Upregulation of leptin receptor gene expression in the anterior pituitary of human growth hormone-releasing hormone transgenic mice

Selective deletion of leptin receptors in gonadotropes reveals activin and GnRH-binding sites as leptin targets in support of fertility

The adipokine, leptin (LEP), is a hormonal gateway, signaling energy stores to appetite-regulatory neurons, permitting reproduction when stores are sufficient. Dual-labeling for LEP receptors (LEPRs) and gonadotropins or GH revealed a 2-fold increase in LEPR during proestrus, some of which was seen in LH gonadotropes. We therefore investigated LEPR functions in gonadotropes with Cre-LoxP technology, deleting the signaling domain of the LEPR (Lepr-exon 17) with Cre-recombinase driven by the rat LH-β promoter (Lhβ-cre). Selectivity of the deletion was validated by organ genotyping and lack of LEPR and responses to LEP by mutant gonadotropes. The mutation had no impact on growth, body weight, the timing of puberty, or pregnancy. Mutant females took 36% longer to produce their first litter and had 50% fewer pups/litter. When the broad impact of the loss of gonadotrope LEPR on all pituitary hormones was studied, mutant diestrous females had reduced serum levels of LH (40%), FSH (70%), and GH (54%) and mRNA levels of Fshβ (59%) and inhibin/activin β A and β B (25%). Mutant males had reduced serum levels of GH (74%), TSH (31%), and prolactin (69%) and mRNA levels of Gh (31%), Ghrhr (30%), Fshβ (22%), and glycoprotein α-subunit (Cga) (22%). Serum levels of LEP and ACTH and mRNA levels of Gnrhr were unchanged. However, binding to GnRH receptors was reduced in LEPR-null LH or FSH gonadotropes by 82% or 89%, respectively, in females (P < .0001) and 27% or 53%, respectively, in males (P < .03). This correlated with reductions in GnRH receptor protein immunolabeling, suggesting that LEP's actions may be posttranscriptional. Collectively, these studies highlight the importance of LEP to gonadotropes with GnRH-binding sites and activin as potential targets. LEP may modulate population growth, adjusting the number of offspring to the availability of food supplies.

Ablation of leptin signaling to somatotropes: changes in metabolic factors that cause obesity

Mice with somatotrope-specific deletion of the Janus kinase binding site in leptin receptors are GH deficient as young adults and become obese by 6 months of age. This study focused on the metabolic status of young (3-4.5 month old) preobese mutant mice. These mutants had normal body weights, lean body mass, serum leptin, glucose, and triglycerides. Mutant males and females showed significantly higher respiratory quotients (RQ) and lower energy output, resulting from a higher volume of CO(2) output and lower volume of O(2) consumption. Deletion mutant females were significantly less active than controls; they had higher levels of total serum ghrelin and ate more food. Mutant females also had lower serum insulin and higher glucagon. In contrast, deletion mutant males were not hyperphagic, but they were more active and spent less time sleeping. Adiponectin and resistin, both products of adipocytes, were increased in male and female mutant mice. In addition, mutant males showed an increase in circulating levels of the potent lipogenic hormone, glucose-dependent insulinotropic peptide. Taken together, these results indicate that mutant mice may become obese due to a reduction in lipid oxidation and energy expenditure. This may stem from GH deficiency. Reduced fat oxidation and enhanced insulin sensitivity (in females) are directly related to GH deficiency in mutant mice because GH has been shown by others to increase insulin sensitivity and fat oxidation and reduce carbohydrate oxidation. Gender-dependent alterations in metabolic signals may further exacerbate the future obese phenotype and affect the timing of its onset. Females show a delay in onset of obesity, perhaps because of their low serum insulin, which is lipogenic, whereas young males already have higher levels of the lipogenic hormone, glucose-dependent insulinotropic peptide. These findings signify that leptin signals to somatotropes are vital for the normal metabolic activity needed to optimize body composition.

The somatotrope as a metabolic sensor: deletion of leptin receptors causes obesity

Leptin, the product of the Lep gene, reports levels of adiposity to the hypothalamus and other regulatory cells, including pituitary somatotropes, which secrete GH. Leptin deficiency is associated with a decline in somatotrope numbers and function, suggesting that leptin may be important in their maintenance. This hypothesis was tested in a new animal model in which exon 17 of the leptin receptor (Lepr) protein was selectively deleted in somatotropes by Cre-loxP technology. Organ genotyping confirmed the recombination of the floxed LepR allele only in the pituitary. Deletion mutant mice showed a 72% reduction in pituitary cells bearing leptin receptor (LEPR)-b, a 43% reduction in LEPR proteins and a 60% reduction in percentages of immunopositive GH cells, which correlated with reduced serum GH. In mutants, LEPR expression by other pituitary cells was like that of normal animals. Leptin stimulated phosphorylated Signal transducer and activator of transcription 3 expression in somatotropes from normal animals but not from mutants. Pituitary weights, cell numbers, IGF-I, and the timing of puberty were not different from control values. Growth curves were normal during the first 3 months. Deletion mutant mice became approximately 30-46% heavier than controls with age, which was attributed to an increase in fat mass. Serum leptin levels were either normal in younger animals or reflected the level of obesity in older animals. The specific ablation of the Lepr exon 17 gene in somatotropes resulted in GH deficiency with a consequential reduction in lipolytic activity normally maintained by GH and increased adiposity.

Localization of leptin and leptin receptor in the bovine adenohypophysis

The present study was carried out to detail the cellular localization of leptin (Lep) and the leptin receptor (LepR) in the bovine adenohypophysis. Lep immunoreactivity (Lep-ir) was found in about 30% of adenohypophysial cells in the gland. Immunochemistry of Lep and specific hormones using serial sections revealed that Lep-ir was present in 60.4% of somatotrophs, 15.9% of gonadotrophs, 6.5% of mammotrophs, 6.5% of thyrotrophs and 2.4% of corticotrophs. Both the common short isoform (OBRa) and the long isoform (OBRb) of LepR mRNA were expressed in the bovine adenohypophysis. LepR immunoreactivity (LepR-ir) was found in only 2.8% of the adenohypophysial cells and over 50% of LepR-ir cells were gonadotrophs, in which most of the cells were distributed in the zona tuberalis. The findings on Lep and LepR in the adenohypophysial cells indicate that Lep may regulate gonadotroph function through autocrine/paracrine pathway in the bovine adenohypophysis.

Growth hormone and ghrelin receptor genes are differentially expressed between genetically lean and fat selection lines of sheep

The objective of this study was to determine whether differences in mRNA levels of key pituitary genes that regulate GH production, pituitary development, and growth were present and/or associated with divergent body composition phenotypes observed between sheep from genetically divergent lean and fat selection lines. Real-time PCR transcription profiles for pituitary specific transcription factor 1, prophet of pit1, GH, GH receptor, GH secretagogue receptor, GHRH receptor, leptin receptor, and somatostatin receptors 1 and 2 were determined in pituitary tissue. There was a difference in the amount of both GH (P < 0.001) and GH secretagogue receptor (P < 0.001) mRNA between the selection lines (5 females and 5 males per line; 20 wk of age); the lean line had greater abundance than the fat line, irrespective of which endogenous control gene was used. The results obtained for GHRH receptor were equivocal but suggestive; there were greater GHRH receptor mRNA levels (P < 0.001) in the lean line using beta-2-microglobulin as the endogenous control but not when hypoxanthine phosphoribosyltransferase and glyceraldehyde-3-phosphate dehydrogenase were used. No difference in pituitary specific transcription factor 1, prophet of pit1, GH receptor, leptin receptor, or somatostatin receptors 1 and 2 mRNA concentration was observed between the lines. The greater abundance of GH mRNA in the pituitary somatotropes from genetically lean animals appears to be associated with increased levels of GH secretagogue receptor mRNA and possibly GHRH receptor mRNA. This suggests that the difference in GH secretion between the lines may be due to differences in the afferent signals, such as ghrelin and/or GHRH, arising from the hypothalamus, or as a result of differential pituitary sensitivity to these hormones.

Interactions of Leptin, GH, and Cortisol in Normal Children

Leptin is the product of the ob gene located in humans on chromosome 7q31.3. It is a 16-kDa protein named after the Greek "leptos," meaning lean, to indicate the function that this adipocyte-secreted protein was thought to have. Since its discovery, in fact, most of the research focused on the role of leptin in body-weight regulation, aiming to elucidate the pathophysiology of human obesity. However, more and more data show that leptin is not only important in the regulation of food intake and energy balance, but it also functions as a neuroendocrine hormone. It is involved in glucose metabolism, as well as in normal sexual maturation and reproduction, and interacts with the hypothalamic-pituitary-adrenal (HPA) and the growth hormone (GH) axes.

In vitro effect of human recombinant leptin and expression of leptin receptors on growth hormone-secreting human pituitary adenomas

OBJECTIVE AND STUDY DESIGN

Leptin is a circulating hormone secreted by adipose tissue and a few other tissues. It has recently been demonstrated that leptin and leptin receptors are expressed in normal and adenomatous pituitary cells. The aim of this study was to investigate the effect of recombinant human leptin on GH release from adenomatous GH-secreting cells in culture. Specimens were obtained from 10 patients with acromegaly who had undergone selective transsphenoidal adenomectomy. Cells (2 x 10(5)/well) preincubated for 24 h with leptin (10(-10)-10(-8) m) or control medium were exposed to GHRH for 2 h. The GH released into the medium was measured before and after GHRH incubation. The expression of leptin receptor isoforms was evaluated by reverse-transcriptase polymerase chain reaction (RT-PCR) in cells obtained from five adenomas.

RESULTS

After the first incubation, there was a slight dose-dependent leptin-induced decrease in GH released into the medium. A significant increase in GH release after GHRH was noted from cells previously exposed to leptin in comparison with cells incubated with medium alone. Expression of leptin receptors was found in two out of five GH-secreting adenomas evaluated.

CONCLUSIONS

Our data confirm that some, but not all, GH-secreting adenomas express leptin receptors. Leptin seems to exert both a slight inhibitory effect on spontaneous GH secretion and a stimulatory effect on GHRH-stimulated GH secretion from GH-secreting adenomatous tissue.

Direct modification of somatotrope function by long-term leptin treatment of primary cultured ovine pituitary cells

Leptin is produced primarily in adipocytes and regulates body energy balance. A close link between leptin and pituitary hormones, including GH, has been reported. The mechanisms employed by leptin to influence somatotropes are not clear, however. Here we report a direct action of recombinant ovine leptin on primary cultured ovine somatotropes by analyzing the levels of mRNA encoding for GH or the receptors for GHRH (GHRH-R) and GH-releasing peptides (GHRP). Treatment of ovine somatotropes with leptin (10(-7)-10(-9) M) for 1-3 d reduced the mRNA levels encoding GH and GHRH-R, but increased GHRP receptor mRNA levels in a time- and dose-dependent manner. Three-day treatment of cells with leptin decreased the GH response to GHRH stimulation, but the GH response to GHRP-2 stimulation was increased. The combined effect of GHRH and GHRP-2 on GH secretion was not altered by treatment of the cells with leptin. These results demonstrated a direct action of leptin on ovine pituitary cells, leading to a reduced sensitivity of somatotropes to GHRH. It is also suggested that GHRP may be useful to correct the decrease in GHRH-induced GH secretion by leptin.

Leptin, cortisol, and GH secretion interactions in short normal prepubertal children

The hormonal regulation of the ob gene and leptin secretion in humans is still unclear. To investigate the interactions among leptin, cortisol, and GH, we analyzed and time-cross-correlated their spontaneous 24-h secretion in 12 short normal prepubertal children of both sexes (6 females and 6 males). Time-cross-correlation analyses demonstrated that leptin and cortisol were correlated in both a negative and positive fashion. The negative correlation, with cortisol leading leptin by 4 and 3 h for boys and girls, respectively, might reflect the stimulatory effect of CRH on the sympathetic system, which inhibits leptin secretion; the positive correlation, with leptin leading cortisol by 6 and 5 h for boys and girls, respectively, might reflect a direct effect of leptin on CRH secretion in the hypophyseal portal system. Time-cross-correlation analyses also showed a strong positive correlation between GH and leptin concentrations, with GH leading leptin by 5 and 2 h for boys and girls, respectively, suggesting a possible direct leptin-releasing effect of GH on adipocytes. We conclude that cross-correlation analyses of 24-h hormone secretions under baseline physiological conditions suggest that the hypothalamic-pituitary-adrenal axis might have a prevailing inhibitory effect on leptin secretion, whereas leptin might exert a positive effect on the hypothalamic-pituitary-adrenal axis. The relation between GH and leptin could be a direct one and characterized prevalently by a positive effect of GH on leptin secretion. Further investigations using different experimental systems are needed to ascertain the validity of these mathematically educed conclusions.

The release of leptin and its effect on hormone release from human pituitary adenomas

BACKGROUND

Leptin is the protein product of the obese gene, known to play an important role in body energy balance. The leptin receptor exists in numerous isoforms, the long isoform being the major form involved in signal transduction. Leptin expression has recently been demonstrated in the human pituitary, both in normal tissue and in pituitary adenomas. The long isoform of the leptin receptor has also been shown to be present in pituitary adenomas; however, contrasting results have been obtained regarding its expression in the normal human pituitary.

AIM

The aim of this study was (i) to investigate the presence and pattern of distribution of leptin mRNA and the long isoform of its receptor mRNA in the normal pituitary and in different types of pituitary adenomas with RT-PCR; (ii) to study leptin secretion from human pituitary tumours in culture and (iii) to assess in vitro pituitary hormone release following stimulation with human leptin.

RESULTS

Leptin receptor long isoform expression was detected in 2/4 GH-secreting adenomas, 12/17 non-functioning adenomas, 5/9 ACTH-secreting adenomas, 1/2 prolactinomas, 2/2 FSH-secreting adenomas and 5/5 normal pituitaries. The receptor long isoform did not segregate with any particular tumour type, and varying levels of expression were detected between the tissues studied. Leptin mRNA was detected at a low level of expression in 2/7 GH-secreting adenomas, 9/14 non-functioning adenomas, 2/3 ACTH-secreting adenomas, 1/3 prolactinomas and 1/3 FSH-secreting adenomas. We were unable to detect leptin mRNA in any of the five normal pituitaries removed at autopsy; however, immunostaining of a non-tumorous pituitary adjacent to an adenoma removed at transsphenoidal surgery showed scattered leptin positive cells. Culture of pituitary adenomas showed that 16/47 released leptin into the incubation media. Leptin release did not correlate with tumour type or with any of the other pituitary hormones released. In vitro leptin stimulation of pituitary tumours caused stimulation of FSH and alpha-subunit secretion from a non-functioning adenoma and TSH secretion from a somatotroph adenoma.

CONCLUSION

We conclude that not only is leptin stored within the pituitary, but it may also be released from pituitary cells and modulate other pituitary hormone secretion. Pituitary leptin may therefore be a novel paracrine regulator of pituitary function.

Influence of cortisol status on leptin secretion

The discovery of the adipocyte-produced hormone leptin has changed the field of obesity research and our understanding of energy homeostasis. It is now accepted that leptin is the afferent loop informing the hypothalamus about the states of fat stores, with hypothalamic efferents regulating appetite and energy expenditure. I addition, leptin has a role as a metabolic adaptator in overweight and fasting states. New and previously unsuspected neuroendocrine roles have emerged for leptin. Leptin participates in the expression of CRH in the hypothalamus, interacts at the adrenal level with ACTH, and is regulated by glucocorticoids. Since leptin and cortisol show an inverse circadian rhythm, it has suggested that a regulatory feedback is present. However glucocorticoids appears to play a modulatory, but not essential roles in generating leptin diurnal rhythm. Glucocortiocids act directly on the adipose tissue and increase leptin synthesis and secretion in humans. Leptin levels are markedly increased in Cushing's syndrome patients and in other pseudo-Cushing's syndrome states. Glucocorticoids appears to act as a key modulator of body weight and food intake, promoting leptin secretion by adipocytes, limiting central leptin induced effects and favoring those of the NPY. Furthermore the modulatory role of glucocorticoids could be altered in obesity, but the precise mode of action remains to be established. The relevance of this finding merits further studies.

Leptin and leptin receptor in anterior pituitary function

Leptin is a 16 kDa protein that exerts important effects on the regulation of food intake and energy expenditure by interacting with the leptin receptor in the brain and in many other tissues. Although leptin is produced mainly by white adipose tissue, several laboratories have shown low levels of leptin production by a growing number of tissues including the anterior pituitary gland. Many studies have implicated leptin in anterior pituitary function including the observation that homozygous mutations of the leptin receptor gene led to morbid obesity, lack of pubertal development and decreased GH and TSH secretion. In addition, leptin functions as a neuroendocrine hormone and regulates many metabolic activities. Leptin also interacts with and regulates the hypothalamic-pituitary-adrenal, the hypothalamic-pituitary-thyroid and the hypothalamic-pituitary-gonadal axes. All of the anterior pituitary cell types express the leptin receptor. However, leptin has been localized in specific subtypes of anterior pituitary cells indicating cell type-specific production of leptin in the anterior pituitary. Subcellular localization of leptin indicates co-storage with secretory granules and implicates hypothalamic releasing hormones in leptin secretion from anterior pituitary hormone cells. Leptin signal transduction in the anterior pituitary has been shown to involve the janus protein-tyrosine kinase (JAK)/signal transducer and activation of transcription (STAT) as well as suppressor of cytokine signalling (SOCS). These proteins are activated by tyrosine-phosphorylation in anterior pituitary cells. The various steps in pituitary leptin signal transduction remain to be elucidated.

Leptin signal transduction in the HP75 human pituitary cell line

Leptin is an adipocyte-derived cytokine with many functions including signaling the status of body energy stores through activation of the leptin receptor (OBR). Activation of the long form of OB-R (OB-Rb) results in JAK2 phosphorylation, activation of STATs, and subsequent gene expression. Activated STAT3 induces SOCS-3 expression in some cell types, which in turn down-regulates the JAK/STAT pathway. Although both leptin and OB-R are expressed in pituitary cells, the mechanism of signal transduction and its regulation in this organ has not been studied extensively. In these experiments we show that leptin reduces proliferation in a human pituitary cell line (HP75) and also increased apoptosis in these cells. Leptin also increased SOCS-3 mRNA and protein expression and tyrosine-phosphorylation in the HP75 human pituitary cell line. These findings suggest that SOCS-3 plays an important role in the inhibition of proximal leptin signal transduction in the anterior pituitary.

Fetal pancreatic islets express functional leptin receptors and leptin stimulates proliferation of fetal islet cells

OBJECTIVE

Previous studies have demonstrated that leptin can stimulate proliferation of insulin-secreting tumor cell lines. The objective of this study was to characterize whether leptin could stimulate proliferation of primary beta-cells too. Since adult beta-cells have very limited capacity for replication, we examined the effect of leptin on islets of Langerhans obtained from fetal rats, in a tissue culture system.

METHODS

Leptin receptor mRNA and c-fos mRNA were measured by RT-PCR. Proliferation of fetal rat islet cells was measured by a WST-1 colorimetric assay and [3H]-thymidine incorporation assay.

RESULTS

Leptin stimulated proliferation of serum-deprived fetal rat islet cells, as indicated by increased formation of formazan dye from a tetrazolium salt WST-1. Leptin stimulated DNA synthesis in islet cells, as indicated by increased [3H]-thymidine incorporation into DNA. The effect of leptin on islet cell proliferation was on average 39-50% of the effect obtained with 10% fetal bovine serum. Leptin increased c-fos mRNA expression by 2.8-fold in isolated fetal islets after 30 min treatment. In fetal pancreatic islets, both the common extracellular portion (OB-R) and the intact long form (OB-Rb) of the leptin receptor were readily detected by reverse transcriptase polymerase chain reaction.

CONCLUSION

Functional leptin receptors are expressed in pancreatic islet cells, as early as during the fetal stage of development of these microorgans. Leptin stimulates proliferation of fetal islet cells and might play a role in determining islet cell mass at birth.

Subcellular localization of leptin in non-tumorous and adenomatous human pituitaries: an immuno-ultrastructural study

Leptin is a key mediator in the maintenance of neuroendocrine homeostasis. Recently, leptin and leptin receptor expression were demonstrated in non-tumorous and adenomatous human pituitaries. This study was performed to determine the subcellular localization of leptin in human adenohypophyses (n = 3) and in various types of pituitary adenoma (n = 16). Immunoelectron microscopy showed leptin immunolabeling in most hormone-producing cells of the human non-tumorous adenohypophysis, but not in stellate cells. Labeling was noted over secretory granules. Immunocytochemistry using double labeling revealed leptin expression in GH-, ACTH-, TSH-, and FSH/LH-containing cells but not in PRL cells. The percentage of immunopositive cells and the intensity of immunostaining varied considerably among the various cell types. Immunoelectron microscopy with double gold labeling showed co-localization of leptin and adenohypophysial hormones in the same secretory granules. Among pituitary tumors, leptin immunolabeling was evident only in corticotroph adenomas. Compared to non-tumorous corticotrophs, leptin immunoexpression was less abundant in corticotroph adenomas. The presence of leptin and adenohypophysial hormones in the same secretory granules suggests that leptin is secreted concomitantly with various adenohypophysial hormones and that its release is under the control of hypothalamic stimulating and inhibiting hormones.

Differential regulation of leptin receptor expression by insulin and leptin in neuroblastoma cells

Leptin exerts its effects by interacting with specific membrane receptors (Ob-R). We studied the exact localization of long intracellular domain form (Ob-Rb) in human brain. In addition, we analyzed the regulatory features of Ob-Rb expression in two neuroblastoma cell lines. The Ob-Rb mRNAs were abundant in putamen, frontal lobe, medulla, cerebral cortex, cerebellum, thalamus, hippocampus, corpus callosum, caudate nucleus, and amygdala, indicating that Ob-Rb transcripts are expressed differently from that of other Ob-R isoforms. In SK-N-MC cells, the expression of Ob-Rb mRNA was induced by increasing doses of insulin, and the maximum amount of mRNA expression was 9.4-fold higher in the presence of insulin (100 nM for 24 h), compared to the absence of insulin. In IMR32 cells, the transcripts were increased 4.0-fold when cells were incubated with 1 nM of insulin for 48 h. In contrast, Ob-Rb expression in IMR32 cells decreased to 18% of control following a 24-h incubation period with 50 ng/mL of leptin, compared to incubation in the absence of leptin. These results indicate that expression of Ob-Rb is differentially regulated by inhibitory signals of energy balance in neuroblastoma cells. The identification of the novel regulatory mechanisms involving the Ob-Rb isoform by insulin and leptin now makes it possible to elucidate the underlying mechanisms involving increased food intake and uncontrolled energy balance associated with leptin resistance in obese individuals.

Leptin and leptin receptor expression in rat and mouse pituitary cells

Leptin is a circulating hormone secreted mainly by adipose tissue. Recent studies have shown leptin production by other tissues, including the placenta, stomach, and mammary tissues. Various reports have suggested that the anterior pituitary may have a role in the regulatory effects of leptin. We recently localized leptin in the human anterior pituitary, but analysis of leptin in rodent pituitary has not been previously reported. In this study we examined rat and mouse pituitary tissues and various cell lines for leptin by RT-PCR, immunohistochemistry, and Western blotting. Leptin receptor messenger RNA was also examined in these tissues by RT-PCR. Leptin was present in a small percentage of rat (4.8 +/- 0.7%) and mouse (7 +/- 2%) pituitary cells. Colocalization studies with leptin and pituitary hormones showed leptin expression mainly in TSH cells (24 +/- 2% of TSH cells in the rat pituitary and 31 +/- 1% of TSH cells in the mouse pituitary). A folliculo-stellate (FS) cell line, TtT/GF, also expressed leptin. The long isoform of leptin receptor (OB-Rb) was present in normal pituitary and in various pituitary cell lines, including FS, GH3, and alphaT3-1 cells. Treatment of GH3 and FS cells with leptin (1 x 10(-8) M) inhibited cell proliferation assessed by [3H]thymidine incorporation in GH3, but not in FS, cells. These findings show for the first time that leptin is expressed in rat and mouse anterior pituitaries mainly by TSH cells and by a mouse FS cell line. The finding of leptin and of the long isoform of leptin receptor in normal rat and mouse pituitaries and in various cell lines implicates an autocrine/paracrine loop in the production and regulation of leptin and leptin receptor in the rodent pituitary.

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.

Pituitary hormone gene expression and secretion in human growth hormone-releasing hormone transgenic mice: focus on lactotroph function

The human GH-releasing hormone (hGHRH) transgenic mouse has a hyperplastic anterior pituitary gland that eventually develops into an adenoma. We showed previously that the number of lactotrophs in the male hGHRH transgenic mouse is increased 2-fold, yet there is no concomitant increase in plasma levels of PRL. To further elucidate underlying changes in lactotroph function in the hGHRH transgenic mouse, the objectives of this study were to 1) examine the relative differences in PRL gene expression in transgenic mice and their siblings, 2) quantify PRL secretion at the level of the individual cell, 3) determine whether tyrosine hydroxylase gene expression and/or activity are altered in the hypothalamus of transgenic mice, and 4) assess dopamine receptor gene expression and functional sensitivity in lactotrophs of transgenic mice. Total PRL messenger RNA (mRNA) levels were increased nearly 5-fold in the hGHRH transgenic mouse, whereas the concentrations of PRL mRNA (PRL mRNA per microg total RNA) were unchanged. In contrast, total PRL contents were unchanged, whereas the concentrations of PRL (micrograms of PRL per mg total protein) were decreased 3-fold. Hypothalamic tyrosine hydroxylase steady state mRNA levels were not altered in the hGHRH transgenic mice, but hypothalamic tyrosine hydroxylase activity was increased 2-fold in transgenic mice. Dopamine D2 receptor mRNA concentrations in the anterior pituitary were increased 2.5-fold in hGHRH transgenic mice, and total pituitary D2 receptor mRNA levels were increased nearly 10-fold. Furthermore, the basal secretory capacity of lactotrophs from transgenic mice was increased significantly at the level of the single cell, and dopamine inhibited the secretion of PRL to a greater extent in hGHRH transgenic mice. Thus, although the total number of lactotrophs is increased 2-fold in hGHRH transgenic mice, the present data are consistent with the hypothesis that increased hypothalamic dopamine synthesis and release coupled with an increase in D2 dopamine receptor gene expression and functional sensitivity in the pituitary result in normal plasma levels of PRL.

Targeted overexpression of galanin in lactotrophs of transgenic mice induces hyperprolactinemia and pituitary hyperplasia

We generated transgenic mice that carry 4.6 kb of the mouse galanin gene fused to 2.5 kb of the rat PRL promoter. In all transgenic lines that carried and transmitted the transgene, there were significant increases in galanin messenger RNA and peptide levels in the anterior pituitary in both male and female transgenic mice, and the elevation of galanin was restricted to the anterior lobe. Furthermore, galanin release from pituitary cells in vitro of both male and female transgenic mice was dramatically increased compared with that in control mice. At 2-4 months of age, pituitary PRL contents in female transgenic mice were increased compared with those in normal controls. Moreover, PRL messenger RNA levels were increased in female transgenic mice. However, plasma levels of PRL in female transgenic mice were not significantly higher until 6 months of age. By 11 months of age, cell numbers in the anterior pituitary were increased in female, but not male, transgenic mice. The percentage of lactotrophs in female transgenic mice as well as PRL gene expression per cell were significantly higher. No differences were detected in PRL content, gene expression, or release between normal and transgenic male mice. Six weeks of estrogen treatment significantly increased anterior pituitary weights and PRL secretion in male transgenic mice compared with that in normal male mice. In addition, anterior pituitary weights and PRL secretion were decreased in female transgenic mice compared with controls 6 weeks after ovariectomy. We conclude that overexpression of galanin in lactotrophs stimulates PRL synthesis and secretion and acts as a growth factor resulting in the formation of pituitary hyperplasia and hyperprolactinemia. Furthermore, estrogen appears critical for these galanin-mediated events.

Neuroendocrine regulation and actions of leptin

The discovery of the adipocyte-produced hormone leptin has greatly changed the field of obesity research and our understanding of energy homeostasis. It is now accepted that leptin is the afferent loop informing the hypothalamus about the state of fat stores, with hypothalamic efferents regulating appetite and energy expenditure. In addition, leptin has a role as a metabolic adaptator in overweight and fasting states. New and previously unsuspected neuroendocrine roles have emerged for leptin. In reproduction, leptin is implicated in fertility regulation, and it is a permissive factor for puberty. Relevant gender-based differences in leptin levels exist, with higher levels in women at birth, which persist throughout life. In adult life, there is experimental evidence that leptin is a permissive factor for the ovarian cycle, with a regulatory role exerted at the hypothalamic, pituitary, and gonadal levels, and with unexplained changes in pregnancy and postpartum. Leptin is present in human milk and may play a role in the adaptive responses of the newborn. Leptin plays a role in the neuroendocrine control of GH secretion, through a complex interaction at hypothalamic levels with GHRH and somatostatin. Leptin participates in the expression of CRH in the hypothalamus, interacts at the adrenal level with ACTH, and is regulated by glucocorticoids. Since leptin and cortisol show an inverse circadian rhythm, it has been suggested that a regulatory feedback is present. Finally, regulatory actions on TRH-TSH and PRL secretion have been found. Thus leptin reports the state of fat stores to the hypothalamus and other neuroendocrine areas, and the neuroendocrine systems adapt their function to the current status of energy homeostasis and fat stores.

The human growth hormone-releasing hormone transgenic mouse as a model of modest obesity: differential changes in leptin receptor (OBR) gene expression in the anterior pituitary and hypothalamus after fasting and OBR localization in somatotrophs

We reported previously an increase in leptin receptor (OBR) gene expression in the anterior pituitary of human GH-releasing hormone (hGHRH) transgenic mice. The primary goal of this study was to investigate the possible mechanisms regulating OBR expression in these mice. Compared with normal sibling controls, hGHRH transgenic mice had significantly greater amounts of abdominal fat, higher levels of leptin messenger RNA (mRNA), and a 2-fold increase in plasma leptin concentrations. Despite normal plasma glucose levels, hGHRH transgenic mice had 4.5-fold elevated levels of plasma insulin. Using a ribonuclease protection assay, we measured the mRNA levels of the OBR long form (OBR(L)) in the anterior pituitary and hypothalamus after 48 h of fasting. In the anterior pituitary, food deprivation induced dramatic increases in OBR(L) mRNA levels in both normal and transgenic mice. In contrast, in the hypothalamus, fasting resulted in a significant decrease in OBR(L) gene expression in normal mice, and no changes were detected in hGHRH transgenic mice. Using dual in situ hybridization, OBR(L) mRNA was detected in somatotrophs. Moreover, the number of OBR(L)-positive pituitary cells as well as the percentage of OBR(L)-positive cells that express GH mRNA were increased in transgenic mice. In conclusion, 1) the modest obesity in hGHRH transgenic mice is associated with increases in leptin synthesis and secretion as well as insulin secretion; 2) GH and/or GHRH as well as leptin and insulin may differentially contribute to the changes in OBR(L) gene expression in the anterior pituitary and the hypothalamus; 3) the response of OBR(L) gene expression in the hypothalamus to fasting is absent in the modestly obese hGHRH transgenic mice; and 4) somatotrophs are target cells for leptin, and the increase in OBR(L) gene expression in the pituitary of hGHRH transgenic mice is due at least in part to the increase in the number of cells expressing OBR(L).

Leptin and leptin receptor expression in normal and neoplastic human pituitary: evidence of a regulatory role for leptin on pituitary cell proliferation

Leptin is a circulating hormone secreted by adipose and a few other tissues. The leptin receptor consists of a single transmembrane-spanning polypeptide that is present as a long physiologically important form as well as in several short isoforms. Recent studies have suggested that the anterior pituitary may have a role in the regulatory effects of leptin in animal models. To test this possibility in human pituitaries, we examined the expression of leptin and OB-R in normal and neoplastic pituitaries, and the possible functions of leptin in the pituitary were also analyzed. Leptin was present in 20-25% of anterior pituitary cells and was expressed in most normal anterior pituitary cells, including ACTH (70% of ACTH cells), GH (21%), FSH (33%), LH (29%), TSH (32%), and folliculo-stellate cells (64%), but was colocalized with very few PRL cells (3%), as detected by double labeling immunohistochemistry with two different antileptin antibodies. In addition, leptin expression was detected by RT-PCR in some pituitary tumors, including ACTH (three of four), GH (one of four), null cells (two of four), and gonadotroph (one of four) tumors as well as in normal pituitary. Immunohistochemical staining showed greater immunoreactivity for leptin in normal pituitaries compared to adenomas. Treatment of an immortalized cultured anterior pituitary cell line, HP75, with leptin stimulated pancreastatin secretion in vitro. Leptin also inhibited cell growth in the human HP75 and in the rat pituitary GH3 cell lines. Both long (OB-Rb) and common (OB-Ra) forms of the leptin receptor messenger ribonucleic acid and leptin receptor protein were expressed in normal and neoplastic anterior pituitary cells. These findings show for the first time that leptin is expressed by most human anterior pituitary cell types and that there is decreased leptin protein immunoreactivity in pituitary adenomas compared to that in normal pituitary tissues. We also show that OB-Rb is widely expressed by normal and neoplastic anterior pituitary cells, implicating an autocrine/paracrine loop in the production and regulation of leptin in the pituitary.

Galanin within the normal and hyperplastic anterior pituitary gland: localization, secretion, and functional analysis in normal and human growth hormone-releasing hormone transgenic mice

Studies evaluating estrogen-induced anterior pituitary tumors revealed a strong direct correlation between expression of the peptide galanin and tumor growth. To evaluate further the potential roles of galanin in the hyperplastic pituitary, we used a model of estrogen-independent anterior pituitary tumor formation, the male human GH-releasing hormone (hGHRH) transgenic mouse. Pituitaries of hGHRH transgenic mice are characterized by a hyperplasia of somatotrophs and contain markedly elevated levels of galanin. We examined the population of galanin-producing pituitary cells in 4- to 6-month-old male hGHRH transgenic mice and their nontransgenic siblings. The percentage of galanin-containing pituitary cells was significantly increased within the anterior pituitaries of hGHRH transgenic mice. By using the cell immunoblot assay we found that the basal secretion of galanin and GH from individual pituitary cells of hGHRH transgenic mice was significantly greater than that from pituitary cells of nontransgenic mice. By modifying the cell immunoblot assay, we determined that somatotrophs from both hGHRH transgenic and normal mice that were positive for galanin immunoreactivity secreted significantly greater amounts of GH than those somatotrophs devoid of galanin immunoreactivity. Moreover, immunoneutralization of galanin significantly decreased GH secretion from pituitary cells obtained from hGHRH transgenic mice. Thus, we now show that the increased levels of galanin peptide within the hyperplastic pituitaries of hGHRH transgenic mice are due to an increase in the population of cells containing galanin, and that galanin participates in the augmented secretion of GH from hyperplastic proliferating pituitary cells.