Ischaemic bowel perforation secondary to a gravid uterus in a patient with treated inflammatory bowel disease and an ileoanal pouch: a case report

The diagnosis of visceral perforation during pregnancy is often delayed and the management complex. A 32-year-old primigravid woman in her second trimester presented with abdominal pain and a pre-existing ileoanal pouch. Initial imaging was negative but later imaging was suggestive of serious pathology. At laparotomy, a caesarean section was performed. Peritonitis was encountered secondary to two discrete perforations in the small bowel separate from her pouch. Histology found an ischaemic perforation secondary to a pressure effect from the gravid uterus. In pregnancy, ileoanal pouches may make the interconnected bowel vulnerable to the pressure effect of the gravid uterus and perforation. Pregnant women with such a surgical history who develop symptoms suggestive of bowel perforation should have rapid imaging and their clinical team should consider early definitive surgical intervention.

Catalytic domains of the LAR and CD45 protein tyrosine phosphatases from Escherichia coli expression systems: purification and characterization for specificity and mechanism

The cytoplasmic domains of two human transmembrane protein tyrosine phosphatases (PTPases), LAR and CD45, have been expressed in Escherichia coli, purified to near-homogeneity, and compared for catalytic efficiency toward several phosphotyrosine-containing peptide substrates. A 615-residue LAR fragment (LAR-D1D2) containing both tandemly repeated PTPase domains shows almost identical specific activity and high catalytic efficiency as the 40-kDa single-domain LAR-D1 fragment, consistent with a single functional active site in the 70-kDa LAR-D1D2 enzyme. A 90-kDa fragment of the human leukocyte CD45 PTPase, containing two similar tandemly repeated PTPase domains, shows parallel specificity to LAR-D1 and LAR-D1D2 with a high kcat/Km value for a phosphotyrosyl undecapeptide. Sufficient purified LAR-D1 and LAR-D1D2 PTPases were available to demonstrate enzymatic exchange of 18O from 18O4 inorganic phosphate into H2(16)O at rates of approximately 1 x 10(-2) s-1. The oxygen-18 exchange probably proceeds via a phosphoenzyme intermediate. Brief incubation of all three PTPase fragments with a [32P]phosphotyrosyl peptide substrate prior to quench with SDS sample buffer and gel electrophoresis led to autoradiographic detection of 32P-labeled enzymes. Pulse/chase studies on the LAR 32P-enzyme showed turnover of the labeled phosphoryl group.

A prospective single centre study comparing computed tomography pneumocolon against colonoscopy in the detection of colorectal neoplasms

BACKGROUND AND AIMS

To determine the sensitivity and specificity of computed tomography (CT) pneumocolon in the detection of colorectal neoplasms.

METHODS

A total of 201 consecutive patients with colorectal symptoms or requiring surveillance for colorectal neoplasms underwent both conventional colonoscopy and CT pneumocolon.

RESULTS

On conventional colonoscopy 13 invasive colorectal carcinomas were detected in 13 patients, and 118 polyps in 63 patients (14 polyps were > or =1 cm in diameter, 25 were 6-9 mm, and 79 were < or =5 mm). CT pneumocolon detected all 13 cancers, two false positive cancers, but only 20 polyps (seven were > or =1 cm). This resulted in a sensitivity of 100% (95% confidence interval (CI) 87-100%) and specificity of 99% (95% CI 97-100%) for detection of invasive carcinoma, and a sensitivity of 73% (95% CI 56-90%) and specificity of 94% (95% CI 91-98%) for detection of invasive carcinoma and/or > or =1 cm polyps. CT pneumocolon also identified invasive carcinoma not seen at colonoscopy because of incomplete examination in three patients, and detected metastases in six colorectal carcinoma patients and extracolonic carcinoma in a further seven patients.

CONCLUSIONS

CT pneumocolon had a high sensitivity and specificity for detection of invasive colorectal carcinoma but not colorectal polyps. CT pneumocolon may be suitable for initial investigation of patients with symptoms of colorectal malignancy.

Transport function and regulation of mitochondrial uncoupling proteins 2 and 3

Uncoupling protein 1 (UCP1) dissipates energy and generates heat by catalyzing back-flux of protons into the mitochondrial matrix, probably by a fatty acid cycling mechanism. If the newly discovered UCP2 and UCP3 function similarly, they will enhance peripheral energy expenditure and are potential molecular targets for the treatment of obesity. We expressed UCP2 and UCP3 in Escherichia coli and reconstituted the detergent-extracted proteins into liposomes. Ion flux studies show that purified UCP2 and UCP3 behave identically to UCP1. They catalyze electrophoretic flux of protons and alkylsulfonates, and proton flux exhibits an obligatory requirement for fatty acids. Proton flux is inhibited by purine nucleotides but with much lower affinity than observed with UCP1. These findings are consistent with the hypothesis that UCP2 and UCP3 behave as uncoupling proteins in the cell.

Fatal septicaemia after fibroid embolisation

Uterine artery embolisation is a new technique for the treatment of uterine fibroids. We report a death after this procedure.

Metabolic, gastrointestinal, and CNS neuropeptide effects of brain leptin administration in the rat

To investigate whether brain leptin involves neuropeptidergic pathways influencing ingestion, metabolism, and gastrointestinal functioning, leptin (3.5 micrograms) was infused daily into the third cerebral ventricular of rats for 3 days. To distinguish between direct leptin effects and those secondary to leptin-induced anorexia, we studied vehicle-infused rats with food available ad libitum and those that were pair-fed to leptin-treated animals. Although body weight was comparably reduced (-8%) and plasma glycerol was comparably increased (142 and 17%, respectively) in leptin-treated and pair-fed animals relative to controls, increases in plasma fatty acids and ketones were only detected (132 and 234%, respectively) in pair-fed rats. Resting energy expenditure (-15%) and gastrointestinal fill (-50%) were reduced by pair-feeding relative to the ad libitum group, but they were not reduced by leptin treatment. Relative to controls, leptin increased hypothalamic mRNA for corticotropin-releasing hormone (CRH; 61%) and for proopiomelanocortin (POMC; 31%) but did not reduce mRNA for neuropeptide Y. These results suggest that CNS leptin prevents metabolic/gastrointestinal responses to caloric restriction by activating hypothalamic CRH- and POMC-containing pathways and raise the possibility that these peripheral responses to CNS leptin administration contribute to leptin's anorexigenic action.

Role of the CNS melanocortin system in the response to overfeeding

The voluntary suppression of food intake that accompanies involuntary overfeeding is an effective regulatory response to positive energy balance. Because the pro-opiomelanocortin (POMC)-derived melanocortin system in the hypothalamus promotes anorexia and weight loss and is an important mediator of energy regulation, we hypothesized that it may contribute to the hypophagic response to overfeeding. Two groups of rats were overfed to 105 and 116% of control body weight via a gastric catheter. In the first group, in situ hybridization was used to measure POMC gene expression in the rostral arcuate (ARC). Overfeeding increased POMC mRNA in the ARC by 180% relative to levels in control rats. For rats in the second group, the overfeeding was stopped, and they were infused intracerebroventricularly with SHU9119 (SHU), a melanocortin (MC) antagonist at the MC3 and MC4 receptor, or vehicle. Although SHU (0.1 nmol) had no effect on food intake of control rats, intake of overfed rats increased by 265% relative to CSF-treated controls. This complete reversal of regulatory hypophagia not only maintained but actually increased the already elevated weight of overfed rats, whereas CSF-treated overfed rats lost weight. These results indicate that CNS MCs mediate hypophagic signaling in response to involuntary overfeeding and support the hypothesis that MCs are important in the central control of energy homeostasis.

The mahogany protein is a receptor involved in suppression of obesity

Genetic studies have shown that mutations within the mahogany locus suppress the pleiotropic phenotypes, including obesity, of the agouti-lethal-yellow mutant. Here we identify the mahogany gene and its product; this study, to our knowledge, represents the first positional cloning of a suppressor gene in the mouse. Expression of the mahogany gene is broad; however, in situ hybridization analysis emphasizes the importance of its expression in the ventromedial hypothalamic nucleus, a region that is intimately involved in the regulation of body weight and feeding. We present new genetic studies that indicate that the mahogany locus does not suppress the obese phenotype of the melanocortin-4-receptor null allele or those of the monogenic obese models (Lep(db), tub and Cpe(fat)). However, mahogany can suppress diet-induced obesity, the mechanism of which is likely to have implications for therapeutic intervention in common human obesity. The amino-acid sequence of the mahogany protein suggests that it is a large, single-transmembrane-domain receptor-like molecule, with a short cytoplasmic tail containing a site that is conserved between Caenorhabditis elegans and mammals. We propose two potential, alternative modes of action for mahogany: one draws parallels with the mechanism of action of low-affinity proteoglycan receptors such as fibroblast growth factor and transforming growth factor-beta, and the other suggests that mahogany itself is a signalling receptor.

Leptin receptor long-form splice-variant protein expression in neuron cell bodies of the brain and co-localization with neuropeptide Y mRNA in the arcuate nucleus

Reduced leptin (Ob protein) signaling is proposed to be a stimulus for the activation of neuropeptide Y (NPY) gene activity and increased expression of mRNA for the long form of the leptin receptor (Ob-Rb) in the hypothalamic arcuate nucleus. To determine if Ob-Rb protein is expressed in arcuate nucleus NPY neurons, we developed an affinity-purified polyclonal antibody against amino acids 956-1102 of human Ob-Rb. This antibody specifically recognizes the cytoplasmic tail of Ob-Rb and does not react with shorter leptin-receptor variants. Western immunoblots of Ob-Rb-transfected COS cells showed a single 150-kD band, and immunofluorescence revealed intense perinuclear staining in the cytoplasm. A 150-kD band was also present in Western immunoblots of hypothalamus. Immunocytochemical staining of brain slices revealed immunoreactive Ob-Rb protein concentrated in many neuronal cell bodies in the same regions of the forebrain that also express Ob-Rb mRNA. In the hypothalamus, Ob-Rb-positive cell bodies were abundant in the arcuate nucleus and ventromedial nucleus, with lesser numbers in the dorsomedial nucleus and paraventricular nucleus. Immunostaining was also detected in cell bodies of pyramidal cell neurons of the pyriform cortex and cerebral cortex, in neurons of the thalamus, and on the surface of ependymal cells lining the third ventricle. The choroid plexus, which expresses the short Ob-Ra form, was negative. Combined immunocytochemistry for Ob-Rb protein and fluorescence in situ hybridization for NPY mRNA identified arcuate nucleus neurons containing both NPY mRNA and Ob-Rb protein. The present finding of Ob-Rb protein in neurons that express NPY mRNA supports the hypothesis that arcuate nucleus NPY neurons are direct targets of leptin and play an important role in regulation of food intake and body weight.

Identification and functional analysis of novel human melanocortin-4 receptor variants

Inactivation of the melanocortin-4 receptor (MC4-R) by gene-targeting results in mice that develop maturity-onset obesity, hyperinsulinemia, and hyperglycemia. These phenotypes resemble common forms of human obesity, which are late-onset and frequently accompanied by NIDDM. It is not clear whether sequence variation of the MC4-R gene contributes to obesity in humans. Therefore, we examined the human MC4-R gene polymorphism in 190 individuals ascertained on obesity status. Three allelic variants were identified, including two novel ones, Thr112Met and Ile137Thr. To analyze possible functional alterations, the variants were cloned and expressed in vitro and compared with the wild-type receptor. One of the novel variants, Ile137Thr, identified in an extremely obese proband (BMI 57), was found to be severely impaired in ligand binding and signaling, raising the possibility that it may contribute to development of obesity. Furthermore, our results also suggest that sequence polymorphism in the MC4-R coding region is unlikely to be a common cause of obesity in the population studied, given the low frequency of functionally significant mutations.

Melanocortin-4 receptor: a novel signalling pathway involved in body weight regulation

For many years, genetically obese mouse strains have provided models for human obesity. The Avy/-agouti mouse, one of the oldest obese mouse models, is characterized by maturity-onset obesity and diabetes as a result of ectopic expression of the secreted protein hormone, agouti protein. Agouti protein is normally expressed in hair follicles to regulate pigmentation through antagonism of the melanocortin-1 receptor, but in-vitro studies have demonstrated that the hormone also has potent antagonist activity for the melanocortin-4 receptor (MC4-R). Subsequent development of the MC4-R knockout mouse model demonstrated that MC4-R plays a role in weight homeostasis as these mice recapitulated the metabolic defects of the agouti mouse. Further evidence for this hypothesis was obtained from pharmacological studies utilizing peptides with MC4-R agonist activity, that inhibited food intake (when administered intracerebrally). Additional studies with peptide antagonists have now implicated the MC4-R in the leptin signalling pathway. Finally, evidence that the MC4-R may play a role in human obesity has been obtained from the identification of a dis-functional variant of the receptor in genetically obese subjects.

Embolization of uterine fibroids

14 patients have undergone uterine artery embolization of uterine fibroids. No complications related to the procedure have occurred. In all six of the cases with available data, there has been a reduction in fibroid volume as measured with MRI at 2 months post procedure (average reduction 43%). Technical aspects of this promising procedure are described.

Response of melanocortin-4 receptor-deficient mice to anorectic and orexigenic peptides

Mutations reducing the functional activity of leptin, the leptin receptor, alpha-melanocyte stimulating hormones (alpha-MSH) and the melanocortin-4 receptor (Mc4r) all lead to obesity in mammals. Moreover, mutant mice that ectopically express either agouti (Ay/a mice) or agouti-related protein (Agrp), antagonists of melanocortin signalling, become obese. These data suggest that alpha-MSH signalling transduced by Mc4r tonically inhibits feeding; however, it is not known to what extent this pathway mediates leptin signalling. We show here that Mc4r-deficient (Mc4r-/-) mice do not respond to the anorectic actions of MTII, an MSH-like agonist, suggesting that alpha-MSH inhibits feeding primarily by activating Mc4r. Obese Mc4r-/-mice do not respond significantly to the inhibitory effects of leptin on feeding, whereas non-obese Mc4r-/- mice do. These data demonstrate that melanocortin signalling transduced by Mc4r is not an exclusive target of leptin action and that factors resulting from obesity contribute to leptin resistance. Leptin resistance of obese Mc4r-/- mice does not prevent their response to the anorectic actions of ciliary neurotrophic factor (CNTF), corticotropin releasing factor (CRF), or urocortin; or the orexigenic actions of neuropeptide Y (NPY) or peptide YY (PYY), indicating that these neuromodulators act independently or downstream of Mc4r signalling.

Melanocortin and leptin signaling systems: central regulation of catabolic energy balance

The recent cloning of the ob gene (leptin) has revolutionized our understanding of obesity and the underlying factors that govern weight homeostasis. There is growing evidence that long term food intake regulation is controlled by the central nervous system by a number of peptide hormones in response to changes in leptin levels. Studies of these hormones, using both genetic and pharmacological approaches, have provided a foundation for decoding the molecular logic of the neuronal circuits which regulate food intake control and energy balance. A review of the current progress in the melanocortin-4 receptor pathway, with particular emphasis on its relation to leptin, neuropeptide Y and other obesity hormones known to modulate weight homeostasis, is presented.

Splice variants of the OB receptor gene are differentially expressed in brain and peripheral tissues of mice

A high affinity receptor for OB protein was recently cloned from the choroid plexus of mice. At least six alternatively spliced forms of the OB receptor (OB-R) gene have been described, all of which encode proteins containing the OB-R extracellular domain. One splice variant encodes a receptor with a long intracellular domain, OB-RL, that has been implicated in OB-R signaling. Here, we have used in situ hybridization to examine the localization of OB-R splice variants in brain and peripheral tissues of adult and newborn mice. Using a probe hybridizing with all known splice variants, we confirmed that OB-R mRNA was widely distributed in the adult tissues. In the CNS, choroid plexus was the major site of expression. We now demonstrate that OB-R mRNA is expressed in peripheral tissues; primarily associated with connective tissues. In addition, OB-R mRNA was detected at higher levels in peripheral tissues of newborn mice than in adult mice. With a probe specific for OB-RL, we confirmed that high mRNA expression was detected in hypothalamic nuclei, while low levels were observed in choroid plexus. We now report that in peripheral tissues of adult mice, OB-RL mRNA expression was either very low or undetectable. In newborn mice, the pattern of OB-RL message expression in the CNS was similar to that of adult mice, while bone was the site of highest OB-RL message expression in the peripheral tissue. These data suggest different biological roles for OB-R splice variants encoding the short and long forms of OB-R. The localization of OB-RL to hypothalamic nuclei supports the idea that OB-RL is the brain receptor that mediates OB protein signaling and actions. In addition, the expression of OB-R message in newborn mice also suggests a biological role of OB-R during development in mice.

Brain administration of OB protein (leptin) inhibits neuropeptide-Y-induced feeding in ob/ob mice

OB protein (or leptin) administration causes a long-lasting reduction in food intake and body weight in obese ob/ob mice. Neuropeptide Y, a stimulator of feeding, has been proposed to be a major mediator of the biological actions of OB protein. To test this hypothesis, the interaction of brain administration of exogenous OB protein and NPY on the feeding behavior of ob/ob mice was examined. Human OB protein, in a dose-dependent manner, partially or completely blocked feeding induced by exogenous NPY. These results demonstrate that OB protein can functionally antagonize and dominate the actions of exogenous NPY on feeding.

Strategies and potential molecular targets for obesity treatment

Obesity is an increasingly prevalent and important health problem. Although treatment is available, the long-term maintenance of medically significant weight loss (5 to 10 percent of initial body weight) is rare. Since 1995 there has been an explosion of research focused on the regulation of energy balance and fat mass. Characterization of obesity-associated gene products has revealed new biochemical pathways and molecular targets for pharmacological intervention that will likely lead to new treatments. Ideally, these treatments will be viewed as adjuncts to behavioral and lifestyle changes aimed at maintenance of weight loss and improved health.

Modular folding and evidence for phosphorylation-induced stabilization of an hsp90-dependent kinase

The de novo folding of the individual domains of the src family kinase p56(lck) was examined within the context of full-length p56(lck) molecules produced in rabbit reticulocyte lysate containing active chaperone machinery. The catalytic domain required geldanamycin-inhibitable heat shock protein 90 (hsp90) function to achieve its active protease-resistant conformation, but the src homology 2 (SH2) domain acquired phosphopeptide-binding competence independently of hsp90 function. The SH2 domain of hsp90-bound p56(lck) was folded and functional. In addition to the facilitation by hsp90 of kinase biogenesis, a conditional role in maintenance folding could be demonstrated; although wild type p56(lck) molecules with a negative-regulatory C-terminal tyrosine matured to a nearly hsp90-independent state, p56(lck) molecules with a mutated C-terminal tyrosine continued to require hsp90-mediated maintenance. De novo folding could be distinguished from maintenance folding on the basis of proteolytic fingerprints and the effects of different temperatures on folding behavior. Results indicate that during p56(lck) biogenesis, the SH2 domain rapidly folds independently of hsp90 function, followed by the slower hsp90-dependent folding of the catalytic domain and suggest the final stabilization of p56(lck) structure by phosphorylation-mediated interdomain interactions.

Csk-mediated phosphorylation of substrates is regulated by substrate tyrosine phosphorylation

Csk is a cellular protein tyrosine kinase (PTK) that has been shown to specifically regulate the activity of Src kinase family members by phosphorylation of a carboxy-terminal tyrosine residue. The molecular mechanisms controlling Csk regulation and its substrate specificity have not been elucidated. Here we report a novel type of overlay kinase assay that allows to probe for Csk-mediated phosphorylation of cellular substrates separated by polyacrylamide gel electrophoresis and transferred to nitrocellulose filters. Most of the cell lines analyzed with this method revealed only a few potential Csk substrates. However, an increased number of Csk substrates was detected in NIH3T3 cells expressing a constitutively activated form of the Src kinase Lck or in PC12 and NIH3T3 cells that had been treated with pervanadate. These cells all display an increased level of cellular protein tyrosine phosphorylation which led to the conclusion that Csk preferentially phosphorylates tyrosine-phosphorylated proteins. To verify this hypothesis we analyzed Csk-mediated phosphorylation of recombinant Lck, a known Csk substrate. Results demonstrated that autophosphorylation of Lck (at Tyr394) facilitates Csk-mediated phosphorylation of Lck at its regulatory site (Tyr505). Subsequent peptide binding studies revealed that Csk can bind to a peptide corresponding to the Lck-autophosphorylation site only when it is phosphorylated. These findings suggest that autophosphorylation of Lck at Tyr394 triggers an interaction with Csk and thereby facilitates subsequent phosphorylation and inactivation of Lck. The phosphorylation of other cellular Csk substrates may be regulated by a similar mechanism.

Obesity genes: molecular genetic approaches to drug target identification

The environment for developing novel therapeutic agents has undergone dramatic change over the past decade. Innovative strategies for identifying and utilizing molecular targets linked to particular human diseases are replacing the classic approach of screening chemical compounds for potential therapeutic action on unknown targets. Since genetic components are involved in many known diseases, mouse and human genetics, positional cloning and other molecular biology-based approaches are now used to identify genes that are associated with these diseases. It is thought that identification of these disease-linked genes may lead to the discovery and understanding of the physiologically relevant biochemical pathways underlying the disease processes. Clearly, a knowledge of these biochemical pathways will provide future molecular targets, enzymes or receptors, that will offer opportunities to apply modern methods of high throughput screening, medicinal chemistry, parallel synthesis and combinatorial chemistry for drug discovery. In this manuscript, we illustrate how mouse genetics and molecular biology-based approaches have led to the identification of all five known single gene mutations that cause obesity in mice. Additionally, we describe how identification of these genes has helped unravel underlying biochemical pathways that regulate behavioral, metabolic and neuroendocrine responses in rodents.

Efficient secretion of biologically active recombinant OB protein (leptin) in Escherichia coli, purification from the periplasm and characterization

The genes encoding the mature forms of mouse (mOB) and human OB (hOB) protein (also called leptin) were fused to the secretion signal coding sequence of the Escherichia coli outer membrane protein A (sOMP A). The hybrid genes were preceded by a ribosome binding site (RBS) and were expressed under transcriptional control of both the lipoprotein promoter (Plpp) and the lac promoter-operator (POlac). The recombinant fusion proteins were efficiently expressed and exported into the periplasmic compartment of E. coli cells from where they were recovered by osmotic shock as soluble mature polypeptides with the sOMP A precisely removed. Recombinant mOB and hOB proteins were also produced in Sf9 insect cells using the baculovirus expression system. Milligram quantities of both proteins were purified to homogeneity using ion-exchange, hydrophobic interaction chromatography and gel filtration and were found to be biologically active and to have antiobesity effects upon testing in genetically obese ob/ob mice.

Central infusion of melanocortin agonist MTII in rats: assessment of c-Fos expression and taste aversion

Like leptin (OB protein), central infusion of the nonspecific melanocortin agonist MTII reduces food intake for relatively long periods of time (i.e., 12 h; W. Fan, B. A. Boston, R. A. Kesterson, V. J. Hruby, and R. D. Cone, Nature; 385: 165-168, 1997). To test the hypothesis that MTII may influence ingestive behavior via mechanisms similar to those that mediate the effects of leptin, we infused a single dose of MTII into the third ventricle (i3vt) of Long-Evans rats and examined three dependent measures that have been studied following i3vt infusion of leptin: 1) effects on long-term food intake and body weight (48 h), 2) patterns of c-Fos expression in the brain, and 3) conditioned taste aversion learning. Similar to leptin, MTII reduced 48-h food intake (1.0 nmol dose), reduced body weight at 24 and 48 h (0.1 and 1.0 nmol doses, respectively), and induced c-Fos expression in the paraventricular nucleus of the hypothalamus and the central nucleus of the amygdala. In contrast to leptin, MTII was found to produce conditioned taste aversions. These results are consistent with the hypothesis that MTII may influence regulatory behavior via mechanisms similar to those that mediate the effects of leptin.

Leptin increases hypothalamic pro-opiomelanocortin mRNA expression in the rostral arcuate nucleus

Melanocortins are peptides, cleaved from the pro-opiomelanocortin (POMC) precursor, that act in the brain to reduce food intake and are potential mediators of leptin action. In the forebrain, melanocortins are derived from POMC-containing neurons of the hypothalamic arcuate nucleus. To test the hypothesis that these POMC neurons are regulated by leptin, we used in situ hybridization to determine whether reduced leptin signaling (as occurs in fasting), genetic leptin deficiency (in obese ob/ob mice), or genetic leptin resistance (in obese db/db mice) lower expression of POMC mRNA. We further hypothesized that leptin administration would raise hypothalamic POMC mRNA levels in leptin-deficient animals, but not in mice with defective leptin receptors. In wild-type mice (n = 12), fasting for 48 h lowered POMC mRNA levels in the rostral arcuate nucleus by 53%, relative to values in fed controls (n = 8; P < 0.001). Similarly, arcuate nucleus POMC mRNA levels were reduced by 46 and 70% in genetically obese ob/ob (n = 6) and db/db mice (n = 6), respectively, as compared with wild-type mice (n = 5) (P < 0.01 for both comparisons). Five daily intraperitoneal injections of recombinant murine leptin (150 microg) raised levels of POMC mRNA in the rostral arcuate nucleus of ob/ob mice (n = 8) by 73% over saline-treated ob/ob control values (n = 8; P < 0.01), but was without effect in db/db mice (n = 6). In normal rats, two injections of a low dose of leptin (3.5 microg) into the third cerebral ventricle (n = 15) during a 40-h period of fasting also increased POMC mRNA levels in the rostral arcuate nucleus to values 39% greater than those in vehicle-treated controls (n = 14; P = 0.02). We conclude that reduced central nervous system leptin signaling owing to fasting or to genetic defects in leptin or its receptor lower POMC mRNA levels in the rostral arcuate nucleus. The finding that leptin reverses this effect in ob/ob, but not db/db, mice suggests that leptin stimulates arcuate nucleus POMC gene expression via a pathway involving leptin receptors. These findings support the hypothesis that leptin signaling in the brain involves activation of the hypothalamic melanocortin system.

Central leptin stimulates corticosterone secretion at the onset of the dark phase

Leptin, a hormone secreted by adipose tissue in proportion to body adiposity, is proposed to be involved in the central nervous regulation of food intake and body weight. In addition, evidence is emerging that leptin regulates neuroendocrine and metabolic functions as well, presumably via its action in the central nervous system (CNS). To investigate this regulatory effect of leptin, we infused 3.5 microg of human leptin directly into the third cerebral ventricle (i3vt) of lean male Long-Evans rats, 90 min before the onset of their dark phase. Before and after infusion, blood samples were withdrawn through indwelling catheters for assessment of hormonal (plasma corticosterone, insulin, leptin), autonomic (plasma norepinephrine, epinephrine), and metabolic (plasma glucose) parameters. I3vt leptin caused an increase in plasma corticosterone and plasma leptin levels relative to the control condition. The effects of i3vt leptin on corticosterone secretion became particularly apparent after the onset of the dark phase. The results of the present study indicate that i3vt leptin stimulates the hypothalamo-pituitary-adrenal (HPA) axis, particularly when rats normally encounter their largest meals. These results are consistent with the possibility that high circulating leptin levels may underlie the increased activity of the HPA axis that is generally characteristic of human obesity and most animal models of obesity.

Co-localization of Fyn with CD3 complex, CD45 or CD28 depends on different mechanisms

The Src family protein tyrosine kinase Fyn (p59fyn) plays an important role in thymocyte development and T cell receptor (TCR) signal transduction. Fyn has been shown to associate with the TCR-CD3 complex, the protein tyrosine phosphatase CD45 and several co-receptors such as CD28 which are crucial for initiating T cell activation and proliferation. The molecular basis of how Fyn is associated with these transmembrane proteins is largely unknown. To investigate the Fyn association with the TCR-CD3 complex, CD45 and CD28 at the molecular level, various Fyn/beta-galactosidase fusion proteins were constructed and expressed in Jurkat cells. Co-localization experiments applying antibody-induced co-capping and double immunofluorescence staining techniques were used to study the association of these fusion proteins with the TCR-CD3 complex, CD45 and CD28. Our results revealed that co-localization of Fyn with the TCR-CD3 complex requires the unique N terminus whereas co-localization with CD45 depends on the unique N terminus, the Src homology (SH)3- and a functional SH2 domain. CD28 co-localizes with Fyn molecules that contain the N terminus and a functional SH2 domain. These results suggest that Fyn association with the TCR-CD3 complex, CD45 and CD28 is mediated by different molecular mechanisms.

Protein-tyrosine phosphatases PTP1B and syp are modulators of insulin-stimulated translocation of GLUT4 in transfected rat adipose cells

The protein-tyrosine phosphatases PTP1B and Syp have both been implicated as modulators of the mitogenic actions of insulin. However, the roles of these protein-tyrosine phosphatases in the metabolic actions of insulin are not well characterized. In this study, we directly assessed the ability of PTP1B and Syp to modulate insulin-stimulated translocation of the insulin-responsive glucose transporter GLUT4 in a physiologically relevant insulin target cell. Primary cultures of rat adipose cells were transiently transfected with either wild-type PTP1B (PTP1B-WT), wild-type Syp (Syp-WT), or the catalytically inactive mutants PTP1B-C/S or Syp-C/S. The effects of overexpression of these constructs on insulin-stimulated translocation of a co-transfected epitope-tagged GLUT4 were studied. Cells overexpressing either PTP1B-C/S or Syp-WT had insulin dose-response curves similar to those obtained with control cells expressing only epitope-tagged GLUT4. In contrast, for cells overexpressing PTP1B-WT the level of GLUT4 on the cell surface at each insulin dose (ranging from 0 to 60 nM) was significantly lower than that observed in the control cells. Interestingly, cells overexpressing the dominant inhibitory mutant Syp-C/S also had a small but statistically significant impairment in insulin responsiveness. At a maximally stimulating concentration of insulin (60 nM), cell surface epitope-tagged GLUT4 was approximately 20% less than that of the control cells. It is possible that effects from high level overexpression of Syp and PTP1B constructs may not reflect what occurs under physiological conditions. Nevertheless, our data raise the possibility that PTP1B may be a negative regulator of insulin-stimulated glucose transport, while Syp may have a small role as a positive mediator of the metabolic actions of insulin.

Central infusion of GLP-1, but not leptin, produces conditioned taste aversions in rats

Leptin (ob protein) and glucagon-like peptide-1-(7-36) amide (GLP-1) are peptides recently proposed to be involved in the regulation of food intake. Although the ability of exogenous leptin and GLP-1 to modulate consummatory behavior is consistent with the suggestion that these peptides are endogenous regulatory agents, central administration of these peptides may have aversive side effects, which could explain the anorexia. In the present experiment, exposure to a saccharine taste was immediately followed by central administration of leptin or GLP-1 to determine if these drugs could produce a conditioned taste aversion (CTA) in rats. At doses equated for producing comparable reductions in short-term food intake, GLP-1, but not leptin, generated a robust CTA. Although leptin caused no aversion, this peptide was the only drug to cause relatively long-term reductions in food consumption (16 h) and body weight (24 h). Hence, the results indicate that central GLP-1 produces aversive side effects, and it is argued that these nonspecific effects may explain the anorectic actions of GLP-1.

Targeted disruption of the melanocortin-4 receptor results in obesity in mice

The melanocortin-4 receptor (MC4-R) is a G protein-coupled, seven-transmembrane receptor expressed in the brain. Inactivation of this receptor by gene targeting results in mice that develop a maturity onset obesity syndrome associated with hyperphagia, hyperinsulinemia, and hyperglycemia. This syndrome recapitulates several of the characteristic features of the agouti obesity syndrome, which results from ectopic expression of agouti protein, a pigmentation factor normally expressed in the skin. Our data identify a novel signaling pathway in the mouse for body weight regulation and support a model in which the primary mechanism by which agouti induces obesity is chronic antagonism of the MC4-R.

The OB protein (leptin) pathway--a link between adipose tissue mass and central neural networks

OB protein (also known as leptin), a previously unknown protein signal, is secreted from adipose tissue, circulates in the blood, probably bound to a family of binding proteins, and acts on central neural networks that regulate ingestive behavior and energy balance. OB protein provides a communication link from fat tissue and the brain. Rapidly accumulating evidence suggests that OB protein appears to play a major role in the control of body fat stores through coordinated regulation of feeding behavior, metabolism, autonomic nervous system and body energy balance in rodents, primates and humans. The field has rapidly moved from cloning of the ob gene to demonstration of complex regulation of ob gene expression in adipose tissue in rats and humans, and then the demonstration of potent biological activity of OB protein in ob/ob, diet-induced, and lean mice as well as obese and lean rats but not in db/db obese mice. A significant milestone was our demonstration that central administration of OB protein lead to reductions in food intake, body weight and alterations in metabolism consistent with activation of the autonomic nervous system. These findings were followed by the identification of a central binding site for labelled OB protein in the choroid plexus in ob/ob, db/db and lean mice as well as lean and obese Zucker rats. The expression cloning of a central receptor, OB-R, from the mouse choroid plexus soon followed. The OB-R receptor was found to be expressed in the choroid plexus, the hypothalamus as well as several peripheral tissues. OB-R exists in multiple forms; the two major forms are a short form (with a truncated intracellular domain) and long form (with the complete intracellular domain). The long form is thought to be the form that signals and mediates the biological effects of OB protein. Initial in situ hybridization studies have demonstrated the mRNA for the long form OB-R receptor to be localized to the hypothalamus as well as peripheral sites. Recently, it was demonstrated that the db gene encodes the OB-R receptor. Evidence has been provided for a specific transport system for OB protein to cross the blood-brain-barrier and enter the brain of mice, rats and humans. The rate of transport can be decreased by high plasma concentrations of OB protein. Thus, reduced entry of OB protein to the brain may be one of the mechanisms of reduced sensitivity of the OB protein pathway in obese individuals. OB protein appears to also play a role in the important neuroendocrine adaptive responses to fasting and in the control of reproduction. Therapeutic approaches to the treatment of obesity based on OB protein ranging from OB protein by injection to OB-R receptor agonists and to upregulation of OB signalling pathways are under intense investigation.

Intraventricular leptin reduces food intake and body weight of lean rats but not obese Zucker rats

The protein encoded by the obese (ob) gene, leptin, is secreted from adipose tissue and is proposed to act in the brain as an important regulator of food intake and body weight. To investigate the direct effects of leptin within the CNS, we injected 3.5 microg of either mouse or human leptin into the third ventricle (ICV) of lean Long-Evans rats or obese (fa/fa) Zucker rats, in which obesity results from a mutation in the leptin receptor gene. ICV administration of leptin reduced 4-h food intake in both deprived and non-deprived lean rats. In addition, repeated ICV administration produced a long-lasting reduction in body weight while peripheral administration of the same dose had no effect. ICV administration of the same dose of leptin into the third ventricle of obese Zucker rats did not reduce food intake. These results are consistent with the hypothesis that leptin has direct actions in the CNS as an afferent signal related to the state of energy stores in adipose tissue. Furthermore, insensitivity to these central effects of leptin may be an important determinant of obesity.

The inositol 5'-phosphatase SHIP binds to immunoreceptor signaling motifs and responds to high affinity IgE receptor aggregation

Immunoreceptors such as the high affinity IgE receptor, FcepsilonRI, and T-cell receptor-associated proteins share a common motif, the immunoreceptor tyrosine-based activation motif (ITAM). We used the yeast tribrid system to identify downstream effectors of the phosphorylated FcepsilonRI ITAM-containing subunits beta and gamma. One novel cDNA was isolated that encodes a protein that is phosphorylated on tyrosine, contains a Src-homology 2 (SH2) domain, inositolpolyphosphate 5-phosphatase activity, three NXXY motifs, several proline-rich regions, and is called SHIP. Mutation of the conserved tyrosine or leucine residues within the FcepsilonRI beta or gamma ITAMs eliminates SHIP binding and indicates that the SHIP-ITAM interaction is specific. SHIP also binds to ITAMs from the CD3 complex and T cell receptor zeta chain in vitro. SHIP protein possesses both phosphatidylinositol-3,4,5-trisphosphate 5'-phosphatase and inositol-1,3,4,5-tetrakisphosphate 5'-phosphatase activity. Phosphorylation of SHIP by a protein-tyrosine kinase, Lck, results in a reduction in enzyme activity. FcepsilonRI activation induces the association of several tyrosine phosphoproteins with SHIP. SHIP is constitutively tyrosine-phosphorylated and associated with Shc and Grb2. These data suggest that SHIP may serve as a multifunctional linker protein in receptor activation.

Hsp90-mediated folding of the lymphoid cell kinase p56lck

Several lines of evidence suggest that members of the 90-kDa family of heat shock proteins (hsp90) may support the folding of various homologues of the src kinase family. In this work, we utilized pulse-chase analyses in rabbit reticulocyte lysate to demonstrate that hsp90-bound intermediates existed for the majority of newly synthesized p56lck molecules. The hsp90-binding drug geldanamycin disrupted the association of p56lck with hsp90, prevented the kinase from demonstrating a protease-resistant conformation, and caused decreases in kinase specific activity. Requirements for geldanamycin-inhibitable hsp90 function and physical interactions between hsp90 and p56lck persisted during chase periods. Consistent with the effects observed in rabbit reticulocyte lysate, application of geldanamycin to fibroblasts caused specific reversion of lck-mediated transformation concomitant with loss of p56lck activity and protein. However, geldanamycin had no direct effect on purified p56lck. Also consistent with functional linkages between hsp90 and p56lck, physical interactions between these proteins were detected in cytoplasmic, but not membrane, fractions of LSTRA cells. Although hsp90 functions in both the initial de novo folding and the reiterative support of p56lck structure in rabbit reticulocyte lysate, the specific occurrence of complexes between hsp90 and p56lck in the cytoplasm of T cells suggests that hsp90 primarily folds nascent molecules of p56lck in vivo.

Central infusions of leptin and GLP-1-(7-36) amide differentially stimulate c-FLI in the rat brain

Recently, glucagon-like peptide-1-(7-36) amide (GLP-1) and leptin have been implicated in the regulation of food intake. In the present study, we compared the effects of third ventricular administration (i3vt) of leptin (3.5 micrograms) and GLP-1 (10.0 micrograms) on short-term food intake and c-Fos-like immunoreactivity (c-FLI) in hypothalamic, limbic, and hindbrain areas in the rat. Relative to controls, infusion of leptin or GLP-1 (3 h before lights off) significantly reduced food intake over the first 2 h in the dark phase (53 and 63%, respectively). In different rats, infusion of leptin or GLP-1 elevated c-FLI in the paraventricular hypothalamus and central amygdala. Furthermore, leptin selectively elevated c-FLI in the dorsomedial hypothalamus, whereas GLP-1 selectively elevated c-FLI in the nucleus of the solitary tract, area postrema, lateral parabrachial nucleus, and arcuate hypothalamic nucleus. The fact that most of the c-FLI after leptin or GLP-1 administration was observed in separate regions within the central nervous system (CNS) suggests different roles for leptin and GLP-1 in the CNS regulation of food intake and body weight.

Differential and multiple binding of signal transducing molecules to the ITAMs of the TCR-zeta chain

A biotin-streptavidin-based technique was developed for high affinity, unidirectional, and specific immobilization of synthetic peptides to a solid phase. Biotinylated 23-mer carboxamide peptides corresponding to the three immunoreceptor tyrosine-based activation motifs (ITAMs) of the T cell antigen receptor associated zeta-chain (TCR-zeta) in their bis-, mono-, or unphosphorylated forms were used to study the binding of cellular proteins from human Jurkat T cells to these signal transduction motifs. The protein tyrosine kinase ZAP-70 bound specifically to all bisphosphorylated peptides but not to the mono- or unphosphorylated peptides. In contrast, Shc, phosphatidylinositol 3-kinase (Pl3K), Grb2, and Ras-GTPase activating protein (GAP) bound with different affinities to the bis- or monophosphorylated peptides, while the Src family protein tyrosine kinase (PTK) Fyn did not bind specifically to any of the tested peptides. The different preferences of the studied signaling molecules for distinct ITAMs, and in particular the binding of some of them preferentially to monophosphorylated peptides, suggests that the TCR-zeta may bind multiple signaling molecules with each ITAM binding a unique set of such molecules. In addition, partial phosphorylation of the ITAMs may result in recruitment of different proteins compared to double phosphorylation. This may be crucial for coupling of the TCR to various effector functions under different conditions of receptor triggering.

Identification of targets of leptin action in rat hypothalamus

The hypothesis that leptin (OB protein) acts in the hypothalamus to reduce food intake and body weight is based primarily on evidence from leptin-deficient, ob/ob mice. To investigate whether leptin exerts similar effects in normal animals, we administered leptin intracerebroventricularly (icv) to Long-Evans rats. Leptin administration (3.5 microg icv) at the onset of nocturnal feeding reduced food intake by 50% at 1 h and by 42% at 4 h, as compared with vehicle-treated controls (both P < 0.05). To investigate the basis for this effect, we used in situ hybridization (ISH) to determine whether leptin alters expression of hypothalamic neuropeptides involved in energy homeostasis. Two injections of leptin (3.5 microg icv) during a 40 h fast significantly decreased levels of mRNA for neuropeptide Y (NPY, which stimulates food intake) in the arcuate nucleus (-24%) and increased levels of mRNA for corticotrophin releasing hormone (CRH, an inhibitor of food intake) in the paraventricular nucleus (by 38%) (both P < 0.05 vs. vehicle-treated controls). To investigate the anatomic basis for these effects, we measured leptin receptor gene expression in rat brain by ISH using a probe complementary to mRNA for all leptin receptor splice variants. Leptin receptor mRNA was densely concentrated in the arcuate nucleus, with lower levels present in the ventromedial and dorsomedial hypothalamic nuclei and other brain areas involved in energy balance. These findings suggest that leptin action in rat hypothalamus involves altered expression of key neuropeptide genes, and implicate leptin in the hypothalamic response to fasting.

OB protein binds specifically to the choroid plexus of mice and rats

Binding studies were conducted to identify the anatomical location of brain target sites for OB protein, the ob gene product. 125I-labeled recombinant mouse OB protein or alkaline phosphatase-OB fusion proteins were used for in vitro and in vivo binding studies. Coronal brain sections or fresh tissue from lean, obese ob/ob, and obese db/db mice as well as lean and obese Zucker rats were probed to identify potential central OB protein-binding sites. We report here that recombinant OB protein binds specifically to the choroid plexus. The binding of OB protein (either radiolabeled or the alkaline phosphatase-OB fusion protein) and its displacement by unlabeled OB protein was similar in lean, obese ob/ob, and obese db/db mice as well as lean and obese Zucker rats. These findings suggest that OB protein binds with high affinity to a specific receptor in the choroid plexus. After binding to the choroid plexus receptor, OB protein may then be transported across the blood-brain barrier into the cerebrospinal fluid. Alternatively, binding of OB protein to a specific receptor in the choroid plexus may activate afferent neural inputs to the neural network that regulates feeding behavior and energy balance or may result in the clearance or degradation of OB protein. The identification of the choroid plexus as a brain binding site for OB protein will provide the basis for the construction of expression libraries and facilitate the rapid cloning of the choroid plexus OB receptor.

Towards unraveling the complexity of T cell signal transduction

Activation of resting T lymphocytes through the T cell antigen receptor complex is initiated by critical phosphorylation and dephosphorylation events that regulate the function and interaction of a number of signaling molecules. Key elements in these reactions are members of the Src, Syk and Csk families of protein tyrosine kinases (PTKs) and the phosphotyrosine phosphatases (PTPases) that regulate and/or counteract them, such as CD45. The PTKs can autophosphorylate and phosphorylate each other at multiple sites and, as the result of these interactions, they are induced to phosphorylate other cellular proteins. These phosphorylation events lead to modulation of enzymatic activities and/or serve as binding sites for other signaling molecules having phosphotyrosine-binding Src homology 2 (SH2) domains. As a result, these proteins translocate to the receptor complexes and are juxtaposed to the kinases that phosphorylate them. Some of the SH2-domain-containing polypeptides lack enzymatic activities and, instead, serve as adapter molecules that couple the signal to downstream effectors, such as regulators of the Ras proteins, and further into serine/threonine-specific protein kinase cascades. Through largely unknown steps these reactions lead to the transcription of previously silent genes, activation of lymphocyte effector functions, progression through the cell cycle and cell proliferation.

The role of the catalytic base in the protein tyrosine kinase Csk

A potential distinguishing feature between protein tyrosine kinases and homologous serine/threonine kinases is the function of the catalytic base in these enzymes. In this study, we show that a peptide containing the unnatural amino acid trifluorotyrosine shows remarkably similar efficiency as a substrate of the tyrosine kinase Csk (C-terminal Src kinase) compared with the corresponding tyrosine-containing peptide despite a 4-unit change in the phenolic pKa. These results argue against the importance of early tyrosine deprotonation by a catalytic base in Csk. To further explore the role of the proposed catalytic base, the Csk mutant protein D314E was produced. This mutant displayed a significant reduction in kcat (approximately 10(4)) but relatively little effect on substrate Km values compared with wild-type Csk. Examination of the thio effect (kcat-ATP/kcat-adenosine 5'-O-(thiotriphosphate)) for D314E Csk led to the suggestion that a role of aspartate 314 may be to enhance the reactivity of the gamma-phosphate of ATP toward electrophilic attack. These results may have significant impact on protein tyrosine kinase inhibitor design.

Recombinant mouse OB protein: evidence for a peripheral signal linking adiposity and central neural networks

The recent positional cloning of the mouse ob gene and its human homology has provided the basis to investigate the potential role of the ob gene product in body weight regulation. A biologically active form of recombinant mouse OB protein was overexpressed and purified to near homogeneity from a bacterial expression system. Peripheral and central administration of microgram doses of OB protein reduced food intake and body weight of ob/ob and diet-induced obese mice but not in db/db obese mice. The behavioral effects after brain administration suggest that OB protein can act directly on neuronal networks that control feeding and energy balance.

Regulation of the p70zap tyrosine protein kinase in T cells by the CD45 phosphotyrosine phosphatase

Two classes of protein tyrosine kinases (PTK) are utilized by the T cell antigen receptor (TcR)/CD3 complex for initiation of the signaling cascade, the Src-family PTK p56lck and p59fyn, and the Syk-family PTK p70zap and p72syk. In addition, the CD45 phosphotyrosine phosphatase (PTPase) is required for the induction of tyrosine phosphorylation by the TcR/CD3, presumably by positively regulating Src-family PTK. Here we report that CD45 also regulates the Syk-family PTK p70zap (or ZAP-70). In CD45-negative T cells, p70zap was constitutively phosphorylated on tyrosine and co-immunoprecipitated with the TcR-zeta chain. In resting wild-type CD45-positive cells, p70zap was mainly unphosphorylated, but it was rapidly phosphorylated on tyrosine upon treatment of the cells with anti-CD3 or PTPase inhibitors. Finally, p70zap co-distributed with CD45 in intact T cells, and tyrosine phosphorylated p70zap was dephosphorylated by CD45 in vitro. These findings suggest that CD45 plays an important role, direct or indirect, in the regulation of p70zap and its function in TcR/CD3 signaling.