Epidermal growth factor (EGF) is mitogenic to preantral follicles, and transforming growth factor β (TGFβ) influences ovarian cell functions in a variety of species. Although an interaction of these ligands during preantral folliculogenesis is likely, whether EGF influences TGFβ action on preantral follicles by modulating TGFβ receptor (TβR) gene transcription and translation is not known. To determine whether EGF influenced TβR mRNA and protein levels in granulosa cells during preantral folliculogenesis, hamster preantral follicles at stages 1–6 were cultured in the absence or presence of EGF and follicular TβR mRNA, and protein levels were monitored by semiquantitative reverse transcription polymerase chain reaction and immunoblotting, respectively. Both TβR type I (TβRI) and TβR type II (TβRII) mRNA and protein were present in preantral follicles, and their expression was up-regulated by EGF in a stage-dependent manner. However, EGF effect on the expression of TβRI and TβRII was differential. In contrast to TβRI, EGF-stimulation of follicular TβRII mRNA expression was evident from stages 1 and 2 onwards, and more than twofold induction was noted for stages 4–6. Moreover, significant increases in thecal TβR mRNA levels were noted for stage 6 follicles. Follicles at smaller stages appeared to be more sensitive to EGF than were larger preantral follicles. Despite an increase in the cytosolic form of TβRI protein for most of the stages and TβRII protein for follicles at stages 4 and 5, EGF-stimulation of the membrane-associated form of the receptor was restricted to follicles at stage 6. Functionally, TGFβ1 attenuated EGF-induced DNA synthesis for follicles at stages 1–3 and 6 without affecting EGF-induced progesterone production for most of the stages. Administration of α-amanitin resulted in a significant reduction of EGF-induction of TβR mRNA levels, suggesting that increased receptor protein levels were a consequence of mRNA synthesis. These results indicate that an interaction between EGF and TGFβ forms an important regulatory mechanism for preantral folliculogenesis. The effect of EGF on TβRI and TβRII gene transcription and translation are differential, and follicular response to EGF depends on the developmental status of the follicles.
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