Tolle, Virginie

Germline mutations in the aryl hydrocarbon receptor-interacting protein ( AIP ) gene predispose humans to pituitary adenomas, particularly of the somatotroph lineage. Mice with global heterozygous inactivation of Aip ( Aip +/− ) also develop pituitary adenomas but differ from AIP -mutated patients by the high penetrance of pituitary disease. The endocrine phenotype of these mice is unknown. The aim of this study was to determine the endocrine phenotype of Aip +/− mice by assessing the somatic growth, ultradian pattern of GH secretion and IGF1 concentrations of longitudinally followed male mice at 3 and 12 months of age. As the early stages of pituitary tumorigenesis are controversial, we also studied the pituitary histology and somatotroph cell proliferation in these mice. Aip +/− mice did not develop gigantism but exhibited a leaner phenotype than wild-type mice. Analysis of GH pulsatility by deconvolution in 12-month-old Aip +/− mice showed a mild increase in total GH secretion, a conserved GH pulsatility pattern, but a normal IGF1 concentration. No pituitary adenomas were detected up to 12 months of age. An increased ex vivo response to GHRH of pituitary explants from 3-month-old Aip +/− mice, together with areas of enlarged acini identified on reticulin staining in the pituitary of some Aip +/− mice, was suggestive of somatotroph hyperplasia. Global heterozygous Aip deficiency in mice is accompanied by subtle increase in GH secretion, which does not result in gigantism. The absence of pituitary adenomas in 12-month-old Aip +/− mice in our experimental conditions demonstrates the important phenotypic variability of this congenic mouse model.

Ghrelin, a gut hormone originating from the post‐translational cleavage of preproghrelin, is the endogenous ligand of growth hormone secretagogue receptor 1a (GHS‐R1a). Within the growth hormone (GH) axis, the biological activity of ghrelin requires octanoylation by ghrelin‐O‐acyltransferase (GOAT), conferring selective binding to the GHS‐R1a receptor via acylated ghrelin. Complete loss of preproghrelin‐derived signalling (through deletion of the Ghrl gene) contributes to a decline in peak GH release; however, the selective contribution of endogenous acyl‐ghrelin to pulsatile GH release remains to be established. We assessed the pulsatile release of GH in ad lib. fed male germline goat−/− mice, extending measures to include mRNA for key hypothalamic regulators of GH release, and peripheral factors that are modulated relative to GH release. The amount of GH released was reduced in young goat−/− mice compared to age‐matched wild‐type mice, whereas pulse frequency and irregularity increased. Altered GH release did not coincide with alterations in hypothalamic Ghrh, Srif, Npy or Ghsr mRNA expression, or pituitary GH content, suggesting that loss of Goat does not compromise canonical mechanisms that contribute to pituitary GH production and release. Although loss of Goat resulted in an irregular pattern of GH release (characterised by an increase in the number of GH pulses observed during extended secretory events), this did not contribute to a change in the expression of sexually dimorphic GH‐dependent liver genes. Of interest, circulating levels of insulin‐like growth factor (IGF)‐1 were elevated in goat−/− mice. This rise in circulating levels of IGF‐1 was correlated with an increase in GH pulse frequency, suggesting that sustained or increased IGF‐1 release in goat−/− mice may occur in response to altered GH release patterning. Our observations demonstrate that germline loss of Goat alters GH release and patterning. Although the biological relevance of altered GH secretory patterning remains unclear, we propose that this may contribute to sustained IGF‐1 release and growth in goat−/− mice.

AbstractGhrelin is a gut hormone processed from the proghrelin peptide acting as the endogenous ligand of the GH secretagogue receptor 1a. The regulatory role of endogenous ghrelin on pulsatile GH secretion and linear growth had to be established. The aim of the present study was to delineate the endogenous actions of preproghrelin on peripheral and central components of the GH axis. Accordingly, the ultradian pattern of GH secretion was measured in young and old preproghrelin-deficient males. Blood samples were collected by tail bleeding every 10 minutes over a period of 6 hours. Analysis of the GH pulsatile pattern by deconvolution showed that GH was secreted in an ultradian manner in all genotypes, with major secretory peaks occurring at about 3-hour intervals. In older mice, the peak number was reduced and secretion was less irregular compared with younger animals. Remarkably, in young Ghrl−/− mice, the amplitude of GH secretory bursts was significantly reduced. In older mice, however, genotype differences were less significant. Changes in GH pulsatility in young Ghrl−/− mice were associated with a tendency for reduced GH pituitary contents and plasma IGF-I concentrations, but with only a minor impact on linear growth. In Ghrl+/− mice, despite reduced Acyl ghrelin to des-acyl ghrelin ratio, GH secretion was not impaired. Ghrelin deficiency was not associated with a reduction in hypothalamic GHRH content or altered response to GHRH stimulation. Therefore, reduction in GHRH production and/or sensitivity do not primarily account for the altered GH pulsatile secretion of young Ghrl−/− mice. Instead, GHRH expression was elevated in young but not old Ghrl−/− mice, suggesting that differential compensatory responses resulting from the absence of endogenous ghrelin is occurring according to age. These results show that endogenous ghrelin is a regulator of GH pulse amplitude in growing mice but does not significantly modulate linear growth.