Thyroxine Ameliorates Oxidative Stress by Inducing Lipid Compositional Changes in Human Lens Epithelial Cells

Abstract

Lipid saturation and sphingolipids make model membranes less susceptible to oxidation. A human lens epithelial cell line, HLE B-3, was treated with thyroxine to determine whether this treatment increases lipid saturation and membrane sphingolipids, as it does in other tissues, and if so, to see whether the treatment ameliorates the affects of lipid oxidation. One group of HLE B-3 cells was treated with thyroxine, and another group was not. Cells were then grown in a normoxic (20% O(2)), or hyperoxic (80% O(2)), atmosphere. Phospholipid composition was determined by matrix-assisted laser desorption ionization time-of-flight mass spectrometry and (31)P nuclear magnetic resonance spectroscopy. Cell viability was determined with a trypan blue dye assay. A chromogenic reagent was used to measure the secondary products of lipid oxidation. After 6 days of growth in a hyperoxic atmosphere, the thyroxine-treated cells were 20 times more viable than were the untreated cells. As a result of thyroxine treatment, the phosphatidylcholine (PC)-to-sphingolipid molar ratio decreased significantly (by 52%), and the PCs were eight times more unsaturated than were the sphingomyelins. The decrease in the amount of PCs coupled with a 33% decrease in the average unsaturation of the sphingolipids resulted in a phospholipid membrane with fewer double bonds. Products of lipid oxidation were three times higher in untreated cells after growth in a hyperoxic atmosphere than in untreated cells grown in a normoxic atmosphere. Thyroxine treatment reduced the amount of lipid oxidation products by approximately 60% compared with that in untreated cells. A 100% increase in cardiolipin with thyroxine treatment may contribute to a decrease in reactive oxygen species generated by the mitochondria. The total antioxidant power was not affected by thyroxine. Therefore, thyroxine-induced fluctuations in antioxidant levels are unlikely to influence increased cell viability and a concomitant decrease in the amount of lipid oxidation products in thyroxine-treated cells. The results support the idea that membranes containing more cardiolipin and more sphingolipids and having higher levels of saturation are more resistant to oxidation and protect cells from oxidative stress. Development of a therapy to increase sphingomyelins and lipid saturation in the lens may delay the onset of cataract.