The regulation of ovarian granulosa cell death by pro- and anti-apoptotic molecules

In the mammalian ovary, follicular development and atresia are closely regulated by cell death and survival-promoting factors, including hormones (gonadotropins) and intraovarian regulators (gonadal steroids, cytokines, and intracellular proteins). Several hundred thousand primordial follicles are present in the mammalian ovary; however, only a limited number of primordial follicles develop to the preovulatory stage and ovulate. The others, more than 99% of follicles, will be eliminated via a degenerative process known as "atresia". The endocrinological regulatory mechanisms involved in follicular development and atresia have been characterized to a large extent, but the precise temporal and molecular mechanisms involved in the regulation of these events have remained largely unknown. Recent studies suggest that the apoptosis of ovarian granulosa cells plays a major role in follicular atresia. In this review, we provide an overview of development and atresia of follicles, and apoptosis of granulosa cells in mammals.     

cFLIP regulates death receptor-mediated apoptosis in an ovarian granulosa cell line by inhibiting procaspase-8 cleavage

More than 99% of follicles in mammalian ovaries undergo atresia, but the mechanisms regulating the strict selection process are still unclear. Granulosa cell apoptosis is considered the trigger of follicular atresia, which occurs in advance of the death of an oocyte. Cellular FLICE-like inhibitory protein (cFLIP), a homologue of procaspase-8 (also called FLICE), is an intracellular anti-apoptotic protein. It is expressed in granulosa cells of porcine ovaries, where its levels decreases during follicular atresia. We hypothesized that cFLIP regulates granulosa cell apoptosis by acting as a pro-survival factor. In the present study, to further reveal the function of cFLIP in granulosa cells, we examined the anti-apoptotic mechanism of cFLIP using KGN, a human granulosa tumor cell line. Fas-mediated apoptosis was induced by co-treatment with anti-Fas antibody (CH-11), which acts as an agonist of Fas-ligand, and cycloheximide (CHX). When cFLIP was stably expressed in KGN cells following transfection of an expression vector, the Fas-mediated apoptosis was inhibited. Suppression of cFLIP by small interfering RNA (siRNA) spontaneously induced cell death. Silencing of cFLIP promoted cleavage of procaspase-8, and the cell death caused by cFLIP siRNA was completely blocked by a caspase-8 inhibitor (Z-IETD-FMK), indicating that cFLIP regulates apoptosis in KGN cells by inhibiting cleavage of procaspase-8. In conclusion, cFLIP is an essential pro-survival factor for granulosa cells, and it prevents granulosa cell apoptosis by inhibiting procaspase-8 activation.