HUMAN CHORIONIC-GONADOTROPIN DECREASES INSULIN-LIKE GROWTH-FACTOR-I GENE-TRANSCRIPTION IN RAT LEYDIG-CELLS

Insulin-like growth factor-I (IGF-I) and hCG have synergistic effects on Leydig cell steroidogenesis in primary culture. In the present stud y, we investigated the effects of hCG on IGF-I gene transcription in L eydig cells. Purified Leydig cells (8-10 x 10(6) cells/100-mm dish) ob tained from 50- to 65-day-old male Sprague-Dawley rats were cultured f or 24 h. After medium change, hCG (0.1-10 ng/ml) or 8-bromo-cAMP (0.1 mM) was added, and cultures were continued for varying periods of time . In response to stimulation with hCG, there was a marked increase in the expression of cholesterol side-chain cleavage cytochrome P450 mRNA . In contrast, hCG caused time- and dose-dependent decrements in IGF-I mRNA levels. Both large 7.5-kilobase (kb) and small (0.8- to 1.2-kb ) species of IGF-I mRNAs were markedly decreased 6 h after treatment w ith hCG. hCG in a concentration of 0.1 ng/ml did not alter IGF-I mRNA levels. Higher concentrations of hCG (1 and 10 ng/ml) markedly decreas ed both 7.5- and 0.8- to 1.2-kb IGF-I mRNAs (80% and 56% reductions, r espectively). 8-Bromo-cAMP (0.1 mM) also markedly reduced IGF-I mRNA l evels. Finally, we evaluated the effects of hCG on the stability and t ranscription rates of IGF-I mRNA. We found that t(1/2) of IGF-I mRNA f or control Leydig cells was 3.86 h, which was not significantly differ ent from that of hCG-treated cells (t(1/2) = 3.41 h). This indicates t hat treatment with hCG did not change the stability of IGF-I mRNA. The average transcription rate per h for IGF-I mRNA decreased from 1 (for control cells) to 0.74 (for hCG-treated cells). The t(1/2) values and rates of transcription for beta-actin were 7.39 and 7.16 h, and 1 and 0.94 for control and hCG-treated cells, respectively, showing that RN A stability and rates of transcription did not change significantly fo r the P-actin transcript. In conclusion, we have unequivocally demonst rated that hCG decreases the expression and transcription of IGF-I mRN A in Leydig cells.