Role of Ras, ERK, and Akt in glucocorticoid-induced differentiation of embryonic rat somatotropes in vitro.

This study investigated the roles of Ras, ERK, and Akt in the glucocorticoid-induced differentiation of growth hormone-producing pituitary cells in vitro. Pituicytes isolated from day-18 rat embryos were cultured with 50 mM dexamethasone in addition to specific inhibitors of Ras (manumycin; 0.5, 5, 50 nM), ERK (U0126, 10 μM), or Akt (LY294002, 25 μM). Differentiation was assessed using immunofluorescent staining of intracellular growth hormone. Radioimmunoassay and Western blot analyses were used to determine levels of secreted and intracellular growth hormone, respectively. Manumycin reduced the fraction of growth hormone-positive cells and dexamethasone-induced growth hormone secretion in a dose-dependent manner (both P < 0.001). In the absence of dexamethasone, LY294002 and U0126 did not alter the fraction of growth hormone-positive cells or intracellular growth hormone protein expression or secretion. Both LY294002 and U0126 alone significantly attenuated the fraction of dexamethasone-treated GH-positive cells and the secretion of GH compared to those of cells treated only with dexamethasone (50 nM for 44 h or 48 h) (all P < 0.05). Dexamethasone treatment alone did not change GH protein levels. Treatment of cells with a combination of LY294402 and U0126 significantly attenuated the fraction of dexamethasone-treated GH-positive cells, GH protein levels, and GH secretion compared to cells treated with dexamethasone alone (all P < 0.05). Moreover, dexamethasone-induced phosphorylation of GTP-Ras, ERK, and Akt was significantly attenuated by exposure to the respective inhibitors (P < 0.05). Taken together, our results indicate that Ras, ERK, and Akt are key effectors in the glucocorticoid-induced differentiation of growth hormone-secreting cells.