Dexamethasone inhibits both growth hormone (GH)-induction of insulin-like growth factor-I (IGF-I) mRNA and GH receptor (GHR) mRNA levels in rat primary cultured hepatocytes

Glucocorticoids are potent inhibitors of growth. In this work, we investigated whether glucocorticoids inhibit the stimulatory action of GH on IGF-I gene expression in rat hepatocytes. GH increased IGF-I mRNA fevers Ii-fold after 24 h, whereas high doses of DXM (10(-6) M) caused a slight (2.6-fold) increase of IGF-I mRNA levels. However, high doses of DXM (10(-6) M) inhibited the induction of IGF-I mRNA by GH. To assess the role of GHR in this inhibition, we investigated the regulation of GHR expression. High doses of DXM decreased GHR mRNA levels. This effect was already detectable 6 h after addition of 10(-6) M DXM and was dose-dependent, with a maximal inhibition observed at a concentration of 10(-6) M. In conclusion, our results show that high doses of DXM inhibits the GH-induced IGF-I gene expression and the GHR gene expression. The parallel decrease of GHR and GH-induced IGF-I mRNA suggests that the GH resistance caused by DXM is mediated by diminished GH receptor synthesis. (C) 1999 Harcourt Publishers Ltd.