Changes in Ribonucleic Acid Polymerase Activities in Gonadotropin-Treated Leydig Cells: An Estradiol-Mediated Process*

Abstract

The effects of desensitizing doses of hCG [human chorionic gonadotropin] on the activities of Leydig cell DNA-dependent RNA polymerases I, II and III were investigated after optimization of the enzyme assay. Individual activities were obtained by taking advantage of their different sensitivities to .alpha.-amanitin. RNA polymerase III was a minor component of the .alpha.-amanitin-resistant activity at 0.10 .mu.g/ml, and was therefore measured together with RNA polymerase I. In adult rats treated with 10 .mu.g hCG, s.c. RNA polymerases II and I plus III activities of Leydig cells rose within 45 min to 180 .+-. 6% and 162 .+-. 2% of the control value, and then decreased to control values at 60 min. The initial stimulation of polymerase activities was coincident with maximal increases in testosterone and estradiol levels in plasma. A 2nd and more sustained increase in polymerases II and I plus III activities occurred between 12 and 24 h (212 .+-. 12% and 180 .+-. 10% of the control value) and was maintained until 48 h after hCG injection. The hCG-induced rise in polymerase activities was due to activation of the enzymes, since chromatin template capacity was unaltered. In animals treated with the antiestrogen tamoxifen, stimulation of RNA polymerase activity by hCG was completely inhibited. Also, hCG did not stimulate polymerase activity in animals treated with amino glutethimide, which blocks steroid synthesis from cholesterol, or in those treated with androstatriendione, which inhibits aromatization of testosterone leading to estradiol. Increases in RNA polymerase activities were also achieved by the administration of lower doses of hCG (0.1 and 1 .mu.g) and the administration of estradiol (2 .mu.g), resembling the pattern seen with 10 .mu.g hCG. The hCG-induced RNA polymerase activation in the Leydig cell is mediated through nuclear actions of estradiol, since stimulation of the enzymes was prevented by tamoxifen and inhibition of steroid biosynthesis, and was induced by estradiol administration.