Oxidized lipoprotein (a) induces cell adhesion molecule Mac-1 (CD 11b) and enhances adhesion of the monocytic cell line U937 to cultured endothelial cells

Because of structural similarities between low density lipoproteins (LDL) and lipoprotein (a) (Lp(a)), we have investigated the properties and the functional activities of oxidized Lp(a) and focused on whether oxidized Lp(a), like oxidized LDL, can induce monocyte differentiation and adhesion of monocytic cells to endothelial cells grown in culture. Oxidized Lp(a), prepared in vitro by cupric ion oxidation, gave absorption curves of conjugated dienes with a lag-phase of 61.7 ± 6.6 min (mean ± S.D.) as compared to 85.2 ± 7.2 min (n = 6, P < 0.01) for oxidized LDL from the same donors and at equimolar concentrations. Degradation of oxidized 125I Lp(a) by the monocytic cell line U937 at 37 °C was 1.6 ± 0.3 nmol/g of cell protein, significantly (P < 0.01) greater than the degradation of oxidized 125I-LDL, which was 1.15 ± 0.2 nmol/g of cell protein. Equimolar concentrations of oxidized Lp(a) and LDL inhibited the growth of U937 by 82 ± 8.2% and 64 ± 7.1%, respectively, when compared with the effect (negligible) produced by native Lp(a) and LDL. In addition, equimolar concentrations of oxidized Lp(a) and LDL induced adhesion molecule, Mac-1 (CD 11b), expression in U937 by 64 ± 7.1% and 58 ± 6.1% (P > 0.05), respectively, of the effect produced by phorbol esters (PMA) (P < 0.01). U937 cells incubated with oxidized Lp(a) and LDL, showed an adherence to cultured endothelial cells at 42 ± 5.2% and 34 ± 4.8%, respectively (P < 0.05), of the adherence shown by the same cells activated by PMA (P < 0.01). Our results suggest that oxidized Lp(a) like oxidized LDL plays an important role in the development of atherogenesis by inducing adhesion of monocytes to the arterial intimal and by stimulating intimal monocytes to differentiate into macrophages.