The role of macrophage phenotypes in atherosclerosis

Abstract

Background and aim: The rupture of atherosclerotic plaque is the key underlying cause of cardiovascular episodes including heart attack and stroke. A key contributor to plaque instability is the macrophage. We found that while M1 macrophages are associated with plaque instability, M2 macrophages are not; rather they produce matrix molecules such as collagen that provide structural integrity to the cap. Modulating the balance of macrophage phenotypes in the plaques may hence be a treatment option. However, since some matrices, particularly the proteoglycans, can retain lipoproteins in the arterial wall, this study examined what proteoglycans M1 and M2 macrophages produce, and how is their degree of lipid binding. Method: Human monocyte-like THP-1 cells or the monocytes isolated from healthy donors were differentiated into M1 and M2 macrophages (and foam cells). The structure and low-density lipoprotein (LDL) binding ability of the proteoglycans were assessed. Results: M1 and M2 macrophages secreted a range of proteoglycans such as biglycan, perlecan and versican, with M2 macrophages producing more perlecan and versican. The glycosaminoglycan (GAG) chains decorating the core protein of these proteoglycans included chondroitin sulphate and heparan sulphate. The chondroitin sulphate produced by M1 macrophages were more sulphated than M2. Heparan sulphate from both cells included non-sulphated and sulphated disaccharides, with the latter being more abundant in the M1. The M1 proteoglycans bound more LDL than those of M2 and this was mediated by heparan sulphate. In contrast, LDL binding sites were found on the M2 proteoglycan core proteins, such as perlecan, with increased LDL binding evident upon cleavage of GAGs. M2 macrophages were co-localised with perlecan and lipid accumulation in plaques, suggesting that they may help in LDL retention via perlecan. When M1 and M2 macrophages transformed into foam cells, they produced less proteoglycans. However, the proteoglycans did not differ in the degree of LDL binding between the phenotypes. Conclusion: Both M1 and M2 macrophages produced proteoglycans that could bind LDL, but in different ways: the M1 macrophage-derived proteoglycans bound LDL via heparan sulphate, whereas those produced by M2 macrophages used core proteins to bind LDL. Therefore, M1 and M2 macrophages are likely to contribute to LDL retention during the development of atherosclerosis.