Growth Hormone-Releasing Hormone in Endothelial Inflammation

The discovery of hypothalamic hormones propelled exciting advances in pharmacotherapy and improved life quality worldwide. Growth hormone-releasing hormone (GHRH) is a crucial element in homeostasis maintenance, and regulates the release of growth hormone from the anterior pituitary gland. Accumulating evidence suggests that this neuropeptide can also promote malignancies, as well as inflammation. Our review is focused on the role of that 44 - amino acid peptide (GHRH) and its antagonists in inflammation and vascular function, summarizing recent findings in the corresponding field. Preclinical studies demonstrate the protective role of GHRH antagonists against endothelial barrier dysfunction, suggesting that the development of those peptides may lead to new therapies against pathologies related to vascular remodeling (eg, sepsis, acute respiratory distress syndrome). Targeted therapies for those diseases do not exist.

Individuals with isolated congenital GH deficiency due to a GHRH receptor gene mutation appear to cope better with SARS-CoV-2 infection than controls

PURPOSE

Several interactions exist between the GH/IGF axis and the immune system, including effects on innate immunity and humoral and cellular response. Acquired GH deficiency (GHD) has been recently proposed as a risk factor for severity of COVID-19 infections. However, acquired GHD is often associated to other factors, including pituitary tumors, surgery, radiotherapy, and additional pituitary hormones deficits and their replacements, which, together, may hinder an accurate analysis of the relationship between GHD and COVID-19. Therefore, we decided to assess the seroprevalence of SARS-CoV-2 antibodies and the frequency of symptomatic cases of COVID-19 in adults subjects with untreated isolated GHD (IGHD) due to a homozygous null mutation in the GHRH receptor gene.

METHODS

A cross-sectional study was carried out in 27 adult IGHD subjects and 27 age- and gender-matched local controls. Interview, physical examination, bio-impedance, hematological and SARS-CoV-2 IgM and IgG antibodies were analyzed.

RESULTS

There was no difference in the prevalence of positivity of anti-SARS-CoV-2 IgM and IgG antibodies between the two groups. Conversely, no IGHD individual had a previous clinical diagnosis of COVID-19 infection, while 6 control subjects did (p = 0.023).

CONCLUSION

The production of anti-SARS-CoV-2 antibodies was similar between IGHD subjects due to a GHRH receptor gene mutation and controls, but the evolution to symptomatic stages of the infection and the frequency of confirmed cases was lower in IGHD subjects than in GH sufficient individuals.

Genetic evaluation of short stature

After a proper medical history, growth analysis and physical examination of a short child, followed by radiological and laboratory screening, the clinician may decide to perform genetic testing. We propose several clinical algorithms that can be used to establish the diagnosis. GH1 and GHRHR should be tested in children with severe isolated growth hormone deficiency and a positive family history. A multiple pituitary dysfunction can be caused by defects in several genes, of which PROP1 and POU1F1 are most common. GH resistance can be caused by genetic defects in GHR, STAT5B, IGF1, IGFALS, which all have their specific clinical and biochemical characteristics. IGF-I resistance is seen in heterozygous defects of the IGF1R. If besides short stature additional abnormalities are present, these should be matched with known dysmorphic syndromes. If no obvious candidate gene can be determined, a whole genome approach can be taken to check for deletions, duplications and/or uniparental disomies.

[Recent trends in the pathophysiology and treatment of pituitary adenomas]

Recent molecular pathological investigations have elucidated the cytodifferentiation of pituitary cells and identified several transcriptional factors that regulate this cytodifferentiation of pituitary cells. The patterns of cytodifferentiation are closely related to the pathogenesis of pituitary adenomas. Meanwhile, the role of hypothalamic hormones in the development of pituitary adenomas has recently attracted the attention of investigators. The expression of growth hormone-releasing hormone and corticotrophin releasing hormone in corticotroph adenomas have been demonstrated in somatotroph adenomas and corticotropin adenomas, respectively. This finding indicates that the endogenous expression of hypothalamic hormones and their receptors in human pituitary adenoma cells has ample significance in the autocrine or paracrine regulation of pituitary hormone production and tumor extension induced by hypothalamic hormones produced by adenoma cells. The recent progress in surgical techniques for treatment of pituitary adenomas has provided several alternatives: transsphenoidal surgery vs. transcranial surgery, sublabial approach vs. endonasal approach, and microsurgery vs. endoscopic surgery. There have also been developments in the medical treatment of pituitary adenomas. The frequently used dopamine agonist, cabergoline, is very effective for treating prolactin-producing adenoma. Long-acting octreotide and pegvisomant are now available for the treatment of growth hormone producing adenoma. Cabergoline is also used for growth hormone producing adenoma. Temozolomide has recently been used for atypical adenomas or pituitary carcinomas. Adult growth hormone deficiency sometimes occurs in postoperative patients with pituitary adenomas. Growth hormone replacement is recommended to maintain the quality of life of these patients.

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.

Molecular basis of the little mouse phenotype and implications for cell type-specific growth

The molecular basis for the little (lit) mouse phenotype, characterized by a hypoplastic anterior pituitary gland, is the mutation of a single nucleotide that alters Asp 60 to Gly in the growth hormone releasing factor receptor. Detailed analysis of the lit mouse anterior pituitary reveals spatially distinct proliferative zones of growth hormone-producing stem cells and mature somatotrophs, each regulated by a different trophic factor. This sequential growth factor requirement for a specific cell type may exemplify a common strategy for regulating cellular proliferation in other mammalian organs.

Proteolytic degradation of rat growth hormone-releasing factor(1-29) amide in rat pituitary and hypothalamus

The identification of peptide bonds vulnerable to tissue peptidases is a valuable approach to design peptide agonists which exhibit a longer duration of action than the native molecules. Therefore, the kinetic of disappearance of rat growth hormone-releasing factor (rGRF(1-29)NH2) and the identification of its metabolites were studied in rat pituitary and hypothalamus. Synthetic rGRF(1-29)NH2 (10 microM) was incubated (0-120 min, 37 degrees C) in the presence of a pituitary (237 +/- 51 micrograms protein/ml) or hypothalamus homogenate (576 +/- 27 micrograms protein/ml). Using analytical high pressure liquid chromatography (HPLC), apparent half-lives of 22 +/- 3 min and 25 +/- 4 min were found in pituitary and hypothalamus, respectively. In both tissues, three degradation products, all less hydrophobic than the native peptide, were detected and isolated by preparative HPLC. The identification of the purified metabolites was ascertained by amino acid analysis, sequencing and chromatography with synthetic homologs. These results indicate that the main sites of cleavage in the pituitary and hypothalamus are Lys21-Leu22 (trypsin-like cleavage site), Leu14-Gly15 and Tyr10-Arg11 (chymotrypsin-like cleavage sites). TLCK and leupeptin did not affect the formation of fragment (1-21)OH while TPCK blocked the cleavage of Leu14-Gly15. The low affinity of fragment (1-21)NH2 for pituitary GRF binding sites suggests that hydrolysis of the Lys21-Leu22 bond inactivates rGRF(1-29)NH2 in this target tissue.

GHRH receptor of little mice contains a missense mutation in the extracellular domain that disrupts receptor function

The growth hormone-releasing hormone receptor (GHRHR) is a member of the family of G protein-coupled receptors that is expressed on pituitary somatotrope cells and mediates the actions of GHRH in stimulating growth hormone (GH) synthesis and secretion. We report that the Ghrhr gene is located in the middle of mouse chromosome 6 in the same region as the little mutation. Mice homozygous for this mutation have reduced GH secretion and a dwarf phenotype. A missense mutation was identified in the extracellular domain of the little GHRHR that disrupts receptor function, suggesting that the growth deficit in these mice results from a defect in the GHRHR. Similar alterations in GHRHR might explain some isolated GH deficiencies in humans.

Molecular cloning and expression of a human anterior pituitary receptor for growth hormone-releasing hormone

GH-releasing hormone (GHRH), acting through the GHRH receptor (GHRH-R), plays a pivotal role in the regulation of GH synthesis and secretion in the pituitary. It is possible that GHRH may serve other roles in other tissues. Here we report the cloning of a cDNA encoding a human GHRH-R from an acromegalic pituitary cDNA library. The isolated cDNA encodes a 423-amino acid protein that has seven putative transmembrane domains characteristic of G-protein-coupled receptors. It is a member of the secretin family of G-protein-coupled receptors and has 47%, 42%, 35%, and 28% identity with receptors for vasoactive intestinal peptide, secretin, calcitonin, and PTH, respectively. Transient expression of this cDNA in COS cells induced saturable, high affinity, GHRH-specific binding and also stimulated intracellular cAMP accumulation in response to physiological concentrations of GHRH. A specific GHRH antagonist blocked both binding and second messenger response. Northern analysis indicated that GHRH-R mRNA was most abundant in extracts of pituitary and was not detected in other tissues.

Pit-1-dependent expression of the receptor for growth hormone releasing factor mediates pituitary cell growth

In Snell (dw) and Jackson (dwJ) dwarf mice, mutations in the gene encoding Pit-1, a tissue-specific POU-domain transcription factor, lead to the absence of somatotroph, lactotroph and thyrotroph cells. Pre-somatotroph proliferation is stimulated by increased intracellular levels of cyclic AMP, normally induced by growth hormone releasing factor (GRF; refs 7-17). Here we report the cloning of mouse and rat complementary DNAs encoding a new member of the seven-transmembrane-helix, G-protein-coupled receptor family restricted to the pituitary gland, which mediates increases in intracellular cAMP and cAMP-dependent gene transcription in response to GRF. The receptor is expressed in a spatial and temporal pattern corresponding precisely to growth hormone gene expression, and neither is expressed in dw/dw mice. The pituitary hypoplasia in these mice thus appears to be due, at least in part, to the absence of GRF receptor, which is in turn due to the absence of functional Pit-1.

Molecular cloning and expression of a pituitary-specific receptor for growth hormone-releasing hormone

A novel cDNA was isolated from rat pituitary mRNA using the polymerase chain reaction to amplify sequences encoding G protein-coupled receptors. The human homolog of this cDNA was isolated and expressed in human kidney 293 cells, and membrane fractions from these cells were found to bind human GH-releasing hormone (GHRH) with high affinity and specificity. GHRH also stimulates intracellular cAMP production in these transfected cells. The encoded receptor protein contains seven potential membrane-spanning domains, a hallmark of G protein-coupled receptors, and is homologous to previously identified receptors for secretin and vasoactive intestinal peptide, ligands that are related to GHRH. The rat GHRH receptor mRNA is expressed predominantly, if not exclusively, in the anterior pituitary gland, the major target for GHRH action. These results define a mechanism for cellular signaling by GHRH and provide the opportunity to examine the role of the GHRH receptor in growth abnormalities that involve the GH axis.

Role of selected endogenous peptides in growth hormone-releasing hexapeptide activity: analysis of growth hormone-releasing hormone, thyroid hormone-releasing hormone, and gonadotropin-releasing hormone

The purpose of this study was to evaluate the contribution of endogenous GH-releasing hormone (GHRH) to exogenous GH-releasing hexapeptide (GHRP-6) activity, and to determine whether TRH or GnRH are endogenous analogs of GHRP-6. The activity of GHRP-6, a synthetic GH secretagogue, was significantly attenuated in rats administered GHRH antiserum or alpha-methyl-rho-tyrosine to reduce endogenous GHRH concentrations, and also in rats administered 5-50 micrograms/kg of [N-Ac-Tyr1,D-Arg2]-GRF 1-29 amide to block pituitary GHRH receptors. However, GHRP-6 activity was potentiated in rats administered 150 micrograms/kg [N-Ac-Tyr1,D-Arg2]-GRF 1-29 amide, presumably due to partial agonist activity of the GHRH receptor antagonist at the higher dose. These data show that endogenous GHRH contributes to full expression of exogenous GHRP-6 activity in vivo. Like TRH, a subthreshold dose of GHRP-6 was significantly more effective in hypothyroid rats than in euthyroid rats. However, suprathreshold doses of GHRP-6 were less effective in hypothyroid rats. Unlike TRH, GHRP-6 had no effect on GH and prolactin release from GH3 cells, and TRH and GnRH were poor competitors for 3H-GHRP-6 binding sites on pituitary membranes. A GnRH receptor antagonist did not block GHRP-6 activity in vivo, and GnRH administered alone or in combination with GHRP-6, did not stimulate GH release. The results of this study suggest that synergy between GHRH and GHRP-6 seen in pharmacological studies is physiologically relevant, and that TRH and GnRH are not endogenous analogs of GHRP-6.

Alteration of somatostatin but not growth hormone-releasing factor pituitary binding sites in obese Zucker rats

The present study was designed to determine whether the diminution of growth hormone (GH) secretion that occurs in obese Zucker rats is related to alterations of GH-releasing factor (GRF) or somatostatin (SRIF) pituitary binding sites. Cold saturation studies were performed in pituitary homogenates of 4-month-old lean and obese rats, using [125I-Tyr10]hGRF(1-44)NH2 as radioligand and [127I-Tyr10]hGRF-(1-44)NH2 as competitor, and in pituitary membrane preparations, using [125I-Tyr0, D-Trp8]SRIF14 as radioligand and [127I-Tyr0, D-Trp8]SRIF14 as competitor. In lean rats, analysis of the curves by the Ligand program revealed the presence of two distinct classes of GRF binding sites, the first being of high affinity (0.74 +/- 0.11 nM) and low capacity (118 +/- 31 fmol/mg protein), the second being of lower affinity (880 +/- 240 nM) and higher capacity (140 +/- 35 pmol/mg protein), and of a single class of SRIF binding sites (affinity: 0.40 +/- 0.12 nM; capacity: 24 +/- 6 fmol/mg protein). In obese rats, no difference was observed in GRF binding parameters for both classes of sites, but the concentration of somatostatin binding sites was reduced by 67% when compared to their lean littermates. These findings suggest that the SRIF pituitary receptors are down-regulated in obese Zucker rats and indicate that no alteration of GRF pituitary binding sites contribute to the blunted GH secretion observed in this model of obesity.

Growth hormone releasing hormone receptors on thymocytes and splenocytes from rats

In the present study, we determined that rat mononuclear leukocytes possess specific receptors for growth hormone releasing hormone (GHRH). The results show that the binding of 125I-labeled GHRH to spleen and thymic cells was saturable and of a high affinity, approximately 3.5 and 2.5 nM for thymus and spleen cells, respectively. The Scatchard analysis revealed a binding capacity of approximately 54 and 35 fmol per 10(6) cells on thymus and spleen, respectively. The binding of GHRH was not competed by 10(-6) M growth hormone, corticotropin releasing factor, substance P or luteinizing hormone releasing hormone and vasointestinal peptide (VIP). Partial characterization of the receptor was accomplished by crosslinking 125I-labeled GHRH to thymus cells with disuccinimidyl suberate and polyacrylamide gel electrophoresis. Autoradiography of dried gels showed two major components in leukocytes and pituitary cells at approximately 42 and 27 kDa which could be diminished by unlabeled GHRH. The treatment of leukocytes with GHRH (10 nM) rapidly increased the intracellular free calcium concentration from a basal level of 70 +/- 20 nM to a plateau value of 150 +/- 20 nM in 6 min after stimulation. The functional activity of GHRH receptors was studied further by measuring lymphocyte proliferative responses and the increase in the level of cytoplasmic GH RNA. The presence of GHRH alone resulted in a dose-dependent increase in thymidine and uridine incorporation and a dose-dependent increase in the levels of GH RNA in the cytoplasm. Taken together, the results show that lymphocytes contain specific receptors for GHRH that are coupled to important biological responses and further support the concept of bidirectional communication between the immune and neuroendocrine tissues.

Demonstration and characterization of the specific binding of growth hormone-releasing peptide to rat anterior pituitary and hypothalamic membranes

Since the growth hormone-releasing peptide (GHRP), His-D-Trp-Ala-Trp-D-Phe-Lys-NH2, was found to specifically release growth hormone by a complementary but yet not clearly defined action on the pituitary as well as the hypothalamus, in vitro studies have been performed to demonstrate and characterized GHRP binding sites on peripheral membranes of both the rat anterior pituitary and hypothalamus. Optimum binding assay conditions were established using [125I]Tyr-Ala-GHRP as the radioligand. The membrane binding sites were specific, reversible, saturable and time, temperature, pH and concentration dependent. Computerized analyses of competition experiments suggested two classes of binding sites in both pituitary and hypothalamic membranes. The maximum specific binding was observed at pH 5.0 than the physiological pH in both tissues. Pretreatment of the membranes with trypsin prevented specific binding. The increase in Bmax was statistically significant and showed a 2.0- to 8.9-fold and 5.8- to 11.2-fold in pituitary and hypothalamus, respectively, whereas the affinity constants (Kds) were not significant. Of the synthetic and natural neuropeptides that influence the release of GH from somatotrophs, only (D-Lys3)GHRP, substance P antagonists and growth hormone-releasing factor analog were potent inhibitors of GHRP binding in both tissues.

GRF analogs and fragments: correlation between receptor binding, activity and structure

GH-releasing activity in vitro was directly correlated with GRF receptor binding affinity for all hGRF analogs examined. hGRF(1-29)-NH2 analogs with Ala15-substitution (for Gly15) displayed 4-5 times higher affinity for the GRF receptor relative to hGRF(1-44)-NH2. Replacement of Gly15 with Sar15 resulted in a dramatic loss of activity and receptor binding. The present data supports the proposal that Ala15-substitution increases receptor affinity, and hence potency, due to increased amphiphilic alpha-helical interactions. Fragments of hGRF, representative of DPP-IV and trypsin-like cleavage, are inactive as a consequence of greatly diminished GRF receptor binding. These results provide a comprehensive analysis of the structural features required for both GRF receptor binding and activation.

Alterations of pituitary growth hormone-releasing factor binding sites in aging rats

This study was designed to determine whether growth hormone (GH)-releasing factor (GRF) binding sites are altered in parallel to the diminution of GH secretion that occurs in aging. Using [125I-Tyr10] hGRF (1-44)NH2 as radioligand, we performed cold saturation studies in 2, 8, 14 and 18-month-old Sprague-Dawley male rat pituitary homogenates. In young rats (2 months), analysis by Ligand revealed the presence of two distinct classes of binding sites (KDs = 0.86 +/- 0.15 and 400 +/- 210 nM; BMAXS = 269 +/- 47 fmol and 42 +/- 19 pmol/mg protein, respectively). In 8 and 14-month old rats, there was a concomitant decrease in capacity of the high affinity class of sites (P less than 0.01) and increase in capacity of the low affinity class of sites (P less than 0.05). In parallel, a transient increase in affinity of the high affinity class of sites was observed in 14-month-old rats (P less than 0.05). In old rats (18 months), the data were no longer statistically analyzed with a two site-model, indicating a severe blunting of the high affinity sites. As the GRF-induced GH secretion is still not diminished at 8 month of age in these animals, our results indicate: 1) that alterations of GRF binding sites precede the GH impairment, thus probably participate in the initiating of this phenomenon and 2) that the biological actions of GRF on GH secretion are likely mediated by the high affinity class of sites.

Effect of lead on TRH and GRF binding in rat anterior pituitary membranes

The effect of lead on binding of the hypothalamic peptides thyroid releasing hormone (TRH) and growth hormone releasing factor (GRF) to rat anterior pituitary receptors was examined in this study. Concentrations of lead ranging from 0.01 to 1 microM did not alter [3H]TRH binding; concentrations above 1 microM increased TRH association with pituitary receptors. A previously uncharacterized ligand, [125I]GRF (human 1-44 amide), was used to examine the binding of GRF to anterior pituitary receptors. A high affinity site (GRFH = 18.1%, KH = 11.5 pM) was displaced by human growth hormone releasing factor (hGRF) (1-44)-NH2 or hGRF (1-29)-NH2 but not by rat growth hormone releasing factor (rGRF) (1-29)-NH2. Use of this ligand also revealed a class of low affinity binding sites (GRFL = 81.9%, KL = 0.39 microM) which has not been previously described. The low affinity site could be displaced by hGRF (1-44)-NH2, hGRF (1-29)-NH2 and rGRF (1-29)-NH2. A synthetic growth hormone releasing peptide (GHRP) also interacted with the low affinity GRF binding site. Lead dose-dependently displaced the binding of [125I]GRF to its pituitary receptors. The IC50 of lead for inhibiting [125I]GRF binding was 0.195 mM added lead or 52 pM free lead. These data suggest that one mechanism by which lead may affect pituitary function is through inhibition of receptor binding.

Characterization of [125I-Tyr10]human growth hormone-releasing factor (1-44) amide binding to rat pituitary: evidence for high and low affinity classes of sites

A sensitive binding assay was developed to determine binding characteristics of commercially available [125I-Tyr10]human growth hormone-releasing factor (hGRF) (1-44)NH2 in rat pituitary using 0.1 gland homogenate (70-75 micrograms protein) per incubation tube. Under standard assay conditions, addition of 5 mM EDTA efficiently prevented the degradation of both human and rat GRF for at least 3 h. Association of the ligand was time-dependent: equilibrium was reached within 30 min of incubation at 23 degrees C and remained stable for an additional 150 min (K1 = 5.01 +/- 0.86 nM-1.min-1). Specific binding increased linearly with the amount of protein present in the assay, from 15 to 170 micrograms per incubation tube. This binding was reversible, dissociation occurring almost completely after a 120-min period (K-1 = 8.13 +/- 0.29 x 10(-3) min-1). A concentration of 5-10 mM Mg2+ was required for optimal specific binding whereas 50 mM Mg2+ or monovalent cations such as Na+, K+, Li+ decreased it. Scatchard analysis of cold saturation studies by the Ligand program statistically revealed the presence of two distinct classes of binding sites; the first was of high affinity (0.68 +/- 0.11 nM) and low capacity (140 +/- 22 fmol/pituitary), the second was of lower affinity (590 +/- 347 nM) and higher capacity (38.7 +/- 18.7 pmol/pituitary). Similar values were obtained with various bovine serum albumin (BSA) concentrations and when using crude or washed pituitary homogenates, suggesting that the second low affinity site was not BSA or a soluble protein from the homogenate.(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular mechanism of caprylic acid-induced growth hormone suppression

To determine the dynamic secretory pattern of growth hormone (GH) in the presence of free fatty acids (FFA), we studied the effect of caprylic acid on basal and rGRF(1-29)NH2 (GRF)-induced GH secretion in acutely dispersed and perifused rat pituitary cells. At a concentration of 3.0 mmol/L, caprylic acid inhibited both basal (P less than .05) and GRF-stimulated GH secretions (P less than .01), except when the maximal (near the EC100) GRF concentration of 100 pmol/L was used. Lower concentrations of caprylic acid such as 0.3 and 1.0 mmol/L significantly inhibited, in a concentration-dependent manner, GH secretion induced by a 6.25-pmol/L GRF. However, at a GRF concentration of 25 pmol/L, this inhibitory effect was abolished. The time-course of GH response to GRF was similar in both control and caprylic acid-treated cells. To elucidate the mechanism(s) of action of the caprylic acid-induced blockade of GH secretion, in 3-day cultured rat pituitary cells, the effect of caprylic acid on basal and GRF-stimulated GH and 3',5'-cyclic adenosine monophosphate (cAMP) release. We also tested its effect on the Ca2+ ionophore, A23187-induced GH release. Caprylic acid (0.3 to 3.0 mmol/L) significantly inhibited basal GH release and GRF- or A23187-induced GH secretion. Furthermore, it decreased both basal and GRF-stimulated cAMP release (P less than .05). In addition, the effect of caprylic acid on rGRF(1-29)NH2 affinity to GRF pituitary binding sites was determined using [125I-Tyr10]hGRF(1-44)NH2 as radioligand.(ABSTRACT TRUNCATED AT 250 WORDS)

Internalization of growth hormone-releasing factor by rat anterior pituitary cells: inhibition by cerulenin, an inhibitor of fatty acid acylation

The GH-releasing factor (GRF) analogue [His1,Nle27]-GRF(1-29) amide was used to study GRF receptor internalization in cultured rat anterior pituitary cells. The half-life of occupied receptors on the surface was approximately 10 min. Uptake of the analogue was followed by lysosomal breakdown, and receptors taken up were replaced to some extent by newly synthesized receptors, as indicated by reduced surface binding in the presence of cycloheximide. 2,3-Epoxy-4-oxo-7,10-dodecadienamide (cerulenin) inhibited internalization without affecting breakdown of the reduced amount of GRF analogue that entered the cells. The effect was half-maximal at 3 micrograms/ml for 1 h. Cerulenin inhibits fatty acid acylation of proteins. One explanation for its effect on GRF receptor internalization is that fatty acid acylation of a protein (possibly the receptor) is necessary for internalization, because cerulenin also inhibited internalization of the transferrin receptor which is known to be acylated. Cerulenin did not affect internalization of the somatostatin receptor present on the same cells, indicating the specificity of the inhibition.

[Growth hormone-releasing hormone. The physiopathologic aspects and its diagnostic-therapeutic use]

The object of the present study is to review all that in the last years has been discovered about growth hormone-releasing hormone (GHRH), in order to point out both its physiopathological characteristics and its possible diagnostic and therapeutic use. In the first section are summarily reviewed the different studies that culminated in 1982 with the identification of three GRF: GRF(1-37)-OH, GRF(1-40)-OH and GRF(1-44)-NH2, the last of which, by immunohistochemical methods, resulted to be similar to the hypothalamic hGHRH. Then we describe the anatomic distribution of GHRH in man, and its mechanism of action at both receptor and postreceptor levels. On the other hand, the control of the GHRH secretion by peptidergic hypothalamic neurons occurs through four principal monoaminergic systems such as dopaminergic, noradrenergic, adrenergic and serotoninergic ones, and also by cholinergic fibers and by endogenous opiates, all acting to cause the release, into the hypothalamo-hypophyseal portal circulation, of GHRH. In the second section is attracted attention on the GHRH as a diagnostic agent in the two diseases that represent the main alterations of the GH secretion: acromegaly and short stature. According to the different studies considered, it may be concluded that GHRH testing has limited diagnostic usefulness in the clinical evaluation of acromegaly, but allows to discriminate acromegalic patients with ectopic production of GHRH from those with pituitary tumors. For what concerns short stature, the results of observation realized both in adult subjects and in children, all with GH deficiency, by exogenous administration of GHRH, have pointed out that the majority of the GH deficiency patients have hypothalamic disregulation, and not a pure pituitary deficiency as it has been supposed before GHRH discovery. In the third section is attracted attention on the GHRH as a therapeutic agent. Its possible use in the therapy of the children with GH deficiency is of considerable interest, above all in relation to the hypothalamic pathogenesis of their short stature.

Blockade of growth hormone-releasing factor (GRF) activity in the pituitary and hypothalamus of the conscious rat with a peptidic GRF antagonist

Microinjection of synthetic GRF into the cerebroventricles or hypothalamus of the rat produces a number of neural effects, including the suppression of GH secretion, possibly representing a negative ultrashort loop autoregulation of GRF and/or stimulation of somatostatin neurosecretion. To demonstrate that such neuromodulation acts physiologically through endogenous GRF activity, the peptidic GRF antagonist (N-Ac-Tyr1,D-Arg2)GRF-(1-29)-NH2 was used to block the action of GRF on its presumed receptors in the hypothalamus. First, to establish the efficacy of the antagonist to block GRF receptors in the anterior pituitary, we injected the antagonist iv at doses of 2, 20, and 50 micrograms or saline (controls) into conscious male rats fitted with jugular cannulae. Sequential blood sampling every 15 min for 6 h between 1000-1600 h showed that 50 micrograms antagonist, iv, significantly suppressed the two periods of spontaneous release of radioimmunoassayable GH in controls in the morning and afternoon. A dose of 20 micrograms, iv, lowered mean plasma GH between 1400-1500 h (P less than 0.025), while the 2-microgram dose was without effect. The GRF antagonist was then microinjected into the third ventricle (3V) of conscious male rats at doses of 0.5 and 8.0 ng in 2 microliter sterile saline. The 8.0-ng dose of 3V antagonist elicited a 3-fold increase in the morning peak of GH (nanograms per ml): 3V antagonist, 159.0 +/- 62.0; 3V control, 51.0 +/- 21.9 (P less than 0.05). The 0.5-ng dose was without effect. Finally, we observed that pretreatment with the GRF antagonist 3V (10 ng), followed 15 min later by 10 ng rat GRF administered 3V, completely blocked the GRF-induced suppression of pulsatile GH release observed earlier. Both the systemic and central effects of the antagonist were specific to the control of GH, since PRL concentrations were unaltered. These results 1) have demonstrated the ability of a peptidic GRF antagonist to specifically suppress pulsatile GH release after its systemic administration, presumably by acting on pituitary GRF receptors, and 2) support the notion that GRF receptors are also present in the hypothalamus and are available for the physiological mediation of GRF-induced inhibition of GH release by a central mechanism.

Nucleotide regulation of growth hormone-releasing factor binding to rat pituitary receptors

The specific binding of a GRF radioligand, [His1,125I-Tyr10,Nle27]hGRF-1-32NH2, to rat pituitary homogenates is reduced by the addition of GTP and its nonhydrolyzable analogs 5'-guanylylimidodiphosphate (GppNHp) and guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S). GDP and cAMP had no effect while the nonhydrolyzable ATP analogs 5'-adenylylimidodiphosphate and adenosine 5'-O-(3-thiotriphosphate) did elicit a significant reduction in GRF binding. The effect of GppNHp was half-maximal at 0.2 microM, and the maximum inhibition achieved was 85%. The effect of 0.1 microM GppNHp on GRF competitive displacement experiments indicated a significant reduction in affinity for the ligand (Kd = 0.51 +/- 0.11 nM in the absence of GppNHp and 2.1 +/- 1.1 nM in its presence) without an effect on receptor number. The GRF radioligand dissociates slowly from its receptor (t1/2 = 250 +/- 50 min), but the addition of 0.1 microM GppNHp converts approximately half of the receptors present to a more rapidly dissociating form (t1/2 = 9 +/- 10 min). These results are consistent with existing models for receptor-G-protein interactions, and thus, we conclude that transduction of the GRF response across the cell membrane involves a guanine nucleotide-binding protein, presumably Gs.

Cross-linking of a growth hormone releasing factor-binding protein in anterior pituitary cells

Growth hormone-releasing factor (GRF) stimulates the release of growth hormone from the anterior pituitary and is related to the peptides of the glucagon/secretin family. Although the mechanism of action of this hormone has been studied in considerable detail, little is known concerning the GRF receptor itself. We have attempted to label the GRF receptor by chemically coupling the 125I-GRF analog [His1, Nle27]-hGRF(1-32)-NH2 (GRFa) (where Nle is norleucine) to plated rat anterior pituitary cells with the protein cross-linker disuccinimidyl suberate (DSS) (0.1 mM). Verification of biological activity of the 125I-GRFa was confirmed prior to the cross-linking experiments using the reverse hemolytic plaque assay. Whole cell extracts prepared from the cross-linked cells were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by autoradiography of the dried gels. Four bands of 72, 50, 30, and 26 kDa were detected in autoradiograms from cells exposed to the labeled analog for 20 min (22 degrees C) followed by exposure to DSS for 2 min. The 72-kDa band was interpreted to be bovine serum albumin, which was used as a carrier in initial studies. The 50- and 30-kDa bands were very faint and probably represent nonspecific binding sites since they were unchanged in the presence of excess unlabeled GRFa. The 26-kDa band was diminished in a concentration-dependent manner by unlabeled rat GRF, GRFa, and to a lesser extent by vasoactive intestinal peptide (VIP). It is unlikely, however, that GRFa was acting at a VIP receptor since the labeled analog did not induce prolactin secretion (VIP is a prolactin secretagogue). GRFa also increased cellular cAMP to levels similar to GRF and greater than VIP. Autoradiographs from gels run under nonreducing conditions revealed the 26-kDa band as the major species, indicating that, if a polymeric form of this binding protein exists, it does not involve disulfide linkages. Thus, the best candidate for the putative GRF receptor is the 26-kDa band. We have further demonstrated that the higher concentrations of DSS used previously (5 mM) result in diffuse autoradiograms with multiple bands, suggesting that caution should be exercised when interpreting cross-linking data under these conditions.

Dopamine receptor modulation by Pro-Leu-Gly-NH2 analogues possessing cyclic amino acid residues at the C-terminal position

The synthesis of several analogues of L-prolyl-L-leucylglycinamide (PLG) was carried out in which the glycinamide residue was replaced with the following cyclic amino acid residues: L- and D-prolinamide, (+)- and (-)-thiazolidine-2-carboxamide, L- and D-3,4-dehydroprolinamide, L-azetidine-2-carboxamide, L-piperidine-2-carboxamide, and L-thiazolidine-4-carboxamide to give PLG analogues 2-10, respectively. The ability of these analogues to enhance the binding of the dopamine agonist ADTN (2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene) to dopamine receptors was determined by using bovine brain tissue. All of the PLG analogues synthesized in this study enhanced the binding of ADTN to central dopamine receptors. The percent enhancement of ADTN binding produced by analogues 2,3, and 7-10 at various concentrations was comparable to the percent enhancement produced by PLG. The PLG analogues Pro-Leu-(+)-thiazolidine-2-carboxamide (4), Pro-Leu-(-)-thiazolidine-2-carboxamide (5), and Pro-Leu-L-3,4-dehydroprolinamide (6), however, produced significantly greater enhancement (2-3-fold) in ADTN binding than did PLG.

Desensitization to growth hormone-releasing factor (GRF) is associated with down-regulation of GRF-binding sites

The time course, concentration dependence, and mechanism of rat anterior pituitary desensitization to GRF were studied. Chronic stimulation of cultures of rat anterior pituitary cells with rat GRF (rGRF) resulted in desensitization to a subsequent challenge with the peptide. Despite a slight enhancement of GH synthesis, prolonged exposure to GRF caused substantial depletion of cellular GH pools. As a result, acute secretory responses were markedly blunted. Depletion was accompanied by a time-dependent decrease in sensitivity to rGRF; GRF EC50 values for GH release of 0.5 nM rGRF-pretreated cells were 24.8 +/- 6 (+/- SEM) pM after 2 h, 46.2 +/- 2.4 pM after 4 h, and 154.7 +/- 31 pM after 8 h compared to 9.2 +/- 0.6 pM for control cells. The process of desensitization was complete within 8 h, as cells pretreated for 24 h exhibited sensitivity to rGRF comparable to that of cells pretreated for 8 h. Desensitization was associated with a time-dependent decrease in rat anterior pituitary cell GRF-binding capacity; a 48% loss of binding sites was evident after a 2-h pretreatment with 0.5 nM rGRF, with a maximum loss occurring after 8 h. The dose of rGRF required to produce an attenuation of responsiveness did not completely correlate with the dose requirement for down-regulation of binding sites. The decrease in GRF-binding sites was not associated with any alteration of apparent Kd values, which were 0.36 (0.18-0.72) nM in control and 0.1 (0.01-0.82) nM after 8 h of exposure to 0.5 nM rGRF. Both the reduction in GRF-binding capacity and decreased sensitivity to GRF were reversible after 24 h, although cellular GH pools were not restored to control levels. These results suggest that rat anterior pituitary cells become desensitized to rGRF after chronic stimulation with a maximal concentration of the peptide. One mechanism for this decrease in apparent sensitivity to rGRF may be the pronounced reduction or down-regulation of GRF-binding sites.

Receptor-associated resistance to growth hormone-releasing factor in dwarf "little" mice

Anterior pituitaries from the dwarf mouse strain "little" did not release growth hormone or accumulate adenosine 3',5'-monophosphate (cyclic AMP) in response to human and rat growth hormone-releasing factor (GRF). Dibutyryl cyclic AMP, as well as the adenylate cyclase stimulators forskolin and cholera toxin, markedly stimulated growth hormone (GH) release. The basis of the GH deficiency in the little mouse may therefore be a defect in an early stage of GRF-stimulated GH release related either to receptor binding or to the function of the hormone-receptor complex.

Covalent cross-linking of growth hormone-releasing factor to pituitary receptors

We have used a bifunctional cross-linker, disuccinimidyl suberate, to covalently attach [125I]human pancreatic GH-releasing factor (GHRF) (-1-40)OH to bovine pituitary membranes and rat anterior pituitary cells. Covalently radiolabeled membrane and cell preparations were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing and nonreducing conditions. In the former case, we observed the specific labeling of a polypeptide with an apparent mol wt of 75,000 +/- 3,000. The labeling of this species was specific for GHRF, as evidenced by the fact that it was inhibited in a dose-dependent fashion with increasing concentration of unlabeled GHRF. Furthermore, the radiolabeling was inhibited in the presence of excess unlabeled GHRF analogs but not unrelated peptides such as insulin and rat GH. The size of the radiolabeled band was the same in both bovine pituitary membranes and rat anterior pituitary cells. The extent of radiolabeling was dependent on the amount of membrane or the number of cells present during the binding reaction. These observations indicate that the mol wt 75,000 species is a ligand-binding subunit of the GHRF receptor in the pituitary. Under nonreducing conditions, a species much larger than mol wt 200,000 was specifically radiolabeled, again in both bovine pituitary membranes and rat cells. This result suggests the possibility that the ligand-binding subunit might be disulfide-linked to other subunit(s) forming homo- and heterooligomers.

Comparative structural requirements of thirty GRF analogs for interaction with GRF- and VIP receptors and coupling to adenylate cyclase in rat adenopituitary, liver and pancreas

The ability of 30 synthetic GRF(1-29)-NH2 analogs to stimulate adenylate cyclase activity was investigated in membranes from rat adenopituitary, rat liver and rat pancreas. In adenopituitary membranes, GRF and GRF analogs interacted with specific GRF receptors, whereas in liver and pancreatic membranes, they interacted with VIP receptors. The C-terminal moiety of GRF was responsible for GRF receptor recognition as the hybrid analog (His1, D-Ala2)-GRF(1-9), VIP(10-28) stimulated pituitary adenylate cyclase through the occupancy of VIP receptors only. When GRF or VIP receptors were occupied by GRF analogs, the N-terminal part of the ligand appeared critical for adenylate cyclase activation. This was established by testing 30 GRF analogs mono-, bi- or tri-substituted in positions 1 to 10. Major observations included: (a) the characterization of (N-Ac-Tyr1, D-Arg2)-GRF(1-29)-NH2 as an antagonist of GRF-stimulated pituitary adenylate cyclase; (b) the discovery of the (N-Ac-Tyr1, D-Phe2)-, (His1, D-Ala2, D-Ser3, NLeu27)-, and (His1, D-Ala2, D-Thr7, NLeu27)-derivatives of GRF(1-29)-NH2 as specific antagonists of VIP receptors in rat pancreatic membranes; (c) the importance of the free NH2 function of amino acid residue 1 for pancreatic adenylate cyclase activation, and (d) the decreased efficiency of iodinated (Tyr1)-GRF(1-29)-NH2 as opposed to the non iodinated form, in all systems tested.

Down-regulation of growth hormone releasing factor receptors following continuous infusion of growth hormone releasing factor in vivo

The in vivo chronic infusion of growth hormone releasing factor (GRF) results in a loss of the pituitary growth hormone (GH) response to GRF as well as in a substantial depletion of pituitary GH content. To evaluate if the loss in response is due to the down-regulation of GRF receptors the specific GRF binding capacity of pituitary homogenates prepared from rats infused with saline or GRF (1 or 15 micrograms/h for 24 h) was determined. The pituitary binding capacity of animals infused with GRF was significantly reduced as compared to animals infused with saline.

Structural requirements for the activation of rat anterior pituitary adenylate cyclase by growth hormone-releasing factor (GRF): discovery of (N-Ac-Tyr1, D-Arg2)-GRF(1-29)-NH2 as a GRF antagonist on membranes

The efficacy and potency of 14 GH-releasing factor (GRF) analogs, substituted in position 1 to 7, on adenylate cyclase activation in crude homogenates from rat anterior pituitary were related to those of human pancreatic GRF(1-29)-amide and vasoactive intestinal peptide. Among several D-amino acid substitutions, that in position 2 was the only one to yield a super-agonist [with a Kact (concentration required for half-maximal adenylate cyclase activation) 2 times lower than that of GRF(1-29)-NH2]. By contrast, D-isomer substitution in position 1 and 3 was without effect and D-isomer substitution in position 4, 6, or 7 decreased the affinity of the analog. The N-acetylated analog of GRF was as potent and active as the parent peptide, and the identity of the amino acid in position 2 of (N-Ac-Tyr1)-GRF(1-29)-NH2 proved to be determining for enzyme activation, with D-Phe2 and D-Trp2 derivatives acting as partial agonists and the (N-Ac-Tyr1,D-Arg2) analog being an efficient competitive antagonist of GRF(1-29)-NH2. With use of this antagonist, it was possible to demonstrate that GRF and vasoactive intestinal peptide receptors represent distinct entities in the rat anterior pituitary.

Somatocrinin receptor coupled with cAMP-dependent protein kinase on anterior pituitary granules

The molecular mechanism of growth hormone release by synthetic somatocrinin was investigated on purified hog anterior pituitary secretory granules; the granules were found to contain a cAMP-dependent protein kinase that catalyzed [gamma-32P]-ATP histone phosphorylation with maximal rates ranging from 1 to 5 nmol of Pi incorporated per mg of protein per 20 min. The activity of this enzyme was further stimulated by somatocrinin. Stimulation was observed at concentrations as low as 0.3 pM, and the half-maximal effect was obtained with 35 +/- 8 pM (n = 4). Michaelis-Menten analysis of phosphorylation kinetics suggested that the peptide did not change significantly the reaction's Vmax, but produced a dramatic increase in enzyme affinity for cAMP: the apparent Km for the nucleotide decreased from 400 X 10(-9) M under unstimulated conditions to 15 X 10(-9) M in the presence of 100 pM somatocrinin. Furthermore, a Hill plot of concentration-dependence curve indicated the existence of negative cooperativity. At the concentration of 35 pM, the less potent analogs of somatocrinin [designated hpGRF-44 to indicate source (human pancreas, hp), activity (growth hormone-releasing factor, GRF), and amino acid composition], hpGRF-(1-37) and [Phe1]hpGRF-(1-40) had 20% and 7%, respectively, of the effect of somatocrinin. The biologically inactive analog hpGRF-(2-40) had no evident effect at concentrations up to 0.1 microM. Therefore, we suggest that somatocrinin stimulation of growth hormone release involves activation of exocytosis through a phosphorylation mechanism mediated by a granular receptor coupled with a cAMP-dependent protein kinase.

Characterization of binding sites for N-Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) in rat brain

Binding sites for N-Tyr-Pro-Leu-Gly-NH2 (Tyr-MIF-1), a novel peptide structurally related but immunoreactively different from Pro-Leu-Gly-NH2 (MIF-1), were investigated. The presence of Tyr-MIF-1-like material in brain tissue has previously been demonstrated by RIA and its levels were shown to vary with the diurnal cycle and after pinealectomy. We now demonstrate a high affinity binding site in rat brain that is saturable and specific for Tyr-MIF-1. Crude P2 synaptosomal fractions from rat brains were incubated with 125I-Tyr-MIF-1 in the presence or absence of 10 microM unlabeled Tyr-MIF-1 (nonspecific binding). Binding reached equilibrium at 30-40 min at 23 degrees C and at about 4 hr on ice, after which it was relatively stable for at least 18 hr. None of the other peptides (including MIF-1) or amino acid residues tested were found to effectively compete for 125I-Tyr-MIF-1 binding. Binding was linear with protein from 280 micrograms to at least 1.1 mg protein per tube. Scatchard analysis of the striatum-thalamus revealed the presence of binding sites with an apparent Kp of 91 nM and maximum number of sites in the range of 45 fmol/mg tissue. Analysis of several brain areas revealed a differential distribution of the binding sites with relatively high concentrations in cortex, striatum, and amygdala and low concentrations in pons-medulla. Together with the previously published RIA results, the demonstration of a receptor for Tyr-Pro-Leu-Gly-NH2 supports the concept of the presence of this novel peptide and its receptor in the brain.

CNS putative L-prolyl-L-leucyl-glycinamide (PLG) receptors, brain and lymphocyte dopamine receptors

1. In view of previously demonstrated modulatory effects of PLG on the sensitivity of central dopamine receptors, we developed a radioligand binding assay to identify specific binding sites of PLG in rat and normal human brain. 2. 3H-PLG binds specifically to rat striatum exhibiting high affinity (KD = 4.69 +/- 0.50 nM) saturability (Bmax = 9.20 +/- 0.30 fmoles/mg protein) and reversibility; the highest density of specific PLG binding sites occurring in the striatum, followed by the hypothalamus and cerebral cortex. 3. Saturable, high-affinity binding sites of PLG were identified in human striatum. The substantia nigra was enriched with the highest level of specific PLG binding sites. 4. Dopamine receptors were identified in human lymphocytes. 5. The results are compatible with the hypothesis that differential modulation of CNS dopamine receptors by PLG is functionally associated with interacting with specific PLG binding sites in the rat and human brain, and pose implications for Parkinson's disease and tardive dyskinesia.