Binding and uptake of oxidized low density lipoprotein (LDL) by macrophage scavenger receptors are enhanced by substrate-bound fibronectin

Defense of Living Body against Oxidative Damage

Living bodies may experience oxidative stress induced by reactive oxygen species and heavy metal ions, which may damage components in the body and cause aging and disorders. In addition to the known defense systems against oxidative damage, the author describes new defense systems. Lipid peroxidation in living bodies, which has hitherto been thought to increase oxidative damage, was found to attenuate oxidative stress-induced DNA damage. Red blood cells become senescent due to oxidative stress during circulation, where membrane band 3 becomes aggregated to anti-band 3 IgG and macrophages attached through poly-N-acetyllactosaminyl sugar chains, and the sugar chain attachment to macrophages is stimulated by oxidative stress in macrophages. Oxidized protein hydrolase that preferentially hydrolyzes proteins damaged by oxidative stress was newly discovered, which may play an important role in saving cells from oxidative damage.

Participation of the arachidonic acid cascade pathway in macrophage binding/uptake of oxidized low density lipoprotein

Arachidonic acid cascade inhibitors, including phospholipase A2 inhibitors, dexamethasone and quinacrine (mepacrine), cyclooxygenase inhibitors, indomethacin and aspirin, and lipoxygenase inhibitor AA861, prevented foam cell formation and cholesterol accumulation in the incubation of thioglycollate-induced mouse peritoneal macrophages with oxidized low density lipoprotein (LDL) at 37 degrees C for 24 h. These inhibitors similarly prevented foam cell formation of fibronectin- and Ca ionophore A23187-stimulated macrophages. Binding and/or uptake of Dil (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine)-acetyl LDL by macrophages at 37 degrees C for 3h and arachidonic acid release from macrophages at 37 degrees C for 4h were inhibited by dexamethasone. Exogenously added phospholipase A2 of bee venom and Crotalus adamanteous venom increased arachidonic acid release during incubation for 2 h, and increased macrophage binding and/or uptake of Dil-acetyl LDL at 37 degrees C for 3 h, and foam cell formation at 37 degrees C for 24 h. Protein kinase inhibitors, ML-9 and staurosporine, that inhibited macrophage binding and/or uptake of Dil-acetyl LDL did not inhibit arachidonic acid release, indicating that protein phosphorylation was not involved in the arachidonic acid pathway in the macrophage scavenger receptor activation. Nordihydroguaiaretic acid that inhibited arachidonic acid release prevented binding and/or uptake of Dil-acetyl LDL. The release of arachidonic acid was not enhanced by fibronectin-stimulation, indicating that Ca influx-dependent stimulation of macrophage activity was not through the activation of phospholipase A2. These results indicate that, as well as the fibronectin-stimulated Ca influx pathway and protein phosphorylation pathway, the arachidonic acid pathway participated in the activation of macrophages to bind and take up oxidized LDL.

Substrate-bound fibronectin enhances scavenger receptor activity of macrophages by calcium signaling

We have previously found that ability of mouse macrophages to bind and take up oxidized low-density lipoprotein (oxLDL) through scavenger receptors is significantly enhanced when the cells are plated on fibronectin (FN)-coated culture substrates. Here, the mechanisms of the enhancement of the scavenger receptor activity by the substrate-bound FN was investigated using thioglycollate-induced mouse peritoneal macrophages. A Ca(2+) channel blocker diltiazem and a calmodulin inhibitor W-7 reduced the scavenger receptor activity of the macrophages plated on FN-coated substrate to the level of the cells plated on uncoated substrate, as assessed by oxLDL binding, while the scavenger receptor activity of the macrophages on uncoated substrate was little affected. Similarly, FN-induced enhancement of the scavenger receptor activity assessed by oxLDL uptake was selectively inhibited by Ca(2+) channel blockers (diltiazem, nifedipine, verapamil) and calmodulin inhibitors (W-7, trifluoperazine). Intracellular free Ca(2+) level of the macrophages was increased, depending on extracellular Ca(2+), when plated on FN-coated substrate. This increase in the Ca(2+) level was inhibited by diltiazem and RGD-containing peptides present in cell adhesive region of FN. Like the substrate-bound FN, Ca(2+) ionophore A23187 enhanced the scavenger receptor activity of binding and taking up of oxLDL. These results indicate that substrate-bound FN enhances scavenger receptor activity of macrophages by increasing channel-dependent Ca(2+) influx. A microtubule disruptor, colchicine, and an actin filament disruptor, cytochalasin B, inhibited the FN-induced enhancement of the scavenger receptor activity, suggesting that these cytoskeletal structures are required for transmission of the adhesion signal of FN. The number of the scavenger receptors was found to increase by 1.4-fold upon adhesion signal of FN. We suggest that substrate-bound FN increases the number of the macrophage scavenger receptors as a result of induction of Ca(2+) influx and causes increased accumulation of oxLDL within the cells, rendering the cells more susceptible to conversion into foam cells.