In Vitro Effects of Epidermal Growth Factor, Insulin-Like Growth Factor-I, Fibroblast Growth Factor, and Follicle-Stimulating Hormone on Hamster Follicular Deoxyribonucleic Acid Synthesis and Steroidogenesis

Abstract

Preantral (stages 1–6) and antral (stages 7–10) follicles from proestrous hamsters were exposed for 24 h to 1, 5, 10, 50, and 100 ng/ml of epidermal growth factor (EGF), insulin-like growth factor-I (IGF-I), and fibroblast growth factor (FGF) and 1 µCi [³H]thymidine to determine the rate of DNA replication. FSH (100 ng/ml) was used as a positive control. Synergism was also studied by using suboptimal doses of growth factors (1 ng/ml) and FSH (5 ng/ml). Optimal doses (50 ng/ml) of EGF, IGF-I, and FGF, and 100 ng/ml FSH significantly enhanced follicular DNA replication, whereas suboptimal doses of FSH, EGF, and IGF-I affected only a few preantral stages, and FGF was totally ineffective. FGF significantly inhibited DNA synthesis induced by a suboptimal dose of EGF but did not affect IGF-I-induced DNA replication. Paradoxically, a combination of suboptimal doses of all three growth factors significantly (p < 0.05) stimulated DNA synthesis for all stages; FSH (5 ng/ml) had no additive effect. FSH significantly stimulated follicular progesterone (P₄), androstenedione (A), and estradiol-17β (E₂) accumulation, but significant P₄ accumulation from stages 3–10 was observed only after optimal EGF exposure; A and E₂ accumulations were unaffected by any of the growth factors. These results indicate that EGF, IGF-I, and FGF stimulate DNA replication to hamster preantral and antral follicles, and may play roles as intraovarian factors regulating folliculogenesis. Moreover, the threshold to initiate the S phase of the cell cycle depends on the total magnitude of stimulation delivered either by a single threshold stimulus or a combination of multiple subthreshold stimuli. It is possible that growth factors and FSH may involve a cascade of events that culminates in the initiation of DNA duplication. From the results of the present and our previous studies, it appears that EGF is involved in proliferation as well as P₄ biosynthesis of hamster follicular cells depending on the stage of development.