DUAL CA-2-PEROXIDATION OF LOW-DENSITY-LIPOPROTEIN BY ACTIVATED HUMAN MONOCYTES( REQUIREMENT FOR OPTIMAL LIPID)

The oxidative modification of LDL seems a key event in atherogenesis a nd may participate in inflammatory tissue injury. Our previous studies suggested that the process of LDL oxidation by activated human monocy tes / macrophages required O2- and activity of intracellular lipoxygen ase. Herein, we studied the mechanisms involved in this oxidative modi fication of LDL. In this study, we used the human monocytoid cell line U937 to examine the role of Ca2+ in U937 cell-mediated lipid peroxida tion of LDL. U937 cells were activated by opsonized zymosan. Removal o f Ca2+ from cell culture medium by EGTA inhibited U937 cell-mediated p eroxidation of LDL lipids. Therefore, Ca2+ influx and mobilization wer e examined for their influence on U937 cell-mediated LDL lipid peroxid ation. Ca2+ channel blockers nifedipine and verapamil blocked both Ca2 + influx and LDL lipid peroxidation by activated U937 cells. The inhib itory effects of nifedipine and verapamil were dose dependent. TMB-8 a nd ryanodine, agents known to prevent Ca2+ release from intracellular stores, also caused a dose-dependent inhibition of LDL lipid peroxidat ion by activated U937 cells while exhibiting no effect on Ca2+ influx. Thus, both Ca2+ influx through functional calcium channels and Ca2+ m obilization from intracellular stores participate in the oxidative mod ification of LDL by activated U937 cells. Ca-45(2+) uptake experiments revealed profound Ca2+ influx during the early stages of U937 cell ac tivation, however, the Ca2+ ionophore 4-bromo A23187 was unable to ind uce activation of U937 cells and peroxidation of LDL lipids. Release o f intracellular Ca2+ by thapsigargin only caused a suboptimal peroxida tion of LDL lipids. Our results indicate that although increases in in tracellular Ca2+ levels provided by both influx and intracellular Ca2 mobilization are required, other intracellular signals may be involve d for optimal peroxidation of LDL lipids by activated human monocytes.