FSH stimulates DNA synthesis by hamster preantral follicles both in vivro and in vitro, and the in vitro mitogenic effect of FSH is effectively reproduced by epidermal growth factor (EGF). To determine whether follicular EGF is the intracellular transducer of FSH action on hamster preantral follicles, intact follicles at stages 1 to 7 (stages 1-4 = preantral follicles 1-4 layers of granulosa cells, respectively, and no theca; stages 5-6 = 5-6 and 7-8 layers granulosa cells, respectively, and developing theca; and stage 7 = follicles with incipient antrum) were cultured for 24 h in a serum-free culture medium in the absence or presence of 100 ng FSH, 50 ng EGF, 50 ng transforming growth factor-α (TGFα), or 1 μmol 8-Br-cAMP and challenged with 50 μl polyclonal antimurine EGF antiserum; the rate of DNA synthesis was determined by [3H]thymidine incorporation. FSH, EGF, and TGFα significantly (P < 0.05) stimulated follicular DNA synthesis; TGFα per se was less effective than either FSH or EGF. However, both FSH- and EGF- induced DNA synthesis was drastically attenuated by EGF antiserum; TGFα effect remained undisturbed. Interestingly antibody inhibition of FSH-induced DNA synthesis was totally reversed by coexposure to TGFα. Follicular DNA synthesis for most stages was stimulated by Br-cAMP, but the effect was significantly (P < 0.05) inhibited by EGF antibody. Moreover, follicles at different stages responded to EGF with different latency. These results strongly suggest that FSH-induced follicular DNA synthesis in the hamster is mediated by follicular EGF and the pathway of events is FSH action → cAMP production → EGF synthesis → cell proliferation.
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