Diseases of liporegulation: new perspective on obesity and related disorders

Obesity-related diseases now threaten to reach epidemic proportions in the United States. Here we review in a rodent model of genetic obesity, the fa/fa Zucker diabetic fatty (ZDF) rat, the mechanisms involved in the most common complications of diet-induced human obesity, i.e., noninsulin-dependent diabetes mellitus, and myocardial dysfunction. In ZDF rats, hyperphagia leads to hyperinsulinemia, which up-regulates transcription factors that stimulate lipogenesis. This causes ectopic deposition of triacylglycerol in nonadipocytes, providing fatty acid (FA) substrate for damaging pathways of nonoxidative metabolism, such as ceramide synthesis. In beta cells and myocardium, the resulting functional impairment and apoptosis cause diabetes and cardiomyopathy. Interventions that lower ectopic lipid accumulation or block nonoxidative metabolism of FA and ceramide formation completely prevent these complications. Given the evidence for a similar etiology for the complications of human obesity, it would be appropriate to develop strategies to avert the predicted epidemic of lipotoxic disorders.

Lipotoxicity of the pancreatic beta-cell is associated with glucose-dependent esterification of fatty acids into neutral lipids

Prolonged exposure of isolated islets to supraphysiologic concentrations of palmitate decreases insulin gene expression in the presence of elevated glucose levels. This study was designed to determine whether or not this phenomenon is associated with a glucose-dependent increase in esterification of fatty acids into neutral lipids. Gene expression of sn-glycerol-3-phosphate acyltransferase (GPAT), diacylglycerol acyltransferase (DGAT), and hormone-sensitive lipase (HSL), three key enzymes of lipid metabolism, was detected in isolated rat islets. Their levels of expression were not affected after a 72-h exposure to elevated glucose and palmitate. To determine the effects of glucose on palmitate-induced neutral lipid synthesis, isolated rat islets were cultured for 72 h with trace amounts of [14C]palmitate with or without 0.5 mmol/l unlabeled palmitate, at 2.8 or 16.7 mmol/l glucose. Glucose increased incorporation of [14C]palmitate into complex lipids. Addition of exogenous palmitate directed lipid metabolism toward neutral lipid synthesis. As a result, neutral lipid mass was increased upon prolonged incubation with elevated palmitate only in the presence of high glucose. The ability of palmitate to increase neutral lipid synthesis in the presence of high glucose was concentration-dependent in HIT cells and was inversely correlated to insulin mRNA levels. 2-Bromopalmitate, an inhibitor of fatty acid mitochondrial beta-oxidation, reproduced the inhibitory effect of palmitate on insulin mRNA levels. In contrast, palmitate methyl ester, which is not metabolized, and the medium-chain fatty acid octanoate, which is readily oxidized, did not affect insulin gene expression, suggesting that fatty-acid inhibition of insulin gene expression requires activation of the esterification pathway. These results demonstrate that inhibition of insulin gene expression upon prolonged exposure of islets to palmitate is associated with a glucose-dependent increase in esterification of fatty acids into neutral lipids.

Cloning and characterization of a novel human leptin receptor overlapping transcript-like 1 gene (LEPROTL1)

A new full-length cDNA encoding a novel protein was isolated from our human fetal brain cDNA library. The cDNA consists of 2701 bp and has a putative open reading frame encoding 131 amino acids which possesses a JAK binding site (Pro(46)-Ile-Pro(48) which is preceded by a cluster of hydrophobic residues) and is highly homologous to the leptin receptor gene-related protein (OB-RGRP). Northern blot analysis showed that this new gene is widely expressed in human tissues and radiation hybrid mapping placed the gene to human chromosome 8p21.1-8p21.2.

Mechanisms for LEPR-mediated regulation of leptin expression in brown and white adipocytes in rat pups

To investigate the underlying mechanisms for leptin receptor (LEPR)-mediated regulation of leptin gene (Lep) expression in brown (BAT) and white (WAT) adipose tissue and resultant effects on plasma leptin concentrations (plasma-LEP), we examined effects of sympathetic nervous system (SNS) activity, caloric balance, and body fat content on leptin mRNA levels in BAT and WAT in 10-day-old rat pups segregating for Lepr(fa). In mother-reared pups, Lep mRNA levels were fa/fa > +/fa = +/+ in BAT and was fa/fa > +/fa > +/+ in WAT. The genotype effects on Lep expression in BAT and plasma-LEP were virtually eliminated when the differences in SNS activity between fa/fa and +/fa pups were equalized by artificial rearing of pups under thermoneutral conditions with or without oral norepinephrine (NE) administration. NE administration alone had little effect on the Lepr(fa)-dependent stratification of Lep expression in WAT. BAT-Lep mRNA was the main determinant of plasma-LEP. Metabolic rate, a surrogate indicator of SNS activity, explained 87% of the variation in BAT-Lep mRNA (R(2) = 0.93), whereas caloric balance (40%) and body fat mass (6%) accounted for most of the variation in WAT-Lep mRNA (R(2) = 0.53). We conclude that feedback regulation of Lep expression in BAT is primarily via central nervous system-mediated effects of leptin on SNS activity, whereas the control of leptin expression in WAT is more likely via mechanisms not directly dependent on SNS activity.

Expression of the transcription factor STAT-1 alpha in insulinoma cells protects against cytotoxic effects of multiple cytokines

Destruction of pancreatic islet beta-cells in type 1 diabetes appears to result from direct contact with infiltrating T-cells and macrophages and exposure to inflammatory cytokines such as interferon (IFN)-gamma, interleukin (IL)-1 beta, and tumor necrosis factor TNF-alpha that such cells produce. We recently reported on a method for selection of insulinoma cells that are resistant to the cytotoxic effects of inflammatory cytokines (INS-1(res)), involving their growth in progressively increasing concentrations of IL-1 beta plus IFN-gamma, and selection of surviving cells. In the current study, we have investigated the molecular mechanism of cytokine resistance in INS-1(res) cells. By focusing on the known components of the IFN-gamma receptor signaling pathway, we have discovered that expression levels of signal transducer and activator of transcription (STAT)-1 alpha are closely correlated with the cytokine-resistant and -sensitive phenotypes. That STAT-1 alpha is directly involved in development of cytokine resistance is demonstrated by an increase of viability from 10 +/- 2% in control cells to 50 +/- 6% in cells with adenovirus-mediated overexpression of STAT-1 alpha (p < 0.001) after culture of both cell groups in the presence of 100 units/ml IFN-gamma plus 10 ng/ml IL-1 beta for 48 h. The resistance to IL-1 beta plus IFN-gamma in STAT-1 alpha-expressing cells is due in part to interference with IL-1 beta-mediated stimulation of inducible nitric-oxide synthase expression and nitric oxide production. Furthermore, overexpression of STAT-1 alpha does not impair robust glucose-stimulated insulin secretion in the INS-1-derived cell line 832/13. We conclude that expression of STAT-1 alpha may be a means of protecting insulin-producing cell lines from cytokine damage, which, in conjunction with appropriate cell-impermeant macroencapsulation devices, may allow such cells to be used for insulin replacement in type 1 diabetes.

Chronic effects of different fatty acids and leptin in INS-1 cells

The effects of long-term exposure of a pancreatic beta cell line, INS-1, to major free fatty acids (FFA; palmitic acid, oleic acid and linoleic acid) and leptin on insulin secretion and cell viability by C,N-diphenyl-N'-4,5 dimethylthiazol 2-yl tetrazolium bromide (MTT) assay were examined. The cells were incubated with 1 mmol/l of each FFA and 25 or 100 ng/ml leptin, alone or in combination, for 4, 24 or 48 h before the insulin secretion experiments. Palmitic acid (C 16:0) significantly suppressed cell viability, and suppressed insulin secretion at 24 h. Treatment with oleic acid (C 18:1) or linoleic acid (C 18:2) enhanced basal insulin secretion and diminished glucose-stimulated insulin secretion (GSIS) at 48 h. In these groups, there were no differences in cell viability as compared to cells treated without FFA. Leptin did not affect insulin secretion at 4, 24 and 48 h, and in the cells co-treated with FFA and leptin, leptin did not ameliorate lipotoxicity. These results suggest that, in INS-1 cells, different FFA have different patterns of lipotoxicity with chronic exposure, and leptin has little direct effect on insulin secretion.

The role of leptin resistance in the lipid abnormalities of aging

Leptin resistance has been implicated in the pathogenesis of obesity-related complications involving abnormalities of lipid metabolism that resemble those of old age. To determine whether development of leptin resistance in advancing age might account for such abnormalities, we compared the effects of hyperleptinemia (>40 ng/ml) induced in 2-month-old and 18-month-old lean wild-type (+/+) Zucker diabetic fatty rats by adenovirus gene transfer. The decline in food intake, body weight, and body fat in old rats was only 25%, 50%, and 16%, respectively, of the young rats. Whereas in young rats plasma free fatty acids fell 44% and triacylglycerol (TG) 94%, neither changed in the rats. In hyperleptinemic young rats, adipocyte expression of preadipocyte factor 1 increased dramatically and leptin mRNA virtually disappeared; there was increased expression of acyl CoA oxidase, carnitine palmitoyl transferase 1, and their transcription factor peroxisome proliferator-activated receptor alpha, accounting for the reduction in body fat. These hyperleptinemia-induced changes were profoundly reduced in the old rats. On a high-fat diet, old rats consumed 28% more calories than the young and gained 1.5x as much fat, despite greater endogenous hyperleptinemia. Expression of a candidate leptin resistance factor, suppressor of cytokine signaling 3 (SOCS-3), was compared in the hypothalamus and white adipocytes of young and old rats before and after induction of hyperleptinemia; hypothalamic SOCS-3 mRNA was approximately 3x higher in old rats before, whereas it was 3x higher in WAT after, hyperleptinemia. We conclude that the anorexic and antilipopenic actions of leptin decline with age, possibly through increased SOCS-3 expression, and that this could account for the associated abnormalities in lipid metabolism of the elderly.

Transgenic mice overexpressing leptin accumulate adipose mass at an older, but not younger, age

Sensitivity to leptin is associated with a normal regulation of the adipose mass, whereas decreased leptin sensitivity results in elevated adipose tissue stores. To address whether the effects of chronic hyperleptinemia are sustained with age, we generated transgenic mice that overexpress leptin under the control of the fat specific aP2 promoter/enhancer. At 6-9 weeks of age, transgenic mice overexpressed 5-fold more human leptin than endogenous mouse levels and had consistently low body weights, with reduced brown and white fat depots characterized by adipocytes either devoid of or containing minute lipid droplets. However, at 33-37 weeks, despite continuous secretion of human leptin, the transgenic mice showed a rebound effect characterized by an increase in body weight, accumulation of adipose mass, and lipid-filled adipocytes. Thus, this mouse model exhibits a two-stage phenotype, with respect to fat accumulation. In addition, plasma glucose, triglycerides, and cholesterol levels were markedly depressed in young, but not older, transgenic mice. A detrimental consequence of early hyperleptinemia was a failure of the transgenic mice to acclimatize to the cold, as a result of depleted fat stores within their brown adipocytes. Cold exposure was tolerated after a 2-week high-fat diet or at an older age when fat depots had naturally accumulated. Treatment of the older transgenic mice with large doses of leptin stimulated weight loss, demonstrating that the leptin pathway still responds to pharmacological levels of leptin. Overall, these studies show that moderate hyperleptinemia in normal mice results in a sensitivity of the adipose mass to leptin at a younger (but not older) age. The mechanisms that lead to the accumulation of fat at an older age remain largely unknown, and this hyperleptinemic mouse model will allow the uncovering of at least some of these mechanisms.

Cell-permeable peptide inhibitors of JNK: novel blockers of beta-cell death

Stress conditions and proinflammatory cytokines activate the c-Jun NH2-terminal kinase (JNK), a member of the stress-activated group of mitogen-activated protein kinases (MAPKs). We recently demonstrated that inhibition of JNK signaling with the use of the islet-brain (IB) 1 and 2 proteins prevented interleukin (IL)-1beta-induced pancreatic beta-cell death. Bioactive cell-permeable peptide inhibitors of JNK were engineered by linking the minimal 20-amino acid inhibitory domains of the IB proteins to the 10-amino acid HIV-TAT sequence that rapidly translocates inside cells. Kinase assays indicate that the inhibitors block activation of the transcription factor c-Jun by JNK. Addition of the peptides to the insulin-secreting betaTC-3 cell line results in a marked inhibition of IL-1beta-induced c-jun and c-fos expression. The peptides protect betaTC-3 cells against apoptosis induced by IL-1beta. All-D retro-inverso peptides penetrate cells as efficiently as the L-enantiomers, decrease c-Jun activation by JNK, and remain highly stable inside cells. These latter peptides confer full protection against IL-1beta-induced apoptosis for up to 2 weeks of continual treatment with IL-1beta. These data establish these bioactive cell-permeable peptides as potent pharmacological compounds that decrease intracellular JNK signaling and confer long-term protection to pancreatic beta-cells from IL-1beta-induced apoptosis.

Leptin and other components of the Metabolic Syndrome in Mauritius--a factor analysis

OBJECTIVE

To use factor analysis to examine the putative role of leptin in the Metabolic Syndrome, and to define better the associations among observed variables and the identified factors.

DESIGN

Factor analysis of cross-sectional data from a 1987 survey.

SUBJECTS

Non-diabetic residents of Mauritius who participated in population-based surveys in 1987 and 1992 (1414 men and 1654 women).

MEASUREMENTS

Fasting and 2 h plasma glucose and insulin following a 75 g oral glucose load; seated blood pressure; body mass index (BMI); waist-to-hip ratio (WHR); and fasting serum triglycerides, HDL-cholesterol, leptin and uric acid concentrations.

RESULTS

Principal components factor analysis revealed three factors for men and women that explained between 54 and 55% of the observed variance of the 12 measured variables. General features of these factors were as follows: factor 1, WHR, BMI, leptin, fasting and 2 h insulin, triglycerides, and HDL-cholesterol; factor 2, systolic and diastolic blood pressure, uric acid (men only), and fasting glucose (women only); and factor 3, fasting and 2 h glucose and insulin. Only three variables loaded on more than one factor with a loading > or = 0.4 (fasting and 2 h insulin, fasting glucose in women only). Leptin loaded on one factor only in both men and women.

CONCLUSIONS

Since multiple factors underlie the Metabolic Syndrome, and since no observed variable loads on all three factors, more than one mechanism might account for the observed clustering of risk characteristics. Leptin does not unite features of this syndrome due to its loading on one factor only. Uric acid is related to a different factor in men and women. The absence of gender differences in factor loadings argues for similar mechanisms for the Metabolic Syndrome in men and women in Mauritius. International Journal of Obesity (2001) 25, 126-131

Leptin in pituitary adenomas--a novel paracrine regulatory system

A growing number of physiological and pathophysiological processes have been shown to be influenced by leptin apart from its first recognised role as a modulator of hypothalamic appetite and weight control centers. We investigated the presence and pattern of distribution of leptin mRNA and the mRNA of the long isoform of the leptin receptor in the normal pituitary and in different types of pituitary adenomas. We also studied leptin secretion from human pituitary tumors in culture, and the in vitro pituitary hormone release following stimulation with human leptin. Leptin mRNA expression was detected at a low level of expression in 50% of tumors but in none of the normal pituitaries. By immunohistochemistry, leptin was present in occasional scattered cells in the normal pituitary and in pituitary tumors. The leptin receptor long isoform was detected in the majority (65%) of pituitary tumors and in all normal pituitaries. It did not segregate with any particular tumor type, and varying levels of expression were detected between the tissues studied. 34% of pituitary adenomas showed leptin release into the incubation media during in vitro culture. Leptin mRNA, the mRNA of the long isoform of the receptor, or in vitro leptin release, did not correlate with tumor type or with any of the other pituitary hormones released. In vitro leptin stimulation of pituitary tumors caused stimulation of FSH and a-subunit secretion from a non-functioning adenoma and TSH secretion from a somatotroph adenoma. As the co-localisation of ACTH and leptin in corticotroph cells was previously suggested, we investigated whether in vivo ACTH release is accompanied by a simultaneous plasma leptin level rise (i) in peripheral plasma samples after food intake-induced ACTH rise in healthy obese and nonobese individuals and (ii) in petrosal sinus samples after CRH injection in Cushing's disease patients. Our data suggest that a rise in ACTH levels is not accompanied by detectable rise in leptin levels in peripheral and in petrosal sinus blood samples. In summary, leptin is synthesized and stored within the pituitary and may modulate other pituitary hormone secretion, although probably it does not contribute to plasma leptin level changes. Pituitary leptin may therefore be a novel paracrine regulator of pituitary function.

Limited brain access for leptin in obesity

Obesity is a major health problem that contributes to the development of type 2 diabetes, hypertension, dyslipidemia, and cardiovascular disease. The current pharmacological therapies for obesity are limited and may have significant side effects. Leptin therapy was shown to effectively cause weight loss in obese rats, however its effectiveness in humans is still under investigation. Obese humans have significantly elevated plasma leptin concentrations compared with lean individuals. Plasma leptin concentrations strongly correlated with percentage of body fat. Leptin concentration in the cerebrospinal fluid (CSF) is correlated, in a nonlinear manner, with plasma leptin levels and body mass index (BMI). The ratio of CSF leptin levels to serum leptin levels was 4 times greater in lean individuals than in obese individuals. One interpretation of this finding is that human obesity could be secondary to a central resistance to leptin action, causing a relative leptin deficiency in the CNS. Six years after the discovery of leptin we still do not have a clear understanding of how leptin accesses its targets in the brain, or whether there is defect in this process in the brain of obese individuals. In this manuscript we will review the different leptin gateways to the brain and the potential sites where a defect in leptin action may be present, as well as some potential clinical implications of leptin. A better understanding of how leptin reaches the brain and how it modulates the release of hypothalamic neuropeptides will be important in understanding the role that leptin plays in the pathophysiology of obesity.

Leptin administration improves skeletal muscle insulin responsiveness in diet-induced insulin-resistant rats

In addition to suppressing appetite, leptin may also modulate insulin secretion and action. Leptin was administered here to insulin-resistant rats to determine its effects on secretagogue-stimulated insulin release, whole body glucose disposal, and insulin-stimulated skeletal muscle glucose uptake and transport. Male Wistar rats were fed either a normal (Con) or a high-fat (HF) diet for 3 or 6 mo. HF rats were then treated with either vehicle (HF), leptin (HF-Lep, 10 mg. kg(-1). day(-1) sc), or food restriction (HF-FR) for 12-15 days. Glucose tolerance and skeletal muscle glucose uptake and transport were significantly impaired in HF compared with Con. Whole body glucose tolerance and rates of insulin-stimulated skeletal muscle glucose uptake and transport in HF-Lep were similar to those of Con and greater than those of HF and HF-FR. The insulin secretory response to either glucose or tolbutamide (a pancreatic beta-cell secretagogue) was not significantly diminished in HF-Lep. Total and plasma membrane skeletal muscle GLUT-4 protein concentrations were similar in Con and HF-Lep and greater than those in HF and HF-FR. The findings suggest that chronic leptin administration reversed a high-fat diet-induced insulin-resistant state, without compromising insulin secretion.

Slow recovery of body fat lost during adenovirus-induced hyperleptinemia

In normal rats, adenovirus-induced hyperleptinemia causes disappearance of visible body fat, downregulation of lipogenic enzymes, and upregulation of oxidative enzymes and thermogenic proteins. In addition, preadipocyte markers replace mature adipocyte markers, suggesting dedifferentiation. In weight loss induced by caloric restriction, by contrast, the lipogenic machinery is essentially intact. To determine if the radical changes induced by leptin would slow the reappearance of body fat, we compared normal lean rats made hyperleptinemic by infusing an adenovirus-leptin construct with diet-matched littermates. Initially, in plasma leptin the hyperleptinemic rats averaged approximately 50x the controls and, although it declined progressively, it was still slightly elevated at 150 days (P < 0.05). In the hyperleptinemics, body fat mass, quantified by magnetic resonance spectroscopy, remained below the pretreatment value for 60 days, while in diet-matched controls it exceeded the pretreatment value. Epididymal fat pad weight in hyperleptinemics was still 28% below paired controls at 150 days posttreatment. Histologic examination revealed adipocytes of hyperleptinemic animals to be smaller 60 days after treatment. At 60 days, adipose tissue UCP-2 gene expression in hyperleptinemics was still above controls, but expression of other lipogenic and oxidative enzymes had returned to baseline expression levels. We conclude that in normal rats recovery of body fat following adenovirus-induced hyperleptinemia is much slower than after caloric restriction, possibly because of persistent upregulation of adipocyte UCP-2.

Development of leptin resistance in rat soleus muscle in response to high-fat diets

Direct evidence for leptin resistance in peripheral tissues such as skeletal muscle does not exist. Therefore, we investigated the effects of different high-fat diets on lipid metabolism in isolated rat soleus muscle and specifically explored whether leptin's stimulatory effects on muscle lipid metabolism would be reduced after exposure to high-fat diets. Control (Cont, 12% kcal fat) and high-fat [60% kcal safflower oil (n-6) (HF-Saff); 48% kcal safflower oil plus 12% fish oil (n-3)] diets were fed to rats for 4 wk. After the dietary treatments, muscle lipid turnover and oxidation in the presence and absence of leptin was measured using pulse-chase procedures in incubated resting soleus muscle. In the absence of leptin, phospholipid, diacylglycerol, and triacylglycerol (TG) turnover were unaffected by the high-fat diets, but exogenous palmitate oxidation was significantly increased in the HF-Saff group. In Cont rats, leptin increased exogenous palmitate oxidation (21.4 +/- 5.7 vs. 11.9 +/- 1.61 nmol/g, P = 0.019) and TG breakdown (39.8 +/- 5.6 vs. 27.0 +/- 5.2 nmol/g, P = 0.043) and decreased TG esterification (132.5 +/- 14.6 vs. 177.7 +/- 29.6 nmol/g, P = 0.043). However, in both high-fat groups, the stimulatory effect of leptin on muscle lipid oxidation and hydrolysis was eliminated. Partial substitution of fish oil resulted only in the restoration of leptin's inhibition of TG esterification. Thus we hypothesize that, during the development of obesity, skeletal muscle becomes resistant to the effects of leptin, resulting in the accumulation of intramuscular TG. This may be an important initiating step in the development of insulin resistance common in obesity.

Lipoatrophy revisited

The lipoatrophy syndromes are a heterogeneous group of syndromes characterized by a paucity of adipose tissue. Severe lipoatrophy is associated with insulin-resistant diabetes mellitus (DM). The loss of adipose tissue can have a genetic, immune, or infectious/drug-associated etiology. Causative mutations have been identified in patients for one form of partial lipoatrophy--Dunnigan-type familial partial lipodystrophy. Experiments using lipoatrophic mice demonstrate that the diabetes results from the lack of fat and that leptin deficiency is a contributing factor. Thiazolidinedione therapy improves metabolic control in lipoatrophic patients; the efficacy of leptin treatment is currently being investigated.

Acyl-CoAs are functionally channeled in liver: potential role of acyl-CoA synthetase

Acyl-CoA synthetase (ACS) catalyzes the activation of long-chain fatty acids to acyl-CoAs, which can be metabolized to form CO(2), triacylglycerol (TAG), phospholipids (PL), and cholesteryl esters (CE). To determine whether inhibiting ACS affects these pathways differently, we incubated rat hepatocytes with [(14)C]oleate and the ACS inhibitor triacsin C. Triacsin inhibited TAG synthesis 70% in hepatocytes from fed rats and 40% in starved rats, but it had little effect on oleate incorporation into CE, PL, or beta-oxidation end products. Triacsin blocked [(3)H]glycerol incorporation into TAG and PL 33 and 25% more than it blocked [(14)C]oleate incorporation, suggesting greater inhibition of de novo TAG synthesis than reacylation. Triacsin did not affect oxidation of prelabeled intracellular lipid. ACS1 protein was abundant in liver microsomes but virtually undetectable in mitochondria. Refeeding increased microsomal ACS1 protein 89% but did not affect specific activity. Triacsin inhibited ACS specific activity in microsomes more from fed than from starved rats. These data suggest that ACS isozymes may be functionally linked to specific metabolic pathways and that ACS1 is not associated with beta-oxidation in liver.

High-fat diet-induced muscle insulin resistance: relationship to visceral fat mass

It has been variously hypothesized that the insulin resistance induced in rodents by a high-fat diet is due to increased visceral fat accumulation, to an increase in muscle triglyceride (TG) content, or to an effect of diet composition. In this study we used a number of interventions: fish oil, leptin, caloric restriction, and shorter duration of fat feeding, to try to disassociate an increase in visceral fat from muscle insulin resistance. Substituting fish oil (18% of calories) for corn oil in the high-fat diet partially protected against both the increase in visceral fat and muscle insulin resistance without affecting muscle TG content. Injections of leptin during the last 4 days of a 4-wk period on the high-fat diet partially reversed the increase in visceral fat and the muscle insulin resistance, while completely normalizing muscle TG. Restricting intake of the high-fat diet to 75% of ad libitum completely prevented the increase in visceral fat and muscle insulin resistance. Maximally insulin-stimulated glucose transport was negatively correlated with visceral fat mass (P < 0.001) in both the soleus and epitrochlearis muscles and with muscle TG concentration in the soleus (P < 0.05) but not in the epitrochlearis. Thus we were unable to dissociate the increase in visceral fat from muscle insulin resistance using a variety of approaches. These results support the hypothesis that an increase in visceral fat is associated with development of muscle insulin resistance.

Subcutaneous and visceral adipose tissue: their relation to the metabolic syndrome

Methods for assessment, e.g., anthropometric indicators and imaging techniques, of several phenotypes of human obesity, with special reference to abdominal fat content, have been evaluated. The correlation of fat distribution with age, gender, total body fat, energy balance, adipose tissue lipoprotein lipase and lipolytic activity, adipose tissue receptors, and genetic characteristics are discussed. Several secreted or expressed factors in the adipocyte are evaluated in the context of fat tissue localization. The body fat distribution and the metabolic profile in nonobese and obese individuals is discussed relative to lipolysis, antilypolysis and lipogenesis, insulin sensitivity, and glucose, lipid, and protein metabolism. Finally, the endocrine regulation of abdominal visceral fat in comparison with the adipose tissue localized in other areas is presented.

Glucose down-regulates the expression of the peroxisome proliferator-activated receptor-alpha gene in the pancreatic beta -cell

To better understand the action of glucose on fatty acid metabolism in the beta-cell and the link between chronically elevated glucose or fatty acids and beta-cell decompensation in adipogenic diabetes, we investigated whether glucose regulates peroxisomal proliferator-activated receptor (PPAR) gene expression in the beta-cell. Islets or INS(832/13) beta-cells exposed to high glucose show a 60-80% reduction in PPARalpha mRNA expression. Oleate, either in the absence or presence of glucose, has no effect. The action of glucose is dose-dependent in the 6-20 mm range and maximal after 6 h. Glucose also causes quantitatively similar reductions in PPARalpha protein and DNA binding activity of this transcription factor. The effect of glucose is blocked by the glucokinase inhibitor mannoheptulose, is partially mimicked by 2-deoxyglucose, and is not blocked by the 3-O-methyl or the 6-deoxy analogues of the sugar that are not phosphorylated. Chronic elevated glucose reduces the expression levels of the PPAR target genes, uncoupling protein 2 and acyl-CoA oxidase, which are involved in fat oxidation and lipid detoxification. A 3-day exposure of INS-1 cells to elevated glucose results in a permanent rise in malonyl-CoA, the inhibition of fat oxidation, and the promotion of fatty acid esterification processes and causes elevated insulin secretion at low glucose. The results suggest that a reduction in PPARalpha gene expression together with a rise in malonyl-CoA plays a role in the coordinated adaptation of beta-cell glucose and lipid metabolism to hyperglycemia and may be implicated in the mechanism of beta-cell "glucolipotoxicity."

Effects of short-term leptin exposure on triglyceride deposition in rat liver

Leptin has recently been suggested to play a role in the pathogenesis of hepatic steatosis. Consequently, this study was designed to examine the direct effects of portal leptin on the intrahepatic lipid contents in the postabsorptive state. Rat livers (n = 6 per group) were perfused in a recirculating system and portally infused with leptin (0.5 nmol/L, 5 nmol/L, and 25 nmol/L), insulin (10 nmol/L), leptin (5 nmol/L) plus insulin (10 nmol/L), glucagon (1 nmol/L), or vehicle (control). Intrahepatic contents of triglycerides, free cholesterol, cholesteryl esters, and free fatty acids were determined from the lipid extract of frozen livers by capillary gas chromatography. Short-term leptin infusion increased total triglycerides in a concentration-dependent (0.5 nmol/L: 2.8 +/- 0.4 mg/g, 5 nmol/L: 7.0 +/- 0.5 mg/g, 25 nmol/L: 8.3 +/- 1.0 mg/g) and time-dependent manner. Total triglycerides also rose during exposure to insulin plus leptin (7.2 +/- 0.6 mg/g) but fell during glucagon infusion (2.6 +/- 0.2 mg/g; control: 4.3 +/- 0.3 mg/g; P <.05). Leptin, insulin, and glucagon increased intrahepatic free cholesterol (P <.05). Free fatty acids were also higher during leptin exposure (0.5 nmol/L: 1.28 +/- 0.08 mg/g, 5 nmol/L: 0.47 +/- 0.01 mg/g, 25 nmol/L: 0.48 +/- 0.04 mg/g, control: 0.38 +/- 0.03 mg/g; P <.05). In conclusion, hyperleptinemia increases hepatic triglyceride content and may therefore contribute to hepatic steatosis in hyperleptinemic obese patients.

Kinetics of intramuscular triglyceride fatty acids in exercising humans

A pulse ([(14)C]palmitate)-chase ([(3)H]palmitate) approach was used to study intramuscular triglyceride (imTG) fatty acid and plasma free fatty acid (FFA) kinetics during exercise at approximately 45% peak O(2) consumption in 12 adults. Vastus lateralis muscle was biopsied before and after 90 min of bicycle exercise; (3)H(2)O production, breath (14)CO(2) excretion and lipid oxidation (indirect calorimetry) rates were measured during exercise.

RESULTS

during exercise, 8.2+/-1.2 and 8.4+/-0.7 micromol x kg(-1) x min(-1) of imTG fatty acids and plasma FFA, respectively, were oxidized according to isotopic measurements. The sum of these two values was not different (P = 0.6) from lipid oxidation by indirect calorimetry (15.4 +/-1.6 micromol x kg(-1) x min(-1)); the isotopic and indirect calorimetry values were correlated (r = 0.79, P<0.005). During exercise, imTG turnover rate was 0.32+/-0.07%/min (6.0+/-2.0 micromol of imTG x kg wet muscle(-1) x min(-1)) and plasma FFA were incorporated into imTG at a rate of 0.7+/-0.1 micromol x kg wet muscle(-1) x min(-1). The imTG pool size did not change during exercise. This pulse-chase, dual tracer appears to be a reasonable approach to measure oxidation and synthesis kinetics of imTG.

Adipose tissue is required for the antidiabetic, but not for the hypolipidemic, effect of thiazolidinediones

There is uncertainty about the site(s) of action of the antidiabetic thiazolidinediones (TZDs). These drugs are agonist ligands of the transcription factor PPAR gamma, which is abundant in adipose tissue but is normally present at very low levels in liver and muscle. We have studied the effects of TZDs in A-ZIP/F-1 mice, which lack white adipose tissue. The A-ZIP/F-1 phenotype strikingly resembles that of humans with severe lipoatrophic diabetes, including the lack of fat, marked insulin resistance and hyperglycemia, hyperlipidemia, and fatty liver. Rosiglitazone or troglitazone treatment did not reduce glucose or insulin levels, suggesting that white adipose tissue is required for the antidiabetic effects of TZDs. However, TZD treatment was effective in lowering circulating triglycerides and increasing whole body fatty acid oxidation in the A-ZIP/F-1 mice, indicating that this effect occurs via targets other than white adipose tissue. A-ZIP/F-1 mice have markedly increased liver PPAR gamma mRNA levels, which may be a general property of fatty livers. Rosiglitazone treatment increased the triglyceride content of the steatotic livers of A-ZIP/F-1 and ob/ob mice, but not the "lean" livers of fat-transplanted A-ZIP/F-1 mice. In light of this evidence that rosiglitazone acts differently in steatotic livers, the effects of rosiglitazone, particularly on hepatic triglyceride levels, should be examined in humans with hepatic steatosis.

Leptin resistance of adipocytes in obesity: role of suppressors of cytokine signaling

Liver-derived hyperleptinemia induced in normal rats by adenovirus-induced gene transfer causes rapid disappearance of body fat, whereas the endogenous adipocyte-derived hyperleptinemia of obesity does not. Here we induce liver-derived hyperleptinemia in rats with adipocyte-derived hyperleptinemia of acquired obesity caused by ventromedial hypothalamus lesioning (VMH rats) or by feeding 60% fat (DIO rats). Liver-derived hyperleptinemia in obese rats caused only a 5-7% loss of body weight, compared to a 13% loss in normoleptinemic lean animals; but in actual grams of weight lost there was no significant difference between obese and lean groups, suggesting that a subset of cells remain leptin-sensitive in obesity. mRNA and protein of a putative leptin-resistance factor, suppressor of cytokine signaling (SOCS)-1 or -3, were both increased in white adipose tissues (WAT) of VMH and DIO rats. Since transgenic overexpression of SOCS-3 in islets reduced the lipopenic effect of leptin by 75%, we conclude that the increased expression of SOCS-1 and -3 in WAT of rats with acquired obesity could have blocked leptin's lipopenic action in the leptin-resistant WAT population.

Insulin secretion and differential gene expression in glucose-responsive and -unresponsive MIN6 sublines

We have established two sublines derived from the insulin-secreting mouse pancreatic beta-cell line MIN6, designated m9 and m14. m9 Cells exhibit glucose-induced insulin secretion in a concentration-dependent manner, whereas m14 cells respond poorly to glucose. In m14 cells, glucose consumption and lactate production are enhanced, and ATP production is largely through nonoxidative pathways. Moreover, lactate dehydrogenase activity is increased, and hexokinase replaces glucokinase as a glucose-phosphorylating enzyme. The ATP-sensitive K(+) channel activity and voltage-dependent calcium channel activity in m14 cells are reduced, and the resting membrane potential is significantly higher than in m9 cells. Thus, in contrast to m9, a model for beta-cells with normal insulin response, m14 is a model for beta-cells with impaired glucose-induced insulin secretion. By mRNA differential display of these sublines, we found 10 genes to be expressed at markedly different levels. These newly established MIN6 cell sublines should be useful tools in the analysis of the genetic and molecular basis of impaired glucose-induced insulin secretion.

The role of high-fat diets and physical activity in the regulation of body weight

The prevalence of obesity is increasing in westernized societies. In the USA the age-adjusted prevalence of BMI > 30 kg/m2 increased between 1960 and 1994 from 13% to 23% for people over 20 years of age. This increase in the prevalence of obesity has been attributed to an increased fat intake and a decreased physical activity. However, the role of the impact of the level of dietary fat intake on human obesity has been challenged. High-fat diets, due to their high energy density, stimulate voluntary energy intake. An increased fat intake does not stimulate its own oxidation but the fat is stored in the human body. When diet composition is isoenergetically switched from low to high fat, fat oxidation only slowly increases, resulting in positive fat balances on the short term. Together with a diminished fat oxidation capacity in pre-obese subjects, high-fat diets can therefore be considered to be fattening. Another environmental factor which could explain the increasing prevalence of obesity is a decrease in physical activity. The percentage of body fat is negatively associated with physical activity and exercise has pronounced effects on energy expenditure and substrate oxidation. High-intensity exercise, due to a lowering of glycogen stores, can lead to a rapid increase in fat oxidation, which could compensate for the consumption of high-fat diets in westernized societies. Although the consumption of high-fat diets and low physical activity will easily lead to the development of obesity, there is still considerable inter-individual variability in body composition in individuals on similar diets. This can be attributed to the genetic background, and some candidate genes have been discovered recently. Both leptin and uncoupling protein have been suggested to play a role in the prevention of diet-induced obesity. Indeed, leptin levels are increased on a high-fat diet but this effect can be attributed to the increased fat mass observed on the high-fat diet. No effect of a high-fat diet per se on leptin levels is observed. Uncoupling proteins are increased by high-fat diets in rats but no data are available in human subjects yet. In conclusion, the increased intake of dietary fat and a decreasing physical activity level are the most important environmental factors explaining the increased prevalence of obesity in westernized societies.

Comparison of the effects of sucrose and fructose on insulin action and glucose tolerance

The purpose of the present study was to determine whether fructose is the nutrient mediator of sucrose-induced insulin resistance and glucose intolerance. Toward this end, male rats were fed a purified starch diet (68% of total calories) for a 2-wk baseline period. After this, rats either remained on the starch (ST) diet or were switched to a sucrose (SU, 68% of total calories), fructose/glucose (F/G, 34/34% of total calories), or fructose/starch (F/ST, 34/34% of total calories) diet for 5 wk. Rats then underwent either an intravenous glucose tolerance test (n = 10/diet) or a euglycemic, hyperinsulinemic clamp (n = 8 or 9/diet). Incremental glucose and insulin areas under the curve in SU, F/G, and F/ST were on average 61 and 29% greater than ST, respectively, but not significantly different from one another. During clamps, glucose infusion rates (mg. kg(-1). min(-1)) required to maintain euglycemia were significantly lower (P < 0.05) in SU, F/G, and F/ST (13.4 +/- 0.9, 9. 5 +/- 1.7, 11.3 +/- 1.3, respectively) compared with ST (22.8 +/- 1. 1). Insulin suppression of glucose appearance (mg. kg(-1). min(-1)) was significantly lower (P < 0.05) in SU, F/G, and F/ST (5.6 +/- 0.5, 2.2 +/- 1.2, and 6.6 +/- 0.7, respectively) compared with ST (9.6 +/- 0.4). Insulin-stimulated glucose disappearance (mg. kg(-1). min(-1)) was significantly lower (P < 0.05) in SU, F/G, and F/ST (17. 9 +/- 0.6, 16.2 +/- 1.3, 15.3 +/- 1.8, respectively) compared with ST (24.7 +/- 1.2). These data suggest that fructose is the primary nutrient mediator of sucrose-induced insulin resistance and glucose intolerance.

Leptin stimulates uncoupling protein-2 mRNA expression and Krebs cycle activity and inhibits lipid synthesis in isolated rat white adipocytes

The treatment of rats and mice with leptin causes dramatic body fat reduction and in some cases even disappearance of fat tissue. Here, we report the effects of leptin (10 and 100 ng.mL-1) on isolated rat adipocytes maintained for 15 h in culture. Leptin decreased the incorporation of acetate into total lipids by 30%. A reduction in this incorporation (42%) was still observed after the leptin-cultivated adipocytes were exposed to a supra-physiological insulin concentration (10 000 microU.mL-1). On the other hand, leptin increased acetate degradation by 69% and the maximal activity of citrate synthase by 50% in isolated adipocytes. It also increased oleate degradation by 35 and 50% at concentrations of 10 and 100 ng. mL-1, respectively. Eventually, leptin upregulated the uncoupling protein-2 (UCP2) mRNA level by 63% and had no effect on uncoupling protein-3 (UCP3) mRNA in isolated adipocytes. The upregulation of UCP2 mRNA might have contributed to the stimulation of acetate and fatty acid degradation by leptin. The peripheral effects of leptin observed in this study are in line with the general energy dissipating role postulated for this hormone and for UCP2. They suggest mechanisms by which adipocytes regulate their fat content by an autocrine pathway without the participation of the central nervous system.

Gonadal steroids and energy homeostasis in the leptin era

Gonadal steroids influence food intake and body weight. Although the specific mechanisms underlying these effects are not clear, a consideration of their effects in the context of current models of energy homeostasis may ultimately lead to the identification of these mechanisms. When compared with leptin, the prototypical humoral signal of energy balance, sex steroids share many common properties related to food intake and body weight. Specifically, gonadal steroids circulate in proportion to fat mass and current energy balance, and administration of these compounds influences food intake, energy expenditure, body weight, and body composition. Moreover, both estrogens and androgens modulate central nervous system effectors of energy homeostasis that are targets for the action of leptin, including pathways that contain neuropeptide Y, pro-opiomelanocortin, or melanin-concentrating hormone. Sex steroids and leptin also regulate one another's production. Although gonadal steroids, unlike leptin, are clearly not critical to the maintenance of normal energy homeostasis, they do appear to function as physiologic modulators of this process. Identifying the specific central mediators of their effects will contribute to our understanding of their role in energy homeostasis.

Leptin

The discovery of the adipose-derived hormone leptin has generated enormous interest in the interaction between peripheral signals and brain targets involved in the regulation of feeding and energy balance. Plasma leptin levels correlate with fat stores and respond to changes in energy balance. It was initially proposed that leptin serves a primary role as an anti-obesity hormone, but this role is commonly thwarted by leptin resistance. Leptin also serves as a mediator of the adaptation to fasting, and this role may be the primary function for which the molecule evolved. There is increasing evidence that leptin has systemic effects apart from those related to energy homeostasis, including regulation of neuroendocrine and immune function and a role in development.

Palmitate oxidation rate and action on glycogen synthase in myoblasts from insulin-resistant subjects

Elevated plasma lipid and nonesterified fatty acid concentrations reduce insulin-mediated glucose disposal in skeletal muscle. Cultured myoblasts from 21 subjects were studied for rates of palmitate oxidation and the effect of palmitate on glycogen synthase activity at the end of an 18-h incubation in serum- and glucose-free media. Oxidation rates of 40 microM palmitate in cultured myoblasts correlated with the fasting glucose (r = 0.71, P = 0.001), log fasting insulin (r = 0.52, P = 0.03), and insulin-mediated glucose storage rate (r = -0.50, P = 0.04) of the muscle donors. Myoblast glycogen synthase activity can be regulated by 240 microM palmitate, but the changes are associated with the basal respiratory quotient and not with the insulin resistance of the muscle donor. These results indicate that myoblasts producing elevated palmitate oxidation rates in vitro can be used to identify skeletal muscle abnormalities which are primary contributors to insulin resistance in vivo. Effects of 240 microM palmitate on myoblast glycogen synthase activity appear to be mechanistically different from the relationship between myoblast palmitate oxidation rates and insulin resistance of the muscle donor.

Tumor necrosis factor-alpha regulates secretion of the adipocyte-derived cytokine, leptin

The seminal observation that secretion of the adipocyte-derived hormone leptin was induced by inflammatory challenge has been expanded upon to demonstrate the importance of the pro-inflammatory cytokines, especially tumor necrosis factor (TNF)-alpha, in inflammatory hyperleptinemia. Initially, it was thought that cytokine-induced hyperleptinemia might somehow be involved in the anorexia and cachexia that often accompany chronic infectious, neoplastic, and autoimmune disease. While the role of leptin in disease-associated anorexia and cachexia appears tenuous in light of recent findings, there is evidence that the hyperleptinemia induced by cytokines is an integral part of the acute phase response and necessary for comprehensive immunocompetence. This hints at the existence of an integrated communication network, wherein the energy status of the animal impacts its ability to fight pathogens.

Leptin physiology: a second look

It is widely believed that the primary physiologic role of leptin is to prevent obesity by regulating food intake and thermogenesis through actions on hypothalamic centers. Here we sugest that the first premise, the anti-obesity role, is untenable, and present evidence for an alternative physiologic role, namely antisteatotic activity in which fatty acid overaccumulation in nonadipose tissues is prevented by leptin-mediated regulation of beta-oxidation. The second premise, namely that leptin acts exclusively on the hypothalamus, is confirmed in normal lean animals with plasma leptin concentrations below 5 ng/ml; their correlation with cerebrospinal fluid levels supports the classical concept of leptin-mediated hypothalamic regulation of food intake. However, when chronic hyperleptinemia exceeds 15 ng/ml, as in obesity, a further rise in plasma leptin does not raise cerebrospinal leptin levels or reduce food intake. Nevertheless, the peripheral antisteatotic action of leptin in acquired obesity continues, suggesting that at chronically hyperleptinemic levels the hormone acts primarily on peripheral tissues and that its hypothalamic action has reached a plateau.

Leptin is a potent stimulator of bone growth in ob/ob mice

Leptin, the product of the obese gene, is a circulating hormone secreted primarily from adipocytes. The lack of leptin in ob/ob mice, who are homozygous for the obese gene, results in hyperglycemia, hyperinsulinemia, hyperphagia, obesity, infertility, decreased brain size and decreased stature. To this end, we investigated the role of leptin as a hormonal regulator of bone growth. Leptin administration led to a significant increase in femoral length, total body bone area, bone mineral content and bone density in ob/ob mice as compared to vehicle treated controls. The increase in total body bone mass was a result of an increase in both trabecular and cortical bone mass. These results suggest that the decreased stature of the ob/ob mouse is due to a developmental defect that is readily reversible upon leptin administration. Our demonstration that the signalling or long form (Ob-Rb) of the leptin receptor is present in both primary adult osteoblasts and chondrocytes suggests that the growth promoting effects of leptin could be direct. In summary, these results indicate a novel role for leptin in skeletal bone growth and development.

Chronic leptin administration promotes lipid utilization until fat mass is greatly reduced and preserves lean mass of normal female rats

Leptin is a hormone synthesized and secreted from adipose tissue. To study the physiologic effects of chronic leptin treatment, normal adult female Sprague-Dawley rats were injected subcutaneously for 35 days. Twice daily injections (250 microgram/day, b.i.d.) resulted in a significant (P<0.05) decrease in food intake that was maintained for 10 days before gradually returning to control level by day 21. Leptin decreased body weight by a maximum of 12% of the initial body weight on day 22 and remained reduced for the duration of the treatment. After 35 days of treatment, visible peritoneal adipose tissue was not detected. Body composition analysis showed that chronic injection of leptin resulted in a dramatic decrease in fat content (28+/-2 to 4+/-2 g, P<0.05; mean+/-SEM) while the lean content remained unchanged. Rats pair-fed to the leptin-treated group but treated with vehicle had the same body composition (23+/-3 g fat mass) as that measured for the ad libitum fed controls. Using indirect calorimetry we observed that leptin decreased respiratory quotient and thus increased fat oxidation. Leptin also prevented energy expenditure reduction typically associated with food restriction. Leptin treatment for 35 days decreased plasma triglyceride (0.75+/-0.07 to 0.30+/-0.03 mM, P<0.05), free fatty acid (0.56+/-0.06 to 0.32+/-0.04 mM) and insulin (3.2+/-0.5 to 1. 4+/-0.4 ng/ml, P<0.05) concentrations despite the fact that food intake was normalized by day 35. Withdrawal of leptin triggered hyperphagia indicating that leptin biology remained throughout the duration of the chronic treatment. These data suggest that leptin reduces fat mass by initially decreasing appetite and by maintaining enhanced fat utilization even when food intake has returned to that of vehicle-treated control.

Long-term differential modulation of genes encoding orexigenic and anorexigenic peptides by leptin delivered by rAAV vector in ob/ob mice. Relationship with body weight change

We investigated the long-term effects of physiological levels of leptin produced by gene therapy on body weight (BW) and expression of genes that encode orexigenic and anorexigenic peptides in the hypothalamus. Recombinant adeno-associated viral vector (rAAV), a non-pathogenic and non-immunogenic vector, encoding leptin (betaOb) was generated and administered iv to ob/ob mice lacking endogenous leptin. Whereas the lowest dose of rAAV-betaOb (6x10(9) particles) was ineffective, the middle dose (6x10(10) particles) curbed BW gain without affecting food consumption for 75 days of observation. A ten-fold higher dose (6x10(11) particles) resulted in increased blood leptin levels and suppressed both BW gain and food consumption throughout the duration of the experiment. rAAV-betaOb doses that either curbed BW without affecting food consumption or evoked BW loss and reduced food intake, decreased the expression of genes encoding the orexigenic peptides, neuropeptide Y and agouti-related peptide in the ARC, and the two doses were equally effective. Concomitantly, the expression of genes encoding the anorexigenic peptide, alpha-melanocyte stimulating hormone and cocaine-and-amphetamine regulatory transcript, was augmented with the latter gene displaying a dose-dependant response. These results document the efficacy of delivering biologically active leptin for extended periods by an iv injection of rAAV-betaOb and show that physiological leptin concentrations simultaneously exert a tonic inhibitory effect on orexigenic and a stimulatory effect on anorexigenic signaling in the hypothalamus. This intricate dynamic interplay induced by leptin regulates BW with or without an effect on food intake in leptin-deficient ob/ob mice. Further, these results suggest that gene therapy is an effective mode of delivery to the hypothalamus of those therapeutic proteins that cross the blood-brain barrier to ameliorate neuroendocrine disorders.

Electrospray ionization mass spectrometric analyses of changes in tissue phospholipid molecular species during the evolution of hyperlipidemia and hyperglycemia in Zucker diabetic fatty rats

The Zucker diabetic fatty (ZDF) rat is a genetic model of type II diabetes mellitus in which males homozygous for nonfunctional leptin receptors (fa/fa) develop obesity, hyperlipidemia, and hyperglycemia, but rats homozygous for normal receptors (+/+) remain lean and normoglycemic. Insulin resistance develops in young fa/fa rats and is followed by evolution of an insulin secretory defect that triggers hyperglycemia. Because insulin secretion and insulin sensitivity are affected by membrane phospholipid fatty acid composition, we have determined whether metabolic abnormalities in fa/fa rats are associated with changes in tissue phospholipids. Electrospray ionization mass spectrometric analyses of glycerophosphocholine (GPC) and glycerophosphoethanolamine (GPE) molecular species from tissues of prediabetic (6 wk of age) and overtly diabetic (12 wk) fa/fa rats and from +/+ rats of the same ages indicate that arachidonate-containing species from heart, aorta, and liver of prediabetic fa/fa rats made a smaller contribution to GPC total ion current than was the case for +/+ rats. There was a correspondingly larger contribution from species with sn-2 oleate or linoleate substituents in fa/fa heart and aorta. The relative contributions of arachidonate-containing GPC species increased in these tissues as fa/fa rats aged and were equal to or greater than those for +/+ rats by 12 wk. For heart and aorta, relative contributions from GPE species with sn-2 arachidonate or docosahexaenoate substituents to the total ion current increased and those from species with sn-2 oleate or linoleate substituents fell as fa/fa rats aged, but these tissue lipid profiles changed little with age in +/+ rats. GPC and GPE profiles for brain, kidney, sciatic nerve, and red blood cells were similar among fa/fa and +/+ rats at 6 and 12 wk of age, and pancreatic islets from fa/fa and +/+ rats exhibited similar GPC and GPE profiles at 12 wk of age. Under-representation of arachidonate-containing GPC and GPE species in some fa/fa rat tissues at 6 wk could contribute to insulin resistance, but depletion of islet arachidonate-containing GPC and GPE species is unlikely to explain the evolution of the insulin secretory defect that is well-developed by 12 wk of age.

Subcellular localization of leptin in non-tumorous and adenomatous human pituitaries: an immuno-ultrastructural study

Leptin is a key mediator in the maintenance of neuroendocrine homeostasis. Recently, leptin and leptin receptor expression were demonstrated in non-tumorous and adenomatous human pituitaries. This study was performed to determine the subcellular localization of leptin in human adenohypophyses (n = 3) and in various types of pituitary adenoma (n = 16). Immunoelectron microscopy showed leptin immunolabeling in most hormone-producing cells of the human non-tumorous adenohypophysis, but not in stellate cells. Labeling was noted over secretory granules. Immunocytochemistry using double labeling revealed leptin expression in GH-, ACTH-, TSH-, and FSH/LH-containing cells but not in PRL cells. The percentage of immunopositive cells and the intensity of immunostaining varied considerably among the various cell types. Immunoelectron microscopy with double gold labeling showed co-localization of leptin and adenohypophysial hormones in the same secretory granules. Among pituitary tumors, leptin immunolabeling was evident only in corticotroph adenomas. Compared to non-tumorous corticotrophs, leptin immunoexpression was less abundant in corticotroph adenomas. The presence of leptin and adenohypophysial hormones in the same secretory granules suggests that leptin is secreted concomitantly with various adenohypophysial hormones and that its release is under the control of hypothalamic stimulating and inhibiting hormones.

Tumor necrosis factor-alpha regulates secretion of the adipocyte-derived cytokine, leptin

The seminal observation that secretion of the adipocyte-derived hormone leptin was induced by inflammatory challenge has been expanded upon to demonstrate the importance of the pro-inflammatory cytokines, especially tumor necrosis factor (TNF)-alpha, in inflammatory hyperleptinemia. Initially, it was thought that cytokine-induced hyperleptinemia might somehow be involved in the anorexia and cachexia that often accompany chronic infectious, neoplastic, and autoimmune disease. While the role of leptin in disease-associated anorexia and cachexia appears tenuous in light of recent findings, there is evidence that the hyperleptinemia induced by cytokines is an integral part of the acute phase response and necessary for comprehensive immunocompetence. This hints at the existence of an integrated communication network, wherein the energy status of the animal impacts its ability to fight pathogens.

Role of adipose tissue in body-weight regulation: mechanisms regulating leptin production and energy balance

Adipose tissue performs complex metabolic and endocrine functions. Among the endocrine products produced by adipose tissue are tumour necrosis factor alpha, interleukin 6, acylation-stimulating protein and leptin. The present review will focus primarily on mechanisms regulating leptin production and leptin action, and the implications of this regulation in the control of energy balance. Leptin acts in the central nervous system where it interacts with a number of hypothalamic neuropeptide systems to regulate feeding behaviour and energy expenditure. The presence of extreme obesity in animals and human subjects with mutations of the leptin gene or the leptin receptor demonstrates that normal leptin production and action are critical for maintaining energy balance. Insulin is the major regulator of leptin production by adipose tissue. Insulin infusions increase circulating leptin concentrations in human subjects. Plasma leptin levels are markedly decreased in insulin-deficient diabetic rodents, and the low leptin levels contribute to diabetic hyperphagia. Based on in vitro studies, the effect of insulin to stimulate leptin production appears to involve increased glucose metabolism. Blockade of glucose transport or glycolysis inhibits leptin expression and secretion in isolated adipocytes. Evidence suggests that anaerobic metabolism of glucose to lactate does not stimulate leptin production. Alterations in insulin-mediated glucose metabolism in adipose tissue are likely to mediate the effects of energy restriction to decrease, and refeeding to increase, circulating leptin levels. Changes in glucose metabolism may also explain the observation that high-fat meals lower 24h circulating leptin levels relative to high-carbohydrate meals in human subjects, suggesting a mechanism that may contribute to the effects that high-fat diets have in promoting increased energy intake, weight gain and obesity. The decreased circulating leptin observed during energy restriction is related to increased sensations of hunger in human subjects. Thus, decreases in leptin during energy-restricted weight-loss regimens may contribute to the strong propensity for weight regain. A better understanding of the precise mechanisms regulating leptin production and leptin action may lead to new approaches for managing obesity.

Individual severity of dietary obesity in unselected Wistar rats: relationship with hyperphagia

We investigated the relative importance of overeating, thermogenesis, and uncoupling protein (UCP) expression in determining the severity of obesity in male Wistar rats fed a highly palatable diet. After 2 wk of feeding, body weight did not differ significantly from controls (248 +/- 4 vs. 229 +/- 3 g; P > 0.3), but rectal temperature, brown adipose tissue (BAT) mass, UCP3 expression in gastrocnemius muscle, and UCP2 expression in white adipose tissue (WAT) were all elevated in diet-fed animals. In a further study, rats fed a palatable diet for 8 wk exhibited higher energy intake and rectal temperature than controls. Dietary-obese rats were divided into high (427-490 g; n = 8) and low (313-410 g; n = 10) weight gainers. The high gainers ate significantly more than the low gainers, and energy intake was positively correlated with weight gain (r(2) = 0.72, P < 0.01). UCP2 and UCP3 mRNA levels in gastrocnemius muscle were significantly increased above lean controls in all diet-fed animals, whereas UCPs in WAT and BAT did not differ significantly from controls. Whereas rats fed palatable food exhibited a thermogenic response, there was no significant difference in core temperature between high and low gain groups (37. 5 +/- 0.1 vs. 37.6 +/- 0.1 degrees C; P > 0.5). We conclude that a higher energy intake is the critical factor determining susceptibility to dietary obesity in unselected Wistar rats.

Relative hypoleptinemia in patients with type 1 and type 2 diabetes mellitus

OBJECTIVE

To determine the relation between plasma leptin concentrations and metabolic control in human diabetes mellitus.

DESIGN AND SUBJECTS

Cross sectional study consisting of 156 patients with diabetes mellitus type 1 (n=42), type 2 (n=114), and non-diabetic subjects (n=74).

RESULTS

Plasma leptin concentrations were lower (P<0.05) in type 1 (8.3+/-1.7 ng/ml) and type 2 diabetic (14.9+/-1.8 ng/ml) than in non-diabetic humans (18.3+/-1.9 ng/ml). Only female type 1 and type 2 diabetic subjects also had decreased leptin/BMI ratios (P<0.05 vs non-diabetic females). The log rank test identified age-adjusted correlation of plasma leptin concentration with sex (P<0.0004) and body mass index (P<0.0218), but not with glycosylated haemoglobin A1c (P>0.5) in all groups. Plasma leptin was correlated with age (P<0.0058) and serum triglycerides (P<0.0199) in type 1 diabetic patients, and with serum cholesterol (P<0.0059) and LDL (P<0.0013) in type 2 diabetic patients.

CONCLUSIONS

Defective leptin production and/or secretion might be present independently of metabolic control in female patients with type 1 or type 2 diabetes mellitus.

Leptin, troglitazone, and the expression of sterol regulatory element binding proteins in liver and pancreatic islets

Overaccumulation of lipids in nonadipose tissues of obese rodents may lead to lipotoxic complications such as diabetes. To assess the pathogenic role of the lipogenic transcription factor, sterol regulatory element binding protein 1 (SREBP-1), we measured its mRNA in liver and islets of obese, leptin-unresponsive fa/fa Zucker diabetic fatty rats. Hepatic SREBP-1 mRNA was 2.4 times higher than in lean +/+ controls, primarily because of increased SREBP-1c expression. mRNA of lipogenic enzymes ranged from 2.4- to 4.6-fold higher than lean controls, and triacylglycerol (TG) content was 5.4 times higher. In pancreatic islets of fa/fa rats, SREBP-1c was 3.4 times higher than in lean +/+ Zucker diabetic fatty rats. The increase of SREBP-1 in liver and islets of untreated fa/fa rats was blocked by 6 weeks of troglitazone therapy, and the diabetic phenotype was prevented. Up-regulation of SREBP-1 also occurred in livers of Sprague-Dawley rats with diet-induced obesity. Hyperleptinemia, induced in lean +/+ rats by adenovirus gene transfer, lowered hepatic SREBP-1c by 74% and the lipogenic enzymes from 35 to 59%. In conclusion, overnutrition increases and adenovirus-induced hyperleptinemia decreases SREBP-1c expression in liver and islets. SREBP-1 overexpression, which is prevented by troglitazone, may play a role in the ectopic lipogenesis and lipotoxicity complicating obesity in Zucker diabetic fatty rats.

Conjugated linoleic acid supplementation in humans: effects on circulating leptin concentrations and appetite

Conjugated linoleic acid (CLA) has been demonstrated to reduce body fat in animals. However, the mechanism by which this reduction occurs is unknown. Leptin may mediate the effect of CLA to decrease body fat. We assessed the effects of 64 d of CLA supplementation (3 g/d) on circulating leptin, insulin, glucose, and lactate concentrations in healthy women. Appetite was assessed as a physiological correlate of changes in circulating leptin levels. Analysis of plasma leptin concentrations adjusted for adiposity by using fat mass as a covariate showed that CLA supplementation significantly decreased circulating leptin concentrations in the absence of any changes of fat mass. Mean leptin levels decreased over the first 7 wk and then returned to baseline levels over the last 2 wk of the study in the CLA-treated group. Appetite parameters measured at around the time when the greatest decreases in leptin levels were observed showed no significant differences between supplementation and baseline determinations in the CLA-supplemented group or between the CLA and placebo-supplemented groups. There was a nonsignificant trend for mean insulin levels to increase toward the end of the supplementation period in CLA-treated subjects. CLA did not affect plasma glucose and lactate over the treatment period. Thus, 64 d of CLA supplementation in women produced a transient decrease in leptin levels but did not alter appetite. CLA did not affect these parameters in a manner that promoted decreases of adiposity.

In vivo administration of leptin activates signal transduction directly in insulin-sensitive tissues: overlapping but distinct pathways from insulin

To determine whether leptin signal transduction is exerted directly upon insulin-sensitive tissues in vivo, we examined the ability of iv leptin to acutely stimulate phosphorylation of STAT3, STAT1, and MAPK, and activities of PI 3-kinase and Akt, in insulin-sensitive tissues of normal rats. Both leptin (1 mg/kg iv x 3 min) and insulin (10 U/kg iv x 3 min) stimulated tyrosine phosphorylation of STAT3 5.6- to 6.0-fold and of STAT1 4.0-fold in adipose tissue. Leptin tended to increase STAT3 phosphorylation in liver and muscle. Both hormones also increased MAPK phosphorylation: leptin increased it 3.2- to 3.8-fold in adipose tissue and liver, whereas insulin stimulated MAPK phosphorylation 5.0-fold in adipose tissue, 6.8-fold in liver, and 2.5-fold in muscle. Leptin was much less effective than insulin at stimulating IRS pathways. Leptin increased IRS-1-associated PI 3-kinase activity in adipose tissue only 2.0-fold (P < 0.01) compared with the 10-fold effect of insulin. IRS-2-associated PI 3-kinase activity was increased 1.7-fold (P < 0.01) by leptin in liver and 6-fold by insulin. Akt phosphorylation and activity were not changed by leptin but increased with insulin. Lower concentrations of leptin (10 and 50 microg/kg) also stimulated STAT3 phosphorylation in fat. These effects appear to be direct because 3 min after leptin intracerebroventricular injection, phosphorylation of STAT3, STAT1, and MAPK were not stimulated in hypothalamus or adipose tissue. Furthermore, leptin activated STAT3 and MAPK in adipose tissue explants ex vivo and in 3T3-L1 adipocytes. Leptin did not activate STAT3 or MAPK in adipose tissue of db/db mice. Thus, leptin rapidly activates signaling pathways directly at the level of insulin sensitive tissues through the long-form leptin receptor, and these pathways overlap with, but are distinct from, those engaged by insulin.

Leptin regulation of neuroendocrine systems

The discovery of leptin has enhanced understanding of the interrelationship between adipose energy stores and neuronal circuits in the brain involved in energy balance and regulation of the neuroendocrine axis. Leptin levels are dependent on the status of fat stores as well as changes in energy balance as a result of fasting and overfeeding. Although leptin was initially thought to serve mainly as an anti-satiety hormone, recent studies have shown that it mediates the adaptation to fasting. Furthermore, leptin has been implicated in the regulation of the reproductive, thyroid, growth hormone, and adrenal axes, independent of its role in energy balance. Although it is widely known that leptin acts on hypothalamic neuronal targets to regulate energy balance and neuroendocrine function, the specific neuronal populations mediating leptin action on feeding behavior and autonomic and neuroendocrine function are not well understood. In this review, we have discussed how leptin engages arcuate hypothalamic neurons expressing putative orexigenic peptides, e.g., neuropeptide Y and agouti-regulated peptide, and anorexigenic peptides, e.g., pro-opiomelanocortin (precursor of alpha-melanocyte-stimulating hormone) and cocaine- and amphetamine-regulated transcript. We show that leptin's effects on energy balance and the neuroendocrine axis are mediated by projections to other hypothalamic nuclei, e.g., paraventricular, lateral, and perifornical areas, as well as other sites in the brainstem, spinal cord, and cortical and subcortical regions.

Linkage analysis of candidate obesity genes among the Mexican-American population of Starr County, Texas

Recent advances in the molecular basis of body fat regulation have identified several genes in which genetic variation may influence obesity and related measures in human populations. Genes that have been shown to have a regulatory function in the control of body fat utilization, eating behavior, and/or metabolic rate in rodent models of obesity include leptin (LEP), leptin receptor (LEPR), neuropeptide Y (NPY), NPY Y1 receptor (NPYY1), glucagon-like peptide-1 (GLP-1), GLP-1 receptor (GLP1R), and uncoupling protein 1 (UCP1). We have typed microsatellite markers located within or near these seven candidate obesity genes in 302 non-diabetic individuals from 59 Mexican-American families from Starr County, Texas. Sib pair linkage analysis was used to examine linkage between these genes and obesity status (obese siblings only; n = 170 pairs) and several obesity-related quantitative variables (all siblings; n = 545 total sibling pairs). Significant linkage (P = 0.042) was found between obesity and NPY within the obese sibling pairs. No other candidate gene was linked to obesity status in this subsample. Consistent with the obese sib pair linkage results, NPY showed evidence of linkage to body weight (P = 0.020), abdominal circumference (P = 0.031), hip circumference (P = 0.012), diastolic blood pressure (P = 0.005), and a composite measure of body mass and size (P = 0.048) in the entire sibling sample. Other significant linkages observed were between LEP and waist/hip ratio (P = 0.010), total cholesterol (P = 0.030), and HDL cholesterol (P = 0.026) and between LEPR and fasting blood glucose (P = 0.018) and diastolic blood pressure (P = 0.003). These results support further investigation of NPY, LEP, and LEPR to identify genetic variation that may influence obesity status, glucose and lipid metabolism, and blood pressure in Mexican Americans.