Effects of cycloheximide on hepatic and uterine insulin-like growth factor-I mRNA

To investigate differences in growth hormone (GH) and estrogen activation of insulin-like growth factor-I (IGF-I) expression, we have examined the effects of cycloheximide on hepatic and uterine IGF-I mRNA abundance in response to GH and 17 beta-estradiol (E2) respectively. In hypophysectomized (hypox) rats a single injection of GH significantly increased hepatic IGF-I mRNA 3.65 +/- 0.68-fold, P less than 0.005, 6 h after injection. Administration of cycloheximide 30 min prior to GH injection completely abolished this response. In contrast, in pituitary intact rats killed 6 h after administration of cycloheximide, hepatic IGF-I mRNA abundance was not significantly different from untreated control rats although the serum IGF-I concentration was significantly reduced; 119.9 +/- 11.8 vs. 270.2 +/- 48.7 ng/ml, P less than 0.005. In immature rats, injection of E2 (1 micrograms/100 g body weight) significantly increased uterine IGF-I mRNA 4.1 +/- 0.4-fold. Cycloheximide did not block the E2-induced increase in IGF-I mRNA but rather significantly enhanced the IGF-I response. These data indicate that continuing protein synthesis is required for GH induction of hepatic IGF-I mRNA in the hypox rat but is not required for E2 induction of uterine IGF-I mRNA. Furthermore, in pituitary-intact rats protein synthesis is not required for maintenance of hepatic IGF-I mRNA levels.