Novel ghrelin analogs with improved affinity for the GH secretagogue receptor stimulate GH and prolactin release from human pituitary cells

Abstract

Objective: Ghrelin, a recently identified 28-amino acid peptide is a potent GH secretagogue (GHS) produced predominantly by the stomach. Ghrelin stimulates GH secretion through binding to the GHS receptor in the hypothalamus and pituitary. In addition to the GH-releasing action, ghrelin has been found to be a powerful orexigenic factor. To assess the direct in vitro effects of ghrelin on human pituitary hormone secretion we have produced a panel of novel ghrelin analogs (molecular weight, 3323 -3384; human native ghrelin, 3371) with enhanced affinity for the human GHS receptor (IC 50 0.38-1.09 nM; human ghrelin, 1.2-2.2 nM). Methods: The peptidic analogs were tested for their effect on GH secretion using dispersed human fetal pituitaries (21 to 23 weeks of gestation) and cultured GH-and prolactin (PRL)-secreting adenomas. The expression of the GHS receptor in normal (fetal and adult) human pituitary tissues, GH-and PRLcell adenomas was established using RT-PCR. Results: The effects of ghrelin, its analogs and GH-releasing hormone (GHRH) alone or in combination on GH and PRL secretion were compared at various concentrations. The ghrelin analogs stimulated GH release by 35-60% from human fetal pituitary cells (1 -10 nM; P , 0.05) and by 50 -75% from cultured pituitary adenomas (10 nM; P , 0.05). This releasing effect was dose-dependent, achieving maximal stimulation with analog concentrations at 100 nM. Human ghrelin was less potent as compared with its analogs in stimulating human GH, in keeping with the improved binding affinity of the analogs for the GHS-1a receptor. The ghrelin analogs and GHRH had comparable effects on GH secretion from both normal and adenomatous cells, and in combination produced an additive stimulatory effect on GH (150%; P , 0.0001). In contrast, ghrelin and its analogs induced a comparable increase in PRL release ranging between 25 and 40% (P , 0.05) from fetal cells and 30 and 70% (P , 0.001) from cultured PRL-cell and mixed GH -PRL adenomas. Conclusions: Our results have demonstrated for the first time that ghrelin analogs with enhanced affinity for the GHS receptor are potent stimulators of GH secretion from human pituitary cells, and thus may possess potential clinical therapeutic benefits.European Journal of Endocrinology 151 787-795

Ghrelin gene products, receptors, and GOAT enzyme: biological and pathophysiological insight

“…Surprisingly, a previous study showed that ghrelin mRNA levels in somatotropinomas were negatively correlated with tumor size and, indeed, high-grade tumors had lower levels of ghrelin mRNA than low-grade adenomas (Kim et al 2001). On the other hand, both GHSR1a and GHSR1b have been found to be highly expressed in somatotropinomas compared with NPs (Barlier et al 1999, Korbonits et al 2001, Rubinfeld et al 2004, which together with the previous data mentioned in this section, might suggest the existence of an autocrine/paracrine role of the ghrelin system in GHproducing adenomas. Moreover, studies in a rat pituitary somatotrope cell line indicate that both AG and UAG can stimulate cell proliferation (Nanzer et al 2004).…”

“…Although the presence of ghrelin and its receptor has been confirmed in prolactinomas (Kim et al 2001, Korbonits et al 2001, Rotondo et al 2011, limited studies have been carried out to elucidate the direct pathological role of the ghrelin system in these tumors. Specifically, a previous study revealed that AG treatment elicited PRL release in prolactinomas and GH-PRL mixed adenoma cell cultures (Rubinfeld et al 2004), suggesting an involvement of the ghrelin system in the development and progression of prolactinomas. However, to the best of our knowledge, the role of other ghrelin gene-derived peptides, such as UAG or obestatin, on prolactinomas has not been explored so far.…”

“…AG enhances GH release in GH-producing adenomas in vitro (Rubinfeld et al 2004), and has been shown to be locally produced in somatotropinomas (Kim et al 2001, Korbonits et al 2001; however, no significant correlation between circulating GH levels before surgery and ghrelin mRNA expression in the tumors has been observed (Korbonits et al 2001). Surprisingly, a previous study showed that ghrelin mRNA levels in somatotropinomas were negatively correlated with tumor size and, indeed, high-grade tumors had lower levels of ghrelin mRNA than low-grade adenomas (Kim et al 2001).…”

Ghrelin treatment protects lactotrophs from apoptosis in the pituitary of diabetic rats

“…Indeed, ghrelin has been reported to stimulate PRL synthesis and secretion both in animals (Kaiya et al, 2003;Riley et al, 2002) and in humans (Arvat et al, 2001;Baldelli et al, 2004;Broglio et al, 2003;Rubinfeld et al, 2004). It is also possible that ghrelin inhibits the increase in lactotroph cell death seen in poorly controlled diabetes.…”

Heterogeneity of Ghrelin/Growth Hormone Secretagogue Receptors

“…This is consistent with observations that ghrelin affects synaptogenesis and development of neuronal circuits, as well as modulation of memory processes, sleep patterns and behavior [44,45,46,47,48,49,94,95,96]. Within the hypothalamus-pituitary axis, GHS-R1a mediates ghrelin/GHS modulation of GH, PRL, CRH/ACTH and GnRH/gonadotropin secretion [26,27,28,97,98,99,100,101,102]. Orexigenic effects of ghrelin/GHS are likely mediated by GHS-R1a expressed by hypothalamic neurons containing neuropeptide Y (NPY)/Agouti-related protein [34,103,104,105,106].…”