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Dietary Flavonoids Afford Cellular Protection
It is increasingly clear that new approaches to treating AMD should focus on both preventing the initial insults that lead to disease progression, and rescuing the retinal pigment epithelium (RPE) and photoreceptor cells that have been damaged. Mounting evidence suggests that chronic oxidative stress may damage the retinal-RPE and predispose it to developing age-related macular degeneration (AMD).
Flavonoids are a large class of polyphenolic compounds found in fruits, vegetables, teas and wine that have evolved to protect plants from the oxidative damage caused by chronic UV exposure. Flavonoids are potent antioxidants that can also influence many cellular enzyme functions. They directly neutralize reactive oxygen species (ROS), and they modulate cell-signaling pathways. In particular they can induce expression of phase-2 proteins that enhance the cell's natural defenses against oxidative stress.
Large-scale observational studies have linked greater dietary intake of certain flavonoids with a lower risk of cardiovascular disease and some cancers. In addition, many of the foods associated with a reduced risk of AMD in epidemiologic studies contain high concentrations of flavonoids, and at least one flavonoid has been identified in the mammalian retina. Earlier experiments report that specific flavonoids can protect retinal ganglion cells from oxidative stress.
Taken together, these observations led investigators from the Scripps Research Institute to determine whether specific flavonoids found in common fruits and vegetables can protect human RPE cells from oxidative stress-induced death.
Research Questions and Methods
A series of experiments were conducted utilizing cultured adult human RPE cells to address the following questions: 1) Can specific dietary flavonoids protect RPE cells from oxidative stress induced cell death? 2) Can these flavonoids prevent cell death after the exposure has occurred? 3) Can these flavonoids act through an intracellular route to reduce the accumulation of ROS? 4) Do flavonoids activate Nrf2 in human RPE cells and induce the expression of phase-2 genes? The techniques employed to answer these questions included cytotoxicity assays, microscopy, isolated RNA analysis, delayed response cyto-protection assay, determination of ROS production and Western blot analysis of HO-1 and Nrf2 expression.
All six flavonoid classes were tested, and the most effective were the flavones and flavonols. Specific members of these two classes which protected against oxidative-stress induced death with good efficacy and low toxicity include quercetin, fisetin, luteolin and EGCG.
These same flavonoids and several others induced the expression of phase-2 detoxification enzymes in the cultured RPE cells. Compared with control cells, quercetin was more protective of RPE cells than vitamin C or various vitamin E compounds (see figure below), although both of these vitamins are known to be better absorbed in vivo than quercetin.
These findings raise the possibility that flavonoids may be among the natural compounds that contribute to the ocular benefits associated with foods identified in epidemiologic studies. Spinach, for example, is not only a source of the more familiar lutein and zeaxanthin, but is a rich source of quercetin and luteolin which were two of the most effective flavonoids in this study. Quercetin is also found red wine, and is highly concentrated in foods such as onions, peppers, currants and black tea, which is also a rich source of EGCG.
Reference Hanneken A, et al. Flavonoids protect human retinal pigment epithelial cells from oxidative stress-induced dealth. Inv Ophthalmol Vis Sci. 47:3164-77, 2006.
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