Selective lack of growth hormone (GH) response to the GH-releasing peptide hexarelin in patients with GH-releasing hormone receptor deficiency

Growth Hormone Deficiency: Health and Longevity

The important role of GH in the control of mammalian longevity was first deduced from extended longevity of mice with genetic GH deficiency (GHD) or GH resistance. Mice with isolated GHD (IGHD) due to GHRH or GHRH receptor mutations, combined deficiency of GH, prolactin, and TSH, or global deletion of GH receptors live longer than do their normal siblings. They also exhibit multiple features of delayed and/or slower aging, accompanied by extension of healthspan. The unexpected, remarkable longevity benefit of severe endocrine defects in these animals presumably represents evolutionarily conserved trade-offs among aging, growth, maturation, fecundity, and the underlying anabolic processes. Importantly, the negative association of GH signaling with longevity extends to other mammalian species, apparently including humans. Data obtained in humans with IGHD type 1B, owing to a mutation of the GHRH receptor gene, in the Itabaianinha County, Brazil, provide a unique opportunity to study the impact of severe reduction in GH signaling on age-related characteristics, health, and functionality. Individuals with IGHD are characterized by proportional short stature, doll facies, high-pitched voices, and central obesity. They have delayed puberty but are fertile and generally healthy. Moreover, these IGHD individuals are partially protected from cancer and some of the common effects of aging and can attain extreme longevity, 103 years of age in one case. We think that low, but detectable, residual GH secretion combined with life-long reduction of circulating IGF-1 and with some tissue levels of IGF-1 and/or IGF-2 preserved may account for the normal longevity and apparent extension of healthspan in these individuals.

Raman Spectroscopic Analysis of Signaling Molecules-Dopamine Receptors Interactions in Living Cells

The selective interaction of signaling compounds including neurotransmitters and drugs with the dopamine receptors (DARs) is extremely important for the treatment of neurodegenerative diseases. Here, we report a method to probe the selective interactions of signaling compounds with D1 and D2 DARs in living cells using the combined approach of theoretical calculation and surface-enhanced Raman spectroscopy (SERS). When signaling compounds such as DA, amphetamine, methamphetamine, and methylenedioxypyrovalerone interact with D1 dopamine receptors (DRD1), the intracellular cyclic adenosine monophosphate (cAMP) level is increased. However, the intracellular level of cAMP is decreased when D2 dopamine receptors (DRD2) interact with the abovementioned signaling compounds. In our experiments, we have internalized the silica-coated silver nanoparticles (AgNP@SiO₂) in living cells to adsorb biologically generated cAMP which was probed by using SERS. Besides adsorptions of cAMP, AgNP@SiO₂ has a crucial role for the enhancement of Raman cross section of the samples. We observed the characteristic SERS peaks of cAMP when DRD1-overexpressed cells interact with the signaling compounds; these peaks were not observed for other cells including DRD2-overexpressed and DRD1-DRD2-coexpressed cells. Our experimental approach is successful to probe the intracellular cAMP and characterize the selectivity of signaling compounds to different types of DARs. Furthermore, our experimental approach is highly capable for in vivo studies because it can probe intracellular cAMP using a low input power of incident laser without significant cell damage. Our experimental results and density functional theory calculations showed that 780 and 1503 cm^-1 are signature Raman peaks of cAMP. The SERS peak at 780 cm^-1 is associated with C-O, C-C, and C-N stretching and symmetric and asymmetric bending of two O-H bonds of cAMP, whereas the SERS peak at 1503 cm^-1 is contributed by the O₉-H₃ bending mode.

Enhanced Pulsatile Growth Hormone Secretion and Altered Metabolic Hormones by in Vivo Hexarelin Treatment in Streptozotocin-Induced Diabetic Rats

Significant growth hormone (GH) reductions have been reported in diabetic animal models with disturbed metabolic balance coinciding with GH deficiency. Therefore, enhanced GH secretion may have beneficial effects in controlling diabetes. Thus, we aim to investigate the effect of hexarelin, a synthetic GH secretagogue (GHS), on GH secretion in streptozotocin (STZ, 65 mg/kg)-induced diabetic rats. Daily hexarelin (100 μg/kg) treatment was performed for two weeks in four-week-long STZ-diabetic and vehicle control rats. Pulsatile GH secretion in STZ-rats was significantly reduced in total, pulsatile, basal, and mass of GH secretion per burst. In addition, impaired GH secretion was followed by an increase in fasting-level free fatty acids (FFAs) and a decrease in insulin-like growth factor 1 (IGF-1) compared to control rats. After hexarelin treatment, pulsatile GH secretion in STZ-rats was significantly increased in total, pulsatile, and basal, but not in the mass GH secretion per burst, compared to STZ-rats without hexarelin treatment. However, there was no significant elevation in GH secretion in the hexarelin-treated control group. In addition, hexarelin-treated STZ-rats showed a significant decrease in fasting level FFAs, whereas suppression of fasting level for IGF-1 was maintained. These results suggest that STZ-induced diabetic rats have impaired pulsatile GH secretion, causing increased FFAs and decreased IGF-1 levels in circulation. Hexarelin injections for two weeks is able to normalize impaired pulsatile GH secretion with normal fasting levels of FFAs, but fails to recover IGF-1 levels.

Growth hormone response to growth hormone-releasing peptide-2 in growth hormone-deficient little mice

OBJECTIVE

To investigate a possible direct, growth hormone-releasing, hormone-independent action of a growth hormone secretagogue, GHRP-2, in pituitary somatotroph cells in the presence of inactive growth hormone-releasing hormone receptors.

MATERIALS AND METHODS

The responses of serum growth hormone to acutely injected growth hormone-releasing P-2 in lit/lit mice, which represent a model of GH deficiency arising from mutated growth hormone-releasing hormone-receptors, were compared to those observed in the heterozygous (lit/+) littermates and wild-type (+/+) C57BL/6J mice.

RESULTS

After the administration of 10 mcg of growth hormone-releasing P-2 to lit/lit mice, a growth hormone release of 9.3±1.5 ng/ml was observed compared with 1.04±1.15 ng/ml in controls (p<0.001). In comparison, an intermediate growth hormone release of 34.5±9.7 ng/ml and a higher growth hormone release of 163±46 ng/ml were induced in the lit/+ mice and wild-type mice, respectively. Thus, GHRP-2 stimulated growth hormone in the lit/lit mice, and the release of growth hormone in vivo may be only partially dependent on growth hormone-releasing hormone. Additionally, the plasma leptin and ghrelin levels were evaluated in the lit/lit mice under basal and stimulated conditions.

CONCLUSIONS

Here, we have demonstrated that lit/lit mice, which harbor a germline mutation in the Growth hormone-releasing hormone gene, maintain a limited but statistically significant growth hormone elevation after exogenous stimulation with GHRP-2. The present data probably reflect a direct, growth hormone-independent effect on Growth hormone S (ghrelin) stimulation in the remaining pituitary somatotrophs of little mice that is mediated by growth hormone S-R 1a.

Growth hormone-releasing hormone as an agonist of the ghrelin receptor GHS-R1a

Ghrelin synergizes with growth hormone-releasing hormone (GHRH) to potentiate growth hormone (GH) response through a mechanism not yet fully characterized. This study was conducted to analyze the role of GHRH as a potential ligand of the ghrelin receptor, GHS-R1a. The results show that hGHRH(1-29)NH(2) (GHRH) induces a dose-dependent calcium mobilization in HEK 293 cells stably transfected with GHS-R1a an effect not observed in wild-type HEK 293 cells. This calcium rise is also observed using the GHRH receptor agonists JI-34 and JI-36. Radioligand binding and cross-linking studies revealed that calcium response to GHRH is mediated by the ghrelin receptor GHS-R1a. GHRH activates the signaling route of inositol phosphate and potentiates the maximal response to ghrelin measured in inositol phosphate turnover. The presence of GHRH increases the binding capacity of (125)I-ghrelin in a dose dependent-fashion showing a positive binding cooperativity. In addition, confocal microscopy in CHO cells transfected with GHS-R1a tagged with enhanced green fluorescent protein shows that GHRH activates the GHS-R1a endocytosis. Furthermore, the selective GHRH-R antagonists, JV-1-42 and JMR-132, act also as antagonists of the ghrelin receptor GHS-R1a. Our findings suggest that GHRH interacts with ghrelin receptor GHS-R1a, and, in consequence, modifies the ghrelin-associated intracellular signaling pathway. This interaction may represent a form of regulation, which could play a putative role in the physiology of GH regulation and appetite control.

Regulation of ghrelin secretion and action

The pulsatile release of growth hormone (GH) from anterior pituitary gland is regulated by the interplay of at least two hypothalamic hormones, GH-releasing hormone (GHRH) and somatostatin, via their engagement with specific cell surface receptors on the anterior pituitary somatotroph. Furthermore, release of GH in vivo may also be controlled by a third type of receptor, the growth hormone secretagogue receptor, a G-protein-coupled receptor, called GHS receptor type 1a (GHSR1a), which was identified in the pituitary and the hypothalamus in humans using a nonpeptidyl growth hormone secretagogue (MK-0677). Ghrelin, the endogenous ligand for the GHS-R1a, is a 28-amino-acid peptide isolated from human stomach that is modified by a straight chain octanoyl group covalently linked to Ser3, which is essential for its endocrine activity. This hormone, predominantly expressed and secreted by the stomach, has a dual action on GH secretion and food intake, showing interdependency between these actions. The finding that fasting and food intake, respectively, increase and decrease the secretion of ghrelin suggests that this hormone may be the bridge connecting somatic growth and body composition with energy metabolism, and appears to play a role in the alteration of energy homeostasis and body weight in pathophysiological states such as hypothyroidism and hyperthyroidism. Despite this, little is known about the intracellular signaling through which ghrelin exerts its regulatory actions. Activation of intracellular calcium mobilization is one of the earliest known cellular signals elicited by ghrelin. In HEK- 293 cells expressing the GHS-R1a, ghrelin induces a biphasic cytosolic calcium elevation characterized by a spike phase of the response, which reflects Ins(1,4,5)P3- dependent calcium mobilization of intracellular stores, and a sustained phase of the response, which is due to calcium influx across the plasma membrane triggered by aperture of capacitative calcium channels (store-operated calcium channels). Upon repeated administration, ghrelin showed a marked suppression of ghrelin-mediated elevations of intracellular calcium. This homologous desensitization represents an important physiological mechanism that modulates receptor responsiveness and acts as an information filter for intracellular signaling system. The discovery of ghrelin adds a new component to the complex machinery responsible for regulation of GH secretion in connection with the regulation of appetite and energy homeostasis.

Growth hormone-releasing hormone and growth hormone secretagogue-receptor ligands: focus on reproductive system

Growth hormone-releasing hormone (GHRH) and somatostatin are the most important hypothalamic neurohormones controlling growth hormone (GH) secretion. Several neurotransmitters and neuropeptides also play an important role in the control of GH secretion, mainly acting via modulation of GHRH and somatostatin. In the past two decades, particular attention has been given to a new family of substances showing a strong GH-releasing effect: GH secretagogues (GHSs). GHSs increase GH secretion in a dose- and age-related manner after iv and even oral administration. The endocrine effects of GHSs, are not fully specific for GH; they show, in fact, prolactin- (PRL), adenocorticotropic hormone- and cortisol-releasing effects. Specific GHS receptors are present in both the central nervous system and peripheral tissues, where they mediate several extraendocrine effects of GHSs. The isolation of these "orphan" receptors suggested the existence of an endogenous GHS-like ligand that could be represented by a recently discovered gastric peptide, named ghrelin. The interaction between GHSs and GHRH at the central level and in the pituitary gland, but not at peripheral level, has clearly been shown. Because GHRH and GHS receptors share the same localization in some peripheral tissues, they may have some interactions even at this level.