ESTRADIOL-MEDIATED SUPPRESSION OF APOPTOSIS IN THE RABBIT CORPUS-LUTEUM IS ASSOCIATED WITH A SHIFT IN EXPRESSION OF BCL-2 FAMILY MEMBERS FAVORING CELLULAR-SURVIVAL

In the rabbit, estradiol is the primary luteotropic hormone. Estradiol withdrawal results in a rapid decline in serum progesterone and event ually in corpus luteum (CL) regression. The objective of this study wa s to determine whether estradiol modulates luteal cell apoptosis. In t he first experiment, rabbits were randomly assigned to one of five exp erimental groups. An empty capsule (control) or estradiol-filled Silas tic capsule was inserted s.c. on Day 0 of pseudopregnancy (day of hCG administration). On Day 11 of pseudopregnancy, some of the group I (co ntrol) and group II (estradiol capsule) rabbits were subjected to lapa rotomy, and one ovary from each rabbit was perfused in vitro to determ ine progesterone secretion rates. The CL from the contralateral ovary were dissected, snap-frozen, and stored at -70 degrees C until analyze d for internucleosomal DNA cleavage (apoptosis), Estradiol-containing capsules were removed from some of the remaining rabbits on Days 8, 9, and 10 to initiate estradiol deprivation. Rabbits were then subjected to laparotomy 24, 48, or 72 h after capsule removal (groups III, IV, and V, respectively), and ovaries or CL were processed as described ab ove. Deprivation of estradiol for 24 (group III), 48 (group IV), or 72 (group V) h in vivo reduced in vitro progesterone secretion rates by more than 90% as compared to that in ovaries collected from estradiol capsule-intact animals. After in vivo endogenous estradiol suppression , withdrawal of exogenous estradiol resulted in luteal cell apoptosis, which increased in a time-dependent manner. Northern blot analysis re vealed an increase in bar mRNA levels and a decrease in bcl-x mRNA lev els coincident with luteal cell apoptosis induced by estradiol withdra wal. These data demonstrate that changes in progesterone production ca used by estradiol exposure and deprivation are in part related to lute al cell apoptosis, and alterations in the expression of bcl-2 gene fam ily members may be one of the mechanisms by which estradiol exerts its luteotropic effect in the rabbit CL.