Hypothalamic growth hormone secretagogue receptor regulates growth hormone secretion, feeding, and adiposity

Abstract

IntroductionGrowth hormone secretagogues (GHSs), which were developed from met-enkephalin based on conformational energy calculations, peptide chemistry, and biological activity, stimulate GH secretion via a specific receptor (1). An intracerebroventricular (ICV) injection of GHS also stimulates food intake in freely feeding rats (2). The GHS receptor (GHS-R) was cloned and found to be a member of the G protein-coupled receptor superfamily (3). The expression of GHS-R mRNA is observed by in situ hybridization or an RNase protection assay mainly in the arcuate nucleus (Arc) and ventromedial nucleus of the hypothalamus and in the pituitary (4). Ghrelin, an endogenous ligand for GHS-R, has recently been isolated from stomach extracts of rats and subsequently cloned in rats and humans (5). Ghrelin-producing cells are found in the hypothalamus as well as in the stomach (5). As expected, ICV administration of ghrelin stimulated GH secretion and food intake in rats (6, 7). Daily peripheral administration of ghrelin caused weight gain by reducing fat utilization in mice and rats (8). However, the physiological role of endogenous ghrelin in the hypothalamus is still unknown.To attenuate GHS-R expression in vivo, we attempted to create transgenic (Tg) rats with impaired GHS-R function in the hypothalamus, especially in the Arc. For this purpose, we used a construct that expresses GHS-R-specific antisense RNA under the control of the promoter for tyrosine hydroxylase (TH). TH is the rate-limiting enzyme in catecholamine biosynthesis and is a marker for the dopaminergic neurons in the hypothalamus. TH-like immunoreactivity is present in most neurons in the ventral part of the Arc that contain GH-releasing hormone (GHRH) (9). GHS-R mRNA hybridizing cells show an extensive overlap with GHRH-expressing neurons (10). These results suggest that a certain number of GHS-R-expressing neurons in the Arc also contain TH. Tg mice bearing a fusion gene containing the TH promoter and the coding region of the human GH gene have been generated, and these Tg mice showed human GH-like immunoreactivity in all the catecholaminergic neurons in the hypothalamus (11). Based on this report, an antisense GHS-R mRNA under the control of the TH promoter would be expected to suppress GHS-R expression in the Arc. Therefore, in the present study we have generated Tg rats that express an antisense GHS-R mRNA under the control of the TH promoter to determine the physiological role of the ghrelin/GHS-R system in the hypothalamus. MethodsGeneration of Tg rats. To construct the antisense GHS-R fusion gene, a synthetic 108-nucleotide DNA fragment spanning the 5′ extracellular region of GHS-R was cloned in the antisense orientation into the vector 4.5THpAL+ (kindly provided by Dona Chikaraishi), which contains 4.5 kb of the upstream region of the rat TH gene (11). The antisense orientation of the cloned fragments was verified by DNA sequencing. A 4.8-kb Growth hormone secretagogues (GHSs) stimulate GH secretion and food intake. GHS receptor (GHS-R) mRNA has b...

Sequencing analysis of the ghrelin receptor (growth hormone secretagogue receptor type 1a) gene

“…Another theory could be that SNP might be able to change a binding site for transcription factors and, in this way, may also lead to altered expression levels of the protein product. There is evidence that decreased expression levels of GHSR1a lead to decreased GH and IGF-I values in transgenic female mice (20); however, the relationship between altered GHS-R1a expression levels and IGFBP-1 levels has yet to be elucidated.…”

“…Anorectic patients have elevated ghrelin (18) and IGFBP-1 levels but subnormal free IGF-I levels, while their GH levels are within the normal range (19). There is evidence that the role of ghrelin in physiology may be sexually dimorphic; decreased expression levels of the ghrelin receptor GHS-R1a gene in transgenic mice result in decreased GH and IGF-I levels more often in female than in male mice (20). This also suggests that a connection between GHS-R1a and IGF-I levels might exist.…”

Family trios analysis of common polymorphisms in the obestatin/ghrelin, BDNF and AGRP genes in patients with Anorexia nervosa: Association with subtype, body-mass index, severity and age of onset

“…Inhibiting ghrelin-receptor expression in the hypothalamus of transgenic rats by expressing anti-sense ghrelin receptor mRNA decreases food intake, and body fat mass (Shuto et al, 2002), suggesting the importance of the hypothalamic ghrelin receptors in energy homeostasis. Ghrelin is an upstream regulator of the orexigenic peptides NPY and AgRP and acts as a natural antagonist to leptin's effects on NPY/AgRP-expressing neurons, resulting in an increase in feeding and body weight (Zigman and Elmquist, 2003).…”

The negative association between plasma ghrelin and IGF-I is modified by obesity, insulin resistance and type 2 diabetes

“…Ghrelin concentrations have been reported to depend on sex only in young subjects [52]. The sex difference in the role of ghrelin in the regulation of GH has been previously shown in animal models [53,54].…”