THE ROLE OF ESTROGEN IN THE CONTROL OF RAT OSTEOCYTE APOPTOSIS

We have previously shown that estrogen withdrawal by gonadotrophin-rel easing hormone analogs (GnRHa) induces osteocyte death via apoptosis i n human bone. Although it is likely that the increase in osteocyte dea th via apoptosis was related to the loss of estrogen, these experiment s could not rule out a direct role for the GnRHa. Therefore, in this s tudy, we have used a rat model of ovariectomy (OVX) to determine wheth er the effect of estrogen withdrawal extends to other species and to c larify the role of estrogen in the maintenance of osteocyte viability. Twelve 9-week-old rats were divided into three treatment groups: sham operated (SHAM) (n = 4), OVX (n = 4), and OVX + estrogen (E2) (25 mu g/day) (n = 4). At 3 weeks following the start of treatment, tibial bo nes were removed. The percentage of osteocytes displaying DNA breaks, using an in situ nick-translation method, was significantly higher in the OVX group compared with the SHAM control in both cortical bone (10 .04% vs. 2.31%, respectively; p < 0.0001) and trabecular bone (6.44% v s. 1.58%, respectively; p = 0.003). Addition of estrogen in the OVX an imals completely abrogated the increase in osteocyte apoptosis in cort ical bone (0.78%) and trabecular bone (1.17%). The percentage of apopt otic osteocytes decreased with increasing distance from the primary/se condary spongiosa interface below the growth plate in the OVX model an d the OVX + E2 model. Nuclear morphology and electrophoresis of DNA co nfirmed the presence of apoptotic cells in the samples. In conclusion, OVX in the rat results in an increase in osteocyte apoptosis as a dir ect or indirect result of E2 loss. Addition of estrogen in the OVX ani mals prevents this increase in osteocyte apoptosis. These data confirm an important role for estrogen in the control of osteocyte apoptosis and the maintenance of osteocyte viability. Estrogen deficiency might, through compromising the viability of osteocyte networks, reduce the ability of bone to respond appropriately to loading.