Kaiya. Hiroyuki H; Konno. Norifumi N; Kangawa. Kenji K; Uchiyama. Minoru M; Miyazato. Mikiya M
We identified two ghrelin receptor isoforms, the ghrelin receptor type-1a (GHS-R1a) and its alternative splice form (GHS-R1b) for West African lungfish, Protopterus annectens. Lungfish GHS-R1a and 1b comprised 361 and 281 amino acids, respectively. Lungfish GHS-R1a showed the highest identity to coelacanth GHS-R1a (80.4%). The highest expression of GHS-R1a mRNAs was seen in the brain, liver, ovary, heart, intestine, and gills. GHS-R1b mRNAs were also detected in the same tissues with GHS-R1a, but their expression level was 1/20 that of GHS-R1a. In human embryonic kidney 293 cells transiently expressing lungfish GHS-R1a, rat and bullfrog ghrelin, and two GHS-R1a agonists, GHRP-6 and hexarelin, increased intracellular Ca(2+) concentrations. The intensity of the Ca(2+) increases induced by GHS-R1a agonists was twice when compared to that induced by ghrelin, although the median effective doses (ED50) were similar, suggesting a long-lasting effect of GHS-R1a agonists with similar affinity. We also examined changes in the GHS-R gene expression during an eight-week estivation. Body weight was slightly lowered, but plasma sodium and glucose concentrations decreased; plasma urea concentration increased significantly 4weeks after the start of estivation. Overall, expression of GHS-R1a mRNA decreased, but changes in GHS-R1b mRNA expression were inconsistent with those of GHS-R1a during estivation, suggesting an involvement of GHS-R in energy homeostasis, as seen in mammals. Our results suggest that the ghrelin-GHS-R1a system is present in this lungfish although ghrelin has not yet been found. The structure of GHS-R1a is closer to that of tetrapods than Actinopterygian fish, indicating a process of evolution that follows the Crossopterygii such as coelacanth.
Zhou. Cheng-Hua CH; Li. Xin X; Zhu. Yang-Zi YZ; Huang. He H; Li. Jing J; Liu. Lu L; Hu. Qian Q; Ma....
Neuropathic pain is related to the sustained activation of neuroglial cells and the production of proinflammatory cytokines in the spinal dorsal horn. Ghrelin, the endogenous ligand for growth hormone secretagogue receptor 1a (GHSR-1a), has been shown to inhibit the activation of microglia and the release of proinflammatory cytokines. The purpose of this study was to investigate the role of ghrelin/GHSR-1a signaling in neuropathic pain and to understand the associated mechanisms.
A rat model of neuropathic pain was established by chronic constriction injury (CCI) of the sciatic nerve. Hyperalgesia and allodynia were evaluated by observing the mechanical withdrawal threshold and the thermal withdrawal latency. The expression levels of ghrelin and GHSR-1a were detected by semiquantitative reverse transcriptase-polymerase chain reaction and Western blot analysis. The levels of interleukin-1β (IL-1β), IL-6, and tumor necrosis factor-α were detected using an enzyme-linked immunosorbent assay. The expression levels of p-p38 mitogen-activated protein kinases (p38 MAPK) and nuclear factor-κB (NF-κB) p65 were determined by Western blot and immunohistochemistry analysis.
Both ghrelin and GHSR-1a were expressed in the spinal dorsal horns of normal rats and were not significantly different from that of sham rats. However, rats in the CCI model group developed severe hyperalgesia and allodynia, as well as significantly downregulated expression of ghrelin and GHSR-1a. Compared with CCI model rats, intrathecal injection of ghrelin clearly delayed thermal hyperalgesia and mechanical allodynia at 3, 5, and 7 days after CCI; reduced the levels of IL-1β, IL-6, and tumor necrosis factor-α; and inhibited the phosphorylation of p38 MAPK and the activation of NF-κBp65 in the spinal dorsal horn. In addition, the effect of ghrelin could be blocked by [D-Lys]-GHRP-6, a GHSR-1a antagonist.
Our present study demonstrated that ghrelin alleviated neuropathic pain through a GHSR-1a-mediated suppression of the p38 MAPK/NF-κB pathway.
Spiliotis. Bessie E BE; Papadimitriou. Dimitrios T DT; Alexandrides. Theodore K TK; Karystianos. Chr...
The combined growth hormone-releasing hormone and growth hormone-releasing peptide-6 (GHRH + GHRP-6) test is most potent in evaluating GH secretion.
To assess its capability in children with GH deficiency and low spontaneous GH secretion (GH neurosecretory dysfunction).
35 children with GH <10 microg/l after levo-dopa/clonidine (GHD), 15 with normal provocative tests but abnormal 24-hour spontaneous GH secretion (GHND), and 20 controls (C) were given 1 microg/kg of GHRH and GHRP-6 i.v. and GH (microg/l) was measured at -15, 0, 5, 10, 15, 30, 45 and 60 min.
Six were nonresponders to the combined test, with significantly lower peak GH 20.7 (7.8-31.8) than C and the rest of the patients (responders). Peak GH was similar between prepubertal (PP) controls 167 +/- 88, GHD 202 +/- 110 and GHND 155 +/- 83. Pubertal (P) controls had higher peak GH 328 +/- 149 than P-GHD 203 +/- 105 and P-GHND 186 +/- 105. While P-C had higher peak GH than PP-C, PP and P children had similar responses within the GHD and GHND groups.
The GHRH + GHRP-6 test detects children with severe GH insufficiency. Patients with GHD respond similarly to those with GHND, indicating a possible hypothalamic GH neuroregulatory dysfunction in GHD. Responders to the combined test may be eligible for treatment with a synthetic GH secretagogue.
Kobashi. Motoi M; Xuan. Song-Yu SY; Fujita. Masako M; Mitoh. Yoshihiro Y; Matsuo. Ryuji R
The effect of ghrelin on rhythmic reflex swallowing was examined in urethane-chloralose anesthetized rats. Swallowing was monitored by recording electromyographic activities of the suprahyoid muscle. Fourth ventricular administration of ghrelin decreased swallowing frequency during electrical stimulation of the central cut end of the superior laryngeal nerve (SLN stimulation). A significant decrease in swallowing frequency was observed after ghrelin administration at doses of 3, 10, 30 and 100 pmol. The administration of ghrelin with growth hormone secretagogue receptor antagonist ([D-Lys(3)] GHRP-6) did not change swallowing frequency during SLN stimulation. Neither mean blood pressure nor heart rate changed after the administration of 10 pmol ghrelin. Bilateral vagotomy did not disrupt the ghrelin response. These observations indicate that the ghrelin response does not depend on either cardiovascular or abdominal responses. Microinjection of ghrelin (0.3 pmol) into the vicinity of the solitary tract inhibited swallowing induced by SLN stimulation. Fourth ventricular administration of orexin-A (3 nmol) also inhibited reflex swallowing elicited by SLN stimulation. These results suggest that ghrelin and other orexigenic peptides inhibit reflex swallowing by modifying neural activities of the dorsal medulla where the swallowing center is housed.
Popovic. Vera V; Pekic. Sandra S; Doknic. Mirjana M; Micic. Dragan D; Damjanovic. Svetozar S; Zarkov...
The objective of this study is to investigate the performance of two novel tests in diagnosing growth hormone deficiency in adults.
Fifty patients presenting with long-standing hypopituitarism and two control groups consisting of 50 healthy subjects in each group [age-, sex- and body mass index (BMI)-matched] participated in a multicentre study. All underwent two provocative tests on different days within 3 months apart: (i) a combined GHRH + GHRP-6 test and (ii) arginine + GHRH test. Main outcome measures were sensitivity, specificity and areas under receiver operating characteristic curves (ROC) for GHRH + GHRP-6 and arginine + GHRH tests in the diagnosis of GH deficiency in adults.
A peak GH level after GHRH + GHRP-6, under 15.0 microg/l had a sensitivity of 94.4% and specificity of 98.8% for diagnosing GHD and area under the ROC curve was 0.99 (95% confidence interval 0.95-0.99). For peak GH levels after arginine + GHRH, a level under 9.6 microg/l had a sensitivity of 88.4% and specificity of 95% and area under the ROC curve of 0.96 (95% confidence interval 0.91-0.98). The relationship between the increment in serum GH following GHRH + GHRP-6 and arginine + GHRH test was positive, i.e. r = 0.749, P = 0.001.Thus, there was high concordance between both tests.
Both GHRH + GHRP-6 and/or arginine + GHRH test may serve as an alternative to the ITT in diagnosing GHD in adults.
Deng. Bin B; Fang. Li L; Chen. Xiaobin X; Chen. Meifang M; Xie. Xiumei X
To determine the effect of ghrelin on protecting the human umbilical vein endothelial cells (HUVEC) from injury by angiotensin II (Ang II) in vitro.
(1) HUVEC was incubated for 24 h with AngII whose final concentration in the medium varied from 10⁻⁹ to 10⁻⁶ mol/L or pretreated with 10⁻⁹ to 10⁻⁶ mol/L ghrelin for 2 h before incubation for 24 h with Ang II whose final concentration in the medium was 10⁻⁶ mol/L. HUVECs were harvested to measure the cell vitality and cell apoptosis. The cell vitality was determined by MTT and cell apoptosis rates were measured by Annexin V-FITC apoptosis detection kit. (2) HUVECs were incubated for 3, 6, 12, or 24 h with 10⁻⁹, 10⁻⁸, 10⁻⁷, or 10⁻⁶ mol/L Ang II, respectively. Before HUVECs were incubated with 10⁻⁶ mol/L Ang II for 24 h, ghrelin (10⁻⁹, 10⁻⁸, 10⁻⁷, and 10⁻⁶ mol/L) was used to pretreat the cells for 2 h. Growth hormone secregogue receptor 1a blocker [D-Lys³]GHRP-6 was added to the cells which were incubated for 24 h with 10⁻⁶ mol/L Ang II and pretreated with 10⁻⁶ mol/L ghrelin for 2 h. Cell reactive oxygen species were measured by dichlorofluorescin (DCF) fluorescence probe method. (3) HUVECs were incubated for 24 h with 10⁻⁹, 10⁻⁸, 10⁻⁷, or 10⁻⁶ mol/L Ang II and ghrelin, respectively,and then were incubated with 10⁻⁶ mol/L of Ang II for 3, 6, 12, or 24 h. Furthermore, HUVECs were pretreated with 10⁻⁹, 10⁻⁸, 10⁻⁷, or 10⁻⁶ mol/L ghrelin for 30 min,1 h, or 2 h, and then were incubated with the inhibitor of mitogen-activated protein kinase /extracellular regulated kinase (MAPK/ERK1/2),PD98059, the inhibitor of phosphoinositide-3-kinase/serine threonine kinase( PI3K/Akt)wortmannin, and [D-Lys³]GHRP-6 for 24 h. NO production was compared among groups. HUVECs were pretreated with ghrelin, PD98059, wortmannin, and [D-Lys³]GHRP-6 for 2 h and co-cultured with 10⁻⁶ mol/L Ang II for 24 h, or pretreated with ghrelin plus PD98059, wortmannin, and [D-Lys³]GHRP-6 before incubation with Ang II for 24 h. NO was measured in the endothelial cell supernatants by Griess method. (4) HUVECs were cultivated with blank or Ang II with or without pretreatment with ghrelin or both ghrelin and wortmannin. The protein expression of eNOS and phospho-protein expression of Akt were measured by Western blot analysis.
Ang II injuried the endothelial cell vitality,increased the cell apoptosis rates in HUVECs, and decreased NO production in HUVEC supernatants,whereas ghrelin protected HUVECs from Ang II injury. Ghrelin decreased the reactive oxygen species in HUVECs induced by Ang II. The effect could be attenuated by [D-Lys³]GHRP-6 pretreatment; PD98059 alleviated Ang II inhibition of NO production in HUVEC supernatants. Wortmannin and [D-Lys³]GHRP-6 could abolish protection of ghrelin from reducing NO production in HUVEC supernatants. Ang II reduced the expression of eNOS,but ghrelin increased eNOS expression. Wortmannin could cancel this effect of ghrelin. Ghrelin increased p-Akt expression and reached the peak in 10 and 20 min.
Ghrelin may protect HUVECs from Ang II induced injury, which is related to decreasing oxidative stress, increasing the protein expression of eNOS, and activating PI3K/Akt signal pathway through GHSR1a receptor.
DeVita. R J RJ
In 1981, Bowers reported that xenobiotic peptides, derived from the Leu- and Met- enkephalins, demonstrated novel growth hormone (GH) secretory activity. The most potent peptide reported, GH releasing peptide-6 (GHRP-6), was shown to release GH by a different pathway to the known signalling peptide, growth hormone releasing hormone (GHRH). The discovery of a peptidyl GH secretagogue laid the foundation for the search for smaller, orally active mimetics of GHRP-6, as well as for its mechanism of action. This review focuses on the recent developments in the field of small molecule GH secretagogues from a medicinal chemistry perspective, and discusses various structural classes of mimetics recently reported in the literature.
Lau. Pui Ngan PN; Chow. Kevin B S KB; Chan. Chi-Bun CB; Cheng. Christopher H K CH; Wise. Helen H
The ghrelin receptor (GHS-R1a) displays a high level of constitutive signaling through a phospholipase C/protein kinase C-dependent pathway. Therefore, we have investigated the role of agonist-dependent and agonist-independent signaling of GHS-R1a in apoptosis using the seabream GHS-R1a stably expressed in human embryonic kidney 293 cells (HEK-sbGHS-R1a cells). Cadmium-induced activation of caspase-3 was significantly attenuated in HEK-sbGHS-R1a cells compared to wild-type HEK293 cells, while the apoptotic responses to the protein kinase C inhibitor staurosporine were similar. GHS-R1a ligands had no effect on caspase-3 activation or on cell proliferation. Concentrations of the inverse agonist [d-Arg(1),d-Phe(5),d-Trp(7,9),Leu(11)]-substance P sufficient to inhibit constitutive inositol phosphate generation did not enhance caspase-3 activity, suggesting a possible role of phosphatidylcholine-specific phospholipase C in the anti-apoptotic activity of GHS-R1a. In conclusion, our data suggests that the constitutive activity of sbGHS-R1a may be sufficient alone to attenuate apoptosis via a protein kinase C-dependent pathway.
Salgado. Luiz Roberto LR; Fragoso. Maria Candida B Villares MC; Knoepfelmacher. Mirta M; Machado. Ma...
Ectopic ACTH syndrome (EAS) occurs in about 5-10% of all patients with ACTH-dependent hypercortisolism with most of them caused by intrathoracic neoplasms. It may be associated with overt malignancies or with occult and indolent tumors. We assessed the accuracy of dynamic tests, inferior petrosal sinus sampling (IPSS) using desmopressin, and imaging in the work-up diagnosis of EAS.
Tumor markers, imaging, and outcome data from 25 patients (13F/12M) aged 18-72 years. High dexamethasone suppression test (HDDST), desmopressin test, GHRP-6 test, corticotropin-releasing hormone (CRH) test, IPSS, computed tomography (CT), magnetic resonance imaging (MRI), and (111)In-pentetreotide scintigraphy were revised.
In 5 out of 20 patients HDDST was positive. In 13 patients who underwent desmopressin test, ACTH- and cortisol-positive responses were seen in six and five patients respectively. GHRP-6 test was positive in two out of three cases. Two patients underwent CRH test with negative response. In the seven patients submitted to IPSS using desmopressin in six of them, none had ACTH gradients. CT was positive in 15 out of 21 patients and MRI in 8 out of 17 cases. (111)In-pentetreotide scintigraphy was positive in three out of five patients. Fourteen patients had intrathoracic tumors, five had pheochromocytomas, three had pancreatic tumors, one had a glomic tumor, and had three occult tumors. Six out of 11 patients with metastasis died and 3 others without metastasis died.
IPSS with desmopressin was helpful for differential diagnosis. Patients initially harboring occult carcinoids may also exhibit severe hypercortisolism and those harboring tymic carcinoids had poor prognoses when compared with bronchial carcinoids and pheocromocytomas.
Gauna. Carlotta C; Kiewiet. Rosalie M RM; Janssen. Joop A M J L JA; van de Zande. Bedette B; Delhant...
Acylated and unacylated ghrelin (AG and UAG) are gut hormones that exert pleiotropic actions, including regulation of insulin secretion and glucose metabolism. In this study, we investigated whether AG and UAG differentially regulate portal and systemic insulin levels after a glucose load. We studied the effects of the administration of AG (30 nmol/kg), UAG (3 and 30 nmol/kg), the ghrelin receptor antagonist [D-Lys(3)]GHRP-6 (1 micromol/kg), or various combinations of these compounds on portal and systemic levels of glucose and insulin after an intravenous glucose tolerance test (IVGTT, d-glucose 1 g/kg) in anesthetized fasted Wistar rats. UAG administration potently and dose-dependently enhanced the rise of insulin concentration induced by IVGTT in the portal and, to a lesser extent, the systemic circulation. This UAG-induced effect was completely blocked by the coadministration of exogenous AG at equimolar concentrations. Similarly to UAG, [D-Lys(3)]GHRP-6, alone or in combination with AG and UAG, strongly enhanced the portal insulin response to IVGTT, whereas exogenous AG alone did not exert any further effect. Our data demonstrate that, in glucose-stimulated conditions, exogenous UAG acts as a potent insulin secretagogue, whereas endogenous AG exerts a maximal tonic inhibition on glucose-induced insulin release.
Kamegai. Jun J; Tamura. Hideki H; Shimizu. Takako T; Ishii. Shinya S; Tatsuguchi. Atsushi A; Sugihar...
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Miura. Tohru T; Maruyama. Keisuke K; Shimakura. Sei-Ichi S; Kaiya. Hiroyuki H; Uchiyama. Minoru M; K...
Intracerebroventricular (ICV) administration of ghrelin, orexin and neuropeptide Y (NPY) stimulates food intake in goldfish. Orexin and NPY interact with each other in the regulation of feeding, while ghrelin-induced feeding has also shown to be mediated by NPY in the goldfish model. To investigate the interaction between ghrelin and orexin, we examined the effects of a selective orexin receptor-1 antagonist, SB334867, and a growth hormone secretagogue-receptor antagonist, [D-Lys(3)]-GHRP-6, on ghrelin- and orexin-A-induced feeding. Ghrelin-induced food intake was completely inhibited for 1h following ICV preinjection of SB334867, while [D-Lys(3)]-GHRP-6 attenuated orexin-A stimulated feeding. Furthermore, ICV administration of ghrelin or orexin-A at a dose sufficient to stimulate food intake increased the expression of each other's mRNA in the diencephalon. These results indicate that, in goldfish, ghrelin and orexin-A have interacting orexigenic effects in the central nervous system. This is the first report that orexin-A-induced feeding is mediated by the ghrelin signaling in any animal model.
Micić. D D; Sumarac-Dumanović. M M; Macut. Dj Dj; Kendereski. A A; Zoric. S S; Popovi&#x...
Controversial data were reported on GH response to different provocative stimuli in obese patients with polycystic ovary syndrome (PCOS). The objective of our study was to assess the effect of short-term fasting on GH response to combined stimulus with GHRH+GH-releasing peptide-6 (GHRP-6) in obese patients with PCOS and possible relation with leptin and insulin changes during fasting. Twelve obese PCOS women and nine obese control women participated in 3-day fasting. GH response, IGF-I, insulin and leptin were measured after GHRH+ GHRP-6, before and after short-term fasting. Obese PCOS patients had significantly greater GH peak after GHRH+GHRP-6 before fasting. Enhanced response to GH stimulation was found after fasting without substantial differences between obese PCOS and obese controls. Insulin and leptin significantly decreased, while insulin sensitivity significantly improved in both groups during fasting. In conclusion, obese PCOS patients have peculiar type of GH response to GHRH+GHRP-6 before fasting, possibly due to enhanced sensitivity of somatotrophs. Observed changes in insulin and leptin may participate in modulation of enhanced GH response after short-term fasting to GHRH+GHRP-6 in PCOS and obese controls.
Bhatti. Sofie F M SF; Duchateau. Luc L; Van Ham. Luc M L LM; De Vliegher. Sarne P SP; Mol. Jan A JA;...
In a study on young and old dogs, the peptide GHRP-6 did raise growth hormone (GH) levels, but it wasn’t as strong as ghrelin in young dogs or GHRH in older dogs. Importantly, GHRP-6 didn’t cause spikes in other hormones like ACTH, cortisol, or prolactin, suggesting a fairly specific GH effect. The age‑related drop in GH response was biggest for ghrelin, while GHRP-6’s drop was smaller.
Rocha-Sousa. A A; Saraiva. J J; Henriques-Coelho. T T; Falcão-Reis. F F; Correia-Pinto. J J; Le...
Scientists found that the hormone ghrelin, which is known for stimulating growth hormone, can also relax the muscles that control pupil size in the eye. This effect is local (produced inside the eye) and works through different pathways depending on the muscle type.
Lall. Sabrina S; Balthasar. Nina N; Carmignac. Danielle D; Magoulas. Charamboulos C; Sesay. Abdul A;...
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Johnstone. Louise E LE; Srisawat. Rungrudee R; Kumarnsit. Ekkasit E; Leng. Gareth G
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Chan. Chi-Bun CB; Leung. Po-Ki PK; Wise. Helen H; Cheng. Christopher H K CH
We have recently cloned the full-length cDNAs of the two growth hormone secretagogue receptor (GHSR) subtypes from a teleost species, the black seabream (Acanthopagrus schlegeli) [Mol. Cell. Endocrinol. 214 (2004) 81], namely sbGHSR-1a and sbGHSR-1b. Functional expression of these two receptor constructs in human embryonic kidney 293 (HEK293) cells indicated that stimulation of sbGHSR-1a by growth hormone secretagogues (GHS) could evoke increases in intracellular Ca2+ concentration ([Ca2+]i), whereas sbGHSR-1b appeared to play an inhibitory role on the signal transduction activity of sbGHSR-1a. In the present study, we have further investigated the signal transduction mechanism of sbGHSR-1a. The peptide GHS GHRP-6 and the non-peptide GHS L163,540 were able to trigger a receptor specific and phospholipase C (PLC)-dependent elevation of [Ca2+]i in HEK293 cells stably expressing sbGHSR-1a. This GHS-induced calcium mobilization was also dependent on protein kinase C activated L-type calcium channel opening. It was found that sbGHSR-1a could function in an agonist-independent manner as it exhibited a high basal activity of inositol phosphate production in the absence of GHS, indicating that the fish receptor is constitutively active. In addition, the extracellular signal-regulated kinases 1 and 2 (ERK1/2) were found to be activated upon stimulation of sbGHSR-1a by GHRP-6. This observation provides direct evidence in the coupling of sbGHSR-1a to ERK1/2 activation. Neither Gs nor Gi proteins are coupled to the receptor, as GHS did not induce cAMP production nor inhibit forskolin-stimulated cAMP accumulation in the sbGHSR-1a bearing cells. Furthermore, the ability of the GHSR antagonist D-Lys3-GHRP-6 to inhibit basal PLC and basal ERK1/2 activity suggests that this compound is an inverse agonist. In summary, the sbGHSR-1a appears to couple through the G(q/11)-mediated pathway to activate PLC, resulting in increased IP3 production and Ca2+ mobilization from both intracellular and extracellular stores. Moreover, sbGHSR-1a may trigger multiple signal transduction cascades to exert its physiological functions.
Thompson. Nichola M NM; Davies. Jeffrey S JS; Mode. Agneta A; Houston. Pamela A PA; Wells. Timothy T
The peptide hormone ghrelin binds to the GH secretagogue receptor (GHS-R), stimulates GH secretion, and promotes adipogenesis. However, continuous GHS infusion does not stimulate skeletal growth and is associated with desensitization to further GH secretagogue treatment. In this study, 7-d intermittent (i.e. every 3 h) infusion of ghrelin, or the GH secretagogue, GH-releasing peptide-6, in the moderately GH- deficient transgenic growth-retarded rat, augmented GH secretion, leading to a sustained acceleration in skeletal growth. In contrast, continuous infusion of ghrelin, or GH-releasing peptide-6, suppressed the amplitude of spontaneous GH secretory episodes and produced only a transient increase in body weight gain. The reduction in GH secretion seen with continuous GHS-R activation was not associated with a desensitization of the pituitary to GH-releasing factor or to down-regulation of hypothalamic GHS-R mRNA expression. Continuous ghrelin treatment elicited an increase in somatostatin mRNA expression in the periventricular nuclei. Thus, exposure to continuously elevated circulating ghrelin may be responsible for the suppression of GH secretion reported in rats after prolonged starvation.
Dasgupta. P P; Singh. A T AT; Mukherjee. R R
Chimeric peptides consisting of growth hormone releasing peptide (GHRP-6) linked to somatostatin (6-11) via an amide bond to provide the effector parts of both the peptides were synthesized. The anti-proliferative, cytotoxic, and GH-inhibitory activities of these chimeric peptides were determined in vitro in the rat pituitary adenoma cell line GH3. One of the chimeric peptides, GSD, exhibited significantly greater (p < 0.001) anti-neoplastic and GH-inhibitory activity, as compared to RC-160. The hybrid peptides displayed high affinity binding to somatostatin receptors on GH3 cells. The bioactivity of GSD was found to be mediated by the stimulation of tyrosine phosphatase, involving a cGMP-dependent pathway, through pertussis toxin-sensitive G-proteins. Such potent GH-inhibitory chimeric peptides may be of potential importance in the therapy of acromegaly, as well as provide novel tools to study the regulation of GH secretion by GHRP and somatostatin.