[D-Lys3]-GHRP-6

IC50: 0.9 μM

[D-Lys3]-GHRP-6 is an antagonist at the ghrelin receptor (GHS-R1a) and weakly binds to melanocortin receptors (Ki = 26-120 μM).

In vitro: [D-Lysa]GHRP-6 used similar affinities to bind to all the four MC receptors, to which the structurally related Met-enkephalin and the functionally related GHuRH, together with LHRH and somatostatin-14 did not bind. This (the low affinity of the GH-releasing/enkephalin peptides) may indicate that they do not interact with the MC receptors at pharmacological concentrations. [DLys3] GHRP-6 only used slightly higher affinity than GHRP-6 to bind to the MC1 receptor. [1].

Compared with GHRP-6, [D-Lys3]GHRP-6 has higher affinity for all the MC receptors, especially for the MC3 and MC4 receptors. Interestingly, [D-Lys 3] GHRP-6 binds to the MC1 receptor with only slightly higher affinity compared with GHRP-6, which may indicate that the basic hydrophilic residue in position 8 is not important for the MC 1 receptor (as the same as he other MC receptor subtypes) [1,2]. The peptidic GHS-receptor antagonist (D-Lys3)-GHRP-6 (10-4 M) antagonized ghrelin (10-7 M) weakly, showing a much weaker affinity (IC50, 0.9×10-6 M) to the GHS-receptor than ghrelin (IC50, 0.3×10-9 M). Ghrelin increased the electrical activity in 76% of all cells inhibited by leptin (n=17). These data reveal that ghrelin interacts with the leptin hypothalamic network in the arcuate nucleus. Leptin and ghrelin oppositely effect on neurons in the arcuate nucleus, which may serve as a neurophysiological correlate of the orexigenic and anorectic effects of them [2].

In vivo: The administration of D-Lys3-GHRP-6 (an antagonist of GHS receptors) alone had no significantly influence on GH secretion. Oppositely, pretreatment with this antagonist efficiently inhibited the stimulatory effect of AMPA and NMDA on GH secretion. To confirm this contention, further experiments to evaluate different protocols (doses, times, additional GHS receptor antagonists) of administration of D-Lys3-GHRP-6 are carried out. Nevertheless, it is possible that ghrelin may only intervene in situations of hypo- or hypersecretion of GH but not involved in the control of basal GH secretion [2].

Clinical trial: So far, no clinical study has been conducted.

References:
[1].  Schith HB, Muceniece R, Wikberg JE. Characterization of the binding of MSH-B, HB-228, GHRP-6 and 153N-6 to the human melanocortin receptor subtypes. Neuropeptides. 1997 Dec;31(6):565-71.
[2].  Traebert M, Riediger T, Whitebread S, Scharrer E, Schmid HA. Ghrelin acts on leptin-responsive neurones in the rat arcuate nucleus. J Neuroendocrinol. 2002 Jul;14(7):580-6.
[3].  Pinilla L, Barreiro ML, Tena-Sempere M, Aguilar E.Role of ghrelin in the control of growth hormone secretion in prepubertal rats: interactions with excitatory amino acids. Neuroendocrinology. 2003 Feb;77(2):83-90.

The GHS-R blocker D-[Lys3] GHRP-6 serves as CCR5 chemokine receptor antagonist.[Pubmed:22211090]

Int J Med Sci. 2012;9(1):51-8. Epub 2011 Nov 18.

[D-Lys3]-Growth Hormone Releasing Peptide-6 (DLS) is widely utilized in vivo and in vitro as a selective ghrelin receptor (GHS-R) antagonist. This antagonist is one of the most common antagonists utilized in vivo to block GHS-R function and activity. Here, we found that DLS also has the ability to modestly block chemokine function and ligand binding to the chemokine receptor CCR5. The DLS effects on RANTES binding and Erk signaling as well as calcium mobilization appears to be much stronger than its effects on MIP-1alpha and MIP-1beta. CCR5 have been shown to act as major co-receptor for HIV-1 entry into the CD4 positive host cells. To this end, we also found that DLS blocks M-tropic HIV-1 propagation in activated human PBMCs. These data demonstrate that DLS may not be a highly selective GHS-R1a inhibitor and may also effects on other G-protein coupled receptor (GPCR) family members. Moreover, DLS may have some potential clinical applications in blocking HIV infectivity and CCR5-mediated migration and function in various inflammatory disease states.

The effect of ghrelin antagonist (D-Lys3) GHRP-6 on ovariectomy-induced obesity in adult female albino rats.[Pubmed:25110211]

Endocr Regul. 2014 Jul;48(3):126-34.

OBJECTIVE: We aimed to investigate the effect of ghrelin antagonist (D-Lys3) GHRP-6 on the treatment of ovariectomy-induced obesity as compared to hormone replacement therapy with estradiol. METHODS: Twenty eight rats were divided into four groups: control sham operated (C), ovariectomized non-treated (OVX), ovariectomized+estradiol-treated (OVX+E) groups, and ovariectomized+ghrelin antagonist-treated group (OVX+GA). Rats were allowed free water and commercial standard diet ad libitum for 5 weeks after surgery. Body mass index (BMI) was determined at the beginning and the end of the experiment. Rats were sacrificed by decapitation and blood samples were collected for measurements of serum lipid profile, insulin, and glucose levels. Gastrocolic omental fat (GCOF) was removed and weighed. RESULTS: Ovariectomy was accompanied with a significantly higher body weight, food intake, BMI, GCOF, serum total cholesterol (TC), low density lipoprotein-cholesterol (LDL-C), glucose, insulin, and homeostatic model assessment - insulin resistance (HOMA-IR), with a significant decrease in high density lipoprotein-cholesterol (HDL-C) and triglycerides (TGs) in comparison with C group. Estradiol reversed the ovariectomy-induced changes except that of TGs. Administration of ghrelin antagonist was effective in treating the ovariectomy-induced obesity as evidenced by normalization of body weight, food intake, BMI, and GCOF weight, serum levels of insulin, glucose, HOMA-IR, and HDL-C. The serum levels of TC, LDL-C and TGs were improved but did not reach the control values. CONCLUSION: Although estradiol succeeded in the prevention of almost all ovariectomy-induced disturbances, it had a potential cardiovascular risk due to a marked increase in serum TGs. Ghrelin antagonist was effective in ameliorating ovariectomy-induced obesity, so it may be used as a promising treatment for postmenopausal obesity, irrespective of hormonal replacement.

Identification of ghrelin receptor blocker, D-[Lys3] GHRP-6 as a CXCR4 receptor antagonist.[Pubmed:22211109]

Int J Biol Sci. 2012;8(1):108-17. Epub 2011 Nov 26.

[D-Lys3]-Growth Hormone Releasing Peptide-6 (DLS) is widely utilized in vivo and in vitro as a selective ghrelin receptor (GHS-R) antagonist. Unexpectedly, we identified that DLS also has the ability to block CXCL12 binding and activity through CXCR4 on T cells and peripheral blood mononuclear cells (PBMCs). Moreover, as CXCR4 has been shown to act as a major co-receptor for HIV-1 entry into CD4 positive host cells, we have also found that DLS partially blocks CXCR4-mediated HIV-1 entry and propagation in activated human PBMCs. These data demonstrate that DLS is not the specific and selective antagonist as thought for GHS-R1a and appears to have additional effects on the CXCR4 chemokine receptor. Our findings also suggest that structural analogues that mimic DLS binding properties may also have properties of blocking HIV infectivity, CXCR4 dependent cancer cell migration and attenuating chemokine-mediated immune cell trafficking in inflammatory disorders.

[D-Lys3]-GHRP-6 exhibits pro-autophagic effects on skeletal muscle.[Pubmed:25450862]

Mol Cell Endocrinol. 2015 Feb 5;401:155-64.

[D-Lys3]-GHRP-6 is regarded as a highly selective growth-hormone secretagogue receptor (GHSR) antagonist and has been widely used to investigate the dependency of GHSR-1a signalling mediated by acylated ghrelin. However, [D-Lys3]-GHRP-6 has been reported to influence other cellular processes which are unrelated to GHSR-1a. This study aimed to examine the effects of [D-Lys3]-GHRP-6 on autophagic and apoptotic cellular signalling in skeletal muscle. [D-Lys3]-GHRP-6 enhanced the autophagic signalling demonstrated by the increases in protein abundances of beclin-1 and LC3 II-to-LC3 1 ratio in both normal muscle and doxorubicin-injured muscle. [D-Lys3]-GHRP-6 reduced the activation of muscle apoptosis induced by doxorubicin. No histological abnormalities were observed in the [D-Lys3]-GHRP-6-treated muscle. Intriguingly, the doxorubicin-induced increase in centronucleated muscle fibres was not observed in muscle treated with [D-Lys3]-GHRP-6, suggesting the myoprotective effects of [D-Lys3]-GHRP-6 against doxorubicin injury. The [D-Lys3]-GHRP-6-induced activation of autophagy was found to be abolished by the co-treatment of CXCR4 antagonist, suggesting that the pro-autophagic effects of [D-Lys3]-GHRP-6 might be mediated through CXCR4. In conclusion, [D-Lys3]-GHRP-6 exhibits pro-autophagic effects on skeletal muscle under both normal and doxorubicin-injured conditions.

Role of ghrelin in the control of growth hormone secretion in prepubertal rats: interactions with excitatory amino acids.[Pubmed:12624529]

Neuroendocrinology. 2003 Feb;77(2):83-90.

Ghrelin is a 28-amino-acid peptide, with an essential n-octanoyl modification at Ser3, that elicits growth-hormone (GH) secretion in rats and humans. At present, the mechanisms of ghrelin action and its interactions with other systems controlling GH secretion remain poorly characterized. In this context, the present study was undertaken to obtain information about ontogeny and possible gender differences in the GH-releasing activity of ghrelin, and to delineate its primary site(s) of action at the hypothalamus and/or pituitary. In addition, the interactions between ghrelin and other relevant signals in the control of GH secretion, such as excitatory amino acids (EAAs), nitric oxide (NO) and serotonin, were assessed. Experiments were carried out in infantile-prepubertal animals, when GH pulsatility is not yet established. Systemic administration of ghrelin (25 nmol/rat, i.p.) to 5-, 10- and 23-day-old male and female rats increased plasma GH levels from day 10 onwards. This action was NO dependent, since it disappeared in 23-day-old males after pretreatment with an inhibitor of NO synthase (NAME). Similarly, central infusion of ghrelin (3 nmol/rat, i.c.v.) elicited GH responses in 10- and 23-day-old animals significantly higher than after systemic administration. By contrast, in vitro challenge of pituitary tissue with increasing doses of ghrelin (10(-9)-10(-7) M) failed to enhance GH release into the incubation medium, whereas stimulation with GH-releasing hormone (GHRH; 10(-7) M) or GHRP-6 (10(-7) M) was effective. Finally, effects of ghrelin were blocked by pretreatment with MK-801 and NBQX antagonists of EAA ionotropic receptors and after manipulation of endogenous serotoninergic tone. In addition, the potent releasing activity of EAA agonists NMDA and AMPA was blunted by pretreatment with D-Lys3-GHRP-6, a selective antagonist of the cognate ghrelin receptor, i.e. the GH-secretagogue receptor. In conclusion, our results demonstrate that GH-releasing activity of ghrelin appears early in the infantile period, is NO dependent and involves a primary hypothalamic site of action. The data also demonstrate for the first time the existence of a cross-talk between ghrelin and other neurotransmitter systems, such as EAAs and serotonin, in precise control of GH secretion.

Ghrelin acts on leptin-responsive neurones in the rat arcuate nucleus.[Pubmed:12121496]

J Neuroendocrinol. 2002 Jul;14(7):580-6.

Leptin decreases food intake and increases energy expenditure in rodents by inhibiting neurones in the hypothalamic arcuate nucleus. The growth hormone secretagogue (GHS) ghrelin is known to stimulate food intake and to be the endogenous ligand for the GHS-receptor, which is strongly expressed in the arcuate nucleus, like the leptin receptor (Ob-R). In this study, we analysed the effect of systemic ghrelin administration on Fos expression in the arcuate nucleus on neurones expressing Ob-R. Injection of ghrelin (0.2 mg/kg, i.p) significantly increased the number of neurones expressing Fos protein in the ventromedial arcuate nucleus. Fifty-seven percent of all Fos-positive cells in the ventromedial arcuate nucleus were also positive for Ob-R staining. Furthermore, we investigated electrophysiologically the effect of ghrelin and leptin on the activity of arcuate neurones in an in-vitro slice preparation. Ghrelin stimulated the electrical activity dose-dependently in 80% of all cells tested (n=49) with a threshold concentration of 10(-11) M; only 8% were inhibited and 12% did not respond. The effect of ghrelin (10(-7) M) was weakly antagonized by the peptidic GHS-receptor antagonist (D-Lys3)-GHRP-6 (10(-4) M), which also showed a much weaker affinity (IC(50), 0.9 x 10(-6) M) to the GHS-receptor than ghrelin (IC(50), 0.3 x 10(-9) M). Ghrelin increased the electrical activity in 76% of all cells which were inhibited by leptin (n=17). These data show that ghrelin interacts with the leptin hypothalamic network in the arcuate nucleus. The opposite effect of leptin and ghrelin on neurones in the arcuate nucleus may serve as a neurophysiological correlate of the orexigenic and anorectic effects of ghrelin and leptin.

Characterization of the binding of MSH-B, HB-228, GHRP-6 and 153N-6 to the human melanocortin receptor subtypes.[Pubmed:9574823]

Neuropeptides. 1997 Dec;31(6):565-71.

We determined the binding affinities of the MSH analogues MSH-B, HP-228 and 153N-6 and of the enkephalin analogue GHRP-6 on a single eukaryotic cell line transiently expressing the human MC1, MC3, MC4 and MC5 receptors. Moreover, we tested the binding and cAMP response of MSH-B in comparison with alpha-MSH on murine B16 melanoma cells. Our results indicate that MSH-B has a potency similar to that of alpha-MSH and that these two peptides induce similar cAMP responses in murine B16 melanoma cells. HP-228 has its highest affinity for the MC1 receptor. For the other receptors, it has slightly higher affinity for the MC5 receptor than for the MC3 and MC4 receptors. 153N-6 was found to be selective for the MC1 receptor. GHRP-6 was found to bind to the MC1 and the MC5 receptors despite its low structural homology with alpha-MSH. [D-Lys3]GHRP-6 bound to all the four MC receptors with similar affinities. The structurally related Met-enkephalin and the functionally related GHRH, as well as LHRH and somatostatin-14 did not bind to these MC receptors. The low affinity of the GH-releasing/enkephalin peptides may indicate that they do not interact with the MC receptors at pharmacologically relevant concentrations.