Effects of Fas ligand on Ischemia/Reperfusion Injury of Cardiomyocytes

We tested the hypothesis that shedding of membrane FasL is a mechanism for downregulating FasL/Fas signaling and both membrane and soluble FasL are involved in cardiomyocyte hypoxia/ reoxygenation (H/R) injury. We examined the relative importance of mitochondrial damage and direct cleavage of the executioner caspases by activated initiator caspase-8 in the propagation of FasL/Fas signaling activated by either recombinant membrane FasL or H/R. In neonatal rat cardiomyocytes maintained under normal culture conditions, recombinant human soluble FasL increased caspase-3 activation by twofold but did not reduce cell viability. In contrast, infection with a recombinant adenoviral vector expressing the non-cleavable human FasL (Ad2/ nchFasL) resulted in cardio- myocyte death that was attenuated by soluble FasL. H/R increased the mRNA levels of both FasL and Fas and activated caspases-8, -9 and -3, indicating the activation of FasL/Fas signaling. Z-IETD-fmk and Z-LEHD-fmk, selective inhibitors for caspases-8 and-9, respectively, abolished caspase-3 activation induced by Ad2/nchFasL or H/R. Z-IETD-fmk also significantly reduced Ad2/nchFasL- or H/R- induced cardiomyocyte death. H/R potentiated membrane FasL-induced cell death. Shedding of membrane FasL downregulates FasL/Fas signaling in cardiomyo- cytes and both membrane and soluble FasL contribute to H/R injury.Activation of FasL/Fas signaling by either recombinant membrane FasL under normal culture conditions or H/R causes cardiomyocyte death mainly through the mitochondrial damage/caspase -9 activation pathway.