Ghrelin

Discovery

In 1999, Kojima et al. isolated GHRL from rat stomach extracts by the orphan receptor strategy using the growth hormone secretagogue receptor type-1a (GHS-R1a). The name originates from its growth hormone-releasing activity, with reference to the Proto-Indo-European word “ghre,” meaning “grow,” and growth hormone release +in (suffix of inducing) to give “ghrelin.”

Structure

The serine residue at position 3 (Ser-3) is modified by middle-chain fatty acids, and GHRL usually refers to the octanoylated form (acylated ghrelin) . While n-decanoic acid contributes to the modification, species difference was seen. Only in the genus Rana (frogs) of the anuran amphibian was acylated aa substituted from Ser-3 to threonine. The first seven aa sequence (GSSFLSP) is highly conserved across animal species. Teleost GHRLs have an amide structure at the C-terminus.The elasmobranch stingray GHRL has O-glycosylation at Ser-10 and/or Thr-11 in addition to the acylation at Ser-3.  Mammalian GHRLs consist of 28 aa in general, but a 27-aa molecule lacking the 14th glutamine residue by alternative splicing, namely des-Gln14 ghrelin, has also been identified. Amino acid sequences have been determined in nonmammalian vertebrates, including cartilaginous fish, bony fish, amphibians, and reptiles and birds. The number of aa ranges from 16 (stingray) to 28 residues (frogs) depending on the species.

Gene, mRNA, and precursor

The mRNA from the GHRL gene (chromosomal location: 3p25.3 for humans, 6 E3j6 52.84 cM for mice, and 4q42 for rats) codes for a 117-aa preproGHRL, consisting of six exons (the presence of extended exon 0 and novel exon -1 was found,5 thereby the composition of the GHRL gene was reviewed). Mammalian GHRL mRNAs are about 480–510 bp in length. The prohormone convertase 1/3 (PC1/3) cleaves a 28-aa unacylated ghrelin and a C-peptide in which a GHRL gene-derived peptide, obestatin, is also cleaved from proGHRL. Several natural antisense transcripts, termed ghrelinOS (ghrelin opposite strand) transcripts, are demonstrated. Semi et al. identified a novel human exon 2-deleted variant, and this splice variant and intron 1-GHRL encode a C-terminally truncated form of the GHRL peptide (GHRL1–13), termed miniGHRL. These variants are present in humans, mice, and other nonmammalian vertebrates.

Properties

Human GHRL: Mr 3371.9. Lyophilized material is stable at -30°C, but -80°C is more desirable. Soluble in water. Because of high adsorptivity, it is desirable to maintain the storage concentration at >10^-4M, or to dilute with saline containing 5% mannitol or 0.2% BSA for treatment.

Synthesis

Gastric GHRL expression increases by fasting, possibly through decreased plasma carbohydrates. In the rat stomach or in thyroid removal, insulin or leptin stimulates and growth hormone (GH) treatment inhibits GHRL gene expression. Expression in the rat stomach appears from the first day after birth, and increases with age, reaching a constant level around 40 days after birth. Ghrelin-O-acyltransferase (GOAT) contributes to acylation.

Receptors

The ghrelin receptor (GHS-R1a) is a member of the Class A GPCR family. The GHRL receptor gene (GHSR), 3q26.31 in humans, comprises two exons and one intron. Two isoforms, GHS-R1a and GHS-R1b, exist in the same gene. GHS-R1a acts as a functional GHRL receptor,10 and GHS-R1b is a mRNA splicing variant that acts as a dominant negative mutant for GHS-R1a. Human GHS-R1a and 1b have 366 aa and 289 aa, respectively. GHS-R1b lacks the structure after TMD 5 of GHS-R1a, and 24 aa that originate from the intron sequence elongate after the 265th leucine at the C-terminus of TMD 6.

Biological functions

GHRL receptors are expressed in the hypothalamic neuropeptide Y (NPY), somatostatin (SST), growth hormone-releasing hormone (GHRH), proopiomelanocortin (POMC), orexin, and Agouti-related peptide (AgRP) neurons in the pituitary, lung, heart, stomach, intestine, pancreas, thymus, gonads, thyroid, kidney, and adipose tissue. GHRL’s functions include stimulation of GH release, feeding, anxiety, learning and memory, gastrointestinal contraction, fat accumulation, depression, gastric acid secretion, sleep duration, and inhibition of drinking. Species differences are seen— for example, feeding is stimulated in goldfish and frogs, but inhibited in chickens and rainbow trout.MiniGHRL appears to have similar actions to full-length GHRL, as treatment with the exogenous miniGHRL peptide stimulates appetite and feeding in mice. Exon 2-deleted prepro GHRL overexpression stimulates the proliferation and cell migration of the PC3 prostate cancer cell.

Agonists and Antagonists

Des-Gln14-GHRL, [His1 , Lys6 ]-GHRP-6, hexarelin, D-Ala-β-[2-Nal]-D-Ala-Trp-D-Phe-Lys-amide (KP-102), Aib-His-D-2-Nal-D-Phe-Lys-amide, wFw-Isn-amide, L-692.585, capromorelin, tabimorelin, adenosin, anamorelin, examorelin, ibutamoren, ipamorelin, macimorelin, pralmorelin, relamorelin, SM-103.686, teaghrelins, and ulimor. [D-Lys3]-GHRP-6, Cortistatin-8 [D-Arg1 , D-Phe5 , D-Trp7,9, Leu11]-Substance P, YIL781, [D-Arg1 , D-Phe5 , D-Trp7,9, Leu11]-Substance P, GSK1614343, PF-05190457, BIM28163, YIL781, JMV3008, liverexpressing antimicrobial peptide-2(LEAP-2), and Abbott14c.

Clinical implications

In humans, an association between variations of the GHRL gene and obesity has been suggested. GHRL gene polymorphisms (604G/A, Arg51Gln, Leu72Met, and Leu90Gln) seem to be associated with cardiovascular disease, carcinoma, type 2 diabetes, and Alzheimer’s disease. There is a possibility that the Brachman-de Lange syndrome with poor growth occurs due to the overlap or translocation of the chromosome on which the GHSR gene is located. Furthermore, abnormal ghrelin responses may play important roles in various GI disorders such as chronic gastritis, Helicobacter pylori infection, irritable bowel syndrome, functional dyspepsia, cachexia, celiac disease, and inflammatory bowel disease.

Description

A unique fatty acid modification mainly with n-octanoic acid at the third serine residue of ghrelin is essential for eliciting ghrelin’s activity. Ghrelin is the first peripheral hormone found to stimulate feeding behavior. Now, multiple functions as hormonal secretagogues as well as lipid and glucose regulators have been demonstrated.

Uses

The application of GHRL for treatments is still in at the clinical trial stage. The expectation is that it might be used as an appetite inducer, for treatment of GH deficiency and improvement of cardiac function, to stimulate breathing in chronic obstructive pulmonary disease (COPD), to stimulate gastrointestinal motility, and in the treatment of obesity and type 2 diabetes.