Progesterone maintains large rat granulosa cell viability indirectly by stimulating small granulosa cells to...

Ovarian follicles are composed of small and large granulosa cells (GCs). Since progesterone (P4) inhibits large GCs from undergoing apoptosis, studies were designed to determine whether both sizes of GCs bind P4. These studies revealed that fluorescein isothiocyanate-BSA-P4 bound only to the surface membranes of small GCs. This binding was steroid-specific and inhibited by an antibody directed against the ligand-binding domain of the nuclear P4 receptor (PR). In addition, a cell-impermeable derivative of P4, BSA-conjugated P4, was as effective as P4 in preventing apoptosis. Quantitative in situ hybridization studies showed that P4 increased the relative amount of basic fibroblast growth factor (bFGF) mRNA expressed per cell as well as the percentage of small GCs that expressed bFGF. To determine whether the anti-apoptotic action of P4 was mediated by bFGF, GCs were cultured in control medium supplemented with either P4, a neutralizing antibody to bFGF, or both P4 and the bFGF antibody. The results from this study demonstrated that P4 suppressed apoptosis and that this effect was attenuated in presence of the bFGF antibody. Basic FGF also prevented GC apoptosis, and its action was not influenced by either the PR antagonist (RU-486), an inhibitor of P4 synthesis (aminoglutethimide), or a PR antibody. Finally, FGF receptors were detected on both small and large GCs. Collectively, these data support the hypothesis that P4 acts through a putative membrane receptor on small GCs to stimulate the synthesis of bFGF. Basic FGF then activates its receptors within large GCs, and this initiates a signal transduction pathway that maintains large GC viability.