Hyperphagia and central mechanisms for leptin resistance during pregnancy

The purpose of this work was to study the central mechanisms involved in food intake regulation and leptin resistance during gestation in the rat. Sprague Dawley rats of 7, 13, and 18 d of pregnancy [days of gestation (G) 7, G13, and G18] were used and compared with nonpregnant animals in diestrus-1. Food intake was already increased in G7, before hyperleptinemia and central leptin resistance was established in midpregnancy. Leptin resistance was due to a reduction in leptin transport through the blood-brain barrier (BBB) and to alterations in leptin signaling within the hypothalamus based on an increase in suppressor of cytokine signaling 3 levels and a blockade of signal transducer and activator of transcription-3 phosphorylation (G13), followed by a decrease in LepRb and of Akt phosphorylation (G18). In early gestation (G7), no change in hypothalamic neuropeptide Y (NPY), agouti-related peptide (AgRP), or proopiomelanocortin (POMC) expression was shown. Nevertheless, an increase in NPY and AgRP and a decrease in POMC mRNA were observed in G13 and G18 rats, probably reflecting the leptin resistance. To investigate the effect of maternal vs. placental hormones on these mechanisms, we used a model of pseudogestation. Rats of 9 d of pseudogestation were hyperphagic, showing an increase in body and adipose tissue weight, normoleptinemia, and normal responses to iv/intracerebroventricular leptin on hypothalamic leptin signaling, food intake, and body weight. Leptin transport through the BBB, and hypothalamic NPY, AgRP and POMC expression were unchanged. Finally, the transport of leptin through the BBB was assessed using a double-chamber culture system of choroid plexus epithelial cells or brain microvascular endothelial cells. We found that sustained high levels of prolactin significantly reduced leptin translocation through the barrier, whereas progesterone and β-estradiol did not show any effect. Our data demonstrate a dual mechanism of leptin resistance during mid/late-pregnancy, which is not due to maternal hormones and which allows the maintenance of hyperphagia in the presence of hyperleptinemia driven by an increase in NPY and AgRP and a decrease in POMC mRNA. By contrast, in early pregnancy maternal hormones induce hyperphagia without the regulation of hypothalamic NPY, AgRP, or POMC and in the absence of leptin resistance.

Cellular localization of orexin receptors in human pituitary

Orexins-A and -B are hypothalamic peptides derived from a precursor called prepro-orexin and relationated with the stimulation of food intake. They act on G protein receptors named orexin receptor 1 (OX(1)R) and orexin receptor 2 (OX(2)R), respectively. In the present study, we used RT-PCR and immunohistochemical techniques to detect the presence of OX(1)R and OX(2)R in human pituitary. A band of the expected size for both OX(1)R and OX(2)R was shown in human pituitary by RT-PCR. The cellular localization of OX(1)R and OX(2)R was carried out using histological techniques. By consecutive sections we demonstrated that OX(1)R was present in acidophil, diffusely distributed cells, which represent the half of the total adenohypophysis cell population. As was expected, these cells were shown to coexpress GH. OX(2)R was found in the pars intermedia and in clusters of basophil cells of the anterior pituitary, which coexpress ACTH. These results were confirmed by double immunofluorescence techniques. We also found focal positivity in axon terminals of neurohypophysis, more intense for OX(2)R than for OX(1)R. In conclusion, these results demonstrated for the first time that OX(1)R and OX(2)R were expressed by somatotrope and corticotrope cells, respectively.

Role of growth hormone receptor in HL-60 cell survival

Although it is presently well established that locally produced growth hormone (GH) plays a major role in the regulation of survival mechanisms in hemopoietic cells, the responsible mechanisms are poorly understood, and the involvement of the GH receptor (GHR) has not even been demonstrated to date. In this work we investigated the presence of GHR in the human promyelocytic leukemia cell line HL-60, as well as the ability of GH treatment to stimulate both GHR and survival signaling pathways downstream GHR. Our results demonstrate that (1) both GHR mRNA and GHR immunoreactivity are present in HL-60 cells; (2) GH treatment results in an increase in the phosphorylation of the GHR-associated Jak2 and Stat3 proteins, indicating the ability of the hormone to induce receptor activation; and (3) activation of GHR increases the activity of Akt, a serine/threonine kinase that plays a prominent role in the regulation of cell survival. Taken together, these results demonstrate that GHR activation promotes survival of HL-60 cells, thus suggesting that GH plays a major role in the regulation of cell survival in the hemopoietic system, via an autocrine/paracrine mechanism.

Expression of growth hormone receptor in the human brain

This study was designed to investigate the presence of growth hormone receptor (GHR) expression in the human brain tissue, both normal and tumoral, as well as in the human glioblastoma cell line U87MG. Reverse transcription-polymerase chain reaction revealed the presence of GHR mRNA in all brain samples investigated and in U87MG cells. GHR immunoreactivity was also detected in this cell line using both immunocytochemistry and western blotting. All together, our data demonstrate the existence of GHR expression within the central nervous system (CNS), thus supporting a possible role for GH in the CNS physiology.

Leptin regulation of prepro-orexin and orexin receptor mRNA levels in the hypothalamus

The aim of this study was to determine the effects of leptin treatment on prepro-orexin and orexin receptor expression in the rat hypothalamus. Adult male rats, food-deprived for 48 and 72 h, were treated one time with vehicle or leptin (10 microg, icv). Prepro-orexin mRNA content was measured by semiquantitative RT-PCR, Northern blot, and in situ hybridization; orexin receptor 1 and 2 mRNA content was quantified by Northern blot and/or semiquantitative RT-PCR. Our results indicate that leptin inhibits a fasting-induced increase in prepro-orexin mRNA and orexin receptor 1 mRNA levels in the rat hypothalamus, while orexin receptor 2 mRNA levels were unchanged in all situations evaluated. These data provide direct evidence for an additional mechanism of adaptation of the hypothalamus to food deprivation and for a new effect of leptin in the regulation of food intake.

Orexin receptors are expressed in the adrenal medulla of the rat

Two recently discovered hypothalamic peptides, orexin-A and orexin-B, play a role as mediators in the central mechanisms that regulate feeding behavior and sleep control. These peptides bind and activate two orexins receptors that belong to the G-protein coupled receptor superfamily. Morphological studies have detected mRNA expression of orexin receptors exclusively in the rat central nervous system. In this paper we demonstrate a strong level of expression of orexin receptor 1 and 2 in the adrenal medulla of the rat by RT-PCR and immunohistochemistry. The results of the present study provide the first evidence showing that the adrenal medulla expresses orexin receptors, and thus appears to be a target tissue for orexins. This could open a new loop in which the central and autonomous nervous system may be involved in body weight homeostasis and sleep control.

Prolactin stimulates leptin secretion by rat white adipose tissue

Leptin, the obese (Ob) gene product, is an adipocyte-derived satiety factor that is involved in the regulation of food intake and body weight. Leptin signals nutritional status to several other physiological systems and modulates their function. As PRL is involved in energy and lipid metabolism, this study was undertaken to investigate the role of PRL on in vivo regulation of leptin serum concentration and Ob messenger RNA expression in white adipose tissue in rats. It was found that increased serum PRL levels, obtained by pituitary graft or exogenous injected ovine PRL (oPRL, 5 mg/kg), significantly stimulate serum leptin concentration. A significant increase (P < 0.01) in serum leptin concentration was present in hyperprolactinemic animals (4.7+/-0.4 microg/liter) in comparison to controls (1.2+/-0.1 microg/liter and 1.09+/-0.09 microg/liter of intact sham operated and ovariectomized rats, respectively). Similar results were obtained in oPRL-treated animals where leptin levels were 5.4+/-0.1 microg/liter vs. 1.1+/-0.1 microg/liter and 0.8+/-0.08 microg/liter of intact sham operated rats and ovariectomized, respectively (P < 0.001). This stimulatory effect of PRL on serum leptin levels was significantly reduced by food deprivation (P < 0.01) where serum leptin levels were 12.5+/-0.65 microg/liter in grafted animals vs. 3.2+/-0.36 microg/liter of grafted animals subjected to 48 h of food deprivation. Moreover, in vivo, PRL was able to induce leptin messenger RNA levels in several areas of rat white adipose tissue. The data demonstrate that PRL acts on the adipose tissue increasing leptin synthesis and secretion, suggesting a new role for this lactogenic hormone in the regulation of food intake.

Correlation of Pit-1 gene expression and Pit-1 content with proliferation and differentiation in human myeloid leukemic cells

The transcription factor pituitary-1 (Pit-1) is a homeodomain-containing protein that is expressed mainly in the pituitary, where it drives the expression of growth hormone, prolactin, and thyroid-stimulating hormone beta chain genes. In addition, Pit-1 is required for adequate pituitary cell growth and may be involved in the pathogenesis of pituitary adenomas. Pit-1 expression has been also reported in nonpituitary tissues, where it might be involved in the control of cell proliferation. In order to elucidate such a possibility, we have investigated the changes in both Pit-1 mRNA and Pit-1 immunoreactivity in HL-60 cells following the addition of several differentiating agents. Our results show that while high Pit-1 levels are found in exponentially growing HL-60 cells, a significant decrease occurs after induction of cells to differentiate along the macrophage lineage with 12-O-tetradecanoylphorbol-13-acetate (TPA). In contrast no changes were observed when cells were treated with interferon-alpha, which also induces differentiation of HL-60 cells that, at odds with TPA, is not accompanied with growth arrest. In all, these findings suggest that Pit-1 expression is specifically associated with proliferation in HL-60 cells, thus supporting the idea that one of the functions of nonpituitary Pit-1 may be the control of cell proliferation.

Synthesis of leptin in human placenta

Gender-based differences in serum leptin levels have been reported in umbilical cord blood, and leptin has been detectedin human amniotic fluid. In order to understand if leptin may be directly synthesized by human placentae an analysis made up of several steps was performed. First at all RT-PCR analysis from placenta-derived RNA was used to detect human leptin mRNA. The leptin-like immunoradioactivity detected in placentae extracts was identical to human leptin according to the criteria of charge, immunorecognition, SDS-PAGE analysis and blotting, indicating that intact leptin was found and no variants in size, charge or immunoactivity were present in the placentae. Finally an immunohistochemical analysis showed the presence of leptin in the cytoplasm of syncytiotrophoblast cells but not in the core of villi.

IN CONCLUSION

leptin is synthesized as a single molecular variant identical to human recombinant leptin in human placentae at delivery.