Effects of antibodies to adipocytes on body weight, food intake, and adipose tissue cellularity in obese rats

Female Wistar rats were fed on a high fat diet for 18 weeks, during which their energy intake increased by 25% and body weight by 50% due to a doubling of adipose tissue tissue stores. Animals were then treated with increasing doses of a sheep polyclonal antiserum to rat adipocytes on days 1-4 and 7 after which they remained untreated for 14 weeks. Antibody treatment reduced body weight by 10% and the weight of parametrial and subcutaneous adipose tissue by 30-40%. This decrease was explicable entirely in terms of a decrease in the number of adipocytes presumably due to adipocyte lysis. These favourable changes in body fat mass were accompanied by improvement in at least one metabolic factor associated with obesity - serum leptin concentrations were significantly reduced in treated animals compared with high fat controls. Genetically obese Zucker rats also showed decreases in the number of adipocytes after treatment with antibodies but in contrast to diet-induced obese rats, they showed a compensatory increase in adipocyte volume which attenuated the effects on body fat mass. These results demonstrate for the first time, the potential to treat diet-induced obesity with antibodies to adipocytes by producing long-term reductions in the number of adipocytes, with minimal side-effects.     

Multihormonal control of ob gene expression and leptin secretion from cultured human visceral adipose tissue: increased responsiveness to glucocorticoids in obesity

The direct role of hormones on leptin synthesis has not yet been studied in cultured adipose cells or tissue from lean and obese subjects. Moreover, this hormonal regulation has never been addressed in human visceral fat, although this site plays a determinant role in obesity-linked disorders. In this study, we investigated the hormonal control of ob expression and leptin production in cultured visceral adipose tissue from lean and obese subjects. We more particularly focused on the interactions between glucocorticoids and insulin. We also briefly tackled the role of cAMP, which is still unknown in man. Visceral (and subcutaneous) adipose tissues from eight obese (body mass index, 41 +/- 2 kg/m2) and nine nonobese (24 +/- 1 kg/m2) subjects were sampled during elective abdominal surgery, and explants were cultured for up to 48 h in MEM. The addition of dexamethasone to the medium increased ob gene expression and leptin secretion in a time-dependent manner. Forty-eight hours after dexamethasone (50 nmol/L) addition, the cumulative integrated ob messenger ribonucleic acid (mRNA) and leptin responses were, respectively, approximately 5- and 4-fold higher in obese than in lean subjects. These responses closely correlated with the body mass index. The stimulatory effect of the glucocorticoid was also concentration dependent (EC50 = approximately 10 nmol/L). Although the maximal response was higher in obese than in lean subjects, the EC50 values were roughly similar in both groups. Unlike dexamethasone, insulin had no direct stimulatory effect on ob gene expression and leptin secretion. Singularly, insulin even inhibited the dexamethasone-induced rise in ob mRNA and leptin release. This inhibition was observed in both lean and obese subjects, whereas the expected stimulation of insulin on glucose metabolism and the accumulation of mRNA species for the insulin-sensitive transporter GLUT4 and glyceraldehyde-3-phosphate dehydrogenase occurred in lean patients only. This inhibitory effect was already detectable at 10 nmol/L insulin and was also observed in subcutaneous fat. Although a lowering of intracellular cAMP concentrations is involved in some of the effects of insulin on adipose tissue, this cannot account for the present finding, because the addition of cAMP to the medium also decreased ob mRNA and leptin secretion (regardless of whether dexamethasone was present). In conclusion, glucocorticoids, at physiological concentrations, stimulated leptin secretion by enhancing the pretranslational machinery in human visceral fat. This effect was more pronounced in obese subjects due to a greater responsiveness of the ob gene and could contribute to the metabolic abnormalities associated with central obesity by para/endocrine actions of hyperleptinemia on adipocytes and liver. Unlike dexamethasone, insulin had no direct stimulatory effect on ob gene expression and leptin secretion, and even prevented the positive response to dexamethasone by a cAMP-independent mechanism that remained functional despite insulin resistance.     

Effects of leptin adipose tissue lipoprotein lipase in the obese ob/ob mouse

OBJECTIVE: To characterize the adaptations of lipid metabolism, with special emphasis on tissue lipoprotein lipase, to negative energy balance brought by chronic treatment of obese ob/ob mice with leptin. DESIGN: According to a 2 x 2 factorial analysis, lean and obese C57BL/6J mice were subcutaneously infused with leptin (100 micrograms.kg-1.day-1) or vehicle (phosphate-buffered saline) during seven days. RESULTS: Cumulative food intake and final body weight of vehicle-infused obese mice were twofold higher than in lean controls. Leptin decreased cumulative food intake and body weight of obese, but not lean mice. Lipoprotein lipase (LPL) activity in white inguinal and epididymal and brown interscapular adipose tissues of control obese mice was at least twofold higher than in lean mice, but comparable in the vastus lateralis muscle. Leptin treatment of obese mice significantly lowered LPL activity to that of lean mice in all tissues examined. Vehicle-infused obese mice had higher liver triglyceride content and were hypertriglyceridemic compared to lean mice, and triglyceride concentrations in plasma and liver were decreased proportionally after leptin treatment. Leptin lowered glycemia and insulinemia of obese mice to lean levels and decreased plasma corticosterone. Leptin infusion had no notable effect on tissue lipoprotein lipase nor plasma variables of lean mice. CONCLUSIONS: Leptin infusion abolished hyperinsulinemia in the ob/ob mouse, an effect that was probably responsible for the concomitant normalization of adipose LPL activity. This study shows that decreased LPL activity, plasma triglyceride concentrations and hepatic triglyceride production constitute some of the adaptive peripheral adaptations of lipid metabolism, which accompany the reduction in fat mass accretion brought by leptin treatment of the obese ob/ob mouse.     

Cocaine disposition in saliva following intravenous, intranasal, and smoked administration

BACKGROUND: In our previous studies, chronic treatment of rats with a new beta 3-adrenoceptor agonist, CL 316,243, retarded diet-induced obesity and promoted thermogenesis in young animals and reversed established diet-induced obesity in older animals that continued to eat a high fat diet. Reversal of obesity was associated with shrinking of enlarged white adipocytes but the number of mature white adipocytes, which had not been increased by the diet, was not reduced. Drug-treatment induced appearance of abundant brown adipocytes in white adipose tissue (WAT) depots as well as hypertrophy of brown adipose tissue (BAT) in both lean and diet-induced obese rats. OBJECTIVES: To find out whether the known hyperplasia of white adipocytes in the obese fa/fa rat could be reversed by CL 316,243-treatment and whether the grossly enlarged WAT depots of the obese fa/fa rat contain precursors to brown adipocytes. RESULTS: CL 316,243 infusion (1 mg/kg/d) reduced abdominal fat. The loss of fat was due to a decrease in white adipocyte size, with no loss of the markedly elevated number of adipocytes in the fa/fa rats. Resting metabolic rate increased by 40% in lean rats, by 70% in fa/fa rats. Food intake decreased in the hyperphagic fa/fa rats but did not change in lean rats, in both lean and fa/fa rats, a marked increase in protein content of retroperitoneal WAT was associated with appearance of abundant densely-stained brown adipocytes expressing uncoupling protein (UCP) but total number of cells (from DNA content) actually decreased. Hyperinsulinemia and hyperglycemia of fa/fa rats were reduced by treatment, indicating improved sensitivity to insulin. CONCLUSIONS: Abundant precursors to brown adipocytes are present in WAT depots of fa/fa rats and much of the exaggerated increase in resting metabolic rate induced by CL 316,243 occurs in these cells. This beta 3-adrenoceptor agonist is an effective anti-obesity and anti-diabetic agent in fa/fa rats. It does not bring about disappearance of mature white adipocytes but does bring about a remodelling of WAT, with a marked change in cell composition.     

Adipose tissue cell size and lipolysis in the rat: response to exercise intensity and food restriction

This experiment was designed to determine if the adaptive increase in adipose tissue epinephrine-stimulated lipolysis (ESL) observed in exercise trained rats is related to decreased adipocyte size or a direct response to exercise. Two levels of treadmill exercise and three levels of food restriction were imposed on male rats over a 12 week experimental period to create a distribution of adipose tissue cell sizes. Epinephrine-stimulated lipolysis was subsequently measured in the isolated adipocytes from rats trained at two different exercise levels and in untrained rats fed either ad libitum or 16%, 27%, or 35% dietary restriction. Energy restriction was effective in reducing body weight and to some extent epididymal fat pad weight; however, adipocyte size and number were not significantly affected. Exercise in both groups of trained rats was effective in reducing adipocyte size; however, cell size did not differ between training groups. The group receiving the greatest amount of daily exercise had significantly greater ESL indicating that the adaptive increase in lipolytic potential seen in adipose tissue of exercise trained rats is a true metabolic adaptation not secondary to reduced cell size.     

Chronic protein restriction does not alter energetic efficiency or brown adipose tissue thermogenic capacity in genetically obese (fa/fa) Zucker rats

Attenuated regulatory thermogenesis in genetically obese (fa/fa) Zucker rats involves an impaired capacity to increase sympathetic drive to brown adipose tissue in response to dietary stimuli. Young, growing lean rats fed a low protein diet reduce energetic efficiency to compensate for elevated energy intake; however, it is not known if obese rats adapt similarly to chronic protein restriction by decreasing energetic efficiency and whether this would be accompanied by increased brown adipose tissue thermogenic capacity. Lean (Fa/Fa) and obese Zucker rats were either protein-restricted (protein 8% of total energy) or fed a control diet (21% protein) starting at age 5 wk. At 9 wk, oxygen consumption (VO2) was measured in response to an intubated meal of mixed macronutrient composition. Mass-adjusted food intake (i.e., food intake/body weight 0.67) was greater in protein-restricted than in control lean rats, but not different due to diet in obese rats. Mass-adjusted brown adipose tissue uncoupling protein levels were more than 100% greater in protein-restricted vs. control lean rats, but not different between protein-restricted and control obese rats. The uncoupling protein level was not significantly different in control lean vs. obese rats. Energetic efficiency was 40% lower in protein-restricted vs. control lean, but not different in obese rats; however, the efficiency of protein utilization was significantly greater in obese protein-restricted than in obese control rats. Meal-induced energy expenditure (VO2) did not differ significantly due to diet or genotype. In conclusion, protein restriction led to overfeeding in lean rats which appeared to enhance brown adipose tissue thermogenic capacity and decrease energetic efficiency. Protein efficiency increased to more than two times its original value in obese (fa/fa) rats, but otherwise no metabolic accommodation in the capacity for regulatory thermogenesis was observed in this genotype.     

Quenching of chlorophyll fluorescence by quinones

Obese (Lepr(fa)/Lepr(fa)) Zucker rats have a missense mutation in the leptin receptor gene. One amino acid substitution in the extracellular domain common to all known leptin receptor proteins results from this mutation. Obese Zucker rats are unable to respond behaviorally to leptin which is peripherally administered. However, conflicting reports exist on whether obese Zucker rats can respond to centrally administered leptin. The purpose of this study was to determine whether obese Zucker rats responded behaviorally and metabolically to intracerebroventricularly (i.c.v.) administered leptin and to compare the responses of lean and obese Zucker rats. We found that both lean and obese Zucker rats had similar body weight and food intake responses when administered a single i.c.v. leptin injection in a range of doses (1.25, 2.5, 5, and 10 microg), as well as daily i.c.v. administered leptin for five consecutive days. Both single and daily leptin administration also decreased respiratory quotient (RQ) similarly in lean and obese Zucker rats, indicating mobilization of fat as an energy source for leptin-treated rats. After withdrawal of daily leptin treatment, lean and obese Zucker rats exhibited different recovery responses. It is concluded that obese Zucker rats can respond to exogenous leptin when leptin is delivered into the brain ventricles.     

Noncompensation of adipose mass in partially lipectomized mice and rats

The effects of surgical ablation of adipose tissue were studied in normal mice and rats. It was found that: 1) restortation of adipose tissue does not occur locally in epididymal fat pads of young rats. 2) Bilateral epididymal fat pad removal in mice disrupts the testes and causes the other fat depots to accumulate excess lipid, but these effects are not sustained; After a sufficient recovery period, testes appear normal and no excess lipid is found in the remaining depots; 3) Temporary enlargement of remaining depots is probably due specifically to epididymal pad removal. It does not occur in response to inguinal depot removal, nor in response to disruption of the testes alone; 4)The quantity of lipid stored by a rapidly growing mouse depends on the number of intact depots in the mouse. These results suggest that surgical removal of fat does not lead to compensatory growth of fat. Autoregulation of adipose tissue mass, if it occurs, most likely operates through detection of adipocyte size rather than adipocyte number or total fat mass.     

Effect of caloric restriction on basal insulin levels and the in vivo lipogenesis and glycogen synthesis from glucose in the Koletsky obese rat

Fasting plasma immunoreactive insulin levels increased with age in hyperinsulinemic Koletsky obese rats, being almost four times as high as in lean siblings at 3 mo (40 +/- 5 muU/ml) and rising steadily to 82 +/- 4 muU/ml at 6 mo (about seven times higher than lean siblings). Restricting the food intake of the obese rats markedly reduced but did not normalize the hyperinsulinemia, which in these rats was accompanied by normal plasma glucose concentrations. The incorporation in vivo of D-U-14C-glucose into tissue lipids and glycogen was measured 1 hr after the intravenous injection of 1 g glucose (containing 100 muDi D-U-14C-glucose) per kg body weight in obese rats eating ad libitum, obese rats after 3 mo on a restricted food intake, and lean siblings. All tissues (heart, diaphragm, skeletal muscle, and adipose tissues and liver) of obese rats exhibited a significantly greater lipogenesis from glucose than those of lean siblings. Dietary restriction of the obese rats reduced the 14C incorporation into lipid to levels not significantly different from lean controls in all tissues except skeletal muscle and liver, where, although greatly reduced, lipogenesis was still significantly higher than in lean rats. Glycogen synthesis tended to be greater in all tissues of obese rats than in lean animals. Dietary restriction of obese rats did not greatly affect glycogen synthesis.     

Induction of uncoupling protein expression in brown and white adipose tissue by leptin

Deposition of excess body fat occurs when energy intake chronically exceeds energy expenditure. In ob/ob mice, the absence of leptin affects both components of the energy balance equation, and the mice become morbidly obese after weaning. Treatment of ob/ob mice with exogenous leptin reduces body weight by decreasing food intake and stimulating energy utilization, but even when saline- and leptin-injected ob/ob mice are pair-fed, mice receiving leptin lose significantly more weight. Therefore, the purpose of the present study was to test the hypotheses that uncoupling protein-1 (UCP1) expression is reduced in adipose tissue from ob/ob mice and is restored by treatment with exogenous leptin. Lean and ob/ob mice (5-6 weeks old) were housed at 23 C and treated with leptin (20 microg/g BW x day) for 3 days before they were killed. Compared with levels in lean littermates, UCP1 messenger RNA (mRNA) and protein levels were lower in brown adipose tissue (BAT) and retroperitoneal white adipose tissue (WAT) from ob/ob mice. Treatment of ob/ob mice with leptin reduced body weight and produced a 4- to 5-fold increase in UCP1 mRNA levels in both interscapular BAT and retroperitoneal WAT. The increases in UCP1 mRNA were accompanied by comparable increases in UCP1 protein in mitochondrial preparations from each tissue. Given that the sole known function of UCP1 is to uncouple oxidative phosphorylation, the present results are consistent with the conclusion that leptin stimulates energy utilization in ob/ob mice by increasing thermogenic activity and capacity (UCP1). In addition, the present results suggest that decreased UCP1 expression in BAT and WAT of ob/ob mice is in part responsible for their increased metabolic efficiency and propensity to become obese.     

The effects of a high fat diet on leptin mRNA, serum leptin and the response to leptin are not altered in a rat strain susceptible to high fat diet-induced obesity

Osborne-Mendel (OM) and S5B/Pl rats differ in their sensitivity to develop obesity when fed a high fat (HF) diet; OM rats become obese, whereas S5B/Pl rats remain thin. We have investigated the possibilities that either an impaired leptin response or resistance to leptin action underlies the sensitivity to this form of obesity in OM rats. In Experiment 1, OM and S5B/Pl rats fed a nonpurified diet were killed at d 0 or were fed either a HF (56% fat energy) or a low fat (LF, 10% fat energy) diet for 2 or 7 d. The HF diet increased serum leptin significantly by d 2 to levels that were similar in both rat strains. At 7 d, leptin levels were lower than at d 2 but remained higher than levels in the d 0 control groups. The leptin mRNA:18S RNA ratio in epididymal adipose tissue increased to higher levels in HF-fed OM rats than in S5B/Pl rats fed that diet. However, although the LF diet had no effect in S5B/Pl rats, it increased leptin mRNA levels in epididymal adipose tissue of OM rats compared with the controls fed the nonpurified diet. In Experiment 2, OM and S5B/Pl rats were fed HF or LF diets for 5 wk. At that time, their feeding response to a range of leptin doses (0, 1, 5 or 10 microgram) given intracerebroventricularly was tested after overnight food deprivation. There was a similar dose-dependent reduction in energy intake in response to leptin in both OM and S5B/Pl rats. These responses were independent of the diet. The data suggest that the susceptibility of OM rats to HF diet-induced obesity is not related to either a loss of central sensitivity to leptin or a failure to enhance leptin production acutely, although the failure to maintain chronically increased levels of serum leptin could contribute to the obesity.     

Is incarceration during pregnancy associated with infant birthweight?

To address the hypothesis that tumor necrosis factor (TNF)-alpha has a role in obesity-associated insulin resistance or the regulation of in vivo lipid metabolism, mice with targeted disruption of the TNF-alpha gene were generated and studied. The absence of TNF-alpha protein in TNF-null (-/-) mice was confirmed. Lean or obese (gold-thioglucose [GTG]-injected) homozygous (-/-) mice were compared with lean or obese age- and sex-matched wild-type (+/+) mice derived from the same line at 13, 19, and 28 weeks of age. The following parameters were significantly affected in lean -/- versus +/+ mice: Body weight was not affected until week 28 (decreased by 14%); epididymal fat pad weight also decreased (25%) at this time, as did percentage body fat (16%), while percentage body protein was increased 13%. Fed plasma insulin levels decreased 47% (28 weeks), triglyceride levels decreased (all three ages; maximum 35% at 19 weeks), and fed plasma leptin decreased 33% (28 weeks). Fasting glucose was slightly (10%) reduced, but the glucose response to an oral glucose tolerance test (OGTT) was not affected. There was a trend (NS) toward increased total adipose tissue lipoprotein lipase in -/- versus +/+ mice. GTG-treatment resulted in obese -/- and +/+ mice with equal mean body weights (42 and 58% increased weight versus lean mice). The following parameters were significantly different in obese -/- mice: fasting plasma glucose decreased 13% (28 weeks), fed plasma insulin decreased 67% (28 weeks), and insulin response to OGTT was decreased by 50%. For both groups of obese mice, glucose levels during the OGTT were substantially increased compared with those in lean mice; however, mean stimulated glucose levels were 20% lower in obese -/- versus +/+ mice. We conclude 1) that TNF-alpha functions to regulate plasma triglycerides and body adiposity and 2) that although TNF-alpha contributes to reduced insulin sensitivity in older or obese mice, the absence of TNF-alpha is not sufficient to substantially protect against insulin resistance in the GTG hyperphagic model of rodent obesity.     

Chronic central leptin infusion enhances insulin-stimulated glucose metabolism and favors the expression of uncoupling proteins

Continuous (4 days) intracerebroventricular leptin infusion (12 microg/day) was performed in lean rats, and its hormonometabolic effects were determined. Intracerebroventricular leptin administration did not result in leakage of the hormone into the peripheral circulation. Thus, its effects were elicited by its presence within the central nervous system. Intracerebroventricular leptin infusion produced marked decreases in food intake and body weight gain relative to vehicle-infused fed ad libitum rats. Because decreases in food intake alter hormonometabolic homeostasis, additional control rats pair-fed to the amount of food consumed by leptin-infused ones were included in the study. Intracerebroventricular leptin-infused and vehicle-infused pair-fed rats were characterized, relative to vehicle-infused ad libitum-fed animals, by decreases in body weight and insulinemia and by increases in insulin-stimulated overall glucose utilization and muscle and brown adipose tissue glucose utilization index. Brown adipose tissue uncoupling protein (UCP)1, UCP2, and UCP3 mRNA levels were markedly decreased in pair-fed animals relative to those of fed ad libitum control animals, as were liver and white adipose tissue UCP2 and muscle UCP3 mRNA levels. In marked contrast, intracerebroventricular leptin administration was accompanied by the maintenance of high UCP1, UCP2, and UCP3 expression in all these tissues. Thus, despite analogies between leptin's effects and those of pair-feeding with regard to glucose handling, their respective underlying mechanisms differ. While leptin maintains or favors energy-dissipating mechanisms (UCP1, UCP2, and UCP3), the latter are markedly depressed in pair-fed rats. This effect of leptin may prevent subsequent excessive storage processes, thereby maintaining normal body homeostasis.     

Leptin is a physiologically important regulator of food intake

OBJECTIVE: These studies were designed to test the hypothesis that endogenous leptin, acting within the brain plays a physiologically important role in the control of food intake in lean rats. DESIGN: Antibodies directed against mouse leptin were raised in rabbits. The purified IgG fractions prepared from pre-immune and immune sera were injected into the right lateral ventricle of lean Sprague-Dawley rats and obese Zucker fatty fa/fa rats. Changes in food intake were measured over the following 20 h period. RESULTS: The anti-leptin antibodies recognized a major epitope in the C-terminal region of the leptin molecule. The antibodies bound both mouse and rat leptin with high affinity, but did not bind human leptin, or a selected range of other hormones and neurotransmitters known to affect food intake. In competition studies, the binding of mouse, but not human leptin to the human Ob-Rb receptor was prevented by the antibodies. This indicates that the antibodies can block the action of leptin by preventing its binding to the ob-Rb receptor. Injection of the anti-leptin antibodies into the brain of lean rats led to an increase in food intake during the first hour after injection which was not compensated during the following 19 h period. Injection of the anti-leptin antibodies did not affect food intake in Zucker fatty fa/fa rats which express an abnormal ob-Rb receptor. CONCLUSION: Endogenous leptin acting within the brain plays a physiologically important role in the control of food intake in lean rats.     

Lipoprotein metabolism in the fat Zucker rat: reduced basal expression but normal regulation of hepatic low density lipoprotein receptors

Hyperlipoproteinemia is one of the phenotypic characteristics of the fat Zucker rat that carries a mutation in the leptin receptor gene. In the present study, we studied the regulation of hepatic low density lipoprotein (LDL) receptor expression in lean and fat Zucker rats. Compared with lean rats, the fat ones had a pronounced (approximately 60%) reduction in hepatic LDL receptor expression, whereas the levels of receptor messenger RNA (mRNA) were not reduced. Fat rats had increased levels of very low density lipoproteins and high density lipoproteins, but their plasma apo B100 within LDL was reduced. Challenge with 2% dietary cholesterol for 8 days suppressed hepatic LDL receptor expression in lean animals to similar levels as seen in fat ones, whereas the reduction in mRNA levels was much less pronounced. Treatment with ethynylestradiol (5 mg/kg BW per day) for 4 days strongly stimulated hepatic LDL receptor expression in both lean and fat rats; this treatment also increased LDL receptor mRNA levels, but to a lesser extent. In conclusion, the basal expression of hepatic LDL receptors is reduced in fat Zucker rats, but the capacity for the regulation of the receptors remains intact.     

Adipose tissue leptin production and plasma leptin kinetics in humans

Abdominal adipose tissue leptin production was determined in vivo by arteriovenous balance in 14 lean and obese men (mean BMI 27.0 +/- 1.9, range 21.4-45.2). Blood samples were taken simultaneously from an abdominal vein that drains subcutaneous adipose tissue and from a radial artery. Adipose tissue blood flow was measured by xenon washout. Abdominal vein leptin concentrations (mean 8.9 +/- 2.4 ng/ml, range 2.1-36.5 ng/ml) were consistently greater than arterial values (mean 6.6 +/- 1.9 ng/ml, range 1.7-28.2 ng/ml) (P < 0.001). The net rate of abdominal adipose tissue leptin production (mean 3.2 +/- 0.5 ng x 100 g(-1) x min(-1)) correlated directly with percentage body fat (rs = 0.59, P = 0.016). Estimated whole-body leptin production rate (797 +/- 283 ng x person(-1) x min(-1)) correlated directly with percent body fat (rs = 0.93, P < 0.0001) and with regional leptin production (rs = 0.81, P < 0.001). In contrast, the rate of leptin clearance from plasma (mean 1.50 +/- 0.23 ml x kg(-1) x min(-1)) and plasma leptin half-life (mean 24.9 +/- 4.4 min) was unrelated to adiposity (rs = 0.06, P = 0.30; rs = 0.16, P = 0.30, respectively). These results provide direct evidence that leptin is produced by adipose tissue in humans and that the rate of production is directly related to adiposity. A combination of greater leptin production per unit of body fat and increased production from expanded total body fat mass, rather than alterations in leptin clearance, account for the increase in plasma leptin concentrations observed in obese humans.     

Effects of gender, body composition, and menopause on plasma concentrations of leptin

Circulating concentrations of leptin ([leptin]) vary directly with body mass index and percentage body fat, and may thus constitute an afferent limb of a system regulating body fatness. We tested the hypotheses that: 1) Plasma [leptin] vary more directly with absolute fat mass than with fractional body fatness per se: and 2). The relationship between fat mass and [leptin] is significantly affected by gender and by menopausal status. [Leptin] in the post-absorptive state was examined in 67 subjects (26 male, 20 premenopausal female, 21 postmenopausal females; 43 never-obese, 24 obese) at usual body weight. Body composition was determined by hydrodensitometry, and [leptin] was determined by a double antibody ELISA assay. In male and pre-menopausal female subjects, subcutaneous adipose tissue aspirations were performed for determination of adipocyte volume by the osmium fixation method, and a 3 hour oral glucose tolerance tests was performed. At usual body weight, ([leptin]) was better correlated with absolute fat mass than with body mass index (BMI) or percentage body fat. BMI and % body fat did not account for any of the variance in [leptin] beyond that attributable to FM, per se. The regression equations relating FM to [leptin] did not differ significantly between obese and never-obese subjects. [Leptin] and fasting serum insulin concentrations were significantly correlated in males only. [Leptin] was significantly higher in pre- and post-menopausal females compared to males, even when [leptin] was corrected for differences in body composition (pre-menopausal females > post-menopausal females > males). While plasma [leptin], corrected for FM, declines significantly in women post-menopause, this decline is not sufficient to account for the striking sexual dimorphism in the relationship of leptin to fat mass. This sexual dimorphism is apparently also due, in part, to a suppressive effect of circulating androgens on [leptin].     

Dietary fat type and energy restriction interactively influence plasma leptin concentration in rats

To investigate whether dietary fat source and energy restriction interactively influence plasma leptin levels and its association of leptin with insulin action, rats were fed diets containing either fish, safflower oil, or beef tallow (20% wt/wt) for 10 weeks. Groups of rats consumed each diet ad libitum or at 85% or 70% of ad libitum energy intake in a design that held fat intake constant. Graded levels of energy restriction caused body weight to decrease (P < 0.001) differently according to the dietary fat provided. Plasma leptin concentrations were 60% higher (P < 0.05) in the groups fed fish oil and safflower oil ad libitum compared with those in the beef tallow group, despite smaller perirenal fat mass and fat cell size in the fish oil-fed animals. Energy restriction resulted in a 62% decrease (P < 0.05) in leptin levels in fish oil- and safflower oil-fed rats, whereas no changes were observed in beef tallow-fed animals. Plasma insulin levels were lower (P < 0.05) in the fish oil group fed ad libitum compared with those in the two other diet groups. These data demonstrate a hyperleptinemic effect in animals consuming diets rich in polyunsaturated fatty acid, which can be normalized to the level of saturated fat consumption by mild energy restriction. Thus, dietary fatty acid composition, independent of adipose tissue mass, is an important determinant of circulating leptin level in diet-induced obesity.