Development of cytochrome P450 17\g=a\-hydroxylase (P450c17) mRNA and enzyme activity in neonatal ovaries of normal and hypogonadal (hpg) mice S A Gray, M A Mannan and P J O'Shaughnessy ABSTRACT The cytochrome P450 enzyme 17\g=a\-hydroxylase (P450c17) is required for androgen synthesis and therefore regulates substrate supply for aromatiz- ation. In this study, changes in P450c17 activity and mRNA levels were measured during ovarian development in the normal mouse and in the hypogonadal (hpg) mouse which lacks circulating gonadotrophins. At birth, low levels of P450c17 activity and mRNA were detectable in normal ovaries. This basal level of expression did not change until after day 10 at which time both enzyme activity and mRNA levels increased by six- to eightfold. In the hpg mouse, levels of P450c17 mRNA were normal at birth but did not change significantly during subsequent development and were significantly less than normal by day 15. Results show that there is a low level of gonadotrophin-independent expression of P450c17 in the ovary at birth and that gonadotrophins are required for the subsequent increase in expression between days 10 and 15. In the ovary, P450c17 is expressed solely in the thecal/interstitial compart- ment and interstitial cells arise in the mouse ovary around day 11. Changes in P450c17 are likely, therefore, to be related to gonadotrophin-dependent development of the interstitial tissue in the mouse. Treatment of adult hpg mice with LH and FSH showed that both gonadotrophins can act to increase P450c17 activity. Since FSH acts only on the granulosa cell compartment of the ovary it is likely that FSH acts through a paracrine mechanism to regulate thecal/interstitial cell activity. Journal of Molecular Endocrinology (1996) 17, 55-60 INTRODUCTION In the ovary of most mammalian species, produc¬ tion of oestrogens requires both thecal and granulosa cells (Hillier et al. 1994). The thecal compartment is responsible for the production of androgens through the actions of the cytochrome P450 enzymes cholesterol side-chain cleavage (P450scc) and 17a-hydroxylase (P450cl7). The P450scc enzyme converts cholesterol to pregnen- olone while the P450cl7 enzyme catalyses con¬ version of C21 steroids to C19 androgens through two mixed function oxidase reactions, 17a- hydroxylation and C17—C20 cleavage. This androgen is then converted by cytochrome P450 aromatase (P450arom) to oestrogen in the granulosa cells. Each of these enzymes has been shown to be gonadotrophin-dependent in mature ovaries and follicles (Bogovich & Richards 1982, Funkenstein et al. 1984, Goldring et al. 1987, Fitzpatrick & Richards 1991) but, until recently, factors regulat¬ ing the expression of these enzymes during early follicular and ovarian development were largely unknown. We have begun to examine the develop¬ mental regulation of the steroidogenic P450 en¬ zymes in the ovary using the hypogonadal (hpg) mouse which lacks circulating gonadotrophins (Cattanach et al. 1977, Mason et al. 1986). We have shown, using the hpg mouse, that there is early gonadotrophin-independent expression of P450scc and P450arom in the ovary when only primor¬ dial follicles and stromal tissue are present (O'Shaughnessy & Mannan 1994, Gray et al. 1995). As primary follicles develop, however, there is increased, gonadotrophin-dependent expression of P450arom while expression of P450scc becomes gonadotrophin-dependent as the secondary follicles develop. Changes in P450cl7 expression are Veterinary Reproduction Research Group, Department of Veterinary Physiology, University of Glasgow Veterinary School, Bearsden Road, Glasgow G61 1QH, UK (Requests for offprints should be addressed to P J O'Shaughnessy)