Expression and regulation of cytochrome P450 17 alpha-hydroxylase messenger ribonucleic acid levels and androstenedione production in hen granulosa cells.

Studies were conducted to evaluate the expression and regulation of cytochrome P450 17a-hydroxylase (P450 17a-OH) mRNA levels and androstenedione production in 6-8-mm-diameter follicle granulosa cells from the domestic hen. Although P450 17a-OH mRNA was detected within granulosa cells from follicles at all stages of development (3-12-mm and preovulatory follicles), 812-fold higher levels were found in small, less developed follicles (3-12 mm in diameter) compared to the three largest follicles within the preovulatory hierarchy (F 3 to F, follicles). By comparison, P450 17a-OH mRNA levels were 25-89-fold higher in theca tissue compared to the granulosa layer at comparable stages of development. Despite detection of P450 17a-OH mRNA in granulosa cells from 6-8-mm follicles, androstenedione production was low to nondetectable when cells were cultured in the presence of exogenous progesterone (10 ng/ml). Treatment with FSH increased levels of P450 17a-OH mRNA (by 6-fold after 16 h of treatment) and induced androstenedione production in cultured granulosa cells; these actions were mimicked by the cAMP analog, 8-bromo-cAMP. By contrast, addition of the growth factors transforming growth factor a (TGFa), epidermal growth factor (EGF), and basic fibroblast growth factor (bFGF) completely suppressed the stimulatory effects of FSH on both mRNA levels and androstenedione production, while insulin-like growth factor I (IGF-I) inhibited only androstenedione production. We conclude that in the granulosa layer of developing ( 12 mm) hen follicles the expression of P450 17a-OH mRNA does not directly reflect P450 17a-OH enzyme activity. Moreover, P450 17a-OH mRNA levels dramatically decrease in granulosa cells from preovulatory (compared to developing) follicles, and in the theca layer from the F, preovulatory follicle (compared to remaining follicles). It is proposed that FSH initiates P450 17a-OH enzyme activity (and perhaps to a lesser extent regulates mRNA levels) at the time a follicle is recruited into the preovulatory hierarchy, and that this action is mediated, at least in part, by the adenylyl cyclase/cAMP second messenger pathway. On the other hand, growth factors (including TGFa, EGF, bFGF, and possibly IGF-I) may act to prevent premature expression of P450 17a-OH activity in the granulosa layer of follicles that have yet to enter the rapid growth phase of follicle development.