Relationship between dietary retinol and lipofuscin in the retinal pigment epithelium

A variety of evidence suggests that autoxidation of cellular components probably plays a significant role in the age-related accumulation of lipofuscin, or age-pigment, in the mammalian retinal pigment epithelium (RPE). Among the likely candidates for conversion into RPE lipofuscin fluorophores via autoxidative mechanisms are vitamin A compounds, which are present in the retina and RPE in high concentrations. Vitamin E, an important lipid antioxidant, is likely to inhibit vitamin A autoxidation. Experiments were conducted to evaluate the significance of vitamin A autoxidation in the deposition of lipofuscin in the RPE. Albino rats were fed diets either supplemented with or lacking vitamin E. Each of these two groups of animals was further subdivided into three groups which were fed different levels of vitamin A palmitate: none, 14.0 mumol/kg diet, and 80.5 mumol/kg diet. After 26 weeks, the animals were killed and the RPE lipofuscin contents were determined by both fluorescence measurements and quantitative ultrastructural morphometry. Vitamin A palmitate deficiency led to significant reductions in RPE lipofuscin deposition, relative to the amounts of this pigment present in the groups receiving vitamin A palmitate in their diets. The relative magnitude of the vitamin A effect was greater in the vitamin E-supplemented groups than in the groups fed the diets deficient in vitamin E. This finding suggests that vitamin E interacts with vitamin A ester metabolites in vivo in a more complex manner than simply acting as an antioxidant protectant. Rats fed the diets containing the higher level of vitamin A palmitate failed to display elevated RPE lipofuscin contents relative to those in the rats fed 14.0 mumol of vitamin A palmitate/kg diet. Failure of high vitamin A intake to enhance RPE lipofuscin deposition may have been due to the fact that intake of vitamin A above normal levels did not lead to an elevation in vitamin A content of the retinal tissue. Establishing an effect of vitamin A deficiency on RPE lipofuscin deposition and characterization of the interactions between vitamins E and A are important steps toward defining precisely the molecular and cellular mechanisms underlying age-pigment accumulation in the RPE.