Abstract
Purpose: :
Gyrate atrophy (GA) is a type of progressive chorioretinal atrophy characterized by hyperornithinemia related to deficiency of ornithine-Δ-aminotransferase (OAT). Experiments using genetically engineered mice lacking OAT have revealed that retinal pigment epithelial (RPE) cells are the initial sites of insult in GA, while several clinical studies have demonstrated that reduction of ornithine by an arginine-restricted diet restrains the progression of chorioretinal atrophy. However, the mechanisms of RPE cell damage and protection remain unclear. We established an in vitro model of GA by addition of ornithine to OAT-deficient human RPE cells using a specific irreversible inhibitor (5-fluoromethylornitine; 5-FMO) that induced cell death. We found upregulation of arginase (ARG) II (a mitochondrial enzyme) in OAT-deficient RPE cells by ornithine using microarray analysis and attempted to clarify the role of ARGII with our in vitro GA model.
Methods: :
The expression of ARGII mRNA in cultured human RPE cells (TERT-RPE) was examined using real-time RT-PCR. The effects of ARGII gene silencing using siRNA on RPE cell viability were investigated by morphologic observations and MTT colorimetric assays. Since arginine is a precursor for synthesis of NO, the production of NO was quantified using the Griess method.
Results: :
(1) The expression of ARGII mRNA was increased by ornithine in both time- and dose-dependent manners. (2) ARGII silencing resulted in increased RPE cell death. (3) NO production was increased in our in vitro GA model and ARGII silencing resulted in greater production. (4) NO donors induced RPE cell death.
Conclusions: :
These results suggest that upregulation of ARGII by ornithine plays a critical role in cytoprotection against RPE cell damage in GA. Furthermore, insult to RPE cells caused by downregulation of ARGII might be the result of increased NO production.
Keywords: retinal pigment epithelium • gene/expression • nitric oxide