Inhibition of ghrelin activity by the receptor antagonist [D-Lys3]-GHRP-6 enhances hepatic fatty acid oxidation and gluconeogenesis in a growing pig model

Despite its central role in regulating energy intake and metabolism, ghrelin is little understood when it comes to its effects on hepatic lipid and glucose metabolism. Growing pigs were intravenously injected with ghrelin receptor antagonist [D-Lys3]-GHRP-6 (DLys; 6 mg/kg body weight) for seven days to determine whether ghrelin plays a role in glucose and lipid metabolism. DLys treatment significantly reduced body weight gain and adipose histopathology found that DLys treatment dramatically reduced adipocyte size. DLys treatment significantly increased serum NEFA and insulin levels, hepatic glucose level and HOMA-IR, and significantly decreased serum TBA level of growing pigs after fasting. Moreover, DLys treatment changed the dynamics of serum metabolic parameters, including glucose, NEFA, TBA, insulin, GH, leptin, and cortisol. Liver transcriptome showed that DLys treatment affected the metabolism-related pathways. Compared with the control group, adipose tissue lipolysis (the adipose triglyceride lipase level was significantly increased), hepatic gluconeogenesis (the G6PC protein level was significantly increased) and fatty acid oxidation (the CPT1A protein level was significantly increased) were promoted in the DLys group. DLys treatment expanded degrees of oxidative phosphorylation in the liver, coming about in a higher NAD+ /NADH proportion and enactment of the SIRT1 signaling pathway. Additionally, the liver protein levels of the DLys group were significantly higher than those of the control group for GHSR, PPAR alpha, and PGC-1. To summarize, inhibition of ghrelin activity can significantly affect metabolism and alter energy levels by enhancing fat mobilization, hepatic fatty acid oxidation and gluconeogenesis without affecting fatty acid uptake and synthesis in the liver.

Limited short-term effects on human prostate cancer xenograft growth and epidermal growth factor receptor gene expression by the ghrelin receptor antagonist [D-Lys3]-GHRP-6

PURPOSE

The ghrelin axis regulates many physiological functions (including appetite, metabolism, and energy balance) and plays a role in disease processes. As ghrelin stimulates prostate cancer proliferation, the ghrelin receptor antagonist [D-Lys³]-GHRP-6 is a potential treatment for castrate-resistant prostate cancer and for preventing the metabolic consequences of androgen-targeted therapies. We therefore explored the effect of [D-Lys³]-GHRP-6 on PC3 prostate cancer xenograft growth.

METHODS

NOD/SCID mice with PC3 prostate cancer xenografts were administered 20 nmoles/mouse [D-Lys³]-GHRP-6 daily by intraperitoneal injection for 14 days and tumour volume and weight were measured. RNA sequencing of tumours was conducted to investigate expression changes following [D-Lys³]-GHRP-6 treatment. A second experiment, extending treatment time to 18 days and including a higher dose of [D-Lys³]-GHRP-6 (200 nmoles/mouse/day), was undertaken to ensure repeatability.

RESULTS

We demonstrate here that daily intraperitoneal injection of 20 nmoles/mouse [D-Lys³]-GHRP-6 reduces PC3 prostate cancer xenograft tumour volume and weight in NOD/SCID mice at two weeks post treatment initiation. RNA-sequencing revealed reduced expression of epidermal growth factor receptor (EGFR) in these tumours. Further experiments demonstrated that the effects of [D-Lys³]-GHRP-6 are transitory and lost after 18 days of treatment.

CONCLUSIONS

We show that [D-Lys³]-GHRP-6 has transitory effects on prostate xenograft tumours in mice, which rapidly develop an apparent resistance to the antagonist. Although further studies on [D-Lys³]-GHRP-6 are warranted, we suggest that daily treatment with the antagonist is not a suitable treatment for advanced prostate cancer.

Incretin dysfunction and hyperglycemia in cystic fibrosis: Role of acyl-ghrelin

BACKGROUND

Insulin secretion is insufficient in cystic fibrosis (CF), even before diabetes is present, though the mechanisms involved remain unclear. Acyl-ghrelin (AG) can diminish insulin secretion and is elevated in humans with CF.

METHODS

We tested the hypothesis that elevated AG contributes to reduced insulin secretion and hyperglycemia in CF ferrets.

RESULTS

Fasting AG was elevated in CF versus non-CF ferrets. Similar to its effects in other species, AG administration in non-CF ferrets acutely reduced insulin, increased growth hormone, and induced hyperglycemia. During oral glucose tolerance testing, non-CF ferrets had responsive insulin, glucagon like peptide-1 (GLP-1) and gastric inhibitory polypeptide (GIP) levels and maintained normal glucose levels, whereas CF ferrets had insufficient responses and became hyperglycemic. Interestingly in wild-type ferrets, the acyl-ghrelin receptor antagonist [D-Lys3]-GHRP-6 impaired glucose tolerance, and abolished insulin, GLP-1, and GIP responses during glucose tolerance testing. By contrast, in CF ferrets [D-Lys3]-GHRP-6 improved glucose tolerance, enhanced the insulin-to-glucose ratio, but did not impact the already low GLP-1 and GIP levels.

CONCLUSIONS

These results suggest a mechanism by which elevated AG contributes to CF hyperglycemia through inhibition of insulin secretion, an effect magnified by low GLP-1 and GIP. Interventions that lower ghrelin, ghrelin action, and/or raise GLP-1 or GIP might improve glycemia in CF.

Effects of Ghrelin on Sexual Behavior and Luteinizing Hormone Beta-subunit Gene Expression in Male Rats

BACKGROUND

The hormones of hypothalamo-pituitary-gonadal (HPG) axis have facilitative effects on reproductive behavior in mammals. Ghrelin as a starvation hormone has an inhibitory effect on HPG axis' function. Hence, it is postulated that ghrelin may reduce the sexual behavior through inhibiting of HPG axis. The aim of this study was to examine the effects of ghrelin and its antagonist, [D-Lys(3) ]-GHRP-6, on sexual behavior and LH beta-subunit gene expression in male rats.

METHODS

In this experimental study, 128 male Wistar rats were divided into two groups. Each group was further subdivided into eight subgroups (n=8 rats/subgroup) including the animals that received saline, ghrelin (2, 4 or 8 nmol), [D-Lys(3) ]-GHRP-6 (5 or 10 nmol) or co-administration of ghrelin (4 nmol) and [D-Lys(3) ]-GHRP-6 (5 or 10 nmol) through the stereotaxically implanted cannula into the third cerebral ventricle. The sexual behavior of male rats encountering with females and the hypo-physeal LH beta-subunit gene expression were evaluated at two different groups. Data were analyzed by ANOVA and p<0.05 was considered statistically significant.

RESULTS

Ghrelin injection (4 and 8 nmol) significantly (p<0.01) increased the latencies to the first mount, intromission and ejaculation as well as the post-ejaculatory interval. Also, 4 and 8 nmol ghrelin significantly (p<0.05) increased the number of mount and decreased the number of ejaculation. In co-administrated groups, [D-Lys(3) ]-GHRP-6 antagonized the effects of ghrelin. Ghrelin injection (4 and 8 nmol) reduced the LH beta-subunit gene expression while pretreatment with [D-Lys(3) ]-GHRP-6 improved the gene expression.

CONCLUSION

Ghrelin decreased the sexual behavior and LH beta-subunit gene expression in male rats, whereas [D-Lys(3) ]-GHRP-6 antagonizes these effects.