Characterization of Somatotrope Cell Expansion in Response to GHRH in the Neonatal Mouse Pituitary

In humans and mice, loss-of-function mutations in growth hormone-releasing hormone receptor (GHRHR) cause isolated GH deficiency. The mutant GHRHR mouse model, GhrhrLit/Lit (LIT), exhibits loss of serum GH, but also fewer somatotropes. However, how loss of GHRH signaling affects expansion of stem and progenitor cells giving rise to GH-producing cells is unknown. LIT mice and wild-type littermates were examined for differences in proliferation and gene expression of pituitary lineage markers by quantitative reverse transcription polymerase chain reaction and immunohistochemistry at postnatal day 5 (p5) and 5 weeks. At p5, the LIT mouse shows a global decrease in pituitary proliferation measured by proliferation marker Ki67 and phospho-histone H3. This proliferative defect is seen in a pituitary cell expressing POU1F1 with or without GH. SOX9-positive progenitors show no changes in proliferation in p5 LIT mice. Additionally, the other POU1F1 lineage cells are not decreased in number; rather, we observe an increase in lactotrope cell population as well as messenger RNA for Tshb and Prl. In the 5-week LIT pituitary, the proliferative deficit in POU1F1-expressing cells observed neonatally persists, while the number and proliferative proportion of SOX9 cells do not appear changed. Treatment of cultured pituitary explants with GHRH promotes proliferation of POU1F1-expressing cells, but not GH-positive cells, in a mitogen-activated protein kinase-dependent manner. These findings indicate that hypothalamic GHRH targets proliferation of a POU1F1-positive cell, targeted to the somatotrope lineage, to fine tune their numbers.