Epidermal growth factor and TGFalpha promote zebrafish oocyte maturation in vitro: potential role of the ovarian activin regulatory system.

Epidermal growth factor (EGF) and TGFalpha are well known for their activities in the ovary. Both factors initiate signal transduction by binding to the common EGF receptor that has been demonstrated in the ovary across vertebrates from fish to humans. Using zebrafish as the model, we demonstrated in the present study that recombinant human EGF and TGFalpha significantly enhanced final maturation of the fully grown, follicle-enclosed oocytes (0.58-0.65 mm) in vitro in a clear time- and dose-dependent manner. The effect of EGF/TGFalpha was additive to that of hCG at low concentrations, but the additivity diminished when the concentration increased. Both actinomycin D and cycloheximide completely blocked the effect of EGF/TGFalpha, indicating that the promotion of oocyte maturation by EGF/TGFalpha requires de novo mRNA transcription and protein synthesis. Interestingly, the effect of EGF/TGFalpha could be blocked by cotreatment with follistatin, a potent binding protein for activin, an ovarian growth factor belonging to the TGFbeta superfamily. Semiquantitative RT-PCR assays showed that both EGF and TGFalpha significantly stimulated the expression of activin betaA and activin type II receptor in the cultured zebrafish ovarian follicle cells in a clear time- and dose-dependent manner. This together with our previous report that activin had a potent stimulatory effect on zebrafish oocyte maturation strongly suggests that the intrinsic ovarian activin system is probably a downstream mediator of EGF/TGFalpha actions in the zebrafish ovary.