Studies on the molecular basis of hormone action during granulosa cell differentiation included evaluation of the functions and mechanisms of action of growth factors and plasminogen activator. TGF-beta was previously found to exert bifunctional actions on the maturation of granulosa cells, and to modulate FSH-induced stimulation of cAMP formation, steroidogenesis, and LH receptor expression in a concentration-dependent manner. TGF-beta amplified gonadotropin responses in the presence of small amounts of FSH, but had less effect or even inhibited the action of higher FSH levels in the presence of insulin. A novel action of TGF-beta on ovum maturation was observed in oocytes of immature gonadotropin- secreted rats. The growth factor accelerated the maturation of both follicle-enclosed oocytes and cumulus-oocyte complexes, with significant increases in the rate of germinal vesicle breakdown. This effect of TGF-beta was manifested with unusual rapidity, being detectable after one hour, and required the presence of the surrounding cumulus cells. Other growth factors including IGF-I, IGF-II, and EGF also stimulated germinal vesicle breakdown. These actions of growth factors, in conjunction with the effects of gonadotropic hormones, may regulate the meiotic maturation of oocytes during follicle development. Studies on the actions of gonadotropins on granulosa cell function revealed that FSH regulates the biosynthesis of a cell-associated tissue plasminogen activator. The enzyme was produced during the initial hours of granulosa cell maturation and was localized in the extracellular matrix laid down by the cells, suggesting that its presence in the basement membrane could be an important factor in the acquisition of the epithelial phenotype by granulosa cells during differentiation.
