Endogenously produced lipoprotein lipase enhances the binding and cell association of native, mildly oxidized and moderately oxidized low-density lipoprotein in mouse peritoneal macrophages

It has been well established that purified lipoprotein lipase (LPL) can facilitate the cellular uptake of various native and modified lipoproteins when added exogenously to macrophages. Because activated macrophages express LPL endogenously, it was the aim of this study to investigate the effect of macrophage-produced LPL on the uptake of native low-density lipoprotein (LDL) and LDL that has been modified to various degrees by Cu2+-mediated oxidation. Cell binding and uptake of Eu3+-labelled native and oxidized LDL was determined in mouse peritoneal macrophages (MPM) from normal mice and induced mutant mice that lack LPL expression in MPM. We found that LPL expressed by MPM was able to increase cell binding and association of native LDL (by 121% and 101% respectively), mildly oxidized LDL (by 47% and 43%) and moderately oxidized LDL (by 30% and 22%). With increased levels of lipoprotein oxidation, the relative proportion of LPL-mediated LDL uptake decreased. This decrease was not due to weakened binding of LPL to oxidized LDL. The drastically increased uptake of highly oxidized LDL in MPM by scavenger-receptor-mediated pathways might dominate the simultaneous exogenous or endogenous LPL-mediated uptake of this lipoprotein. Competition experiments with positively charged poly(amino acids) furthermore suggested that histidine, arginine and lysine residues in LPL are important for the interaction between LDL and LPL. Our results imply that physiological levels of LPL produced by macrophages facilitate the uptake of native LDL as well as mildly and moderately oxidized LDL. This process might, in the micro-environment of arteries, contribute to the accumulation of macrophage lipids and the formation of foam cells.