Abstract
In order to elucidate the mechanism of thyroid hormone action on the ovary, direct effects of L-thyroxine (T4) or L-triiodothyronine (T3) on steroidogenic enzyme induction were investigated in vitro using a monolayer culture system of porcine granulosa cells obtained from small follicles.
The cells were cultured in the absence or presence of porcine FSH (20ng/ml) for 6 days, with or without T4 or T3, under sparsely (4%) serum supplemented condition. The mechanism of thyroid hormone action on the granulosa cells was studied by testing the capability of thyroid hormone to enhance the steroidogenesis in response to exogenously provided substrates. Concomitant treatment with FSH (20ng/ml) and T4 (10-7 M) caused a further increased production of progesterone in response to the addition of pregnenolone compared to that in the absence of pregnenolone. The same treatment with FSH and T4 also caused a further increased production of estrone in response to the addition of androstenedione. Concomitant treatment with 10-9 M T3 demonstrated similar stimulatory effects on the steroidogenesis in cultured granulosa cells. T4 or T3 alone without FSH was incapable of exhibiting these stimulatory effects.
Furthermore, aromatase activity in cultured granulosa cells assessed by the release of tritiated water from [1β-3H, 4-14 C] androstenedione was significantly higher in the cells treated concomitantly with FSH (20ng/ml) and T4 (10-7M) than that in the cells treated with FSH alone.
These results suggest that thyroid hormone synergizes with FSH and increases FSH-mediated induction of 3β-hydroxysteroid dehydrogenase (3β-HSD) and aromatase activity in immature granulosa cells. Since the effective dose of T4 and T3 observed in our studies is in the physiological range of circulating total levels of T4 and T3, it can be concluded that the synergism between FSH and thyroid hormone is of physiological importance to the full expression of FSH actions in the functional differentiation of immature granulosa cells.
