PURPOSES. Oxidative processes may play important roles in agerelated macular degeneration. Previous studies have suggested that enhancing proteasome activity by pretreatment with low doses of proteasome inhibitors reduces injury from oxidative damage in neuronal cultures. The objective of the current study was to determine whether proteasome inhibitors could ameliorate the toxicity from oxidative stresses in ARPE-19 cells and to dissect the pathways that may mediate these protective effects. METHODS. The toxicity of oxidative stressors menadione (VK3) and 4-hydroxynonenal (4-HNE) and the protective effects of proteasome inhibitors, including MG-132 and clasto-lactacystin- b-lactone (LA), were studied in ARPE-19 cells. Binding and activation of the peroxisome proliferator-activated receptors (PPARs) family of transcription factors were studied using electrophoretic mobility shift assay (EMSA) and a peroxisome proliferator-activated response element (PPRE)-driven dualluciferase reporter gene. RESULTS. An 18-hour pretreatment with 30 to 300 nM MG-132 or 300 to 1000 nM LA reduced the toxicity of menadione or 4- HNE in ARPE-19 cells. The protective effects of MG-132 pretreatment were partially reversed by the PPARα antagonist GW6471 but not by the PPARγ antagonist GW9662; in contrast, neither agent reduced the protective effects of LA. MG-132 but not LA induced increased expression of a PPREdriven luciferase reporter gene in a dose-dependent manner. Nuclear proteins isolated from ARPE-19 cells treated by MG- 132 had increased binding to PPRE sequences as measured by EMSA. CONCLUSIONS. Our data suggest that pretreatment with proteasome inhibitors reduces oxidative injury in ARPE-19 cells and that the underlying mechanisms are different for different proteasome inhibitors, with PPARα-dependent effects for MG- 132 and PPAR-independent effects for LA.