Dexamethasone, OB gene, and leptin in humans; effect of exogenous hyperinsulinemia

This study was undertaken to investigate temporal association between dexamethasone administration, OB gene expression, and leptin response in humans in the presence and absence of exogenous hyperinsulinemia. Six healthy males (age 24.5 +/- 1.0 yr, body mass index 26.4 +/- 1.0, body fat 16.2 +/- 1.8%) received 10 mg oral dexamethasone in five divided doses twice (protocols A and B) 1-2 weeks apart beginning at 0800 h on day 1 and ending at 0700 h on day 2. The dexamethasone administration was combined with two subcutaneous abdominal fat biopsies performed at 0800 h before and after dexamethasone administration (protocol A), or 4-h isoglycemic hyperinsulinemic (300 mU/m2 BSA/min, protocol B) clamp carried out between 0900 and 1300 h on day 2. OB gene expression (protocol A) did not change. In both protocols on day 2, the 0800 h leptin levels nearly doubled (P < 0.001), whereas 1300 h levels nearly quadrupled (P < 0.001). The elevation in leptin persisted until 0800 h of day 3 (24 h after last dexamethasone dose) with its subsequent rapid normalization. The short-term isoglycemic hyperinsulinemia (protocol B) had no additional effect on the postdexamethasone leptin response. We summarize that: 1) 24-h administration of dexamethasone has a marked stimulatory effect on circulating leptin levels but not on OB gene expression in the subcutaneous abdominal fat. 2) The effect is sustained for the next 24 h. 3) Short-term hyperinsulinemia has no additional effect. We conclude that dexamethasone is a powerful stimulator of leptin production in vivo through a mechanism that appears to be independent of OB gene transcription in the human subcutaneous abdominal fat.

Synthesis of leptin in human placenta

Gender-based differences in serum leptin levels have been reported in umbilical cord blood, and leptin has been detectedin human amniotic fluid. In order to understand if leptin may be directly synthesized by human placentae an analysis made up of several steps was performed. First at all RT-PCR analysis from placenta-derived RNA was used to detect human leptin mRNA. The leptin-like immunoradioactivity detected in placentae extracts was identical to human leptin according to the criteria of charge, immunorecognition, SDS-PAGE analysis and blotting, indicating that intact leptin was found and no variants in size, charge or immunoactivity were present in the placentae. Finally an immunohistochemical analysis showed the presence of leptin in the cytoplasm of syncytiotrophoblast cells but not in the core of villi.

IN CONCLUSION

leptin is synthesized as a single molecular variant identical to human recombinant leptin in human placentae at delivery.

Improved sensitivity to insulin in obese subjects following weight loss is accompanied by reduced protein-tyrosine phosphatases in adipose tissue

Insulin resistance in adipose tissue in human obesity is associated with increased protein-tyrosine phosphatase (PTPase) activity and elevated levels of the PTPases leukocyte common antigen-related PTPase (LAR) and PTP1B. To determine whether the improved insulin sensitivity associated with weight loss in obese subjects is accompanied by reversible changes in PTPases, we obtained subcutaneous adipose tissue from seven obese subjects (mean body mass index [BMI], 40.4 kg/m2) before and after a loss of 10% of body weight and again after a 4-week maintenance period. Weight loss was accompanied by an 18.5% decrease in overall adipose tissue PTPase activity (P = .015) that was further reduced to 22.3% of the control value (P = .005) at the end of the maintenance period. By immunoblot analysis, the abundance of LAR was decreased by 21% (P = .04) and abundance of PTP1B was decreased by 40% (P < .004) after the initial weight loss, and the decreases persisted during the maintenance period. Enhanced insulin sensitivity following weight loss, evident from a 26% decrease in fasting insulin levels (P < .05), was also closely correlated with the reduction in the abundance of both LAR (R2 = .80, P < .01) and PTP1B (R2 = .64, P = .03). These results support the hypothesis that LAR and PTP1B may be reversibly involved in the pathogenesis of insulin resistance, and may be therapeutic targets in insulin-resistant states.

Interleukin 1 alpha increases serum leptin concentrations in humans

Leptin, the protein product of the ob gene, regulates appetite and body weight in animals. Endotoxin and cytokines, induced by endotoxin, interleukin (IL) 1 and tumor necrosis factor, increase expression of leptin in mice and hamsters. We measured serum leptin concentrations in patients with cancer before and after administration of recombinant human IL-1 alpha. Fourteen patients received IL-1 alpha at one of three dose levels (0.03, 0.1, or 0.3 microgram/kg.day) for 5 days. Serum leptin concentrations increased in all but two patients within 24 h after the first dose. The increase in leptin was correlated directly with IL-1 alpha dose (P = 0.0030). Despite continued administration of IL-1 alpha, serum leptin concentrations returned to pretreatment levels by day 5 of therapy. An increase in serum leptin concentrations may be one mechanism by which anorexia is induced by IL-1 alpha. However, tachyphylaxis of the leptin response suggests that other mechanisms also are involved.

Leptin receptors in the adrenal medulla of the rat

Leptin is the protein product of the recently cloned obesity gene. Leptin receptor mRNA is found in a number of central and peripheral locations. The hypothalamus is a presumed site of action. However, little is known about the specific locations of the receptor in peripheral organs. Epinephrine has potent anorectic effects and can cause weight loss by a variety of mechanisms. Excretion of epinephrine is reduced in the ob/ob mouse, which lacks leptin, suggesting an effect by leptin on the adrenal medulla. In the current study, the presence of the leptin receptor was identified on epinephrine-secreting cells in the adrenal medulla. Immunohistochemical studies found dense leptin receptor-like immunoreactivity in the adrenal medulla with no labeling in the adrenal cortex. Double immunofluorescent labeling confirmed that the leptin receptor was present on cells that were phenylethanolamine N-methyltransferase-like immunoreactive and therefore were epinephrine-secreting cells. Leptin receptor mRNA in the adrenal medulla was detected by reverse transcriptase-polymerase chain reaction, with the majority of the mRNA coding for the short isoform (Ob-Ra) of the receptor. Finally, autoradiography was performed using 125I-labeled leptin; specific binding was found in the adrenal medulla, with no specific binding in the adrenal cortex. These results suggest that leptin may have a direct effect on epinephrine-secreting cells in the adrenal medulla. Epinephrine may play a role in mediating the effects of leptin to reduce body weight.

Nocturnal rise of leptin in normal prepubertal and pubertal children and in patients with perinatal stalk-transection syndrome

We studied 24-h profiles of circulating leptin levels using a sensitive and specific radioimmunoassay in healthy pre- (Tanner 1) and pubertal boys and girls (Tanner 3-4) as well as in a group of patients with perinatal stalk-transection syndrome. Similar nyctohemeral rhythm in serum leptin levels were found in prepubertal (MESOR: 2.34 +/- 0.2 ng/ml; amplitude 0.32 +/- 0.1 ng/ml) and pubertal boys (MESOR 2.2 +/- 0.2 ng/ml; amplitude 0.38 +/- 0.07 ng/ml). Likewise, no differences were found between prepubertal (MESOR 6.6 +/- 1.07 ng/ml; amplitude 1.67 +/- 0.4 ng/ml) and pubertal girls (MESOR 4.05 +/- 0.5 ng/ml; amplitude 0.95 +/- 0.2 ng/ml). In contrast, higher MESOR (p < 0.002) and amplitude values (p < 0.005) were found in prepubertal and pubertal girls than in prepubertal and pubertal boys. Finally a significant nyctohemeral rhythm in serum leptin levels was found in patients with perinatal stalk-transection syndrome (MESOR: 9.3 +/- 2.3 ng/ml; amplitude 1.46 +/- 0.4 ng/ml). This data shows the existence of sexual dimorphism in the nyctohemeral rhythm in serum leptin levels that are not influenced by the pubertal stage or by pulsatile anterior pituitary hormone secretion.

Placental leptin: an important new growth factor in intrauterine and neonatal development?

BACKGROUND

Leptin, the protein product of the ob gene, is produced by the adipocyte and seems to function as a link between adiposity, satiety, and activity. Leptin has also been found to be necessary for pubertal development, conception, and pregnancy in mice, and is increased in prepubertal children, independent of adiposity, suggesting a role in childhood growth and development. This study investigated 100 mother/newborn pairs to determine the role of leptin in neonatal development. Placental tissue was assayed for leptin mRNA to evaluate it as a source of leptin production in utero.

METHODS

One hundred mother/newborn pairs were enrolled in this study. Radioimmunoassay was performed for leptin on maternal venous and newborn cord blood. Leptin concentrations were measured in 43 children in Tanner stages 1 and 2 as a control group. Placental tissue was obtained from five mothers and assayed for leptin mRNA by reverse transcription/polymerase chain reaction (RT/PCR). Human placental cell lines JAR and JEG-3 were also assayed for leptin mRNA expression.

RESULTS

Leptin was present in all newborns studied at a mean concentration of 8.8 ng/mL (+/-9.6 standard deviations). Leptin concentrations in cord blood correlated with newborn weight (r = .51), body mass index (BMI) (r = .48), and arm fat (r = .42). There was no correlation between leptin and insulin. When statistically covarying for adiposity for newborns and Tanner stages 1 and 2 children, newborns had greater concentrations of leptin (mean, 10.57 ng/mL) than children (mean, 3.04 ng/mL). Leptin was present in all mothers at a mean value of 28.8 ng/mL (+/-22.2 standard deviations). Leptin concentration correlated with prepregnancy BMI (r = .56), BMI at time of delivery (r = .74), and arm fat (r = .73). Maternal leptin correlated with serum insulin (r = .49). There was no correlation between maternal and newborn leptin concentrations. Thirteen percent of newborns had higher leptin concentrations than their mothers. Placental tissue from five separate placentas expressed leptin mRNA at comparable or greater levels than adipose tissue. Two human trophoblastic placental cell lines, JAR and JEG-3, also expressed leptin mRNA.

CONCLUSIONS

The correlation between leptin and adiposity found in children and adults was also found in newborns. Serum leptin concentrations in newborns were increased more than three-fold compared with children in Tanner stages 1 and 2 when controlling for adiposity, suggesting that leptin concentrations in the newborn are not explained by adiposity alone. Maternal leptin concentrations correlated with measures of adiposity at delivery but did not correlate with newborn adiposity or leptin. Leptin mRNA was expressed both in placental tissue and in two human placental cell lines. These data suggest that leptin has a role in intrauterine and neonatal development and that the placenta provides a source of leptin for the growing fetus.

Plasma leptin is partly cleared by the kidney and is elevated in hemodialysis patients

Leptin, the gene product of the ob gene, is important in the control of appetite in rodents and may have an important role in humans. The clearance of leptin from the circulation is unknown. As the leptin receptor is present in the kidney, we evaluated the role of the kidney in removing circulating leptin in humans. We measured leptin in aortic and renal vein plasma in 8 patients with intact renal function and 6 patients with impaired renal function who were undergoing elective cardiac catheterization. Renal blood flow was measured in all patients to calculate net mass balance across the kidney. In patients with intact renal function there is net renal uptake of 12% of circulating leptin, whereas in patients with renal insufficiency there is no renal uptake of leptin. In a separate cohort of 36 patients with end-stage renal failure on hemodialysis, peripheral leptin levels factored for body mass index was increased by > fourfold as compared to a group of healthy controls (N = 338). In addition, plasma leptin is not cleared by hemodialysis with a modified cellulose membrane. Additional studies are required to evaluate the role of leptin in mediating the anorexia of uremia.

Leptin and obesity in humans

It now appears that leptin is the peripheral signal, hypothesized in the lipostasis theory, that informs the central nervous system how much adipose tissue there is in the body. The ability of the leptin signal to regulate body composition and the amount of body fat has been demonstrated in animals. Furthermore, defects in the ob gene and the leptin receptor lead to the development of obesity in rodents. No such defects have been found in humans although it appears that obese humans are resistant to the action of their endogenous leptin. Further characterization of the hormone and ultimately, the administration of leptin to humans, will be necessary to determine the role of the leptin signal system in the development of obesity in humans.

Peroxisome proliferator-activated receptor gene expression in human tissues. Effects of obesity, weight loss, and regulation by insulin and glucocorticoids

The peroxisome proliferator activated receptor (PPAR gamma) plays a key role in adipogenesis and adipocyte gene expression and is the receptor for the thiazolidinedione class of insulin-sensitizing drugs. The tissue expression and potential for regulation of human PPAR gamma gene expression in vivo are unknown. We have cloned a partial human PPAR gamma cDNA, and established an RNase protection assay that permits simultaneous measurements of both PPAR gamma1 and PPAR gamma2 splice variants. Both gamma1 and gamma2 mRNAs were abundantly expressed in adipose tissue. PPAR gamma1 was detected at lower levels in liver and heart, whereas both gamma1 and gamma2 mRNAs were expressed at low levels in skeletal muscle. To examine the hypothesis that obesity is associated with abnormal adipose tissue expression of PPAR gamma, we quantitated PPARgamma mRNA splice variants in subcutaneous adipose tissue of 14 lean and 24 obese subjects. Adipose expression of PPARgamma 2 mRNA was increased in human obesity (14.25 attomol PPAR gamma2/18S in obese females vs 9.9 in lean, P = 0.003). This increase was observed in both male and females. In contrast, no differences were observed in PPAR gamma1/18S mRNA expression. There was a strong positive correlation (r = 0.70, P < 0.001) between the ratio of PPAR gamma2/gamma1 and the body mass index of these patients. We also observed sexually dimorphic expression with increased expression of both PPAR gamma1 and PPAR gamma2 mRNAs in the subcutaneous adipose tissue of women compared with men. To determine the effect of weight loss on PPAR gamma mRNA expression, seven additional obese subjects were fed a low calorie diet (800 Kcal) until 10% weight loss was achieved. Mean expression of adipose PPAR gamma2 mRNA fell 25% (P = 0.0250 after a 10% reduction in body weight), but then increased to pretreatment levels after 4 wk of weight maintenance. Nutritional regulation of PPAR gamma1 was not seen. In vitro experiments revealed a synergistic effect of insulin and corticosteroids to induce PPAR gamma expression in isolated human adipocytes in culture. We conclude that: (a) human PPAR gamma mRNA expression is most abundant in adipose tissue, but lower level expression of both splice variants is seen in skeletal muscle; to an extent that is unlikely to be due to adipose contamination. (b) RNA derived from adipose tissue of obese humans has increased expression of PPAR gamma 2 mRNA, as well as an increased ratio of PPAR gamma2/gamma1 splice variants that is proportional to the BMI; (c) a low calorie diet specifically down-regulates the expression of PPAR gamma2 mRNA in adipose tissue of obese humans; (d) insulin and corticosteroids synergistically induce PPAR gamma mRNA after in vitro exposure to isolated human adipocytes; and (e) the in vivo modulation of PPAR gamma2 mRNA levels is an additional level of regulation for the control of adipocyte development and function, and could provide a molecular mechanism for alterations in adipocyte number and function in obesity.

Regulation of in vivo growth hormone secretion by leptin

Leptin, the product of the ob gene, is a recently discovered hormone secreted by adipocytes that regulates food intake and energy expenditure. Growth hormone (GH) secretion is markedly influence by body weight being markedly suppressed in obesity and underweight. The aim of the present study was to study whether leptin can act as a metabolic signal connecting the adipose tissue with the growth hormone axis. We administered leptin antiserum (10 ul, i.c.v.) or normal rabitt serum (NRS; 10 ul, i.c.v.) to freely moving fed rats. Furthermore we assessed the effect of leptin administration (10 ug, i.c.v.) on fed and fasted rats. Spontaneous GH secretion was assessed over 6 hours with blood samples taken every 15 min. Administration of leptin antiserum led to a decrease in spontaneous GH secretion as assessed by the area under the curve (AUC) (168+/-72 ng/ml/6h) in comparison to NRS-treated rats (813+/-179 ng/ml/6h, p<0.01). While leptin administration (10 ug/rat; i.c.v.) to normal fed rats did not modify spontaneous GH secretion, leptin administration to fasted rats led to a reversal of the inhibitory effect exerted by fasting on GH secretion (AUC, 1650+/-351 ng/ml/6h vs 77+/-32 ng/ml/6h, p<0.01). This data suggests that leptin is a metabolic signal that regulates GH secretion.

Dexamethasone stimulates leptin release from human adipocytes: unexpected inhibition by insulin

In the present study we have examined the effect of dexamethasone on ob gene mRNA expression and leptin release from isolated human subcutaneous adipocytes. Dexamethasone stimulated leptin release from cultured adipocytes in a time- and dose-dependent manner. A two-fold increase in leptin release was detectable by 36 h of treatment with 10(-7) M dexamethasone. Leptin release was preceded by a significant 83 +/- 30% increase in ob mRNA after 24 h exposure to the compound. Co-incubation of cells with dexamethasone (10(-7) M) and insulin (10(-7) or 10(-9) M) completely blocked the dexamethasone-stimulated increase in ob mRNA and leptin release. These data demonstrate that insulin and glucocorticoids regulate leptin synthesis and release from human adipocytes in vitro.

Gender-dependent effects of exercise training on serum leptin levels in humans

Leptin, the product of the ob gene, is elevated in obese humans and appears to be closely related to body fat content. The purpose of the present investigation was to determine the effect of aerobic exercise training on systemic leptin levels in humans. Eighteen sedentary middle-aged men (n = 9) and women (n = 9) who did not differ in aerobic capacity (29.4 +/- 1.2 vs. 27.5 +/- 1.2 ml x kg(-1) x min(-1)) or insulin sensitivity index (3.41 +/- 1.12 vs. 4.88 +/- 0.55) were studied. Fat mass was significantly lower in females vs. males (21.83 +/- 2.25 vs. 26.99 +/- 2.37 kg, P < 0.05). Despite this, fasting serum leptin was significantly higher in the females vs. males (18.27 +/- 2.55 vs. 9.88 +/- 1.26 ng/ml, P < 0.05). Serum leptin concentration decreased 17.5% in females (P < 0.05) after 12 wk of aerobic exercise training (4 day/wk, 30-45 min/day) but was not significantly reduced in males. Fat mass was not altered after training in either group. In contrast, both aerobic capacity (+13% males, +9.1% females) and insulin sensitivity (+35% males, +82% females) were significantly improved subsequent to training. These data suggest that 1) women have higher circulating leptin concentrations despite lower fat mass and 2) exercise training appears to have a greater effect on systemic leptin levels in females than in males.

Serum immunoreactive leptin concentrations in patients with anorexia nervosa before and after partial weight recovery

Leptin, the product of the ob gene, is a recently discovered hormone secreted by adipocytes. Serum leptin concentrations increase in correlation with the percentage of body fat, but besides that little is known about the physiological actions of leptin in humans. In order to understand the role of leptin in severe malnutrition, in the present work 10 patients recently diagnosed with anorexia nervosa were studied both before and 2 months later, after partial weight recovery, and were compared with 18 normal-weight women as controls. Leptin was measured by a newly developed radioimmunoassay and both IGF-I and IGFBP-3 were measured by commercial radioimmunoassays. The mean (+/-SE) serum leptin concentrations (in microgram/liter) were 18.1 +/- 2.0 in control women with BMI of 21.1 +/- 0.3, significantly higher (P < 0.01) than that in the anorexia nervosa patients at diagnosis (2.2 +/- 0.1, BMI 15.3 +/- 0.6). These differences were also observed in IGF-I values (microgram/liter) that were 228.0 +/- 14.6 in controls and 157.4 +/- 28.7 in anorexia nervosa patients (P < 0.02). No differences were observed in IGF-BP3. After treatment, patients with anorexia nervosa experienced an increase in BMI (17.1 +/- 0.5, P < 0.0001 vs before) although they were still underweight. The partial recovery in weight led to a complete normalization of IGF-I levels (214.0 +/- 21.0 micrograms/liter) and to an enhancement in leptin levels (3.3 +/- 0.5 micrograms/liter; P < 0.03 vs before treatment), though still lower than those in normal-weight women (P < 0.05). Individually analyzed, a large dispersion was observed in control subjects, with leptin levels ranging from 5.5 to 38.7 micrograms/liter, while in all anorexia nervosa patients leptin levels were under 3 micrograms/liter. A treatment-induced increase in body weight led to an increase in leptin levels in 7 out of the 10 anorexia nervosa patients studied and the 3 patients with no increase in leptin were all initially under the 14.5 BMI. In conclusion, leptin levels are severely reduced in anorexia nervosa patients with severe malnutrition, and a significant rise occurred after partial weight recovery. There seems to be a level of BMI below which leptin levels do not drop further but also do not increase despite weight gain. While IGF-I reflects the energy intake of the previous few weeks, the serum leptin concentration reflects the true status of the adipose stores, a fact that has useful clinical implications.

Serum leptin concentrations in patients with anorexia nervosa, bulimia nervosa and non-specific eating disorders correlate with the body mass index but are independent of the respective disease

OBJECTIVE

Leptin, the product of the ob gene, is a recently discovered hormone secreted by adipocytes. Serum leptin concentrations increase in correlation with the percentage of body fat, but little else is known about the physiological actions of leptin in humans. The aim of this study was to determine the role of leptin in severe eating disorders, and whether its levels are correlated with the specific disease or exclusively with body weight.

TESTS

Serum concentrations of human leptin were analysed by specific radioimmunoassay and compared with the individual body mass indexes (BMI). The correlations between serum leptin concentrations and BMI, age and height were analysed.

PATIENTS

A total of 65 women were studied: 25 patients with anorexia nervosa, 20 women with bulimia nervosa, 6 women with a diagnosis of nonspecific eating disorder, and 14 normal-weight women who acted as controls. At the time of the study, the patients were non-cured, under treatment, and at different stages of therapeutic evolution.

MEASUREMENTS

Plasma leptin levels were measured by specific radioimmunoassay.

RESULTS

The mean serum leptin in the normal-weight women was 10.5 +/- 1.1 micrograms/l, compared with 7.6 +/- 1.1 micrograms/l in the anorexia nervosa patients (P < 0.05). This reduction in leptin levels was paralleled by the differences in BMI (21.4 +/- 0.4 vs 18.8 +/- 0.2) P < 0.05. These differences between the controls and anorexia nervosa patients were not observed in patients with bulimia nervosa who had a mean serum leptin level of 9.9 +/- 1.4 micrograms/l and BMI of 21.3 +/- 0.6, neither significantly different from controls. On the contrary, patients with non-specific eating disorders showed a large reduction in BMI (17.9 +/- 1.2, P < 0.05 vs control), and a parallel reduction in serum leptin levels, 4.5 +/- 1.0 (P < 0.05 vs controls). When individual values of leptin were plotted against BMI a wide range was observed in all groups; in the control subjects from 5.6 to 17.7 micrograms/l, in anorexia nervosa patients from 2.1 to 28.1 micrograms/l, in patients with bulimia nervosa between 2.6 and 25.9 micrograms/l, and in women with non-specific eating disorder from 2.0 to 8.9. No correlation was observed with the specific disease but in each group a significant correlation was observed only with BMI.

CONCLUSIONS

Serum leptin levels in three groups of patients affected by severe eating disorders are not related to the specific pathology but are correlated with the individual BMI. The analysis of leptin values may be a useful index of assessing the adipose tissue stores in the clinical setting, but will be of no help for diagnosis nor prognosis of severe eating disorders.

Do leptin receptors play a functional role in the endocrine pancreas?

It was recently speculated that leptin may exert a direct inhibitory effect upon insulin release from the pancreatic B-cell. This proposal meets, however, with two objections. First, although the message for leptin receptors is indeed detected in rat pancreatic islets, the short form of this receptor, for which no signalling function is known, represents the major species present in islet cells. Second, in the isolated perfused rat pancreas, leptin (1.0 nM) fails to affect the release of either insulin or glucagon.

Relationship between weight loss maintenance and changes in serum leptin levels

Serum leptin concentrations are higher in obese humans than in lean and are decreased by initial weight loss. This study examined the effects of maintenance of weight loss on leptin concentrations and tested whether leptin concentrations at baseline or after initial weight loss are related to the ability to maintain a reduced body weight. Fifty-two overweight women [body mass index (kg/m2) averaging 31.3] were studied before and after a 4 month weight loss program and at 6 month follow-up. Subjects lost 8.1 kg over the 4 month program, and leptin concentrations decreased from 30.1 to 20.4 ng/ml. Initial leptin level per unit body mass index (r = -0.61, p < 0.0001) and weight loss during months 0 to 4 (r = 0.39, p = 0.004) were both significantly associated with initial changes in leptin, and together explained 60% of the variance in change in leptin. Subjects who maintained their weight losses over the 6-month follow-up maintained their reductions in leptin levels; again, weight changes during follow-up were correlated with changes in serum leptin levels (r = 0.41, p = 0.003). There was no evidence that baseline leptin concentration (or leptin/body mass index) or the changes in leptin which accompanied initial weight loss were predictive of subsequent weight regain. Thus, changes in leptin concentration during weight loss track with changes in weight. However, neither baseline concentrations nor initial changes in leptin predict success at weight loss or maintenance.

Serum immunoreactive-leptin levels are increased in patients with Cushing's syndrome

Leptin, the product of the ob gene, is a recently discovered hormone secreted by adipocytes. Cushing's syndrome is a disease state usually associated with weight gain due to the accumulation of adipose tissue. In order to study the effect of chronic glucocorticoid excess upon serum leptin levels; in the present work, four patients with recently diagnosed Cushing's syndrome and a group of control subjects matched for age, sex and body mass index (BMI) were studied. Serum leptin concentrations, measured by radioimmunoassay, were assessed in samples taken every 60 minutes over a 24 hour period. Assessment of leptin concentrations over 24 hours, by means of the area under curve showed a twofold increase in serum leptin levels in patients with Cushing's syndrome (mean+/-SEM 54.3+/-14) in comparison to control subjects (29.3+/-4.4; p<0.05). In conclusion, our data show that leptin levels are markedly increased in Cushing's syndrome patients. The relevance of this finding to the increased body fat present in patients with Cushing's syndrome merits further studies.

Responses of leptin to short-term fasting and refeeding in humans: a link with ketogenesis but not ketones themselves

We investigated the response of leptin to short-term fasting and refeeding in humans. A mild decline in subcutaneous adipocyte ob gene mRNA and a marked fall in serum leptin were observed after 36 and 60 h of fasting. The dynamics of the leptin decline and rise were further substantiated in a 6-day study consisting of a 36-h baseline period, followed by 36-h fast, and a subsequent refeeding with normal diet. Leptin began a steady decline from the baseline values after 12 h of fasting, reaching a nadir at 36 h. The subsequent restoration of normal food intake was associated with a prompt leptin rise and a return to baseline values 24 h later. When responses of leptin to fasting and refeeding were compared with that of glucose, insulin, fatty acids, and ketones, a reverse relationship between leptin and beta-OH-butyrate was found. Consequently, we tested whether the reciprocal responses represented a causal relationship between leptin and beta-OH-butyrate. Small amounts of infused glucose equal to the estimated contribution of gluconeogenesis, which was sufficient to prevent rise in ketogenesis, also prevented a fall in leptin. The infusion of beta-OH-butyrate to produce hyperketonemia of the same magnitude as after a 36-h fast had no effect on leptin. The study indicates that one of the adaptive physiological responses to fasting is a fall in serum leptin. Although the mediator that brings about this effect remains unknown, it appears to be neither insulin nor ketones.

Leptin is related to body fat content in male distance runners

Leptin, the product of the ob gene, has been reported to be related to body fat in humans (Considine et al. N. Engl. J. Med. 334: 292, 1996). However, little is known about the physiology of this putative satiety signal in humans. The purpose of the present study was to determine whether leptin is related to body fat content in relatively lean endurance-trained adults. In addition, the effect of acute exercise on circulating leptin concentration was studied. Thirteen male runners, whose mean age, height, weight, %fat, and maximal oxygen consumption (VO2max) were 32.2 +/- 2.5 yr, 176.2 +/- 1.6 cm, 71.9 +/- 6.9 kg, 9.7 +/- 0.9%, and 62.9 +/- 2.2 ml.kg-1.min-1, respectively, were studied. Blood samples were obtained after an overnight fast and again immediately after the completion of a 20-mile run at 70% VO2max under controlled environmental conditions. Serum leptin was closely related to fat mass (r = 0.92) in the runners. Acute exercise had no detectable effect on serum leptin levels (PRE = 2.19 +/- 0.32 ng/ml, POST = 2.14 +/- 0.36 ng/ml). These data indicate that, even at a biological extreme of body fat, circulating leptin concentration is closely related to fat content. Furthermore, the data suggest that, in trained individuals with low leptin concentrations, acute exhaustive exercise has no immediate effect on circulating leptin concentration.

Response of leptin to short-term and prolonged overfeeding in humans

As one of the postulated roles of the ob gene product, leptin, is regulation of energy balance and preservation of normal body composition, we investigated the effect of acute and chronic calorie excess (weight gain) on serum leptin in humans. Two protocols were employed: 1) acute (12-h) massive (120 Cal/kg) voluntary overfeeding of eight healthy individuals; and 2) chronic overfeeding to attain 10% weight gain, with its subsequent maintenance for additional 2 weeks, involving six normal males. In the acute experiments (protocol 1), circulating leptin rose by 40% over baseline (P < 0.01) during the final hours of overfeeding; this increase persisted until the next morning. At the point of achievement and the 2-week maintenance of 10% weight gain (protocol 2), a more than 3-fold rise in the basal leptin concentration was observed (P < 0.01). A direct linear relationship was found between the magnitude of the leptin response to weight gain and the percent gain of body fat (r = 0.88; P < 0.01). In summary, 1) in contrast to normal food intake (8), short term massive overfeeding is associated with a moderate elevation of circulating leptin levels that persists until next feeding cycle is initiated; and 2) a 10% weight gain causes different changes in the body composition, and the resulting rise in circulating leptin parallels the increase in the percentage of body fat. In conclusion, these studies document acute elevation of leptin in response to positive energy balance and suggest that developing resistance to leptin is associated with bigger fat deposition during weight gain in humans.

Gender differences in serum leptin levels in humans

Leptin, the product of the ob gene, is an adipose tissue-derived hormone that appears to regulate both satiety and thermogenesis. In the present report, we have reexamined the relationship between circulating leptin concentration and body fat in humans using a more valid measure of adiposity (hydrodensitometry) and have extended these observations to examine the influence of regional body fat distribution and cardiorespiratory fitness. Fasting serum leptin concentration was 6.9 +/- 0.3 ng.ml-1 in males (N = 333) and 15.2 +/- 1.3 ng.ml-1 in females (N = 63). Interestingly, total fat mass did not differ between groups (males 20.5 +/- 0.5 kg; females 20.4 +/- 1.5 kg), suggesting that females have higher leptin levels per unit fat mass. In a multiple regression model, fat mass was the best predictor of serum leptin concentration in males, accounting for 51% of the variance in leptin concentration. In females, percentage body fat was the best predictor of leptin, accounting for 49% of the variance. In both groups, the relationship between leptin and adiposity remained significant after adjusting for age, maximal treadmill time, waist circumference, and fasting insulin concentration. These observations support previous conclusions that circulating leptin is primarily a function of adiposity and demonstrate for the first time that this relationship is independent of fat distribution or cardiorespiratory fitness. The data also suggest that there is a gender dichotomy in the relationship between leptin and body fat mass in humans.

Effect of troglitazone on leptin production. Studies in vitro and in human subjects

Leptin, the product of the ob gene, is a hormone secreted by adipocytes. Animals with mutations in the ob gene are obese and lose weight when given leptin, but little is known about the physiological role of leptin in humans. Obese subjects have higher concentrations of leptin than lean subjects, the strongest correlation being with percentage body fat. Thus, it appears that obese subjects are resistant to the effects of endogenously secreted leptin. We have also shown that insulin stimulates leptin production, chronically but not acutely, presumably through its trophic effect on adipocytes. Troglitazone is an insulin-sensitizing thiazolidinedione, which improves hepatic and skeletal muscle insulin resistance in NIDDM and obesity. This study was undertaken to investigate the effects of troglitazone on leptin production in vitro and in vivo. In the presence and absence of 100 nmol/l insulin and 10 umol/l troglitazone, 72-h primary cultures of isolated abdominal adipocytes were studied. Insulin led to an almost twofold increase in leptin in vitro, and this increase was completely abolished by coincubation with troglitazone. Incubation with troglitazone alone led to a 40% decrease in leptin production. In obese patients administered troglitazone 200 mg twice daily for 12 weeks, there was no significant change in fasting plasma leptin concentrations, despite a 40-50% reduction in fasting and postmeal plasma insulin concentrations. Troglitazone treatment led to a significant increase in insulin sensitivity, and there was a positive correlation between the change in insulin sensitivity and the change in plasma leptin concentration in these subjects. In conclusion, troglitazone treatment had no net effect on plasma leptin concentrations, possibly because of improvement in insulin sensitivity and reduction in plasma insulin concentrations.