Ultraviolet-treated lipoproteins as a model system for the study of the biological effects of lipid peroxides on cultured cells. II. Uptake and cytotoxicity of ultraviolet-treated LDL on lymphoid cell lines

Abstract

The ‘cytotoxicity’ of ultraviolet-treated low-density lipoproteins (LDL) has been investigated using cultured lymphoid cell lines from normal subjects and from a patient with receptor-negative familial hypercholesterolemia. The ultraviolettreated LDL were taken up by control lymphoblasts through the classical apo B/E-receptor pathway, while they were slowly taken up by receptor-negative lymphoblasts by non-specific endocytosis. These LDL were found highly ‘cytotoxic’ on normal lymphoblasts as demonstrated by Trypan blue dye uptake, [ 3H]thymidine incorporation, lactate dehydrogenase release and by electron microscopy. The ‘cytotoxicity’ increased progressively with the concentration of ultraviolet-treated LDL in the culture medium and with the incubation time. In contrast, lymphoblasts from familial hypercholesterolemia were not sensitive to low doses of ultraviolet-treated LDL (up to 150 μg apo-B/ml). The comparison of cells from normals and familial hypercholesterolemia showed that the ‘cytotoxic’ effect occurred subsequently to the LDL uptake, either receptor-mediated or receptor-independent. Experiments combining short-time (5 h) pulse with ultraviolet-treated LDL (labelled with [ 3H]cholesteryl oleyl ether) and a relatively long-chase period (72 h) showed: (1) a relationship between the delay for the appearance of the ‘cytotoxicity’ and the amount of ultraviolet-treated LDL taken up by the cells; and (2) the existence of a minimal dose (threshold dose) for triggering the ‘cytotoxic’ effect. The use of ‘hybrid’ LDL, prepared by partial delipidation of non-treated LDL and reconstitution by re-incorporating the neutral lipid fractions isolated from ultraviolet-treated LDL, demonstrated that the ‘cytotoxic’ effect is mainly mediated by triacylglycerols and cholesterol esters. Scanning electron microscopy showed that the most prominent morphological change resulting from the uptake of ultraviolet-treated LDL was the early blebbing of plasma membranes.