Expression of Inhibitor of Apoptosis Proteins (IAPs) in Rat Granulosa Cells during Ovarian Follicular Development and Atresia

Abstract

The inhibitor of apoptosis proteins (IAPs) constitute a family of highly conserved apoptosis suppressor proteins that was originally identified in baculoviruses. Although IAP homologs have been recently identified and demonstrated to suppress apoptosis in mammalian cells, their expression and role during follicular development and atresia are unknown. The present study was conducted to address these questions. Using established in vivo models for the induction of follicular development and atresia in immature rats, it was possible to compare the immunolocalization of X-link inhibitor of apoptosis protein (Xiap) and human inhibitor of apoptosis protein-2 (Hiap-2), two members of the IAP family, at defined stages of follicular maturation and to relate the differences observed with those of follicular cell proliferation and apoptosis [as determined by proliferating cell nuclear antigen (PCNA) immunohistochemistry and in situ terminal deoxynucleotidyl transferase-mediated dUTP-biotin end labeling (TUNEL), respectively]. In addition, granulosa cell DNA and proteins were assessed for apoptotic fragmentation by 3′-end labeling/agarose gel electrophoresis (DNA ladder) and Hiap-2 and Xiap protein content by Western blot analysis, respectively. Hiap-2 and Xiap expression in both granulosa and theca cells increased with follicular maturation, reaching maximal levels at the antral stage of development. The immunoreactivity for PCNA, Xiap, and Hiap-2 decreased markedly in atretic (TUNEL-positive) follicles at the small to medium sized antral stage of development, suggesting follicular atresia may be associated with decreased granulosa cell IAP protein content and decreased proliferation. Atresia was also associated with a change in the intracellular distribution of IAPs in granulosa cells. Biochemical analysis of DNA fragmentation (DNA ladder) in granulosa cells from preantral and early antral follicles indicates extensive apoptosis that was associated with minimal IAP protein content. Gonadotropin treatment increased Hiap-2 and Xiap protein content and suppressed apoptosis in granulosa cells, resulting in the development of follicles to the antral and preovulatory stages. In addition, gonadotropin withdrawal induced apoptotic DNA fragmentation in granulosa cells in early antral and antral follicles, which is accompanied by a marked decrease in Hiap-2 and Xiap expression. These data suggest that IAPs may be involved in the suppression of granulosa cell apoptosis by gonadotropin in small to medium-sized antral follicles and play an important role in determining the fate of the cells, and thus also the eventual follicular destiny (atresia vs. ovulation).