A positive feedback loop between nitric oxide and amyloid β (1-42) accelerates mitochondrial damage in human lens epithelial cells

We have reported that excessive nitric oxide (NO), like other reactive oxygen species (ROS), causes a decrease in cytochrome c oxidase (CCO) activity and ATP levels (mitochondrial damage) resulting in lens opacity. In addition, previous reports have shown that oxidative stress caused by ROS enhances amyloid β (Aβ) production in mammalian lenses, and that Aβ_1-42 stimulates inducible nitric oxide synthase (iNOS) promoter activity. Based on these reports, we investigated the relationship between NO and Aβ_1-42 production in human lens epithelial (HLE) cells. iNOS was induced by the co-incubation of HLE cells with 1000 IU interferon-γ (IFN-γ) and 100 ng/ml lipopolysaccharide (LPS) for 48 h. This led to enhanced NO release, an increase in the gene expression levels of proteins related to Aβ production, and the cellular accumulation of Aβ_1-42. Moreover, both aminoguanidine (AG, a selective inhibitor of iNOS) and diethyldithiocarbamate (DDC, a nuclear factor-kappa B (NFκB) inhibitor) attenuated these changes in IFN-γ and LPS stimulated HLE cells. Based on our finding that Aβ_1-42 accumulation is induced by co-incubation of HLE cells with both IFN-γ and LPS, we prepared a HLE cell model with Aβ_1-42 accumulation (Aβ-accumulated-HLE cell model) by pre-stimulating cells with IFN-γ and LPS for 48 h. Aβ_1-42 accumulation caused NO production via iNOS, resulting in an enhancement in the mRNA levels for enzymes necessary for the proteolysis of amyloid precursor protein (APP) to Aβ in HLE cells. In addition, excessive NO produced in response to Aβ_1-42 accumulation led to a decrease in CCO activity and ATP levels. Taken together, we hypothesize that excessive NO production in the lens epithelium enhances Aβ_1-42 production, and that this enhancement accelerates NO release. The enhancement in NO production in the lens epithelium based on positive feedback (NO-Aβ positive feedback loop, a vicious cycle) may promote the onset of cataracts (lens opacification) via the decrease in CCO activity and ATP levels. These findings provide significant information that can be used to design further studies aimed at developing anti-cataract drugs.