Mechanisms for regulation of cellular responsiveness to human IFN-beta1a

Interferons (IFNs) are potent, pleiotropic cytokines, and therefore it is likely that the cell has mechanisms to modulate IFN activity in response to excessive or prolonged IFN exposure. To investigate this question, Jurkat T cells were exposed to IFN-beta1a in vitro. The effect of dose and frequency of IFN treatment on receptor expression, the signal transduction pathway, and biologic activity was examined. Results demonstrate that at even modest doses of IFN (60 IU/ml), cell surface expression of the IFN receptor subunit, IFNAR-1, decreased significantly, and the cells were unresponsive to further IFN treatment. More interestingly, after an initial treatment with very low concentrations of IFN (<10 IU/ml), even when receptor levels remained normal and phosphorylation of signaling molecules occurred, cells were still refractory to further IFN treatment. After withdrawal of IFN, full cellular responsiveness was a progressive but surprisingly slow process. Cells retreated 2 days or 4 days after the initial IFN treatment were still refractory to even high doses (500 IU/ml) of IFN. Cells retreated 1 week after the initial IFN treatment were fully responsive. High levels of Stat1 and Stat2 correlated with the block in transcriptional activation of IFN-dependent genes and may be a mechanism by which cells can downmodulate an IFN response. Similar results were obtained when fresh peripheral blood mononuclear cells (PBMC) were treated with IFN and expression of the endogenous IFN-dependent gene, MxA, was examined. Cell surface levels of IFNAR-1 decreased and Stat1 levels increased after IFN-beta treatment, and retreatment with IFN resulted in an attenuated induction of Mx protein expression. In the context of using IFNs as therapeutic agents in the treatment of human disease, our data suggest that increasing the amount or frequency of IFN administration may not yield desired biologic effects. Thus, issues concerning the dosage and the frequency of IFN-beta administration deserve careful consideration.

Disordered fat storage and mobilization in the pathogenesis of insulin resistance and type 2 diabetes

The primary genetic, environmental, and metabolic factors responsible for causing insulin resistance and pancreatic beta-cell failure and the precise sequence of events leading to the development of type 2 diabetes are not yet fully understood. Abnormalities of triglyceride storage and lipolysis in insulin-sensitive tissues are an early manifestation of conditions characterized by insulin resistance and are detectable before the development of postprandial or fasting hyperglycemia. Increased free fatty acid (FFA) flux from adipose tissue to nonadipose tissue, resulting from abnormalities of fat metabolism, participates in and amplifies many of the fundamental metabolic derangements that are characteristic of the insulin resistance syndrome and type 2 diabetes. It is also likely to play an important role in the progression from normal glucose tolerance to fasting hyperglycemia and conversion to frank type 2 diabetes in insulin resistant individuals. Adverse metabolic consequences of increased FFA flux, to be discussed in this review, are extremely wide ranging and include, but are not limited to: 1) dyslipidemia and hepatic steatosis, 2) impaired glucose metabolism and insulin sensitivity in muscle and liver, 3) diminished insulin clearance, aggravating peripheral tissue hyperinsulinemia, and 4) impaired pancreatic beta-cell function. The precise biochemical mechanisms whereby fatty acids and cytosolic triglycerides exert their effects remain poorly understood. Recent studies, however, suggest that the sequence of events may be the following: in states of positive net energy balance, triglyceride accumulation in "fat-buffering" adipose tissue is limited by the development of adipose tissue insulin resistance. This results in diversion of energy substrates to nonadipose tissue, which in turn leads to a complex array of metabolic abnormalities characteristic of insulin-resistant states and type 2 diabetes. Recent evidence suggests that some of the biochemical mechanisms whereby glucose and fat exert adverse effects in insulin-sensitive and insulin-producing tissues are shared, thus implicating a diabetogenic role for energy excess as a whole. Although there is now evidence that weight loss through reduction of caloric intake and increase in physical activity can prevent the development of diabetes, it remains an open question as to whether specific modulation of fat metabolism will result in improvement in some or all of the above metabolic derangements or will prevent progression from insulin resistance syndrome to type 2 diabetes.

Increased insulin and leptin sensitivity in mice lacking acyl CoA:diacylglycerol acyltransferase 1

Acyl coenzyme A:diacylglycerol acyltransferase 1 (DGAT1) is one of two known DGAT enzymes that catalyze the final step in mammalian triglyceride synthesis. DGAT1-deficient mice are resistant to diet-induced obesity through a mechanism involving increased energy expenditure. Here we show that these mice have decreased levels of tissue triglycerides, as well as increased sensitivity to insulin and to leptin. Importantly, DGAT1 deficiency protects against insulin resistance and obesity in agouti yellow mice, a model of severe leptin resistance. In contrast, DGAT1 deficiency did not affect energy and glucose metabolism in leptin-deficient (ob/ob) mice, possibly due in part to a compensatory upregulation of DGAT2 expression in the absence of leptin. Our results suggest that inhibition of DGAT1 may be useful in treating insulin resistance and leptin resistance in human obesity.

Obesity therapy: altering the energy intake-and-expenditure balance sheet

Obesity is associated with numerous health complications, which range from non-fatal debilitating conditions such as osteoarthritis, to life-threatening chronic diseases such as coronary heart disease, diabetes and certain cancers. The psychological consequences of obesity can range from lowered self-esteem to clinical depression. Despite the high prevalence of obesity and the many advances in our understanding of how it develops, current therapies have persistently failed to achieve long-term success. This review focuses on how fat mass can be reduced by altering the balance between energy intake and expenditure.

Thyroid hormone receptor interacting protein 3 (trip3) is a novel coactivator of hepatocyte nuclear factor-4alpha

Mutations of the hepatocyte nuclear factor-4alpha (HNF-4alpha) gene are associated with a subtype of maturity-onset diabetes of the young (MODY1) that is characterized by impaired insulin secretion in response to a glucose load. HNF-4alpha, which is a transcription factor expressed in pancreatic beta-cells, plays an important role in regulating the expression of genes involved in glucose metabolism. Thus, cofactors that interact with HNF-4alpha and modify its transcriptional activity might also play an important role in regulating the metabolic pathways in pancreatic beta-cells, and the genes of such cofactors are plausible candidate genes for MODY. In the present study, we showed, using a yeast two-hybrid screening assay, that thyroid hormone receptor interacting protein 3 (Trip3) interacted with HNF-4alpha, and their interaction was confirmed by the glutathione S-transferase pull-down assay. Human Trip3 cDNA contained an open reading frame for a protein of 155 amino acids, and the gene was expressed in both pancreatic islets and MIN6 cells. Cotransfection experiments indicated that Trip3 could enhance (two- to threefold) the transcription activity of HNF-4alpha in COS-7 cells and MIN6 cells. These results suggest that Trip3 is a coactivator of HNF-4alpha. Mutation screening revealed that variation of the Trip3 gene is not a common cause of MODY/early-onset type 2 diabetes in Japanese individuals. Trip3 may play an important role in glucose metabolism by regulating the transcription activity of HNF-4alpha.

The role of peroxisome proliferator-activated receptor gamma in diabetes and obesity

Peroxisome proliferator-activated receptor gamma (PPAR gamma) is a nuclear receptor, which upon activation with various natural and synthetic ligands, stimulates the transcription of genes responsible for growth and differentiation of adipocytes. Furthermore, PPAR gamma is the receptor for the insulin-sensitizing thiazolidinediones, which are commonly used for the treatment of type 2 diabetes. Rare inactivating mutations of the gene encoding PPAR gamma are associated with insulin resistance type 2 diabetes, and hypertension, whereas a rare gain of function mutation causes extreme obesity. A common polymorphism (Pro12Ala) of the adipose tissue-specific gamma 2 isoform is associated with increased insulin sensitivity and decreased risk of developing type 2 diabetes. These findings indicate a central role of PPAR gamma in fat cell biology and in the pathophysiology of obesity, diabetes, and insulin resistance.

Energy metabolism and expression of uncoupling proteins 1, 2, and 3 after 21 days of recovery from intracerebroventricular mouse leptin in rats

Animals tend to maintain a lower body weight for an extended period after leptin administration has ended. This may be due to an enhancement of metabolic rate that persists after treatment withdrawal. Our objectives were to determine the period of leptin influence, when injected intracerebroventricularly (icv), on food intake, body weight, and energy expenditure. Additionally, the relationship between expressions of UCP1, UCP2, and UCP3 in different adipose tissues and heat production (HP) was assessed. Twenty-four adult male Sprague-Dawley rats were injected intracerebroventricularly with either 10 g mouse leptin or 10 l vehicle once per day for 4 days. At 24 h after the last injection, one group was killed while the other was placed in calorimetry chambers and monitored for 21 days of recovery. Leptin-injected rats exhibited an overshoot of food intake and respiratory quotient (RQ) during recovery, but body weight remained significantly lower up to 6 days. HP decreased in both groups over time but remained higher in the leptin group through recovery. However, retained energy (RE) was significantly greater than control for about 8 days. Overall, UCP expression was reduced at the end of recovery in parallel with the decline in HP. Brown adipose tissue (BAT) was the most responsive to leptin administration by dramatically changing UCP1 and UCP3 mRNA levels. Our data show that leptin has extended effects on energy expenditure but relieves control on food intake and RQ after treatment withdrawal. This translated into a reduced positive energy balance that slowed body weight recovery.

Requirement of helix 1 and the AF-2 domain of the thyroid hormone receptor for coactivation by PGC-1

Although PGC-1 (peroxisome proliferator-activated receptor-gamma coactivator-1) has been previously shown to enhance thyroid hormone receptor (TR)/retinoid X receptor-mediated ucp-1 gene expression in a ligand-induced manner in rat fibroblast cells, the precise mechanism of PGC-1 modulation of TR function has yet to be determined. In this study, we show that PGC-1 can potentiate TR-mediated transactivation of reporter genes driven by natural thyroid hormone response elements both in a ligand-dependent and ligand-independent manner and that the extent of coactivation is a function of the thyroid hormone response element examined. Our data also show that PGC-1 stimulation of TR activity in terms of Gal4 DNA-binding domain fusion is strictly ligand-dependent. In addition, an E457A AF-2 mutation had no effect on the ligand-induced PGC-1 enhancement of TR activity, indicating that the conserved charged residue in AF-2 is not essential for this PGC-1 function. Furthermore, GST pull-down and mammalian two-hybrid assays demonstrated that the PGC-1 LXXLL motif is required for ligand-induced PGC-1/TR interaction. This agonist-dependent PGC-1/TR interaction also requires both helix 1 and the AF-2 region of the TR ligand-binding domain. Taken together, these results support the notion that PGC-1 is a bona fide TR coactivator and that PGC-1 modulates TR activity via a mechanism different from that utilized with peroxisome proliferator activator receptor-gamma.

Mechanism for peroxisome proliferator-activated receptor-alpha activator-induced up-regulation of UCP2 mRNA in rodent hepatocytes

Peroxisome proliferator-activated receptor-alpha (PPARalpha)activators, fish oil feeding, or fibrate administration up-regulated mitochondrial uncoupling protein (UCP2) mRNA expression in mouse liver by 5-9-fold, whereas tumor necrosis factor-alpha (TNFalpha) also up-regulated UCP2 in liver. In this study, the mechanisms for PPARalpha activators-induced up-regulation of UCP2 mRNA, related to TNFalpha and reactive oxygen species (ROS), were investigated. PPARalpha activators-induced UCP2 up-regulation in mouse/rat liver tissues was due to their increases in hepatocytes but not in non-parenchymal cells. Addition of PPARalpha activators, WY14,643 or fenofibrate, to cultured hepatocytes up-regulated UCP2 mRNA by 5-10-fold. PPARalpha activators-induced up-regulation of UCP2 mRNA was not due to increased mRNA stability and required cycloheximide-sensitive short term turnover protein(s). However, expression of PPARalpha/retinoid X receptor-alpha and PGC-1 was not rate-limiting for WY14,643-induced UCP2 up-regulation. In primary hepatocytes, an exogenous oxidant, tert-butyl-hydroperoxide (TBHP), which increased ROS production, up-regulated UCP2 mRNA, whereas WY14,643 treatment did not produce detectable ROS under the condition that fibrate markedly up-regulated UCP2. In in vivo studies, PPARalpha activators moderately up-regulated TNFalpha mRNA expression in mouse liver. An anti-oxidant pyrrolidine dithiocarbamate ammonium salt injection completely prevented their TNFalpha mRNA increases but did not prevent most of their UCP2 mRNA increases. These data indicate that PPARalpha activators up-regulate UCP2 expression in hepatocytes through unknown proteins by increased transcription, and neither ROS nor TNFalpha production are the major causes for PPARalpha activators-induced UCP2 up-regulation.

Diminished hepatic response to fasting/refeeding and liver X receptor agonists in mice with selective deficiency of sterol regulatory element-binding protein-1c

Two treatments, fasting/refeeding and administration of liver X receptor (LXR) agonists, elevate the mRNA for sterol regulatory element-binding protein-1c (SREBP-1c) and enhance lipid synthesis in liver. These treatments do not affect the mRNA for SREBP-1a, an alternative transcript from the same gene. Through homologous recombination, we eliminated the exon encoding SREBP-1c from the mouse genome, leaving the SREBP-1a transcript intact. On a normal diet, livers of SREBP-1c(-/-) mice manifested reductions in multiple mRNAs encoding enzymes of fatty acid and triglyceride synthesis, including acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS). In contrast, SREBP-1c(-/-) livers showed a compensatory increase in hepatic SREBP-2 mRNA, accompanied by increased mRNA levels for cholesterol biosynthetic enzymes. In fasted/refed animals, ACC and FAS mRNAs rose, but not to the same extent as in wild-type livers. The refeeding-induced increase in SREBP-1c(-/-) mice was greater than in mice lacking SREBP cleavage-activating protein (SCAP), in which all nuclear SREBPs are absent. Thus, SREBP-2 and/or SREBP-1a can substitute partially for SREBP-1c in permitting an insulin-mediated increase in ACC and FAS mRNAs. In contrast, mRNAs for several other lipogenic enzymes (glucose-6-phosphate dehydrogenase, malic enzyme, glycerol-3-phosphate acyltransferase, and stearoyl-CoA desaturase-1) showed a complete failure of the normal inductive response to refeeding, indicating specific reliance on SREBP-1c. Moreover, these mRNAs, as well as multiple other lipogenic mRNAs, showed a markedly blunted response to the LXR agonist T090137, indicating an essential role of SREBP-1c in the LXR response.

Chronic leptin administration decreases fatty acid uptake and fatty acid transporters in rat skeletal muscle

Chronic leptin administration reduces triacylglycerol content in skeletal muscle. We hypothesized that chronic leptin treatment, within physiologic limits, would reduce the fatty acid uptake capacity of red and white skeletal muscle due to a reduction in transport protein expression (fatty acid translocase (FAT/CD36) and plasma membrane-associated fatty acid-binding protein (FABPpm)) at the plasma membrane. Female Sprague-Dawley rats were infused for 2 weeks with leptin (0.5 mg/kg/day) using subcutaneously implanted miniosmotic pumps. Control and pair-fed animals received saline-filled implants. Leptin levels were significantly elevated (approximately 4-fold; p < 0.001) in treated animals, whereas pair-fed treated animals had reduced serum leptin levels (approximately -2-fold; p < 0.01) relative to controls. Palmitate transport rates into giant sarcolemmal vesicles were reduced following leptin treatment in both red (-45%) and white (-84%) skeletal muscle compared with control and pair-fed animals (p < 0.05). Leptin treatment reduced FAT mRNA (red, -70%, p < 0.001; white, -48%, p < 0.01) and FAT/CD36 protein expression (red, -32%; p < 0.05) in whole muscle homogenates, whereas FABPpm mRNA and protein expression were unaltered. However, in leptin-treated animals plasma membrane fractions of both FAT/CD36 and FABPpm protein expression were significantly reduced in red (-28 and -34%, respectively) and white (-44 and -56%, respectively) muscles (p < 0.05). Across all experimental treatments and muscles, palmitate uptake by giant sarcolemmal vesicles was highly correlated with the plasma membrane FAT/CD36 protein (r = 0.88, p < 0.01) and plasma membrane FABPpm protein (r = 0.94, p < 0.01). These studies provide the first evidence that protein-mediated long chain fatty acid transport is subject to long term regulation by leptin.

13C NMR isotopomer analysis reveals a connection between pyruvate cycling and glucose-stimulated insulin secretion (GSIS)

Cellular metabolism of glucose is required for stimulation of insulin secretion from pancreatic beta cells, but the precise metabolic coupling factors involved in this process are not known. In an effort to better understand mechanisms of fuel-mediated insulin secretion, we have adapted 13C NMR and isotopomer methods to measure influx of metabolic fuels into the tricarboxylic acid (TCA) cycle in insulinoma cells. Mitochondrial metabolism of [U-13C3]pyruvate, derived from [U-13C6]glucose, was compared in four clonal rat insulinoma cell 1-derived cell lines with varying degrees of glucose responsiveness. A 13C isotopomer analysis of glutamate isolated from these cells showed that the fraction of acetyl-CoA derived from [U-13C6]glucose was the same in all four cell lines (44 +/- 5%, 70 +/- 3%, and 84 +/- 4% with 3, 6, or 12 mM glucose, respectively). The 13C NMR spectra also demonstrated the existence of two compartmental pools of pyruvate, one that exchanges with TCA cycle intermediates and a second pool derived from [U-13C6]glucose that feeds acetyl-CoA into the TCA cycle. The 13C NMR spectra were consistent with a metabolic model where the two pyruvate pools do not randomly mix. Flux between the mitochondrial intermediates and the first pool of pyruvate (pyruvate cycling) varied in proportion to glucose responsiveness in the four cell lines. Furthermore, stimulation of pyruvate cycling with dimethylmalate or its inhibition with phenylacetic acid led to proportional changes in insulin secretion. These findings indicate that exchange of pyruvate with TCA cycle intermediates, rather than oxidation of pyruvate via acetyl-CoA, correlates with glucose-stimulated insulin secretion.

Central leptin gene delivery evokes persistent leptin signal transduction in young and aged-obese rats but physiological responses become attenuated over time in aged-obese rats

The purpose of this study was to determine if long-term leptin treatment desensitizes leptin signal transduction and the subsequent downstream anorexic and thermogenic responses in normal and leptin-resistant age-related obese rats. To this end, we administered, i.c.v., recombinant adeno-associated virus encoding rat leptin cDNA (rAAV-leptin) or control virus into young and aged-obese rats and after 9 or 46 days, examined food intake, oxygen consumption, body weight, serum leptin, STAT3 phosphorylation, hypothalamic NPY and POMC mRNAs, and UCP1 expression and protein level in brown adipose tissue (BAT). In young rats, rAAV-leptin depleted body fat and both anorexic and thermogenic mechanisms contributed to this effect. Moreover, leptin signal transduction was not desensitized, and there were persistent physiological responses. Similarly, in the aged-obese rats, there was unabated leptin signal transduction, however, both the anorexic and thermogenic responses completely attenuated sometime after day 9. This attenuation, downstream of the leptin receptor, may be contributing to the leptin-resistance and age-related weight gain in these aged-obese rats. Finally, in young rats, although the initial responses to rAAV-leptin were dominated by anorexic responses, by 46 days, the predominant response was thermogenic rather than anorexic, suggesting that energy expenditure may be an important component of long-term weight maintenance.

Lipid metabolism and resistin gene expression in insulin-resistant Fischer 344 rats

The interrelationship between insulin and leptin resistance in young Fischer 344 (F344) rats was studied. Young F344 and Sprague-Dawley (SD) rats were fed regular chow. F344 animals had two- to threefold higher insulin and triglyceride concentrations and increased stores of triglycerides within liver and muscle. F344 animals gained more body fat. Both acyl-CoA oxidase (ACO) and carnitine palmitoyltransferase I gene expression were 20-50% less in F344 animals than in age-matched SD animals. Peroxisome proliferator-activated receptor-alpha gene expression was reduced in 70-day-old F344 animals. Finally, resistin gene expression was similar in 70-day-old SD and F344 animals. Resistin gene expression increased fivefold in F344 animals and twofold in SD animals from 70 to 130 days, without a change in insulin sensitivity. We conclude that young F344 animals have both insulin and leptin resistance, which may lead to diminished fatty oxidation and accumulation of triglycerides in insulin-sensitive target tissues. We did not detect a role for resistin in the etiology of insulin resistance in F344 animals.

Atrophic change of rat salivary gland during adenovirus-induced hyperleptinemia

Sustained hyperleptinemia in normal rats induced by infusing a recombinant adenovirus containing the rat leptin cDNA (AdCMV-leptin) exhibited a remarkable reduction in food intake (AdCMV-leptin, 9.3 +/- 2.6 vs untreated, 20.6 +/- 1.0 g/day) and ablated body fat without any significant changes in wet weight of liver and left ventricle. In those hyperleptinemic rats, we found a 52% reduction in wet weight of salivary gland compared with that in the pair-fed AdCMV-beta-gal-treated rats, which received a recombinant virus containing the beta-galactosidase gene (AdCMV-beta-gal) and were fed on the same amount of food as had been consumed by the AdCMV-leptin-treated group on the previous day. Microscopic examination with hematoxylin-eosin staining revealed that atrophic change was induced in both serous and mucous gland only in the AdCMV-leptin-treated group, but not in the pair-fed controls. Thus, the atrophic changes in hyperleptinemic rats were due to neither a decrease of food intake nor disuse of the salivary gland related with anorexia. Our data suggested that size of the salivary gland was controlled, at lease in part, by "non-anorexic" effect of leptin.

Evaluation of liver fatty acid oxidation in the leptin-deficient obese mouse

We hypothesized that liver fatty acid oxidation (FAO) is compromised in the leptin-deficient obese (Lep(ob)/Lep(ob)) mouse model, and that this would be further challenged when these mice were fed a high-fat diet. Obese mice had a 3.8-fold increased body fat content and a 9-fold increased liver fat content as compared to control mice when both groups were fed a low-fat diet. The expression of liver FAO enzymes, carnitine palmitoyltransferase-1a, long-chain acyl-CoA dehydrogenase, medium-chain acyl-CoA dehydrogenase, and short-chain acyl-CoA dehydrogenase, was not affected in obese mice as compared to controls on either a low-fat or a high-fat diet. The expression of very-long-chain acyl-CoA dehydrogenase was elevated in obese mice on the control diet, as compared to control mice. For all measures evaluated, increasing the level of fat in the diet had a smaller effect than leptin deficiency. In summary, despite obese mice having an excess of fat available for mitochondrial beta-oxidation in liver, overall energy balance appeared to dictate that the net liver FAO remained at control levels.

Combinatorial control of gene expression by nuclear receptors and coregulators

The nuclear receptor (NR) superfamily of transcription factors regulates gene expression in response to endocrine signaling, and recruitment of coregulators affords these receptors considerable functional flexibility. We will place historical aspects of NR research in context with current opinions on their mechanism of signal transduction, and we will speculate upon future trends in the field.

PGC-1 functions as a transcriptional coactivator for the retinoid X receptors

Ligand-dependent gene transcription mediated by the nuclear receptors involves the recruitment of transcriptional coactivators to the ligand-binding domain (LBD), which leads to interaction with the basal transcription machinery, and ultimately with RNA polymerase II. Although most of these coactivators are ubiquitously expressed, a tissue-selective coactivator, PGC-1, has recently been characterized. Because PGC-1 and the retinoid X receptors (RXRs) possess an overlapping tissue distribution, we investigated whether PGC-1 is a coactivator for the retinoid X receptors. In a transient transfection assay, PGC-1 augments ligand-stimulated RXR transcription. Furthermore, PGC-1 efficiently enhances the RXR element-driven reporter gene transcription by all three RXR isoforms. An immunoprecipitation assay reveals that PGC-1 and RXRalpha interact in vivo. In addition, a glutathione S-transferase pull-down assay showed that this interaction requires the presence of the LXXLL motif of PGC-1. We demonstrate further, in a mammalian two-hybrid assay, that this physical interaction also requires the presence of the AF-2 region of RXR to interact with the LXXLL motif of PGC-1, which is consistent with our protein-protein interaction results. A time-resolved fluorescence assay shows that a peptide within the NR box of PGC-1 is efficiently recruited by a ligand-bound RXRalpha in vitro. Finally, PGC-1 and TIF2 synergistically enhance ligand-activated RXRalpha transcriptional activity. Taken together, these results indicate that PGC-1 is a bona fide coactivator for RXRalpha.

Over-expression of the glucagon-like peptide-1 receptor on INS-1 cells confers autocrine stimulation of insulin gene promoter activity: a strategy for production of pancreatic beta-cell lines for use in transplantation

To develop transplantable beta-cell lines for the treatment of diabetes mellitus, we have taken advantage of the property of INS-1 cells to synthesize and secrete not only insulin, but also small quantities of the insulinotropic hormone glucagon-like peptide-1 (GLP-1). In INS-1 cells over-expressing the beta-cell GLP-1 receptor (GLP-1-R), we have shown, by radioimmune assay and bioassay of conditioned medium, that an autocrine signaling mechanism of hormone action exists whereby self-secreted GLP-1 acts as a competence factor in support of insulin gene transcription. INS-1 cells also exhibit insulin gene promoter activity, as assayed in cells transfected with a rat insulin gene I promoter-luciferase construct (RIP1-Luc). The GLP-1-R agonist exendin-4 stimulates RIP1-Luc activity in a glucose-dependent manner, an effect mediated by endogenous GLP-1-Rs, and is blocked by the serine/threonine protein kinase inhibitor Ro 31-8220. Over-expression of GLP-1-R in transfected INS-1 cells reduces the threshold for exendin-4 agonist action, whereas basal RIP1-Luc activity increases 2.5-fold in the absence of added agonist. The increase of basal RIP1-Luc activity is a consequence of autocrine stimulation by self-secreted GLP-1 and is blocked by introduction of (1) an inactivating W39A mutation in the N-terminus ligand-binding domain of GLP-1-R or (2) mutations in the third cytoplasmic loop that prevent G protein coupling. No evidence for constitutive ligand-independent signaling properties of the GLP-1-R has been obtained. Over-expression of GLP-1-R increases the potency and efficacy of D-glucose as a stimulator of RIP1-Luc. Thus, INS-1 cells over-expressing the GLP-1-R recapitulate the incretin hormone effect of circulating GLP-1, thereby providing a possible strategy by which beta-cell lines may be engineered for efficient glucose-dependent insulin biosynthesis and secretion.

Accelerated substrate cycling: a new energy-wasting role for leptin in vivo

Simultaneous lipolysis and reesterification form the triacylglycerol/fatty acid (TAG/FA) cycle, a substrate cycle commonly used for thermogenesis. Its rate was measured in vivo by indirect calorimetry and continuous infusion of [2-(3)H]glycerol and [1-(14)C]palmitate, after injection of leptin or vehicle saline in rabbits. Leptin stimulated in vivo lipolysis from 9.66 +/- 0.62 to 14.78 +/- 0.93 micromol x kg(-1) x min(-1), the rate of appearance of FA from 20.69 +/- 2.14 to 29.03 +/- 3.03 micromol x kg(-1) x min(-1), and TAG/FA cycling from 24.82 +/- 1.73 to 37.09 +/- 2.49 micromol FA x kg(-1) x min(-1). This large increase in total cycling was caused by an 85% rise in primary cycling (reesterification without transit in the circulation) and accounted for 14% of the difference in metabolic rate between the controls and the leptin-treated animals. This study shows that leptin causes a strong activation of TAG/FA cycling, lipolysis, and FA oxidation, shifting fuel preference from carbohydrates to lipids. Therefore, the acceleration of substrate cycling is a new mechanism triggered by leptin to increase metabolic rate, besides the known induction of uncoupling proteins.

Prospective function of placental leptin at maternal-fetal interface

Leptin is an endocrine and a growth factor which is important for regulation of body fat, feeding, and energy homeostasis. The anti-obesity function of leptin has been recently extended to reproduction, puberty and pregnancy as an endocrine signal to the hypothalamus. Leptin controls the functional integrity of the feto-placental unit thereby maintaining pregnancy by virtue of its immunomodulatory property via T lymphocytes or other proto-oncogenes. Dysregulation of autocrine/paracrine function of leptin at feto-placento-maternal interface may be implicated in the pathogenesis of recurrent miscarriage gestational diabetes, pre-eclampsia and intra-uterine fetal growth retardation including disturbance of fetal bone turnover. This review will focus on the role of leptin in normal and abnormal pregnancy and fetal growth.

The effects of cerulenin, an inhibitor of protein acylation, on the two phases of glucose-stimulated insulin secretion

The potential role of protein acylation in the control of biphasic insulin secretion has been studied in isolated rat pancreatic islets. The protein acylation inhibitor cerulenin inhibited both phases of glucose-stimulated insulin secretion. However, it did not affect the secretory response to a depolarizing concentration of KCl in either the absence or presence of diazoxide. Therefore, cerulenin has no deleterious effect on the L-type Ca(2+) channels or subsequent events in Ca(2+) stimulus-secretion coupling. Advantage was taken of this to study the effect of cerulenin on the K(ATP) channel-independent pathway of glucose signaling. In the presence of KCl and diazoxide, cerulenin powerfully inhibited the augmentation of insulin release by glucose and palmitate. Similar inhibition of the augmentation of release by glucose and palmitate was seen under Ca(2+)-free conditions in the presence of 12-O-tetradecanoylphorbol-13-acetate and forskolin. As neither glucose oxidation nor the effect of glucose to inhibit fatty acid oxidation is affected by cerulenin, these data suggest that protein acylation is involved in the K(ATP) channel-independent pathway of glucose signaling.

Peroxisome proliferator-activated receptor gamma coactivator 1beta (PGC-1beta ), a novel PGC-1-related transcription coactivator associated with host cell factor

Peroxisome proliferator-activated receptor gamma coactivator-1 (PGC-1) plays a critical role in regulating multiple aspects of energy metabolism, including adaptive thermogenesis, mitochondrial biogenesis, and fatty acid beta-oxidation. Recently, this coactivator of nuclear receptors/transcription factors has been shown to control hepatic gluconeogenesis, an important component of the pathogenesis of both type-1 and type-2 diabetes. We described here the cloning of a novel bona fide homologue of PGC-1, PGC-1beta (PGC-1 was renamed as PGC-1alpha), first identified through searches of new data base entries. Despite the fact that PGC-1alpha and -1beta share similar tissue distributions with highest levels of expression in brown fat and heart, their mRNAs are differentially regulated in the brown adipose tissue upon cold exposure and during brown fat cell differentiation. Like PGC-1alpha, PGC-1beta mRNA levels are increased significantly in the liver during fasting, suggesting a possible role for this factor in the regulation of hepatic gluconeogenesis and/or fatty acid oxidation. Consistent with this, PGC-1beta was shown to physically interact and potently coactivate hepatic nuclear factor 4 and peroxisome proliferator-activated receptor alpha, nuclear receptors that are essential for hepatic adaptation to fasting. Finally, using sequence comparisons between PGC-1alpha and -1beta, we have identified a conserved amino acid motif that serves as a docking site for host cell factor, a cellular protein implicated in cell cycle regulation and viral infection. HCF is shown to bind to both PGC-1alpha and -1beta and augment their transcriptional activity.

Reversal of diet-induced glucose intolerance by hepatic expression of a variant glycogen-targeting subunit of protein phosphatase-1

Glycogen-targeting subunits of protein phosphatase-1 facilitate interaction of the phosphatase with enzymes of glycogen metabolism. Expression of one family member, PTG, in the liver of normal rats improves glucose tolerance without affecting other plasma variables but leaves animals unable to reduce hepatic glycogen stores in response to fasting. In the current study, we have tested whether expression of other targeting subunit isoforms, such as the liver isoform G(L), the muscle isoform G(M)/R(Gl), or a truncated version of G(M)/R(Gl) termed G(M)DeltaC in liver ameliorates glucose intolerance in rats fed on a high fat diet (HF). HF animals overexpressing G(M)DeltaC, but not G(L) or G(M)/R(Gl), exhibited a decline in blood glucose of 35-44 mg/dl relative to control HF animals during an oral glucose tolerance test (OGTT) such that levels were indistinguishable from those of normal rats fed on standard chow at all but one time point. Hepatic glycogen levels were 2.1-2.4-fold greater in G(L)- and G(M)DeltaC-overexpressing HF rats compared with control HF animals following OGTT. In a second set of studies on fed and 20-h fasted HF animals, G(M)DeltaC-overexpressing rats lowered their liver glycogen levels by 57% (from 402 +/- 54 to 173 +/- 27 microg of glycogen/mg of protein) in the fasted versus fed states compared with only 44% in G(L)-overexpressing animals (from 740 +/- 35 to 413 +/- 141 microg of glycogen/mg of protein). Since the OGTT studies were performed on 20-h fasted rats, this meant that G(M)DeltaC-overexpressing rats synthesized much more glycogen than G(L)-overexpressing HF rats during the OGTT (419 versus 117 microg of glycogen/mg of protein, respectively), helping to explain why G(M)DeltaC preferentially enhanced glucose clearance. We conclude that G(M)DeltaC has a unique combination of glycogenic potency and responsiveness to glycogenolytic signals that allows it to be used to lower blood glucose levels in diabetes.

Insulin resistance in morbid obesity: reversal with intramyocellular fat depletion

Obesity is a frequent cause of insulin resistance and poses a major risk for diabetes. Abnormal fat deposition within skeletal muscle has been identified as a mechanism of obesity-associated insulin resistance. We tested the hypothesis that dietary lipid deprivation may selectively deplete intramyocellular lipids, thereby reversing insulin resistance. Whole-body insulin sensitivity (by the insulin clamp technique), intramyocellular lipids (by quantitative histochemistry on quadriceps muscle biopsies), muscle insulin action (as the expression of Glut4 glucose transporters), and postprandial lipemia were measured in 20 morbidly obese patients (BMI = 49 +/- 8 [mean +/- SD] kg x m(-2)) and 7 nonobese control subjects. Patients were restudied 6 months later after biliopancreatic diversion (BPD; n = 8), an operation that induces predominant lipid malabsorption, or hypocaloric diet (n = 9). At 6 months, BPD had caused the loss of 33 +/- 10 kg through lipid malabsorption (documented by a flat postprandial triglyceride profile). Despite an attained BMI still in the obese range (39 +/- 8 kg x m(-2)), insulin resistance (23 +/- 3 micromol/min per kg of fat-free mass; P < 0.001 vs. 53 +/- 13 of control subjects) was fully reversed (52 +/- 11 micromol/min per kg of fat-free mass; NS versus control subjects). In parallel with this change, intramyocellular-but not perivascular or interfibrillar-lipid accumulation decreased (1.63 +/- 1.06 to 0.22 +/- 0.44 score units; P < 0.01; NS vs. 0.07 +/- 0.19 of control subjects), Glut4 expression was restored, and circulating leptin concentrations were normalized. In the diet group, a weight loss of 14 +/- 12 kg was accompanied by very modest changes in insulin sensitivity and intramyocellular lipid contents. We conclude that lipid deprivation selectively depletes intramyocellular lipid stores and induces a normal metabolic state (in terms of insulin-mediated whole-body glucose disposal, intracellular insulin signaling, and circulating leptin levels) despite a persistent excess of total body fat mass.

Regulation of signal transducer and activator of transcription and suppressor of cytokine-signaling gene expression in the brain of mice with astrocyte-targeted production of interleukin-12 or experimental autoimmune encephalomyelitis

Interleukin (IL)-12 and interferon (IFN)-gamma are implicated in the pathogenesis of immune disorders of the central nervous system (CNS). To define the basis for the actions of these cytokines in the CNS, we examined the temporal and spatial regulation of key signal transducers and activators of transcription (STATs) and suppressors of cytokine signaling (SOCS) in the brain of transgenic mice with astrocyte production of IL-12 or in mice with experimental autoimmune encephalomyelitis (EAE). In healthy mice, with the exception of STAT4 and STAT6, the expression of a number of STAT and SOCS genes was detectable. However, in symptomatic transgenic mice and in EAE significant up-regulation of STAT1, STAT2, STAT3, STAT4, IRF9, and SOCS1 and SOCS3 RNA transcripts was observed. Although the increased expression of STAT1 RNA was widely distributed and included neurons, astrocytes, and microglia, STAT4 and STAT3 and SOCS1 and SOCS3 RNA was primarily restricted to the infiltrating mononuclear cell population. The level and location of the STAT1, STAT3, and STAT4 proteins overlapped with their corresponding RNA and additionally showed nuclear localization indicative of activation of these molecules. Thus, in both the glial fibrillary acidic protein-IL-12 mice and in EAE the CNS expression of key STAT and SOCS genes that regulate IL-12 (STAT4) and IFN-gamma (STAT1, SOCS1, and SOCS3) receptor signaling is highly regulated and compartmentalized. We conclude the interaction between these positive and negative signaling circuits and their distinct cellular locations likely play a defining role in coordinating the actions of IL-12 and IFN-gamma during the pathogenesis of type 1 immune responses in the CNS.

Leptin modulates the effects of acyl CoA:diacylglycerol acyltransferase deficiency on murine fur and sebaceous glands

Acyl CoA:diacylglycerol acyltransferase (DGAT) is a ubiquitously expressed enzyme that catalyzes the final reaction in the major pathways of triglyceride synthesis. Mice lacking DGAT1 (Dgat(-/-)) demonstrate significant changes in lipid metabolism in several tissues, including the skin. Here we report the effects of DGAT1 deficiency on fur and sebaceous glands. Adult Dgat(-/-) mice had dry fur and hair loss, which were associated with atrophic sebaceous glands and fur lipid abnormalities. As a result, Dgat(-/-) mice had impaired water repulsion and defective thermoregulation after water immersion. These phenotypes were mostly absent in Dgat(-/-) mice with leptin deficiency, indicating an unexpected role for leptin in modulating the skin phenotype. Our findings indicate that DGAT1 plays an important role in normal fur and sebaceous gland physiology and provide evidence that leptin modulates these processes in the skin.

Two thyroid hormone-mediated gene expression patterns in vivo identified by cDNA expression arrays in rat

Thyroid hormone (T3) is essential for normal development, differentiation and metabolic balance. Only a limited number of T3-target genes have been identified so far and their complex regulation pattern is poorly understood. We performed cDNA expression array hybridisation to identify T3-regulated genes and to investigate their expression pattern after various time points in vivo. Radioactively labelled cDNA was prepared from hepatic RNA of hypothyroid and hyperthyroid rats 6, 24 and 48 h after the administration of T3. Labelled cDNA probes were hybridised to rat Atlas Arrays. Twenty-three of 588 genes were shown to be differentially regulated, 18 of which were previously not known to be regulated by T3. The expression of 19 genes was verified by independent northern blot hybridisation. Two different expression time courses of T3 expression were observed. In a first expression profile ('early' expression) the transcription level of the target genes rises within 6 h, drops by 24 h and increases again within 48 h after the administration of T3. In a second expression profile ('late' expression) the mRNA level rose in the first 6 h and rose further by 48 h, indicating an additional regulation mechanism. Nuclear respiratory factor (NRF)-1 and peroxisome proliferator-activated receptor gamma coactivator 1 (PGC-1), but not NRF-2, were up-regulated within 6 h after T3 administration, suggesting NRF-1 and/or PGC-1 as key regulators for mediating the 'late' expression pattern.

A spermine-coupled cholesterol metabolite from the shark with potent appetite suppressant and antidiabetic properties

OBJECTIVE

We describe the pharmacological properties of a novel spermine-cholesterol adduct, MSI 1436 (3beta-N-1(spermine)-7alpha, 24R-dihydroxy-5alpha-cholestane 24-sulfate), which causes reversible suppression of food and fluid intake in mammals resulting in profound weight loss, not associated with other signs or symptoms of illness, and which exhibits antidiabetic properties in genetically obese mice.

METHODS

Wild-type rodents and strains with genetic obesity were studied. Effects on food and fluid intake, body weight and composition were examined along with pharmacological and toxicological parameters.

RESULTS

MSI-1436 induces profound inhibition of food and fluid intake in rats and mice, resulting in significant weight loss. MSI-1436 is active when introduced directly into the third ventricle of the rat, suggesting the compound acts on central targets. Pair-feeding studies suggest that MSI-1436 causes weight loss by suppressing food intake. Fluid intake is also profoundly reduced but animals remain normally hydrated and defend both water and electrolyte balance from parenteral administration. MSI-1436 is active in ob/ob, db/db, agouti and MC4 receptor knockout mice. MSI-1436 has been administered to ob/ob mice over a 4 month period via a regimen that safely controls body weight, glucose homeostasis and serum cholesterol levels. Following MSI-1436 treatment, db/db mice preferentially mobilize adipose tissue and hyperglycemia is corrected.

CONCLUSION

A naturally occurring spermine metabolite of cholesterol, isolated from the dogfish shark, Squalus acanthias, has been identified that induces profound reduction in food and fluid intake in rodents in a setting where thirst is preserved and fluid and electrolyte homeostasis appears to be functioning normally. MSI-1436 probably acts on a central target involving neural circuits that lie downstream from the leptin and the MC4 receptors. Although long-term administration can be accomplished safely in mice, the utility of this compound as a potential human therapeutic awaits an analysis of its pharmacological properties in man.

Insulin signalling and the regulation of glucose and lipid metabolism

The epidemic of type 2 diabetes and impaired glucose tolerance is one of the main causes of morbidity and mortality worldwide. In both disorders, tissues such as muscle, fat and liver become less responsive or resistant to insulin. This state is also linked to other common health problems, such as obesity, polycystic ovarian disease, hyperlipidaemia, hypertension and atherosclerosis. The pathophysiology of insulin resistance involves a complex network of signalling pathways, activated by the insulin receptor, which regulates intermediary metabolism and its organization in cells. But recent studies have shown that numerous other hormones and signalling events attenuate insulin action, and are important in type 2 diabetes.

Selective deletion of leptin receptor in neurons leads to obesity

Animals with mutations in the leptin receptor (ObR) exhibit an obese phenotype that is indistinguishable from that of leptin deficient ob/ob mice. ObR is expressed in many tissues, including brain, and the relative importance of leptin's effects on central versus peripheral sites has not been resolved. To address this, we generated mice with neuron-specific (ObR(SynI)KO) and hepatocyte-specific (ObR(Alb)KO) disruption of ObR. Among the ObR(SynI)KO mice, the extent of obesity was negatively correlated with the level of ObR in hypothalamus and those animals with the lowest levels of ObR exhibited an obese phenotype. The obese mice with low levels of hypothalamic ObR also show elevated plasma levels of leptin, glucose, insulin, and corticosterone. The hypothalamic levels of agouti-related protein and neuropeptide Y RNA are increased in these mice. These data indicate that leptin has direct effects on neurons and that a significant proportion, or perhaps the majority, of its weight-reducing effects are the result of its actions on brain. To explore possible direct effects of leptin on a peripheral tissue, we also characterized ObR(Alb)KO mice. These mice weigh the same as controls and have no alterations in body composition. Moreover, while db/db mice and ObR(SynI)KO mice have enlarged fatty livers, ObR(Alb)KO mice do not. In summary, these data suggest that the brain is a direct target for the weight-reducing and neuroendocrine effects of leptin and that the liver abnormalities of db/db mice are secondary to defective leptin signaling in the brain.

High glycemic index carbohydrate diet alters the diurnal rhythm of leptin but not insulin concentrations

Morning serum leptin values in humans are inconsistently altered by diet, and the molecular mechanisms controlling the diurnal leptin pattern remain unexplained. We determined whether leptin values after meals or the leptin diurnal pattern was altered by the type of carbohydrate (CHO) ingested in diets containing either 20% or 30% fat. In a randomized, crossover study design, nine healthy lean adults ate one of four isocaloric diets for 8 days. Diets contained 15% protein: A, high glycemic index (GI) CHO, 30% fat; B, low GI CHO, 30% fat; C, high GI CHO, 20% fat; and D, low GI CHO, 20% fat. Serum glucose, insulin, and leptin were measured at intervals on Day 8 for 24 hr, and on Day 9 during an oral glucose tolerance test (GTT). Although the 24-hr glucose and insulin profiles did not differ with the diets, diets A and C altered the serum leptin diurnal pattern. In contrast to the usual evening rise in leptin concentration, which begins after 2200 hr, diets A and C caused a rise in leptin beginning at 1300 hr. The area under the curve for leptin between 1230 and 2400 hr was 17% greater for diets A and C. During the GTT, leptin concentrations were similar for each diet. These results suggest that the pattern and amount of leptin secretion may be altered by high GI CHO or the simple sugar content of the diet, unrelated to differences in insulin concentration, that high GI foods may have little or no effect on serum insulin in the context of a mixed meal, and that a single 0800-hr leptin value may not be sufficient to reveal a diet-induced change in leptin secretion

Overfeeding rapidly induces leptin and insulin resistance

In common forms of obesity, hyperphagia, hyperinsulinemia, and hyperleptinemia coexist. Here, we demonstrate rapid induction of insulin and leptin resistance by short-term overfeeding. After 3 and 7 days on the assigned diet regimen, rats were tested for their biological responses to acute elevations in plasma insulin and leptin concentrations. Severe resistance to the metabolic effects of both leptin and insulin ensued after just 3 days of overfeeding. During the insulin clamp studies, glucose production was decreased by approximately 70% in control rats and 28-53% in overfed rats. Similarly, leptin infusion doubled the contribution of gluconeogenesis to glucose output in control rats but failed to modify gluconeogenesis in overfed animals. These findings demonstrate a paradoxical and rapid collapse of the leptin system in response to nutrient excess. This partial failure is tightly coupled with the onset of insulin resistance.

Positive modulation by Ras of interleukin-1beta-mediated nitric oxide generation in insulin-secreting clonal beta (HIT-T15) cells

In the present study, we have shown that exposure of insulin-secreting clonal beta (HIT-T15) cells to interleukin-1beta (IL-1beta) results in a time- and concentration-dependent increase in nitric oxide (NO) release. These effects by IL-1beta on NO release were mediated by induction of inducible nitric oxide synthase (iNOS) from the cells. Preincubation of HIT cells with Clostridium sordellii lethal toxin-82, which irreversibly glucosylates and inactivates small G-proteins, such as Ras, Rap, Ral, and Rac, but not Cdc42, completely abolished IL-1beta-induced NO release. Pre-exposure of HIT cells to C. sordellii lethal toxin-9048, which monoglucosylates and inhibits Ras, Cdc42, Rac, and Rap, but not Ral, also attenuated IL-1beta-mediated NO release. These data indicate that activation of Ras and/or Rac may be necessary for IL-1beta-mediated NO release. Preincubation of HIT cells with C. difficile toxin-B, which monoglucosylates Rac, Cdc42, and Rho, had no demonstrable effects on IL-mediated NO release, ruling out the possibility that Rac may be involved in this signaling step. Further, two structurally dissimilar inhibitors of Ras function, namely manumycin A and damnacanthal, inhibited, in a concentration-dependent manner, the IL-1beta-mediated NO release from these cells. Together, our data provide evidence, for the first time, that Ras activation is an obligatory step in IL-1beta-mediated NO release and, presumably, the subsequent dysfunction of the pancreatic beta cell. Our data also provide a basis for future investigations to understand the mechanism of cytokine-induced beta cell death leading to the onset of insulin-dependent diabetes mellitus.

gamma-Interferon signaling in pancreatic beta-cells is persistent but can be terminated by overexpression of suppressor of cytokine signaling-1

Proinflammatory cytokines, including gamma-interferon (IFN-gamma), have been implicated in the destruction of beta-cells in autoimmune diabetes. IFN-gamma signaling is transient in some cell types, but there is indirect evidence that it may be prolonged in beta-cells. In this study, we have shown that IFN-gamma signaling, measured by signal transducer and activator of transcription-1 (STAT1) activation and the expression of IFN-gamma-responsive genes, is persistent in beta-cells for as long as the cytokine is present. Because members of the suppressor of cytokine signaling (SOCS) family may regulate the duration of IFN-gamma signaling, their expression was investigated in beta-cells. We found that cytokine-inducible SH2-containing protein, SOCS-1, and SOCS-2 are expressed in primary islets and NIT-1 insulinoma cells, both at the mRNA and protein levels, after treatment with IFN-gamma and other proinflammatory cytokines. Transfected SOCS-1 was found to inhibit responses to IFN-gamma in NIT-1 insulinoma cells, including STAT1 activation, class I major histocompatibility complex upregulation, and IFN-gamma-induced cell death, but only when expressed at levels higher than those found in untransfected cells. Consistent with this, IFN-gamma signaling was not affected in SOCS-1-deficient beta-cells. Therefore, persistent IFN-gamma signaling in beta-cells is associated with SOCS-1 expression that is not sufficient to terminate signaling. Because overexpression of SOCS-1 can suppress responses to IFN-gamma, this may be a useful strategy for protecting beta-cells from cytotoxicity mediated by IFN-gamma and possibly other proinflammatory cytokines.

Leptin and the regulation of food intake, energy homeostasis and immunity with special focus on periparturient ruminants

The biology of leptin has been studied most extensively in rodents and in humans. Leptin is involved in the regulation of food intake, energy homeostasis and immunity. Leptin is primarily produced in white adipose tissue and acts via a family of membrane bound receptors, including an isoform with a long intracellular domain (OB-Rb), and many isoforms with short intracellular domains (Ob-Rs). OB-Rb is predominantly expressed in the hypothalamic regions involved in the regulation of food intake and energy homeostasis. The other isoforms are distributed ubiquitously and are found in most peripheral tissues in far greater abundance than OB-Rb. The effects of leptin on food intake and energy homeostasis are central and are mediated via a network of orexigenic neuropeptides (neuropeptide Y, galanin, galanin-like peptide, melanin-concentrating hormone, orexins, agouti-related peptide) and anorexigenic neuropeptides (corticotropin-releasing hormone, pro-opiomelanocortin, alpha-melanocyte stimulating hormone and cocaine- and amphetamine-regulated transcript). In addition, leptin acts directly on immune cells to stimulate hematopoesis, T-cell immunity, phagocytosis, cytokine production, and to attenuate susceptibility to infectious insults. Emerging data in ruminants suggest that leptin is dynamically regulated by many factors and physiological states. Thus, leptin is secreted in a pulsatile fashion, but without a marked diurnal rhythm. A positive relationship between adiposity and plasma leptin concentration exists in growing and lactating ruminants. The concentration of plasma leptin increases during pregnancy, starts to decline 1--2 wk before parturition, and reaches a nadir in early lactation. The reduction of plasma leptin at parturition is likely to promote centrally mediated adaptations required in periods of energy deficit, but could have negative effects on immune cell function. Future research is needed in ruminants to address the roles played by leptin and the central nervous system in orchestrating metabolism during the periparturient period and during infectious diseases.

Biology of leptin in the pig

The recently discovered protein, leptin, which is secreted by fat cells in response to changes in body weight or energy, has been implicated in regulation of feed intake, energy expenditure and the neuroendocrine axis in rodents and humans. Leptin was first identified as the gene product found deficient in the obese ob/ob mouse. Administration of leptin to ob/ob mice led to improved reproduction as well as reduced feed intake and weight loss. The porcine leptin receptor has been cloned and is a member of the class 1 cytokine family of receptors. Leptin has been implicated in the regulation of immune function and the anorexia associated with disease. The leptin receptor is localized in the brain and pituitary of the pig. The leptin response to acute inflammation is uncoupled from anorexia and is differentially regulated among swine genotypes. In vitro studies demonstrated that the leptin gene is expressed by porcine preadipocytes and leptin gene expression is highly dependent on dexamethasone induced preadipocyte differentiation. Hormonally driven preadipocyte recruitment and subsequent fat cell size may regulate leptin gene expression in the pig. Expression of CCAAT-enhancer binding proteinalpha (C/EBPalpha) mediates insulin dependent preadipocyte leptin gene expression during lipid accretion. In contrast, insulin independent leptin gene expression may be maintained by C/EBPalpha auto-activation and phosphorylation/dephosphorylation. Adipogenic hormones may increase adipose tissue leptin gene expression in the fetus indirectly by inducing preadipocyte recruitment and subsequent differentiation. Central administration of leptin to pigs suppressed feed intake and stimulated growth hormone (GH) secretion. Serum leptin concentrations increased with age and estradiol-induced leptin mRNA expression in fat was age and weight dependent in prepuberal gilts. This occurred at the time of expected puberty in intact contemporaries and was associated with greater LH secretion. Further work demonstrated that leptin acts directly on pituitary cells to enhance LH and GH secretion, and brain tissue to stimulate gonadotropin releasing hormone secretion. Thus, development of nutritional schemes and (or) gene therapy to manipulate leptin secretion will lead to practical methods of controlling appetite, growth and reproduction in farm animals, thereby increasing efficiency of lean meat production.

Modulation of the leptin-induced white adipose tissue lipolysis by nitric oxide

The present study tested the hypothesis that nitric oxide (NO) is involved in the leptin-induced stimulation of lipolysis. The effect of intravenous (iv) administration of leptin (10, 100 and 1000 microg/kg body weight) or vehicle on serum NO concentrations and glycerol release from white adipocytes of Wistar rats was examined. One hour after injection, the three leptin doses tested increased serum NO concentrations 15.1%, 23.4% and 60.0%, respectively (P<.001 vs. baseline). The effect of leptin on NO concentrations was significantly dose dependent on linear trend testing (P=.0001). Simple linear regression analysis showed that the lipolytic rate measured was significantly correlated with serum NO concentrations (P=.0025; r=.52). In order to gain further insight into the potential underlying mechanisms, the effect of leptin on lipolysis was studied in the setting of nitric oxide synthase (NOS) inhibition or acute ganglionic blockade. The stimulatory effect of leptin on lipolysis was significantly decreased (P<.05) under NOS inhibition. On the contrary, the leptin-induced lipolysis was unaltered in pharmacologically induced ganglionic blockade. The lack of effect on isoproterenol-, forskolin- and dibutyryl-cyclic AMP-stimulated lipolysis suggests that leptin does not interfere with the signal transduction pathway at the beta-adrenergic receptor, the adenylate cyclase and the protein kinase A levels. These findings suggest that NO is a potential regulator of leptin-induced lipolysis.

Diabetic ketoacidosis in a case of pheochromocytoma

A 31-year-old woman was admitted to our hospital because of diabetic ketoacidosis (DKA). Ultrasound sonography revealed the existence of the left adrenal tumor and endocrinological examinations established a diagnosis of pheochromocytoma. She had been healthy and there was no evidence for gestational diabetes in her personal history. Characteristic features were not found in her tumor size and the catecholamine levels as compared with typical cases of pheochromocytoma. An overwhelming secretion of catecholamine might suppress insulin secretion, as evidenced by the improvement after the resection of the tumor. However, a significant insulin resistance continued after tumor resection. Obesity and the heterozygosity of beta3-adrenergic receptor gene (Try64Arg) might play a role in insulin resistance, which resulted in DKA at least in part. Literature survey revealed four cases of DKA in the patients with pheochromocytoma including ours, three of which were Japanese. Pancreatic capacity to secrete insulin has been reported to be less than Caucasians, which might be another reason for DKA. Thus, we speculate that both suppressed insulin secretion and insulin resistance deteriorated by obesity or other factor(s) such as abnormality in beta3 adrenergic receptor probably depress beta-cell function resulting in abnormal metabolic imbalance such as DKA.

Susceptibility of insulin-secreting hepatocytes to the toxicity of pro-inflammatory cytokines

The liver has been suggested as a suitable target organ for reversing type I diabetes by gene therapy. Whilst gene delivery systems to the hepatocyte have yet to be optimized in vivo, whether insulin-secreting hepatocytes are resistant to the autoimmune process that kills pancreatic beta-cells has never been addressed. One of the mechanisms by which beta-cells are killed in type I diabetes is by the release of the cytokines interleukin-1beta (IL-1beta), tumour necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma) by immune cells. To test the effect of the cytokines on insulin-secreting hepatocytes in vitro we exposed the betacyte, also called the HEP G2ins/g cell which possesses cytokine receptors and can synthesize, store and secrete insulin in a regulated fashion to a glucose stimulus, to the above mentioned cytokines for 14 days. Viability of the HEP G2ins/g cells was similar to that of other liver cell lines/primary cells which were more resistant to the cytokines than the beta-cell line NIT-1. The cytokines had no adverse effect for the first six days on insulin secretion, content and mRNA levels of the HEP G2ins/g cells and insulin secretion in response to 1-h exposure to 20 mM glucose was enhanced 14-fold. Our results indicate that genetically engineered hepatocytes and primary liver cells are more resistant than pancreatic beta-cells to the adverse effects of cytokines offering hope that insulin secreting hepatocytes in vivo made by gene therapy are less likely to be destroyed by cytokines released during autoimmune destruction.

Leptin increases the viability of isolated rat pancreatic islets by suppressing apoptosis

To test the hypothesis that leptin secreted from adipose tissue is a mediator linking obesity and pancreatic islet hypertrophy, we examined the effects of leptin on proliferative and apoptotic responses in rat islet cells. Rat pancreatic islets were isolated and incubated with 0, 1, 5, or 75 nM leptin for 24 h under serum-deprived conditions. Cell viability was assessed with 2,5-diphenyltetrazolium bromide and trypan blue dye exclusion tests. Cell proliferation and apoptosis were evaluated with 5-bromo-2'-deoxyuridine incorporation into DNA and DNA ladder formation, respectively. Incubation for 24 h with 1 and 5 nM leptin, the concentrations observed in obese subjects, increased the viability of isolated pancreatic islet cells. Five nanomolar concentrations of leptin did not stimulate 5-bromo-2'-deoxyuridine incorporation into incubated islet cells, indicating no influence on cell proliferation, but did inhibit DNA ladder formation, a hallmark of cell apoptosis. Moreover, 5 nM leptin reduced the triglyceride content and suppressed inducible nitric oxide synthase mRNA expression in incubated islets. These results suggest that leptin increased viable cell numbers via suppression of apoptosis in isolated pancreatic islet cells under these experimental conditions. This mechanism might account at least in part for an obesity-induced increase in pancreatic beta-cell mass.

[Leptin-induced regulation of fat metabolism and its accumulation]

Recent findings have shown that supplementation of leptin decreases body weight in leptin-deficient ob/ob mice through its suppressive effect on food intake and accelerating effect on energy expenditure, particularly on peripheral fat lipolysis. When endogenously hyperleptinemic obese rats were further induced to be hyperleptinemic exogeously using adenovirus vector, their body fat mass was reduced but not food intake. These findings implicate a direct lipolytic action of leptin on peripheral adipose tissues in obese rats because leptin transport capacity across the blood-brain barrier is almost saturated by the relative hyperleptinemia. Recovery from excessive body fat accumulation after adenovirus-induced hyperleptinemia is much slower than that after caloric restriction because there may be difference between those treatments in decreased lipogenic enzymes activities and/or increased activities of fatty acid oxidative enzymes and thermogenic uncoupling proteins. The fat melting effects of leptin may show its crucial pharmacologic potencies to design therapeutic strategies against morbid obesity. The studies on leptin provide a better understanding for creative approaches to anti-obesity drug that are efficient for reducing body fat mass without harmful side-effects.

The effect of chronic exposure to fatty acids on gene expression in clonal insulin-producing cells: studies using high density oligonucleotide microarray

Fatty acids affect insulin secretion of pancreatic beta-cells. Investigating gene expression profiles may help to characterize the underlying mechanism. INS-1 cells were cultured with palmitate (0, 50, and 200 microM) for up to 44 d. Insulin secretion and expressions of 8740 genes were studied. We found that basal insulin secretion increased in cells exposed to palmitate. The response to glucose stimulation declined on d 44 in cells cultured at 200 microM palmitate. In response to 50 and 200 microM palmitate exposure, expression was changed in 11 and 99 genes on d 2 and 134 and in 159 genes on d 44, respectively. Genes involved in fatty acid oxidation were up-regulated, whereas those involved in glycolysis were down-regulated with 200 microM palmitate. A suppression of insulin receptor and insulin receptor substate-2 gene expression was found on d 44 in cells cultured at 200 microM palmitate. In conclusion, chronic exposure to low palmitate alters insulin secretion as well as gene expression. The number of genes that changed expression was palmitate dose and exposure time dependent. Randle's fatty acid-glucose cycle seems to be operative on the gene transcription level. A modification of expression of various genes may contribute to the functional changes.