Growth hormone-releasing hormone: synthesis and signaling

Growth hormone-releasing hormone receptor antagonist MIA-602 attenuates cardiopulmonary injury induced by BSL-2 rVSV-SARS-CoV-2 in hACE2 mice

COVID-19 pneumonia causes acute lung injury and acute respiratory distress syndrome (ALI/ARDS) characterized by early pulmonary endothelial and epithelial injuries with altered pulmonary diffusing capacity and obstructive or restrictive physiology. Growth hormone-releasing hormone receptor (GHRH-R) is expressed in the lung and heart. GHRH-R antagonist, MIA-602, has been reported to modulate immune responses to bleomycin lung injury and inflammation in granulomatous sarcoidosis. We hypothesized that MIA-602 would attenuate rVSV-SARS-CoV-2-induced pulmonary dysfunction and heart injury in a BSL-2 mouse model. Male and female K18-hACE2tg mice were inoculated with SARS-CoV-2/USA-WA1/2020, BSL-2-compliant recombinant VSV-eGFP-SARS-CoV-2-Spike (rVSV-SARS-CoV-2), or PBS, and lung viral load, weight loss, histopathology, and gene expression were compared. K18-hACE2tg mice infected with rVSV-SARS-CoV-2 were treated daily with subcutaneous MIA-602 or vehicle and conscious, unrestrained plethysmography performed on days 0, 3, and 5 (n = 7 to 8). Five days after infection mice were killed, and blood and tissues collected for histopathology and protein/gene expression. Both native SARS-CoV-2 and rVSV-SARS-CoV-2 presented similar patterns of weight loss, infectivity (~60%), and histopathologic changes. Daily treatment with MIA-602 conferred weight recovery, reduced lung perivascular inflammation/pneumonia, and decreased lung/heart ICAM-1 expression compared to vehicle. MIA-602 rescued altered respiratory rate, increased expiratory parameters (Te, PEF, EEP), and normalized airflow parameters (Penh and Rpef) compared to vehicle, consistent with decreased airway inflammation. RNASeq followed by protein analysis revealed heightened levels of inflammation and end-stage necroptosis markers, including ZBP1 and pMLKL induced by rVSV-SARS-CoV-2, that were normalized by MIA-602 treatment, consistent with an anti-inflammatory and pro-survival mechanism of action in this preclinical model of COVID-19 pneumonia.

Effects of Astragalus Extract Mixture HT042 on Circulating IGF-1 Level and Growth Hormone Axis in Rats

Astragalus extract mixture HT042 is a standardized functional food granted by the Korean FDA for promoting “Children’s Height Growth”. In this study, we determined whether HT042 affects circulatory Insulin-like growth factor-1 (IGF-1) after administration and investigated whether Growth hormone (GH), Growth hormone-releasing hormone receptor (GHRH-R), and Growth hormone secretagogue receptor (GHS-R) mRNAs are expressed in the pituitary, and whether Growth hormone-releasing hormone (GHRH) and Somatostatin (SST) are expressed in the hypothalamus. We also evaluated the growth effect of HT042 on endochondral bone formation. Male Sprague-Dawley rats in the control and HT042 groups were orally administered a single dose of the control and HT042, respectively, and those in the recombinant human GH (rhGH) group were subcutaneously injected with rhGH. Tetracycline was injected intraperitoneally 72 h prior to sacrifice to decide endochondral bone formation. To determine the endocrine or paracrine/autocrine mechanism, we evaluated the expression of local BMP-2 and IGF-1, an immunohistochemical study after HT042 administration. It was confirmed that the growth-promoting effect of HT042 can be contributed to the increase in serum IGF-1, which can be stimulated by GH secretion. Administration of HT042 modulated the activity of GHRH-R and GHR-S in the pituitary gland and promoted GH secretion, thereby changing longitudinal growth through GH/IGF-1 mediation. Results for GHRH and SST expression demonstrated that the hypothalamus can be influenced and mediated by HT042 through a complex neuroendocrine regulatory system. In addition, it was confirmed by oral administration for 10 days that HT042 increased bone formation in cartilage, which is important for height growth. The effect of HT042 could be owing to upregulation of local Bone morphogenetic protein-2 (BMP-2) and IGF-1 expression in the growth plate, which could be regarded as a GH-dependent autocrine/paracrine pathway, as well as circulatory IGF-1.

Constitutive signal bias mediated by the human GHRHR splice variant 1

The mechanism of functional changes induced by alternative splicing of GHRHR is largely unknown. Here, we demonstrate that GHRH-elicited signal bias toward β-arrestin recruitment is constitutively mediated by SV1. The cryogenic electron microscopy structures of SV1 and molecular dynamics simulations reveal the different functionalities between GHRHR and SV1 at the near-atomic level (i.e., the N termini of GHRHR and SV1 differentiate the downstream signaling pathways, G_s versus β-arrestins). Our findings provide valuable insights into the functional diversity of class B1 GPCRs that may aid in the design of better therapeutic agents against certain cancers.

Alternative splicing of G protein–coupled receptors has been observed, but their functions are largely unknown. Here, we report that a splice variant (SV1) of the human growth hormone–releasing hormone receptor (GHRHR) is capable of transducing biased signal. Differing only at the receptor N terminus, GHRHR predominantly activates G_s while SV1 selectively couples to β-arrestins. Based on the cryogenic electron microscopy structures of SV1 in the apo state or GHRH-bound state in complex with the G_s protein, molecular dynamics simulations reveal that the N termini of GHRHR and SV1 differentiate the downstream signaling pathways, G_s versus β-arrestins. As suggested by mutagenesis and functional studies, it appears that GHRH-elicited signal bias toward β-arrestin recruitment is constitutively mediated by SV1. The level of SV1 expression in prostate cancer cells is also positively correlated with ERK1/2 phosphorylation but negatively correlated with cAMP response. Our findings imply that constitutive signal bias may be a mechanism that ensures cancer cell proliferation.

Tris(1,3-dichloro-2-propyl)phosphate Reduces Growth Hormone Expression via Binding to Growth Hormone Releasing Hormone Receptors and Inhibits the Growth of Crucian Carp

Tris(1,3-dichloro-2-propyl)phosphate (TDCIPP) has commonly been used as an additive flame retardant and frequently detected in the aquatic environment and in biological samples worldwide. Recently, it was found that exposure to TDCIPP inhibited the growth of zebrafish, but the relevant molecular mechanisms remained unclear. In this study, 5 day-old crucian carp (Carassius auratus) larvae were treated with 0.5, 5, or 50 μg/L TDCIPP for 90 days; the effect on growth was evaluated; and related molecular mechanisms were explored. Results demonstrated that 5 or 50 μg/L TDCIPP treatment significantly inhibited the growth of crucian carp and downregulated the expression of growth hormones (ghs), growth hormone receptor (ghr), and insulin-like growth factor 1 (igf1). Molecular docking, dual-luciferase reporter gene assay, and in vitro experiments demonstrated that TDCIPP could bind to the growth hormone releasing hormone receptor protein of crucian carp and disturb the stimulation of growth hormone releasing hormone to the expression of ghs, resulting in the decrease of the mRNA level of gh1 and gh2 in pituitary cells. Our findings provide new perceptions into the molecular mechanisms of developmental toxicity of TDCIPP in fish.

Growth hormone-releasing hormone promotes therapeutic effects of peripheral blood endothelial progenitor cells in ischemic repair

PURPOSE

In peripheral artery disease, blockage of the blood supply to the limbs leads to blood flow attenuation and tissue ischemia. We investigated whether growth hormone-releasing hormone (GHRH) could enhance the biological functions and therapeutic effects of endothelial progenitor cells (EPCs) derived from adult human peripheral blood (PB).

METHODS

EPCs were isolated from human PB (PB-EPCs) and cord blood and expanded in vitro. PB-EPCs incubated with or without GHRH were evaluated for proliferation, migration, and angiogenesis capacity and apoptosis rates under oxidative stress conditions. Activation of STAT3 and Akt pathways was evaluated using Western blot. A hind-limb ischemia (HLI) mouse model was used to study the efficacy of GHRH in improving EPC therapy in vivo.

RESULTS

GHRH enhanced the proliferation, migration, and angiogenesis capacity of PB-EPCs and reduced apoptosis under H₂O₂ stimulation. These beneficial effects were GHRH receptor-dependent and were paralleled by increased phosphorylation of STAT3 and Akt. Transplantation of GHRH-preconditioned EPCs into HLI model mice enhanced blood flow recovery by increasing vascular formation density and enhanced tissue regeneration at the lesion site.

CONCLUSION

Our studies demonstrate a novel role for GHRH in dramatically improving therapeutic angiogenesis in HLI by enhancing the biological functions of EPCs. These findings support additional studies to explore the full potential of GHRH in augmenting cell therapy for the management of ischemia.

Thyrotropin-Releasing Hormone (TRH) and Somatostatin (SST), but not Growth Hormone-Releasing Hormone (GHRH) nor Ghrelin (GHRL), Regulate Expression and Release of Immune Growth Hormone (GH) from Chicken Bursal B-Lymphocyte Cultures

It is known that growth hormone (GH) is expressed in immune cells, where it exerts immunomodulatory effects. However, the mechanisms of expression and release of GH in the immune system remain unclear. We analyzed the effect of growth hormone-releasing hormone (GHRH), thyrotropin-releasing hormone (TRH), ghrelin (GHRL), and somatostatin (SST) upon GH mRNA expression, intracellular and released GH, Ser133-phosphorylation of CREB (pCREB^S133), intracellular Ca²⁺ levels, as well as B-cell activating factor (BAFF) mRNA expression in bursal B-lymphocytes (BBLs) cell cultures since several GH secretagogues, as well as their corresponding receptors (-R), are expressed in B-lymphocytes of several species. The expression of TRH/TRH-R, ghrelin/GHS-R1a, and SST/SST-Rs (Subtypes 1 to 5) was observed in BBLs by RT-PCR and immunocytochemistry (ICC), whereas GHRH/GHRH-R were absent in these cells. We found that TRH treatment significantly increased local GH mRNA expression and CREB phosphorylation. Conversely, SST decreased GH mRNA expression. Additionally, when added together, SST prevented TRH-induced GH mRNA expression, but no changes were observed in pCREB^S133 levels. Furthermore, TRH stimulated GH release to the culture media, while SST increased the intracellular content of this hormone. Interestingly, SST inhibited TRH-induced GH release in a dose-dependent manner. The coaddition of TRH and SST decreased the intracellular content of GH. After 10 min. of incubation with either TRH or SST, the intracellular calcium levels significantly decreased, but they were increased at 60 min. However, the combined treatment with both peptides maintained the Ca²⁺ levels reduced up to 60-min. of incubation. On the other hand, BAFF cytokine mRNA expression was significantly increased by TRH administration. Altogether, our results suggest that TRH and SST are implicated in the regulation of GH expression and release in BBL cultures, which also involve changes in pCREB^S133 and intracellular Ca²⁺ concentration. It is likely that TRH, SST, and GH exert autocrine/paracrine immunomodulatory actions and participate in the maturation of chicken BBLs.

The Multiple Faces of the Metal Transporter ZIP14 (SLC39A14)

The SLC39A family of metal transporters was identified through homologies with the Zrt- and Irt-like (ZIP) proteins from yeast and plants. Of all the ZIP transporters, ZIP14 is arguably the most robustly characterized in terms of function at the integrative level. Mice with a global knockout of Zip14 are viable, thus providing the opportunity to conduct physiologic experiments. In mice, Zip14 expression is highly tissue specific, with the greatest abundance in the jejunum > liver > heart > kidney > white adipose tissue > skeletal muscle > spleen > pancreas. A unique feature of Zip14 is its upregulation by proinflammatory conditions, particularly increased interleukin 6 (IL-6) and nitric oxide. The transcription factors AP-1, ATF4, and ATF6α are involved in Zip14 regulation. ZIP14 does not appear to be zinc-regulated. The Zip14 knockout phenotype shows multiple sites of ZIP14 function, including the liver, adipose tissue, brain, pancreas, and bone. A prominent feature of the Zip14 ablation is a reduction in intestinal barrier function and onset of metabolic endotoxemia. Many aspects of the phenotype are accentuated with age and accompany increased circulating IL-6. Studies with 65Zn, 59Fe [nontransferrin-bound iron (NTBI)] and 54Mn show that ZIP14 transports these metals. At a steady state, the plasma concentrations of zinc, NTBI, and manganese are such that zinc ions are the major substrate available for ZIP14 at the cell surface. Upregulation of ZIP14 accounts for the hypozincemia and hepatic zinc accumulation associated with acute inflammation and sepsis and is required for liver regeneration and resistance to endoplasmic reticulum (ER) stress. Zip14 ablation in mice produces a defect in manganese excretion that leads to excess manganese accumulation in the brain that produces characteristics of Parkinsonism.

Role of growth hormone-releasing hormone in dyslipidemia associated with experimental type 1 diabetes

Dyslipidemia associated with triglyceride-rich lipoproteins (TRLs) represents an important residual risk factor for cardiovascular and chronic kidney disease in patients with type 1 diabetes (T1D). Levels of growth hormone (GH) are elevated in T1D, which aggravates both hyperglycemia and dyslipidemia. The hypothalamic growth hormone-releasing hormone (GHRH) regulates the release of GH by the pituitary but also exerts separate actions on peripheral GHRH receptors, the functional role of which remains elusive in T1D. In a rat model of streptozotocin (STZ)-induced T1D, GHRH receptor expression was found to be up-regulated in the distal small intestine, a tissue involved in chylomicron synthesis. Treatment of T1D rats with a GHRH antagonist, MIA-602, at a dose that did not affect plasma GH levels, significantly reduced TRL, as well as markers of renal injury, and improved endothelial-dependent vasorelaxation. Glucagon-like peptide 1 (GLP-1) reduces hyperglucagonemia and postprandial TRL, the latter in part through a decreased synthesis of apolipoprotein B-48 (ApoB-48) by intestinal cells. Although plasma GLP-1 levels were elevated in diabetic animals, this was accompanied by increased rather than reduced glucagon levels, suggesting impaired GLP-1 signaling. Treatment with MIA-602 normalized GLP-1 and glucagon to control levels in T1D rats. MIA-602 also decreased secretion of ApoB-48 from rat intestinal epithelial cells in response to oleic acid stimulation in vitro, in part through a GLP-1-dependent mechanism. Our findings support the hypothesis that antagonizing the signaling of GHRH in T1D may improve GLP-1 function in the small intestine, which, in turn, diminishes TRL and reduces renal and vascular complications.

Growth Hormone-Releasing Hormone in Diabetes

Growth hormone-releasing hormone (GHRH) is produced by the hypothalamus and stimulates growth hormone synthesis and release in the anterior pituitary gland. In addition, GHRH is an important regulator of cellular functions in many cells and organs. Expression of GHRH G-Protein Coupled Receptor (GHRHR) has been demonstrated in different peripheral tissues and cell types, including pancreatic islets. Among the peripheral activities, recent studies demonstrate a novel ability of GHRH analogs to increase and preserve insulin secretion by beta-cells in isolated pancreatic islets, which makes them potentially useful for diabetes treatment. This review considers the role of GHRHR in the beta-cell and addresses the unique engineered GHRH agonists and antagonists for treatment of type 2 diabetes mellitus. We discuss the similarity of signaling pathways activated by GHRHR in pituitary somatotrophs and in pancreatic beta-cells and possible ways as to how the GHRHR pathway can interact with glucose and other secretagogues to stimulate insulin secretion. We also consider the hypothesis that novel GHRHR agonists can improve glucose metabolism in Type 2 diabetes by preserving the function and survival of pancreatic beta-cells. Wound healing and cardioprotective action with new GHRH agonists suggest that they may prove useful in ameliorating certain diabetic complications. These findings highlight the future potential therapeutic effectiveness of modulators of GHRHR activity for the development of new therapeutic approaches in diabetes and its complications.

Protective effect of Growth Hormone-Releasing Hormone agonist in bacterial toxin-induced pulmonary barrier dysfunction

RATIONALE

Antibiotic treatment of patients infected with G(-) or G(+) bacteria promotes release of the toxins lipopolysaccharide (LPS) and pneumolysin (PLY) in their lungs. Growth Hormone-releasing Hormone (GHRH) agonist JI-34 protects human lung microvascular endothelial cells (HL-MVEC), expressing splice variant 1 (SV-1) of the receptor, from PLY-induced barrier dysfunction. We investigated whether JI-34 also blunts LPS-induced hyperpermeability. Since GHRH receptor (GHRH-R) signaling can potentially stimulate both cAMP-dependent barrier-protective pathways as well as barrier-disruptive protein kinase C pathways, we studied their interaction in GHRH agonist-treated HL-MVEC, in the presence of PLY, by means of siRNA-mediated protein kinase A (PKA) depletion.

METHODS

Barrier function measurements were done in HL-MVEC monolayers using Electrical Cell substrate Impedance Sensing (ECIS) and VE-cadherin expression by Western blotting. Capillary leak was assessed by Evans Blue dye (EBD) incorporation. Cytokine generation in broncho-alveolar lavage fluid (BALF) was measured by multiplex analysis. PKA and PKC-α activity were assessed by Western blotting.

RESULTS

GHRH agonist JI-34 significantly blunts LPS-induced barrier dysfunction, at least in part by preserving VE-cadherin expression, while not affecting inflammation. In addition to activating PKA, GHRH agonist also increases PKC-α activity in PLY-treated HL-MVEC. Treatment with PLY significantly decreases resistance in control siRNA-treated HL-MVEC, but does so even more in PKA-depleted monolayers. Pretreatment with GHRH agonist blunts PLY-induced permeability in control siRNA-treated HL-MVEC, but fails to improve barrier function in PKA-depleted PLY-treated monolayers.

CONCLUSIONS

GHRH signaling in HL-MVEC protects from both LPS and PLY-mediated endothelial barrier dysfunction and concurrently induces a barrier-protective PKA-mediated and a barrier-disruptive PKC-α-induced pathway in the presence of PLY, the former of which dominates the latter.

Mini-review: novel therapeutic strategies to blunt actions of pneumolysin in the lungs

Severe pneumonia is the main single cause of death worldwide in children under five years of age. The main etiological agent of pneumonia is the G+ bacterium Streptococcus pneumoniae, which accounts for up to 45% of all cases. Intriguingly, patients can still die days after commencing antibiotic treatment due to the development of permeability edema, although the pathogen was successfully cleared from their lungs. This condition is characterized by a dramatically impaired alveolar epithelial-capillary barrier function and a dysfunction of the sodium transporters required for edema reabsorption, including the apically expressed epithelial sodium channel (ENaC) and the basolaterally expressed sodium potassium pump (Na+-K+-ATPase). The main agent inducing this edema formation is the virulence factor pneumolysin, a cholesterol-binding pore-forming toxin, released in the alveolar compartment of the lungs when pneumococci are being lysed by antibiotic treatment or upon autolysis. Sub-lytic concentrations of pneumolysin can cause endothelial barrier dysfunction and can impair ENaC-mediated sodium uptake in type II alveolar epithelial cells. These events significantly contribute to the formation of permeability edema, for which currently no standard therapy is available. This review focuses on discussing some recent developments in the search for the novel therapeutic agents able to improve lung function despite the presence of pore-forming toxins. Such treatments could reduce the potentially lethal complications occurring after antibiotic treatment of patients with severe pneumonia.

Arginine induces GH gene expression by activating NOS/NO signaling in rat isolated hemi-pituitaries

The amino acid arginine (Arg) is a recognized secretagogue of growth hormone (GH), and has been shown to induce GH gene expression. Arg is the natural precursor of nitric oxide (NO), which is known to mediate many of the effects of Arg, such as GH secretion. Arg was also shown to increase calcium influx in pituitary cells, which might contribute to its effects on GH secretion. Although the mechanisms involved in the effects of Arg on GH secretion are well established, little is known about them regarding the control of GH gene expression. We investigated whether the NO pathway and/or calcium are involved in the effects of Arg on GH gene expression in rat isolated pituitaries. To this end, pituitaries from approximately 170 male Wistar rats (~250 g) were removed, divided into two halves, pooled (three hemi-pituitaries) and incubated or not with Arg, as well as with different pharmacological agents. Arg (71 mM), the NO donor sodium nitroprusside (SNP, 1 and 0.1 mM) and a cyclic guanosine monophosphate (cGMP) analogue (8-Br-cGMP, 1 mM) increased GH mRNA expression 60 min later. The NO acceptor hemoglobin (0.3 µM) blunted the effect of SNP, and the combined treatment with Arg and L-NAME (a NO synthase (NOS) inhibitor, 55 mM) abolished the stimulatory effect of Arg on GH gene expression. The calcium channel inhibitor nifedipine (3 µM) also abolished Arg-induced GH gene expression. The present study shows that Arg directly induces GH gene expression in hemi-pituitaries isolated from rats, excluding interference from somatostatinergic neurons, which are supposed to be inhibited by Arg. Moreover, the data demonstrate that the NOS/NO signaling pathway and calcium mediate the Arg effects on GH gene expression.

Agonist of growth hormone-releasing hormone reduces pneumolysin-induced pulmonary permeability edema

Aggressive treatment with antibiotics in patients infected with Streptococcus pneumoniae induces release of the bacterial virulence factor pneumolysin (PLY). Days after lungs are sterile, this pore-forming toxin can still induce pulmonary permeability edema in patients, characterized by alveolar/capillary barrier dysfunction and impaired alveolar liquid clearance (ALC). ALC is mainly regulated through Na(+) transport by the apically expressed epithelial sodium channel (ENaC) and the basolaterally expressed Na(+)/K(+)-ATPase in type II alveolar epithelial cells. Because no standard treatment is currently available to treat permeability edema, the search for novel therapeutic candidates is of high priority. We detected mRNA expression for the active receptor splice variant SV1 of the hypothalamic polypeptide growth hormone-releasing hormone (GHRH), as well as for GHRH itself, in human lung microvascular endothelial cells (HL-MVEC). Therefore, we have evaluated the effect of the GHRH agonist JI-34 on PLY-induced barrier and ALC dysfunction. JI-34 blunts PLY-mediated endothelial hyperpermeability in monolayers of HL-MVEC, in a cAMP-dependent manner, by means of reducing the phosphorylation of myosin light chain and vascular endothelial (VE)-cadherin. In human airway epithelial H441 cells, PLY significantly impairs Na(+) uptake, but JI-34 restores it to basal levels by means of increasing cAMP levels. Intratracheal instillation of PLY into C57BL6 mice causes pulmonary alveolar epithelial and endothelial hyperpermeability as well as edema formation, all of which are blunted by JI-34. These findings point toward a protective role of the GHRH signaling pathway in PLY-induced permeability edema.

The zinc transporter SLC39A14/ZIP14 controls G-protein coupled receptor-mediated signaling required for systemic growth

Aberrant zinc (Zn) homeostasis is associated with abnormal control of mammalian growth, although the molecular mechanisms of Zn's roles in regulating systemic growth remain to be clarified. Here we report that the cell membrane-localized Zn transporter SLC39A14 controls G-protein coupled receptor (GPCR)-mediated signaling. Mice lacking Slc39a14 (Slc39a14-KO mice) exhibit growth retardation and impaired gluconeogenesis, which are attributable to disrupted GPCR signaling in the growth plate, pituitary gland, and liver. The decreased signaling is a consequence of the reduced basal level of cyclic adenosine monophosphate (cAMP) caused by increased phosphodiesterase (PDE) activity in Slc39a14-KO cells. We conclude that SLC39A14 facilitates GPCR-mediated cAMP-CREB signaling by suppressing the basal PDE activity, and that this is one mechanism for Zn's involvement in systemic growth processes. Our data highlight SLC39A14 as an important novel player in GPCR-mediated signaling. In addition, the Slc39a14-KO mice may be useful for studying the GPCR-associated regulation of mammalian systemic growth.

PRKAR1A and the evolution of pituitary tumors

Carney complex (CNC) is an inherited tumor predisposition associated with pituitary tumors, including GH-producing pituitary adenomas and rare reports of prolactinomas. This disease is caused by mutations in PRKAR1A, which encodes the type 1A regulatory subunit of the cAMP-dependent protein kinase, PKA. Loss of PRKAR1A causes enhanced PKA signaling, which leads to pituitary tumorigenesis. Mutations in the gene have not been detected in sporadic pituitary tumors, but there is some data to suggest that non-genomic mechanisms may cause loss of protein expression. Unlike CNC patients, mice heterozygous for Prkar1a mutations do not develop pituitary tumors, although complete knockout of the gene in the Pit1 lineage of the pituitary produces GH-secreting pituitary adenomas. These data indicate that complete loss of Prkar1a/PRKAR1A is able to cause pituitary tumors in mice and men. The pattern of tumors is likely related to the signaling pathways employed in specific pituitary cell types.

Multiple roles of Gi/o protein-coupled receptors in control of action potential secretion coupling in pituitary lactotrophs

G(i/o) protein-coupled receptors, signaling through G protein-dependent and protein-independent pathways, have prominent effects on secretion by modulating calcium signaling and regulating the size of the releasable secretory pool, the rates of exocytosis and endocytosis, and de novo synthesis. Pituitary cells fire action potentials spontaneously, and the associated calcium influx is sufficient to maintain prolactin (PRL) release but not gonadotropin release at high and steady levels for many hours. Such secretion, termed intrinsic, spontaneous, or basal, reflects fusion of secretory vesicles triggered by the cell type-specific pattern of action potentials. In lactotrophs, activation of endothelin ET(A) and dopamine D(2) receptors causes inhibition of spontaneous electrical activity and basal adenylyl cyclase activity accompanied with inhibition of basal PRL release. Agonist-induced inhibition of cAMP production and firing of action potentials is abolished in cells with blocked pertussis toxin (PTX)-sensitive G(i/o) signaling pathway. However, agonist-induced inhibition of PRL release is only partially relieved in such treated cells, indicating that both receptors also inhibit exocytosis downstream of cAMP/calcium signaling. The PTX-insensitive step in agonist-induced inhibition of PRL release is not affected by inhibition of phosphoinositide 3-kinase and glycogen synthase kinase-3 but is partially rescued by downregulation of the G(z)alpha expression. Thus, ET(A) and D(2) receptors inhibit basal PRL release not only by blocking electrical activity but also by desensitizing calcium-secretion coupling.

Neuroendocrine hormones such as growth hormone and prolactin are integral members of the immunological cytokine network

Neuroendocrine hormones such as growth hormone (GH) and prolactin (PRL) have been demonstrated to accelerate the recovery of the immune response after chemotherapy and bone marrow transplantation and to enhance the restoration of immunity in individuals infected with HIV and in normal individuals with compromised immune systems associated with aging. As the mechanism of action of these hormones has been elucidated, it has become clear that they are integral members of the immunological cytokine/chemokine network and share regulatory mechanisms with a wide variety of cytokines and chemokines. The members of this cytokine network induce and can be regulated by members of the suppressor of cytokine signaling (SOCS) family of intracellular proteins. In order to take advantage of the potential beneficial effects of hormones such as GH or PRL, it is essential to take into consideration the overall cytokine network and the regulatory effects of SOCS proteins.

Neuropeptides of the pituitary adenylate cyclase-activating polypeptide/vasoactive intestinal polypeptide/growth hormone-releasing hormone/secretin family in testis

Mammalian testicular development and the maintenance of spermatogenesis are hormone-dependent processes that are controlled by the pituitary gonadotropins and testosterone. Recent studies have demonstrated the presence of many neuropeptides and their receptors in the testis, suggesting that these peptides operate as local regulators of testicular germ cell development and function. Among these testicular neuropeptides, the peptides that belong to the pituitary adenylate cyclase-activating polypeptide (PACAP) family, particularly growth hormone-releasing hormone and secretin, appear to show some unique common features in terms of intratesticular localization and the time of expression during the spermatogenic cycle. However, their precise physiologic roles and mechanisms of action remain unknown. This review analyzes the available information on the functional interactions among the testicular cells that appear to be mediated by locally produced neuropeptides, with a special emphasis on the peptides of the PACAP family.

Neuropeptides of the pituitary adenylate cyclase-activating polypeptide/vasoactive intestinal polypeptide/growth hormone-releasing hormone/secretin family in testis

Mammalian testicular development and the maintenance of spermatogenesis are hormone-dependent processes that are controlled by the pituitary gonadotropins and testosterone. Recent studies have demonstrated the presence of many neuropeptides and their receptors in the testis, suggesting that these peptides operate as local regulators of testicular germ cell development and function. Among these testicular neuropeptides, the peptides that belong to the pituitary adenylate cyclase-activating polypeptide (PACAP) family, particularly growth hormone-releasing hormone and secretin, appear to show some unique common features in terms of intratesticular localization and the time of expression during the spermatogenic cycle. However, their precise physiologic roles and mechanisms of action remain unknown. This review analyzes the available information on the functional interactions among the testicular cells that appear to be mediated by locally produced neuropeptides, with a special emphasis on the peptides of the PACAP family.

Disruption of PC1/3 expression in mice causes dwarfism and multiple neuroendocrine peptide processing defects

The subtilisin-like proprotein convertases PC1/3 (SPC3) and PC2 (SPC2) are believed to be the major endoproteolytic processing enzymes of the regulated secretory pathway. They are expressed together or separately in neuroendocrine cells throughout the brain and dispersed endocrine system in both vertebrates and invertebrates. Disruption of the gene-encoding mouse PC1/3 has now been accomplished and results in a syndrome of severe postnatal growth impairment and multiple defects in processing many hormone precursors, including hypothalamic growth hormone-releasing hormone (GHRH), pituitary proopiomelanocortin to adrenocorticotropic hormone, islet proinsulin to insulin and intestinal proglucagon to glucagon-like peptide-1 and -2. Mice lacking PC1/3 are normal at birth, but fail to grow normally and are about 60% of normal size at 10 weeks. They lack mature GHRH, have low pituitary growth hormone (GH) and hepatic insulin-like growth factor-1 mRNA levels and resemble phenotypically the "little" mouse (Gaylinn, B. D., Dealmeida, V. I., Lyons, C. E., Jr., Wu, K. C., Mayo, K. E. & Thorner, M. O. (1999) Endocrinology 140, 5066-5074) that has a mutant GHRH receptor. Despite a severe defect in pituitary proopiomelanocortin processing to mature adrenocorticotropic hormone, blood corticosterone levels are essentially normal. There is marked hyperproinsulinemia but without impairment of glucose tolerance. In contrast, PC2-null mice lack mature glucagon and are chronically hypoglycemic (Furuta, M., Yano, H., Zhou, A., Rouille, Y., Holst, J., Carroll, R., Ravazzola, M., Orci, L., Furuta, H. & Steiner, D. (1997) Proc. Natl. Acad. Sci. USA 94, 6646-6651). The PC1/3-null mice differ from a human subject reported with compound heterozygosity for defects in this gene, who was of normal stature but markedly obese from early life. The PC1/3-null mice are not obese. The basis for these phenotypic differences is an interesting topic for further study. These findings prove the importance of PC1/3 as a key neuroendocrine convertase.

Expression of Pit-1 and growth hormone-releasing hormone receptor mRNA in human pituitary adenomas: difference among functioning, silent, and other nonfunctioning adenomas

We analyzed the expression of Pit-1 and growth hormone-releasing hormone receptor (GHRH-R) mRNA in various types of functioning and nonfunctioning adenomas using a quantitative reverse transcriptase polymerase chain reaction (RT-PCR) method. Among clinically nonfunctioning adenomas, tumors considered as silent adenomas were reclassified on a pathologic basis. Competitive RT-PCR showed that the levels of Pit-1 and GHRH-R mRNA expression in silent somatotroph adenomas and silent prolactinomas were similar to those in the corresponding functioning adenomas. In silent thyrotroph adenomas, both mRNAs showed high levels of expression that were similar to those in functioning and silent somatotroph adenomas. The results suggest that the cause of the silence in these tumors seems to be in the downstream to transcription of Pit-1 gene in the signaling pathway leading to hormone secretion. Competitive RT-PCR assay could distinguish silent adenomas of the Pit-1 group from the other nonfunctioning adenomas in the expression levels of Pit-1 and GHRH-R mRNAs. In the future, precise diagnosis of various adenomas may become possible by assaying transcription factors such as steroidogenic factor-1 and thyrotroph embryonic factor, which are thought to be related to adenohypophyseal cytodifferentiation.

Effects of growth hormone-releasing hormone-related peptide on stem cell factor expression in cultured rat Sertoli cells

The growth hormone-releasing hormone (GHRH) gene produces a precursor molecule that contains GHRH and a carboxyl-terminal peptide that we have named GHRH-related peptide (GHRH-RP). This peptide, like GHRH, stimulates the expression of stem cell factor (SCF), an important reproductive and hematopoietic cytokine, in vitro and in vivo. In the present study, using primary cultures of rat Sertoli cells, we compared the time course of action and the level of SCF stimulation seen following treatment with GHRH-RP and GHRH. Additionally, we investigated the activity of a truncated peptide, p75-92NH2, whose sequence is contained within GHRH-RP. All three of these peptides were shown to stimulate the steady-state levels of SCF mRNA to a comparable degree. However, the time course of action for GHRH-RP differed markedly from that of GHRH. GHRH-RP and p75-92NH2, similar to GHRH, induce SCF expression, at least in part, via the activation of the protein kinase A/cyclic adenosine monophosphate intracellular signaling pathway.

CREB-independent regulation by CBP is a novel mechanism of human growth hormone gene expression

Hypothalamic growth hormone-releasing hormone (GHRH) stimulates growth hormone (GH) gene expression in anterior pituitary somatotrophs by binding to the GHRH receptor, a G-protein-coupled transmembrane receptor, and by mediating a cAMP-mediated protein kinase A (PKA) signal-transduction pathway. Two nonclassical cAMP-response element motifs (CGTCA) are located at nucleotides -187/-183 (distal cAMP-response element; dCRE) and -99/-95 (proximal cAMP-response element; pCRE) of the human GH promoter and are required for cAMP responsiveness, along with the pituitary-specific transcription factor Pit-1 (official nomenclature, POU1F1). Although a role for cAMP-response element binding protein (CREB) in GH stimulation by PKA has been suggested, it is unclear how the effect may be mediated. CREB binding protein (CBP) is a nuclear cofactor named for its ability to bind CREB. However, CBP also binds other nuclear proteins. We determined that CBP interacts with Pit-1 and is a cofactor for Pit-1-dependent activation of the human GH promoter. This pathway appears to be independent of CREB, with CPB being the likely target of phosphorylation by PKA.

Inhibition of growth, production of insulin-like growth factor-II (IGF-II), and expression of IGF-II mRNA of human cancer cell lines by antagonistic analogs of growth hormone-releasing hormone in vitro

Antagonistic analogs of growth hormone-releasing hormone (GHRH) suppress growth of various tumors in vivo. This effect is exerted in part through inhibition of the GHRH-GH-insulin-like growth factor (IGF)-I axis. Nevertheless, because autocrine/paracrine control of proliferation by IGF-II also is a major factor in many tumors, the interference with this growth-stimulating pathway would offer another approach to tumor control. We thus investigated whether GHRH antagonists MZ-4-71 and MZ-5-156 also act on the tumor cells directly by blocking the production of IGF-II. An increase in the IGF-II concentration in the media during culture was found in 13 of 26 human cancer cell lines tested. Reverse transcription-PCR studies on 8 of these cell lines showed that they also expressed IGF-II mRNA. Antagonists of GHRH significantly inhibited the rate of proliferation of mammary (MDA-MB-468 and ZR-75-1), prostatic (PC-3 and DU-145), and pancreatic (MiaPaCa-2, SW-1990, and Capan-2) cancer cell lines as shown by colorimetric and [3H]thymidine incorporation tests and reduced the expression of IGF-II mRNA in the cells and the concentration of IGF-II secreted into the culture medium. Growth and IGF-II production of lung (H-23 and H-69) and ovarian (OV-1063) cancer cells that express mRNA for IGF-II and excrete large quantities of IGF-II also was marginally suppressed by the antagonists. These findings suggest that antagonistic analogs of GHRH can inhibit growth of certain tumors not only by inhibiting the GHRH-GH-IGF-I axis, but also by reducing the IGF-II production and by interfering with the autocrine regulatory pathway.