Estradiol-Mediated Suppression of Apoptosis in the Rabbit Corpus Luteum Is Associated with a Shift in Expression of bcl-2 Family Members Favoring Cellular Survival1

In the rabbit, estradiol is the primary luteotropic hormone. Estradiol withdrawal results in a rapid decline in serum progesterone and eventually in corpus luteum (CL) regression. The objective of this study was to determine whether estradiol modulates luteal cell apoptosis. In the first experiment, rabbits were randomly assigned to one of five experimental groups. An empty capsule (control) or estradiol-filled Silastic capsule was inserted s.c. on Day 0 of pseudopregnancy (day of hCG administration). On Day 11 of pseudopregnancy, some of the group I (control) and group II (estradiol capsule) rabbits were subjected to laparotomy, and one ovary from each rabbit was perfused in vitro to determine progesterone secretion rates. The CL from the contralateral ovary were dissected, snap-frozen, and stored at 2708C until analyzed 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 above. 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 revealed an increase in bax mRNA levels and a decrease in bcl-x mRNA levels coincident with luteal cell apoptosis induced by estradiol withdrawal. These data demonstrate that changes in progesterone production caused by estradiol exposure and deprivation are in part related to luteal cell apoptosis, and alterations in the expression of bclAccepted May 18, 1998. Received October 31, 1997. 1This study was supported by the Rockefeller Foundation (A.M.D.), National Institute of Health Grants R01-HD19430 (A.M.D.), R01HD34226 (J.L.T.), and R01-AG12279 (J.L.T.), National Health and Medical Research Council of Australia (A.M.D.), Australian Research Council (A.M.D.), Raine Foundation (A.M.D.), and a Johns Hopkins University Institutional Research Grant. 2Correspondence. FAX: 61–8-9380–1051; e-mail: [email protected] 3Current address: 2804 Fountain Grove Terrace, Olney, MD 20832. 4Current address: Department of Ob/Gyn, The University of Tokyo Faculty of Medicine, 7–3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan. 5Current address: 2001 Marcus Avenue, Suite N213, Lake Success, NY 11042. 6Current address: Department of Gyn/Ob, Ramathibodi Hospital, Mahidol University School of Medicine, Bangkok, Thailand. 2 gene family members may be one of the mechanisms by which estradiol exerts its luteotropic effect in the rabbit CL.