Pituitary gland development is dependent on the pituitary-specific transcription factors, POU1F1 and PROP1. PROP1 is required for Pou1f1 expression and POU1F1 is required for differentiation of somatotropes, lactotropes, and thyrotropes (Gage et al., 1995, 1996; Sornson et al., 1996; Camper et al., 1990; Lin et al., 1992; Ingraham et al., 1988; Li et al., 1990). In mice, Pou1f1 expression begins at approximately embryonic day (e)14.5. NEUROD4, a basic helix-loop-helix transcription factor, is required for the onset of somatotrope differentiation and its expression is apparent in the mouse pituitary gland by approximately e14.5 (Zhu et al., 2006; Ando et al., 2018). While Gh1 mRNA can be observed at e15.5 (Japon et al., 1994), growth hormone (GH) protein is not apparent at this age (Stallings et al., 2018). GH protein is not detected in mice until approximately e16.5 (Kapali et al., 2016), suggesting that additional signals are required for terminal differentiation of these cells.

Glucocorticoids regulate somatotrope differentiation and function (Vakili et al., 2012; McEachern et al., 2011; Jain et al., 2011; Dean et al., 1999; Nogami et al., 1993, 2008; Sato et al., 1998). While chronic stress, Cushing's syndrome, or immunosuppressive therapies inhibit growth, several lines of evidence support a role for physiological levels of glucocorticoids in promoting terminal differentiation of somatotropes (Vakili et al., 2012). First, cases of infants with glucocorticoid deficiency also exhibit GH deficiency which eventually resolves upon glucocorticoid replacement, suggesting that differentiation and proliferation of somatotropes had been impaired in the absence of glucocorticoids (McEachern et al., 2011; Jain et al., 2011). Secondly, establishment of a functional hypothalamic-pituitary-adrenal axis coincides with somatotrope terminal differentiation in the mouse (Vakili et al., 2012; Keeney et al., 1995; Jaskoll et al., 1994). Finally, treatment with exogenous glucocorticoids can induce premature somatotrope appearance in fetal rats and chick embryos, demonstrating that glucocorticoids can stimulate somatotrope terminal differentiation (Dean et al., 1999; Nogami et al., 1993). There is evidence to suggest that glucocorticoids stimulate Gh1 expression indirectly via an intermediate factor (Strobl et al., 1989) while directly stimulating expression of the receptor for GH releasing hormone (Ghrhr) (Nogami et al., 2002). While Pou1f1 expression does not appear to be regulated by glucocorticoids, POU1F1 is required for glucocorticoid-induced Ghrhr expression (Nogami et al., 2005).

Another transcription factor that is important for somatotrope differentiation is the forkhead transcription factor, FOXO1. FOXO1 is required for normal expression of Gh1, Ghrhr, and Neurod4 (Kapali et al., 2016; Stallings et al., 2022, 2024). FOXO1 deficient animals exhibit delayed differentiation and impaired function of somatotropes (Kapali et al., 2016). Deletion of both Foxo1 and its closely related family member, Foxo3, reduces Pou1f1 expression (Stallings et al., 2022), however Pou1f1 expression is unaffected when Foxo1 alone is deleted (Kapali et al., 2016). Thus, both glucocorticoids and FOXO1 play roles in regulating somatotrope terminal differentiation.

The goal of the current investigation is to explore the interplay of FOXO1 and glucocorticoid signaling during terminal differentiation of somatotropes. To this end, we found that in addition to the markers of somatotrope terminal differentiation, Gh1 and Ghrhr, glucocorticoids induce Foxo1 expression both in vitro and in vivo. Loss of FOXO1 impairs the ability of glucocorticoids to prematurely induce terminal differentiation of mouse somatotropes in vivo. These studies provide new insights into the mechanisms regulating somatotrope terminal differentiation.