Compartmentalization of type 117β-hydroxysteroid oxidoreductase in the human ovary

The steroid-metabolizing enzyme, type I 17β-hydroxysteroid oxidoreductase (17β-HSOR) also called 17β-hydroxysteroid dehydrogenase (17β-HSD) plays a key role in ovarian synthesis of 17β-estradiol. This is the only enzyme in the steroid-metabolizing pathway which has not been localized in the human ovary by immunohistochemistry. In this study, using antibody directed against human placenta! cytosolic 17β-HSOR (type I), a single protein band with a relative molecular mass of approximately 34 kDa was demonstrated by Western analysis in both human luteinized granulosa cells and placental tissue. In placental tissue, immunoreactive type I 17β-HSOR was demonstrated within the syncytiotrophoblast using immunohistochemistry. In human ovary, immunoreactive type I 17β-HSOR was localized exclusively in granulosa cells of developing follicles, ranging from primary follicles with a single layer of cuboidal-shaped granulosa cells, preantral follicles with multiple layers of granulosa cells, and large antral follicles. No immunoreactivity was detected in spindle-shaped granulosa cells of primordial follicles, theca interna, theca externa or surrounding stroma. In the corpus luteum, type I 17β-HSOR immunoreactivity was localized solely in granulosa-lutein cells. For comparison, immunoreactive 3β-hydroxysteroid dehydrogenase (3β-HSD) was examined in the same tissues. Both theca interna and granulosa cells of preantral and antral follicles exhibited 3β-HSD staining. Primary follicles did not exhibit detectable 3β-HSD in either granulosa or theca cells. This study serves to demonstrate that in the human ovary, type I 17β-HSOR is compartmentalized in granulosa cells of the developing follicles and granulosa-lutein cells. In addition, the expression of type I 17β-HSOR appears to be present prior to that of 3β-HSD at the stage of primary follicle. With the strategic location of type I 17β-HSOR and the ability to aromatize theca-derived androstenedione to estrone, granulosa cells have the capacity to synthesize and maintain the high intrafollicular levels of 17β-estradiol, which are essential for normal follicular development.