Inhibition of Leydig Tumor Cell Steroidogenesis by 10-Propargylestr-4-ene-3,17-dione, an Irreversible Aromatase Inhibitor¹

The murine Leydig cell tumor (M5480A) was assayed for the presence of aromatase activity and for the effects of 10-propargylestr-4-ene-3,17-dione (PED), an aromatase inhibitor, on steroidogenesis. Microsomal preparations from these tumors contained low levels of aromatase activity which was PED sensitive. In addition, these Leydig tumor cells were maintained in primary culture and incubated under basal and gonadotropin-stimulated conditions with various doses of PED. Medium levels of progesterone, a major product of these cells, were found to decrease in a dose- and time-dependent manner upon addition of PED. To determine whether the observed effect was due to reduced synthesis or to increased metabolism of progesterone, tritiated progesterone was added to these cell cultures. Analysis of culture medium by high-performance liquid chromatography suggested that PED dramatically reduced the conversion of labeled progesterone to testosterone. Furthermore, examination of medium pregnenolone levels revealed diminished amounts of this steroid as well. Taken together, these results suggest that PED or its metabolites inhibit Leydig tumor cell steroidogenesis at several sites. Thus, in addition to its role as an aromatase inhibitor, this agent also has effects prior to pregnenolone production, as well as other effects in the pathway between progesterone and testosterone.