Differential regulation of GHRH-receptor and GHS-receptor expression by long-term in vitro treatment of ovine pituitary cells with GHRP-2 and GHRH

The allostery and modification of hGHRH molecules and specific dimer produced significant fertility effect by proliferating and activating in-situ ovarian mesenchymal stem cells

The negative coordination of growth hormone secretagogue receptor (GHS-R) and growth hormone-releasing hormone receptor (GHRH-R) involves in the repair processes of cellular injury. The allosteric U- or H-like modified GHRH dimer Grinodin and 2Y were comparatively evaluated in normal Kunming mice and hamster infertility models induced by CPA treatment. 1-3-9 µg of Grinodin or 2Y per hamster stem-cell-exhaustion model was subcutaneously administered once a week, respectively inducing 75-69-46 or 45-13-50 % of birth rates. In comparison, the similar mole of human menopausal gonadotropin (hMG) or human growth hormone (hGH) was administered once a day but caused just 25 or 20 % of birth rates. Grinodin induced more big ovarian follicles and corpora lutea than 2Y, hMG, hGH. The hMG-treated group was observed many distorted interstitial cells and more connective tissues and the hGH-treated group had few ovarian follicles. 2Y had a plasma lifetime of 21 days and higher GH release in mice, inducing lower birth rate and stronger individual specificity in reproduction as well as only promoting the proliferation of mesenchymal-stem-cells (MSCs) in the models. In comparison, Grinodin had a plasma lifetime of 30 days and much lower GH release in mice. It significantly promoted the proliferation and activation of ovarian MSCs together with the development of follicles in the models by increasing Ki67 and GHS-R expressions, and decreasing GHRH-R expression in a dose-dependent manner. However, the high GH and excessive estrogen levels in the models showed a dose-dependent reduction in fertility. Therefore, unlike 2Y, the low dose of Grinodin specifically shows low GHS-R and high GHRH-R expressions thus evades GH and estrogen release and improves functions of organs, resulting in an increase of fertility.

Growth hormone releasing hormone in the unfolded protein response context

The effects of Growth Hormone Releasing Hormone in human pathophysiology are not limited to those mediated by the Growth Hormone Releasing Hormone-Growth Hormone-Insulin-like Growth Factor-I axis. Receptors specific for this neuropeptide are expressed in a diverse variety of human tissues, to initiate multifarious signaling cascades, regulators of cellular homeostasis and survival. The Unfolded Protein Response is in charge of adaptive responses towards a plethora of challenges, able to trigger cellular repair or death. The possible involvement of Growth Hormone Releasing Hormone and its agonistic and antagonistic analogs in those events, may deliver exciting possibilities in the treatment of human disease, including the Acute Respiratory Distress Syndrome.

Effect of the Orally Active Growth Hormone Secretagogue MK-677 on Somatic Growth in Rats

PURPOSE

Growth hormone secretagogues (GHSs) possess the ability to release growth hormone (GH) in the body. This study aimed to investigate the effects of MK-677, an orally active GHS, on somatic growth in rats.

MATERIALS AND METHODS

The serum levels of GH were measured after oral administration of MK-677 to confirm GH stimulatory effects. Body weight, body length, tibia length, epiphyseal plate width, and serum levels of insulin-like growth factor (IGF)-I were measured after oral administration of 4 mg/kg of MK-677 for 6 weeks to investigate growth-promoting effects.

RESULTS

Oral administration of MK-677 at 4 mg/kg increased peak GH concentrations by 1.8-fold, compared to baseline. However, oral administration of MK-677 for 6 weeks did not increase body growth or serum levels of IGF-I. At 6 weeks after treatment, the GH response to MK-677 was abolished. Pituitary GH mRNA and hypothalamic GH-releasing hormone mRNA, and GH secretagogue receptor (GHSR) mRNA expression in the pituitary and hypothalamus did not differ between the control and treatment group. Somatostatin (SST) mRNA expression in the hypothalamus was markedly increased in the treatment group, whereas SST receptor (SSTR)-2 mRNA expression in the pituitary gland was decreased. Protein expression of hypothalamic GHSR, SST, and pituitary SSTR-2 showed patterns similar to those for mRNA expression.

CONCLUSION

Our results suggest that prolonged administration of MK-677 in rats does not promote growth despite the GH stimulatory effect of MK-677, which may be related to increased expression of SST in the hypothalamus. Further studies are needed to overcome the observed desensitization to GHS.

Influence of season and nutritional status on the direct effects of leptin, orexin-A and ghrelin on luteinizing hormone and growth hormone secretion in the ovine pituitary explant model

The aim of this study was to examine whether leptin (anorexigenic peptide), orexin-A, and ghrelin (orexigenic peptides) could directly (ie, independently of hypothalamic influences) affect the secretion of luteinizing hormone (LH) and growth hormone (GH) by adenohypophyseal (AP) explants obtained from normally fed or fasted (48 h) ewes during the breeding and nonbreeding seasons. In addition, a specific ovine super leptin antagonist (SLAN-3) was used to assess the interactions between leptin and ghrelin and/or orexin-A. Pituitary glands from 16 ovariectomized Polish Longwool ewes that had received estradiol-releasing subcutaneous implants were collected in the breeding (November; n = 8) and nonbreeding (May; n = 8) seasons. The AP explants were incubated for 240 min in a gas-liquid interface and treated with leptin (50 ng/mL), ghrelin (100 ng/mL), orexin-A (100 ng/mL), and SLAN-3 (500 ng/mL) with orexin-A or ghrelin. Treatments with leptin and SLAN-3 + orexin-A increased (P < 0.05) LH concentrations in the cultures of AP explants from fasted animals in the breeding season. Orexin-A increased (P < 0.05) LH secretion by AP explants from both fasted and fed animals in the breeding season. Ghrelin stimulated (P < 0.05) GH secretion by AP explants collected from fasted animals in nonbreeding season and from normally fed ewes in both seasons. Leptin decreased (P < 0.05) GH secretion by AP explants collected from fasted ewes in both seasons and from nonfasted ewes in the breeding season. However, the treatment with SLAN-3 + ghrelin resulted in greater (P < 0.05) GH concentrations compared with leptin treatment of AP explants from fasted ewes in the breeding season and from normally fed ewes in nonbreeding season. In summary, leptin, orexin-A, and ghrelin exerted direct effects on AP secretory function in an ex situ model and both the reproductive season and nutritional status of the animals impinged on the direct effects of the peptides on LH and GH release. Specifically, orexin-A was more potent than leptin in directly stimulating LH secretion in cycling ewes, whereas ghrelin and leptin generally had opposing effects on the secretory function of somatotrophs in sheep.

Gonadotropin-inhibitory hormone (GnIH) secretion into the ovine hypophyseal portal system

GnIH was first identified in avian species, and there is now strong evidence that it is operant in mammals as an inhibitor of reproduction. Mammalian gonadotropin-inhibitory hormone (GnIH)-3 is encoded by the RFRP gene in neurons of the dorsomedial nucleus. These neurons project to the median eminence, predicting a role as a secreted neurohormone and regulation of the pituitary gonadotropes. To determine whether GnIH-3 is a secreted neurohormone, we measured its concentration in hypophyseal portal blood in ewes during the nonbreeding (anestrous) season and during the luteal and follicular phases of the estrous cycle in the breeding season. Paired portal and jugular blood samples were collected and plasma prepared for RIA using an ovine GnIH-3 antibody. Pulsatile GnIH-3 secretion was observed in the portal blood of all animals. Mean GnIH-3 pulse amplitude and pulse frequency was higher during the nonbreeding season. GnIH-3 was virtually undetectable in peripheral blood plasma. There was a lack of association between secretory pulses of GnIH-3 (portal) and LH (peripheral). To determine the role of secreted GnIH-3, we examined its effects on GnRH-stimulated LH secretion in hypothalamo-pituitary-disconnected ewes; a significant reduction in the LH response to GnRH was observed. Finally, to identify cellular targets in the pituitary, the expression of GnIH receptor [G protein-coupled receptor 147 (GPR147)] in fractions enriched for gonadotropes somatotropes, and lactotropes was examined; expression was observed in each cell type. These data show GnIH-3 is secreted into portal blood to act on pituitary gonadotropes, reducing the action of GnRH.

Nanobiology and physiology of growth hormone secretion

Growth hormone (GH) secretion is controlled by hypothalamic releasing hormones from the median eminence together with hormones and neuropeptides produced by peripheral organs. Secretion of GH involves movement of secretory vesicles along microtubules, transient ‘docking’ with the porosome in the cell membrane and subsequent release of GH. Release of GH is stimulated by GH releasing hormone (GHRH) and inhibited by somatostatin (SRIF). Ghrelin may be functioning to stimulate GH release from somatotropes acting via the GH secretagogue (GHS) receptor (GHSR). However, recent physiological studies militate against this. In addition, ghrelin does influence GH release acting within the hypothalamus. Release of GH from the somatotropes involves the GH-containing secretory granules moving close to the cell surface followed by transitory fusion of the secretory granules with the porosomes located in multiple secretory pits in the cell membrane. Other peptides/proteins can influence GH secretion, particularly in species of non-mammalian vertebrates.

Homologous and heterologous in vitro regulation of pituitary receptors for somatostatin, growth hormone (GH)-releasing hormone, and ghrelin in a nonhuman primate (Papio anubis)

Secretion of GH by pituitary somatotrophs is primarily stimulated by GHRH and ghrelin and inhibited by somatostatin through the activation of specific receptors [GHRH receptor (GHRH-R), GH secretagogue receptor (GHS-R) and somatostatin receptors (sst1-5), respectively]. However, we have shown that somatostatin, at low doses, can also stimulate GH release, directly and specifically, in primary pituitary cultures from a nonhuman primate (baboons, Papio anubis) and pigs. To determine whether somatostatin, GHRH, and ghrelin can also regulate the expression of their receptors in primates, pituitary cultures from baboons were treated for 4 h with GHRH or ghrelin (10(-8) m) or with high (10(-7) m) and low (10(-15) m) doses of somatostatin, and GH release and expression levels of all receptors were measured. GHRH/ghrelin decreased the expression of their respective receptors (GHRH-R and GHS-R). Both peptides increased sst1, only GHRH decreased sst5 expression, whereas sst2 expression remained unchanged. The effects of GHRH/ghrelin were completely mimicked by forskolin (adenylate cyclase activator) and phorbol 12-myristate 13-acetate (protein kinase C activator), respectively, indicating the regulation of receptor subtype levels by GHRH and ghrelin involved distinct signaling pathways. In contrast, high-dose somatostatin did not alter GH release but increased sst1, sst2, and sst5 expression, whereas GHRH-R and GHS-R expression were unaffected. Interestingly, low-dose somatostatin increased GH release and sst1 mRNA but decreased sst5 and GHRH-R expression, similar to that observed for GHRH. Altogether, our data show for the first time in a primate model that the primary regulators of somatotroph function (GHRH/ghrelin/somatostatin) exert both homologous and heterologous regulation of receptor synthesis which is dose and subtype dependent and involves distinct signaling pathways.

Comparative analyses of sequence structure, evolution, and expression of four somatostatin receptors in orange-spotted grouper (Epinephelus coioides)

Somatostatins (SSs) and somatostatin receptors (SSTRs) play important roles in the growth, development and metabolism of vertebrates. In the present study, four SSTRs were isolated from orange-spotted grouper (Epinephelus coioides), a coral fish of high commercial value cultivated in Southeast Asia. Phylogenetic tree analysis grouped the four SSTRs as two distinct groups of SSTR1 and SSTR2/3/5. Four SSTRs exhibited high homology across the vertebrates. The expression of four grouper SSTR mRNAs was studied in 11 tissues. The highest level of SSTR1 mRNA was found in forebrain. The mRNAs of SSTR2 and SSTR3 were highly expressed in pituitary, forebrain and liver. The levels of SSTR5 mRNA were low in most tissues except for pituitary and intestine. The expression of four grouper SSTR mRNAs was investigated in seven embryonic stages and five early larval development stages. The highest levels of SSTR1 and 2 mRNAs appeared during hatching, while the highest levels of SSTR3 and 5 mRNAs were found in brain vesicle stage. Intraperitoneal injection of SS14 significantly increased the levels of all four SSTR mRNAs in pituitary and SSTR1, 3 mRNAs in liver in a dose-dependent manner, but no effect on SSTR2 and 5 in liver. These observations contribute to the understanding of the evolution of SSTR family and offer information on structure, distribution and function of fish SSTRs.

Resistin regulates pituitary somatotrope cell function through the activation of multiple signaling pathways

The adipokine resistin is an insulin-antagonizing factor that also plays a regulatory role in inflammation, immunity, food intake, and gonadal function. Although adipose tissue is the primary source of resistin, it is also expressed in other tissues and organs, including the pituitary. However, there is no information on whether resistin, as described previously for other adipokines such as leptin and adiponectin, could regulate this gland. Likewise, the molecular basis of resistin actions remains largely unexplored. Here we show that administration of resistin to dispersed rat anterior pituitary cells increased GH release in both the short (4 h) and long (24 h) term, decreased mRNA levels of the receptor of the somatotrope regulator ghrelin, and increased free cytosolic Ca(2+) concentration in single somatotropes. By means of a pharmacological approach, we found that the stimulatory action of resistin occurs through a Gs protein-dependent mechanism and that the adenylate cyclase/cAMP/protein kinase A pathway, the phosphatidylinositol 3-kinase/Akt pathway, protein kinase C, and extracellular Ca(2+) entry through L-type voltage-sensitive Ca(2+) channels are essential players in mediating the effects of resistin on somatotropes. Taken together, our results demonstrate for the first time a regulatory role for resistin on somatotrope function and provide novel insights on the intracellular mechanisms activated by this protein.

Regulation of cell invasion and signalling pathways in the pituitary adenoma cell line, HP-75, by reversion-inducing cysteine-rich protein with kazal motifs (RECK)

Degradation and remodelling of the extracellular matrix has been investigated, with the main focus on the balance between matrix metalloproteinases (MMP) and tissue inhibitor of metalloproteinases (TIMP). Recent reports disclose the presence of a novel MMP-inhibiting cell membrane-anchored glycoprotein designated 'reversion-inducing cysteine-rich protein with Kazal motifs' (RECK). Our main aim in this study was to elucidate the role of RECK in cell invasion of pituitary adenomas and its contribution to signal transduction. The function of RECK in cell invasion was investigated by comparing data obtained from full-length RECK clone transfection and gene silencing with RECK mRNA-targeting siRNA. RECK expression was confirmed using real-time RT-PCR and Western blotting. Levels of matrix metalloproteinases (MMP-2 and -9) and TIMP-1 were measured by zymography and reverse zymography, respectively. Cell invasion was examined with a 3-D invasion assay. The signal cascade was investigated by cDNA microarray analysis. As expected, expression of RECK was elevated upon cDNA transfection, and diminished using siRNA. We observed elevation of MMP-2 and -9 expression and consequent 3-D cell invasion in cells under-expressing RECK. However, TIMP expression was not affected by RECK. Analysis with cDNA microarray revealed that RECK additionally upregulates growth hormone-releasing hormone receptor (GHRHR) and latrophilin 2 at the transcriptional level. Our findings collectively suggest that RECK regulates the cell signalling pathway, playing a critical neuroendocrinological role in the pituitary adenoma cell line.

Kisspeptin is present in ovine hypophysial portal blood but does not increase during the preovulatory luteinizing hormone surge: evidence that gonadotropes are not direct targets of kisspeptin in vivo

There is strong evidence that kisspeptin acts to regulate GnRH secretion, but whether there is also a component of action on the gonadotropes is not clear. Using quantitative RT-PCR, we found that G protein-coupled receptor-54 mRNA is expressed in ovine pituitary cell fractions enriched for gonadotropes as well as in somatotropes and lactotropes. To test whether kisspeptin acts directly on the pituitary gonadotropes, we first examined LH release from primary ovine pituitary cell cultures treated with kisspeptin. We found that kisspeptin treatment increased the concentration of LH in culture media by 80%, compared with control, but only in pituitary cultures from ewes during the follicular phase of the estrous cycle. After this, we determined whether kisspeptin acts on the pituitary gland in vivo. Using GnRH-replaced ovariectomized hypothalamo-pituitary-disconnected ewes, we were not able to achieve any effect of kisspeptin on LH under steady-state conditions or during the period of an estrogen-induced LH surge. Finally, we collected hypophysial portal blood samples from ovariectomized ewes and measured kisspeptin levels. Low but detectable amounts of kisspeptin were found in portal plasma, but levels were similar in ovariectomized ewes that were untreated or given estrogen to elicit an LH surge. Thus, although we observed an effect of kisspeptin on LH release in vitro in some situations, similar findings were not obtained in vivo. Moreover, the low concentrations of kisspeptin in hypophysial portal blood and the lack of any change during the period of an estrogen-induced GnRH/LH surge suggest that action on the pituitary gland is not of major consequence in terms of LH release.

Evidence that ghrelin is as potent as growth hormone (GH)-releasing hormone (GHRH) in releasing GH from primary pituitary cell cultures of a nonhuman primate (Papio anubis), acting through intracellular signaling pathways distinct from GHRH

Ghrelin is more effective than GHRH in stimulating GH release in normal adult humans and monkeys in vivo. This robust effect of ghrelin has been largely attributed to regulation of hypothalamic input, whereas the direct effect of ghrelin on pituitary GH release has been minimized by the observation that ghrelin has only a modest impact on GH release, compared with GHRH, in cultures prepared from human fetal pituitaries and GH-producing adenomas, as well as pituitaries from nonprimate species. However, comparable in vitro studies have not been performed to test the direct effect of ghrelin on normal adult primates. Therefore, in the present study, primary pituitary cell cultures from female baboons (Papio anubis) were used as a model system to test the direct effects of ghrelin on primate somatotrope function. In this model, both ghrelin and GHRH increased GH release in a dose-dependent fashion. Surprisingly, at maximal concentrations (10 nM), both ghrelin and GHRH elicited a robust increase in GH release (4 and 24 h, respectively), and both up-regulated GH secretagogue-receptor and GHRH-receptor mRNA levels (24 h). Combined treatment with ghrelin and GHRH resulted in an additive effect on GH release, suggesting that distinct intracellular signaling pathways are activated by each ligand, as confirmed by the use of specific inhibitors of intracellular signaling. Together, these results present the first evidence that a direct effect of ghrelin on somatotrope function may play a major role in stimulating GH release in primates.