Robert V. Considine

Decreased cerebrospinal-fluid/serum leptin ratio in obesity: a possible mechanism for leptin resistance

BACKGROUNDnA receptor for leptin has been cloned from the choroid plexus, the site of cerebrospinal-fluid (CSF) production and the location of the blood/cerebrospinal-fluid barrier. Thus, this receptor might serve as a transporter for leptin. We have studied leptin concentrations in serum and (CSF).nnnMETHODS AND FINDINGSnWe demonstrated by radioimmunoassay and western blot the presence of leptin in human CSF. We then measured leptin in CSF and serum in 31 individuals with a wide range of bodyweight. Mean serum leptin was 318% higher in 8 obese (40.2 [SE 8.6] ng/mL) than in 23 lean individuals (9.6 [1.5] ng/mL, p < 0.0005). However, the CSF leptin concentration in obese individuals (0.337 [0.04] ng/mL) was only 30% higher than in lean people (0.259 [0.26] ng/mL, p < 0.1). Consequently, the leptin CSF/serum ratio in lean individuals (0.047 [0.010]) was 4.3-fold higher than that in obese individuals (0.011 [0.002], p < 0.05). The relation between CSF leptin and serum leptin was best described by a logarithmic function (r = 0 x 52, p < 0.01).nnnINTERPRETATIONnOur data suggest that leptin enters the brain by a saturable transport system. The capacity of leptin transport is lower in obese individuals, and may provide a mechanism for leptin resistance.

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 influenced 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 rabbit 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/6 h, p < 0.01). While leptin administration (10 ug/rat; i.c.v.) to normal fed rats did not modify sp...

Leptin and the regulation of body weight

Leptin has received considerable attention as a newly recognized metabolic hormone and for its potential for therapeutic use in the treatment of human obesity. Furthermore, defects in the leptin signal pathway that result in obesity in animal models have raised the possibility of a similar etiology for obesity in humans. This review will summarize the current findings on leptin in both humans and rodents. These findings will be discussed with respect to our view of the physiology and potential for pathophysiology in leptin-mediated regulation of body weight and composition.

The hypothalamic leptin receptor in humans : Identification of incidental sequence polymorphisms and absence of the db/db mouse and fa/fa rat mutations

Leptin-receptor gene expression in hypothalamic tissue from lean and obese humans was examined. The full-length leptin receptor, that is believed to transmit the leptin signal, is expressed in human hypothalamus. There was no difference in the amount of leptin-receptor mRNA In seven lean (BMI 23.3 +/- 0.9 kg/m2) and eight obese (BMI 36.9 +/- 1.5) subjects as determined by reverse transcription-polymerase chain reaction. A sequence polymorphism (A-->G) was detected at position 668 of the leptin receptor cDNA. This second base substitution changed a glutamine to an arginine at position 223 of the leptin receptor protein. Of 15 subjects analyzed, 11 were heterozygous for this base change and 3 were homozygous. The occurrence [correction of occurance] of the polymorphic allele(s) did not correlate with BMI in the population studied. The mutation responsible for the defect in the leptin receptor in db/db mice was not detected in any obese human, nor was the fa/fa rat mutation. These results provide evidence that the leptin resistance observed in obese humans is not due to a defect in the leptin receptor.

Protein kinase C is increased in the liver of humans and rats with non-insulin-dependent diabetes mellitus: an alteration not due to hyperglycemia.

We tested the hypothesis that liver protein kinase C (PKC) is increased in non-insulin-dependent diabetes mellitus (NIDDM). To this end we examined the distribution of PKC isozymes in liver biopsies from obese individuals with and without NIDDM and in lean controls. PKC isozymes alpha, beta, epsilon and zeta were detected by immunoblotting in both the cytosol and membrane fractions. Isozymes gamma and delta were not detected. There was a significant increase in immunodetectable PKC-alpha (twofold), -epsilon (threefold), and -zeta (twofold) in the membrane fraction isolated from obese subjects with NIDDM compared with the lean controls. In obese subjects without NIDDM, the amount of membrane PKC isozymes was not different from the other two groups. We next sought an animal model where this observation could be studied further. The Zucker diabetic fatty rat offered such a model system. Immunodetectable membrane PKC-alpha, -beta, -epsilon, and -zeta were significantly increased when compared with both the lean and obese controls. The increase in immunodetectable PKC protein correlated with a 40% elevation in the activity of PKC at the membrane. Normalization of circulating glucose in the rat model by either insulin or phlorizin treatment did not result in a reduction in membrane PKC isozyme protein or kinase activity. Further, phlorizin treatment did not improve insulin receptor autophosphorylation nor did the treatment lower liver diacylglycerol. We conclude that liver PKC is increased in NIDDM, a change that is not secondary to hyperglycemia. It is possible that PKC-mediated phosphorylation of some component in the insulin signaling cascade contributes to the insulin resistance observed in NIDDM.

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.

LEPTIN : GENES, CONCEPTS AND CLINICAL PERSPECTIVE

Obesity is a complex disease which results from the interaction of multiple genes and the environment. The recently discovered genes for leptin (ob gene) and the leptin receptor appear to play a major regulatory role in body energy balance and adipose tissue deposition. Furthermore, defects in the ob gene and leptin receptor gene have been demonstrated to be the cause of obesity in several rodent models. These observations raise the possibility that human obesity may also be due to defects in the leptin signal system. This review will summarize the current findings on the ob gene, leptin and the leptin receptor in both animals and humans. These observations will be discussed in the context of potential defects in the system and the possibility that these defects result in obesity in humans.

Serum leptin levels in women throughout pregnancy and the postpartum period and in women suffering spontaneous abortion

In pregnancy, important changes occur in the body weight of the mother, caused by sodium and water retention and by an increase in body fat tissue, but the mechanisms that regulate maternal and foetal changes in fat mass are poorly understood. Leptin is a hormone produced by adipocytes in order to regulate food intake and energy expenditure at the hypothalamic level in man. In order to verify whether leptin participates in the changes in body composition during pregnancy and postpartum, 630 healthy women were studied at specific time periods and leptin and auxological parameters were determined.

Intensity of acute exercise does not affect serum leptin concentrations in young men

Purpose: We examined the effects of exercise intensity on serum leptin levels. nMethods: Seven men (age = 27.0 yr; height = 178.3 cm; weight = 82.2 kg) were tested on a control (C) day and on 5 exercise days (EX). Subjects exercised (30 min) at the following intensities: 25% and 75% of the difference between the lactate threshold (LT) and rest (0.25 LT, 0.75 LT), at LT, and at 25% and 75% of the difference between LT and peakOV2? (1.25 LT, 1.75 LT). nResults: Kcal expended during the exercise bouts ranged from 150 ± 11 kcal (0.25 LT) to 529 ± 45 kcal (1.75 LT), whereas exercise + 3.5 h recovery kcal ranged from 310 ± 14 kcal (0.25 LT) to 722 ± 51 kcal (1.75 LT). Leptin area under the curve (AUC) (Q 10-min samples) for all six conditions (C + 5 Ex) was calculated for baseline (0700–0900 h) and for exercise + recovery (0900–1300 h). Leptin AUC for baseline ranged from 243 ± 33 to 291 ± 56 ng~mL-1 X min; for exercise + recovery results ranged from 424 ± 56 to 542 ± 99 ng~mL-1 X min. No differences were observed among conditions within either the baseline or exercise + recovery time frames. Regression analysis confirmed positive relationships between serum leptin concentrations and percentage body fat (r = 0.94) and fat mass (r = 0.93, P < 0.01). nConclusion: We conclude that 30 min of acute exercise, at varying intensity of exercise and caloric expenditure, does not affect serum leptin concentrations during exercise or for the first 3.5 hours of recovery in healthy young men.

Association of leptin and hunger-satiety ratings in obese women.

OBJECTIVE: To measure leptin, insulin and cholecystokinin (CCK) concentrations in obese women on calorie restriction and to determine their correlation with hunger-satiety ratings. Although it has been proposed to play a role in appetite regulation, the effects of physiological concentrations of these hormones on hunger-satiety in humans have not yet been well established.DESIGN: Prospective metabolic study. A two week `wash-in period’ followed by a three-week observation period, during which each subject underwent six measurements of satiety, blood parameters and body weight.SETTING: Energy Metabolism Research Unit, Department of Nutrition Sciences, University of Alabama at Birmingham, Birmingham, Alabama, USA.SUBJECTS: 22 moderately to severely overweight women (mean age: 45±8u2005y; body mass index (BMI): 33±6u2005kg/m2).INTERVENTION: Energy restriction, in the form of a 3.3u2005MJ (800u2005kcal) diet during five weeks.MAIN OUTCOME MEASUREMENTS: Fasting blood levels of leptin, insulin, glucose and CCK, fasting hunger-satiety scores and body weight.RESULTS: The mean (±s.d.) fasting serum leptin concentration at the beginning of the observation period was 26.1±15.9u2005ng/ml (range: 6.7–59.8u2005ng/ml). Leptin concentrations correlated positively with body weight (P<0.0001). Furthermore, reductions in body weight were associated with decreases in fasting leptin levels (P=0.002). Leptin concentrations correlated with serum levels of insulin (P=0.0001) and CCK (P=0.06), but in multivariate analysis including insulin, CCK and glucose, only leptin had a significant relationship with satiety (P=0.04). This relationship was linear.CONCLUSIONS: These results confirm the association between leptin levels, body weight and serum insulin. We also showed that higher serum leptin levels correlated with greater feelings of fullness, a relationship which was not blunted in the more obese subjects. These findings suggest that leptin is a satiety hormone that reduces appetite, even in obese individuals, and that weight gain must be due to other factors, overriding this feed-back regulation.