Fas Ligand-Fas Signaling Participates in Light-Induced Apoptotic Death in Photoreceptor Cells

PURPOSE. To investigate the function of Fas in photoreceptors. METHODS. Postmortem human eyes and mouse-derived photoreceptor cells (661W) were examined for Fas expression by in situ hybridization and immunofluorescence. 661W cells were treated with FasL, Fas agonistic antibody, or exposed to light with/without pharmacological manipulation of Fas signaling, followed by apoptosis detection by TUNEL, immunofluorescence and FACS. Fractionated cellular extracts were used to detect protein expression or protein phosphorylation after immunoprecipitation by western blot. RESULTS. Expression of Fas was found in the photoreceptor layer of human retina. Fas and a cleaved form of FasL were found on the cell surface of 661W cells. Treatment with FasL or Fas agonistic antibody induced apoptosis in 661W cells. Blocking the activity of FasL or administration of caspase-8 inhibitor z-IETD inhibited light-induced apoptosis. However, it simultaneously caused induction of necroptosis, which could be blocked by the RIP1 inhibitor, Necrostatin-1. Light exposure in the presence of z-IETD caused hyper-phosphorylation of RIP1. Light exposure did not elevate the expression of Fas, FasL, or FADD. Cells or conditioned medium after light exposure induced apoptosis in dark-adapted cells, which could be attenuated by blockade of Fas. CONCLUSIONS. Fas has a pro-apoptotic role in photoreceptors. Under light stress, soluble and 2 membrane-bound FasL can bind to Fas inducing apoptosis via a paracrine mechanism. Although blocking Fas signaling inhibits apoptosis, it does not improve the overall photoreceptor survival due to a compensatory activation of necroptosis. Hence, prevention of photoreceptor loss from retinal photooxidative stress should target both Fas and RIP1.