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

Abstract

Apoptosis of the ovarian granulosa cells plays a crucial role in the determination of the number of follicles destined to ovulate in each reproductive cycle. While the activation of specific apoptotic pathway or the inactivation of cell survival pathway can initiate apoptosis, the signaling mechanism(s) involved in initiating the onset of apoptosis in granulosa cells is not fully understood. In the present study, using granulosa cells derived from eCG-primed immature rats, we investigated the temporal signaling events involved in the onset of apoptosis in the granulosa cells. The administration of 15 IU of eCG to 21-day-old immature female rats stimulate the growth and development of ovarian follicles until 72 h, after which the granulosa cells of the ovarian follicles undergo apoptosis due to the waning levels of tropic hormonal support. An analysis of the signaling events leading to apoptosis indicates that the DNA fragmentation can be seen in these cells from 96 h. A small increase in the levels of the pro-apoptotic factor Bax 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-1 kinase activity can be discerned from 48 h onwards. A concomitant decrease in the phosphorylated form of Bad can also be detected. These findings taken together, suggest that the loss of tropic hormone support is translated into the attenuation of Raf-1-MEK-ERK signaling pathway and this reduction along with a reduction in the levels of phosphorylated form of Bad triggers the onset of apoptosis in the ovarian granulosa cells.