Epidermal growth factor modulates transforming growth factor receptor messenger RNA and protein levels in hamster preantral follicles in vitro.

Abstract

Epidermal growth factor (EGF) is mitogenic to preantral follicles, and transforming growth factor beta (TGFbeta) influences ovarian cell functions in a variety of species. Although an interaction of these ligands during preantral folliculogenesis is likely, whether EGF influences TGFbeta action on preantral follicles by modulating TGFbeta receptor (TbetaR) gene transcription and translation is not known. To determine whether EGF influenced TbetaR 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 TbetaR mRNA, and protein levels were monitored by semiquantitative reverse transcription polymerase chain reaction and immunoblotting, respectively. Both TbetaR type I (TbetaRI) and TbetaR type II (TbetaRII) 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 TbetaRI and TbetaRII was differential. In contrast to TbetaRI, EGF-stimulation of follicular TbetaRII 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 TbetaR 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 TbetaRI protein for most of the stages and TbetaRII 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, TGFbeta1 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 alpha-amanitin resulted in a significant reduction of EGF-induction of TbetaR mRNA levels, suggesting that increased receptor protein levels were a consequence of mRNA synthesis. These results indicate that an interaction between EGF and TGFbeta forms an important regulatory mechanism for preantral folliculogenesis. The effect of EGF on TbetaRI and TbetaRII gene transcription and translation are differential, and follicular response to EGF depends on the developmental status of the follicles.