Dexamethasone Inhibits Growth Hormone Induction of Insulin-Like Growth Factor-I (IGF-I) Messenger Ribonucleic Acid (mRNA) in Hypophysectomized Rats and Reduces IGF-I mRNA Abundance in the Intact Rat*

Exogenous glucocorticoids are potent inhibitors of skeletal growth in experimental animals and children. The mechanism whereby glucocorticoids exert this effect is unclear, and circulating somatomedin levels have been reported to be low, normal, or high in this situation. Since recent studies have emphasized the importance of autocrine or paracrine insulin-like growth factor I (IGF-I), we have examined the effects of dexamethasone (DXM) on IGF-I gene expression in the hypophysectomized (hypox) and pituitary intact rat. An increase in IGF-I messenger RNA (mRNA) abundance of approximately 5-, 3-, 2-, and 1.5-fold in the liver, proximal tibia, lung, and kidney, respectively, was seen in hypox rats killed 6 h after injection of human GH (100 micrograms/100 g body wt). As little as 1 micrograms/100 g body wt of DXM administered 3 h before GH injection significantly reduced GH induction of IGF-I mRNA in the liver and the proximal tibia (P less than 0.05 and P less than 0.005, respectively). Higher doses of DXM were required to reduce IGF-I mRNA abundance in the kidney and lung. Of the tissues examined the order of sensitivity to DXM was liver greater than tibia greater than kidney greater than lung. In contrast to the effects of DXM on tissue IGF-I mRNA abundance, an approximately 10-fold higher dose of DXM was required to inhibit the GH-induced rise in serum IGF-I concentration. The effect of DXM on steady state IGF-I mRNA abundance in pituitary-intact rats which were killed 9 h after DXM was also examined. The reduction in IGF-I mRNA abundance required higher doses of DXM (6-360 micrograms/100 g body wt) and was less marked in pituitary-intact rats than in GH-treated hypox rats. In the pituitary-intact rats the order of sensitivity to DXM was tibia greater than liver greater than lung greater than kidney. In acute studies serum IGF-I levels were not decreased by any dose of DXM; rather a significant increase in serum IGF-I was apparent in pituitary intact rats, treated with the lowest and highest doses of DXM. When DXM was administered daily for 6 days to pituitary intact rats, body wt gain and hepatic and tibial IGF-I mRNA abundance were significantly reduced. No significant changes were seen in serum IGF-I concentrations.(ABSTRACT TRUNCATED AT 400 WORDS)