PET Imaging of Leptin Biodistribution and Metabolism in Rodents and Primates

We have determined the systemic biodistribution of the hormone leptin by PET imaging. PET imaging using ¹⁸F- and ⁶⁸Ga-labeled leptin revealed that, in mouse, the hormone was rapidly taken up by megalin (gp330/LRP2), a multiligand endocytic receptor localized in renal tubules. In addition, in rhesus monkeys, 15% of labeled leptin localized to red bone marrow, which was consistent with hormone uptake in rodent tissues. These data confirm a megalin-dependent mechanism for renal uptake in vivo. The significant binding to immune cells and blood cell precursors in bone marrow is also consistent with prior evidence showing that leptin modulates immune function. These experiments set the stage for similar studies in humans to assess the extent to which alterations of leptin's biodistribution might contribute to obesity; they also provide a general chemical strategy for ¹⁸F labeling of proteins for PET imaging of other polypeptide hormones.     

Leptin Mediates a Glucose-Fatty Acid Cycle to Maintain Glucose Homeostasis in Starvation

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Leptin Activates Oxytocin Neurons of the Hypothalamic Paraventricular Nucleus in Both Control and Diet-Induced Obese Rodents

The adipocyte-derived hormone leptin acts in the brain to reduce body weight and fat mass. Recent studies suggest that parvocellular oxytocin (OXT) neurons of the hypothalamic paraventricular nucleus (PVN) can mediate body weight reduction through inhibition of food intake and increased energy expenditure. However, the role of OXT neurons of the PVN as a primary target of leptin has not been investigated. Here, we studied the potential role of OXT neurons of the PVN in leptin-mediated effects on body weight regulation in fasted rats. We demonstrated that intracerebroventricular (ICV) leptin activates STAT3 phosphorylation in OXT neurons of the PVN, showed that this occurs in a subpopulation of OXT neurons that innervate the nucleus of the solitary tract (NTS), and provided further evidence suggesting a role of OXT to mediate leptin’s actions on body weight. In addition, our results indicated that OXT neurons are responsive to ICV leptin and mediate leptin effects on body weight in diet induced obese (DIO) rats, which are resistant to the anorectic effects of the hormone. Thus, we conclude that leptin targets a specific subpopulation of parvocellular OXT neurons of the PVN, and that this action may be important for leptin’s ability to reduce body weight in both control and obese rats.     

Modulation of the Leptin Receptor Mediates Tumor Growth and Migration of Pancreatic Cancer Cells

Obesity has been implicated as a significant risk factor for development of pancreatic cancer. In the setting of obesity, a systemic chronic inflammatory response is characterized by alterations in the production and secretion of a wide variety of growth factors. Leptin is a hormone whose level increases drastically in the serum of obese patients. High fat diet induced obesity in mice leads to an overall increased body weight, pancreatic weight, serum leptin, and pancreatic tissue leptin levels. Here we report the contribution of obesity and leptin to pancreatic cancer growth utilizing an in vivo orthotopic murine pancreatic cancer model, which resulted in increased tumor proliferation with concomitant increased tumor burden in the diet induced obese mice compared to lean mice. Human and murine pancreatic cancer cell lines were found to express the short as well as the long form of the leptin receptor and functionally responded to leptin induced activation through an increased phosphorylation of AKT473. In vitro, leptin stimulation increased cellular migration which was blocked by addition of a PI3K inhibitor. In vivo, depletion of the leptin receptor through shRNA knockdown partially abrogated increased orthotopic tumor growth in obese mice. These findings suggest that leptin contributes to pancreatic tumor growth through activation of the PI3K/AKT pathway, which promotes pancreatic tumor cell migration.     

The Leptin,Dopamine and Serotonin Receptors in Hypothalamic POMC-Neurons of Normal and Obese Rodents

The pro-opiomelanocortin (POMC)-expressing neurons of the hypothalamic arcuate nucleus (ARC) are involved in the control of food intake and metabolic processes. It is assumed that, in addition to leptin, the activity of these neurons is regulated by serotonin and dopamine, but only subtype 2C serotonin receptors (5-HT_2CR) was identified earlier on the POMC-neurons. The aim of this work was a comparative study of the localization and number of leptin receptors (LepR), types 1 and 2 dopamine receptors (D₁R, D₂R), 5-HT_1BR and 5-HT_2CR on the POMC-neurons and the expression of the genes encoding them in the ARC of the normal and diet-induced obese (DIO) rodents and the agouti mice (A ^y /a) with the melanocortin obesity. As shown by immunohistochemistry (IHC), all the studied receptors were located on the POMC-immunopositive neurons, and their IHC-content was in agreement with the expression of their genes. In DIO rats the number of D₁R and D₂R in the POMC-neurons and their expression in the ARC were reduced. In DIO mice the number of D₁R and D₂R did not change, while the number of LepR and 5-HT_2CR was increased, although to a small extent. In the POMC-neurons of agouti mice the number of LepR, D₂R, 5-HT_1BR and 5-HT_2CR was increased, and the D₁R number was reduced. Thus, our data demonstrates for the first time the localization of different types of the serotonin and dopamine receptors on the POMC-neurons and a specific pattern of the changes of their number and expression in the DIO and melanocortin obesity.     

Estradiol and Estrogen Receptor Agonists Oppose Oncogenic Actions of Leptin in HepG2 Cells

Obesity is a significant risk factor for certain cancers, including hepatocellular carcinoma (HCC). Leptin, a hormone secreted by white adipose tissue, precipitates HCC development. Epidemiology data show that men have a much higher incidence of HCC than women, suggesting that estrogens and its receptors may inhibit HCC development and progression. Whether estrogens antagonize oncogenic action of leptin is uncertain. To investigate potential inhibitory effects of estrogens on leptin-induced HCC development, HCC cell line HepG2 cells were treated with leptin in combination with 17 β-estradiol (E2), estrogen receptor-α (ER-α) selective agonist PPT, ER-β selective agonist DPN, or G protein-coupled ER (GPER) selective agonist G-1. Cell number, proliferation, and apoptosis were determined, and leptin- and estrogen-related intracellular signaling pathways were analyzed. HepG2 cells expressed a low level of ER-β mRNA, and leptin treatment increased ER-β expression. E2 suppressed leptin-induced HepG2 cell proliferation and promoted cell apoptosis in a dose-dependent manner. Additionally E2 reversed leptin-induced STAT3 and leptin-suppressed SOCS3, which was mainly achieved by activation of ER-β. E2 also enhanced ERK via activating ER-α and GPER and activated p38/MAPK via activating ER-β. To conclude, E2 and its receptors antagonize the oncogenic actions of leptin in HepG2 cells by inhibiting cell proliferation and stimulating cell apoptosis, which was associated with reversing leptin-induced changes in SOCS3/STAT3 and increasing p38/MAPK by activating ER-β, and increasing ERK by activating ER-α and GPER. Identifying roles of different estrogen receptors would provide comprehensive understanding of estrogenic mechanisms in HCC development and shed light on potential treatment for HCC patients.     

Hypothalamic mTORC1 Signaling Controls Sympathetic Nerve Activity and Arterial Pressure and Mediates Leptin Effects

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Highlights? Activation of hypothalamic mTORC1 increases sympathetic traffic and arterial pressure ? The sympathetic and hemodynamic effects of leptin require hypothalamic mTORC1 ? PI3K links the leptin receptor to mTORC1 signaling in the hypothalamus ? Modulating PI3K activity perturbs the sympathetic and hemodynamic functions     

Upregulation of miR21 and Repression of Grhl3 by Leptin Mediates Sinusoidal Endothelial Injury in Experimental Nonalcoholic Steatohepatitis

Sinusoidal endothelial dysfunction (SED) has been found to be an early event in nonalcoholic steatohepatitis (NASH) progression but the molecular mechanisms underlying its causation remains elusive. We hypothesized that adipokine leptin worsens sinusoidal injury by decreasing functionally active nitric oxide synthase 3 (NOS)3 via miR21. Using rodent models of NASH, and transgenic mice lacking leptin and leptin receptor, results showed that hyperleptinemia caused a 4–5 fold upregulation of hepatic miR21 as assessed by qRTPCR. The upregulation of miR21 led to a time-dependent repression of its target protein Grhl3 levels as shown by western blot analyses. NOS3-p/NOS3 ratio which is controlled by Grhl3 was significantly decreased in NASH models. SED markers ICAM-1, VEGFR-2, and E-selectin as assessed by immunofluorescence microscopy were significantly up regulated in the progressive phases of NASH. Lack of leptin or its receptor in vivo, reversed the upregulation of miR21 and restored the levels of Grhl3 and NOS3-p/NOS3 ratio coupled with decreased SED dysfunction markers. Interestingly, leptin supplementation in mice lacking leptin, significantly enhanced miR21 levels, decreased Grhl3 repression and NOS3 phosphorylation. Leptin supplementation in isolated primary endothelial cells, Kupffer cells and stellate cells showed increased mir21 expression in stellate cells while sinusoidal injury was significantly higher in all cell types. Finally miR21 KO mice showed increased NOS3-p/NOS3 ratio and reversed SED markers in the rodent models of NASH. The experimental results described here show a close association of leptin-induced miR21 in aiding sinusoidal injury in NASH.     

Adenylyl Cyclase-Associated Protein 1 Is a Receptor for Human Resistin and Mediates Inflammatory Actions of Human Monocytes

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啮齿类动物中布鲁斯效应的研究进展

布鲁斯效应是一种由陌生雄性以及其化学信号所引起的妊娠终止现象,首次在实验室小鼠(Musmusculus)中发现。学者们对这一现象进行了多角度、多层次的探索,从行为到内分泌,从神经机理到进化意义,都有相关的研究报道。研究热点主要集中在:布鲁斯效应在其他啮齿类物种中的情况,其生物学意义及神经内分泌机制,有效诱发雌鼠妊娠终止的化学信号成分等几个方面。本文就目前对布鲁斯效应已经取得的一些研究进展进行了回顾,并讨论了未来可能的研究趋势。     

Chemotaxonomic significance of volatile compounds in Thymus samius and its related species Thymus atticus and Thymus parnassicus

Thymus samius, a rare species endemic to the island of Samos (East Aegean, Greece), has been considered to be of possible hybrid origin, with Thymus cilicicus, Thymus parnassicus or Thymus zygioides as putative parents. Morphologically it also resembles Thymus atticus. In order to evaluate any possible chemotaxonomic relationships indicated by the qualitative and/or quantitative differentiation of volatile compounds, the essential oils of T. samius, T. parnassicus and T. atticus were analyzed and the oils of T. cilicicus and T. zygioides were used for comparison. T. atticus presents considerable variation in chemical constituents, with all Greek populations being poor in thymol/carvacrol and rich in (E)-nerolidol, germacrene D, (E)-caryophyllene, caryophyllene oxide, 1,8-cineole, α-pinene and camphene, depending on the sample. T. parnassicus shows remarkable qualitative stability in its volatile constituents, and all its populations have a similar profile with (E)-caryophyllene being the most prominent compound. T. samius is characterized by the presence of germacrene D and β-bisabolene, which constitute almost half its oil percentage. This species is also characterized by a low amount of monoterpenes; the latter exist in a considerable percentage in the related species. Our results indicate that T. samius does not show similarity or intermediacy in chemical compounds with any given couple of T. atticus, T. parnassicus, T. cilicicus, or T. zygioides.     

Positive Regulatory Control Loop between Gut Leptin and Intestinal GLUT2/GLUT5 Transporters Links to Hepatic Metabolic Functions in Rodents

Background and Aims

The small intestine is the major site of absorption of dietary sugars. The rate at which they enter and exit the intestine has a major effect on blood glucose homeostasis. In this study, we determine the effects of luminal leptin on activity/expression of GLUT2 and GLUT5 transporters in response to sugars intake and analyse their physiological consequences.

Methodology

Wistar rats, wild type and AMPKα₂ ^−/− mice were used. In vitro and in vivo isolated jejunal loops were used to quantify transport of fructose and galactose in the absence and the presence of leptin. The effects of fructose and galactose on gastric leptin release were determined. The effects of leptin given orally without or with fructose were determined on the expression of GLUT2/5, on some gluconeogenesis and lipogenic enzymes in the intestine and the liver.

Principal Findings

First, in vitro luminal leptin activating its receptors coupled to PKCβII and AMPKα, increased insertion of GLUT2/5 into the brush-border membrane leading to enhanced galactose and fructose transport. Second in vivo, oral fructose but not galactose induced in mice a rapid and potent release of gastric leptin in gastric juice without significant changes in plasma leptin levels. Moreover, leptin given orally at a dose reproducing comparable levels to those induced by fructose, stimulated GLUT5-fructose transport, and potentiated fructose-induced: i) increase in blood glucose and mRNA levels of key gluconeogenesis enzymes; ii) increase in blood triglycerides and reduction of mRNA levels of intestinal and hepatic Fasting-induced adipocyte factor (Fiaf) and iii) increase in SREBP-1c, ACC-1, FAS mRNA levels and dephosphorylation/activation of ACC-1 in liver.

Conclusion/Significance

These data identify for the first time a positive regulatory control loop between gut leptin and fructose in which fructose triggers release of gastric leptin which, in turn, up-regulates GLUT5 and concurrently modulates metabolic functions in the liver. This loop appears to be a new mechanism (possibly pathogenic) by which fructose consumption rapidly becomes highly lipogenic and deleterious.     

Paternal Behavior in Rodents

The literature on paternal behavior in rodents includes reportsof many species; however, paternal behavior appears more prevalentamong muroid and hystricomorph rodents than in the sciurids.Many reports of paternal behavior are based on small numbersof animals observed under special conditions. Where systematicstudies have been conducted, the most salient feature of rodentpaternal behavior is its lability. The occurrence varies withobserver constraints, genotype, experience, social conditionsduring observation, and stimulus characteristics. This intraspecificvariability renders application of the comparative method inunderstanding evolution difficult but may provide a key in theremarkable adaptation of rodents to diverse habitats. Althoughspecies differences exist the evolution of rodent paternal behaviorappears to entail changes in the thresholds and conditions forthe occurrence of a relatively few basic motor patterns of paternalcare.     

The Functions of the Thymus

In rodents the thymus performs at least two functions. It is a major site of lymphopoiesis in the embryo and newborn, with the resulting lymphocytes migrating from the thymus to seed the spleen, lymph nodes and other lymphoid organs. In addition, the thymus produces a hormone which has an immunotrophic effect, i.e. it endows cells having immunological potential with immunological competence. In some animals other organs, in addition to the thymus, are responsible for directing the normal development of the immunological system. These are the bursa of Fabricius in birds and the appendix in rabbits. In humans it has been postulated that the tonsillar tissues may play an analogous role. Animal experiments involving extirpation of the immunotrophic lymphoid tissues have led to a better understanding of immunological deficiency diseases in man.     

Lck和Fyn对T细胞发育过程的影响

胸腺中T细胞的发育及次级淋巴组织中成熟T细胞的活化均需要细胞能够对环境信号分子做出适应性的反应。在共刺激分子及细胞因子受体介导的信号参与下通过TCR(T cell receptor)及其辅助受体CD4和CD8与MHC/抗原肽复合物相互作用,可以诱导TCR信号通路激活并最终导致T细胞免疫反应的发生。Src家族激酶Lck(Lymphocyte-specific protein tyrosine kinase)和Fyn(Proto-oncogene tyrosine-protein kinase)的激活是启动TCR信号通路的关键因素。在T细胞的发育、阳性选择、初始T细胞的外周存活及由淋巴细胞缺失诱导的细胞增殖中都起着关键性的作用。研究显示,虽然这两种信号分子紧密相关,但在某些条件下Lck发挥着比Fyn更重要的作用,并且Fyn仅可以补充Lck的部分功能。文章针对这两个激酶在T细胞发育的整个过程中的作用机制进行了论述。