Thyroid hormone effects on mouse oocyte maturation and granulosa cell aromatase activity

In the present study we evaluated the role of T-3 on the in vitro processes of mouse cumulus cell-oocyte complex expansion, oocyte meiotic maturation, and granulosa cell aromatase activity. Results obtained from cumuli oophori isolated from immature and adult mice ovaries demonstrated that T-3 at all concentrations tested (0.1-100 nM) did not affect basal or FSH-induced cumulus expansion or interfere with oocyte meiotic maturation up to metaphase II stage. On the contrary, T-3 inhibited in a time- and dose-dependent manner FSH-induced aromatase activity in cultured granulosa cells obtained from either adult or immature female mice. The half-maximal dose (ED50) of T-3 inhibition was 0.87 +/- 0.21 nM, which is in agreement with the reported dissociation constant of T-3 nuclear receptor (K-d = 0.4-5 nM) in mammalian granulosa cells. Time-course experiments demonstrated higher sensitivity to T-3 of adult granulosa cells with respect to immature granulosa cells in culture. Indeed, in immature granulosa cells T-3 inhibition became significantly evident only after 6 days of hormonal treatment, whereas in adult granulosa cells the inhibitory effect was present after only 2 days of treatment. (Bu)(2)cAMP- or 3-isobutyl-1-methyl-xanthine-stimulated aromatase activity was also significantly decreased by T-3, thus suggesting that the inhibition was downstream from cAMP formation. Lastly, analysis of aromatase messenger RNA (mRNA) levels by semiquantitative RT-PCR demonstrated the ability of FSH to increase aromatase mRNA level in cultured granulosa cells by 2.4 +/- 0.5-fold. In agreement with the effect on enzyme activity, the stimulatory effect of FSH on aromatase mRNA level was greatly reduced after T-3 cotreatment. In conclusion, T-3 inhibition of aromatase activity may be of physiological relevance in the complex multihormonal regulation of mammalian follicle development and may contribute to explaining the alteration in female reproductive functions after thyroid hormone hypo- or hypersecretion.