Abstract
The aim of this study was to determine the effects of vitamin E (α-tocopherol) on the low density lipoprotein (LDL) receptor, a cell surface protein which plays an important role in controlling blood cholesterol. Human HepG2 hepatoma cells were incubated for 24 hours with increasing amounts of α, δ, or γ-tocopherol. The LDL receptor binding activity, protein and mRNA, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase mRNA, cell cholesterol and cell lathosterol were measured. The effect of α-tocopherol was biphasic. Up to a concentration of 50 μM, α-tocopherol progressively increased LDL receptor binding activity, protein and mRNA to maximum levels 2, 4 and 6-fold higher than control, respectively. The HMG-CoA reductase mRNA and the cell lathosterol concentration, indices of cholesterol synthesis, were also increased by 40% over control by treatment with 50 μM α-tocopherol. The cell cholesterol concentration was decreased by 20% compared to control at 50 μM α-tocopherol. However, at α-tocopherol concentrations higher than 50 μM, the LDL receptor binding activity, protein and mRNA, the HMG-CoA reductase mRNA and the cell lathosterol and cholesterol concentrations all returned to control levels. The biphasic effect on the LDL receptor was specific for α-tocopherol in that δ and γ-tocopherol suppressed LDL receptor binding activity, protein and mRNA at all concentrations tested despite the cells incorporating similar amounts of the three homologues. In conclusion, α-tocopherol, exhibits a specific, concentration-dependent and biphasic "up then down" effect on the LDL receptor of HepG2 cells which appears to be at the level of gene transcription. Cholesterol synthesis appears to be similarly affected and the cell cholesterol concentration may mediate these effects.
Highlights
Nutrition Journal 2003, 2 http://www.nutritionj.com/content/2/1/3 reported that the plasma cholesterol was twofold higher than normal in rabbits made deficient in vitamin E and that dietary replenishment of the vitamin normalised the cholesterol concentration
The low density lipoprotein (LDL) receptor binding activity of the HepG2 cells incubated for 24 h in media containing 0 to 100 μM α-tocopherol was measured as the calcium-dependent binding of colloidal gold-LDL
The upregulation phase was specific for αtocopherol in that only downregulation of the LDL receptor was observed with δ- and γ-tocopherol at comparable concentrations
Summary
Nutrition Journal 2003, 2 http://www.nutritionj.com/content/2/1/3 reported that the plasma cholesterol was twofold higher than normal in rabbits made deficient in vitamin E and that dietary replenishment of the vitamin normalised the cholesterol concentration This effect was later confirmed by others in the rat [2,3,4] as well as in the rabbit [5,6,7]. In animal models of diet-induced hypercholesterolaemia, where the animals are not deficient in vitamin E, α-tocopherol supplementation often decreases plasma cholesterol [8,9,10,11,12] This is not always the case ; in some studies either no change [13,14,15] or even an increase [16] in plasma cholesterol was observed.
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