INCREASED UPTAKE OF LDL BY OXIDIZED MACROPHAGES IS THE RESULT OF AN INITIAL ENHANCED LDL-RECEPTOR ACTIVITY AND OF A FURTHER PROGRESSIVE OXIDATION OF LDL

Iron ions were recently shown to induce cellular lipid peroxidation in macrophages, and these oxidized cells can convert native low-density lipoprotein (LDL) to oxidized LDL (Ox-LDL). The present study demonstr ates that deoxycholic acid (DCA) and angiotensin II (ANG-II) can also induce oxidative modification of macrophages via metal ions independen t mechanisms. Furthermore, incubation of LDL (200 mu g of protein/ml) for 24 h at 37 degrees C with DCA, ANG-II, as well as FeSO4-induced ox idized macrophages, resulted in oxidative modification of the lipoprot ein as evidenced by increased TBARS formation in LDL (by 50, 105, and 258%, respectively), decreased TNBS reactivity (by 45, 56, and 42%, re spectively), and increased cellular uptake (by 60, 166, and 230%, resp ectively). A positive correlation (n =.88) was found between the exten t of the cellular lipid peroxidation and the increment in the cellular uptake of the LDL. The oxidative modification of LDL by oxidized macr ophages was found to be a progressive process. Incubation of LDL with oxidized macrophages for increasing periods of time up to 24 h resulte d in progressive increment in: (1) the electrophoretic mobility of the LDL; (2) the TBARS formation in LDL; (3) the cellular uptake of LDL b y the oxidized macrophages via the Ox-LDL receptor. Upon fractionation on a heparin-sepharose column of LDL that was incubated for different periods of time with oxidized macrophages, a gradual increment in the unbound LDL fraction was obtained, up to 72% after 24 h of incubation . During the first hour of LDL incubation with the oxidized macrophage s a twofold increase in the cellular uptake of LDL by these cells was detected, although no significant oxidation of the lipoprotein occurre d during this short time period. This effect could be attributed to an increased number of LDL receptors on the cell surface of the oxidized macrophages. In conclusion, increased uptake of LDL by oxidized macro phages results from two routes: (1) enhanced uptake via the LDL recept or due to increased LDL receptor activity; (2) lipoprotein uptake via the Ox-LDL receptors due to cellular modification of LDL. Both of thes e processes lead to macrophage cholesterol accumulation and foam cell formation, and thus contribute to accelerated atherosclerosis under ox idative stress. (C) 1997 Elsevier Science Inc.