Apoptosis of ovarian granulosa cells: Correlation with the reduced activity of ERK-signaling module

Apoptosis of the ovarian granulosa cells plays a crucial role in the determ ination of the number of follicles destined to ovulate in each reproductive cycle. While the activation of specific apoptotic pathway or the inactivat ion of cell survival pathway can initiate apoptosis, the signaling mechanis m(s) involved in initiating the onset of apoptosis in granulosa cells is no t fully understood. In the present study, using granulosa cells derived fro m eCG-primed immature rats, we investigated the temporal signaling events i nvolved in the onset of apoptosis in the granulosa cells. The administratio n of 15 IU of eCG to 21-day-old immature female rats stimulate the growth a nd development of ovarian follicles until 72 h, after which the granulosa c ells of the ovarian follicles undergo apoptosis due to the waning levels of tropic hormonal support. An analysis of the signaling events leading to ap optosis indicates that the DNA fragmentation can be seen in these cells fro m 96 h. A small increase in the levels of the pro-apoptotic factor Bar can be seen from 96 h while an increase in the activity of JNK can be seen from 108 h onwards. By contrast, a reduction in ERK signaling can be seen by 48 h. Similar reduction in Raf-l kinase activity can be discerned from 48 h o nwards. A concomitant decrease in the phosphorylated form of Bad can also b e detected. These findings taken together, suggest that the loss of tropic hormone support is translated into the attenuation of Raf-1-MEK-ERK signali ng pathway and this reduction along with a reduction in the levels of phosp horylated form of Bad triggers the onset of apoptosis in the ovarian granul osa cells. (C) 1999 Wiley-Liss, Inc.