Aromatase Activity in a Rat Leydig Cell Tumor*

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Abstract
Male Wistar-Furth rats bearing the transplantable LTW (m) Leydig cell tumor have elevated serum estradiol (E2) concentrations. We measured the ability of these tumors to aromatize testosterone (T) to E2 by two methods. First, tumor minces were incubated with [7-3H]T, and the resultant [3H]E2 and [3H]estrone were purified and measured. In addition, tumor cell cultures were incubated with [1.beta.-3H]T, and the resultant [3H]H2O was determined as a measure of aromatization. Tumor minces aromatized more actively than normal rat testicular tissue (3.30 .+-. 0.15% of the T added was converted to E2 by the tumor vs. 0.30 .+-. 0.25% by normal testis). Most of the aromatizing activity was localized to the microsomes. Using cell cultures the maximum velocity was 6.1 pmol/h .cntdot. 5 .times. 105 cells, and the Km was 98 nM. In neither minces nor cell cultures were we able to show stimulation of aromatization with hCG, (Bu)2cAMP, or phorbol esters, although we could show stimulation by these agents in normal testicular cells. We were unable to inhibit the aromatase activity with human .beta.-endorphin or stimulate it with naloxone. However, we were able to inhibit the aromatase activity with 4-hydroxy-4-androstene-3,17-dione. We conclude that the LTW (m) rat Leydig cell tumor has an active autonomous aromatase system that is not responsive to compounds affecting the adenylate cyclase-cAMP system. It can be inhibited by 4-hydroxy-4-androstene-3,17-dione, a competitive-suicide inhibitor of the aromatase enzyme(s).