as a signal involved in the regulation of energy stores through centrally mediated effects on ingestive behavior and metabolism. To investigate the relationships between appetite-related sensations and circulating leptin in humans, 12 healthy male volunteers were served two test lunches (1200 kcal, 38% carbohydrate, 50% fat, 12% protein) with comparable palatability but resulting in different postprandial hunger ratings, and plasma leptin concentrations were monitored during 8 h after meal ingestion. Whereas postprandial hunger ratings (assessed by visual analog scales) and plasma insulin patterns differed significantly between meals, no significant difference in postprandial plasma leptin concentrations was observed between the two meals. A moderate but significant increase in mean plasma leptin was found over time. We conclude that postprandial plasma leptin concentrations do not significantly differ between two meals inducing different effects on hunger subjective feelings. These data are in agreement with a role for leptin in the regulation of energy balance as a long-term adiposity-related signal rather than a short-term meal-related factor.

Leptin in relation to prostate cancer and benign prostatic hyperplasia

The aim of our study was to determine whether leptin, a hormone implicated in both energy-balance and reproductive function, is involved in the etiology of prostate cancer or benign prostatic hyperplasia (BPH). We compared the serum leptin levels of 43 cases of incident prostate cancer, 41 patients with BPH, and 48 healthy controls, all recruited in Athens, Greece. Multiple logistic regression modeling was used, with adjustment for age, height, body mass index, education, estradiol, testosterone, dihydrotestosterone, dehydroepiandrosterone sulfate, sex hormone-binding globulin and insulin like growth factor 1. Odds ratios per 4 ng/ml increment of leptin were 0.70 [95% confidence interval (CI) (0.32,1.55)] for prostate cancer and 1.06 [95% CI (0.67,1.67)] for BPH. After adjustment for body mass index, serum leptin levels were not significantly correlated with levels of any of the other hormones under study. Leptin levels are unlikely to affect the risk of either prostate cancer or BPH substantially.

Leptin

Leptin (from the Greek leptos=thin) was identified only 3 years ago. It has attracted huge attention both scientifically, with more than 600 publications, and in the media, where this protein has been portrayed as the way to a cure for obesity. Indeed, leptin was first described as an adipocyte-derived signalling factor, which, after interaction with its receptors, induced a complex response including control of bodyweight and energy expenditure. Leptin seems in addition to its role in metabolic control to have important roles in reproduction and neuroendocrine signalling. Human obesity is a complex disorder, with many factors playing a part; the pathophysiology of leptin is not as simple as it seems to be in rodent models of obesity.

Advances in endocrinology

Molecular genetics will continue to help us to make precise diagnoses. At present, the expertise to achieve this for a specific disease is often exclusive to one unit with a research interest. It will be important to establish a coordinated approach at a supraregional level to provide molecular diagnosis for rare disorders as a fast reliable clinical service. In addition understanding the molecular mechanisms of disease is likely to direct a search for new treatments. For instance, calcium channel blockers have been used in nesidoblastosis to reduce the hypersecretion of insulin, as a result of the recognition of the role that calcium has in the function of the beta-cell ATP sensitive K+ channel. Although the potential benefits of hGH are now being clearly defined in a range of growth disorders, the treatment is invasive and expensive. It is likely that future endocrine therapeutic developments could include slow release growth hormone preparations, orally active growth hormone mimetics, or even hormone production from an ectopic viral cDNA vector. The next "advances in endocrinology" will also reveal whether leptin will have a therapeutic role in appetite control or even the modulation of pubertal development.

Efficient secretion of biologically active recombinant OB protein (leptin) in Escherichia coli, purification from the periplasm and characterization

The genes encoding the mature forms of mouse (mOB) and human OB (hOB) protein (also called leptin) were fused to the secretion signal coding sequence of the Escherichia coli outer membrane protein A (sOMP A). The hybrid genes were preceded by a ribosome binding site (RBS) and were expressed under transcriptional control of both the lipoprotein promoter (Plpp) and the lac promoter-operator (POlac). The recombinant fusion proteins were efficiently expressed and exported into the periplasmic compartment of E. coli cells from where they were recovered by osmotic shock as soluble mature polypeptides with the sOMP A precisely removed. Recombinant mOB and hOB proteins were also produced in Sf9 insect cells using the baculovirus expression system. Milligram quantities of both proteins were purified to homogeneity using ion-exchange, hydrophobic interaction chromatography and gel filtration and were found to be biologically active and to have antiobesity effects upon testing in genetically obese ob/ob mice.

Age-related differences in the thermogenic and ponderal effects following the administration of fragment peptides from the rat ob protein

The ob gene encodes a protein, which regulates satiety, metabolic rate and fat storage. The administration of a pool of five 20-amino-acid fragment peptides derived from the carboxy-terminal region of the ob protein produced a statistically significant reduction in body weight gain in adult rats, while rectal temperature showed a statistically significant increase. Administration of the same pool of peptides to young rats did not produce changes in body weight gain, although a statistically significant transient increase in rectal temperature was observed. These results envisage the possibility that small sequences of amino acids derived from the ob protein may mimic the effects of the whole protein on temperature and ponderal regulation. Furthermore, data suggest possible age-related differences in the response to leptin administration.

Leptin, leptin receptors, and the control of body weight

The assimilation, storage, and disposition of nutrient energy constitute a complex homeostatic system central to the survival of both prokaryotic and eukaryotic organisms. In vertebrates, and especially among land dwelling mammalian species, the ability to store large quantities of energy-dense fuel in the form of adipose tissue triglyceride permits survival during prolonged periods of food deprivation. In order to maintain such fuel stores during times of dietary scarcity or surfeit, some balance between energy intake and expenditure must be achieved. Lesions of the hypothalamus alter body weight suggesting that this brain region regulates nutritional state. These and other studies led to the hypothesis that body weight was regulated by a feedback loop in which peripheral signals reported nutritional information to an integratory center in the brain. However, the identity of these nutrition signals proved elusive.