Progesterone receptor in the liver and oviduct of the lizard Podarcis sicula

In this paper we report the presence of a (3)H-Progesterone ((3)H-P) binding moiety, which has the characteristics of a true receptor, in the liver of the female of the lizard Podarcis sicula. (3)H-P binding studies show the presence of one type of binding site with an average Kd value of 6.2 +/- 2.0 nM in the cytoplasm and 6.3 +/- 1.1 nM in the nucleus. Competition experiments showed that progesterone (P) was the best competitor, while testosterone, deoxycorticosterone (DOC), corticosterone, 17 alpha-hydroxyprogesterone; R5020; RU486 and RU26988-5 were poor competitors. We have also investigated the immunological characteristics of progesterone receptor (PR) in both the liver and the oviduct of Podarcis sicula, by Western blotting using the monoclonal antibody PR22 raised against the PR isoforms A and B of chicken. One imunoreactive band of about 70 kDa was detected in cytoplasmic and nuclear extracts of both the liver and the oviduct. PR immunoreactivity was present in the liver during the quiescent phase. In the oviduct PR immunoreactivity increased from the recovery to the full grown phase. P treatment of estrogen-primed females did not affect the presence of PR in the liver, while brought about a PR increase in the oviduct. This study suggests that PR is expressed differently in the liver and the oviduct of Podarcis sicula throughout the reproductive cycle. PR might fulfill different requirements in relation to the different physiological functions of the tissue during the reproductive cycle.     

Localization and role of leptin in the thyroid gland of the lizard Podarcis sicula (Reptilia, Lacertidae)

Leptin, the product of the ob gene, is a hormone secreted by adipocytes that regulates food intake and energy expenditure. The hypothalamus-pituitary-thyroid axis is markedly influenced by the metabolism status, being suppressed during food deprivation. The present study was designed to ascertain whether (1) lizard thyroid gland expresses the long form of leptin receptor (Ob-Rb) and (2) the leptin administration affects the thyroid gland activity in this species (and to verify whether leptin plays a similar role in reptiles as observed in the other vertebrates). The presence of leptin receptor in the thyroid gland of Podarcis sicula was demonstrated by immunohistochemical technique (avidin-biotin-peroxidase complex--ABC method). The role of leptin in the control of thyroid gland activity was studied in vivo using light microscopy (LM) technique coupled to a specific radioimmunoassay for thyroid-stimulating hormone (TSH) and thyroid hormones (T4 and T3). Leptin (0.1 mg/100 g body wt)/day increased T4 and T3 release for 3 days but decreased the plasma concentration of TSH; using LM clear signs of stimulation in the thyroid gland were observed. These findings suggest that systemic administration of leptin stimulates the morphophysiology of the thyroid gland in the lizard through a direct mechanism involving Ob-Rb.     

Leptin expression in the rat ovary depends on estrous cycle.

Leptin is a hormone originally identified in adipocytes. It is involved in the regulation of fat deposition and energy expenditure and in other functions, such as reproduction. The presence of leptin has been reported in several reproductive organs. However, few studies have addressed its expression in the ovary. Moreover, the existing information is not consistent with regard to the particular cell types responsible for leptin expression. In this work we studied the distribution of leptin in the rat ovary by immunohistochemistry (IHC) and in situ hybridization (ISH). Leptin staining was found in steroid-producing cells: thecal, luteal, and interstitial cells. The strongest signal with both techniques was found in the cytoplasm of oocytes. A weak reaction for leptin mRNA was detected in granulosa of all growing follicles, although leptin protein was found only in the mature follicle. Western blotting analysis detects a strongly reactive 16-kD band, giving further support to the presence of leptin in the rat ovary. Variations in this immunoreactive band were found throughout the estrous cycle. Localization of leptin in the ovary may contribute to a better understanding of female reproductive function.     

瘦素在哺乳动物体重调节、繁殖和免疫中的作用

瘦素(Leptin) 主要是由白色脂肪细胞分泌的、肥胖基因编码的、分子量为16 KD 的蛋白类激素。其N 端具有信号肽序列, 引导蛋白质进入分泌途径, 信号肽被切除后成为有生物学功能的成熟蛋白质。瘦素在动物的体重调节、发育与生殖、免疫和糖代谢等方面有重要作用。瘦素已经不仅仅是脂肪细胞分泌的蛋白类激素, 而是一个在许多方面发挥作用的神经内分泌调节因子。本文综述了瘦素在哺乳动物体重调节、繁殖和免疫中的作用及其调控机制, 主要包括: 动物血清瘦素浓度的季节性变化; 光周期、温度和食物等环境因子对瘦素浓度的影响; 瘦素与解偶联蛋白(Uncoupling proteins , UCPs) 在能量代谢和产热中的互作; 瘦素与下丘脑神经肽Y (Hypothalamus neuropeptide Y, NPY) 在体重调节和产热作用中的拮抗; 瘦素与甲状腺激素和胰岛素在能量代谢中的互作以及瘦素在生殖和免疫中的作用。     

Duplicated Leptin Receptors in Two Species of Eel Bring New Insights into the Evolution of the Leptin System in Vertebrates

Since its discovery in mammals as a key-hormone in reproduction and metabolism, leptin has been identified in an increasing number of tetrapods and teleosts. Tetrapods possess only one leptin gene, while most teleosts possess two leptin genes, as a result of the teleost third whole genome duplication event (3R). Leptin acts through a specific receptor (LEPR). In the European and Japanese eels, we identified two leptin genes, and for the first time in vertebrates, two LEPR genes. Synteny analyses indicated that eel LEPRa and LEPRb result from teleost 3R. LEPRb seems to have been lost in the teleost lineage shortly after the elopomorph divergence. Quantitative PCRs revealed a wide distribution of leptins and LEPRs in the European eel, including tissues involved in metabolism and reproduction. Noticeably, leptin1 was expressed in fat tissue, while leptin2 in the liver, reflecting subfunctionalization. Four-month fasting had no impact on the expression of leptins and LEPRs in control European eels. This might be related to the remarkable adaptation of silver eel metabolism to long-term fasting throughout the reproductive oceanic migration. In contrast, sexual maturation induced differential increases in the expression of leptins and LEPRs in the BPG-liver axis. Leptin2 was strikingly upregulated in the liver, the central organ of the reproductive metabolic challenge in teleosts. LEPRs were differentially regulated during sexual maturation, which may have contributed to the conservation of the duplicated LEPRs in this species. This suggests an ancient and positive role of the leptin system in the vertebrate reproductive function. This study brings new insights on the evolutionary history of the leptin system in vertebrates. Among extant vertebrates, the eel represents a unique case of duplicated leptins and leptin receptors as a result of 3R.     

A role for leptin in sexual maturation and puberty?

Leptin, the ob gene product, is involved in the regulation of body weight in rodents, primates and humans. It provides a molecular basis for the lipostatic theory of the regulation of energy balance. White adipose tissue and placenta are the main sites of leptin synthesis. There is also evidence of ob gene expression in brown fat. Leptin seems to play a key role in the control of body fat stores by coordinated regulation of feeding behaviour, metabolic rate, autonomic nervous system regulation and body energy balance. Apart from the function of leptin in the central nervous system on the regulation of energy balance, it may well be one of the hormonal factors that signal to the brain the body's readiness for sexual maturation and reproduction. During late pregnancy and at birth when maternal fat stores have been developed, leptin levels are high. During these developmental stages leptin could be a messenger molecule signalling the adequacy of the fat stores for reproduction and maintenance of pregnancy. At later stages of gestation leptin could signal the expansion of fat stores in order to prepare the expectant mother for the energy requirements of full-term gestation, labour and lactation. Leptin serum concentrations change during pubertal development in rodents, primates and humans. In girls, leptin serum concentrations increase dramatically as pubertal development proceeds. The pubertal rise in leptin levels parallels the increase in body fat mass. In contrast, leptin levels increase shortly before and during the early stages of puberty in boys and decline thereafter. Testosterone has been found to suppress leptin synthesis by adipocytes both in vivo and in vitro. The decline of leptin levels in late puberty in boys accompanies increased androgen production during that time and most likely reflects suppression of leptin by testosterone and a decrease in fat mass and relative increase in muscle mass during late puberty in males. This overview focuses on those topics of leptin research which are of particular interest in reproductive and adolescent medicine.     

Effects of leptin administration on the endocrine pancreas and liver in the lizard Podarcis sicula

In this study, we investigated the presence of leptin receptor in pancreatic islets and the effect of exogenous leptin administration in Podarcis sicula on glucose metabolism. Our data show the presence of leptin receptor immunoreactivity in the endocrine pancreas suggesting that leptin may act at a peripheral level as previously postulated in mammals. The effects of short- and long-term and dose-response treatment with supraphysiological concentrations of leptin on circulating levels of insulin, glucagon and glucose in the blood have been evaluated. Taken together, our results indicate that leptin treatment was followed by an increase in insulin, glucagon and glucose in the blood, depending on the dose of leptin. Moreover, leptin treatment brought about a decrease of glycogen and the appearance of tyrosine-phosphorylated proteins in the liver. This study shows that in the lizard P. sicula leptin is involved in glucose metabolism.     

Cross-talk between estrogen and leptin signaling in the hypothalamus

Obesity, characterized by enhanced food intake (hyperphagia) and reduced energy expenditure that results in the accumulation of body fat, is a major risk factor for various diseases, including diabetes, cardiovascular disease, and cancer. In the United States, more than half of adults are overweight, and this number continues to increase. The adipocyte-secreted hormone leptin and its downstream signaling mediators play crucial roles in the regulation of energy balance. Leptin decreases feeding while increasing energy expenditure and permitting energy-intensive neuroendocrine processes, such as reproduction. Thus, leptin also modulates the neuroendocrine reproductive axis. The gonadal steroid hormone estrogen plays a central role in the regulation of reproduction and also contributes to the regulation of energy balance. Estrogen deficiency promotes feeding and weight gain, and estrogen facilitates, and to some extent mimics, some actions of leptin. In this review, we examine the functions of estrogen and leptin in the brain, with a focus on mechanisms by which leptin and estrogen cooperate in the regulation of energy homeostasis.     

Expression of leptin and its receptor genes in the ovarian follicles of cycling and early pregnant pigs

Leptin is a polypeptide hormone produced primarily by adipocytes. It has been implicated in the regulation of satiety and energy homeostasis. Leptin has been suggested to play a role in reproduction based on its involvement in the regulation of the hypothalamic–pituitary–gonadal axis via endocrine, paracrine and/or autocrine pathways. The aim of the present study was to localize the cellular distribution of leptin and the long isoform of leptin receptor (OB-Rb) genes in porcine ovarian antral follicles and to compare the expression levels of leptin and OB-Rb mRNAs in porcine granulosa cells (GC), theca interna (TIC) and theca externa (TEC) cells during the luteal phase of the estrous cycle and in early pregnancy. The expression of leptin and OB-Rb genes was detected in GC, TIC and TEC. Significantly higher levels of leptin gene expression in GC were observed during the mid- and late-luteal phases of the cycle than on days 30 to 32 of pregnancy. On days 14 to 16 of pregnancy, leptin mRNA expression was higher than that on days 14 to 16 of the cycle. The expression of the OB-Rb gene in GC and TEC increased during pregnancy in comparison with the analyzed luteal phases of the cycle. Our results validate the hypothesis that locally produced leptin plays a role in the regulation of porcine reproduction at the ovarian level and exerts a direct effect on porcine follicles. The differences in OB-Rb gene expression in porcine GC and theca cells also suggest that their sensitivity to leptin varies in the ovaries of pregnant and cyclic pigs.     

Implications of leptin in neuroendocrine regulation of male reproduction

Obesity is a major health problem contributing to increased subfertility in males, as well as increased morbidity for diseases related to a decline in testosterone production with aging. Leptin is a hormone produced by adipose tissue whose production increases with the amount of body fat. Several studies have supported a relationship between increased leptin production and regulation of reproductive function. Indeed, leptin acts at all levels of the hypothalamus–pituitary–gonadal (HPG) axis in males. However, most of the obese individuals become insensitive to increased endogenous leptin production and develop a functional leptin resistance. This deregulation of leptin signaling might result in abnormal endocrine and reproductive functions. Altered leptin dynamics may contribute to male infertility in different ways, leading to hypogonadism. These include leptin resistance or leptin insufficiency at the hypothalamus and leptin modulation of testicular physiology. In this review, we address the mechanisms of action of leptin at different levels of the HPG axis. Moreover, the influences of leptin on steroidogenesis and spermatogenesis, as well as seasonal variations of leptin's action on male reproduction are discussed.     

The effect of leptin on mouse trophoblast cell invasion

The hormone leptin is produced by adipose tissue and can function as a signal of nutritional status to the reproductive system. The expression of leptin receptor and, in some species, leptin, in the placenta suggests a role for leptin in placental development, but this role has not been elucidated. Leptin is required at the time of embryo implantation in the leptin-deficient ob/ ob mouse and has been shown to upregulate expression of matrix metalloproteinases (MMPs), enzymes involved in trophoblast invasion, in cultured human trophoblast cells. This led us to the hypothesis that leptin promotes the invasiveness of trophoblast cells crucial to placental development. We found that leptin stimulated mouse trophoblast cell invasion through a matrigel-coated insert on Day 10, but not Day 18 of pregnancy. Optimal stimulation occurred at a concentration of 50 ng/ml leptin, similar to the peak plasma leptin concentration during pregnancy in the mouse. Leptin treatment did not stimulate proliferation of mouse trophoblast cells in primary culture. Leptin stimulation of invasion was prevented by 25 muM GM6001, an inhibitor of MMP activity. Our results suggest that leptin may play a role in the establishment of the placenta during early pregnancy and that this function is dependent on MMP activity. This effect of leptin may represent one mechanism by which body condition affects placental development.     

Plasma clearance and tissue distribution of radiolabeled leptin in the chicken

Leptin is an adipose and liver tissue-derived secreted protein in chickens that has been implicated in the regulation of food intake and whole-body energy balance. In this study, the metabolic clearance and tissue uptake of leptin were examined in the chicken (Gallus gallus). Four-week-old broiler males were infused with (125)I-labeled mouse leptin. Chromatography of radiolabeled leptin in plasma produced two peaks, one at 16 kDa (free leptin) and a free iodine peak. No leptin binding protein in blood was detected. Leptin was cleared with a half-life estimate of 23 min. In order to investigate the tissue distribution and uptake of radiolabeled leptin, multiple tissues were removed from infused birds at 15 and 240 min post-infusion, and trichloroacetic acid (TCA)-precipitable radioactivity was determined. The amounts of radioactivity at 15 min post-infusion in the tissues in rank order were: kidney, testis, lung, spleen, heart, liver, small and large intestine, gizzard, pancreas, bursa, leg and breast muscle, adrenals, and brain. A slightly different pattern of distribution was observed at 240 min post-infusion. We conclude from these studies that unlike mammals, no circulating leptin binding protein is present in chickens. Leptin is metabolized and cleared very rapidly from blood by the kidney.     

Leptin: a possible link between food intake, energy expenditure, and reproductive function

Several regulatory substances participate in the regulation of both food intake/energy metabolism and reproduction in mammals. Most of these neuropeptides originate and act in the central nervous system, mainly at specific hypothalamic areas. Leptin represents a signal integrating all these functions, but originating from the periphery (adipose tissue) and carrying information mainly to central structures. Observations in rodent models of leptin deficiency have suggested that leptin participates in the control of reproduction, in conjunction with that of food intake and energy expenditure. Indeed, leptin administration resulted in the restoration of normal body weight, food intake, and fertility in the ob mouse, lacking circulating leptin. Specific targets of leptin in the hypothalamus are neurons expressing neuropeptide Y, proopiomelanocortin and gonadotropin-releasing hormone, but the presence of leptin receptors in peripheral reproductive structures suggests that leptin might also act at these sites. Human obesity is often associated with reproductive disturbances. The situation in humans is more complex than in the animal models of leptin deficit and the presence of leptin resistance in these subjects is suggested. In conclusion, leptin fits many requirements for a molecule linking the regulation of energy balance and the control of reproduction.     

Leptin

Leptin is an adipocyte hormone that signals nutritional status to the central nervous system (CNS) and peripheral organs. Leptin is also synthetized in the placenta and in gastrointestinal tract, although its role in these tissues is not yet clear. Circulating concentrations of leptin exhibit pulsatility and circadian rhythmicity. The levels of plasma leptin vary directly with body mass index and percentage body fat, and leptin contributes to the regulation of body weight. Leptin plasma concentrations are also influenced by metabolic hormones, sex, and body energy requirements. Defects in the leptin signaling pathway result in obesity in animal models. Only a few obese humans have been identified with mutations in the leptin gene or in the leptin receptor; however, most cases of obesity in humans are associated with high leptin levels. Thus, in humans obesity may represent a state of leptin resistance. Minute-to-minute fluctuations in peripheral leptin concentrations influence the activity of the hypothalamic-pituitary-ovarian and hypothalamic-pituitary-adrenal axes, indicating that leptin may be a modulator of reproduction, stress-related endocrine function, and behavior. This suggests potential roles for leptin or its antagonists in the diagnosis, pathophysiology and treatment of several human diseases.     

Leptin and metabolic control of reproduction

Leptin treatment prevents the effects of fasting on reproductive processes in a variety of species. The mechanisms that underlie these effects have not been elucidated. Progress in this area of research might be facilitated by viewing reproductive processes in relation to mechanisms that maintain fuel homeostasis. Reproduction, food intake, and fuel partitioning can be viewed as homeostatic responses controlled by a sensory system that monitors metabolic signals. These signals are generated by changes in intracellular metabolic fuel availability and oxidation rather than by changes in the amount of body fat or by changes in any aspect of body composition. Leptin might be viewed as either a mediator or as a modulator of the intracellular metabolic signal. Consistent with its purported action as a mediator of the metabolic signal, leptin synthesis and secretion are influenced acutely by changes in metabolic fuel availability, and these changes might lead to changes in reproductive function. The effects of leptin treatment on reproduction are blocked by treatments that inhibit intracellular fuel oxidation. Metabolic signals that inhibit reproduction in leptin-treated animals might act via neural pathways that are independent of leptin's action. Alternatively, both leptin and metabolic inhibitors might interact at the level of intracellular fuel oxidation. In keeping with the possibility that leptin modulates the metabolic signal, leptin treatment increases fuel availability, uptake, and oxidation in particular tissues. Leptin might affect reproduction indirectly by altering fuel oxidation or other peripheral processes such as gastric emptying. Reproductive processes are among the most energetically expensive in the female repertoire. Because leptin increases energy expenditure while simultaneously inhibiting energy intake, it may have limited use as a long-term treatment for infertility.     

Leptin, adiposity, and testosterone in captive male macaques

Leptin is considered to act as a signal relating somatic energetic status to the reproductive system. However, the nature of that signal and its relationship with male reproductive function across nonhuman primate species are unclear. We suggest that species-specific differences in leptin physiology may be related to the degree of environmental variation and variation in the importance of energy stores for male reproduction. In order to test the role of seasonality in species differences among nonhuman primates, we compared leptin, testosterone, and body composition in male rhesus (n = 69) and pig-tailed (n = 43) macaques. Despite having larger abdominal fat deposits, the rhesus macaques did not exhibit significantly higher leptin levels (rhesus, 2.21 +/- 0.43 ng/ml; pig-tailed, 2.12 +/- 0.39 ng/ml). Both species showed increases in leptin across adolescent, subadult, and adult age-groups (P = 0.036 for rhesus; P = 0.0003 for pig-tailed by ANCOVA). Testosterone was not significantly associated with leptin in either the rhesus (r = 0.039; P = 0.754) or pig-tailed (r = 0.2862; P = 0.066) samples. Comparison of leptin levels across the two species using univariate modeling procedures showed no significant age-group by abdominal fat interaction. These findings suggest little difference in leptin production between these two closely related species, despite the difference in breeding seasonality.     

Leptin, but not immune function, is linked to reproductive responsiveness to photoperiod

Energetic demands are high while energy availability is minimum during winter. To cope with this energetic bottleneck, animals exhibit numerous energy-conserving adaptations during winter, including changes in immune and reproductive functions. A majority of individual rodents within a population inhibits reproductive function (responders) as winter approaches. A substantial proportion of small rodents within a species, however, fails to inhibit reproduction (nonresponders) during winter in the field or in the laboratory when maintained in winter-simulated day lengths. In contrast, immune function is bolstered by short day lengths in some species. The specific mechanisms that link reproductive and immune functions remain unspecified. Leptin is a hormone produced by adipose tissue, and several studies suggest that leptin modulates reproductive and immune functions. The present study sought to determine if photoperiodic alterations in reproductive function and leptin concentrations are linked to photoperiod-modulated changes in immune function. Siberian hamsters (Phodopus sungorus) were housed in either long (LD 16:8) or short (LD 8:16) day lengths for 9 wk. After 9 wk, blood samples were collected during the middle of the light and dark phase to assess leptin concentrations. One week later, animals were injected with keyhole limpet hemocyanin to evaluate humoral immunity. Body mass, body fat content, and serum leptin concentrations were correlated with reproductive responsiveness to photoperiod; short-day animals with regressed gonads exhibited a reduction in these measures, whereas short-day nonresponders resembled long-day animals. In contrast, immune function was influenced by photoperiod but not reproductive status. Taken together, these data suggest that humoral immune function in Siberian hamsters is independent of photoperiod-mediated changes in leptin concentrations.     

Differential role of melanocortins in mediating leptin's central effects on feeding and reproduction

Leptin serves as a humoral link coupling the status of energy reserves to the functional activity of the reproductive system. Leptin is thought to act through melanocortinergic pathways in the brain to regulate ingestive behaviors; however, whether melanocortins mediate leptin's actions on the neuroendocrine-reproductive axis is unknown. We tested this hypothesis first by determining whether the effects of leptin on feeding behavior and reproduction in the ob/ob mouse could be blocked by the melanocortin receptor (MC-R) antagonist SHU9119 and second, by examining the effects of the MC-R agonist MTII on feeding and the endocrine-reproductive system. Administered by intracerebroventricular injections, leptin inhibited food intake, raised plasma gonadotropin levels, and increased seminal vesicle weights compared with controls; SHU9119 (intracerebroventricularly) attenuated leptin's effects on food intake and body weight but did not alter leptin's stimulatory effect on the reproductive axis. MTII (intracerebroventricularly and intraperitoneally) decreased food intake and increased body temperature compared with controls but had no effect on the reproductive-endocrine axis. These results suggest that although leptin acts centrally through melanocortinergic pathways to inhibit ingestive behaviors and stimulate metabolism, leptin's activational effect on the reproductive axis is likely to be mediated by other, unknown neuroendocrine circuits.     

Long form of leptin receptor gene and protein expression in the porcine trophoblast and uterine tissues during early pregnancy and the oestrous cycle

Leptin, the product of the OB gene, is a 16-kDa polypeptide of 146 amino acid residues produced mainly by adipocytes that regulates metabolism and reproduction. The actions of leptin are mediated mainly via the long form of the leptin receptor (OB-Rb). The identification of leptin and OB-Rb mRNAs and proteins in human and mouse endometrium, and placental trophoblast suggests that leptin may be involved in the implantation process. Thus, the aim of this study was to compare the expression levels of porcine OB-Rb mRNA and protein in the endometrium and myometrium during mid- and late-luteal phases of the oestrous cycle (days 10-12 and 14-16, respectively) as well as during two stages of pregnancy respondent to the beginning of the implantation process (days 14-16) and the post-implantation period (days 30-32), and in trophoblast during both periods of pregnancy. OB-Rb gene expression in endometrium during the examined stages of pregnancy and the mid- and late-luteal phases of the cycle was at the same level. In contrast, in myometrium leptin receptor gene expression decreased on days 14-16 of pregnancy compared to both phases of the cycle, and on days 30-32 of pregnancy in relation to late-luteal phase. OB-Rb protein expression in the tissues was lower during the examined stages of pregnancy in comparison to the mid- and late-luteal phases of the cycle. In trophoblast, OB-Rb mRNA and protein expression was higher on days 30-32 than during days 14-16 of pregnancy. In conclusion, our results might suggest that leptin can participate in the control of pig reproduction by exercising its action at the uterine and trophoblast level and have a direct effect on these organ during both the luteal phase of the cycle and early pregnancy. Moreover, changes in OB-Rb gene and protein expression in tissues of pig reproductive tract strongly suggest that their sensitivity to leptin varies throughout luteal phase of the cycle and early pregnancy.