Role of PPAR-γ ligands in neuroprotection against glutamate-induced cytotoxicity in retinal ganglion cells

The peroxisome proliferator-activated receptor-gamma (PPAR-gamma) is the target of the insulin sensitizing thiazolidinediones (TZDs), a class of drugs used in the treatment of type 2 diabetes mellitus. Glaucoma and other retinal disorders are some of the major complications in diabetes. In the present study, the role that PPAR-gamma ligands play in protecting retinal ganglion cells (RGC-5) against glutamate insult was explored. Transformed rat RGC (RGC-5 cells) and two PPAR-gamma agonists, 15-deoxy-D(12,14)-prostaglandin J2 (15d-PGJ2) and troglitazone were used. RGC-5 cells were incubated with either of the PPAR-gamma ligands and were exposed to either L-glutamic acid or buthionine sulfoximine (BSO). Cell viability was determined with the neutral red dye uptake assay. Levels of PPAR-gamma receptor proteins were monitored by immunoblot analysis. Glutamate treatment resulted in RGC-5 cell death, and both 15d-PGJ2 and troglitazone protected the RGC-5 cells from glutamate cytotoxicity. The neuroprotective concentrations of both compounds ranged from approximately 1 to 5 micro M. Troglitazone further protected against BSO toxicity, whereas 15d-PGJ2 did not. Glutamate treatment appears to exert its cytotoxicity through oxidative damage, because pretreatment of RGC-5 cells with the antioxidants N-acetyl cysteine (NAC) and thiourea resulted in the reversal of glutamate cytotoxicity. Furthermore, the glutamate effect was not reversed by pretreatment with MK801 or DL-threo-betabenzyloxyaspartate (DL-TBOA), suggesting that glutamate cytotoxicity is not mediated through the NMDA receptor and/or glutamate transporter, respectively. Levels of PPAR-gamma receptor protein did not show any appreciable change in response to glutamate exposure, with or without 15d-PGJ2 or troglitazone. Two PPAR-gamma ligands, 15d-PGJ2 and troglitazone, protect RGC-5, an established transformed rat retinal ganglion cell line, against glutamate cytotoxicity. The neuroprotective effects of the two compounds appear to be mediated through an antioxidant rather than a PPAR-gamma-dependent pathway. These results suggest that PPAR-gamma agonists, in addition to improving insulin sensitivity, may also provide a valuable antioxidant benefit that could prove valuable in targeting ocular complications including glaucoma.