Prevention of ornithine cytotoxicity by nonpolar side chain amino acids in retinal pigment epithelial cells

Purpose: To investigate the effect of Amino acids on ornithine cytotoxicity in ornithine-delta-aminotransferase (OAT)-deficient human retinal pigment epithelial (RPE) cells as an in vitro model of gyrate atrophy (GA) of the choroid and retina.

Methods: RPE cells were treated with 0.5 mM 5-fluoromethylornithine (5-FMOrn), a specific and irreversible OAT Inhibitor. OAT-deficient RPE cells were incubated with 10 mM ornithine in the presence of 20 mM of 1 of 18 Amino acids or 10 mM 2-amino-2-norbornane-carboxylic acid (BCH), a conventional inhibitor of the amino acid transporter system L. Ornithine cytotoxicity and cytoprotective effects of each amino acid was evaluated with a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) colorimetric assay 72 hours after treatment with ornithine in OAT-deficient RPE cells. Ornithine incorporation into RPE cells was evaluated using DL-[14C]ornithine.

Results: An MTT colorimetric assay revealed that small and large zwitterionic Amino acids, but not acidic or basic Amino acids, decreased ornithine cytotoxicity in OAT-deficient RPE cells. Incorporation of DL-[14C]ornithine by RPE cells decreased to 79% of the control level after incubation for 48 hours with 20 mM leucine, the most effective cytoprotective amino acid. Further, BCH prevented ornithine cytotoxicity in a dose-dependent manner. Both LIGHT and heavy chains of L-type amino acid transporter (LAT)-1, LAT2, y+LAT1, and 4F2hc were expressed in RPE cells.

Conclusions: The present results demonstrate that L-type amino acid transporter(s) may be involved in protection against ornithine cytotoxicity in human RPE cells. Thus, amino acid transportation in RPE cells may be a good target for a new therapy for GA as well as other kinds of chorioretinal degeneration.