Molecular mechanisms of FSH secretion induced by novel

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Previous studies reported that activin alone increased FSH receptor levels by using receptor binding assay and various hormone regulated FSH receptor mRNA levels in cultures rat granulosa cells. In this study, we examined the effect of activin A on FSH receptor mRNA in cultured rat granulosa cells using a FSH receptor cRNA probe. Northern blot analysis with cRNA of FSH receptor detected mRNA transcripts of 5.5- and 2.4-kb. Both transcripts are identical to size which have been reported previously in our in vivo studies. Our data showed that activin alone stimulated FSH receptor mRNA transcripts as well as FSH receptor levels in a concentration-dependent manner. Previous studies reported that FSH may maintain its own receptor mRNA and various growth factors as epidermal growth factor(EGF), basic fibroblast growth factor(bFGF), and insulin-like growth factor-I(IGF-I) did not alter basal levels of FSH receptor mRNA but EGF or bFGF dose-dependently attenuated the stimulatory actions of FSH on FSH receptor mRNA levels. Therefore, our results indicated that the stimulatory action of activin on the levels of the FSH receptor is closely related to the stimulatory action of activin on FSH receptor mRNA levels, and compared with other growth factors, activin may play a important role in the induction of the FSH receptor.To understand the molecular mechanism of activin action, we have cloned the rat type II activin receptor from a rat ovary cDNA library. This result revealed that the receptor intracellular domain was a serine/threonine-specific protein kinase. The results that the granulosa cells contained two predominant rat type II activin receptor mRNA transcripts (6.0 and 3.0 kilobases). Furthermore, in vivo experiments showed that the mRNA levels of activin receptor in immature rat ovary has been induced according to the maturation of follicles and even after ovulation it continued to increase.