Reduction of Caspase-8 and -9 Cleavage Is Associated With Increased c-FLIP and Increased Binding of Apaf-1 and Hsp70 After Neonatal Hypoxic/Ischemic Injury in Mice Overexpressing Hsp70

Abstract

Background and Purpose— Caspase-8 and caspase-9 are essential proteases of the extrinsic and intrinsic apoptotic pathways, respectively. We investigated whether neuroprotection associated with overexpression of heat-shock protein 70 (Hsp70), a natural cellular antiapoptotic protein, is mediated by caspase-8 and caspase-9 signaling in the neonatal mouse brain after hypoxia/ischemia (H/I) injury. Methods— Postnatal day 7 transgenic mice overexpressing rat Hsp70 (Hsp70 Tg) and their wild-type (Wt) littermates underwent unilateral common carotid artery ligation followed by 30 minutes of exposure to 8% O 2 . The expression of apoptotic proteins was quantified by Western blot analysis, and the specific interaction between Hsp70 and apoptotic protease activating factor 1 (Apaf-1) was determined by coimmunoprecipitation. Results— Hsp70 overexpression reduced cytosolic translocation of cytochrome c without affecting the levels of Apaf-1 and pro–caspase-9 24 hours after H/I. The expression of these apoptotic proteins in the naïve neonatal brains was also not affected by Hsp70 overexpression. Reduced caspase-9 cleavage occurred in Hsp70 Tg mice compared with Wt littermates 24 hours after H/I and correlated with increased binding of Hsp70 and Apaf-1. Increased cellular Fas-associated death domain–like interleukin-1β–converting enzyme inhibitory protein (FLIP) expression and decreased caspase-8 cleavage were also observed in Hsp70 Tg compared with Wt mice 24 hours after H/I. Conclusions— Our results suggest that the extrinsic and intrinsic apoptotic pathways mediate the neuroprotective effects of Hsp70 overexpression in neonatal H/I, specifically by upregulating FLIP and sequestering Apaf-1, leading to reduced cleavage of caspase-8 and caspase-9.