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A. Kasus-Jacobi, L. D. Wicker; Role of Retinol Dehydrogenases RDH11 and RDH12 in the Clearance of Toxic 4-Hydroxynonenal. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3999.
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We hypothesize that the physiological role of the two closely related retinol dehydrogenases RDH11 and RDH12 is to protect photoreceptor cells from toxic hydroxy aldehydes, such as 4-hydroxynonenal (4-HNE), produced during oxidative stress. If not cleared, 4-HNE forms adducts with proteins disrupting important cellular functions and leading to cell apoptosis. Here, we investigated the ability of RDH11 and RDH12 to protect against 4-HNE-induced adduct formation and apoptosis in transfected HEK-293 cells and in wild-type and knockout mouse retina.
HEK-293 cells were transfected with Rdh11 or Rdh12 and subjected to increasing concentrations of 4-HNE. Within the population of transfected cells, the percent of cells undergoing apoptosis was determined by flow cytometry. Retinas were dissected from wild-type and Rdh11 and Rdh12 knockout mice and incubated with increasing concentrations of 4-HNE. After ½, 1, 2, 4, 6, and 8 h incubation of retinal explants, 4-HNE-protein adducts were quantified by dot blot using anti-HNE antibody. Quantification of RDH11 and RDH12 was performed by immunoblot analysis.
We found that transfected RDH11 and RDH12 were expressed at similar levels in HEK-293 cells and protected 75% and 50%, respectively, of the transfected cells against 4-HNE-induced apoptosis. In whole mouse retina, RDH12 is expressed at a 7-fold higher level than RDH11. Formation of 4-HNE-protein adducts was not significantly increased in the Rdh11 knockout retinal explants. By contrast, we found that the formation of 4-HNE-protein adducts was significantly increased in the Rdh12 knockout retinal explants.
The protective effect of these enzymes in HEK-293 cells is most likely due to their ability to reduce 4-HNE to alcohol, preventing the formation of protein adducts and apoptosis. In mouse retina RDH12, but not RDH11, is efficient in preventing adduct formation probably because it is expressed at a higher level. Light stress induces the production of 4-HNE in photoreceptor cells. A defective clearance of 4-HNE in Rdh12 knockout photoreceptors could explain the higher sensitivity of these mice to light-induced retinal damage. Accumulation of 4-HNE could also mediate the retinal dystrophy associated with RDH12 mutations in patients with Leber Congenital Amaurosis.
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