OXIDIZED LOW-DENSITY-LIPOPROTEIN REDUCES THROMBOMODULIN TRANSCRIPTIONIN CULTURED HUMAN ENDOTHELIAL-CELLS THROUGH DEGRADATION OF THE LIPOPROTEIN IN LYSOSOMES

Oxidized low density lipoprotein (LDL), a potent atherogenic lipoprote in, has been shown to cause the alteration of various endothelial func tions. We have examined the effect of oxidized LDL on the cofactor act ivity for thrombin-dependent protein C activation and expression of th rombomodulin (TM), a cell surface antithrombotic glycoprotein, on cult ured human umbilical vein endothelial cells. Oxidized LDL prepared by irradiation of LDL with 254-nm ultraviolet light did not directly affe ct the cofactor activity of isolated TM. Exposure of the cells to oxid ized LDL (25-200 mu g/ml), but not native LDL and acetylated LDL, redu ced TM cofactor activity in parallel with its antigen levels on the ce ll surface in an oxidation-, concentration- and time dependent manner. TM mRNA levels were reduced prior to decrease in TM antigen levels an d were 50% of the control levels at 3.0 h after treatment of the cells with oxidized LDL. The apparent half-life time (t(1/2) = 2.8 h) of TM mRNA in the oxidized LDL-treated cells, however, did not significantl y differ from that (t(1/2) = 2.6 h) in the control cells when the cell s were coincubated with 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazo le, a transcriptional inhibitor. Treatment of the cells with bafilomyc in A1, an inhibitor for the proton pump of the lysosomes, inhibited in tracellular degradation of the LDL and prevented down-regulations of t he mRNA and the cell surface TM antigen levels caused by oxidized LDL. The inhibitor molecule in oxidized LDL was shown to be a lipid; organ ic solvent extracts (300 mg/ml cholesterol, an equivalent concentratio n with lipids in 200 mu g/ml oxidized LDL) of oxidized LDL inhibited e xpression of TM antigen to nearly the same extent as the oxidized LDL, although water extracts did not affect TM expression on the cells. Th ese results suggested that down-regulation of TM on endothelial cells exposed to oxidized LDL resulted from inhibition of its transcription mediated by lysosomal degradation of oxidized LDL and that a lipid com ponent in the LDL could be an active species. A decrease in TM express ion on the surface of endothelial cells may contribute to promote thro mbosis in atherosclerotic lesions.