The mechanism of cyclosporine's action in the inhibition of testosterone biosynthesis by rat Leydig cells in vitro.

We have previously demonstrated that cyclosporine inhibits testosterone (T) biosynthesis in vivo. To better understand the mechanism by which CsA inhibits T synthesis, interstitial cells were isolated from rat testes and incubated in the standard medium 199 with or without CsA (0-10 micrograms/ml) in the presence or absence of human chorionic gonadotropin (hCG, 10(-7) M) and 8-bromo cyclic AMP (cAMP, 0.5 mM) for 3 hr at 32 degrees C. The levels of cAMP and T were determined by RIA. CsA did not inhibit the basal secretion of T, but inhibited hCG-stimulated T production in a dose-dependent manner (4 ng/10(6) cells vs. 10 ng/10(6) cells at a CsA dose of 5 micrograms/ml, P less than 0.05). Radioligand binding of 125I-hLH to testicular membranes was not affected by CsA, as CsA did not compete with hCG/LH for binding sites (25-28% binding with or without CsA). Similarly, the MIX-stimulated cAMP production was not affected by CsA (24.03 +/- 1.09 vs. 20.60 +/- 0.38 pmol/10(6) cells), suggesting that CsA does not inhibit the accumulation of the second messenger. However, when interstitial cells were incubated with CsA in the presence of cAMP, a significant dose-dependent decline in T secretion was observed (7 ng/10(6) cells vs. 20 ng/10(6) cells at a CsA dose of 5 micrograms/ml). To determine whether CsA inhibits the steps beyond cAMP stimulation of T secretion, the kinetic parameters (Km and Vmax) of steroidogenic enzymes, delta 4-3 keto-17 alpha hydroxylase (17 alpha-hydroxylase), and delta 4-3 keto-17 beta hydroxy steroid dehydrogenase (17B-HSD) were determined by using Michaelis Menten analysis. Results are shown in the presence of CsA vs. no CsA: Km and Vmax values for 17 alpha-hydroxylase were (2.32 vs. 7.98 microM) and (27.96 vs. 100.97 pmol/mg protein/min), respectively. For 17B-HSD the Km and Vmax were (2.14 vs. 1.52 microM) and (15 vs. 15 pmol/mg protein/min), respectively. These results indicate that CsA inhibits the activity of 17 alpha-hydroxylase uncompetitively and 17B-HSD activity competitively. In conclusion the primary site for CsA inhibition is the cAMP stimulation and, CsA inhibits T synthesis at multiple sites.