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 oophor i isolated from immature and adult mice ovaries demonstrated that T-3 at al l concentrations tested (0.1-100 nM) did not affect basal or FSH-induced cu mulus expansion or interfere with oocyte meiotic maturation up to metaphase II stage. On the contrary, T-3 inhibited in a time- and dose-dependent man ner FSH-induced aromatase activity in cultured granulosa cells obtained fro m 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 d issociation 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 c ulture. Indeed, in immature granulosa cells T-3 inhibition became significa ntly evident only after 6 days of hormonal treatment, whereas in adult gran ulosa cells the inhibitory effect was present after only 2 days of treatmen t. (Bu)(2)cAMP- or 3-isobutyl-1-methyl-xanthine-stimulated aromatase activi ty was also significantly decreased by T-3, thus suggesting that the inhibi tion was downstream from cAMP formation. Lastly, analysis of aromatase mess enger 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 stimula tory effect of FSH on aromatase mRNA level was greatly reduced after T-3 co treatment. In conclusion, T-3 inhibition of aromatase activity may be of ph ysiological relevance in the complex multihormonal regulation of mammalian follicle development and may contribute to explaining the alteration in fem ale reproductive functions after thyroid hormone hypo- or hypersecretion.