Regulation of Fas antigen (Fas, CD95)-mediated apoptosis of bovine granulosa cells by serum and growth factors

Expression and regulation of Fas antigen and tumor necrosis factor receptor type I in hen granulosa cells

It is now well established that vertebrate ovarian follicles undergo atresia via apoptosis, a process that is initiated within the granulosa cell layer of undifferentiated follicles. Although the exact signals, membrane-bound receptors, and associated intracellular signaling pathways leading to apoptosis within granulosa cells have yet to be established, it is evident that multiple and redundant pathways exist. Fas, together with its ligand, has been the most commonly studied death-inducer in the mammalian ovary; however, nothing is currently known regarding expression of either Fas or the related tumor necrosis factor receptor type 1 (TNFR1), in avian species. Based on characterization of a chicken fas partial cDNA, which includes the entire death domain, the deduced amino acid sequence shows 37% identity (53% positive) to human Fas. Northern blot analysis demonstrates low expression of the 2.0-kilobase fas transcript in most tissues, including the granulosa layer, and highest levels are found in the spleen, theca tissue, and the postovulatory follicle. Significantly, fas and tnfr1 mRNA levels are higher in atretic follicles than in nonatretic, prehierarchal (3- to 8-mm diameter) follicles. Moreover, both fas and tnfr1 mRNA levels are up-regulated by twofold to eightfold in granulosa cells following plating in the presence of fetal bovine serum, with the most dramatic increase found in fas expression within prehierarchal follicle granulosa. Coculture with transforming growth factor (TGF) beta attenuates this increase for both receptors, whereas cAMP attenuates only the up-regulation of fas. By comparison, treatment with TGFalpha enhances expression of tnfr1, but not fas, mRNA. Taken together, these data are the first to implicate fas as a mediator of granulosa cell apoptosis in a nonmammalian vertebrate, and to implicate the protein kinase A signaling pathway in down-regulating fas expression. In addition, data provided demonstrate the presence of multiple death domain-containing TNFR family members simultaneously expressed within hen granulosa cells, each of which may be regulated by separate signaling pathways.

Susceptibility of ovarian granulosa cells to apoptosis differs in cells isolated before or after the preovulatory LH surge

The luteinizing hormone (LH) surge initiates the final stages of ovarian follicle development, and induces ovulation and luteinization of preovulatory follicles. To investigate whether exposure to the LH surge alters follicle cell susceptibility to apoptosis, granulosa and theca cells were isolated from bovine preovulatory follicles before and 14 h after injection of GnRH to induce an LH surge. Granulosa cells isolated before the LH surge were susceptible to apoptosis induced by soluble Fas ligand or serum withdrawal, while cells isolated after the LH surge were resistant to apoptosis. Resistance to Fas-mediated apoptosis was not associated with decreased Fas mRNA or protein levels. Pretreatment of granulosa cells isolated after the LH surge with the protein synthesis inhibitor cycloheximide rendered the cells susceptible to Fas-mediated apoptosis, indicating that inhibition of apoptosis was mediated by expression of labile survival factors. Theca cells were sensitive to Fas-mediated apoptosis before and after exposure to the LH surge. Resistance to apoptosis of granulosa cells from preovulatory follicles after the LH surge may be important for normal ovulation and luteinization.

Apoptosis of bovine granulosa cells after serum withdrawal is mediated by Fas antigen (CD95) and Fas ligand

Ovarian follicular atresia occurs by apoptosis of granulosa and theca cells. The Fas antigen (Fas), a cell surface receptor that triggers apoptosis when activated by Fas ligand (FasL), may be involved in this process. A possible role of the Fas pathway in mediating serum withdrawal-induced apoptosis of granulosa cells was examined. Granulosa cells collected from 5- to 10-mm bovine follicles were cultured in DMEM-F12 containing serum for 3 days, deprived of serum, and live cells were counted at various times after serum withdrawal. Cell death increased significantly 6 h after serum withdrawal (21% +/- 7%; P: < 0.05 vs. 0 h) and continued to increase until 24 h (43% +/- 6%). No further increases in cell death were observed through 72 h. Detection of the translocation of phosphatidylserine to the outer surface of the cell membrane by annexin V binding indicated that cells died by apoptosis. Quantitative reverse transcriptase-polymerase chain reaction assays showed no changes in Fas mRNA levels but a 4.7-fold increase in FasL mRNA 3 h after serum withdrawal (P: < 0.05 vs. 0 h). FasL mRNA remained elevated through 24 h and returned to basal levels at 48 h. Immunohistochemical staining showed that both Fas and FasL protein increased on the cell surface within 3 h and remained elevated through 12 h (the last time point tested). Binding of FasL to Fas was blocked with two reagents that bind to the extracellular domain of FasL: an anti-FasL antibody and Fas:Fc, a chimeric protein consisting of the Fc portion of human immunoglobulin G and the extracellular domain of human Fas. Cell death 24 h after serum withdrawal was reduced 55% +/- 10% and 34% +/- 12% by anti-FasL antibody and Fas:Fc, respectively (P: < 0.05 vs. no blocking protein). In conclusion, serum withdrawal-induced apoptosis of bovine granulosa cells is mediated at least partially by Fas/FasL interactions. These results are consistent with a potential role of Fas in an autocrine or paracrine pathway to trigger ovarian follicular atresia.