The rat H540 Leydig
tumor cell is established as a model for acute
lutropin action on the initial step of steroidogenesis, namely the conversion of
cholesterol to
pregnenolone. Herein, we demonstrate that H540 cells express high levels of three
steroid-metabolizing
enzymes which are involved in the further processing of
pregnenolone in the endoplasmic reticulum of the steroidogenic cell. In particular, in addition to expressing
17 alpha-hydroxylase cytochrome P-450 (P-450(17) alpha) and
3 beta-hydroxysteroid dehydrogenase/delta 5----4-isomerase (3 beta-HSD), H540 cells also showed high levels of
steroid 5 alpha-reductase mRNA and activity. The H540 cells therefore exhibit similarity to Leydig cells from sexually immature animals which also demonstrate high
5 alpha-reductase activity. Thus, after 3 beta-HSD-catalyzed formation from
pregnenolone,
progesterone was efficiently converted to 5 alpha-pregnan-3,20-dione (5 alpha-dihydroprogesterone) and subsequent metabolism to the corresponding 17 alpha-hydroxylated derivative and 5 alpha-androstan-3,17-dione in a reaction catalyzed by P-450(17) alpha. H540 cells have apparently very low 17-ketosteroid
reductase activity and, therefore, a principal end-product of the steroidogenic pathway in these cells was 5 alpha-androstan-3,17-dione. H540 cells maintained in primary culture under serum-free conditions accumulated demonstrable levels of
mRNA species for P-540 17 alpha (1.7 kb), 3 beta-HSD (1.6 kb) and
5 alpha-reductase (2.7 kb). This finding suggests that the H540
tumor cell model will not only be of utility in the study of acute
lutropin action but also in the elucidation of mechanisms involved in the regulation of expression of various families of microsomal
steroid-metabolizing
enzymes.