Upregulation of thioredoxin system via Nrf2-antioxidant responsive element pathway in adaptive-retinal neuroprotection in vivo and in vitro

We tested the hypothesis that stress responses mediated by the Nrf2-antioxidant responsive element (ARE) pathway are involved in the initiation of retinal neuroprotection provided by bright-cyclic-light rearing. Albino rats born and raised in dim (5 lux) or bright (400 lux) cyclic light were exposed to damaging light (3000 lux, 6 h). After exposure, the outer nuclear layer thickness and area and the electroretinogram a- and b-wave amplitudes were significantly reduced in the dim-light-reared rats compared to the bright-light-reared rats, demonstrating a light adaptation neuroprotection phenomenon. In bright-cyclic-light-reared rats, the retinal levels of thioredoxin (Trx) (2.4-fold), Trx reductase (TrxR) (2.9-fold), and proteins modified by 4-hydroxynonenal (4-HNE) (1.5-fold) were upregulated by Western blot analyses, and the nuclear translocation of Nrf2 (2.2-fold) and the DNA binding activity of Nrf2, small Maf, and cJun to the ARE were increased as determined by electrophoretic mobility shift assays. In mouse photoreceptor-derived 661W cells, pretreatment with a sublethal dose of 4-HNE protected against H2O2-induced cell damage. Treatment with 4-HNE upregulated cellular Trx, TrxR, and heme oxygenase-1 (HO-1) levels in addition to DNA binding activity of Nrf2, small Maf, and cJun to the ARE. Downregulation of Nrf2 using RNA interference technology diminished 4-HNE-mediated upregulation of Trx and Trx reductase but did not affect the upregulation of HO-1 by 4-HNE. Cytoprotection by 4-HNE pretreatment against H2O2-induced cell damage was not observed in 661W cells with a silenced Nrf2 gene. The results suggest that upregulation of the Trx system by 4-HNE via the Nrf2–ARE pathway may be involved in the molecular mechanism of the retinal neuroprotection phenomenon.