Prostaglandin F_2α-mediated activation of apoptotic signaling cascades in the corpus luteum during apoptosis: Involvement of caspase-activated DNase

Prostaglandin F_2α (PGF_2α) acting via a G protein-coupled receptor has been shown to induce apoptosis in the corpus luteum of many species. Studies were carried out to characterize changes in the apoptotic signaling cascade(s) culminating in luteal tissue apoptosis during PGF_2α-induced luteolysis in the bovine species in which initiation of apoptosis was demonstrable at 18 h after exogenous PGF _2α treatment. An analysis of intrinsic arm of apoptotic signaling cascade elements revealed that PGF_2α injection triggered increased ratio of Bax to Bcl-2 in the luteal tissue as early as 4 h posttreatment that remained elevated until 18 h. This increase was associated with the elevation in the active caspase-9 and -3 protein levels and activity (p < 0.05) at 4-12 h, but a spurt in the activity was seen only at 18 h posttreatment that could not be accounted for by the changes in the Bax/Bcl-2 ratio or changes in translocation of Bax to mitochondria. Examination of luteal tissue for FasL/Fas death receptor cascade revealed increased expression of FasL and Fas at 18 h accompanied by a significant (p < 0.05) induction in the caspase-8 activity and truncated Bid levels. Furthermore, intrabursal administration of specific caspase inhibitors, downstream to the extrinsic and intrinsic apoptotic signaling cascades, in a pseudopregnant rat model revealed a greater importance of extrinsic apoptotic signaling cascade in mediating luteal tissue apoptosis during PGF_2α treatment. The DNase responsible for PGF_2α-induced apoptotic DNA fragmentation was found to be Ca²⁺/Mg²⁺-dependent, temperature-sensitive DNase, and optimally active at neutral pH conditions. This putative DNase was inhibited by the recombinant inhibitor of caspase-activated DNase, and immunodepletion of caspase-activated DNase from luteal lysates abolished the observed DNA fragmentation activity. Together, these data demonstrate for the first time temporal and spatial changes in the apoptotic signaling cascades during PGF _2α-induced apoptosis in the corpus luteum.