Somatotroph recruitment by glucocorticoids involves induction of growth hormone gene expression and secretagogue responsiveness

Abstract

Prior research indicates that growth hormone (GH) cell differentiation can be induced prematurely by treatment with glucocorticoids in vitro and in vivo. However, the nature of these responses has not been fully characterized. In this study, the time course of corticosterone induction of GH-secreting cells in cultures of chicken embryonic pituitary cells, responsiveness of differentiated somatotrophs to GH secretagogues, localization of somatotroph precursor cells within the pituitary gland, and the effect of corticosterone on GH gene expression were determined to better define the involvement of glucocorticoids in somatotroph recruitment during development. Anterior pituitary cells from embryonic day 12 chicken embryos were cultured in 10(-9) M corticosterone for 4 to 48 h and were then subjected to reverse haemolytic plaque assays (RHPAs) for GH. Corticosterone treatment for as short as 16 h increased the percentage of GH cells compared with the control. When corticosterone was removed after 48 h and cells were cultured for an additional 3 days in medium alone, the percentage of GH secretors decreased but remained greater than the proportion of somatotrophs among cells that were never treated with corticosterone. To determine if prematurely differentiated somatotrophs were responsive to GH secretagogues, cells were exposed to corticosterone for 48 h and then subjected to GH RHPAs in the presence or absence of GH-releasing hormone (GHRH) or thyrotropin-releasing hormone (TRH). Approximately half of the somatotrophs induced to differentiate with corticosterone subsequently released more GH in response to GHRH and TRH than in their absence. The somatotroph precursor cells were localized within the anterior pituitary by culturing cells from the caudal lobe and cephalic lobe of the anterior pituitary separately. Corticosterone induction of GH cells was substantially greater in cultures derived from the caudal lobe of the anterior pituitary, where somatotroph differentiation normally occurs. GH gene expression was evaluated by ribonuclease protection assay and by in situ hybridization. Corticosterone increased GH mRNA in cultured cells by greater than fourfold. Moreover, corticosterone-induced somatotroph differentiation involved GH gene expression in cells not expressing GH mRNA previously, and the extent of somatotroph differentiation was augmented by treatment with GHRH in combination with corticosterone. We conclude that corticosterone increases the number of GH-secreting cells within 16 h, increases GH gene expression in cells formerly not expressing this gene, confers somatotroph sensitivity to GHRH and TRH, and induces GH production in a precursor population found primarily in the caudal lobe of the anterior pituitary, a site consistent with GH localization in adults. These findings support the hypothesis that glucocorticoids function to induce the final stages in the differentiation of fully functional somatotrophs from cells previously committed to this lineage.