Abstract
Growth hormone (GH)-releasing peptides (GHRPs) are synthetic peptides which induce strong GH release in both animals and humans. Among them, GHRP-2 is known to stimulate GH release by acting at both hypothalamic and pituitary sites, but also induces adrenocorticotropic hormone (ACTH) release in healthy subjects. GHRP-2 may stimulate ACTH release directly via GHRP receptor type 1a in ACTH-producing tumors. GHRP-2 increases ACTH secretion in rat in vivo, but not ACTH release from rat primary pituitary cells. In the present study, in order to elucidate the mechanism underlying ACTH secretion by GHRPs, mouse pituitary cells were stimulated by GHRP-2. GHRP receptor mRNA was expressed in the mouse pituitary, and GHRP-2 directly stimulated secretion and synthesis of ACTH in the mouse anterior pituitary cells. GHRP-2 increased intracellular cyclic AMP production. H89, a potent protein kinase A (PKA) inhibitor, and bisindolylmaleimide I, a selective protein kinase C (PKC) inhibitor, inhibited the GHRP-2-induced ACTH release, and that H89, but not bisindolylmaleimide I, inhibited the GHRP-2-induced proopiomelanocortin mRNA levels. Together, the GHRP-2-induced ACTH release was regulated via both PKA and PKC pathways in the mouse pituitary cells, while ACTH was synthesized by GHRP-2 only via the PKA pathway.
Introduction
Growth hormone (GH)-releasing peptides (GHRPs) are synthetic peptides which induce strong GH release in both animals and humans [1]. GHRP-6 has been extensively studied in Europe [1], whereas GHRP-2 is currently approved for clinical use in Japan [2]. GHRP-2 is one of the most potent GHRPs, and acts via a specific receptor, in which ghrelin would be a natural ligand [3].
GHRP-2 is known to stimulate GH release by acting at both hypothalamic and pituitary sites [4], but also induces adrenocorticotropic hormone (ACTH) release in healthy subjects [5]. GHRP-2 may be useful for diagnosing secondary adrenal insufficiency such as hypothalamic disorder and pituitary damage [6]. GHRPs or ghrelin stimulates ACTH release via corticotropin-releasing factor (CRF) and/or arginin vasopressin (AVP) in the hypothalamus of rats [7], [8]. GHRPs greatly increase plasma ACTH levels in patients with Cushing's disease [9]. Intravenous administration of GHRP-2 increases ACTH secretion in rat in vivo, but not ACTH release from rat primary pituitary cells [10]. In the present study, in order to elucidate the direct mechanism underlying ACTH secretion by GHRPs in the anterior pituitary (AP) cells, mouse pituitary cells were stimulated by GHRP-2, and then changes in ACTH levels and proopiomelanocortin (POMC) mRNA levels were determined.
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Section snippets
Animals
All mice used in this study were over 8 weeks old and were of C57BL/6J × 129 Sv genetic background. The mice were maintained under 12:12 h light: dark cycles (lights on at 0800h; lights off at 2000h). Food and water were available ad libitum. The mice were acclimatized to their environment before the study to avoid stress effects. The mice were anesthetized and sacrificed by decapitation for tissue collection at the same time of the day to avoid variations. All experiments were conducted in
Expression of GHSR mRNA in mouse AP cells
Expression of GHSR mRNA was examined by RT-PCR. GHSR mRNA was detected in AtT-20 cells and mouse AP cells (Fig. 1). Basal expression levels of GHSR mRNA were similar between them.
Effects of GHRP-2 on ACTH levels in mouse AP cells
To determine whether GHRP-2 stimulates ACTH secretion in mouse AP cells in a time- and dose-dependent manner, AP cells were incubated with GHRP-2. As shown in Fig. 2, ACTH levels in the medium increased gradually from 3 h to 9 h following the addition of 100 nM GHRP-2 (ANOVA; P < 0.001). The ACTH levels significantly
Discussion
It has been shown that GHRP-2 increases ACTH secretion in rat in vivo, but not ACTH release from rat primary pituitary cells [10]. GHRPs prompt ACTH secretion in corticotroph tumor cells with Cushing's disease [14], [15]. In the present study, we first found that GHSR mRNA was expressed in mouse AP cells, and GHRP-2 directly stimulated secretion and synthesis of ACTH in the mouse pituitary. Taken together with the results in our mouse study, this discrepancy may be caused by the differences
Acknowledgments
This work was supported in part by Health and Labour Science Research Grants (Research on Measures for Intractable Diseases) from the Ministry of Health, Labour, and Welfare of Japan, and by a grant to Toshihiro Suda from the Ministry of Education, Science and Culture of Japan (No. 18591014).
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Pituitary tumor with gigantism, acromegaly and preclinical Cushing's disease diagnosed from the 10th row
2013, American Journal of the Medical Sciences
In the present case, a single pituitary tumor was oversecreting both GH and apparently Big ACTH. Growth hormone–releasing peptide-2 stimulates GH’s release, acting on both the hypothalamus and pituitary and may induce ACTH release also in ACTH (and Big ACTH) producing tumors.7 Whether growth hormone–releasing peptide-2 was involved to cause the increase of both ACTH and GH in the circulation as demonstrated in the present case is unknown, but it has been demonstrated that there can be overexpression of the growth hormone–releasing hormone gene within acromegalyassociated pituitary tumors,8 which could lead to overproduction of both GH and Big ACTH as in the present patient.
The effects of ghrelin/GHSs on AVP mRNA expression and release in cultured hypothalamic cells in rats
2011, Peptides
However, there may be species differences in the mechanism of action of GHSs on ACTH secretion. KP-102 stimulates the secretion and mRNA expression of ACTH by cultured mouse pituitary cells in vitro, indicating the direct action of KP-102 on ACTH-secreting cells [25], while in rats, GHRP-6 and l-692,429 increase in vivo ACTH secretion, but not in cultured pituitary cells [7,15]. The role of CRF and AVP in the stimulatory mechanism of ghrelin/GHSs on ACTH secretion is controversial. View full text
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