Rapamycin attenuates visible light-induced injury in retinal photoreceptor cells via inhibiting endoplasmic reticulum stress.

An extended exposure of the retina to visible light may lead to photochemical damage in retinal photoreceptor cells. The exact mechanism of retinal light damage remains unknown, and an effective therapy is still unavailable. Here, we demonstrated that rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR), markedly protected 661W photoreceptor cells from visible light exposure-induced damage at the nanomolar level. We also observed by transmission electron microscopy that light exposure led to severe endoplasmic reticulum (ER) stress in 661W cells as well as abnormal endomembranes and ER membranes. In addition, obvious upregulated ER stress markers were monitored by western blot at the protein level and by quantitative reverse transcription-polymerase chain reaction (RT-PCR) at the mRNA level. Interestingly, rapamycin pretreatment significantly suppressed light-induced ER stress and all three major branches of the unfolded protein response (UPR), including the RNA-dependent protein kinase-like ER kinase (PERK), inositol-requiring enzyme 1 (IRE1), and activating transcription factor 6 (ATF6) pathways both at the protein and mRNA levels. Additionally, the inhibition of ER stress by rapamycin was further confirmed with a dithiothreitol (DTT; a classical ER stress inducer)-damaged 661W cell model. Meanwhile, our results also revealed that rapamycin was able to remarkably inhibit the activation of mTOR and its downstream factors eukaryotic translation initiation factor 4E-binding protein 1 (4EBP1), p-4EBP1, p70, p-p70, and phosphorylated ribosomal protein S6 kinase (p-S6K) in the light-injured 661W cells. Thus, these data indicate that visible light induces ER stress in 661W cells; whereas the mTOR inhibitor, rapamycin, effectively protects 661W cells from light injury through suppressing the ER stress pathway.