Induction of phase 2 genes by sulforaphane protects retinal pigment epithelial cells against photooxidative damage

Abstract

The retinal pigment epithelial cell (RPE cell) layer protects the photoreceptors of the retina against oxidative stress. The decline of this capacity is believed to be a major factor in the impairment of vision in age-related macular degeneration. Exposure of human adult RPE cells to UV light at predominantly 320 -400 nm (UVA light) in the presence of all-trans-retinaldehyde results in photooxidative cytotoxicity. Significant protection of RPE cells was obtained by prior treatment with phase 2 gene inducers, such as the isothiocyanate sulforaphane or a bis-2-hydroxybenzylideneacetone Michael reaction acceptor. The degree of protection was correlated with the potencies of these inducers in elevating cytoprotective glutathione levels and activities of NAD(P)H:quinone oxidoreductase. In embryonic fibroblasts derived from mice in which the genes for the transcription factor Nrf2, the repressor Keap1, or both Nrf2 and Keap1 were disrupted, the magnitude of resistance to photooxidative damage paralleled the basal levels of glutathione and NAD(P)H:quinone oxidoreductase in each cell type. Demonstration of protection of RPE cells against photooxidative damage by induction of phase 2 proteins may shed light on the role of oxidative injury in ocular disease. Moreover, the finding that dietary inducers provide indirect antioxidant protection suggests novel strategies for preventing chronic degenerative diseases, such as age-related macular degeneration. Because oxidative damage is widely believed to contribute to the etiology and progression of many age-related chronic degenerative diseases, the development of methods for antioxidant protection is a priority and may be broadly relevant to disease prevention (1). Here, we describe a strategy for protecting retinal pigment epithelial cells (RPE cells) against photooxidative damage by coordinate elevation (induction) of the activities of a family of phase 2 genes that protect cells against injury by oxidants and electrophiles (2-5). This study complements and extends our recent demonstration that sulforaphane, a potent phase 2 gene inducer, protects human adult RPE cells against the cytotoxicity of the following oxidants: tert-butyl hydroperoxide, menadione, 4-hydroxynonenal, and peroxynitrite (6). The magnitude of this protection is correlated quantitatively with the elevation of phase 2 genes and glutathione (GSH) levels (6). Jones, Sternberg, and colleagues (7,8) have also demonstrated protection of RPE cells against oxidative damage by elevation of GSH levels and induction of the phase 2 response.Oxidative damage has been implicated in the etiology of age-related macular degeneration (AMD), which is the leading cause of blindness among the elderly. AMD entails progressive degeneration of both macular photoreceptors and associated RPE cells. This photoreceptor loss is widely believed to be, in part, secondary to degenerative changes in the RPE layer. Two significant risk indicators for AMD are the formation of drusen and the accumulation of lipofuscin within RPE cells. Dru...

Cinnamoyl-based Nrf2-activators targeting human skin cell photo-oxidative stress

“…In the skin of Nrf2 knockout mice, UVB-irradiation induced a more pronounced sunburn reaction and oxidative DNA damage as compared to wildtype mice, but no alterations in UVB-induced carcinogenesis were observed [40]. However, pharmacological induction of Nrf2 by the chemopreventive activator sulforaphane has been demonstrated to protect retinal pigment epithelial cells against photo-oxidative damage by upregulating NQO1 expression and elevating cellular glutathione content [39]. The same study demonstrated increased photosensitivity of fibroblasts from Nrf2 knockout mouse embryonic fibroblasts.…”

“…Indeed, germ line Nrf2-null mice are highly susceptible to electrophilic and oxidative stress [35]. Moreover, recent studies strongly suggest a role of Nrf2-mediated gene expression in the suppression of acute photo-oxidative damage in skin cells and also in the prevention of epidermal chemical (TPA/DMBA-induced) and UVinduced carcinogenesis [37][38][39]. In the skin of Nrf2 knockout mice, UVB-irradiation induced a more pronounced sunburn reaction and oxidative DNA damage as compared to wildtype mice, but no alterations in UVB-induced carcinogenesis were observed [40].…”

Viewpoint: Mechanisms of Action and Therapeutic Potential of Neurohormetic Phytochemicals

“…Some phytochemicals may activate the transcription factor NF-κB, which is known to mediate adaptive cellular stress responses by inducing the expression of the AOE Mn-superoxide dismutase. BDNF, brainderived neurotrophic factor; CBP, CREB-binding protein; DAG, diacylglycerol; IP3, inositol triphosphate; NGF, nerve growth factor; PLC, phospholipase C. This figure is modified from Mattson and Cheng, 2006. lial cells against photooxidative damage (Gao and Talalay, 2004). In a model of Parkinson's disease, sulforaphane protected dopaminergic neurons (Han et al, 2007).…”

Coordinate regulation of enzyme markers for inflammation and for protection against oxidants and electrophiles

“…Induction of the phase 2 response protects against reactive oxygen species (ROS) arising from exogenous oxidants and oxidative cycling in many cell lines, including ARPE-19 retinal pigment epithelial cells (26,27) and U937 leukemia cells (19). We examined this protection and its dependence on nrf2 gene function by measuring formation of fluorescent products from the oxidationsensitive dye 2Ј,7Ј-dichlorodihydrofluorescein diacetate (DCFH-DA) in RAW264.7 cells and peritoneal macrophages derived from WT and nrf2 Ϫ/Ϫ mice that were exposed to tert-butyl hydroperoxide ( Table 2).…”