α-tocopherol and trolox block the early intracellular events (TBARS and calcium rises) elicited by oxidized low density lipoproteins in cultured endothelial cells

Low-density lipoproteins (LDLs), treated by UV-C radiations under conditions permitting mildly oxidized LDL (6 ± 2 nmol TBARS/mg apoB, without major structural or functional alteration of apoB), have been used for studying their cytotoxicity to cultured bovine aortic endothelial cells and the cytoprotective effect of various analogs of a-tocopherol. Toxic doses of oxidized LDL evoked intracellular events, such as cellular thiobarbituric acid reactive substances (TBARS) and a sustained peak of [Ca2+]i (cytosolic calcium). The sustained [Ca2+]i peak seems to be directly involved in the genesis of cell injury leading to cell death in contrast to cellular TBARS, which seems to be either an earlier step of signal transduction or a side effect, as shown by inhibiting the [Ca2+]i rise by ethylene glycol-O,O′-bis(amino ethyl)-N1N1N1′N′-tetraacetic acid (EGTA) added just before the time of the [Ca2+]i peak. When a-tocopherol or trolox (α short-chain, water-soluble analog of α-tocopherol) were added to the culture medium simultaneously with oxidized LDL, they were able to increase the resistance of endothelial cells against the cytotoxic effect of oxidized LDL, whereas α-tocopheryl acetate and a-tocopheryl succinate were almost completely ineffective because of the liberation of only very low levels of α-tocopherol. Trolox exhibited a more potent cytoprotective effect than α-tocopherol (IC50: 1 ± 0.2and8 ± 2 μmol/l for trolox and α-tocopherol, respectively). As shown by preincubating cells with effective concentrations of α-tocopherol or trolox, the cytoprotective effect was completely independent of any inhibition of LDL oxidation and was remanent for 2 d with α-tocopherol or for 3–4 d with trolox. Cytoprotective concentrations of trolox and α-tocopherol did not inhibit LDL uptake but acted at the cellular level by blocking the formation of cellular TBARS and the sustained [Ca2+]i peak as well. The potential relevance of these data in relation to the prevention of atherosclerosis is discussed.