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Protein Modifications by 4-Hydroxynonenal and 4-Hydroxyhexenal in Light-Exposed Rat Retina

¹From the Departments of Ophthalmology and ⁴Cell Biology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma; the ²Dean A. McGee Eye Institute, Oklahoma City, Oklahoma; and the ³Free Radical Biology and Aging Research Program, Oklahoma Medical Research Foundation, Oklahoma City, Oklahoma.

PURPOSE. 4-Hydroxynonenal (4-HNE) and 4-hydroxyhexenal (4-HHE) are reactive aldehydes derived from the nonenzymatic oxidation of n-6 and n-3 polyunsaturated fatty acids, respectively. Increasing evidence suggests that protein modifications by reactive aldehydes are involved in various diseases. The present study was undertaken to test whether protein modifications by 4-HNE and 4-HHE increase in retinal tissues after exposure of rats to damaging levels of light.

METHODS. Albino rats were exposed to 1 or 5 klux white fluorescent light for 3 hours and, at various times thereafter, the levels and localizations of aldehyde-modified proteins in retinas were assessed by densitometric analysis of semiquantitative Western dot blots and by immunohistochemistry, using 4-HNE- and 4-HHE-specific antibodies. In some rats, the protective antioxidant phenyl-N-tert-butylnitrone (PBN) was injected (50 mg/kg) before exposure to light. To assess retinal damage, outer nuclear layer (ONL) thickness was measured on hematoxylin-eosin (H&E)–stained sections, and apoptosis was semiquantitatively analyzed by TUNEL staining.

RESULTS. By dot blot analysis, 4-HNE- and 4-HHE-modified proteins were significantly increased in retina (both by 1.7-fold) and RPE fraction (1.5- and 1.8-fold, respectively) after 5-klux exposure. In retina, increases in 4-HNE- and 4-HHE-modified proteins were more prominent at 3 hours than at 24 hours or 48 hours after exposure to light. In rod outer segments, only 4-HHE-modified proteins increased significantly (1.4-fold). Retinal thinning, TUNEL staining in ONL, 4-HNE-, and 4-HHE protein modifications were all found in the same retinal regions. PBN treatment inhibited the light-induced increase of 4-HNE and 4-HHE modified proteins in retina and RPE fractions.

CONCLUSIONS. Exposure to intense light increases 4-HNE and 4-HHE protein modifications in the retina, suggesting that free radical initiated, nonenzymatic reactions are involved in this process. These modifications may be early events that precede photoreceptor cell apoptosis.

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