INVOLVEMENT OF GLUCOCORTICOID-INDUCED FACTOR(S) IN THE STIMULATION OFGROWTH-HORMONE EXPRESSION IN THE FETAL-RAT PITUITARY-GLAND IN-VITRO

The mechanism by which glucocorticoids induce GH expression between em bryonic days 18 and 19 (E18-19) in the fetal rat pituitary gland was e xamined with an in vitro organ culture system. Twenty-four hour incuba tion of E18 pituitary glands in serum-free medium containing either de xamethasone (DEX, 5-50 nM) or corticosterone (0.5-5 mu M) resulted in a conspicuous accumulation of GH messenger RNA (mRNA), whereas no spon taneous expression of GH mRNA was noted without glucocorticoid. Triiod othyronine (1 nM) alone weakly induced GH mRNA but increased the effec t of DEX 2-fold. The GH mRNA accumulation was not observed after 5 or 10 h incubation with DEX. However, a 10-h incubation with DEX followed by 14 h chase incubation without DEX resulted in apparent induction o f GH mRNA. The induction of GH mRNA by DEX was completely inhibited by puromycin. These data, taken as a whole, suggest that the induction o f GM mRNA by DEX in the fetal pituitary gland is not a direct effect o f DES on the GH gene but is mediated by a factor that is synthesized i n the pituitary gland in response to DES. Both immunoblot and RNase pr otection assays suggested that this factor is not pit-1, which is know n to be required for GH mRNA expression.