Photoreceptor cell apoptosis induced by the 2-nitroimidazole radiosensitizer, CI-1010, is mediated by p53-linked activation of caspase-3

The nitroimidazole radiosensitizer CI-1010 ((R)-alpha-[[(2-bromoethyl)-amino]methyl]-2-nitro-1H-imidazole-1-ethanol monohydrobromide) causes selective, irreversible, retinal photoreceptor apoptosis in vivo. The mouse 661W photoreceptor cell line was used as a neuronotypic model of CI-1010-mediated retinal degeneration. Exposure to CI-1010 for 24 h induced apoptosis in 661W cells, as determined by ultrastructural analysis, agarose electrophoresis and analysis of TUNEL-positive nuclei. CI-1010 caused a loss of viability in 661W cells, as measured by the reduction of MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide). A clear link was established between the onset of apoptosis and activity of caspase-3 and caspase-8, prior to poly[ADP-ribose]polymerase (PARP) cleavage. Pretreatment with caspase inhibitors, ZVAD.fmk or DEVD-CHO, prevented morphological changes in most CI-1010-treated cells. Evaluation of mitochondrial inner membrane potential (Δψm) in live 661W cells using the fluorescent dye, tetramethylrhodamine methyl ester revealed retention of (Δψm) until after caspase activation. Absence of cytochrome c in the cytoplasm in treated cells further supports the hypothesis of a mitochondrial-independent mechanism of cell death. Significant increase in DNA crosslinks observed in 661W cells correlates with induction and phosphorylation of p53 at multiple serine sites. Cell cycle analysis of 661W cells reveals a G2 arrest in response to CI-1010-induced DNA damage and neuronal cell death. Increased protein expression of Bax, Fas, and FasL, concomitant to the loss of Bcl-xL in treated 661W cells may be modulated by p53. This study evaluates in vitro mechanisms of CI-1010-induced cell death in photoreceptors and provides evidence in support of a p53-linked activation of caspase-3 in response to DNA damage caused by CI-1010.