Estrogen inhibits Fas-mediated apoptosis in experimental stroke

INTRODUCTION. It is well known that 17 beta-estradiol protects against brain injury in experimental stroke, and the mechanism (s) is likely multi-factorial. However, neuronal apoptosis following stroke plays a pivotal role in the pathological processes of brain ischemia and reperfusion[1]. Increasing evidence showed that the Fas/FADD signaling pathway is integral to apoptotic cell death [2]. Recent data show that increased expression of both Fas and Fas ligand in the ischemic rat brain and that infarction and measures of apoptosis are reduced in mice with a loss-of-function mutation in Fas [3]. The aim of our experiments was to determine whether brain injury after middle cerebral occlusion (MCAO) involves Fas/FADD pathway and if estradiol acts as an anti-apoptotic agent via this pathway. METHODS. Ovariectomy in mice is performed 1-2 weeks before the experiment, and estradiol is replaced by implanting a silastic capsule containing 35 ul of 180 ug/ml 17 beta-estradiol (E2, 7 days), as previously described [4]. MCAO was produced using the intraluminal suture technique [5]. Ischemic or contralateral cortex was collected at 3, 6 or 12 hr post-MCAO and analyzed for Fas and FADD mRNA and protein expression using RT-PCR and Western blot, and activation of caspases-8 and caspase-3 was determined fluorometrically. Primary cultured cortical neurons were pretreated with 17 beta-estradiol [6] in the presence and absence of monoclonal anti-Fas antibody for 48 hrs, and apoptotic cells were quantified using fluorescence activated cell sorting (FACS). RESULTS. Fas and FADD mRNA and protein were elevated in ischemic cortex relative to contralateral cortex at 3, 6 and 12 hours post-MCAO (p<0.05), with peak levels at the 6h time point. Estradiol suppressed Fas/FADD mRNA and protein expression at all time points (p<0.05). Cortical caspase-8 and caspase-3 activities were increased at 3, 6 and 12 hours after MCAO (p<0.05) as part of the downstream mechanism of injury. Caspase activity was significantly reduced in estradiol-treated animals. In cultured cells, application of Fas-antibody induced neuronal apoptosis, which was attenuated by estradiol. CONCLUSIONS. Our results indicate that estradiol mitigates Fas/FADD signaling after focal cerebral ischemia and Fas-induced neural apoptosis. We speculate that this mechanism may be an important part of estradiol's anti-ischemic effects in the female brain.