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Biomedical subjects

J S Flier

Publications and source records attributed to J S Flier.

At least 91 records · Page 5Linked to original sources

Leptin concentrations in the polycystic ovary syndrome.

The polycystic ovary syndrome (PCOS) is characterized by menstrual disturbances, chronic anovulation and hyperandrogenism and is associated with insulin resistance and hyperinsulinemia. Leptin, the product of the ob gene, is an adipocyte-secreted molecule that signals the magnitude of energy stores to the brain and has been recently shown to have important effects on the reproductive axis of rodents. To assess the potential contribution of leptin to the pathogenesis of PCOS, we measured leptin levels in 24 obese women with PCOS and 12 weight- and age-matched controls and determined whether alterations in hyperinsulinemia produced by administration of the insulin-sensitizing agent troglitazone had an effect on serum leptin levels. Leptin concentrations at baseline were not different in women with PCOS (38.1 +/- 2.15 ng/mL) and controls (33.12 +/- 2.39 ng/mL). Moreover, leptin concentrations remained unchanged after treatment with troglitazone (38.1 +/- 2.15 vs. 39.21 +/- 2.65 ng/mL). Baseline leptin correlated strongly with body mass index in both controls (r = 0.59; P < 0.05) and women with PCOS (r = 0.70; P = 0.0004). Leptin levels were not associated with baseline insulin, testosterone, non-sex hormone-binding globulin (SHBG)-bound testosterone, dehydroepiandrosterone sulfate, estradiol, or SHBG. Finally, despite significantly reduced insulin, non-SHBG-bound testosterone, and estradiol levels after troglitazone treatment of women with PCOS, their leptin levels remained unchanged. We conclude that circulating leptin levels in patients with PCOS do not differ from those in age- and weight-matched controls. Furthermore, increased circulating insulin due to insulin resistance does not appear to alter circulating leptin levels in women with PCOS.

Adult↗

Effect of birth weight and maternal smoking on cord blood leptin concentrations of full-term and preterm newborns.

Prematurity, maternal smoking, and low birth weight each result in neuroendocrine dysfunction and increased perinatal morbidity and mortality. Leptin, an adipocyte-secreted protein, has provided the first physiological link to the regulatory system controlling starvation-induced neuroendocrine changes in rodents. This study investigated whether leptin concentrations were detectable in cord blood of newborns, and assessed the effect of birth weight, prematurity, and maternal smoking on cord blood leptin concentrations. Fifty consecutively enrolled full-term and 12 preterm newborns born to mothers who smoked during pregnancy were compared to 50 full-term and 12 preterm newborns born to parents who were nonsmokers. RIA for leptin was performed using cord blood samples collected immediately after birth. Leptin concentrations were detectable in newborns and correlated positively with obesity (full-term, r = 0.30, P < 0.01; preterm, r = 0.47, P < 0.05). Maternal smoking during pregnancy was associated with decreased leptin concentrations in the cord blood of both full-term and preterm newborns. This effect was independent of obesity (full-term newborns: 5.25 +/- 2.48 vs. 4.21 +/- 2.71 ng/ml, P = 0.01) and was more pronounced in premature newborns (5.67 +/- 3.6 vs. 2.46 +/- 2.03, P = 0.02), and its magnitude in full-term newborns was directly related to the reported number of cigarettes the mothers of the full-term newborns smoked per day (r = -0.438, P < 0.001). Thus, low birth weight and maternal smoking are both associated with decreased leptin concentrations, and these effects are more pronounced in premature newborns. Future studies will be needed to determine whether administration of leptin might reverse the neuroendocrine dysfunction caused by maternal smoking.

Birth Weight↗

Role of leptin in the neuroendocrine response to fasting.

A total deficiency in or resistance to the protein leptin causes severe obesity. As leptin levels rise with increasing adiposity in rodents and man, it is proposed to act as a negative feedback 'adipostatic signal' to brain centres controlling energy homeostasis, limiting obesity in times of nutritional abundance. Starvation is also a threat to homeostasis that triggers adaptive responses, but whether leptin plays a role in the physiology of starvation is unknown. Leptin concentration falls during starvation and totally leptin-deficient ob/ob mice have neuroendocrine abnormalities similar to those of starvation, suggesting that this may be the case. Here we show that preventing the starvation-induced fall in leptin with exogenous leptin substantially blunts the changes in gonadal, adrenal and thyroid axes in male mice, and prevents the starvation-induced delay in ovulation in female mice. In contrast, leptin repletion during this period of starvation has little or no effect on body weight, blood glucose or ketones. We propose that regulation of the neuroendocrine system during starvation could be the main physiological role of leptin.

Adrenal Glands↗

Regulation of PPAR gamma gene expression by nutrition and obesity in rodents.

The orphan nuclear receptor, peroxisome proliferator-activated receptor (PPAR) gamma, is implicated in mediating expression of fat-specific genes and in activating the program of adipocyte differentiation. The potential for regulation of PPAR gamma gene expression in vivo is unknown. We cloned a partial mouse PPAR gamma cDNA and developed an RNase protection assay that permits simultaneous quantitation of mRNAs for both gamma l and gamma 2 isoforms encoded by the PPAR gamma gene. Probes for detection of adipocyte P2, the obese gene product, leptin, and 18S mRNAs were also employed. Both gamma l and gamma 2 mRNAs were abundantly expressed in adipose tissue. PPAR gamma 1 expression was also detected at lower levels in liver, spleen, and heart; whereas, gamma l and gamma 2 mRNA were expressed at low levels in skeletal muscle. Adipose tissue levels of gamma l and gamma 2 were not altered in two murine models of obesity (gold thioglucose and ob/ob), but were modestly increased in mice with toxigene-induced brown fat ablation uncoupling protein diphtheria toxin A mice. Fasting (12-48 h) was associated with an 80% fall in PPAR gamma 2 and a 50% fall in PPAR gamma mRNA levels in adipose tissue. Western blot analysis demonstrated a marked effect of fasting to reduce PPAR gamma protein levels in adipose tissue. Similar effects of fasting on PPAR gamma mRNAs were noted in all three models of obesity. Insulin-deficient (streptozotocin) diabetes suppressed adipose tissue gamma l and gamma 2 expression by 75% in normal mice with partial restoration during insulin treatment. Levels of adipose tissue PPAR gamma 2 mRNA were increased by 50% in normal mice exposed to a high fat diet. In obese uncoupling protein diphtheria toxin A mice, high fat feeding resulted in de novo induction of PPAR gamma 2 expression in liver. We conclude (a) PPAR gamma 2 mRNA expression is most abundant in adipocytes in normal mice, but lower level expression is seen in skeletal muscle; (b) expression of adipose tissue gamma1 or gamma2 mRNAs is increased in only one of the three models of obesity; (c) PPAR gamma 1 and gamma 2 expression is downregulated by fasting and insulin-deficient diabetes; and (d) exposure of mice to a high fat diet increases adipose tissue expression of PPAR gamma (in normal mice) and induces PPAR gamma 2 mRNA expression in liver (in obese mice). These findings demonstrate in vivo modulation of PPAR gamma mRNA levels over a fourfold range and provide an additional level of regulation for the control of adipocyte development and function.

Adipocytes↗

Overexpression of Ha-ras selectively in adipose tissue of transgenic mice. Evidence for enhanced sensitivity to insulin.

To determine the role of Ras-dependent signaling pathways in adipocyte function, we created transgenic mice that overexpress Ha-ras in adipocytes using the aP2 fatty acid-binding protein promoter/enhancer ligated to the human genomic ras sequence. ras mRNA was increased 8-17-fold and Ras protein 4-5-fold in white and brown fat, with no overexpression in other tissues. The subcellular distribution of overexpressed Ras paralleled that of endogenous Ras. [U-14C]Glucose uptake into isolated adipocytes was increased approximately 2-fold in the absence of insulin, and the ED50 for insulin was reduced 70%, with minimal effect on maximally stimulated glucose transport. Expression of Glut4 protein was unaltered in transgenic adipocytes, but photoaffinity labeling of transporters in intact cells with [3H]2-N-[4-(1-azi-Z,Z,Z-trifluoroethyl)benzoyl]-1,3-bis-(D-mann os-4- yloxy)-2-propylamine revealed 1.7-2.6-fold more cell-surface Glut 4 in the absence of insulin and at half-maximal insulin concentration (0.3 nM) compared with nontransgenic adipocytes. With maximal insulin concentration (80 nM), cell-surface Glut4 in nontransgenic and transgenic adipocytes was similar. Glut1 expression and basal cell-surface Glut1 were increased 2-2.9-fold in adipocytes of transgenic mice. However, Glut1 was much less abundant than Glut4, making its contribution to transport negligible. These in vitro changes were accompanied by in vivo alterations including increased glucose tolerance, decreased plasma insulin levels, and decreased adipose mass. We conclude that ras overexpression in adipocytes leads to a partial translocation of Glut4 in the absence of insulin and enhanced Glut4 translocation at physiological insulin concentration, but no effect with maximally stimulating insulin concentrations.

Adipocytes↗

Down-regulation of the expression of the obese gene by an antidiabetic thiazolidinedione in Zucker diabetic fatty rats and db/db mice.

Obese (ob) is a recently identified gene involved in the regulation of energy balance in the mouse. We report here that AD-5075, a potent thiazolidinedione which lowered plasma glucose and triglyceride in Zucker diabetic fatty (ZDF) rats and db/db mice, decreased the expression of the ob gene in these animal models of obesity and non-insulin-dependent diabetes mellitus. The level of adipose ob mRNA in ZDF rats was 3-fold greater than that detected in the Zucker lean littermates. Chronic treatment with AD-5075 elicited a 67 and 70% reduction of ob mRNA in ZDF and control lean rats, respectively. Furthermore, the amount of adipose ob mRNA in db/db mice was 7 times higher than that detected in lean littermates. Treatment of db/db mice with AD-5075 resulted in a 78% reduction of the level of ob mRNA with parallel changes in circulating level of the ob gene product, leptin. The reduction of the ob mRNA in the Zucker lean rats was accompanied by significantly greater food intake and weight gain. However, in ZDF rats and db/db mice, there was profound increase in body weight without hyperphagia. The results demonstrate that the expression of the ob gene is up-regulated in these two rodent models of diabetes compared to their lean counterparts and that such overexpression is attenuated by treatment with an agent that improves insulin sensitivity and glucose homeostasis in vivo.

Adipose Tissue↗

Insulin regulation of phosphoenolpyruvate carboxykinase gene expression does not require activation of the Ras/mitogen-activated protein kinase signaling pathway.

Expression of phosphoenolpyruvate carboxykinase (PEPCK), the rate-limiting step in hepatic gluconeogenesis, is primarily regulated at the level of gene transcription. Insulin and phorbol esters inhibit basal PEPCK transcription and antagonize the induction of PEPCK gene expression by glucocorticoids and glucagon (or its second messenger cAMP). Insulin activates a signaling cascade involving Ras --> Raf --> p42/p44 mitogen-activated protein (MAP) kinase kinase (MEK) --> p42/p44 MAP kinase (ERK 1 and 2). Recent reports suggest that activation of this Ras/MAP kinase pathway is critical for the effects of insulin on mitogenesis and c-fos transcription but is not required for insulin action on metabolic processes such as glycogen synthesis, lipogenesis, and Glut-4-mediated glucose transport. We have used three distinct approaches to examine the role of the Ras/MAP kinase pathway in the regulation of PEPCK transcription by insulin in H4IIE-derived liver cells: (i) chemical inhibition of Ras farnesylation, (ii) infection of cells with an adenovirus vector encoding a dominant-negative mutant of Ras, and (iii) use of a chemical inhibitor of MEK. Although each of these methods blocks insulin activation of MAP kinase, none alters insulin antagonism of cAMP- and glucocorticoid-stimulated PEPCK transcription. Although phorbol esters activate MAP kinase and mimic the effects of insulin on PEPCK gene transcription, inhibition of MEK has no effect on phorbol ester inhibition of PEPCK gene transcription. Using the structurally and mechanistically distinct phosphatidylinositol 3-kinase (PI 3-kinase) inhibitors, wortmannin and LY 294002, we provide further evidence supporting a role for PI 3-kinase activation in the regulation of PEPCK gene transcription by insulin. We conclude that neither insulin nor phorbol ester regulation of PEPCK gene transcription requires activation of the Ras/MAP kinase pathway and that insulin signaling to the PEPCK promoter is dependent on PI 3-kinase activation.

Alkyl and Aryl Transferases↗

Decreased brown fat markedly enhances susceptibility to diet-induced obesity, diabetes, and hyperlipidemia.

Previous studies have indicated that rodents are relatively resistant to diet-induced obesity and that this resistance may be mediated in part by the capacity for diet-induced thermogenesis in brown adipose tissue (BAT). To test this hypothesis, we fed UCP-DTA transgenic with toxigene-mediated ablation of BAT and their control littermates a "Western diet" [21% (wt/wt) fat] or normal mouse chow [6.5% (wt/wt) fat]. The diets were begun at weaning (19 days old). At the age of 12 weeks, transgenic mice receiving the Western diet were markedly obese. The increased body weight and total body lipid content were significantly greater in transgenic mice receiving the Western diet than were the additive individual effects of Western diet (in control mice) and decreased BAT (in chow-fed mice), suggesting a synergistic interaction between diminished BAT and diet. A synergistic effect of Western diet and BAT ablation was also observed for morbid metabolic complications, such as insulin resistance, hyperglycemia, and hyperlipidemia. These metabolic changes were accompanied by increased expression of tumor necrosis factor-alpha and decreased expression of GLUT4 and beta 3-adrenergic receptor messenger RNA levels in white adipose tissue of UCP-DTA transgenic mice receiving the Western diet compared to those in the other experimental groups. As previously described, transgenic mice with diminished brown fat are hyperphagic. Of note, the degree of hyperphagia in transgenics compared to controls was similar whether the animals were fed chow or a Western diet. Thus, the synergistic effect of Western diet on obesity in transgenic mice was not mediated by a further stimulation of food intake. Overall, this study demonstrates the existence of a synergistic interaction between decreased BAT and Western diet to cause marked obesity and its accompanying disorders, such as insulin resistance and hyperlipidemia, and gives further support for the view that an important function of BAT is protection from diet-induced obesity, diabetes, and insulin resistance.

Adipose Tissue, Brown↗

Transgenic mice with muscle-specific insulin resistance develop increased adiposity, impaired glucose tolerance, and dyslipidemia.

Impaired skeletal muscle insulin receptor function is a feature of common forms of insulin resistance, including obesity and noninsulin-dependent diabetes mellitus. However, the extent to which this defect accounts for impaired muscle glucose disposal or altered in vivo glucose homeostasis remains to be established. We recently showed that transgenic mice that overexpress dominant-negative insulin receptors specifically in striated muscle have a severe defect in muscle insulin receptor-mediated signaling and modest hyperinsulinemia. Here we performed hindlimb perfusion studies to determine the impact of this defect on muscle glucose uptake and metabolism. Maximal rates of insulin-stimulated muscle 3-O-methylglucose transport were reduced by 32-40% in transgenic mice with proportional defects involving total hindlimb [14C]glucose uptake, lactate production, and muscle glycogen synthesis. To address the hypothesis that muscle insulin resistance could promote an increase in the accretion of body fat, carcass analysis was performed using two independent lines of transgenic mice. Although body weights were normal, transgenic mice had a 22-38% increase in body fat, with a reciprocal decrease (10-15%) in body protein. Mean gonadal fat pad weight was also increased in transgenic mice. Skeletal muscle histology and fiber type distribution were not affected. To determine whether muscle-specific insulin resistance was sufficient to cause impaired glucose tolerance, oral glucose tolerance tests were performed with 6-month-old transgenic and control mice. Fasting glucose levels were increased by 25%, and peak values were 22-40% higher in transgenic mice. Transgenic mice also had a 37% decrease in plasma lactate levels and modest increases in levels of plasma triglycerides and FFA (29% and 15%, respectively). We conclude that 1) severe defects in muscle insulin receptor function result in impaired insulin-stimulated glucose uptake and metabolism in this tissue; 2) muscle-specific insulin resistance can contribute to the development of obesity; and 3) a "pure" defect in insulin-mediated muscle glucose disposal is sufficient to result in impaired glucose tolerance and other features of the insulin resistance syndrome, including hyperinsulinemia and dyslipidemia.

Adipose Tissue↗

Examination of the phosphoenolpyruvate carboxykinase gene promoter in patients with noninsulin-dependent diabetes mellitus.

Expression of phosphoenolpyruvate carboxykinase (PEPCK), a rate-limiting enzyme in gluconeogenesis, is under dominant negative regulation by insulin. In this study, we sought to test the hypothesis that mutations in the PEPCK gene promoter may impair the ability of insulin to suppress hepatic glucose production, thereby contributing to both the insulin resistance and increased rate of gluconeogenesis characteristic of NIDDM. The proximal PEPCK promoter region in 117 patients with noninsulin-dependent diabetes mellitus and 20 obese Pima Indians was amplified by PCR and analyzed with single strand conformation polymorphism techniques. In addition, limited direct DNA sequencing was performed on the insulin response sequence and flanking regions. No DNA sequence polymorphisms were found in any patient. This result suggests that mutations in cis-acting PEPCK gene regulatory elements do not constitute a common cause of noninsulin-dependent diabetes mellitus. The significance of genetic variation in promoter regions to human disease is discussed.

Adolescent↗

Evidence for leptin binding to proteins in serum of rodents and humans: modulation with obesity.

Many hormones circulate bound to serum proteins that modulate ligand bioactivity and bioavailability. To understand the biology of leptin action, we investigated the presence of leptin binding proteins in serum. 125I-labeled leptin binds competitively to at least three serum macromolecules with molecular masses of approximately 85, approximately 176, and approximately 240 kDa in rodents and approximately 176 and approximately 240 kDa in humans. The ability to bind appears to involve sulfhydryl/disulfide interactions because it is inhibited under reducing conditions. When serum is added to recombinant 125I-leptin, there is a shift in sedimentation of 125I-leptin as analyzed by sucrose gradient centrifugation from approximately S1.9 to approximately S4.3. This shift is markedly attenuated in serum from obese mice (ob/ob, db/db, brown-fat ablated, gold-thioglucose treated, high-fat fed) compared with that from nonobese controls. The size distribution of endogenous serum leptin as determined by radioimmunoassay (RIA) after sucrose gradient centrifugation is also consistent with saturation of binding in hyperleptinemic obesity. In humans, free leptin increases with BMI. Thus, in lean rodents and humans a large proportion of leptin circulates bound to several serum proteins. Free leptin is increased in serum of obese subjects, which may alter leptin bioactivity, transport, and/or clearance.

Adult↗

Absence of mutations in the human OB gene in obese/diabetic subjects.

The product of the obese (ob) gene, leptin, is a secreted protein that is important in the regulation of body weight. Mice with mutations in the ob gene are obese and diabetic and manifest reduced physical as well as metabolic activity. In this study, we tested the possibility that mutations in the OB gene may contribute to human obesity. We report the isolation and partial sequence of the human OB gene and the screening of 105 obese patients for mutations in the protein coding sequence using the technique of single-strand conformational polymorphism. No coding sequence polymorphism was found, suggesting that mutations in the coding sequence of the OB gene do not constitute a common cause of increased body weight in humans. We also identified a highly polymorphic simple dinucleotide repeat DNA polymorphism in this gene that will be useful for genetic studies.

Amino Acid Sequence↗

Activation of beta(3) adrenergic receptors suppresses leptin expression and mediates a leptin-independent inhibition of food intake in mice.

To examine potential interactions between leptin and the beta3 adrenergic system in the regulation of food intake, we determined the effects of treatment with a selective beta3 adrenergic receptor (AR) agonist (CL 316,243 [1 mg/kg]) on body weight, food intake, and leptin expression. Studies were carried out in C57Bl/6J and FVB male control mice as well as in mice with targeted disruption of the beta3 AR gene. These findings were correlated with measurement of the expression in hypothalamus of neuropeptide Y (NPY) and melanin concentrating hormone (MCH), two neuropeptides that may be involved in the central regulation of food intake. Treatment with CL 316,243 (1 mg/kg) for 12 or 24 h decreased leptin mRNA abundance and circulating levels to 20% of baseline in normal animals. No effect of the CL 316,243 compound was seen in mice with targeted disruption of the beta3 AR gene. Despite the failing leptin levels, beta3 agonist administration acutely suppressed food intake. Finally, the induced suppression of food intake and leptin levels occurred despite unchanged or increased hypothalamic expression of the orexigenic neuropeptides NPY and MCH. Thus, beta3 AR agonists via beta3 ARs suppress leptin levels acutely and simultaneously suppress food intake via a mechanism that operates downstream of leptin and two of its putative central targets.

Adipose Tissue↗

Targeted disruption of the beta 3-adrenergic receptor gene.

beta 3-Adrenergic receptors (beta 3-ARs) are expressed predominantly in white and brown adipose tissue, and beta 3-selective agonists are potential anti-obesity drugs. However, the role of beta 3-ARs in normal physiology is unknown. To address this issue, homologous recombination was used to generate mice that lack beta 3-ARs. This was accomplished by direct injection of a DNA-targeting construct into mouse zygotes. Twenty-three transgenic mice were generated, of which two had targeted disruption of the beta 3-AR gene. Mice that were homozygous for the disrupted allele had undetectable levels of intact beta 3-AR mRNA, as assessed by RNase protection assay and Northern blotting, and lacked functional beta 3-ARs, as demonstrated by complete loss of beta 3-agonist (CL 316,243)-induced stimulation of adenylate cyclase activity and lipolysis. beta 3-AR-deficient mice had modestly increased fat stores (females more than males), indicating that beta 3-ARs play a role in regulating energy balance. Importantly, beta 1 but not beta 2-AR mRNA levels up-regulated in white and brown adipose tissue of beta 3-AR-deficient mice (brown more than white), strongly implying that beta 3-ARs mediate physiologically relevant signaling under normal conditions and that "cross-talk" exists between beta 3-ARs and beta 1-AR gene expression. Finally, acute treatment of normal mice with CL 316,243 increased serum levels of free fatty acids (FFAs) (3.2-fold) and insulin (140-fold), increased energy expenditure (2-fold), and reduced food intake (by 45%). These effects were completely absent in beta 3-AR-deficient mice, proving that the actions of CL are mediated exclusively by beta 3-ARs. beta 3-AR-deficient mice should be useful as a means to a better understanding of the physiology and pharmacology of beta 3-ARs.

Adenylyl Cyclases↗

Genetic variability in the TNF-alpha promoter is not associated with type II diabetes mellitus (NIDDM).

Recent evidence suggests that expression of tumor necrosis factor-alpha (TNF-alpha) by adipocytes is a molecular mediator of insulin resistance in obesity. We have therefore tested the hypothesis that variations within the regulatory region of the TNF-alpha gene, which might cause increased adipocyte or generalized TNF-alpha synthesis, are associated with NIDDM, a state in which insulin resistance is routinely observed. Neither the previously known variants within the TNF-alpha promoter at position -308 and -238, nor two newly identified polymorphisms at position -376 and -163, were found at a significantly higher frequency in Caucasian NIDDM patients compared to non-diabetic controls. No genetic variants were found in Pima Indians. These data make it unlikely that mutations within regulatory elements of the TNF-alpha gene are associated with an increase in the prevalence of NIDDM.

Adult↗

Insulin-like growth factor I (rhIGF-I) as a therapeutic agent for hyperinsulinemic insulin-resistant diabetes mellitus.

Insulin resistance is one of the major underlying abnormalities in NIDDM, however, its pathophysiologic mechanisms are not well understood. Many clues about the mechanisms of insulin action have come from patients with the most severe forms of insulin resistance, including those with genetic abnormalities in the insulin signal transduction cascade. We used rhIGF-I as a probe to differentiate insulin and IGF-I action and to study the therapeutic potential of IGF in states of insulin resistance. To date, we have studied six subjects with varying phenotypes of severe insulin resistance but without mutations in the insulin receptor itself. All subjects underwent baseline physiologic monitoring to quantitate carbohydrate tolerance, insulin secretion, and insulin action prior to receiving rhIGF-I at 100 micrograms/kg body wt s.c. bid for 1 month with interval testing of glycemic control and insulin sensitivity. None of the six subjects noted significant side effects from the rhIGF-I. Four of the six subjects had overt diabetes during control testing; three of these subjects demonstrated normalization of fasting and postprandial blood glucose concentrations during rhIGF-I administration on no other therapy. In the fourth patient, insulin requirements and fasting hypertriglyceridemia decreased without improvement in glycemic control. The two subjects with normal glucose tolerance (two sisters with congenital lipodystrophy) maintained normal glucose tolerance at dramatically lower insulin levels and had a dramatic reduction in triglyceride levels. The efficacy of IGF-I continued to increase over the duration of the study.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood Glucose↗