1 Introduction [1] [2–5] [6,7] [8,9] [10] [11,12] [13] [10–12] [14,15] [16,17] [1] [1] [16] [18] [10] [19] [20] [21] [1] [22] [16] The aim of this study is to investigate the effects of the oxidative state of CMR on their uptake by macrophages and on the accumulation of lipid within the cells, and to determine how oxidation affects the pathways involved in the internalisation of the particles. Chylomicron remnant-like particles (CRLPs) at three different levels of oxidation (CRLPs, oxidized CRLPs (oxCRLPs) and CRLPs containing the antioxidant probucol (pCRLPs)) and macrophages derived from the human monocyte cell line THP-1 were used as the experimental model, and the mechanisms of uptake were evaluated using specific inhibitors of the processes believed to be involved. The findings clearly demonstrate that oxidation of CRLPs reduces the rate of their uptake by THP-1 macrophages and decreases lipid accumulation in the cells, and further show that this is due to differential interaction with apoE dependent receptors. 2 Materials and methods l l 2.1 Preparation of lipoproteins d g d g d d g d d d [23] g d g d d g g g 4 4 2.2 Culture of THP-1 cells 2 [24] g 2.3 Analytical methods Table 1 [25] [26] [27] [28] t 3 Results 3.1 Characteristics of CRLPs Table 2 [15,29] Table 2 d Fig. 1 Fig. 1 3.2 Effect of the oxidative state of CRLPs on the induction of lipid accumulation in macrophages Fig. 2 P P Fig. 2 Fig. 2 3.3 Effect of the oxidative state of CRLPs on their uptake by macrophages Fig. 3 Fig. 3 P P Fig. 3 P Fig. 3 3.4 Role of apoE in the uptake of CRLPs of different oxidative states n Fig. 4 P Fig. 5 Fig. 5 Fig. 5 Fig. 5 P Fig. 5 P P Fig. 5 n P P Fig. 6 3.5 Role of scavenger receptors and phagocytosis in the uptake of CRLPs of different oxidative states [30] Fig. 7 P P Fig. 7 Fig. 7 Fig. 7 Fig. 6 Fig. 7 3.6 Expression of mRNA for the LDLr and LRP in THP-1 macrophages n n 4 Discussion [23] [31–34] [23,35] [24,36] [16] [15] [14] , [24,36,37] [24] Fig. 2 Fig. 3 Table 2 [10,19,38] Figs. 4, 5 Fig. 5 [18] Fig. 5 [39] [39,40] [41] Fig. 7 [22] [42] Fig. 7 [1,20] [43] [44] [21] The results of this study demonstrate that oxidative modification of CMR as compared to LDL has profoundly different effects on the uptake of the particles and the subsequent induction of lipid accumulation in macrophages. Instead of markedly enhancing foam cell formation, oxidation of CMR slows their uptake by macrophages and reduces the amount of lipid subsequently accumulated in the cells. This difference may be due to the different receptor mechanisms involved, since oxidation of LDL shifts the main route of uptake from the regulated LDLr to the unregulated scavenger receptors, while our experiments suggest that CMR are taken up mainly by the LRP with some contribution from the LDLr, with CD36 and phagocytosis playing only minor roles, irrespective of their oxidative state. These findings provide important new information about the way in which oxidation of CMR influences their induction of foam cell formation and the mechanisms involved, and has important implications for the role of dietary factors such as oxidized lipids and antioxidants which are transported in CMR in the promotion of atherosclerosis.