Involvement of double-stranded RNA-dependent protein kinase in ER stress-induced retinal neuron damage.
OBJECTIVE
To clarify whether the activation of double-stranded RNA-dependent protein kinase (PKR) participates in the cell death induced by endoplasmic reticulum (ER) stress, the authors used cultured retinal ganglion cells (RGC-5, a rat ganglion cell line transformed with the E1A virus) in vitro and the effect of a PKR inhibitor (an imidazolo-oxindole derivative) on N-methyl-D-aspartate (NMDA)-induced retinal damage in mice in vivo.


METHODS
In RGC-5 culture, cell damage was induced by tunicamycin (an ER stress inducer), and cell viability was measured by Hoechst 33342, YO-PRO-1, or propidium iodide (PI) double staining or by the resazurin-reduction test. Levels of glucose-regulated protein (GRP) 78/BiP, activating transcription factor 4 (ATF4), C/EBP-homologous protein (CHOP) and the phosphorylated form of PKR were analyzed by immunoblot. The PKR inhibitor and two siRNAs that recognize nonoverlapping sequences of rat PKR were tested for their effects on tunicamycin-induced cell death. In vivo, retinal cell damage was induced by intravitreal injection of NMDA (20 nmol/eye) in mice. To examine its effect in vivo, the PKR inhibitor (1 nmol/eye) was intravitreally injected with NMDA, and ganglion cell layer cell loss and inner plexiform layer thinning were evaluated 7 days after NMDA injection.


RESULTS
Treatment with tunicamycin at 1, 2, and 4 microg/mL for 24 hours increased the number of YO-PRO-1 and PI-positive (apoptosis or necrosis indicator) cells in a concentration-dependent manner. Immunoblotting analysis showed that tunicamycin at 2 microg/mL induced BiP, ATF4, and CHOP protein production and PKR phosphorylation. Both the PKR inhibitor (0.03-1 microM) and the PKR knockdown (using siRNA) inhibited tunicamycin-induced RGC-5 cell death. The same inhibitor also reduced NMDA-induced retinal damage in vivo. The PKR inhibitor reduced the tunicamycin-induced increase in CHOP but not that in BiP protein production.


CONCLUSIONS
These results indicate that inhibiting PKR activation is neuroprotective against ER stress-induced retinal damage, suggesting that PKR activation may be involved in the mechanisms underlying ER stress-induced cell death.
Shimazawa
Masamitsu
M
Department of Biofunctional Molecules, Gifu Pharmaceutical University, Gifu, Japan.
Ito
Yasushi
Y
Inokuchi
Yuta
Y
Hara
Hideaki
H
D000818:Animals; D021461:Animals, Outbred Strains; D000998:Antiviral Agents / Q000633:toxicity; D017209:Apoptosis / Q000187:drug effects / Q000502:physiology*; D002461:Cell Line, Transformed; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000201:enzymology; D018690:Excitatory Amino Acid Agonists / Q000633:toxicity; D006360:Heat-Shock Proteins / Q000378:metabolism; D051379:Mice; D018832:Molecular Chaperones / Q000378:metabolism; D016202:N-Methylaspartate / Q000633:toxicity; D010766:Phosphorylation / Q000187:drug effects; D034741:RNA, Small Interfering; D051381:Rats; D012165:Retinal Ganglion Cells / Q000187:drug effects / Q000201:enzymology* / Q000473:pathology*; D051742:Transcription Factor CHOP / Q000378:metabolism; D014415:Tunicamycin / Q000633:toxicity; D019892:eIF-2 Kinase / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D000091342:Endoplasmic Reticulum Chaperone BiP
D000998:Antiviral Agents; D018690:Excitatory Amino Acid Agonists; D006360:Heat-Shock Proteins; D018832:Molecular Chaperones; D016202:N-Methylaspartate; D034741:RNA, Small Interfering; D051742:Transcription Factor CHOP; D014415:Tunicamycin; D019892:eIF-2 Kinase; C051684:GRP78 protein, rat; D000091342:Endoplasmic Reticulum Chaperone BiP