The mechanisms of transforming growth factor-β (TGF-β) potentiation of FSH action on follicular DNA synthesis in the hamster were investigated by use of TGF-β (β1 and β2) and a potent epidermal growth factor (EGF)- specific polyclonal antibody. Preantral follicles at stages 1-6 and early antral follicles at stage 7 were enzymatically and mechanically isolated from adult golden hamsters on proestrus. Follicles were incubated in Dulbecco's modified Eagle's medium with ITS+ (6.25 μg insulin, 6.25 μg transferrin, 6.25 μg selenium, 1.25 mg BSA, and 5.35 μg linoleic acid/ml) hydrocortisone, and 1 μCi/ml [3H]thymidine. Follicles were exposed for 24 h to FSH (100 ng/ml), TGF-β1, and TGF-β2 (1 and 10 ng/ml), EGF antibody (50 μl/ml), and TGF-α (50 ng/ml) under various experimental conditions. Both TGF-β1 and TGF-β2 significantly potentiated FSH-induced follicular DNA synthesis; however, the effect was drastically attenuated by EGF antibody. EGF antibody also inhibited FSH-induced follicular [3H]thymidine incorporation. TGF-β2 potentiation of FSH action was also significantly inhibited by a second dose of TGF-β2 when added 6 h after the beginning of culture; however, follicles at different stages responded differently to the second dose of TGF-β2. For example, when the second dose of TGF-β2 was added 14 h after the first dose, only the large preantral follicles at stages 4-6 were affected. Interestingly, the second dose of TGF-β2 also inhibited TGF-β2-induced DNA synthesis. The inhibitory effect of EGF antibody on TGF- β2 and FSH synergism was effectively reversed by TGF-α, which was not recognized by the EGF antibody. These results suggest that TGF-β potentiation of FSH-induced follicular [3H]thymidine incorporation involves a latent action of follicular EGF. TGF-β appears to lengthen the G1 phase of the follicular cell cycle and synchronize a significantly higher number of cells at the G1/S boundary, thus allowing more cells to enter the S phase and a resultant increase in net [3H]thymidine incorporation.
ASJC Scopus subject areas
- Cell Biology