Endocrine Abstracts

3beta-hydroxysteroid dehydrogenase (3beta-HSD) expression is essential for the synthesis of all classes of steroid hormones, converting delta⁵beta-hydroxysteroids into hormonally active delta⁴-3-ketosteroids in NAD⁺-dependent reactions. Numerous 3beta-HSD isoforms have been described in mouse, rat, human and other mammals, all with distinct tissue and developmentally specific expression, distinct kinetics, with alternative substrate specificity and catalytic properties demonstrable in some isoforms. Thus rat type I (adrenal/gonadal) and type IV (skin/placenta) 3beta-HSDs, in addition to 3beta-HSD activity, have been shown to exhibit 17beta-HSD-like activity, but only towards the 5alpha-reduced steroid, dihydrotestosterone. Mouse type VI 3beta-HSD is the major isoform expressed during the first half of pregnancy in uterine and embryonic cells, is the only isoform expressed in skin and thus appears to be the orthologue of human type I 3beta-HSD.

We sought to establish whether mouse type I (adrenal/gonadal) and type VI 3beta-HSDs and their respective human orthologues, types II and I respectively, displayed 17beta-HSD-like activity.

Non-steroidogenic HEK 293T cells were transiently transfected with pCMV-based vectors containing cDNAs encoding mouse type I, mouse type VI, human type I and type II 3beta-HSDs. Subsequently the transfected cells were incubated with either radiolabelled testosterone (1 micromolar) or pregnenolone (1 micromolar) for various periods up to 24 hours. While the human 3beta-HSDs were unable to metabolise testosterone, both mouse type I and VI isoforms converted testosterone to androstenedione at rates one-tenth of those of pregnenolone to progesterone in similar 293T cells. The metabolism of dihydrotestosterone was minor compared to that of testosterone.

Our findings demonstrate that the mouse 3beta-HSD I and VI isoforms can inactivate testosterone within intact cells. On the other hand the human 3beta-HSDs do not exhibit 17beta-HSD activity. These findings are noteworthy not only in establishment of structure-function relationships, but also where murine paradigms are used for human developmental/reproductive matters associated with uterine/embryonic/skin disorders.