Evidence that endogenous SST inhibits ACTH and ghrelin expression by independent pathways

Abstract

Corticosterone and total ghrelin levels are increased in somatostatin (SST) knockout mice (Sst Ϫ/Ϫ ) compared with SST-intact controls (Sst ϩ/ϩ ). Because exogenous ghrelin can increase glucocorticoids, the question arises whether elevated levels of ghrelin contribute to elevated corticosterone levels in Sst Ϫ/Ϫ mice. We report that Sst Ϫ/Ϫ mice had elevated mRNA levels for pituitary proopiomelanocortin (POMC), the precursor of adrenocorticotropic hormone (ACTH), whereas mRNA levels for hypothalamic corticotropin-releasing hormone (CRH) did not differ from Sst ϩ/ϩ mice. Furthermore, SST suppressed pituitary POMC mRNA levels and ACTH release in vitro independently of CRH actions. In contrast, it has been reported that ghrelin increases glucocorticoids via a central effect on CRH secretion and that n-octanoyl ghrelin is the form of ghrelin that activates the GHS-R1a and modulates CRH neuronal activity. Consistent with elevations in total ghrelin levels, Sst Ϫ/Ϫ mice displayed an increase in stomach ghrelin mRNA levels, whereas hypothalamic and pituitary expression of ghrelin was not altered. Despite the increase in total ghrelin levels, circulating levels of n-octanoyl ghrelin were not altered in Sst Ϫ/Ϫ mice. Because glucocorticoids and ghrelin increase in response to fasting, we examined the impact of fasting on the adrenal axis and ghrelin in Sst ϩ/ϩ and Sst Ϫ/Ϫ mice and found that endogenous SST does not significantly contribute to this adaptive response. We conclude that endogenous SST inhibits basal ghrelin gene expression in a tissue specific manner and independently and directly inhibits pituitary ACTH synthesis and release. Thus endogenous SST exerts an inhibitory effect on ghrelin synthesis and on the adrenal axis through independent pathways. somatostatin; adrenocorticotropic hormone; hypothalamic-pituitaryadrenal axis; corticotropin-releasing hormone; pituitary; stomach SOMATOSTATIN (SST), OR ITS SYNTHETIC ANALOGS, can reduce circulating adrenocorticotropic hormone (ACTH) and glucocorticoid levels in rats and humans in vivo (19,43,46). The inhibitory actions of SST are believed to be due, at least in part, to a direct effect on the pituitary. This hypothesis is supported by the fact that SST can block corticotropin-releasing hormone (CRH)-mediated ACTH release in cultures of primary rat pituitary cells, mouse pituitary tumor cells (AtT-20), and cultures of human corticotropinomas (20,25,44), where this effect appears to be mediated by the SST receptors sst2 and/or sst5 (20,43,44). Both the in vivo and in vitro inhibitory effects of SST on ACTH release can be masked by glucocorticoids, where glucocorticoids directly inhibit basal and CRH-mediated ACTH release (19,25). However, a recent study (20) demonstrates that an sst5 selective agonist can inhibit ACTH release in vitro in the presence of glucocorticoids. The inhibitory action of exogenous SST on ACTH release has prompted speculation regarding the role of endogenous SST as a corticotropin release-inhibiting factor (14). More recent observations suppo...

Identification of a mouse ghrelin gene transcript that contains intron 2 and is regulated in the pituitary and hypothalamus in response to metabolic stress

“…We have previously reported that fasting up-regulates the expression of native ghrelin in the mouse PIT, but has no effect on HPT mRNA levels (Luque et al 2006b(Luque et al , 2007. Consistent with these findings, in the current report, we observed that the expression of both native ghrelin (Fig.…”

“…This cell line was maintained in our laboratory according to previously described methods (Belsham et al 2004). Primary PIT cell cultures were prepared by enzymatic dispersion of 3-4 pituitaries/experiment from 8-to 10-week-old male mice, as previously described by our laboratory (Luque & Kineman 2006, Luque et al 2006b). …”

“…PCR products were then column-purified and sequenced again to confirm target specificity. To determine the starting copy number of cDNA, RT samples were PCR amplified and the signal compared with that of synthetic cDNA standard curve run on the same plate as previously described (Luque & Kineman 2006, Luque et al 2006b). In addition, total RNA samples that were not reverse transcribed and a non-DNA control were run on each plate to control for genomic DNA contamination and monitor potential exogenous contamination respectively.…”

“…In addition, total RNA samples that were not reverse transcribed and a non-DNA control were run on each plate to control for genomic DNA contamination and monitor potential exogenous contamination respectively. In addition, to control for variations in the amount of RNA used in the RT reaction and the efficiency of the RT reaction, mRNA copy number of the transcript of interest was adjusted by that of cyclophilin A (a peptidyl isomerase) as previously described (Luque & Kineman 2006, Luque et al 2006b). Cyclophilin A mRNA levels did not significantly vary between experimental groups within tissue type.…”

“…Exogenous ghrelin stimulates food intake and promotes energy storage (Tschop et al 2000, Asakawa et al 2001, Nakazato et al 2001. In a number of species including humans, endogenous ghrelin is down-regulated in obesity (Tschop et al 2001, Sugino et al 2002, Moesgaard et al 2004, Luque & Kineman 2006, Martin et al 2006 and up-regulated in response to food deprivation (Toshinai et al 2001, Luque et al 2006b, 2007.…”