title: Fas ligand-Fas signaling participates in light-induced apoptotic death in photoreceptor cells.
abstract: OBJECTIVE
  We investigated 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 or Fas agonistic antibody, or exposed to light with/without pharmacological manipulation of Fas signaling, followed by apoptosis detection by TUNEL, immunofluorescence and fluorescence activated cell scanning (FACS). Fractionated cellular extracts were used to detect protein expression or protein phosphorylation after immunoprecipitation by Western blot.
  
  
  RESULTS
  Fas was expressed 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 receptor-interacting protein 1 (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 the Fas-associated death domain adaptor protein (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 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 photo-oxidative stress should target Fas and RIP1.
authors.affiliation: Department of Ophthalmology & Visual Sciences, University of Illinois at Chicago, Chicago, Illinois, USA.
authors.forename: Qing
authors.initials: Q
authors.lastname: Chang
authors.forename: Marcus E
authors.initials: ME
authors.lastname: Peter
authors.forename: Michael A
authors.initials: MA
authors.lastname: Grassi
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000187:drug effects / Q000502:physiology*; D015153:Blotting, Western; D061945:Caspase Inhibitors; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D053222:Fas Ligand Protein / Q000187:drug effects / Q000378:metabolism*; D006801:Humans; D007093:Imidazoles / Q000494:pharmacology; D047468:Immunoprecipitation; D020287:In Situ Nick-End Labeling; D007211:Indoles / Q000494:pharmacology; D008027:Light / Q000009:adverse effects*; D051379:Mice; D010786:Photoreceptor Cells / Q000187:drug effects*; D019014:fas Receptor / Q000502:physiology*
chemical_list: D061945:Caspase Inhibitors; D015853:Cysteine Proteinase Inhibitors; C491901:FAS protein, human; D053222:Fas Ligand Protein; C491899:Fas protein, mouse; D007093:Imidazoles; D007211:Indoles; D019014:fas Receptor; C507699:necrostatin-1